Neotectonics 2014

Chapter - 2

TECTONIC FRAMEWORK-NEOTECTONICS

AND SEISMOTECTONICS

2.1 GENERAL

The tectonic fabric of the study region is cumulative effects of

geodynamic processes that were in operation and that are in operation. The

mid oceanic Ridge at Indian Ocean drifts the Indian plate at the rate of 4 -

5cm/year at an average and it was 20 cm/ year until it collided with Asia

(Searle, 2005). Being part of actively colliding Indian plate, though covered

with hard crystalline Archaean rocks the prowess of tectonic disturbances are

imminent in the form of adjustment tectonics and reactivation of faults etc. A

reappraisal on tectonic framework will enlighten the tectonic events and

possible places of reactivation.

2.2 TECTONIC FRAMEWORK OF SOUTH INDIA AND STUDY REGION

The Southern Granulite Terrain (SGT) has a complex evolutionary

history from the early Archean to late Neoproterozoic (3500– 550 Ma) with

repeated multiple deformations, anatexis, intrusions and polyphase

metamorphism (Bartlett et al., 1998; Bhaskar Rao et al., 2003). The region

essentially consists of charnockite and khondalite group of rocks and their

magmatic derivatives, supracrustals, gneissic complex, intruded by mafic–

ultramafic rocks, granites and alkaline complexes of various periods. Fermor

was the first to divide the Indian Peninsular shield into “Charnockite and

Non Charnockite” Regions, prior to that both were grouped together as the

Archaean “Dharwar system. The SGT is separated from the Dharwar Craton

by the orthopyroxene isograd known as the Fermor line. However, there

exists a narrow transition zone along which the low-grade greenstone granite

domain transforms to high-grade granulite facies rocks (Swaminath et al.,

1976). The SGT is a mosaic of crustal blocks consisting of highland

charnockite massifs separated from each other by a network of low-lying

shear zones extending in different directions viz. NE–SW, E–Wand NW–SE

Chapter 2 - Tectonic Framework-Neotectonics.....

Geoinformatic Modelling for Certain Georesources and Geohazards of Attur Valley, Tamil Nadu, India.

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The most prominent among the charnockite massifs are the Biligiri Rangan,

the Shevaroy, the Nilgiri and the Kodaikanal hills and they constitute the

northern massif (Nr-M). The most important shear zones of the region are the

Moyar– Bhavani –Salem- Attur (MBSASZ), the Palghat–Cauvery (PCSZ) and

the Achankovil (AKSZ) shear zones. The SGT is composed of rocks of two

different ages, the northern part is of Archaean age, Northern massif or

Northern granulite block (NGT) and the southern part Pandiyan mobile belt

(PMB) is of neoproterozoic age and they are separated by MBSASZ and

PCSZ.The Achankovil Shear Zone marks the southern limit of the Madurai

Block and the northern limit of the Trivandrum Block (TB). The MBSASZ

branches into several curvilinear shear zones in the NE–SW direction.

Prominent among them is the Mettur shear zone (MESZ) (Fig. 2.1).

Based on satellite data and subsequent ground follow-up, Chetty et al.

(2003) termed the network of crustal-scale shear zones as the Cauvery Shear

Zone system (CSZ). They divided the region between the Biligiri Rangan and

Kodaikanal high-grade charnockite massifs into the Moyar–Bhavani- Salem-

Attur shear zone (MBSASZ), the Chennimalai–Noyil shear zone (CNSZ), the

Dharapuram shear zone (DSZ), the Devattur–Kallimandayam shear zone

(DKSZ) and the Karur–Oddanchatram shear zone (KOSZ). The well-known

Palghat–Cauvery Shear Zone broadly coincides with the Chennimalai–Noyil

shear zone. All the shear zones of the CSZ exhibit dextral strike –slip

movement with a maximum lateral displacement of ~80km

(Drury et al., 1984; Chetty et al., 2003). Most of these shear zones are intruded

by the late Neoproterozoic (750–550 Ma) granites and alkaline (carbonatite

and syenite) plutons (Anil Kumar et al., 1998; Santosh et al., 2005). Some of

these are associated with layered anorthosites and mafic/ultramafic

complexes.

The Charnockite group occupying the eastern & central parts of the

Salem district includes charnockite, pyroxene granulite and banded magnetite

quartzite. The charnockite in the Kolli hill and Shevaroy hill is altered to

bauxite & laterite. A number of shear zones traverse E-W trending foothill of



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the Kolli Malai. Pyroxene granulite bands associated with magnetite

quartzites occurs as interbanded sequence with charnockite and form good

marker in deciphering the structure of the area as seen around Attur. The

Satyamangalam group rocks comprising fuchsite quartzite and amphibolites

occur in a linear zone surrounding the Sankari dome.

Some of the important tectonic domains are discussed below;

2.2.1 Northern Massif (Nr-M):

An overall prominent NNE-trend (Dharwar trend) characterizes the

vast expanse of various Northern massifs including the Biligirirangan,

Shevaroy and Kalrayan hills to the north of the Moyar Shear Zone. The north

trending boundary between the older Western Dharwar craton (3.3-3.0Ga)

and younger Eastern Dharwar craton (2.8- 2.6Ga) (Swaminath and

Ramakrishnan, 1981; Naqvi and Rogers, 1987) approximately passes within

the domain of northern massif in the western parts (Fig.2.1).

2.2.2 Moyar Shear Zone (MSZ):

Most distinct rotation of the NNE-trending fabric of the Northern

massifs has been observed along their southern margin, which is demarcated

by the Moyar Shear Zone, having distinct E-W, ESE-WNW and ENE-WSW

trends. The Gradual rotation of the regional trends on either side of the MSZ

reveals its dextral shear character having large strike-slip component in

contrast to the earlier observations by Naha and Srinivasan (1996), who

postulated large-scale up-thrust displacements. In the western parts, the

Moyar River flows along this zone, this is then occupied by the eastward-

flowing Bhavani River around Satyamangalam. The shear zone skirts the

northern margin of Sankaridurg and extends uninterruptedly eastwards

towards Salem and Attur (Srinivasan, 1974; Chetty, 1996; Chetty and

Bhaskar Rao, 1998; Bhadra, 2000).

The MSZ is characterized by strong penetrative mylonitic shear

foliation, which trends almost E-W in the western parts and dips very steeply

both towards the north as well as south. A subordinate trend of foliation



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having N30°E orientations also noteworthy and may correspond to the relict

N-S foliation of the Northern massifs within the shear zone. Numerous shear

criteria like asymmetric mafic boudins within the MSZ reveal distinct ductile

dextral shear sense of movement, with mylonitic foliation characterized by

sub horizontal to gently plunging mineral/stretching lineation. However, the

mylonitic foliation also contains variable sub vertical to steeply down-dip

plunging lineations towards NW or SE as well as towards west. Further, the

gradual rotation of axial surfaces of tight to isoclinal folds due to dextral

shearing within the MSZ .The main orientation of the ductile shear zones

within the MSZ is almost E-W, having both north/ south dips and a minor

component of NE trending sinistral shear zones. Further eastwards in the

Satyamangalam region, orientation of dextral shear zones has slightly

changed to S 80° E with steep northerly dips while sinistral shears remain

oriented at N 30° E with steep E/W dips.In the easternmost region of Attur,

where both the conjugate sets are well developed, dextral shear zones trend S

70 E with very steep to vertical dips and the sinistral ductile shear zones are

oriented N 20° E. All along the MSZ, mineral/stretching lineations are well

developed on the mylonite foliation and within ductile shear zones, and

plunge either steeply down-dip (Naha and Srinivasan, 1996; Bhadra, 2000) or

undergo rotation to become moderate to gentle (Chetty and Bhaskar Rao,

1998).

2.2.3 Kolli -Pachchaimalai Massifs:

The presence of many charnockitic massifs in the eastern parts to the

south and southeast of Salem with high relief Kolli Malai, Pachchaimalai and

surrounding areas are remarkably charnockitic and reveal dominant NE

trends, which coincide with the main foliation. These trends undergo gradual

rotation due to the presence of dextral shear zones along the northern and

southern margins of the massifs.

2.2.4 Palghat-Cauvery Shear Zone (PCSZ):

Large and wide low-lying expanse around Coimbatore-Namakkal-

Tiruchirappalli and further east is characterized by E-W trends in the Palghat-



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Cauvery gap, which was named as the Palghat-Cauvery Shear Zone by Drury

et al. (1984). In this domain, eastward draining Cauvery River flows along

distinct E-W trending major shear zone boundary from south of Namakkal to

Tiruchirapalli.The main foliation within the PCSZ regionally trends almost E-

W with subordinate trends towards NE and north.

Fig 2.1 Tectonic framework of Southern Granulites Terrain (SGT)



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2.2.5 Salem-Attur Shear Zone

It is the eastward continuation of the Moyar-Bhavani shear zone along

Salem and Attur to the Eastern Ghat.

The Godumalai shear zone of Grady (1971) became the Vellar fault of

Srinivasan (1974) and Salem-Attur shear zone (SASZ) of Gopalkrisnan

(1996). Drury et al., (1984), Chetty (1996), Bhadra (2000) have accepted the

SASZ as the eastern continuation of MBSZ. Srinivasan (1974) showed another

fault immediately to the south called the Swetha Nadi fault.

2.3 DEMARCATION OF STUDY WINDOW

The various fracture controlled linear features and geomorphological

features were interpreted for the study area using the raw and digitally

processed satellite data IRS-1A, P24-R60. Geology map of the study area was

prepared from Geology and mineral map of Tamil Nadu and Pondicherry

(GSI-1995). The topographic analysis of the study region was done using

SRTM data. It is quintessential to narrow down the study region in order to

provide greater thrust and to make a detail study pertaining to the objectives.

2.3.1 Topographic Anomalies

The study region is covered with major hills like Yelagiri Shevaroyan

Chitteri, Kalrayan, Kolli, Pachchai hills with intermittent plains and rolling

topography. East and south eastern part of the study region is covered with

younger sedimentary rocks of cretaceous and tertiary periods that are vastly

plains with badland topography at the places of marly limestone and shale.

Significantly, the crystalline rocky terrain is alternated with hills and valleys

and plains.

DEM (Digital elevation model) was created using SRTM data of 30m

resolution (source: http//www.glcc.com). Profiles were drawn along N-S and

E-W directions to bring out the relief variations of the terrain (Fig. 2.2).



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2.3.1.1 Profile A-B

The profile A-B was drawn along Shervaroys to Bodamalai. The cross

section clearly indicated a low lying plain occupying altitude of 350 - 400m

between high hill ranges of 1550m of Shervaroys and 850m high Bodamalai

that was corresponding to easterly extension of Moyar – Bhavani Attur Shear

Zone.

2.3.1.2 Profile C-D

The profile C-D clearly defines the oscillation of altitudes along N-S

direction. The southern foot of the Chitteri hills corresponds to Vellar fault

(Srinivasan, 1974) and northern foot of Kolli hills corresponds to Sweta nadi

fault (Srinivasan, 1974) and low lying central part corresponds to Moyar

Bhavani Attur Shear zone (Srinivasan, 1974). Moreover the hills are also

highly dissected and clearly depict the strong lineaments.

2.3.1.3 Profile E-F

The profile E-F shows higher amplitude at the places of Yelagiri,

Kalrayan and Pachchai hills. The low area between Yelagiri and Kalrayan hills

corresponds to Ponnaiyar river fault. The Vellar fault is relatively deeper than

Ponnaiyar river fault along the N-S profile.

2.3.1.4 Profile G-H

Near E-W profile was drawn along the southern fringe of Shevaroys,

and Kalrayan hills. The profile was gentle and smooth. This profile G-H is

more or less corresponds to Vashista Nadi fault.

2.3.1.5 Profile I-J

Another profile was drawn along the northern fringe of Kolli and

Pachchai hills. The profile is smooth except for few undulations near

Naraikinaru hills and Kudamalai in the east. Naraikinaru hills and Kudamalai

hills are much resistant and stand slightly elevated than the plains

corresponding to Sweta Nadi fault.



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Fig. 2.2 Topographic Analysis Using SRTM and Anomalous Profile Variations



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2.3.2 Significant Lithologies

Though most part of southern peninsula is covered with rocks of age

earlier than late Proterozoic age (550ma before present), the general

assemblages of significant rock types have possess some tectonic history in

them to tell (Fig. 2.3). Alkaline rocks syenites, carbonatites and ultramafic

rocks, granites, mylonite, phyllonites, pegmatites and dykes were taken as

significant lithology since they are associated with major lineaments and rifts.

‟

Fig. 2.3 Regional Study - Significant Lithologies



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The ages of Yelagiri syenites (773±36Ma, Miyazaki et al., 1999),

Samalpatti carbonatite (700±30Ma, Moralev et al. 1975) and Sevattur

carbonatites (767±8Ma, Kumar et al., 1991) suggest the tectonics associated

with their emplacement were contemporary to Pan-African orogeny (Rajesh

et al., 1996).

Rock types varying from Sathymangalam group- equivalent to Sargur

schist belt to Cuddalore Sanstone of Mio-Pliocene sequence is nearly

complete in the latitudes of 11 – 12 degrees that is towards the east of Attur

valley (Fig. 2.3).

Considering the vastness and volume of younger sediments, this

could be possible, when the easterly flowing rivers vigor

enough to erode and supply the sediments.

The intensity of river could be positively correlated with rise of

the plateaus bounding the Attur valley.

2.3.3 Geomorphic Anomalies

The landforms are expression of combined action of tectonics and

geomorphic agents. The tectonic landforms like intermontane valley, fracture

valleys, combination of dissected plateau and undissected plateau, bajada

(high relative relief) structural basins clearly evidences neotectonism and are

taken as anomalies (Fig.2.4).

The Shevaroys, Chitteri, Kalrayan and Kolli and Pachchai hills shows

morphotectonic anomalies like combination of dissection and undissection as

similar as the features observed in Kodai hills by Kumanan (2001).

2.3.4 Drainage anomalies

The crude parallel drainage pattern that is shown by the streams viz.

Sweta and Vashista which are flowing on the southern fringes of Shevaroys,

Chitteri and Kalrayan and northern fringes of Kolli and Pachchai hills

respectively (Fig.2.5). The parallelism is controlled by vellar fault and sweta

fault (Srinivasan, 1974). Moreover, the Toppur ar and Ponnaiyar fault along



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the northern fringe of Shevaroys, Chitteri and Kalrayan takes a southerly

deflection while Kollidam River, a major distributory of river Cauvery flows

south of Kolli and Pachchai hills and takes a northerly deflection along NE-

SW sinistral graben, (Ramsamy, 1989). This suggests a possible development

of deep or transpressional tectonism or block faulting in complement to

cymatogenic arching along Mangalore–Chennai and Cochin–

Ramanathapuram (Ramasamy et al., 1987, 1995a; Ramasamy 1989; and

Subrahmanya, 1994 & 1996).

Fig. 2.4 Regional Study - Geomorphic Anomalies

Extensive gullying east of gangavalli shear zone have connote either

the soft lithology like shale or salt affected land or a topographic high which

results in the formation of badlands.



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Fig. 2.5 Regional Study – Drainage Anomalies

2.3.5 Structural Anomalies

The structures in the study region studied by various people like

Srinivasan (1974) Vemban et al. (1977) Ramasamy (1989,1995b), Ramasamy

et al. (1991, 1998, 2000, 2006a ), Chetty, T.R.K. (1996), Chetty et al. (1998),

Bhadra, (1999, 2000), Naganjaneyulu et al. (2003), Vijaya Rao et al. (2006),

Biswal et al. (2010) and many others. Because of multi phase deformation

history, the structural basins and their alignment along the shear zones are

observed as significant rather than alkaline ring complexes in Samalpatti and

Koratti.

2.3.5.1 Trend line anomalies and structures

The trend lines are the expressions of the structures of the rocks either

primary like bedding or imparted structures during deformation

accompanied with metamorphism. The rocks near Vellalakundam expresses a



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basin structure and major axis of the oval basin trends ENE-WSW direction.

Another fold with north easterly plunge is observed in southern part of

Chitteri hills. The Palaniyapuri basin (Malliyakarai basin) completely aligning

its major axis of the oval towards E-W direction. Hills near Singliyankombai

expresses “S‟ type fold with axis trending NE-SW direction and the loop

structure is observed in Naraikkinaru hills (Fig.2.6a).

2.3.5.2 Lineament Anomalies

Polymodally oriented lineaments show dominance of NW-SE

orientation in the northern part and NE-SW orientation in the southern part of

the study region. The imprints of N-S lineaments are extending from northern

end to southern end of the study region but the conspicuous E-W lineaments

are confined to bounds of Shervaroys, Chitteri and Kalrayan and Kolli and

Pachchai hills. The presence of E-W parallel lineaments at the Attur valley, E-

W and NW-SE parallel lineaments at Shervaroys and Chitteri hills, N-S and

NW-SE parallel lineaments near Yelagiri hills, Shevaroys and Chitteri hills

and branching off lineaments in Kalrayan hills are of prime importance as

they signifies the neotectonism ( Ramasamy, 2006a).

2.3.5.3 Lineament Density

The total length of the lineaments in 5sq.km grid area were measured

and plotted in the respective grid centres and thus the lineament density

diagram was generated. Though the azimuth of the lineaments varies from E-

W, NE-SW, NW-SE, NNE-SSW, NNW-SSE and N-S, the trend of the

lineament density maximas were E-W, NNE-SSW, NE-SW, NNW-SSE, and

NW-SE. The maxima zone of lineament density was concentrated in Attur

valley (Fig.2.6b).



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Fig. 2.6a Regional Study – Structural and Lineament Anomalies



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Fig. 2.6b Regional Study – Lineament Density and Lineament Density

Maxima

2.3.6 Synthesis and Demarcation of Study Window

From the regional study following observations were made,

The Chitteri Kalrayan and Kolli, Pachchai sector shows anomalous

relief variations with a broad valley in between them.

The presence of steep fracture valleys, steep slopes, fault scarp,

combination of dissected and undissected plateaus.

The bajada on all the sides of raised plateaus suggest renovation of

relative relief and thereby increase in sediment deposition.



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Though the alkaline rocks and ultramafic rocks are found in rift

tectonic settings, and in this part of country they are dated to late

proterozoic period

Sheared rocks are strikingly possesses the clues of recent crustal

disturbances.

The parallel drainage pattern shown by Vashista nadi and Sweta

nadi is owing to the E-W trending faults viz., Vellar fault in the

northern fringe and Sweta nadi fault in the southern fringe of broad

Attur valley, (Srinivasan, 1974).

The deflections of drainages are confined to E-W trending Attur

valley. The complementary deep along Attur valley to the

cymatogenic arching along Mangalore –Chennai and Cochin–

Ramanathapuram (Ramsamy et al., 1987, 1995), possesses the

evidences of block faulting.

Lineament density is higher in the latitude of 11-12 degrees

Lineament fabric is nearly East-West, NNE-SSW and N-S

Structural basins are present- Kanjamalai, Palaniyapuri basin and

their conspicuous alignment along E-W shear zone.

The presence of branch off lineaments, parallel lineaments and

curvilinear lineaments indicates the area is highly vulnerable for

neotectonism.

Based on the above conceptions new study window was selected for

studying the neotectonic activity and related geodynamics and geohazards.

The study window ranging from 11º 15‟ to 12º 0‟ latitudes and 78º 0‟ to 79º 0‟

longitudes and includes parts of Salem, Dharmapuri, Tiruvannamalai,

Villupuram, Cuddalore, Perambalur, Trichy, Namakkal districts (Fig.1.1).



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2.4 NEOTECTONIC AND SEISMOTECTONIC ZONATION MAPPING

The Indian Peninsular Shield in general and its southern part in

particular has always been thought of as being inert to younger earth

movements and related seismicities/ earthquakes. Though the Southern

Indian Peninsular Shield has not been studied in great detail with regards to

faults, especially concerning their tectonic alertness, since 1960, a number of

workers have observed, in various parts, possible repetitive tectonism since

the Jurassics.

Some significant observations are: possible Post-Jurassic tectonic

movements along the Palghat graben (Arogyasamy 1963); varying signatures

of Neotectonism of the Mysore plateau (Radhakrishna 1966); possible

repetitive Post- Jurassic tectonic movements in South India (Vaidyanadhan

1967); a positive relation between Neotectonism and petroleum occurrences in

South India (Ermenko 1968); active tectonic graben along the Salem–Attur

valley (Srinivasan 1974); a striking coincidence of historical seismicity data

with NE–SW and ENE–WSW lineaments/faults/lithological boundaries of

South India (Vemban et al., 1977); tectonic wedging and related drainage

reversals in the Dharmapuri region (Suryanarayana and Prabhakar Rao

1981); possible Neotectonism and the related clockwise rotational migration

of Palar in the Chennai region (Rao 1989); Holocene transform faults of ENE–

WSW orientation along the Kerala coast (Nair and Subramainan 1989); N–S

trending cymatogenic arching and related rejuvenation of the Cauvery river

(Radhakrishna 1992); signatures favouring intra plate deformation in South

India (Subrahmanya 1996); dynamic mobile belts in South India (Chetty

1996); multi various evidences favouring Late Quaternary/Holocene earth

movements in South India (Valdiya 1997, 1998, 2001, Valdiya et al., 2000);

and signatures on active tectonic movements in parts of the Western Ghats

(Gunnell and Fleitout 2000), etc. In recent years, Ramasamy et al., (1987, 1991

1993) have carried out interpretation of satellite images and recorded

evidence of possible Neo-active tectonics in parts of South India, with



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possible land arching in the Chennai and Ramanathapuram areas.

Subsequently, Subrahmanya (1994) and Ramasamy and Balaji (1995) also

observed evidence of possible regional cymatogenic arching along the

Mangalore–Chennai region.

Tamil Nadu State is one of the 13 identified seismotectonic zones of

Peninsular India (Umesh Chandra, 1977). Reliable historical earthquake

records for the last 200 years are available with the reputed research

institutions of India through published literature. So far 12 earthquakes of

M>5.0 have occurred in the State (Ganapathy et al., 2010). Bureau of Indian

standards (BIS. 2001) categorized Tamil Nadu under Seismic Zones II and III,

representing an area of 73% and 27% respectively. These disastrous

earthquakes have changed the long held belief of low order seismicity of

Peninsular India and revised the seismic zonation of part of Peninsular India

from moderate to high seismic prone areas according to Bureau of Indian

Standard (BIS), 2001.

The author has taken up detailed studies to identify and interpret

various tectonic, riverine, and coastal geomorphic anomalies from satellite

based remote sensing data and aeromagntic anomalies, further, to spatially

integrate this information to build up a comprehensive picture of Neo-active

tectonics for the study window. After validating with multidepth resitivity

data, alignment of spring, historic seismicity data and field study, the

resources and geohazards were spatially correlated to bring out a

comprehensive report of the influence of neotectonism.

The present study is a newer attempt to identify, analyze, and spatially

amalgamate a large number of anomalies visibly displayed by the tectonic,

fluvial, geophysical, and hydrological systems in remote sensing and ground

based datasets/observations, and to finally paint a fair picture on the active

tectonic scenario of Attur valley.



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2.4.1 General Methodology

The possible anomalies used by many workers around the world as

well as in India for Neotectonic and seismic zonation mapping in respect of

tectonic, fluvial and geomorphic, and geophysical anomalies were cataloged.

The anomalies observed by the earlier workers in parts of South India were

also taken into account. In addition, GIS based visualization techniques were

used in the present study to identify newer and characteristic anomalies

which can signal such Neotectonic zones.

By duly interpreting such raw and digitally enhanced IRS-1C satellite

images, the lineament map was prepared for the study area and from the

same, various anomalies such as fracture swarms, curvilinear lineaments,

branch off lineaments, radial lineaments, etc., were interpreted and probable

zones of Neo–Active tectonics were deduced.

In the same way, various drainage / fluvial and geomorphic

anomalies were interpreted such as radial drainages, palaeochannels,

deflected drainages, compressed meanders, eyed drainages, etc. and the

anomalies seen in landforms like fracture valleys, escarpments, triangular

facets, etc. Topographic expressions studied from shaded relief map

derived from SRTM were interpreted for neotectonic signatures.

Aeromagnetic total intensity anomalous values and break were taken for

neotectonic analysis.

Litho units of tectonic importance and structural trend breaks were

interpreted as weak zones for neotectonism. All these anomalies picked up

from lineaments, drainage, litho units, structural trend and tectonic

geomorphology and aeromagnetic were integrated using Arc-GIS and zones

of coincidence were identified as probable zones of Neotectonics in the study

area (Fig. 2.7).

In addition to building up the concept for Neotectonic mapping, the

Neotectonic model brought out for the study area was also validated with the

help of historical seismicities, alignment of springs, multidepth resistivity

data and field study.



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NEOTECTONIC AND SEISMOTECTONIC ZONATION MAPPING

Data Integration and Detection of NEOTECTONIC

and SEISMOTECTONIC Zones

Detection of Anomalies

Structural Trend

Anomalies

Lineament

Anomalies

Geomorphic

Anomalies

Aeromagnetic

Anomalies

Drainage

Anomalies

Topographic

Anomalies

Significant

Lithology

Model Building

Through Historical Seismicity Data

Model Validation

Through Alignment of Springs

Through Multi Depth Resistivity

Data

Field Validation

Fig. 2.7 Methodology Flow Chart–Neotectonic and Seismotectonic Mapping

2.4.2 Topographic Anomalies from SRTM

2.4.2.1 General

Since the announcement of Shuttle Radar Topography Mission (SRTM)

in 1998 (NASA-JPL, 1998a; NASA-JPL, 1998b) great expectations were

increased within the scientific community for its numerous environmental

applications. Antecedently the most frequently used global DEM was the

Global Digital Elevation Model (GTOPO30). However this system was

restricted by its numerous limitations such as the combination of different

elevation data sources with different vertical errors and the spatial resolution

of 30 arc-seconds. Although some research was carried out using this data



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(Miliaresis and Argialas, 1999; Miliaresis and Argialas, 2002), it presents

constrains especially at the modeling stage. The SRTM elevation data was

produced with the synthetic aperture radar (SAR) interferometry technique.

The use of SRTM elevation data appear as new opportunity for

geomorphometric studies at regional scale and geomorphometric

susceptibility areas for neotectonism (Enrique Castellanos, 2005). DEM data

can be used to derive topographic factors, other than simply elevation,

including slopes, aspects, hill shading, slope curvature, slope roughness,

slope area and qualitative classification of landforms (Fernandez et al., 2003).

La´szlo´ Fodor et al. (2005) have studied the neotectonic structures and

morphotectonics of the western and central Pannonian Basin using SRTM

data.

The topography of a region is a cumulative effect of paleo-tectonism

and neotectonism over characteristically different rock types and the action of

various atmospheric agents. Unless affected and altered by neotectonism in

the form of exhumation and reactivation and adjustment tectonics, the

resultant topography would have smoothness in relief variations, gentle

denudational slope and complete denudation will mask the previously

originated linearity. In the present study the anomalous relief variations, gaps

in between hills, linear shadows, steep slope sides and deeps in the

topographic profiles were interpreted for places of active tectonism. The

topographic sheets of SOI were checked for contour values and spacing.

2.4.2.2 Anomalous Relief Variations, Contour Pattern, Shape and Slope

The study window shows great variations in the relief with many

hillocks and hills cover the entire region. The relief of the entire area ranges

from 80m at the eastern part of the study area to 1649m at Shevaroyan hills.

The sudden raise of the hillocks from the surrounding plains with steep sides

probably escarpments, vast plateau on the hills and steep narrow drops in the

middle, defines steep sided narrow valleys which could be neotectonically



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significant. The altitude of the hills decreases gradually towards the

easternside as well as the southernside of the study window.

2.4.2.2.1 Shevaroys

The peak Sholai Karadu hill measures 1649m and forms the highest

peak in the study window and the stretch of peaks along the NE-SW direction

are known for bauxite cappings. The bauxite capped hills were strikingly

separated by broad valleys on either side with an altitude of 1200m on the

west and 1020m on the east. In the easternside of the hills the contour valleys

goes as low as 780m along an N-S trending linear depressions. The contours

spacing are very close around the hill and the slope measures up to 56º -60º

with steep cliffs. On the contrary, the contour spacing is comparatively broad

on the top of the hills with intermittent breaks.

2.4.2.2.2 Chitteri

The easternside of the Chitteri hills the highest being the

Arunuttumalai peak which measures 1211m and it is a very sharp and narrow

peak. The western side of the Arunuttumalai hill is bounded by

Manjavadighat passes which is a narrow valley separates the Chitteri hills

from the Shevaroys. The contour pattern and the shadows from relief map

(Fig. 2.8a) clearly depict NE-SW ridges. In the middle of the hill, a broad N-S

trending linear valley with the minimum contour value 436m. NW-SE

trending two narrow linear depressions were observed on the middle of the

Chitteri hills. NNE-SSW trending linear depression merges with N-S trending

narrow valley at the southern side of the Chitteri hills. Three more NNE-SSW

trending valleys makes the hill highly dissected appearance. On the northern

fringe the ESE-WSW trending Toppur Ar fault forms the boundary of the

Chitteri hills.

2.4.2.2.3 Kalrayan

The maximum altitude of the Kalrayan hills is 1249m and there are

many peaks at the altitude of 1000m and above in the south-westernside of

the hill. The central part contours are broad and represent the denudational



97

plateau. The Kalrayan hill is separated from Chitteri by the NE-SW trending

Kotapatti shear and along this western side the slope is moderate when

compared to southern and easternside. The outline of the hill is highly

serrated in the southern and easternside.

Two N-S trending narrow valleys converge into a deep valley at the

northernside. The easternside of the hills are steep and having sharp contact

with the plains.

2.4.2.2.4 Kolli Hills

The Kolli hills lies in the southern side of the study window and 1370m

is the highest peak. North and northeasternside of the hills shows serrated

appearance whereas the west and eastside of the hills are steep and sharply

raised above the plains. The top of the hills shows less dissection than

Pachchai hills. The entire hill itself shows a crude „S‟ type folding. Kolli hills

separated from Pachchai hills by a broad valley.

2.4.2.2.5 Pachchai Hills

The Pachchai hills are highly dissected and Gangavalli shear separates

the Manmalai hill (. 971m) from Kambakkal malai hill (.957m) where as the

highest peak is1071m. The northernside and northeasternside of the Pachchai

hills are serrated and NE-SW trending narrow valley further separates the

Pachchai hills into two halves. The slope on the westernside and south

western sides are steep and the altitude decreases gradually towards the east

and northeast.

2.4.2.2.6 Other Hills

Apart from the above major hills discussed, there are many hills with

maximum altitudes of 900m and above like, Kanjamalai, Bodamalai,

Godumalai, Malliyakarai, Manjini, Singliyankombai, Jargumalai and

Tenmalai. These hills shows characteristic steep slopes and their isolated



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distribution along certain shear zones make them sensitive clues for marking

neoactive lineaments.

Fig. 2.8a Study Window - Lineaments from Topographic Analysis

2.4.2.3 Profiles

The profiles provide vertical section view of the terrain of interest and

fundamental exercise for the geoscientists to understand the topographic

variations. This type of representation is often helpful in bringing out the

slope, depth of valleys and frequency of relief variations.

Profiles I to V (Fig.2.8b) was drawn to find out the relationships

between the major hills in the study area and the intervening vast plains

spotted with small structural and residual hills.



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2.4.2.3.1 Shevaroyan Hills – NW-SE Profile –I

Profile I was drawn from west of Shevaroys to Chitteri hills along NW

to SE direction so as to cut perpendicular to the general trend of the rocks.

The west side of the profile rises up to 1560m then drops to 780m in the east

which corresponds to N-S narrow valley. Another sharp drop of altitude to

580m was observed after a flat profile that was corresponds to NE-SW

Manjavadighat pass. The sharp drop can be conveniently classed as

escarpments as the field observations confirm triangular facets on the western

side walls of the valleys.

2.4.2.3.2 Chitteri -Kalrayan hills – NW-SE Profile –II

Profile II was drawn across the general trend of the rocks in Chitteri

and Kalrayan hills. The NW-SE profile shows a drop in altitude from an

average height of 650m to 440m on the west side of Chitteri hills which

corresponds to N-S fracture valley.

2.4.2.3.3 Kalrayan- Pachchai hills – N-S Profile –III

The N-S profile was drawn from Kalrayan to Pachchai hills so as to cut

across the intervening plains between the two hills. The southern fringe of

Kalrayan displays a E-W trending narrow trough. Further south there was a

great plain with an average height of 210m which corresponds to E-W

trending Attur valley. The Attur valley was fringed by E-W trending faults.

2.4.2.3.4 Shevaroyan - Kolli hills – NNW-SSE Profile –IV

The fourth profile was drawn in NNW-SSE direction and to cut across

Shevaroys to Kolli hills. The broad valley corresponds to the E-W trending

Moyar Bhavani Attur shear zone and moreover the intermittent presence of

hills indicates branching nature of the shear zone. The northern fringe of the

Kolli hills corresponds to the Swetha nadi fault.



100

Fig 2.8b Topographic Profiles along Major Hills



101

2.4.2.3.5 Kolli - Pachchai hills – E-W Profile –V

The fifth profile displays the break at NNE-SSW trending Kolli east

Shear and Gangavalli shear zone near Pachchai malai. The Kolli east shear is

relatively broad where as Gangavalli shear shows sharp “V” shaped valley.

2.4.3 Significant Lithology

2.4.3.1 General

Lithology map was prepared from District Resource Map (DRM) of

GSI with the updates from field work, where alkaline rocks, ultramafic rocks,

dykes, granites and pegmatites, pseudotachylytes and mylonites were taken

as significant lithologies and their orientation and location have been

considered as tectonic weak zone or anomalies. Since, these rocks represent

the tectonic history or recurrence of tectonic events these will give us clue on

the orientation of then tectonics weak zones and possible zones of reactivation

(Fig. 2.9).

2.4.3.2 Alkaline Rocks and Ultramafic Rocks

A large number of alkaline and ultramafic suite of rocks have been

observed (Udas and Krishnamurthy, 1970; Borodin et al. 1971;

Krishnamurthy, 1977; Subramanian et al., 1978; Viladkar and Subramanian

1995) in the northern parts of Tamil Nadu and they are found to be located

along a major NE-SW lineament (Grady, 1971).

The study window displays many ultramafic bodies including Chalk

hills where active Magnesite mining is going on. Ultramafic suite with

alkaline rock – shonkinite was found to be aligning with NE-SW lineament.

The Chalk hills shows E-W elongation near north of Nagaramalai and show

strike slip along NW –SE lineament near southern part.

Siddeswaran Kovil near northern flank of Kanjamalai dunite and

peridotite with magnesite occurrences was reported. The original extensions

of the rocks were mapped during the field study by the author during the

year 2008 where peridotite extensions were observed parallel to the E-W



102

lineament (Plate IA). Near Ariyanur (Salem-Erode NH47) on the southern

flank of Kanjamalai, ultramafic rocks emplacement was observed during field

study on the 1008 Shivalinga temple Hillock (.306 m. E 78° 5' 5.94", N 11° 35'

42", Plate IB) and Makkalur hill (.296m. E 78° 3' 25.2", N 11° 34' 58.8").

Another mound of amphibolite (E 78° 3' 25.2", N 11° 34' 19.19") with

intervening mylonites was also observed during the field study near

Chinnasirangapadi village. The amphibolite shows higher degree of alteration

in to asbestos where the hornblende biotite gneisse contains sericite in patches

near the asbestos pockets. Syenite was also found on the way from Omalur

to Mecheri road near Silakardu hill (.426m) which crudely aligning with a NE-

SW lineament and an E-W lineament.

Carbonatites outcrops were observed near Umaiyapurampudur

(E 78° 28' 8.39", N 11° 37' 30", Plate-IC) and near Karippatti (E 78° 17' 13.2", N

11° 39' 28.79"). Near the first location the NW-SE trending dykes were

completely fractured by the intrusion of the carbonatite and the mine was not

operating due to the poor quality of the produce. The carbonatite veins

invading the joints present in the charnockite were observed near Karippatti

and metasomatic effects of the intrusion could be seen on the vein –rock

contact (Plate-ID).

New outcrops of syenite were located in Attur valley during field

study and were reported (Plate-IE) near Vembakavundanpudur (78° 25'

14.09", N 11° 31' 28.27”). These syenites are found to be corundiferous

(Plate-VIIIE) associated with ultramafic rocks and aligning with NE-SW

trending lineament. Lamprophyre was observed in a well cutting near

Koraiyar (E 78° 19' 6.28", N 11° 29' 25.40", Plate-IF and VIIID) during the field

study.

2.4.3.3 Granites, Pegamtites and Migmatites

The granites of Southern Granulite Terrain (SGT) can be grouped

under two broad categories, viz., the Late Archaean / Early-proterozoic

granites and the Late-Proterozoic / Early-Palaeozoic (Pan-African) granites.



103

The older granites are restricted to the northern part of SGT, while the

younger Pan-African granites are mostly found in central and southern parts

of SGT. Geochronological studies have yielded isochron age of 534 ±15 Ma for

the Sankari- Tiruchengodu, 619±35 Ma for the Maruda Malai and 471-475Ma



104

for the Punjai Puliyampatti granites. The field setting, mineralogical and

geochemical characters of most of the Pan-African granitoids of SGT

characterize them as Anorogenic A-type granites (Nathan et al., 2009).

The Granulite-Gneiss terrain of Central Tamil Nadu, representing the

marginal zones of Dharwar craton, witnessed wide spread Neoproterozoic

acid magmatism. This event is marked by the emplacement of several

granitoids (viz. Sankari-Tiruchengode, Punjai Puliyampatti, Karamadai and

Madura Malai granites) in a linear array within the E-W trending Cauvery

Shear Zone / Cauvery Suture Zone (CSZ) which is bound by Moyar-Bhavani

Attur Lineament (MBAL) in the north and Palghat-Cauvery Lineament (PCL)

in the south. The Sankari-Tiruchengode (ST) granite, occurring at the

intersection of the MBAL with the NNE-SSW trending Mettur lineament, is

emplaced within the Bhavani Gneissic Complex and the associated

supracrustal rocks of Sathyamangalam Group. The ST granite comprises two

distinct phases, viz. a leucocratic phase and a pink phase. The leucogranites,

showing grain size variation from medium grained to pegmatoidal, occur in

the peripheral parts of the ST pluton while the pink granites (coarse to

pegmatoidal) occupy the core.

Late Archaean-Early Proterozoic periods in Tamil Nadu and

Pondicherry are characterised by granulitic facies metamorphism with

charnockite formation and concomitant anatexis of earlier rocks. A number of

small granite plutons were emplaced as culmination of migmatisation during

this period. The Migmatite Complex shown in the map at places includes

gneisses and granitoids generated during this period. The Late Archaean

granite is developed along the northern periphery of the state (to the north of

Palar River) around Tiruttani, Sholingar, Bisanattam, Ebbari and Krishnagiri

(Ca 2500 Ma) (Krogstad et al. 1988), while early Proterozoic granite is

recognised around Gingee, Tiruvannamalai and Tirukovilur (2254Ma;

Balasubrahmanyan et.al. 1979).



105

Linear NE-SW trending granite outcrops were marking lineaments

with the similar trend were found in Thoppar Ar valley near Mallapuram and

four exposures were found near Northern part of Chitteri hills. Similar NE-

SW trend of the granitic bodies were observed near Belukurichi, and

Thumbaipatti in the southwestern part of the study area and also near

Mukkanur in the northeastern part of the study area. E-W trending granites

could be observed near central eastern part of the study area.

There are a large number of sheets, bands and lenses of Pegmatoidal

granites confined to southern parts of Attur valley in general and to the

southeastern portions in particular. Maximun concentration of these bodies

were recorded around hill .419 and .513, north of .582, northeast of .600, south

of .601 and west of .626. In most of the cases they intruded into charnockites,

except at hill .419 and southeast of .654, where they are emplaced into a

pyroxene granulite.

The granite bodies vary in size from less than 5 meters to over a km.

long with a thickness range otf 1-25 meters, but on an average, they are a

couple of meters thick. Longest of all pegmatoidal granites are found at hill

.419 (west of Singipuram, ridge west of .626 and northeast of .593), which are

around a km. long.

The exposures of grey granite near Pokkamalai (Δ539m) and pink

granites near Kudamalai (E 78° 35' 31.20", N 11° 27' 28.8") were found and the

pink granite found to be emplaced at the intersection of Gangavalli shear zone

and Swetha nadi fault (Plate-IIA).

West of Chalk hills, pegmatite veins and rock exposures were observed

near Chettipatti and Reddiyur near Omalur and also near Kachchirapalayam

on the east of Kalrayan hills.

Many exposures of pegmatites were found near Taramangalam and

adjoining area where feldspar and quartz are being mined. Good quality beryl

crystals are commonly found in these rocks.



106

Migmatites were found near Sitampundi south west corner of the

study area and also north of Bodamalai. The first one was found to be

trending NE-SW and the other one was trending WSW-ENE direction.

2.4.3.4 Basic dykes

The northern part of Tamil Nadu, north of Noyil-Cauvery Rivers

(north of 11°latitude) is characterised by dyke swarms, in contrast to the areas

south of Noyil-Cauvery Rivers where they are absent. In general, the mafic

dykes trend WNW-ESE and NNE-SSW and rarely N-S and NNW-SSE. In the

central part of Tamil Nadu, ENE-WSW to NE-SW trending mafic dykes are

seen transecting the charnockite and migmatites in Nilgiri and Kolli Hills.

Although most of these mafic dykes show textural characteristics of dolerite,

gabbroic / basaltic variants are not uncommon. The mineral assemblages of

these dykes indicate quartz-gabbro / quartz–dolerite composition with minor

variations to olivine-gabbro/dolerite. Petrochemical studies indicate that the

majority of these dykes are quartz normative tholeiites, while olivine-dolerite

dykes show basaltic komatiite chemistry (Krishna Rao and Nathan, 1999).

The chemical attributes of these dykes suggest that they were emplaced in a

continental tectonic setting. The available K-Ar ages for the mafic dykes of

Tamil Nadu are clustering around 1700Ma (Radhakrishna and Mathew

Joseph, 1993; Sarkar and Mallick, 1995) indicating that they were emplaced

during a major extensional tectonic regime in the Southern Peninsular Shield.

A large number of mafic intrusives comprising medium to coarse

grained dolerites to fine grained basalts are encountered in the study area.

These dyke bodies traverse across almost all the rock types mainly along NE-

SW, E-W and NNW-SSE trends. Local variation of a few degrees in trends of

these dykes was common since most of them are of swerving nature and a

few were branching. The general trend of above sets of dykes roughly follow

the regional fold axes and shear zones; and timing of their emplacement could

also be related to the regional episodes of folding and dislocations. Among

the vast number of dykes encountered in the area, the following were the

major ones:



107

(a) NNW-SSE trending 7.5 km. long discontinuous dolerite dyke

extending from southeast of hill .711 to the south of Karippatti.

(b) NNW-SSE trending 2.5 km. long coarse grained dolerite dyke with

highly varying thickness, passing between hill .601 and hill .413.

(c) NE-SW trending 2.5 km. long dolerite dyke and swerving at two places

extends from NE of Mudiyanur to south of .413.

(d) two NE-SW trending dolerite dykes, about 2 km. long, one to the NE of

Palaniyapuri and the other to the NW of Singipuram.

(e) NE-SW trending highly mylonitized 2 km. long dolerite between hill

.698 and hill .711 showing branching nature.

(f) NE-SW trending discontinuous dolerite dyke extending from SW of

Vembakavundan- - pudur to east of Tirnmanayakkaapatti (combined

length - 5 km.).

(g) A NE-SW trending faulted dolerite dyke south of Pusariyur

(h) NE-SW and NW-SE trending dykes intersect near Umayalpuram-

-pudur (Plate-IIB). NW–SE trending dykes extend from

Puthrakavundadanpalayam up to Tammampattiin in the south.

In addition to the above listed dykes, there are several dolerite dykes

relatively smaller dimensions, extending in both NNW-SSE and NE-SW

directions.

They are generally dark greenish to black (melanocratic), hard and

compact, massive and less jointed, but quite a few show shearing and

mylonitisation, which was obivious by the presence of pseudotachylyte

veins.Most of the dykes, either stands out prominently as low ridges or

exposed as exfoliated boulders in plains. Extremely well developed

exfoliation in coarse grained dolerite dykes could be seen along the road

cutting between hills .601 and .413 and east of hill.413.



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2.4.3.5 Siderite –Ankerite gneiss

A process of carbonatisation, similar to fenitisation, has affected the

country rocks around carbonatites and adjoining area, leading to the

formation of pinkish siderite-ankerite bearing quartzofeldspathic gneisses

(termed as carbonated gneisses). Some mafic rocks are also affected by

carbonatisation giving rise to carbonated mafic rocks and they are of small

dimensions and patchy in occurrence. This type of carbonate metasomatism

has been reported from carbonatite complexes as well as from certain major

shears zones. Similar process of carbonatisation has affected a sizable area

around Kangankunde Carbonatite Complex, west of Lake Chilwa, Nyasaland

(Garson, 1958), where the surrounding rocks were first permeated by

strontianite rich ankerite and siderite material and later remobilized to K-

feldspar-ankerite-siderite rock. Regional carbonatisation connected with Epi-

Hercynian tectogenesis was also reported from southern Tien Shan (Baratov

et al., 1984).

Since siderite- ankerite carbonates which are involved in carbonate

metasomatism in the area are also the primary carbonates of all the

carbonatite bodies in the Attur Valley, the process of carbonatisation seems

related to the emplacement of carbonatites, not only spatially but also

genetically, at least in this area. However, carbon isotope study of carbonates

from these litho units is necessary for confirming their consanguinity.

2.4.3.6 Garnetiferous Gabbro

A group of ultrabasic rocks ranging in composition from dunite,

peridotite, websterite, garnetiferous gabbro, gabbroic anorthosite and

anorthosite occur closely associated with the Sathyamangalam Group in the

central belt of Tamil Nadu, around Mettupalayam and other areas. They also

occur as enclaves within the peninsular gneisses as a part of the dismembered

sequence. Large volume of garnetiferous gabbro and hornblendic anorthosite

with chromitite layers as well as small lenses of eclogitic rocks are the

characteristic features of this suite (Gopalakrishnan, 1994b). They are



109

considered to have been emplaced along reactivated lineaments, shear zones,

fracture zones or as tectonic slice (GSI, 2006).

These rocks were recorded near Mariamman pallam (E 78° 7' 48", N 11°

59' 45") north of Toppal Ar, near Varagur (E 78° 57' 32", N 11° 53' 45") within

Kaduvanthu reserve forest and near Cholambathu (E 78° 55' 19",

N 11° 50' 27”). Around Cholampattu four small rock bodies‟ viz. one on the

south, second on the NE side, third on the east and fourth on the NNW side

of the Cholampattu were recorded by GSI.

2.4.3.7 Mylonites and Cataclasites

The term mylonite refers to rocks with a specific (micro) structure that,

in most cases, can be qualified as follows: (Rudolph A. J. Trouw, et al., 2010)

Presence – of a strong SL (Schistosity –Lineation) fabric

Presence of a fine-grained matrix with porphyroclasts. Minerals

like quartz, chlorite, biotite and muscovite are usually present in

the matrix, either highly strained at low grade, or recrystallised at

higher grades. Minerals like feldspar, garnet, hornblende and

pyroxenes may form porphyroclasts, commonly showing evidence

of crystal-plastic deformation by undulose extinction and/or

partial recrystallisation.

Presence of a certain asymmetry, especially in low-grade

mylonites, in the form of C/S fabric or C´ shear bands, mineral

fish, stair stepping, oblique foliation etc.

A protomylonite has between 10 and 50% matrix, a mylonite has 50-

90% matrix and an ultramylonite has between 90 and 100% matrix. Some

ultramylonites have also been referred to as phyllonites, a term also used for

mylonites rich in mica, derived from schists.

The cataclasites reffered in this chapter are all cohesive fault rocks that

show evidence of brittle fracturing although other processes such as grain

boundary sliding and pressure solution may have played a role in their



110

formation as well. These rocks are usually composed of angular broken rock

fragments embedded in a matrix of quartz, iron oxide, calcite, chlorite and/or

other minerals that precipitated from a fluid. Cohesive cataclasites are

thought to form in the P-T realm where brittle deformation predominates,

that is, approximately in the upper 10 km of the Earths crust, with lithostatic

pressure up to about 3 kbar and temperatures up to about 300 °C. However,

their depth cannot be established with precision because other factors, like the

presence or absence of fluids and the strain rate, also play an important role.

Pseudotachylytes are cohesive glassy or very fine-grained fault rocks

that characteristically occur in veins. They are composed of an extremely fine-

grained or glassy matrix with inclusions of wall-rock fragments. Commonly, a

straight main fault vein (Sibson 1975; Spray 1992) representing a generation

surface is present, from which smaller injection veins branch out. The main

criteria to distinguish pseudotachylytes from cataclasites are:

The presence of injection veins branching from a straight main

fault vein (generation surface).

A sharp transition between pseudotachylyte veins and the wall

rock, with characteristic embayments at the site of mica or

hornblende crystals; in cataclasites the transitions are more

commonly gradual. There is potential confusion, however, where

pseudotachylyte is in contact with cataclasite rather than with

intact wall rock.

The Salem – Attur Shear Zone is about 100 km long and 2 to 5 km wide

and passes through a valley extending from Salem to Attur. The average

trend of the shear zone is EW. Mylonitisation was well at places and this zone

was characterised by 1 to 1.5 km wide zone of Phyllonite. Evidence of dextral

(Chetty, 1996, Bhadra, 2000, Biswal et al., 2009 & 2010) and sinistral

movements has been recorded along this shear zone. The mylonite bands

could be traced from west of study window i.e. Kanjamalai to Godumalai



111

where it takes a northeasterly swerve to Tumbal north west of Attur. At the

same time eastern extension could be traced up to Ettappur but no visible

exposure/ outcrop could be found except well cuttings. Further extension

beyond this location needs field study.



112

So, the branching nature of the shear zone with intermittent isles of

non-strained zones was characteristic feature of Attur valley.

Protomylonite were observed near Perumapuram (E 78º18‟18.9”, N

11º39‟31.1” and E 78º23‟37.91”, N 11º38‟22.1”) where the rocks display bigger

clasts of feldspar within granitoids (Plate-IIC). In most of the places along

Salem –Attur shear zone, the hornblende botite gneiss is mylonitized and they

became highly foliated (Phyllonite) near Sarkar Nattarmangalam adjacent to

Godumalai (Plate-IID).

Cataclasites and Pseudotachylytes were the common brittle shear

indicator as the study area is dominated by charnockites and they were

restricted to charnockites and granites at some places. The NE-SW trending

Gangavalli Shear zone, E-W trending Swetha nadi fault and N-S trending a

set of faults in Kalrayan hills were important locales of cataclasites (Plate-IIE -

insert) and pseudotachylytes (Plate-IIE) where 1m to 1km thick shear zones

with bands of cataclasites and pseudotachylyte could be found. Apart from

these major occurrences there are plenty of locales of smaller veins measuring

less than a millimeter to few centimeters are widespread in the study area

with orientation of NE-SW, E-W and NW-SE.

2.4.3.8 Epidote hornblende gneiss

The epidote-hornblende gneiss is formed due to the progressive

retrogression of charnockite. Enclaves of intermediate charnockite, pyroxene

granulite, meta-pyroxenite and meta-gabbro and conformable bands of

quartzo-feldspathic gneiss occur within the epidote-hornblende gneiss.

Gabbro and dolerite dykes traverse the area along WNW-ESE and NW-SE

directions. The epidote hornblende gneiss which hosts the Molybdenum

characteristically comprises the Neoproterozoic ultramafic – alkali ±

carbonatite plutons as oval shaped complexes viz. Odugattur, Rasimalai,

Elagiri, Koratti, Samalpatti and Pakkanadu.

The alkali complexes show well preserved igneous planar features.

Discordant bodies of felsites, aplitic syenite, pegmatoidal syenites and quartz



113

veins also occur within this belt. Younger quartz reefs cut across all the above

mentioned rock types along NNE-SSW direction.

Fig. 2.9 Significant Lithologies within Study Window (Courtesy: GSI)

The well developed foliation planes strike along NNE-SSW to NE-SW

directions, in general, with steep dips towards NW. The axial plane traces of



114

the regional folds are parallel to the foliation. Epidote-hornblende gneiss

showing compositional variation from granodiorite to quartz-diorite is the

dominant rock type. These rocks were present on the northern limit of the

study area near Manjavadi ghat pass and north of Chitteri hills ((Plate-IIF).

2.4.4 Geomorphic Anomalies

2.4.4.1 General

The dynamism of geomorphic agents is relentless in shaping the earth

and the agents are the immediate respondants of the geotectonic and

geodynamic processes operating on the Earth system. They leave us certain

clues about the onset of neotectonism apart from the past tectonism. Tectonic

landforms are the major and direct indicators of morphotectonic and

morphodynamic processes operating in the planet Earth. Sometimes

intensified sedimentation does explain change in relief of the region due to

neotectonism. Ramasamy (1989) has described the evolution of east and west

coasts of Indian peninsula based on geomorphic and tectonic studies. Richard

Thomas Walker (2006) has made remote sensing based Geomorphological

observations to study the folds and faults in southern Kerman province in

Southeast Iran.

2.4.4.2 Combinations of Dissected and Undissected Plateaus

Plateau is an upland area with relatively a flat topography and most

are erosion surfaces. They may be extensive or dissected until only fragments

are left. They occur on a wide range of rock types including horizontal strata,

metamorphic rocks, granite and massive lava flow sequences. Volcanic and

tectonic processes that raise rocks above sea level are ultimately responsible

for elevating mountain ranges, although normal faulting also may produce

local relief in extensional settings. Erosional processes may limit the total

relief maintained by rock uplift but also cut valleys and produce relief over

shorter length scales. Fluvial and glacial processes that incise the landscape

produce relief, whereas mass-wasting processes (such as soil creep and many



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types of landsliding) tend to reduce relief. The overall relief of a mountain

range ultimately depends on the balance between uplift and erosion, unless

accumulation of crustal material exceeds the mechanical limit supportable by

crustal strength, leading to the growth of a high plateau (Plate-IIIA).

By interpreting satellite image FCC various tectono-geomorphic

anomalies were interpreted such as plateaus, fracture valleys and ridge lines.

The plateaus were by and large found in the Northern and the southern

parts of the study area.

The NW-SE, NE-SW and N-S dissections were well pronounced in

Shevaroys and Chitteri hills than Kalrayan hills. Similarly contrast dissection

among Kolli and Pachchai hills conveys that a NE-SW trending lineament

extending from Vaniyambadi to Virudhunagar separates the dissected

western plateaus from the undissected east. Further Ramsamy and

Sivakumar (1999) have observed that this Vaniyambadi-Virudhunagar

lineament should be morphotectonically active (Fig.2.10).

2.4.4.3 Structural hills

The structural hills are hills with prominent discernible structural

elements. Since these hills possess the evidences of structural disturbances

that had happened in the terrain. The study area possess the hills like

Kanjamalai, Godumalai and Palaniyapuri basin (Malliyakkarai basin) which

are characteristically possess the banded iron formation (BIF), meta gabbro,

quartzo-feldspathic gneisses, pyroxenites, etc. These rocks show

characteristic bedding nature and they were folded in Kanjamalai and

Palaniyapuri basin with elliptical trendlines in satellite image. But no folding

was observed in Godumalai where beds were dipping steeply and trending E-

W. The entire three hills posses the rocks equivalent to Sargur supracrustals

i.e. Sathyamangalam group of rocks which forms isolated hills within a vast

plain of mylonitised gneissic rocks.



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2.4.4.4 Fracture Valleys and Intermontane Valleys

The Shevaroys and Chitteri were separated by a major NE-SW aligned

fault valley. On the contrary, the tectonic valleys interpreted have shown that

these are predominantly falling in N-S and NE-SW directions, followed by E-

W and by NW-SE lineaments. Two broad intermontane valleys were observed

in east of Shevaroys and middle of Chitteri hills with N-S trend. The extension

of the intermontane valley from Shevaroys (Plate-IIIB) could be traced on the

Bodamalai with a right lateral shift.

Two fracture valleys at Kalrayan hills shows N-S trend with a

curvilinear orientation and the convexity towards the east. Moreover the

valleys show branching at the southern part. The fracture valleys at Kolli and

Pachchai hills show NE-SW orientation in contrary to multi orientated

fracture valleys in the northern hills in the study area. The NW-SE dissection

was pronounced in Shevaroys and Chitteri where as in Kalrayan the NW-SE

fracture valleys are thin and displays more spacing. The hills present in the

middle of the Attur valley also show thin linear fracture valleys as they could

be linked to the major fracture valleys in the northern hills (Plate-IVC).

Barren fracture valleys were present in between a set of NW-SE

trending lineaments and filled valleys on SW and NE blocks were observed in

Chitteri hills. N-S lineament in the east of Kalrayan separates the eastern

barren fracture valley and the western filled valleys. Pachchai hills display

barren fracture valleys which probably extend the eastern N-S trending

lineament of Kalrayan to the west of Pachchai hills. So, this lineament has a

curvi-linearity and alignment over Gangavalli shear.

2.4.4.5 Rocky Slope / Cliff

A cliff is a steep slope (usually >40º, often vertical and sometimes

overhanging), exposing rock formations. Cliffs rising 100–500 m above sea

level are termed high cliffs, and those ≥500 m (as in Peru and western Ireland)

megacliffs (Guilcher 1966). Cliffs less than a metre high are termed



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microcliffs. Joints, bedding planes, faults and intrusions influence cliff

morphology, and lateral changes in lithology result in changes in cliff profiles.

Mass movements occur on cliffs where the groundwater load becomes

excessive, where stresses develop, where a massive caprock exerts pressure

on underlying weaker formations, or where there is expansion or base

exchange, weakening clay minerals. Breakaways develop at the cliff crest

where masses of rock topple down the cliff, and slumping produces irregular

topography as rock outcrops disintegrate and material slides, flows or creeps

down the slope towards a basal receding cliff. Cliffs and rocky slopes are

commonly observed in Shevaroys, Chitteri, Kalrayan, Kolli, Pachchai and

other small hillocks. They are very distinct very steep (Plate-IIIC).

2.4.4.6 Triangular Facets and Escarpments

Heights and stages of dissection of triangular facets are indicative of

relative tectonic activity (Bull and McFadden, 1977). Basal sections of

triangular facets may resemble degraded fault planes (Hamblin, 1976;

Menges, 1990; Ellis et al., 1999). Obvious landscape contrasts in the Great

Basin of Obvious landscape contrasts in the Great Basin of west-central

Nevada were used by De Polo and Anderson (2000) to estimate slip rates for

hundreds of normal faults. Rapidly rising mountain fronts have

1) Fault scarps on the piedmont and at the mountain–piedmont

junction and

2) High triangular facets.

Tectonically inactive mountain fronts have neither.

The term escarpment, or scarp, has been applied traditionally to a

steep, often single slope, of considerable length, that dominates a section of

landscape. An escarpment thus can be distinguished from the two flanking

walls of canyons. For example, south of Sydney, Australia, the coastal

escarpment forms a long, virtually continuous wall, but it is outflanked by

canyons which extend more than 100 km further inland. Another notable



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instance of a valley cut well back from an escarpment is the Sognefjord, which

extends about 200 km inland from the coastal edge of the Norwegian

highlands. The lengths of escarpments vary from a few kilometres to the

subcontinental scale of mega-escarpments, or Great Escarpments, such as the

Drakensberg of South Africa, while their heights vary from a few tens of

metres to several thousand metres. A distinction is generally drawn between

denudational escarpments and fault although this may be no simple exercise

in areas of essentially homogeneous crystalline rocks. Less extensive, though

nonetheless impressive, escarpments have developed as a result of regional

uplift in continental interiors. The classic examples are along the margins of

the Mittelgebirge of central Europe, such as the Massif Centrale of France and

the Erzgebirge of Germany (Goudie, 2004).

The triangular facets/ spur facets were very common feature along the

entire western slope of Kolli, south east and southwest slope of Shevaroys,

southwestern slope of Kalrayan hills (Plate-IIID), south western slope of

Pachchai hills and north and southern part of Bodamalai. The lines connecting

the top of apices of the triangles tentatively represent the trend of the

lineaments. So, the facets in the western Shevaroys show N-S trend and in the

eastern side show NE-SW trend. NE-SW trend of lineament could be traced

from Chitteri hills whereas NNW-SSE trend from west and ENE-WSW trend

from northern and southern sides of Bodamalai. N-S trend from northwest,

WNW-ESE from northwest lower side and eastern side and a set of NE-SW

lineaments could be traced from the Kolli hills. In Pachchai hills the trend of

lineaments of NE-SW and NNW-SSE could be traced from the facets.

The splendid triangular facets of the Wasatch Range escarpment in

north-central Utah have been a classic example of a tectonic landform since

the time of Davis, W. M. (1903). Blackwelder (1934), Hamblin (1976), and

Wallace (1978) describe triangular facets as being fault planes that have been

modified by erosion.



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Mapping of escarpment seems appropriate for mountains bounded by

normal faults. The northwestern side and eastern side of the Shevaroys were

bounded by NE-SW trending escarpments (Plate-IIIE).

Fig. 2.10 Geomorphic Anomalies within Study Window



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2.4.4.7 Linear Ridge

Linear ridge are long narrow elevated topography that stretches in a

straight line. It may indicate the presence of a fold or fault. If it is found along

a lateral fault, it may be a shutter ridge or a pressure ridge. Sometimes they

are resulting from differential erosion of different rocks or intrusion of dykes

along linear cracks. Though many dykes were present in the study area the

one near Malliyakkarai with NW-SE trend stood out as a prominent ridge.

Many ridges of lesser elevation were found near west and north of Kolli hills

where the linearity corresponds to banded iron formation (BIF) and dykes

with NE-SW trend in the west and E-W trend in the north.

The Gangavalli shear stands out as a ridge which is essentially a

charnockite with cataclasites and pseudotachylyte in the middle make this

ridge hard and resistant. This ridge displays dextral slip and they may be a

shutter ridge resulting from lateral –dextral slip fault.

Meta-gabbro forms prominent ridge around the entire Kanjamalai at

the base and similar beds forms protruding ridge at different elevation. The

bottom most ridge was dissected by N-S and E-W trending lineaments and

thus appears like hogback (Plate-IIIF).

2.4.4.8 Alluvial fans and Bajada

Alluvial fans occur in two characteristic situations at mountain fronts

and at tributary junctions. In both cases, high sediment loads encounter zones

of reduced stream power, with accommodation space for deposition. These

conditions are controlled by long-term landform evolution, including the

tectonic setting and erosional history. Mountain fronts may be fault-

controlled or erosional, in which case the fans may bury an older pediment

surface. Tributary-junction settings are controlled by the long-term

dissectional history. Pope and Wilkinson (2005) studied the roles of climate

and tectonics in the late quaternary fan development on the Spartan piedmont

Greece.



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In Kolli hills the bajada were observed all along the western and

eastern sides and two patches of alluvial fans on the northern side along

Vachchikai ar and near Periya Kombai. Near the south western side of

Kalrayan and southeastern side of Chitteri hills bajada was observed. Entire

eastern and south eastern side of Kalrayan hills displays bajada zones. Apart

from this there were localized occurrences of alluvial fans near

Vellalakundam (on the foot of .777, .747, .711, .626 peaks). In Bodamalai four

Alluvial fans were observed on the northeastern side, southeastern side and

easternside. A thick alluvial fan was observed on the easternside of .582 hill

near Mudiyanur.

Alluvial fans were strikingly absent around Shevaroys and Pachchai

hills. The alignment of the alluvial fans around Kalrayan and Kolli hills, along

NE-SW clearly indicates the slope modifications of these hills due to rising or

faulting. North of Tenkal malai (.811) the alluvial fan has sharp boundary and

aligning with NE-SW lineament passing through Kottapatti shear. Similar,

abrupt termination of alluvial fans could be observed on the southern and

south western side of Kalrayan hills and they could be attributed to NW-SE

lineament on the southwest side and E-W lineament on the southern side of

the Kalrayan hill. Near Mannur reserve forest the northern one (.392 hill)

could be correlated to E-W lineament and the southern one could be

attributed to ENE-WSW lineament.

2.4.5 Drainage Anomalies

2.4.5.1 General

The nineteenth and early twentieth-century geomorphologist Davis,

W.M (1889, 1899) developed an elaborate scheme to describe the components

of a river drainage network as they related to stages in its physiographic

development. Their pattern and flow dynamics, in other words, their

architecture are greatly dependent on the local lithology and the exposed and

buried geological structures in their mature stage. Miller (1937), Chitale

(1970) and again a large number of workers have brought out exhaustive



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information on how these drainages can be used in mapping the lithology

and geological structures especially from the aerial photographs. In addition,

the rivers as indicate the palaeo and buried geological structures, their

anomalies have unique credentials in mapping the active geological

processes.

The drainage fabric of the study area was brought out using the Survey

of India topographic sheets of 1972 and 1973. These drainages were compared

with the IRS-1C satellite images and necessary updation was done. These

drainages were digitized using ArcGIS software and GIS layers were

generated for the study area.

The drainage zones of various anomalies viz. radial drainages, parallel

drainages, deflected drainages, eyed drainages, anomalous compressed

meanders etc. were marked as probable tectonic weak zones. While the

deflected drainages indicated such probable weak zones predominantly along

N – S, NE – SW and NW – SE alignments, the eyed drainages and other

anomalies indicated the predominance of N – S oriented tectonic weak zones

with marginal variations from NNE – SSW to NNW – SSE and NE – SW and

NW – SE tectonic weak zones. Moreover, the Vellar and Gomukhi drainage

have a constriction in the east by Mio-Pliocene Cuddalore sandstone

uplands so, these river have funnel shaped drainage pattern and cutting

through the uplands via the E- W trending Vellar fault (Fig.2.11).

2.4.5.2 Drainage Anomalies and Related Tectonics

In this process, the following various drainage anomalies were

identified in the study sectors

Radial drainages and Annular drainages

Parallel drainages

Deflected and lineament controlled drainages

Eyed drainages and Braiding of streams

Compressed meanders

Palaeochannels.



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2.4.5.2.1 Radial and Annular Drainages

The radial drainages are the drainages that are radiating away in all

directions from a central point or drainages converging from the periphery

towards a central point. The former is called as centrifugal radial drainage

and the latter as centripetal radial drainage. These have been invariably

interpreted to be the indicators of recent tectonic movements unless otherwise

these are controlled by the topographic features of erosion or the impact

phenomenon.

Such radial centrifugal drainages observed in many parts of the world

were inferred to be due to active tectonic doming (Whitehouse 1941, King

1942, Isachsen 1975, Twidale 2004 and many others). In India too, a lot of

inferences were made by many on the possible active tectonics from such

radial drainages. In Saurashtra Peninsula, Sood et al. (1982) interpreted a

number of such centrifugal radial drainages along with gullying and doubted

for possible Post Deccan trap diapirism. Babu (1975) while has inferred that

the anomalous radial cum annular drainages indicate possible

morphostructures related to hydrocarbon in Krishna–Godavari delta.

Ramasamy et al., (2006a) have observed network of radial cum annular

drainages in Cauvery delta and inferred them to be the reflection of recent

subsurface doming.

Hence, the detailed scanning of the enlarged formats of the drainage

fabric in the computer has lead to the detection of radial drainages in a

number of places. These radial drainages were analysed in conjunction with

various topographic features such as hills, erosional plateaus, prominent

depressions, etc. and only the radial drainages that do not fall in these

features were identified as radial drainages related to probable Neo–Active

tectonics.

Such an interpretation has lead to the identification of radial drainages

in two locations; one was near west of Pathakurichi and another one near

Kandamattan (near E 78º 50‟, N 11º 30‟).



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The drainages were radiating in all directions centrifugally. Wherever

different arms of radial drainages showed rectilinearity those were marked as

active lineaments / faults. These two radial drainages were buffered out as

probable Neotectonic zones.

Annular pattern of drainages were observed on the northern side of

Pachchai hill where the Swetha nadi displays a curvature from near

Gangavalli up to near Krishnapuram where it confluence with Vellar.

Vashista nadi was showing a curvature after Kattukottai and parallelism

could be observed with Swetha nadi. A set of arcuate drainages were seen

near Kumbapadi on the Kalrayan hill. Another set of arcuate drainages were

observed east of the Kalrayan hill top.

Two more arcuate drainages were observed one near Sankapuram and

other southwest of the previous one. Strikingly they show parallelism and

bend towards the south was evident. SE corner of the study area near Chittall

an annular drainage with western convexity was observed. On the top of

Chitteri hill the drainage system follows the fold where the axis of the

curvature was pointing NE-SW direction.

2.4.5.2.2 Parallel / Rectilinear Drainages

A system of co-linear drainages is called as “parallel drainages”,

whereas the long and straight flow paths of the drainages are called as

rectilinear drainages. The parallel or co-linear drainages are normally seen in

dune fields in between co-linear ridges (Miller 1937). In general, the

rectilinear flow of drainages is normally attributed to the faults, whether

active or dormant. In fact, most of the lineaments and faults are interpreted by

the Geoscientists only from such rectilinear flow of drainages (Twidale 2004).

In India, the rectilinear flow of Narmada River for over 1000 km in Central

India was inferred to be due to a major crustal dislocation (Oldham et al.,

1901, West, 1962, Yellur, 1968, Murty and Mishra 1981 and many others). In

parts of Tamil Nadu also, most of the easterly flowing near rectilinear rivers



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namely Palar, Ponnaiyar, Cauvery in parts, Vellar, Vagai etc. were inferred to

flow along WNW–ESE transverse faults, out of which some of them are

inferred to be active (Vemban et al., 1977). Again the northeasterly rectilinear

flow of river Coleroon (Cauvery) from east of Tiruchirappalli and upto its

point of confluence was observed to follow a NE–SW Pleistocene sinistral

fault by Ramasamy and Balaji (1995). Closely spaced parallel drainages were

observed to indicate fracture swarms in general by many. Chamyal et al.

(2003) have observed the closely spaced parallel drainages in Saurashtra

Peninsula to indicate tectonic upliftment.

The Vashista nadi that flows on the southern flanks of Chitteri and

Kalrayan hills and Swetha nadi that flows on the northern fringe of Kolli and

Pachchai hills which are nearly 40 km. apart exhibit parallelism. This

strengthen the view of Srinivasan (1974) who has described that the Attur

valley as a graben. In fact these rivers were mainly controlled by E-W running

Vellar fault in the north and Swetha nadi fault in the south.

Parallel drainages were observed in Shevaroys corresponding to NW-

SE lineaments, N-S lineaments and NE-SW lineaments. Fold pattern was

explicitly defined by the parallel drainages in the Chitteri hills. Parallel

drainages of smaller dimension could be traced in Kalrayan which are mostly

N-S for shorter distance and trend of many smaller streams collectively

project a N-S trend.

2.4.5.2.3 Deflected and Lineament Controlled Drainages

The slope controlled and the lineament controlled drainages which

were sharply deflected by the lineaments were interpreted as deflected

drainages. In the case of slope controlled drainages, the geometry of the

lineaments which have deflected the drainages were interpreted as the related

lineaments. Whereas, in the case of lineament controlled drainages, the

geometry of both the lineaments along which the drainages were originally

flowing and the lineaments which have deflected the drainages were taken as



127

the lineaments related to such drainage anomalies. Such mega deflected

drainages were interpreted in 643 locations in different parts of study area.

Whereas in the case of fracture controlled drainages, both the

lineaments (controlling one and the deflected ones) were inferred as probable

Neo-Active tectonic lineaments.

These types of deflected drainages were interpreted in many riverine

systems the world over (Bowler and Harford 1966, Panizza 1978, Reid 1992,

Saintot et al., 1999, Kusky and El–Baz 2000, Twidale 2004 and many others).

For example Twidale (2004) has observed many such deflections in Murray

river of Australia and identified many lineaments / faults related to them. In

India too, a lot of workers have utilized the drainage deflection phenomenon

as a tool for detecting the land stability and Neo-Active tectonics. Babu (1975)

has observed anomalous drainage deflections in river Godavari, Andhra

Pradesh and attributed these to various lineaments related to tectonic

upliftments. The rectangular flow of Bhramaputra river, Assam valley has

been explained by NE–SW, NW–SE, E–W and N–S faults (Murty and Mishra

1981). Amalkar (1988) has explained the complex drainage pattern in Luni

basin, Rajasthan through lineaments of various orientations. Radhakrishna

(1992) has observed multiple deflections in river Cauvery in Biligirirangan–

Hogenekkal area (south of Bangalore) and on the basis of such acute and

rectangled deflections, identified a spectrum of N–S dextral and sinistral

faults. Ramasamy et al. (2006b) attributed deflection of Araniyar and

Korattaliyar to Late Holocene lineaments. Ramasamy et al. (1992) recorded

that the river Cauvery has migrated during 2700–2300 B.P. towards

Tiruchirappalli – Thanjavur plains and its left out trace was occupied by river

Ponnaiyar after it. Hence, obviously the drainages, the deflections and the

related lineaments of these deflected drainages of Ponnaiyar River should be

younger to 2300 years B.P.

By taking the above analogy, the drainages which were deflected and

controlled by lineaments could have neotectonic significant and hence they



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were marked. The analyses of the controlling and the deflecting lineaments

have shown that N–S, NE–SW and NW–SE and E-W oriented lineaments

have contributed to the maximum in such drainage anomalies (Plate-

IVA&B).

Fig. 2.11 Drainage Anomalies within Study Window



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2.4.5.2.4 Eyed Drainages (Anastomosis) and Braiding of Streams

An anabranching alluvial river is a system of multiple channels

characterized by vegetated or otherwise stable alluvial islands that divide

flows at discharges up to bankfull. The islands may be developed from

within-channel deposition, excised by channel avulsion from extant

floodplain, or formed by prograding distributary channel accretion on splays

or deltas. A specific subset of distinctive low-energy anabranching systems

associated with mostly fine-grained or organic sedimentation are defined as

anastomosing rivers (Smith and Smith 1980; Knighton and Nanson 1993;

Makaske 2001). Neither of these terms now applies to braided rivers where

divided flow is strongly stage dependent around bars that are

unconsolidated, ephemeral, poorly vegetated and overtopped at less than

bankfull. However, some confusion remains because an individual low-flow

channel in a braided system is sometimes referred to as an anabranch. The

islands in an anabranching river are about the same elevation as the adjacent

floodplain, persist for decades to centuries, have relatively resistant banks,

and support mature vegetation. Anabranching bedrock rivers can occur

where the individual channels follow joint and fracture patterns. However,

bankfull flow is unclearly defined making such rivers difficult to compare to

their alluvial counterparts.

Van Niekerk et al. (1999) found that bedrock anabranching channels

on the Sabie River in South Africa have a significantly greater potential to

transport sediment than do the other entire channel types along that river. At

present, relatively little is known about bedrock anabranching systems.

Anabranching is not a mutually exclusive category for it occurs in association

with other

The drainages flow as a single stream, branch off into two and rarely

into four or five, run co-linearly or curvilinearly and meet after a few hundred

meters or kilometers, thus ultimately giving a shape of an eye or biconvex

lens. Because of such morphology, such drainage anomalies were interpreted



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as “eyed drainages”. Smith et al. (1997) have called this phenomenon as

“Anastomosis”. Thornbury (1985) has long back propounded that vertical

cutting / incisement of drainages indicates tectonic emergence and the

splitting up of the drainages suggests land subsidence. Ramasamy and

Kumanan (2000) have found similar eyed drainages in few places in Tamil

Nadu along the easterly flowing Bay of Bengal bound rivers and drainages

and these were invariably found to be either bisected by lineaments or

confined within two or more sub parallel lineaments. On the basis of this,

they have further demonstrated ongoing land subsidence along these

lineaments / faults. Ramasamy and Karthikeyan (1998) have observed a

mega lens shaped drainage (eyed drainage) in river Cauvery in

Tiruchirappalli, South India with an eye length of 15–20 km and the same was

found to fall exactly within a NE–SW trending 300 km long Holocene graben

extending from Pondicherry in the northeast to Kambam valley in the

southwest. Ramasamy and kumanan (2000) have observed eyed drainages in

parts of Tamil Nadu, with eye length of 5–30 km and explained them to be

due to tectonic subsidence in Holocene period. They have observed „S‟ shaped

dragging in eye shaped drainages, from which they have suspected sinistral

strike slip movements along these lineaments / faults which bisect such eyed

drainages or the sub parallel lineaments which bound such eyed drainages.

Ramasamy and Ramesh (1999) have observed an eyed drainage in Coleroon

river, east of Tiruchirappalli having a broad rectangular shaped caught up

island during 1930 AD and the modification of the same into a trapezohedran

shape during 1992 AD. From the same, they have visualized sinistral

movements of the NE–SW fault in the recent years along which the Coleroon

river is flowing. Similar drainage anomalies were also reported by many from

outside the country. Significant amongst them are the observations made by

Smith et al. (1997) in Okavango river, Botswana that the river on reaching a

graben split up into four channels and ultimately rejoin after crossing the

graben. They have called this tectonically induced phenomenon as

“anastomosis”.



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The hallmark of braided rivers is the presence of multiple active

channels that divide and rejoin to form a pattern of gently curved channel

segments separated by exposed bars. Braided rivers are marked equally by

temporal dynamism: gradients in sediment flux associated with the complex



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spatial topography change in local slopes, leading the flow to continually

adjust its path as it picks its way through the network. Even when external

conditions are constant, the braided pattern is continually changing, yet

statistically consistent: a true dynamic equilibrium. It has been suggested that

braiding was the dominant river pattern on Earth before the first appearance

of land plants in late Silurian time (Schumm 1968). Historically, a major step

in analysis of the causes of river patterns like braiding came with the

application of stability analysis to the problem (Fredsoe 1978; Parker 1976). In

stability analysis, one asks mathematically how a system responds to small

perturbations.

The braiding of streams usually happens when the gradients drops

near confluence points. Excluding alluvial fans, if the braiding starts at much

earlier stages of stream then it could be due to drop in gradient which is

anomalous manifestation of land subsidence along a fault.

These eyed drainages were invariably found to be either bisected by

the orthogonally / obliquely aligned lineaments or confined within two sub

parallel / oblique lineaments. Such eyed drainages were viewed critically in

conjunction with lineament data. Three eyed drainages were observed near

Gomukhi reservoir east of Kalrayan hills and 8 eyed drainages were found

near eastern side of study area where the Swetha and Vashista nadies

confluence to form Vellar. Two were found along the Kottapatti shear zone

and at four places Tirumanimuttar nadi displays eyed pattern. So, totally

some 17 eyed drainages, have been identified through visual interpretation

and the size varies from place to place.

In Tirumanimuttar nadi the lineaments set corresponds to eyeing have

NE-SW and NW-SE orientations and near Thummal sector NW-SE and E-W

lineaments control the anabranching of streams. Those in Gomukhi River near

eastern Kalrayan were controlled by ENE-WSW lineaments, the drainages

near Pachchai hills and in Vellar the controlling lineament have NNW-SSE,

NW-SE and NNE-SSW lineaments.



133

Radhakrishna (1992) has observed such N–S trending system of

dextral and sinistral faults in Sivasamudram area (south of Bangalore and

west of present study area) and concluded that such faults only have caused

rejuvenation of the river Cauvery. Ramasamy and Balaji (1995) have also

observed that most of the NE–SW faults of South India are sinistral strike slip

faults of Pleistocene parentage related to the post collision tectonics. Hence all

these eyed drainages were filtered out as the probable zones of Neo–Active

tectonics.

2.4.5.2.5 Compressed Meanders

The otherwise normally flowing drainages when exhibit compressed

meandering abruptly, the same is interpreted as compressed meanders.

These types of anomalous occurrences of compressed meanders in otherwise

normally flowing drainage systems have been demonstrated to be indicative

of active tectonics in such zones of compressions (Bakliwal and Sharma

1980, Murthy and Sastri 1981, Barooah and Bhattacharya 1989, Ramasamy

et al., 1991, Smith et al., 1997, Valdiya 2001, Jain and Sinha 2005 and many

others).

Many workers explained the anomalous sinuosities of the river Yamuna

right from its outlet from Himalayas and upto it‟s joining point with Ganges in

Indo-Gangetic plain to be due to active tectonics related to post collision

phenomenon. On the contrary, Bakliwal and Sharma (1980) have explained

the intense, acute and restricted compressed meandering in river Yamuna in

Agra region of the Indo–Gangetic plains to active scissor fault tectonics along

two sub parallel lineaments of the Great Boundary Fault System. Murthy and

Sastri (1981), Barooah and Bhattacharya (1989) and many others have

interpreted a large number of drainage anomalies in the form of compressed

meandering in Brahmaputra river and explained them to be due to still

ongoing collision of the Indian plate. The anomalous compressed meandering

in otherwise 1000 km long rectilinear Narmada river in its western end near

Broach area, Western India, was demonstrated to be due to ongoing tectonic



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activities by three major faults which occur in triangular pattern and caused

compressed meandering within them (Ramasamy et al., 1991).

Ramasamy et al. (1995a) observed anomalies like compressed meanders

in Veller river of Cuddalore region, Tamil Nadu and explained them to be

scissor fault movements along two sub parallel N–S trending lineaments.

Valdiya (2001) has observed compressed meandering in the drainages

belonging to major river Cauvery in the area of west of Bangalore and brought

out a number of active faults from them. Jain and Sinha (2005) have attributed

the acute compressed meandering in river Baghmati, Himalayan foreland

basin due to active block faulting.

The interpretation has revealed such compressed meanders in 14

locations out of which Veppadi ar or Thoppai ar is very strikingly show

meandering over a length of nearly 30km within the Toppur ghat section up

to Mallapuram. This could be correlated to NE-SW trending lineaments with

divergent step have opened sag ponds or pull apart basins or rhomb shaped

grabens (Fig 2.11CM). The Vaniyar Ar ENE-WSW and NE-SW lineament set

have the control in an alternative fashion and produce step like appearance

until NE-SW trending Manjavadighat lineament took control in further north.

The Piniyar Ar which flows near Pappireddipatti has NE-SW orientation and

aligning with Manjavadighat lineament where it confluences with Vaniyar

Ar. Small scale meandering was observed near Kottapatti in Kallar which

was aligning themselves to the NE-SW trending Kottapatti shear and

Sholiyar Ar a tributary of Kallar aligning with NW –SE trending lineaments.

Anaimaduvu Ar which is originating from southern Chitteri hills display

meandering parallel to NE-SW lineaments and similar pattern and orientation

could be observed in Thumbal Ar from near Thumbal to Ramanattam. The

Anaimaduvu Ar takes SE turn along a NW-SE lineament. East of Thumbal

meandering was observed in the Thumbal Ar along ENE-WSW lineaments.

Apart from deflections Tirmanimutthar show meandering south of Ilupili and

also near Sellipalayam that was along NE-SW lineaments (Plate-IVE). NE-SW

lineaments control Singipuram Ar till Singipuram then NW-SE lineaments



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deflects and sag the drainage and then ENE-WSW lineaments make the

drainage meander until it reaches the Vashista nadi near Etappur cross road.

Swetha nadi meander along ENE-WSW near Kudamalai and a

tributary originating in Pachchai hills joins Swetha nadi near Krisnapuram

(Ganagvalli) with meandering along NW-SE lineaments. East of the study

area the Vellar river shows compressed meandering near east of Neyveli.

In this interpretation, all the related lineaments were also brought out

by superposing the vector layer of such drainage map over the imagery. The

details of such compressed meanders and the related lineaments were taken as

symptomatic neotectonic zones.

2.4.5.2.6 Palaeochannels

The occurrence of palaeochannels indicates that the river has left these

traces and migrated away. The palaeochannels mapping and the evaluation of

phenomenon of river migration give excellent information on the Quaternary

geological and climatological events viz:

Active tectonic movements

Sea level changes

Climatic changes

Flood dynamics

Littoral currents, etc.

Among these the active tectonic movements seem to play a greater role

in river migration when these rivers show preferential migrations only in one

direction, the same is inferred to indicate that the land located opposite to the

direction of migration is undergoing tectonic emergence (Ramasamy et al.,

1992). Many workers have used this as a tool to understand and map the recent

tectonic movements. The phenomenal anticlockwise rotational migration of

river Sarasvati and its burial in the northern part of the Great Indian Desert was

attributed to the rise of Aravalli Mountains (Yashpal et al., 1980, Valdiya

1997, Radhakrishna 1998, Rajawat et al., 2003 and Gupta et al., 2004).



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Ramasamy and Karthikeyan (1998) have observed the southerly

migration of Ponnaiyar River in Pondicherry due to ongoing Holocene graben

to its south. The anticlockwise rotational migration of river Cauvery in its

deltaic regime in Tamil Nadu was explained by the Phenomenon of block

faulting of the Mio-Pliocene Sandstone and its upliftment from almost during

the last 6000 years (Ramasamy et al., 2006b). The northerly migrations of

Manimuttar and Vaigai rivers were inferred to be migrating by the E–W

cymatogenic arching to their south in Ramanathapuram–Rameswaram area

(Ramasamy et al., 1987). Preferential migration of rivers have been used as

an indicator for active tectonic movements around the world viz: southerly

migration of Echuca river, New South Wales (Bowler and Hard ford 1966,

Pels 1966), Charwell river in New Zealand (Bull and Knuepfer 1987), Po

river in Italy (Castaldini 1990), Murry river in Australia (Twidale 2004), etc.

These palaeodrainages / palaeochannels / buried rivers show

spectacular ribbon like, loop like, linear, curvilinear and contorted features

with black tone in black and white panchromatic aerial photographs and

reddish tone in satellite colour coded FCC images.

A faint palaeochannel was observed with an ENE-WSW orientation

along Idappadi- Salem-Vazhapadi and connects the present day Cauveri with

the present day Vashista nadi which is tributary of Vellar (Fig.2.11). A

palaeochannel was observed along Veppadi ar or Thoppai ar near

Mallapuram where the stream was taken by ENE-WSW trending sinistral

fault. Tirumanimuthar river has developed palaeochannel near Ezizebeth

pettai on the southwest flank of Shevaroys where the river originates (Plate-

IVD). Thick pile of alluvial sediments was observed near Kandashramam

where the small streamlet viz. Kannimar odai flows and this could be the

alluvial deposits formed by the same sreamlet (Plate-IVF). Further

downstream Vellar shows numerous palaeochannels in its deltaic region near

Chidambaram (Ramasamy et al., 1992). The ongoing E–W grabening to its

north was inferred to be the cause for northerly migration of the river Vellar

along Pudukkottai coast, Tamil Nadu (Ramasamy 2006a).



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2.4.6 Structure and Trend line Anomalies

2.4.6.1 Trendline Anomalies

Structure and trendline are the direct evidences of bygone tectonism

and they follow or align themselves parallel to the dragging forces of crustal

movement and they get folded when the dragging forces were acting across

to them. These rocks leave trails on the top and prominently reflected in the

topography as a linear line which thus indicates the trend line (Fig.2.12).

Fig. 2.12 Structure and Trend line Anomalies



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Ramsamy et al. (1993) used the trend line features for groundwater

targeting. Mădălina Nicoleta Frînculeasa and Alexandru Istrate (2010) have

mapped structural geological elements from the area of Dâmbovicioara

corridor of Romania using LANDSAT TM data for updating the already

existing structural and geological map.

The Salem-Attur Shear Zone exhibits imprints of multiphase shearing

and domains of meso- to micro-fabrics indicating opposing shear sense,

resulting in conflicting interpretations of the regional kinematics (Satheesh

Kumar and Prasannakumar, 2009). Since the sense of movement is a major

constraint in matching the shear zones in Gondwana fragments, a substantial

theory could unravel the enigma of opposing sense of shear.

If it is assumed that the Trendline was produced by a tectonic event

then their trend tends to remain the same unless it gets disturbed.so, it is

presumed that the trend lines are interrupted or detached by later events

otherwise it will have a linearity or curvi linearity attained by much earlier

events. So, the breaks and shift in the alignments were traced for lineaments.

2.4.6.2 Sense of Shear

Fig.2.13a Schematic Model on Lineaments Pattern and Sense of Shear



139

Fig. 2.13b Tectonic Model indicating Sense of Shear

Visual interpretation of lineaments clearly indicates sense of shear

where many of the earlier lineaments were displaced by later one. NE-SW

lineaments show sinistral sense of shear whereas NW-SE and N-S lineaments



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generally display dextral sense of shear. NE-SW trending Chitteri east

lineament aligning with the Kottapatti shear zone show dextral sense of shear

and suggesting a block faulting which was reflected at the south western end

in the deflection of Cauvery River near south of Sitampundi complex

(Fig 2.11).

Fig. 2.14 Lineaments from Anomalous Sense of Shear



141



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Ramsamy and Balaji (1995) have studied the Active tectonics of south

India and they came with model indicating NE-SW trending lineaments with

sinistral sense and NW-SE with dextral sense are Holocene lineaments and

these were resulted due to northerly compression of Indian plate.With this

idea and field observation a schematic model was constructed to picturise the

lineament pattern and sense of shear in relation to northerly compression

(Fig. 2.13a). NE-SW lineaments with dextral sense and NW-SE lineaments

with sinistral sense are much earlier or may be coeval to late Proterozoic

alkaline and ultramafic emplacement (Fig. 2.13b). Hence, based on the above

model, lineaments with sinistral sense of shear along NE-SW direction and

dextral sense of shear along NW-SE were considered anomalous and traced as

Neotectonic lineaments (Fig. 2.14).

Field study at selected windows also confirms the above model and

that the NE-SW lineaments with sinistral sense NW-SE lineaments with

dextral sense are Neotectonic lineaments (Plate-VI A-F).

2.4.7 Lineament Anomalies

2.4.7.1 General

A topographic line that is structurally controlled is called a lineament

(Billings 1972). Kelley (1955) has defined the lineament as a rectilinear

feature of considerable extent on the surface of the earth and a tectonic

lineament may be defined as either a general alignment of structural features

or a boundary between contrasting structural features. Lineament

identification is often questioned due to the difficulty in showing the

structural significance of either individual lineaments, or the observed pattern

(Huntington and Raiche, 1978). But, the lineaments have been the matter of

greater attraction to the Geoscientists from all over the world, especially more

after the advent of modern Remote Sensing technology, as such remotely

sensed satellite pictures spectacularly display the lineaments as linear /

curvilinear features represented by rectilinear topography, vegetation

alignments, rectilinear pattern of river courses, soil tonal linearities, etc. These



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lineaments have been studied by the scientists, the world over, for

understanding the general geodynamics of the Earth, of course, primarily on

the geodynamics of the older Precambrian rocks and to some extent in the

field of Neo– Active tectonics and the related seismic vulnerabilities. But not

much detailed work has been carried out on various lineament anomalies.

Rubke (1974) was the first person to bring out a holistic picture on the

lineament system and the geological evolution of Lesser Himalaya. Das and

Ray (1977) have prepared the lineament map for entire Deccan volcanic

province of Maharastra and therefrom brought out the first time picture on

the Post Cretaceous tectonics of Deccan plateau. Bakliwal and Ramasamy

(1987) have prepared a lineament map for entire Rajasthan and Gujarat using

Landsat imagery and brought out a comprehensive tectonic picture for the

Western India on 1:5,00,000 scale. Varadharajan and Ganju (1989) have

interpreted the lineament fabric of entire east and west coasts of India and

analyzed their signatures in general and also with reference to Quaternary

tectonics. Rakshit and Prabhakar Rao (1989) have prepared mega lineament

map for Indian Peninsula and in which they have classified the lineaments

into four azimuthal groups viz: ENE–WSW, NE–SW, N–S and NW–SE and

further they observed a remarkable coincidence in between the regional

lineaments and the earthquakes / geothermal activities in Koyna–Cambay

region, Hazaribagh–Bahneswar area, Bhatrachalam region, Narmada–Son rift

and Gondwana graben system and also Puga–Manikaran zones in Himalayas.

Haman (1961) has demonstrated a new technique of how palaeo-stress

environment of a region can be brought out by preparing lineament density,

lineament intersection density, etc. This technique was widely followed in

India too and significant amongst them are, the studies of Bakliwal (1978) to

bring out the stress pattern of Ranthambhore Quartzites of Rajasthan;

Ramasamy et al. (1983) to evaluate the stress / forces related to the origin of

Ishwarakuppam dome, Cuddapah basin, Andhra Pradesh; Ramasamy (1995c)

has evaluated the palaeo stress environment and therefrom the tectonic

evolution of Vindhyan basin of western India and again by



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Ramasamy et al. (1999) have brought out the tectonic evolution of Eastern

and Western Ghats fold belts of South India; Kumanan (1998) and

Ramasamy (1995c, 2006a & 2006b) have done remote sensing and GIS based

lineament studies and they identified the possible zones of Neotectonics in

parts of Tamil Nadu. Again, from amongst various studies in India, the Son–

Narmada lineament of Central India was studied by many for its pulsatory

tectonism and related issues (Oldham et al., 1901, West 1962, Yellur 1968,

Crawford 1978, Murty and Mishra 1981 and many others). However a large

number of scientists have used lineaments for various natural resources

explorations.

Christine A. Powell et al. (1994) has monitored microearthquakes for

ten years with a regional seismic network and has revealed the presence of a

well-defined, linear zone of seismic activity in eastern Tennessee. The

lineaments derived from the satellite data were also studied in many parts of

the world to understand the Neotectonic movements also (Blanchet 1957,

Gay 1973, Qiang and Zhang 1984, Rust and Stewart 1996, Machette 2000,

Calamita et al., 2000, Han et al., 2003 and many more). Hence in the present

study, first to start with, the lineaments were interpreted and detailed

lineament map was prepared using the raw and digitally processed IRS-1C

data for the study window and from such lineament maps, various lineament

anomalies viz: the zones of curvilinear lineaments, branch off lineaments,

radial lineaments, lineament bundles, parallel lineaments, lineament density

maximas, lineament intersection maxima and lineament number maximas

were interpreted and such zones were buffered out as independent GIS

layers, integrated together and the probable Neo-Active tectonic zones were

brought out (Fig.2.15).

2.4.7.2 Lineament Fabric

As stated earlier, primarily by IRS-1C satellite data and supplemented

by their digitally processed counter parts, the lineament map was prepared.

The lineaments were interpreted on the basis of tonal linearities /

http://www.sciencemag.org/search?author1=Christine+A.+Powell&sortspec=date&submit=Submit



145

curvilinearities, drainage linearities, soil tonal linearities, vegetation

linearities, etc. These lineaments were subjected to regional studies in

conjunction with published map of Geological Survey of India (Anon 2000)

and also the maps prepared by various earlier workers. Detailed field surveys

were also undertaken to verify the lineaments and their extensions. The

lineaments interpreted so, were mostly observed to be lithological contacts,

linear and vertical escarpments in the hills, fracture valleys, rectilinear river

courses, inferred faults of Geological Survey of India (Anon 2000), etc. in most

of the places. Finally the lineaments related to tectonics were filtered out and

the same is shown in Fig.2.15. These lineaments interpreted in general have

shown NE–SW, NNE–SSW, and NNW–SSE, NW–SE, E–W and N–S

orientations.

2.4.7.3 Curvilinear Lineaments

Lineaments in general will be rectilinear and on certain tectonic

conditions they exhibit curvilinear manifestations too. Normally, when the

faults are low dipping, then they show curvilinear expressions in the surface.

While evaluating the tectonic evolution of the Fennoscandian–Baltic Shield in

Denmark, Liboriussen et al. (1987) have inferred the curvilinear lineaments

to signify Late Cretaceous tectonics. In parts of South India too, such

curvilinear lineaments were inferred to be related to post drift kinematics of

the Indian Peninsula (Prabaharan et al., 1995). Valdiya (2001) has inferred a

number of peripheral / curvilinear faults in Anamalai, Palani and Nilgiri hill

areas and observed them to be the reflection of recent horst and graben

structures on the basis of various geomorphic anomalies.

Qureshy (1964) and Gubin (1969) have also observed a number of

E–W linear and curvilinear faults in southern part of Tamil Nadu in

Anamalai–Palghat–Nilgiri hills and on the basis of geophysical anomalies

observed them to be the boundaries of recent horst and graben structures.

Ramasamy and Balaji (1995) and Ramasamy et al. (1998) have also

reported mud eruption associated with seismic tremors along curivilinear



146

lineament near Tiruppattur. Ramasamy and Kumanan (2000) have inferred

tectonic subsidence along this curvilinear lineament on the basis of eyed

drainage in river Palar, east of Gudiyattam.

Fig. 2.15 Study Window - Lineament Anomalies

The curvilinear lineaments trending E–W graben are very significant

and nearly forms the boundary of the Attur valley which impart adequate

evidence to call it Attur graben as proposed by Srinivasan (1974). These

lineaments with E-W fabric were very unique to this generally NE-SW terrain.

Two of these lineaments, traces the trend of MBSZ along Salem-Attur sector

running north and south of Kanjamalai. Another curvilinear lineament with

similar trend was running south of Malliyakkarai basin.



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A lineament with southern curvature, a distinctive character

differentiates it from other E-W trending curvilinear lineament where they

display northern curvature was traced along the northern foot of Kolli and

Pachchai hills and follows the Swetha nadi fault of Srinivasan (1974). Another

lineament with similar trend was observed that was running on the southern

foot of Shevaroys, Chitteri and cutting the southern part of Kalrayan hills via

Thummal, Karumanthurai and north of Gomukhi nadi in the east.

A set of lineaments with curvilinearity cuts Shevaroys, Chitteri and

Kalrayan with ENE-WSW trend were observed running north and south of

Manimukta nadi in Kalrayan which shows compressed meandering. Steep

escarpments and alluvial fans could be observed on north and south side of

Manimukta nadi. Toppal Ar –Ponniayar Ar lineament with ESE-WSW trend

and passing through a spring at Tirthamalai show curvilinearity. NW-SE

trending curvilinear lineaments extending from Pachchai hills to Shevaroys

hills in the north impose a NW-SE trend in the Attur valley (Fig. 2.15a).

2.4.7.4 Branch off Lineaments

The sub parallel lineaments that meet at acute angles, such branch off

lineaments or converging lineaments were interpreted as tear faults, aligned

orthogonal to the folds related to the recent fault followed folds in Wheeler

ridge of southwestern California by Mueller and Talling (1997). Ramasamy

et al. (2009) have observed the coincidence of recent earth tremors along such

acutely joining or the branching off lineaments in parts of Tamil Nadu. These

Y‟- shaped structure was supposed to branch off from near Ponnaiyar (E 78°

44' 38.40", N 12° 7' 22.80"), near Mel Nilavur (E 78° 42' 14.40", N 11° 53' 56.40")

and near Karumantur (E 78° 37' 44.4", N 11° 50' 13.20"). These lineaments

were having a consanguineous character of eastern convexity and thereby

possibly signify an anticlockwise rotation of southern peninsular after the

near saturated collision with Eurasia. Such three set of branch off lineaments

were buffered out from Kalrayan area (Fig. 2.15b).



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2.4.7.5 Parallel lineaments

Ramasamy et al. (1999), in their remote sensing based Precambrian

tectonic model of South India, observed that the E–W fracture swarms of the

Bangalore–Chennai region do not fit in with Precambrian orogeny. Whereas,

Chakrapani Naidu and Jayakumar (1979) have doubted the Post Tertiary

origin of these dykes filling these fracture swarms. While Ghosh (1976)

attributed the E–W to ENE–WSW fracture swarms of the Saurashtra

Peninsula (Western India) to the E–W aligned Amerli cymatogenic arch of

Post Trappean age, Sychanthavong (1985) and Ramasamy (1995a) have also

advocated that these fracture swarms of the Saurashtra Peninsula are related

to Post Trappean cymatogenic arching connected to the collision of the Indian

Plate with the Eurasian Plate. Ramasamy et al. (1987, 1995a), Ramasamy

(1989), and Subrahmanya (1994, 1996) have also doubted possible

cymatogenic arching in the Mangalore–Chennai region. Similarly, Kumanan

(1998) have observed the E–W fractures fracture swarms in the Varushanad

hills which coincide with sub parallel E–W fractures observed in the area by

Ramasmy (2006a) who attributing this to cymatogenic arching along Cochin–

Ramanathapuram.

Ramasamy (2006a) has observed bundles of NW–SE trending sub

parallel lineaments from the IRS satellite FCC data of at different places of

Tamil Nadu controlling Pambar river at its northwestern, the flow of the

Ponnaiyar river in its matured and old stages, deflects the Vellar river,

delimits the Jayamkondam Mio-Pliocene Sandstone, and also causes

conspicuous compressed meandering in the otherwise northeasterly flowing

Coleroon/Cauvery river and along the coast, it abruptly cuts off the beach

ridges. Further, he observed wider flood plain of Suruliar in Kambam valley

which was a well defined tectonic valley by NE-SW sub parallel lineaments.

Ramasamy (2006a) has reiterated N-S and NNE-SSW parallel and sub

parallel lineaments, namely, the Stanley reservoir–Tevaram, Krishnagiri–Cape

Comorin, Gudiyattam–Cape Comorin, Tanjore–Avadaiyarkoil, and



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Kumbakonam–Muttupet lineaments are tectonically active. He also opined

the extension block faulting morphology to N-S lineaments wrench faulting

signatures to NE-SW and NW-SE faults.

Similar, tendency could be observed and applied to this study window.

The parallel and sub parallel lineaments in this area could be classified on the

basis of orientations in to E-W trending with an azimuth of 86º to 92º, N-S

trending with an azimuth of 174º -186º, NE-SW trending with an azimuth of

42º-50º, NW –SE trending with an azimuth of 46º - 48ºand 58º- 62º, and NNE-

SSW trending with an azimuth of 27º-30º (Fig. 2.15c).

2.4.7.6 Lineament Density Maxima

The total length of lineaments per unit area is called as lineament

density. This type of lineament density diagrams were used as potential guide

to understand the palaeo stress environment, stress maxima zones and also in

assessing the compressive forces involved in generating the folded structures

by many around the world (Blanchet 1957, Harris et al 1960, Haman 1961,

Marrs and Raines 1984, Berhe and Rothery 1987).

Haman (1961) was the first person to demonstrate this technique of

lineament analysis for evaluating the palaeo stress environment. Many earlier

workers have utilized this tool for groundwater and hydrocarbon

explorations in India (Raiverman et al., 1966, Ermenko 1968, Kumar 1983,

Usha et al., 1989, Kumanan and Ramasamy 2001, etc.). Bakliwal (1978),

Ramasamy et al. (1983), Nair (1990), Ramasamy (1995b) and others have

extensively used this method for evaluating the palaeo stress related to the

tectonic evolution of respective fold belts. Nair (1990) has even used this

technique to identify the pattern of the folds in the Western Ghats of Kerala

and he inferred that the circular stress fields were correlatable to surfacial and

sub surfacial domal structures and elongated elliptical stress maxima zones

indicated the elliptical anticlinal structures. Similarly, Ramasamy et al.

(1995b) have identified the pattern of stress on the basis of the shapes of the



150

lineament density contours and related them to different fold styles in

Aravalli mountains of western India, such as circular stress fields to broad

regional domes and basins, elliptical stress fields to anticlines and synclines

and rectilinear stress maxima zones to long and linear tight anticlines and

synclines. Kumanan (1998) and Ramasamy (2000, 2006a) have used

lineament density maxima to identify the Neotectonic zones in parts of

Western Ghats, South India.

Hence, in the present study, the total lengths of lineaments were

counted per five sq km area and plotted in the respective grid centres and

contoured using surfer software. The lineament maxima axes were drawn

along the crest of the elliptical contours of maximum values. Such lineament

axes so drawn mostly fell along E–W directions, NE–SW directions near NW

part of study window and NW–SE directions in south centre to NW part of

study window. The axes were mostly oriented in E-W and sparsely in N–S

directions. These lineament density maxima axes were buffered out and GIS

image was generated (Fig.2.16 a).

2.4.7.7 Lineament Number Maxima

In the same way, the total numbers of lineaments were counted per five

sq km grid, plotted in the respective grid centres and such values were

contoured, called as “isofracture map” or “lineament number density map”.

Again the zones of elliptical contours of maximum values were studied and

maxima axes were drawn along crest of such elliptical contours. Isofracture

pattern in the study window display the general trend of prominent N-S, E-W

and NW-SE trend with little NE-SW fabric. Finally the lineament number

maxima axes were buffered out as a separate GIS layer (Fig 2.16b).

2.4.7.8 Lineament Intersection Maxima

The total numbers of lineaments intersection were counted per five sq

km grid, plotted in the respective grid centres and such values were

contoured. The zones of elliptical contours of maximum values were studied



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and maxima axes were drawn along crest of such contours. A prominence of

NW-SE fabric was observed and it was followed by E-W, N-S and NE-SW

fabrics (Fig 2.16c).

Fig. 2.16 Lineament Anomalies from Density, Frequency and Intersection



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2.4.8 Aeromagnetic Anomalies

2.4.8.1 General

Aeromagnetic survey is considered as one of the fastest and most

economical method of undertaking geophysical reconnaissance of any

unexplored or inaccessible regions. Though the use of the method was

initially to target ore deposits, its utility in interpreting regional structure was

widely realized and hence, it is used for regional tectonic studies. Zietz et al.

(1969) have described the existence of the Towanta lineament during an

aeromagnetic investigation of crustal structure in western United States. It

provides information on structural trends, the position of faults even in areas

of extensive soil cover as well as in areas where in the crystalline basement is

overlain by sedimentary sequences (Harikumar et al., 2000).

Aeromagnetic surveys in India dates back to early nineteen fifties and

an area of 18,000 sq. km was covered in Bengal Basin by the Standard

Vacuum Oil Co., of USA for Oil exploration during 1951-52, which was

followed by a survey in Brahmaputra valley in upper Assam in 1953-54 (Hari

Narain, 1965; Bahuleyan, 1997). Atomic Minerals Division (AMD)

conducted radiometric survey in 1955. The aeromagnetic survey on crystalline

terrains using multi-sensors was achieved by AMSE Wing of GSI between

1967 and 1971. At the same time, i.e. in 1968, an area of about 25,000 sq.

kilometres was flown under the UNDP Mineral Development Programme in

northeastern Tamil Nadu. The entire country was covered by systematic

aeromagnetic survey by the Geological Survey of India under a National

Programme in a period of 15 years starting from 1980. In addition AMD and

National Geophysical Research Institute have conducted detailed surveys

over areas of mineral deposits.

Earlier surveys have generated contour maps and visual interpretation

was made to bring out the regional structures and anomalies formed due to

ore deposits. While iron' ores were successfully delineated by the surveys,



153

other non-magnetic mineral deposits were identified by the structural control

of ore localisation and the kimberlite pipes were targeted by the circular

features formed. The advent of computer processing has helped in generation

of colour - coded images and HariKumar et al. (2000) and Rajaram et al.

(2001) have attempted the aeromagnetic study of Peninsular India.

The aeromagnetic total intensity anomaly data used in the present

study is obtained from AMSE division of GSI in 1: 250,000 scale with the

contour interval of 39900 to 40660 gamma and from the Department of

Geology and Mining, Government of Tamil Nadu, as a total intensity

contoured map in 1:63,360 scale with the contour interval of 50 gamma and 10

gamma contours and the magnetic values range from 42000 to 49200gamma.

The survey was made in 1968 under UNDP mineral development programme

by Hunting Geology and Geophysics Ltd., England. with a mean flying height

of 150 m and mean flight line spacing of 1 km. The flying was carried out in

NW-SE direction across the regional trend and flights in NE-SW at mean

spacing of 15 kilometres were flown for tie lines. The necessary corrections

carried out during Aeromagnetic survey were already made for the

preparation of contour map. A mosaic of the six sheets was done and the map

was redrawn with the contour interval of 50 gammas, as the 10 gamma

contours were too dense to be reproduced.

2.4.8.2 Aeromagnetic Pattern of the Study Area

The striking feature of the total intensity aeromagnetic map is the

sparsely distributed contour pattern in a linear zone extending diagonally in

NE-SW in a northeastern part of the area. This zone coincides with the

Kottapatti shear, which divides the Kalrayan and Chitteri hills. Another zone

characterized by few contours is curvilinear extending from the southeastern

margin to the center up to the foothills of the Shevaroys. The eastern

extension of the zone is just south of the Kalrayan massifs and this zone is

possibly the Moyar-Bhavani-Salem-Attur Shear zone (MBSASZ). The western

extension of this particular shear however, is not clearly brought out, which



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can be attributed to the complex pattern formed due to the BMQ deposit at

Kanjamalai and intrusion of the Chalk hills ultramafic complex. In addition,

three parallel NW-SE trending linear magnetic anomalies dissect the area and

the magnetic trends in these linear zones are oblique to the regional magnetic

trends and these are considered as major magnetic breaks. Further, the iron

ore occurrence in Godumalai and Kanjamalai produced a strong dipole

anomaly pattern, which is typical of iron ore Provinces.

2.4.8.3 Interpretation of the Aeromagnetic –Total Intensity Anomaly Data

Total intensity value contour map, was used to generate image

(Fig. 2.17) as well as shaded relief maps (Fig. 2.18). In general, the southern

part of the area in the vicinity of the MBSASZ is characterized by the lower

values relative to the area located in the north. Curvilinear magnetic trends in

east - west direction are also faintly brought out. The NW-SE trending linears

are also faintly displayed in the image, but the breaks are very prominent.

The shaded relief map produced with illumination located in the NW

at an angle of 45° and an altitude of 30º gives a smooth picture (Fig. 2.18). The

dark areas in the figure correspond to lower values and the bright areas are

higher in magnetic susceptibility. The major feature observed is the influence

of the BMQ occurrence in Kanjamalai. A strong negative linear is observed

east of Salem town. The area south of Attur also displays alternating magnetic

low and high axes.

The Shevaroys and charnockite massifs generally characterized by a

smooth pattern, on the other hand alternating lows and highs trending NE-

SW to NNE-SSW are observed in Kalrayan massif. The area south of the

massifs is characterised by curvilinear trend lines in E- W to ENE-WSW

direction with alternating highs and lows. The Kottapatti shear is

characterized by a subdued fabric forming a NE-SW trending zone. When the

illumination is changed to SW, in addition to the features displayed

prominent NW-SE trending lineament are visible. These lineaments cross-cut

Kottappatti shear and extend into the Attur area suggesting that they are



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younger to Attur as well as Kottappatti shears. However, offsetting of these

lineament are noticed when they extend from the massifs into the Attur shear.

The lower magnetic susceptibility of the rocks in Attur area is clearly seen.

Fig. 2.17 Aeromagnetic Total Intensity Anomaly Map

The shaded relief map with the illumination at NW gives a very good

picture with intricate details of the magnetic trends and breaks when

compared with the earlier map (which includes 'Kanjamalai data). The

Kottappatti' shear is clearly brought out and is characterised by subdued

fabric. The change in magnetic trends to E-W in MBSASZ is also well

illustrated. East-west trending magnetic break appears on the northern



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margin of the area, which coincides, with the northern margin of the massif.

Few east-west lineaments are also noticed in the satellite imageries in this

locality. Magnetic linears trending NW-SE are also observed in the shaded

relief map. These lineaments are found as parallel lines cross-cutting and

deflecting the magnetic trends. The lineament extends diagonally from NW to

SE and is prominent in the northern part comprising the charnockites and

becomes faint in the Attur area. Its influence is also visible in the Kottappatti

shear area suggesting that the NW-SE trending breaks are younger. Study of

the aerial photographs reveals that the massifs as well as Shevaroys are

dissected by NW-SE trending faults along which the marker beds are

displaced.

As in the image using the total magnetic field, the Kanjamalai BMQ

occurrence is distinctly seen as a basin. The NE-SW regional grain in the

northern part of the area is obvious. The southern part of the area is

characterised by curvilinear E-W to ENE-WSW trend, with well defined

alternating high and low magnetic axes. Similar low - high pairing of

magnetic axes is also found in Kalrayans and Chitteri hills. The Kottappatti

shear forms a zone with minimum variation in the residuals. NW-SE trending

lineaments are also observed faintly.

In addition to the magnetic lineament noticed in the image generated

with total magnetic intensity, NNW-SSE trending magnetic breaks are noticed

in the eastern margin of the area. These breaks truncate the magnetic axes and

deflect them and transect the MBSASZ as well as the charnockite massifs.

Among these the most prominent is located in the eastern margin of the area.

From the southeastern margin the break has a NNW-SSE trend and when it

dissects the Kalrayan, a minor dextral shift is observed. A number of breaks

are found parallel to the one described are also noticed. This magnetic

break/linear coincides with the set of N-S trending lineaments are noticed in

the eastern margin of the area in the Kalrayan massif. Similar N-S breaks are

also noticed in the central part of the area on the eastern part of the area

crossing hills and also along the eastern flanks of Kanjamalai.



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Fig. 2.18 Aeromagnetic Total Intensity Anomaly – Shaded Relief Map

The magnetic pattern is grossly controlled by structure and

charnockites, gneisses, mafic granulites and dolerite dykes cannot be

separated. This is mainly due to the fact that the mafic granulites and dolerite

dykes though they are mafic rocks with high magnetic content occur as

narrow bands, rarely exceeding 200m in width. Hence, they are not separable

in the aeromagnetic data measured at the present image.

2.4.8.4 Interpretation of Magnetic Profiles

2.4.8.4.1 Profile A-B

The N-S profile (Fig. 2.19a) in the eastern boundary from Kalrayan

Pachchai hills near 78°42' shows that the magnetic intensities are

characterised by alternating highs and lows upto lat. 11°38', and further



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south, the values dropped up to 41050 gamma corresponding to Attur valley.

Many deep valleys were observed on the top of Kalrayan which were

representing NE-SW lineaments. The Gangavalli shear does not display

prominent anomaly and a gently sloping magnetic low is noticed to the

southeast of the shear. This coincides with a prominent N-S magnetic break

observed in the image.

2.4.8.4.2 Profile C-D

Magnetic profile drawn across the Kanjamalai hill (Fig. 2.19b) along

the anomalies located on the west shows that the peak-to-peak amplitude of

the anomaly is 41700 gammas in the north and 42,400 gammas in the south.

The valley may represent dipping nature of the beds.

2.4.8.4.3 Profile E-F

The Chitteri- Kolli profile (Fig. 2.19c) the first two dents in the profile

represent NE-SW trending lineaments and further south the values were

reaching as low as 41300 gamma corresponding to Attur valley.

The Swetha nadi fault was strongly displayed by the very steep and

deep valley at the south of the profile.

2.4.8.4.4 Profile G-H

The profile G-H across Chalk hill ultramafic complex displays a

distinct magnetic high, which corresponds with the serpentinised dunites

(Fig. 2.19d). The ultramafic body is bounded by two magnetic lows with the

magnetic values reducing to 41350 gamma and values steeply rise to 41800

gammas in the ultramafic body. Further south, the magnetic values reduce

with fluctuations and a prominent magnetic low was observed which

coincide with the Attur valley passing through Salem. The Attur valley that

was observed south of Chalk hills, exhibit branching (Anastomosis) nature of

the MBSASZ. South of Bodamalai, there is a steep decline in the value up to

41050 gamma represent ENE-WSW lineament south of Bodamalai.



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Fig. 2.19 Aeromagnetic Total Intensity Anomaly Map -Profiles



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2.4.8.4.5 Profile I-J

The magnetic profile across the Shevaroys -Kalrayan (Fig. 2.19e) shows

varying amplitude of the anomaly. The western Shevaroys was marked with

valleys of NW-SE trend and eastern side with NE-SW lineaments. Relatively

higher anomaly values were observed for Chitteri hills and easternside of this

hill a broad valley of Kottapatti shear zone was observed. The steep valley at

the eastern side of profile corresponds to NE-SW trend lineament in Kalrayan.

2.4.9 Demarcations of Neotectonic Lineaments

The present study encompass the analysis of significant lithology,

analysis of geomorphic anomalies and topography, drainage anomalies,

structural trend analysis, visually interpreted lineament analysis and

aeromagnetic anomalies to identify the neotectonic lineaments.

This DEM constitutes the basis for geomorphic analysis at regional

scale. The faults we have mapped can be considered as the main ones because

they are clearly displayed on such an image. The topographic analysis with

the shaded relief map from SRTM was used to study the anomalous relief

variations, gaps in between hills, linear shadows, steep slope sides and deeps

in the topographic profiles were interpreted for places of active tectonism.

Alkaline rocks, ultramafic rocks, dykes, granites and pegmatites,

pseudotachylytes and mylonites were taken as significant lithologies and

their orientation and location have been considered as representations of the

tectonic history or recurrence of tectonic events these will give us clue on the

orientation of then tectonics weak zones and possible zones of reactivation.

Neotectonic faults can be identified by (1) their morphology, forming

asymetric ranges with one side corresponding to breaks in slope or scarps, (2)

the displacement of recent sediment boundaries, structural or erosional

surfaces, and (3) the occurrence of straight lines of several tens of kilometers

in length dislocating and controlling the drainages. Image of the study

window was systematically compared with geological maps in order to

carefully separate the scarps formed by fault planes (active) from those



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resulting from differential erosion of contrasted lithology (ancient). The active

fault scarps, even eroded, are much higher and longer than the scarps formed

by lithological contrasts.

When possible the strike-slip, normal or reverse nature of the faults

was identified. Strike-slip faults have rectilinear traces and they locally bound

push-up hills or extensional basins at step-over or bends of the fault trace.

They can be associated with typical patterns such as tail-crack or horse-tail

structures at fault ends. Reverse faults have sinuous traces and they are

associated with half-cylindrical-shaped hills of the uplifted blocks due to drag

folds deforming ancient planar erosion surface in the hanging wall. Normal

faults are recognized by the following geomorphic characters: (1) they

generally have a widely arched trace, concave (mainly) or convex towards the

footwall, in contrast to the strike-slip faults whose trace is generally straighter;

(2) they bound tilted plateaus (tilted blocks); (3) as is also the case for the

strike-slip faults, they are not related to half-cylindrical-shaped hills

corresponding to recent drag folds, which accompany active reverse faulting.

Harikumar et al. (2000), through long wavelength magnetic anomaly

studies up to 17ºN, identified charnockitic rocks as the main source of

magnetic anomalies in the Southern Granulite Terrane and Banded Iron

Formation (BIF) in the Dharwar craton. Reddy et al. (1988) analysed the

aeromagnetic data up to 12ºN and showed the usefulness of aeromagnetic

data in deciphering the crustal structure. From the study of aeromagnetic data

from 12º to 17ºN, Anand and Rajaram (2002) showed that the difference in

magnetic signatures of the Eastern and Western Dharwar craton was related

to the difference in metamorphic grades. So, the aeromagnetic data acts as an

excellent tool to decipher litho contact and magnetic lineaments which in

agreement with morphological expressions were traced as neotectonic

lineaments. Magnetic break has provided the geometry and trace of NW-SE

lineaments than any other tools we have used in the present study.



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Fig. 2.20 Neotectonic Lineament Map

Thus, 99 lineaments were deduced from various anomalies and the

same were taken for all the subsequent studies by considering their

importance in control over natural resources and natural hazards.

Neotectonic map was produced by filtering out 53 lineaments (Fig.2.20)

which were significantly showing coincidences of atleast three or more



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anomalous characters observed in topography, geomorphology, drainage,

lithology, trendline, shear sense, lineaments and aeromagnetic data

(Table 2.1).

2.5 VALIDATION OF NEOTECTONIC MODEL

2.5.1 General

Neotectonic zonation mapping cannot be used to provide a firm

baseline data for earthquake mitigation unless it is properly validated. The

various anomalies studied have been corroborated with each other and a new

methodology was evolved to detect the Neotectonic zones. Neotectonic

model deduced from the study was validated with historical seismicity data,

alignment of springs and multi-depth resistivity data.

2.5.2 Historical Seismicity and Identification of Seismotectonic Lineaments

The historical seismicity data is again one of the best tools for

validating such neotectonic models. Hence, such historical seismicity data

published by Geological Survey of India (Anon 2000) was scanned and over

250 epicenters of more than 2.5 magnitudes were picked out and seismic data

published by Ramalingeswara Rao (1992) are compiled and a GIS data base

was generated for the study area as well as for the adjoining area (Fig.2.21).

These data were compared with neotectonic lineaments and the

lineaments which cutting the epicenters and or proximal to the lineaments

were taken as seismotectonic lineaments (Fig. 2.21). The earthquake

epicentres of the study area showed a dominant N-S, NE–SW and WNW–ESE

to NW–SE alignments confirming respectively active sinistral and dextral

faults.

Isoseismal lines were drawn by feeding the above epicentres data into

the “Surfer 8” for entire study area (Fig.2.22). Then the resultant data was

exported to ARCGIS environment, from such isoseismal lines, isoseismal

maxima axes were drawn along the crest of maximum values.



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Fig. 2.21 Seismotectonic Lineament Map

Again using “3D Analyst Module” of ArcGIS, 3D visualized GIS image

was generated on the isoseismal pattern and similar isoseismal maxima axes

were drawn along the elliptical isoseismal ridges.

Amongst these, the N–S to NNE–SSW oriented isoseismal maxima axes

were found to have great agreement with the fall in parallelism and proximity

to N–S / NNE–SSW trending regional Pleistocene faults inferred by

Ramasamy and Balaji (1995). Whereas the NE–SW oriented isoseismal

maximas fell in proximity and parallelism to the NE–SW Pleistocene sinistral

faults.



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Fig. 2.22 Iso-Seismal Map and Iso-Seismal Maxima

Thus, the historical seismicity data have shown excellent validation of

the Neotectonicmodel evolved in the present study and the various active

faults of N–S, NE–SW and NW–SE azimuthal frequencies (Table 2.1). Though

the maxima axes were restricted to northeastern side of the study window,

the minimum value recorded is 2.8 and hence no area can be left as

seismically safe.The recent tremor in parts of Salem-Namkkal (Mw2.9),

Krishnagiri (Mw3.12, Annexure-IA), Thalaivasal (Annexure-IB), Ambur



166

earthquake (Mw3.8, 07-06-2008, E 78° 47' 59.94", N 12° 48' 0", Annexure-IC),

largest instrumented Earthquake in Tamil Nadu & Puducherry, - Off the coast

of Puducherry, (Mw5.5, 26–09-2001, E80°13'30", N11°59'2.4", Annexure-ID&E)

have confirmed that the Neotectonic lineaments and their northern

extensions have a greater coincidences with lineaments NL19, NL46-NL47,

NL37and NL1 respectively.

2.5.3 Neotectonic Lineaments Vs Springs

Springs are points where ground water, recharged at higher elevations,

emerges at the surface. Depending on the nature of the recharge and of the

storage/transmission characteristics of the aquifer through which the water

has flowed, they may be permanent (perennial), seasonal or intermittent.

Springs are found at many elevations from high in mountains to beneath sea

level, the Vrulja of the Mediterranean being an example of the latter (Goudie,

2004).

Besides this springs are one of the best geomorphic indicators of strike

slip faulting. Springs were mapped from topographic sheets and the

neotectonic lineament map was overlaid and their alignment with the springs

provides the evidences of strike slip faults (Fig.2.23, Table 2.1).

2.5.4 Multi-depth Resistivity data

Many people have identified the lineaments based on geophysical

resistivity data. Having realizied the credentials of geophysical resistivity in

evaluating the subsurface geological and structural data, the geophysical

resistivity data were also analysed in the present study. Again, a new

attempt was made to visualize the multi-depth resistivity data three

dimensionally using GIS.

The geophysical resistivity data collected for 30m, 50m, 80m 100m and

150m depths from 1300 number of locations for Salem region were analyzed

by preparing isoresistivity contours of different depths using SURFER 8 and

exported to ARCGIS. From such 3D GIS images the rectilinear resistivity

lows and breaks were buffered and correlated with neotectonic lineaments.

http://asc-india.org/lib/20010925-pondicherry.htm

http://asc-india.org/lib/20010925-pondicherry.htm



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The same has revealed that the NE – SW, NW – SE and N – S trends

indicating probable tectonic weak zones in these directions.

Fig. 2.23 Neotectonic Lineaments Vs Springs

2.5.4.1 Resistivity at 30 Meter Depth

Isoresistivity contours were drawn in SURFER 8 and exported to

ARCGIS, then the 3D GIS image generated for 30m depth was interpreted in

the computer, (Fig.2.24) and the resistivity lows and breaks were compared

with neotectonic lineaments.



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Fig. 2.24 Neotectonic Lineaments Vs 30m depth Isoresistivity

The resistivity lows and breaks interpreted from the isoresistivity

contours and 3D GIS have mostly corroborated with neotectonic lineaments.

The lineaments L1, L8, L9, L10, L12, L17, L22, L27, L28, L32, L33, L36, L38,

L39, L40, L41, L42, L46, L53, L63, L65, L66, L67, L69, L70, L71, L72, L74, L75,

L78, L79, L82, L84, L87, L89, L97 and L99 were aligning with resistivity

valleys and breaks (Fig. 2.24a).


Similar isoresistivity contours drawn in SURFER 8 was exported to

ARCGIS and the 3D GIS image generated there from for the apparent

resistivity values at 50m depth are shown in Fig2.25b. Again the lows and

breaks were correlated with Neotectonic lineaments (Table 2.1).



169

The broad resistivity valleys were observed with NE-SW, NW-SE and

E W trend and these valleys represent the concentration of lineaments in these

directions.

Fig 2.25 Neotectonic Lineaments Vs 50m and 80m depth Isoresistivity



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The generated 3D image of 80m depth isoresistivity show higher

degree of correlation with the neotectonic lineaments and has good

agreement with that of 30m and 50m lows and breaks. An NW-SE trending

valley was observed along L74 and L75 lineaments with intermittent

resistivity hills. E-W trending resistivity valley was observed along L5 and

L81 which in turn coincides with Swetha nadi fault (Fig.2.25c). Similar valley

was observed along NE-SW trend and aligning with L47. Other lineament

matching with resistivity breaks were also observed and tabulated

(Table 2.1).


Similar resistivity breaks and lows were interpreted from 100m depth

isoresistivity and compared with Neotectonic lineaments and their alignment

with Neotectonic lineaments was much sharper than shallower depth

isoresistivity data. The valley with contour values of 50 Ohm was observed

which must be corresponding to zones of intersection of multi-oriented

lineaments (Fig.2.26e).

The N–S oriented anomalies were mostly concentrated in central

parts, NE–SW in south eastern part of the area and NW–SE in western parts

of area (Table 2.1).


150m depth resistivity isolines were prepared as said above and

correlated with the Neotectonic lineaments. Higher degree of coincidence

could be observed and the broad valleys in the central part of the study area

was aligning with L67 and L38 whereas the L 77 and L83 aligning with the

valley in the northern part of study area (Fig.2.26f). L12 was aligning with

NW-SE trending valley and L32 and L79 were aligning with NE-SW trending

valleys. Value as low as 100 ohms was observed in these valleys. The

lineaments which are characteristically aligning with the resistivity lows and

breaks were tabulated (Table 2.1).



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Fig. 2.26 Neotectonic Lineaments Vs 100m and 150m depth Isoresistivity



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2.5.5 Field Validation

Attur valley is a physiographic expression of a major deep seated

lineament, hence the name "Attur valley Lineament"; named after a small

town located in the eastern part of the lineament.

This is considered as important lineament as it forms a part of the

complex shear system which practically divides Tamil Nadu into northern

and southern blocks - an observation based on physiography. Srinivasan

(1974) considered the Attur valley as a rift zone between Shevaroys-Chitteri-

Kalrayan and Kolli - Pachchai hill massifs. Drury and Holt (1980) interpreted

Attur Valley lineament as a branch of their complete curvilinear shear system,

which incorporates Moyar-Bhavani, Palghat-Cauvery and several other

northerly trending lineaments.

Though the importance of Attur lineament is widely known in Tamil

Nadu Geology, no detailed work on how this lineament has affected the

earlier lithologies, structure and associated features is available. The earlier

analyses were based on LANDSAT imagery and aerial photo interpretations

without any ground control (Grady, 197l; Srinivasan, 1974; Katz, l978; and

Drury and Holt, 1980).

The area was manifesting excellent structures and exotic lithologies

and the field based study would provide valid evidences for the reactivation

of tectonic zones.

Five locations were selected so as to cover the Attur valley in part and

adjoining area. In these locations most of the earlier structures like fold, faults,

shears and other minor feature were modified by the later structures.

Reactivation involves the accommodation of geologically separable

displacement events (intervals >1 Ma) along pre-existing structures. The

definition of a significant period of quiescence is central to this

phenomenological definition and the duration of the interval chosen

represents the resolution limit of reactivation criteria found in most ancient

settings. In neotectonic environments, reactivation can be further defined as



173

the accommodation of displacements along structures that formed prior to the

onset of the current tectonic regime. This mechanistic definition cannot

always be applied to ancient settings due to the uncertainties in constraining

relative plate motion vectors. Four sets of criteria may be used to recognize

reactivation in the geological record:

Stratigraphic,

Structural,

Geochronological and

Neotectonic.

Some structural criteria may not be reliable if used in isolation to

identify reactivated structures. Much of the previously published evidence

cited to invoke structural inheritance is equivocal as it uses similarities in

trend, dip or three-dimensional shape of structures. Numerous fault and

shear zone processes can cause significant weakening both synchronously

with deformation and in the long term and may be invoked to explain

reactivation. The collage of fault-bounded blocks forming most continents

therefore carries a long-term architecture of inheritance which can explain

much of the observed complexity of continental deformation zones.

2.5.5.1 Tiruchengode

2.5.5.1.1 Thrust structure in Amphibolite near Tiruchengode

The study site lies between 110 36‟50‟‟ to 110 27‟30‟‟ latitudes and 770 58‟

20‟‟ to 780 00‟ 50‟‟ longitudes forms parts of Toposheet no 58 E/14 and E/15

published by government of India in 1972. The area is well connected by

Salem-Tiruchengode-Erode road and railways. The nearest railway station is

Sankagiri RS.

2.5.5.1.2 Kinematic Analysis

The area is mainly covered with amphibolite, granitic gneiss;

hornblende biotite gneiss and younger granites .Extensive field work were

made and traversed parallel to foliation and across the strike of the rocks. The



174

samples were collected for thin section study.The rocks were mapped and

structural details are plotted on the base map to produce structural map of the

area (Fig.2.27).

The amphibolites strike N 100 to 200 degrees and dip 150 to 300 degrees

towards east. The pitch of the lineation varies from 300 to 350 degrees towards

NE. but there is striking variation in the dip direction two km west of

Semmampalayam, where the amphibolite rocks lie at the vicinity of road.

There the rocks show N150W strike and dipping towards the NE with 300 to

400. The 100 symmetrical rose diagram of the strike trends N100 to 200. The

scatter plot of the foliation planes and 1% area contour are concentrating near

the centre of the primitive circle. This suggests very low dipping or sub-

horizontal nature of the amphibolite rocks. In well cuttings near Kallukadai

shows layered amphibolite and hornblende biotite gneiss with dip amount

much steeper than the surface outcrops.

The hornblende biotite gneiss shows concordant bedding nature with

amphibolite. The general trend of the hornblende biotite gneiss varies from

N100 W to N200E and the dip amount ranges from 650 to 800 E. The pitch of

the lineation ranges from 180 to 340 NE. The 100 symmetrical rose diagram of

strike shows N100W to N200E. The scatter plot and 1%area contour shows

bimodal symmetry and suggesting a broad open and northerly plunging fold

structure.

The granitic gneiss lies west of amphibloite and is having direct contact

with younger granites further west. The general trend of the granitic gneiss is

N 00 to N140 and the dip is 220 to 640 E. the pitch of the lineation is 120 to 45 0

NE and the plunge ranges from 90 to 410 NE. The 100 symmetrical rose

diagram indicates the strike ranges from 00 to 300 N. The scatter plot and 1%

area contour show great variation in clustering of foliation values. This

suggest the granitic gneiss is syntectonic in origin and lit par lit injected



175

granitic melt into amphibolite with shear texture of “δ” “Ώ” and “σ” type

clasts are noted and are showing dextral sense near Morepalayam

(Plate VI A-F).

Fig 2.27 Thrust Structure in Amphibolite near Morepalayam, Tiruchengode



176

The mylonite zone near Morepalayam village, Namakkal District is ~5

km length and ~ 50- 75 m width and narrows down in the south and it trends

NNE-SSW and dips nearly vertical. In field the sense of shear is identified by

very big feldspar porphyroclast, S-C fabric and asymmetric fold where as in

thin section it‟s not showing any kind of sense of shear, it suggests that after

shearing one more phase of metamorphism has happened and the micro

clasts are recrystallized and the mesostructures are remained as such.

Conjugate Kink bands are also observed in the same outcrops with the

orientations of 45º with the dip of 69º and 95 ºwith the dip of 68º. The acute

bisectrix of the conjugate orients 70º i.e. σ1 the primary stress field has the

above said orientation. This evidence suggests later to Sankagiri granite

emplacement there must be reactivation.

2.5.5.1.3 Geometric model of Amphibolite Thrust

The structural orientation of all the kinds of rocks mapped near

Tiruchengode superficially shows concordant strike orientation but the dip of

all three rocks varies. The amphibolites are dipping very gently towards east

possibly represent a thrust structure. This schematic block diagram is

constructed based on its structural orientation. (Fig.2.27)

The sheet of rocks might have formed by the recumbent folding of

amphibolite and hornblende biotite gneiss rocks due to the thrusting

emplacement of Sankagiri Granite from SW side of the terrain pushing the

rocks NE part of the area to ride over the igneous plutons and thus creating a

ramp thrust sheet.



177



178

2.5.5.2 Western Kanjamalai

2.5.5.2.1 Structure of the Area

The hill has a basin shaped structure, as already observed by King

and Foote (1864). The average dip of the rocks was about 55º but on the

northern side it may be anything from 500 upwards. The long axis of the

structure lies just north of the main ridge at the top of the hill. The dip was

eastward on its western side, while in the ridge marked by the north of the

Siddheswaran kovil, the dip varies from 75 º to 80 º towards the S and SE

direction.

An irregular depression is found on the east of Sidhar kovil, and this

depression is backed by the main mass of Kanjamalai in the easterly direction.

A low broken ridge forms the northern boundary of this depression while

“Chinnakanjamalai” (Small Kanjamalai) is its southern boundary rising to the

heights of 200 to 300 m forming the continuation of the main ridge of the

mountain. The slope on the northern side is somewhat gentle and has less

vegetation. Contrary to the northern slope, the southern slope of the hill is

clothed thickly with thorny bushes and shrubs. The slope is steeper and

scarred by ridges and valleys thus presenting a diversified morphological

aspect Kanjamalai, by virtue of its situation, stands out as gigantic hill on the

plains of the Attur valley. The bold relief of the flanks of the hill exhibits dark

bands which stand out as ribs.

The first detailed account on the structure and geology of Kanjamalai

was given by King and Foote (1864). They interpreted the structure of

Kanjamalai as a basin and dunite pockets with veins of magnesite regarded as

the later intrusive in the earlier gneisses. Some sort of dislocation is noticed

near Sidhar kovil adjacent to dunite.

From all the directions the rocks dip towards the mountain, the

amount of dip varies from 600 to 900. Rapid variation is noticed in the amount

of dip of the iron ore bands along the northern portion of Kanjamalai. There is

a small anticlinical structure exposed on the north-eastern foot hills of

Kanjamalai.



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The appearance of anticlinal structure at the border can be explained

owing to the intrusion of dunite and later granites. The force of intrusion

might have buckled the northern limb of the already folded syncline. The

plunging nature of the syncline is further supported by the outcrop pattern on

the eastern end of the foot hills. Here the outcrops on either side of the flanks

are not connected up around the terminal slopes, but they are separated by

the country rocks. This is an evidence for the easterly plunge of the fold axis.

In some cases, great difficulty is experienced in ascertaining the dip from

lineation direction on a more or less elliptical and rounded surface of the area.

Sidhar kovil is situated on the north-western foot hills. Here the hill shows

clearly that the rocks around the places are eroded away to give a notch like

look to the land. Southern cliffs of the hill show scarps of the prominent rock

bands. The iron one bands of these western peaks suddenly end in this cliff.

Above all, east of this area is depression, there is a dunite intrusion. This

dunite intrusion is in line with the chalk Hills (North East of Kanjamalai) and

this was observed from the top of the Shevaroy. Hence it is assumed that the

north-western part of the hill around Sidhar kovil was uplifted by the dunite

intrusion. In this process the plunging structure of Kanjamalai seems to be

disrupted but broken up owing to faulting. The absence of the iron band in

this area is easily explained as due to the erosion of the uplifted masses.

The evidence for the dunite intrusion was also very well observed in

Nagaramalai northeast of Kanjamalai. The dunite of the southern portion of

the chalk hills (near Nagaramalai) has been reported to cut across the country

rocks including the garnet pyroxene rock bands. The separation of, originally

adjacent points on the bands are now more than half a kilometer apart

separated by the dunitic intrusion.

The gneisses occur extensively in the plains and often carry varigated

lenticular patches of older rocks and often show interaction along their

contacts with older rocks which has given rise to migmatites. The formation

of these peninsular gneisses has brought about certain retrogressive changes

in all the older igneous members of Dharwars.



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Granites, pegmatites and aplites veins are very scarce and outcrop

predominantly along the disturbed one of the north-western portion of

Kanjamalai. The intrusion of these younger granites further induced

retrogressive changes in older Dharwarian rocks. Dolerites are very rare and

petrologically unimportant by the absence of its differentiated members. The

general structure of the study area in doubly plunging syncline fold with the

fold axes F1, F2 and F3. The associated rocks charactistically exhibits „shear

fold‟ structure (Plate-VIIE). Folds and faults often occur together and small

folds are often related to drag effects along the faults in this area, most of

these combinations occur in the area near Veerapandi, Sevampalayam and

near Perumampatti.

(i) The maximum stress axis was in the N-S direction and

intermediate, minimum stress axes are EW and vertical

downwards.

(ii) The later folding activity was directed in SE and SW direction of

maximum stress axis.

(iii) F3 fold axis was represented in the same direction as the F2 axis,

this type of folding activity is responsible for the high amplititude

and unharmonic folding commonly found in both mylonitised

hornblende biotite gneiss (Plate- VII A-F).

2.5.5.2.2 Fracture Patterns Related to Folds

Well defined fractures are often displayed by BIF. The fractures are

found on folds formed by a type of folding mechanism in which individual

layers in a sequence flex, and bedding plane slippage occurs as folding

proceeds to allow development of concentric folds.

The fracture direction occurs parallel to the axis along the strike and in

particularly well developed along the crest. „Sheeting‟ fractures also found in

BIF which is referred to as a form of large scale exfoliation.



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Among the ideas which have been put forward to explain the origin of

these fractures are,

a) Earth tides (these appear too weak at the present time to cause

fracture initially, but they might be effective in propagating

basement fractures upward into the cover and tides may have been

stronger in early Pre-Cambrian)

b) Oscillatory response to a non-oscillatory force such as earthquakes,

c) Crustal compression at depth.

d) Isostatic adjustment causing extension.

e) Expansion of the earth‟s interior causing extension in the crust.

2.5.5.2.3 Structural Pattern

The characters of many strike slip and normal faults are variable in the

areas, in and around Veerapandi, Sevampalayam and Perumampatti.

Establishment of strike-slip displacement on faulting is most conclusive when

linear features are cut and displayed by the fault. The movements have taken

place near Sevampalayam, Veerapandi and Perumampatti. Later

displacement in the BIF is noted by the physiographical effects, i.e., Stream

terrace deposits. The strike-slip faults are mainly seen in western part of the

foot hill of the Kanjamalai crossing through the dunite outcrop to

Siddeswaran kovil and opened a way in the ring of metagabbros (Plate-IIIF).

2.5.5.2.4 Macroscopic Fabrics

The fabric elements most commonly used to define macroscopic fabrics

are macroscopic S-Surfaces lineations and axes and axial planes of folds.

Microscopic structures, though less intensively studied by many geologists,

may also build arrays that are homogenous on a large scale, thus contributing

to the fabric of macroscopic domain.

Rarely homogeneity on the macroscopic scale is demonstrable by

simple inspection. By contrast, in many deformed bodies superposed



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structures are so complex that simple inspection of an outcrop or of a

structural map reveals an apparently chaotic structure. Yet even in such

bodies the existence of a macroscopic sub fabric, homogenous with respect to

one or more kinds of structure, can emerge from a well-planned geometric

analysis.

1. Data are collected from a large number of stations uniformly

distributed (within the limits imposed by the nature of the

exposure) on the topographic surface.

2. A number of small domains are chosen, each encompassing some

20 or 50 stations and having a uniform or simple outcrop pattern

on the map.

3. Strong preferred orientation in any diagram shows that the

domain is homogenous with respect to the corresponding

structure.

4. Comparison of the sub-fabrics of the various domains now reveals

which structures, if any, maintain a constant orientation

throughout the whole body.

5. At suitable locations N-S traverses were made to understand the

nature of repetition of beds.

2.5.5.2.5 Structure of the Western Kanjamalai

The various structural elements present in the area under investigation.

The following structural elements have been discussed.

(1) Planar Structures

(2) Folds and joints and

(3) Linear structures

Bedding planes are observed in various metasedimentary units.

Bedding plane was observed in Banded Magnetite Quartzite in the area. It

was identified on the basis of color banding and crenulated folded bandings.



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The bedding plane in BIF has been identified on the basis of color banding

both the sides of Kanjamalai area.

Gneissosity is observed mainly in metagabbros. Due to regional

metamorphism, some granitic rocks are converted into gneissose rocks and

gneissosity is also developed in metagabbros. The general strike trends of the

rocks fall in N800- N850 direction. Poles of different parts of the Kanjamalai

area are plotted in Schmidt‟s equal area net. It indicates that the foliation is of

the same generation in both the metagabbros and gneissose rocks. The

orientation of 154 reading of western side of the Kanjamalai area in plotted in

Schmidt‟s equal area net (Fig.2.28) and then density contour diagrams are

prepared using STEREO software (Fig. 2.28 insert).

It is inferred from these foliation plots indicate that the plunging

syncline structure exist in western Kanjamalai.

The rocks in the area under investigation exhibit megascopic fold of

both the small scale and those of regional scale which have been inferred after

interpretation of structural data. Many of the „Ptigmatic folds‟ are present in

the Banded Magnetite Quartzite rocks. Western Kanjamalai shows clear

evidence of folding. The fold axis is very clear on NNE-SSW direction

(Fig.2.28).

The minor folds are very common in BIF and also in garnetiferous

metagabbros and mylonitised Hb-biotite gneiss (Plate-VII A-F). Rootless

fold and inter-folial fold are other characteristic features present as an

evidence of multi phase deformations (Plate-VIID).

Joints are observed in quartzites, garnet bearing meta gabbro rocks and

gneisses. Mylonites which occur west of Kanjamalai, has closely spaced

horizontal joints with joint spacing ranging from few cm to 20 cm or so, and

the contact is very clearly seen in shear zones.

Mineral lineation is commonly observed in gneissose rocks which

occur near the Sidhar kovil. The mineral lineation is defined by the preferred



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orientation of elongated felsic grains. It generally reads 400-500 SE near

Siddheswaran kovil and 450-550 NE. and the plunge of the lineation is 500-600

in average towards SE. Another fault trending 3500 N is observed along which

50m (approx.) down throw in the garnetiferous meta- gabbro is noticed.



185

Fig. 2.28 Structure of Western Kanjamalai, Salem

Detailed field study revealed the Kanjamalai was bounded by

mylonitised gneisses on either side at the basement for a width of nearly 2 km

in the southern side and 1-1.5 km in the northern side with near E-W trend.

F1, F2 and F3 foldings are prevalent in the mylonitised gneisses on the

southernside of Kanjamali near Chinnasirangapadi. The F2 fold axis trend is

55º-65º dipping 65º-70º NW and with a rake of N45ºE and with the plunge of



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80º. The F1 folds form rootless folds or boudinaged folds and the axial planes

of which are aligning parallel to the S2 foliations. F2 folds are open folds and

the S2 foliations were developed parallel to the F2 folding. These second

generation folds seem to be corresponding to the structural fabric of the

region. F3 folding are having conjugate fold axes and develops box folds with

the trend of 358º and 82º SW and another axis of the trend 40º and 84º NE. At

places F3 folds forms explosion folds (high amplitude folds) in mafic

components and lesser stretching in felsic components and thus the

differential stretching produces cavities in the hinges and filled with Gash

veins of quartz. The axis orients 92º with 72º northern dip and with the rake of

72º and plunge of 84º for the lineation (Plate-VII A-F).

Shear bands orients 100º and dip of 75º and with the rake of lineation

measures 85º were observed near the GPS location E 78º 02‟ 40.3” and N 11º

34‟ 45.6”. Micro faults with sinistral sense were observed near E 78º 2‟ 59.4”

and N 11º 36‟ 12.2” with the trend of 345º and dip of 85º west. Pyrite

mineralization was observed along these micro faults.

2.5.5.3 Udayappatti

The traverse was made along the stream in Namam hill around

Udayappatti, Kandasamam temple and Masinyakanpatti in NE – SW

direction at different locations, which is ~ 7-8 km away from Salem town. The

traverse was taken along nala section near temple (N 11°39‟38.6‟‟, E 78°12‟

37.3‟‟). This location contains highly weathered & sheared mylonites rocks

that are in contact with chlorite schist along the eastern flank of ridge. The

chlorite schists are greenish in nature & schistosity is not well developed. The

mylonitic foliation shows trend of NE-SW direction and dips 40°-80° towards

NW direction. Some lineation has been observed that shows plunge towards

NE direction in varying amounts 40°-60°.In another location of Namam hill

(N 11°39‟36.1‟‟, E 78°12‟39.2‟‟) and also along the nala section mylonites are

exposed, which are highly weathered & sheared. Foliation of NE-SW & dips

toward south direction & lineation 35°-80° toward SE direction. In another



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location on the same hill (N 11°39‟21.3‟‟, E 78°12‟37.1‟‟) charnockite shows

gneissic foliation and close contact with mylonites. Near Kandasramam

temple (N 11°38‟56.5‟‟, E 78°12‟38.1‟‟) gneissic banding is well developed at

the foothills and rocks are highly Mylonitised.

Fig. 2.29 Study on Mylonites of Udayarppatti (Biswal et al., 2010)



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Charnockites is intruded by granitic as well as pegmatite veins. The

granitic & quartz veins are more or less parallel to the foliation plane.

Pegmatite veins showing cross cut relationship to charnockites. Foliation

plane have a NW-SE trend & dipping towards SW in varying amount 10°-50°.

Some lineations show 15°-30° plunge toward SE direction. Within the

mylonites small patches of charnockites are present which contain dark

coloured basic enclaves. Next traverse was taken along Namam hill (N

11°38‟37.3‟‟, E 78°12‟7.7‟‟) towards SW direction from Kandasramam temple.

The road cutting shows mylonite rock having contact with charnockite in the

foothills. At this location rocks are intruded by quartz veins trending NE-SW.

In last location SW direction from Kandasramam temple (N 11°38‟18.4‟‟, E

78°11‟53.7‟‟) along the hill pseudotachylyte veinlets has been observed within

the charnockite. This pseudotachylyte veinlet shows sharp boundaries,

having length less than 1cm -to 1 m. The host rock shows trend NE-SW

dipping toward NW direction & intruded by quartz veins (Fig.2.29).

2.5.5.4 Sarkarnattar Mangalam

The Sarkarnattar Mangalam (N 11°41.349‟& E 78°17.756‟) is ~ 24 km

away from Salem town in NE direction .The traverse was taken along the

eastern end of Godumalai hill called Sarkarnattar Mangalam & Chinna

Agraharam. It is also shows weathered & sheared mylonite. In Sarkarnattar

Mangalam (N 11°41‟10.2‟‟& E 78°17‟17.8‟‟) along the foothill mylonite shows

NE-SW & EW trend, dipping toward NW to North direction in varying

amount of 20°-80°.The mylonite shows well developed stretching lineation

which is down dipping. Along the foothill, lineation measured shows amount

20°-65° toward NE-SE direction. At another location (N11°41‟13.9 &

E78°17‟27.4‟‟) the right side of road, an open pit quarry shows good outcrop.

The foliation plane shows E-W trend & dips towards North in almost

30° -40° dip amount. One recumbent, tight fold observed shows plunge of 20°

towards SE & axial plane orientation of 130°. In the same outcrop, Z-shaped



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fold observed in charnockite. This outcrop is rich in garnet. In this outcrop

lineations are well developed, shows nearly 20° -30° plunge in NW direction.

Fig. 2.30 Study on Mylonites of S.Nattarmangalam (Biswal et al., 2010)

Mylonites are highly weathered, sheared and intruded by pegmatite &

quartz veins, one has trend nearly E-W & another N-S direction. The mylonite



190

contains porphyroclast of quartz & feldspar with quartz ribbons. Another

traverse was taken from Agraharam (N 11°40‟56.5‟‟& E 78°17‟25.9‟‟) towards

the south direction through Karumapuram, Minnampalli, Karipatti

(N 11°39‟55.4‟‟& E 78°17‟3.3‟‟) & Chinna Kavundapuram (N 11°39.423‟&

E 78°16.042‟) places across the strike of shear zone. The mylonitised zone

extends from Sarkarnattar Mangalam to Karipatti area and has E-W trending

foliation plane dips 40° -80° towards north. Again at this location the lineation

shows 50° -60° plunge toward NW direction. In Chinna Kavundapuram area

only charnockite exposed shows trends NE-SW dips 30° -75° towards NW

direction. Some exposer shows SE dipping. A recumbent fold shows 20°

plunge towards 105° direction. In this outcrop, small pyroxene porphyroclast

occur within the Charnockite 34 shows right lateral shearing. The S-C angle

measured 32° towards SW direction. One set of joint are observed, shows

trend 35° - 40° dipping 70°-72° towards SE (Fig.2.30).

2.5.5.5 Attur (Gangavalli Shear Zone)

Field work was carried in the Attur area ~60 km away from Salem

town. Traverse were taken following locations Kattukkottai (N 11°36.410‟& E

78°40.414‟), Chenni Malai (N 11°34.858‟& E 78°39.955‟), Odiyattur L-14 (N

11°33‟.58.9‟‟& E 78°39‟19.6‟‟) & Kannndiyan Malai (N 11°32‟55.4‟‟& E

78°38‟33‟‟). All These locations have pseudotachylytes which shows trend

East to SW direction. In Kattukkottai village, highly jointed & sheared

pseudotachylytes has exposed in a river section, which are highly jointed &

sheared. The pseudotachylyte is dark coloured, highly fractured and jointed.

Foliation plane shows NW-SE trend, dipping 40°-45° toward NE direction and

joint pattern trends NE-SW dipping towards NW. The Vellar river section

shows pink granite intrusions. The North Chenni Malai hill range shows full

length of pseudotachylyte as well as cataclastic brecciated rock which is

sheared and fractured shows quartz and feldspars porphyroclast. In

Chennimalai near the Government College, exposer of pseudotachylyte vein

shows trend of NE –SW dips 70°-80° toward NW. It has an S-C angle of 50°

towards SE. The outcrop shows two prominent joint directions, one in NW &

another in NE making acute angle. Some veins are detached due to left lateral



191

shearing. The joints are trending E-W & NW-SE direction. At certain location

(N 11°34‟19.8‟‟ E 78°39‟24.7‟‟) brecciated rocks are exposed (Fig.2.31).

Fig. 2.31 Study on Mylonites of Gangavalli (Biswal et al., 2010)

In Odiyattur, well rounded porphyroclast are observed within the

pseudotachylyte that has contact with charnockites. The porphyroclast is big

in size, ranging < 1mm to > 3cm .The foliation plane shows NE-SW direction ,

dips 70°-72° toward SE direction. At location-15 Odiyattur (N 11°33‟3.5‟‟E

78°38‟41.4‟‟) highly foliated magnetite bearing charnockite has been observed



192

shows NW-SE foliation & dips 30°-70° towards NE direction. At the last

location-17 (N 11°32‟55.3‟‟E 78°38‟33‟‟), several small veins of pseudotachylyte

has been found within the charnockites. Few vein detached shows right

lateral shearing in small scale. Along with two sets of joint observed, one has

NE-SW trend dips vertically and another has NW-SE trend with SW dips. The

pseudotachylyte veins show prominent NE-SW trend within the charnockite.

2.5.5.6 Small Scale Structures

The charnockites are marked by penetrative ENE-WSW gneissic fabric

(S1) which is axial planar to a set of isoclinal recumbent folds (F1) that have

folded the primary foliation (layering). Another set of recumbent folds,

though more open than the former, have folded the gneissic foliation within

the shear zone and shear fractures (C -fabric) are developed parallel to the

axial plane of such recumbent folds along ENE – WSW directions . These

folds are identified as SF1 folds and explained to be developed on the gneissic

bandings of the charnockites during shearing. The C-fabric in the shear zone

is developed parallel to such shear fractures and remains absolutely

horizontal at several sections as in Udayarppatti. The gneissic fabric (S1) and

the C-fabric are involved in open to tight upright F2 folding along E-W axial

plane. A crude axial planar fabric (S2) is associated with F2 folds along E-W

direction. The mylonitic foliation is marked by lineations in the form of

grooves and ribs which are more akin to ductile slickenside striae (Lin et al.

2007) than stretching lineation. However, the trend of the ductile slickenside

striae and the stretching lineation is parallel in the study area. Hence, the term

mylonitic lineation is used hereafter to describe these linear features. The

mylonitic lineations have a low plunge towards NE or are subhorizontal in

that direction on subhorizontal C-surfaces or parting planes. However, where

the linear features are folded by F2 folds, the lineations plunge down dip on

F2 limbs in these situations, the F2 fold axis and intersection lineation

produced from the intersection between S2 and S1, and S2 and C fabric are

subhorizontal. Thus two types of lineation namely mylonitic lineation and

intersection lineation, coexists at several places. F2 folds show plunge reversal



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and at places they are vertical. In such localities, the intersection lineation and

fold axis become vertical while mylonitic lineation remains horizontal.

Therefore it is quite difficult to distinguish these two sets of lineations purely

based on orientation. At several localities, pseudotachylites occur as

millimeter to centimeter scale dark coloured bands or veins along fractures in

the charnockite. At places they occur as angular patches and are cut by close

spaced fractures. In hand specimen and at the outcrop scale angular clast

fragments of charnockitic composition set in a dark colored matrix could be

identified within the pseudotachylyte. Under the microscope microlites are

identified (Plate-VIIIC). From the bulk chemistry determined by XRF

(Thirukumaran et al., 2010) the pseudotachylites are inferred to have been

formed as a result of nearly complete melting of the former quartz, feldspar,

pyroxene and mica dominated rock i.e., from the charnockite.

2.5.5.7 Micro fabric study of Mylonite

The microfabric analysis of the mylonites was carried out on thin

sections oriented parallel to the mylonitic lineation and perpendicular to the

foliation, which is referred as the “Vorticity Profile Plane” (Passchier and

Coelho, 2006). The clasts are dominantly alkali feldspars, which have

undergone both plastic and plastic–cataclastic deformation.

Hence, many feldspar porphyroclasts are observed along the mylonitic

foliation (Plate-VIIIA). The porphyroclasts show rotation indicative of dextral

top-to-the-NE sense of shear. Microfaults inside the feldspar porphyroclast, as

they are at high angle to C planes, show sinistral shearing antithetic to main

shearing. In phyllonites, hornblende fish are observed showing distinct tails

that also suggests dextral top-to-the-NE sense of shear. In addition to these,

S-C fabric is observed in phyllonite where the C-fabric is defined by shear

bands marked by growth of mica and polygonized thin quartz ribbons (Plate-

VIIIB) while the S-fabric is defined by an oblique growth of quartz and biotite

to C-fabric. The angularity between S- and C- suggests a thrust slip shearing.

This is further substantiated by asymmetric folds developed in the quartz

ribbons where the S-fabric remains axial planar to the folds.



194



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2.5.5.8 Large scale structures

`The Salem-Attur shear zone has been mapped in detail in two areas,

namely Udayappatti and S. Nattarmangalam. The Udayappatti area shows

the charnockitic rocks forming very high hills. Along the hill slopes the

mylonitic rocks are exposed at several places, more distinctly at one locality

where they occur as a low dipping zone. The variations in orientation are due

to F2 folding of the foliation. This is clearly seen in a stereogram of poles to

foliation. The plot shows a girdle distribution with E-W axial plane and

westerly plunging axis (fold axis). The lineations detail collected from the

shear zone as well as the country rock and plotted in the show diverse

orientation; this is partly due to F2 folding and partly to the association of

intersection with mylonites. At Udayappatti, the shear zone appears to be

bifurcating; in fact this is due to gently dipping shear zone intersecting steep

topography. The area around S. Nattarmangalam, farther east of Udyappatti,

is dominated by charnockite. The Salem- Attur shear zone passes within the

charnockite and trends in an E-W direction. Due to extremely weathered

character, the shear zone occupies low lying topography.

A stereogram of foliation poles (Biswal et al., 2010.) shows the E-W

stike distributed on a girdle. This is because of F2 folding which has an axial

plane striking E-W and â-axis plunging to WSW. The lineations plunge due E

or W. This suggests that in this area, the lineations are more uniform than in

the Udayappatti area. Farther towards the east, near Chenni Malai Hill, the

Gangavalli shear zone is exposed. It is a NNE-SSW trending brittle shear zone

where the charnockite is extremely fractured and shows various sets of shear

fractures. Rose diagram reveals that the approximately E-W and N-S oriented

fractures dominate (Fig.2.32, Biswal et al., 2010). Pseudotachylite veins and

bands are emplaced along these shear fractures and in turn they are also

sheared. The pseudotachylites are derived from the melting of the

charnockitic rock and contain microlites (Plate-VIIIC). It is inferred that the

Gangavalli shear zone developed subsequent to the upliftment of the

charnockites to the upper part of the crust.



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2.5.5.9 Tectonic Implications

Considering all these observations, an attempt has been made to

interpret the kinematics of the Salem-Attur shear zone. Where the mylonitic

foliation is subhorizontal, it shows northeasterly oriented lineations. This

implies that the Salem- Attur shear zone is a northeasterly verging

subhorizontal thrust (Biswal et al., 2010). The overprinting of the gneissic

foliation by a mylonitic fabric suggests that the thrusting event postdates the

granulite facies metamorphism, so that the charnockites were uplifted due to

thrusting.

The thrust zone is fairly wide with splays that branch and rejoin,

encompassing lenses of low strain charnockitic blocks within the mylonites.

Since the thrusting developed on a well banded charnockite, shear folds (SF1)

were developed on the bandings due to buckling instability followed by

shearing, and subhorizontal shear fractures were developed parallel to the

axial plane of the folds. Subsequently, the thrust plane was folded by E –W

trending upright fold (F2) which has resulted in variation in attitude of the

thrust plane and mylonitic foliation. Thus the mylonitic foliations show dip

variation from north to south. The mylonitic lineations are also folded to

show down dip plunge where the mylonitic foliation is steep.

Superimposition of F2 fabric on mylonitic zones has complicated the

interpretation of shear sense indicators. This is reflected in the earlier work

where various models have been suggested for the Salem-Attur shear zone.

2.6 SYNTHESIS

Thus, the Neotectonic lineaments were derived from various anomalies

like Geomorphology, Topography, Drainage, Aeromagnetic, Structural trend,

Lineaments and Siginificant lithologies and unique tectonic model has been

developed for the Attur valley with the E-W thrusting towards north, N-S

open fractures NE-SW and NW- SE wrench faults.

Validation was done with geophysical multi-depth isoresistivity data,

alignment of springs and with historic seismicities data and thereby Seismo-



197

tectonic lineaments were identified. 40 out of 53 Neotectonic lineaments are

significantly seismogenic in characters and they align with the epicenter of

recent earthquakes. Field study was made at selective windows and evidences

of reactivation in terms reversal of shear sense and microfaulting of sheared

rocks and related changes were observed. The table 2.1 displays the

characteristic anomalies on which the lineaments were designated as

Neotectonic and their validation tool and depth of Lineaments.

A) B)

Total 99 azimuthal values Total 53 azimuthal values

Largest Petal 8 values Largest Petal 5 values

Largest Petal 7% of all values Largest Petal 8% of all values

C) D)

Total 40 azimuthal values

Largest Petal 4 values

Largest Petal 10% of all values

Fig. 2.32 Rose diagram- Azimuth of Significant, Neotectonic and Seismoectonic Lineaments and Structures Associated with Transpression

Rose diagram of the azimuth of the significant lineaments numbering

99 show the dominant E-W trend and NW–SE (Fig. 2.32A) followed by N-S

and NE-SW lineaments. Out of 53 Neotectonic lineaments mapped, as



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maximum as 5 are aligning along N-S direction (Fig. 2.32B). E-W trending

and NW-SE neotectonic lineaments were also dominant in Attur valley. Rose

diagram (Fig. 2.32C) clearly indicates E-W and NW-SE trending lineaments

are dominantly seismogenic. This clearly drew certain conclusions that,

N-S trending lineaments are acting as stress releaser and they

may be the tear faults resulting from the thrust related to

arching of this part of region. Evidences of decompression

structures (Plate-VIIIF) and Eclogite –Gabbro contact rocks

along Palghat –Cauvery Shear Zone for about 70km arcuate

stretch, suggests thrusting and exhumation of deep seated

rocks.

The E- W trend seems very significant and this region possess

some associated structures like folds, thrust faults tensional

frctures, pull-apart basin or rhomb shaped grabens (Sag ponds,

Fig. 2.11CM) and riedal shears and hence they may be

attributed to transpression related to crustal shortening (Fig.

2.32 D) and the dominance of E-W trending seismogenic and

neotectonic lineaments confirms the same.



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Table 2-1 Characteristic Anomalies and Orientations of Significant and Neotectonic Lineaments.

S.N

o

Lin

ea

me

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Characteristic Anomalies

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Lin

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Se

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Sp

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Depth Resistivity

Lin

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Le

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m

Az

imu

th

30

m

50

m

80

m

10

0m

15

0 m

1 L1 ED,LD,FV,UM,

CL NL1 SL

121.12 88

2 L2 AM,LD,FV,E NL2 SL 71.71 175

3 L3 LD,AD,T, FV NL3 SL 188.65 66

4 L4 LD, AR 66.97 157

5 L5 LD,AM 145.08 266

6 L6 LD,AM ,E,

AF,GR NL4

DL

243.88

56

7 L7 CM,LD,T,FV, AF, CAR,PL

NL5 SL 127.62

30

8 L8 PD,AM , E,

MY,CL NL6 SL

DL

191.29

93

9 L9 PD,AM ,E, MY,

M,CG,CL NL7 SL

DL

98.30

86

10 L10 LD,AM , FV, CL NL8 SL 66.66 131

11 L11 LD,AM ,FV,CL NL9 SL 78.62 133

12 L12 LD,AM, T,AF,PL

NL10 SL DL

275.41

143

13 L13 E,FV,PL NL11 SL DL

98.03

46

14 L14 LD,AR 172.81 50

15 L15 LD,AM ,FV NL12 DL

93.80

123

16 L16 LD,ED,AD,AM,

FV NL13

DL

109.39

125

17 L17 AM ,AF,PL NL14 58.07 140

18 L18 LD,AR 53.85 84

19 L19 AM,AR 20.26 84

20 L20 FV,AR 13.69 183

21 L21 LD,AR 9.50 181

22 L22 CM, PL 60.37 179

23 L23 LD,CM,AM, E,AF,FV,CL

NL15 SL 86.24

266

24 L24 AM ,PL 0 41.12 129

25 L25 LD,E,PL 0 131.73 128

26 L26 CM,AM ,CL NL16 SL DL

86.32

72



200

27 L27 LD,E,FV,BL NL17 SL DL

164.40

179

28 L28 AD,LD,BD,AM,

FV,P,BL NL18 SL

DL

120.16

179

29 L29 PL,AR 75.50 49

30 L30 LD,AR DL

73.11

70

31 L31 LD,AR 44.52 119

32 L32 CM,LD,AF,MY,

EG,PL NL19

142.76 29

33 L33 LD,T,FV,CL NL20 SL DL

171.19

182

34 L34 PL,AR 76.19 49

35 L35 PL,AR 120.56 43

36 L36 LD,FV,SY,CL NL21 SL 166.77 180

37 L37 LD,FV,GG,PL NL22 159.79 41

38 L38 ED,LD,AM ,E, P,

UM NL23 SL

DL

118.53

99

39 L39 LD,AM ,FV,CL NL24 SL 156.23 176

40 L40 LD,AM ,E, T, UM,CAR,CL

NL25 SL DL

127.89

91

41 L41 LD,T,AF,FV, NL26 SL 119.40 194

42 L42 LD,FV,SY,UM,

CL NL27

107.37 186

43 L43 LD,T,

UM,GG,PL NL28 SL

DL

141.53

41

44 L44 CM,LD,AD, AM, FV,UM

NL29 SL 143.90

62

45 L45 CM,FV,MY,PL NL30 SL 114.90 27

46 L46 LD,AM,

Sy,CG,GR NL31

DL

103.76

233

47 L47 T,PL 96.90 43

48 L48 LD,CM 68.00 84

49 L49 AM ,AR 50.83 128

50 L50 PL,AR,AM NL32 SL 47.75 126

51 L51 PL,AR 109.60 127

52 L52 FV,PL 41.44 128

53 L53 E, LD,AM ,CL NL33 SL 84.35 135

54 L54 E,FV,P DL

62.18

208

55 L55 T,PL DL

58.49

50

56 L56 LD,AR DL

63.98

20

57 L57 PD,LD,P 60.62 90

58 L58 P, BL DL

77.97

187

59 L59 LD,ED 153.99 200



201

60 L60 LD,AF 148.41 8

61 L61 E, LD,T, PL NL34 51.51 44

62 L62 ED,LD 60.94 159

63 L63 CM,AM DL

81.97

144

64 L64 LD,AR 42.36 141

65 L65 LD,AM 209.03 29

66 L66 CM,E 66.04 80

67 L67 LD,MY, AM NL35 SL 94.32 75

68 L68 ED,LD 79.94 133

69 L69 PD,LD,T,PL NL36 SL DL

207.17

209

70 L70 LD,CM,PD,AM

,FV, E,EG,PL NL37 SL

DL

191.16

210

71 L71 LD,AM ,T,FV,

PL NL38 SL

169.17 182

72 L72 FV,BL DL

66.72

5

73 L73 LD,AM ,FV NL39 151.35 60

74 L74 LD,AM ,PL NL40 SL 75.56 138

75 L75 LD,AM,

AF,UM,PL NL41 SL

116.37 139

76 L76 ED,LD,AM ,E NL42 DL

111.54

89

77 L77 LD,CM NL43 SL 76.16 78

78 L78 LD,AD,AM

,FV,CL NL44 SL

100.39 82

79 L79 ED,LD,FV, CAR NL45 SL DL

127.38

244

80 L80 CM,FV,BL NL46 SL DL

86.24

180

81 L81 LD,BD,AM ,T, 108.73 273

82 L82 LD,FV,PL NL47 66.55 181

83 L83 E, AF,CL 88.71 85

84 L84 PD,LD,AM ,T,

E,FV,EG,PL NL48 SL

137.06 213

85 L85 CM,AM ,CL NL49 SL DL

151.05

69

86 L86 LD,MY,AM,SS NL50 SL DL

56.30

16

87 L87 LD,PD,E,PL NL51 105.57 212

88 L88 LD,AM 67.12 125

89 L89 E, UM 95.35 32

90 L90 ED,BL 43.27 168

91 L91 LD,FV 137.49 178

92 L92 LD,T 38.49 197

93 L93 CM,AM 92.48 89

94 L94 E,PL 58.31 147

95 L95 FV,E, 64.48 54



202

96 L96 LD,E, 45.63 49

97 L97 LD,AM 41.19 238

98 L98 LD,AM NL52 SL 127.53 71

99 L99 LD,CM,FV,EG,

PL NL53 SL

182.33 210

Read as: AM-Aeromagnetic; E- Escarpment; LD-Lineament Controlled/Deflected Drainage; PD-Parallel Drainage; ED-Eyed Drainage; CM-Compressed Meander; PC-Palaeochannel; AD-Annular Drainage; T-Triangular facets; FV-Fracture Valley; AF-Alluvial Fan; UM-Ultramafics; GR-Granite; CAR-Carbonatite; My-Mylonite: P-Pseudotachylyte; CG-Carbonate gneiss; EG-Epidote-Hornblende Gneiss; M-Migmatite; GG- Garnet Gabbro; Sy-Syenite; NL-Neotectonic lineaments; PL- Parallel Lineaments; CL-Curvilinear Lineaments; BL- Branch off Lineaments; AR- Anomalous Relief; DL –Discharge Lineament; SS-Sense of Shear/Structure.

Documents

Neotectonics 2014