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A published paper on the tectonic evolution of the Tripura-Mizoram fold belt
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Quart. Jour. Geol. Min. Met. Soc. India,
Vol. 55, No. 4, p. 186-194, 1983
TECTONIC EVOLUTION OF TRIPURA-MIZORAM FOLD BELT,
SURMA BASIN, NORTHEAST INDIA
D. R. NANDY
1, SUJIT DASGUPTA
1, KALYAN SARKAR
1 AND ANIRUDDHA GANGULY
2
1. Geological Survey of India, Calcutta-700016
2. Department of Geo1ogical Sciences, Jadavpur University, Calcutta-700032
Abstract
The Tripura-Mizoram area, a part of Neogene Surma basin, comprises a belt of elongated folds
with marked sub-meridian trend and arcuate shape with westward convexity. This fold belt, parallel and
subjacent to the Arakan Yoma subduction/ suture zone, is spectacular in the Indian sub-continent and has
evolved within a compressive stress field generated by the eastward drift of the Indian plate during late
Tertiary. The style of folding and outcrop pattern in the region, show that there was minor rotation of the
stress axes which in turn corroborates rotation of the Indian plate during late Tertiary while it drifted
eastward. Geological mapping aided by photogeological and remote sensing studies reveals that the
deformation of the sedimentary units was first initiated by tangential E-W compressive stress resulting in
shortening, principally by folding and strike faulting and adjustment along conjugate shear fractures with
strike slip movements. The present study also reveals that the hypothesis of vertical tectonics as
postulated by earlier workers for the tectonic evolution of the region is not tenable.
Introduction
The area forms the major part of the
Neogene Surma basin to the west of the
Arakan Yoma subduction-collision zone,
which represents the northward extension of
the Sumatra-Java trench—the eastern
margin of the Indian plate. The basinal
sediments are folded into long arcuate belt
in a series of linear narrow anticlines and
synclines forming a unique foreland fold
belt in the Indian sub-continent like the Jura
mountain and the “valley and ridge
province” of the Appalachian.
Though the area is very significant in
respect of regional tectonic evolution and its
set-up in relation to the development of the
eastern margin of the Indian plate, no
comprehensive work has so far been carried
out to unravel the evolutionary process and
history of this fold belt.
As the area is highly inaccessible it
remained terra incognita till recently. In
order to gather a comprehensive idea of the
geology and structure of the area as a whole,
188 D. R. NANDY, SUJIT DASGUPTA, KALYAN SARKAR AND ANIRUDDHA GANGULY
photogeological map of the States of Tripura
and Mizoram, India, was prepared aided by
the knowledge of field mapping and check
traverses (Banerjee et al, 1979; Das Gupta et
al, 1979 ; Nandy et al, 1972 and 1973)
which is generalised and reproduced in
figure 1. Regional structural elements have
been studied in LANDSAT imagery mosaic
(Fig. 2). With the knowledge of regional
tectonic setting of the Surma Basin (Nandy,
1980, 1981 and 1982) and with the results
obtained from the photogeological and
remote sensing studies an attempt has been
made here to work out and synthesise the
geodynamics of this fold belt.
Geological Setting
The Neogene Surma basin of which the
present area is a part, is limited by (a) the
post Barail unconformity (close of Oli-
gocene), subsequently faulted, to the east,
(b) the E-W Dauki fault and the Disang
Thrust to the north and (c) the Sylhet fault
(Das Gupta, et a!., 1982) and the Barisal-
Chandpur high (Sengupta, 1966), concealed
below the alluvium of Bangladesh, to the
west and northwest. The folded sediments of
the Surma Group continue to the south up to
Ramri Island of Burma. Within this vast
terrain the arcuate fold belt continues for
550 km along strike and about 200 km
across at its widest part.
Syndeformation deposition of the
subflysch-molasse sediments commenced at
the close of Oligocene over the basin-floor
formed of the Barails or coeval rocks.
Stratigraphy
The entire sedimentary column of the
area is constituted of sandstone, siltstone,
shale, mudstone, sand rock, silt and rare
pockets of shell-limestone, which is divided
into four major stratigraphic units based
mostly on lithologic characteristics.
Sequentially they are, (1) Barail (Oligocene;
3000 m) sandstone and shale, (2) Surma
Group (Miocene) consisting mainly of (a)
arenaceous Lower Bhuban Formation (+
9000m, (b) argillaceous Middle Bhuban
Formation (3000m), (c) arenaceous Upper
Bhuban Formation (1100 m) and (d)
argillaceous Bokabili Formation (1000 m),
overlain by (3) Tipam Group (Pliocene; +
1300 m) consisting. of feldspathic sand with
fossil wood and minor silt. The youngest (4)
Dupi Tila Formation (not shown in figure 1)
consisting of mottled clay, fine silt and
laterite occupies the synclinal valleys to the
west and overlies the Tipams over a
pronounced unconformity. Older and older
rocks crop out toward east across the strike.
Sediments are characterised by various
types of primary sedimentary structures
indicating shallow marine to deltaic
environment (Sarkar et al., 1977). Scanty
faunal and floral record indicates that the
sediments were deposited in shallow paralic
environment (Ganguly, 1975). Details of the
faunal assemblages of the area are given by
Das Gupta (1982) suggesting Mio-Pliocene
age of these sediments.
Structure
There are 15 major long and arcuate
anticlines and corresponding synclines in the
TECTONIC EVOLUTION OF TRIPURA-MIZORAM FOLD BELT 191
area under consideration, trending NNW-
SSE to N-S to NNE-SSW from south to
north with convexity towards west (Figs. 1
and 2). Many of the anticlines bifurcate to
form two anticlinal ridges with sub-parallel
axial trends. In some cases such split axes
merge again to enclose an elliptical synclinal
valley (a common feature in Chittagong
hills) giving rise to flattened lensoid
structural domes and basins. These
structures might have developed as a result
of the compression axis changing orientation
where ‘a’ kinematic axis of the later evolved
folds was very close to the axial plane and
‘a’ kinematic axis of the earlier formed
folds, viz., type 1 of Ramsay (1962).
However, the point that should be borne in
mind in this context is that in a single
exposure or hand specimen of a fold no
interference pattern could be detected and
that the above comments are based solely on
interpretation of structures and linears of
mesoscopic scale. There are more than one
axial culminations and depressions in each
long structure with regional plunge varying
from 5° to 25°. Width of the synclines
gradually diminishes from west to east
where the folds become appressed. In the
westcentral part of the area the major folds
show varying geometry The Baramura,
Langtarai, Machmara and Hachecktlang
anticlines are flat crested and box shaped,
whereas the other anticlines (Fig. 1) in the
neighbourhood are sharp crested. Though all
the synclinal fold forms in the western and
central part of the area are broad and wide,
the Damchhera syncline east of Bansul
anticline is appressed and steeply plunging
(Fig. 1). The fold belt is narrowed down to
the south and gradually widens to the north.
Faults, whose traces run parallel to
subparallel with the fold axes are moderate
to high angle reverse faults and often swerve
to follow the curvature of the fold axes in
plan. The fault traces are located mainly
along the hinge zone of the fold forms.
There are numerous conjugate faults, mostly
intersecting in the zones of culminations and
depressions.
Structural complexity and also the
intensity of deformation gradually increase
from west to east, i.e., from Bangladesh end
to Burma end. To the east the folds are much
compressed, overturned and isoclinal; the
outcrop patterns have been made
complicated by a number of strike faults,
thrusts and transverse faults; while in the
west folds are open, canoe shaped showing
broad and less intricate outcrop pattern.
The mesoscopic folds of the area are
varied in style. They were preserved mostly
in the thinly laminated multilayers of silt
shale alternations of both Bhuban and
Bokabil formations. The wave lengths of
these folds are less than their amplitudes.
Two types of folds are common, viz, sharp
crested (p1 values of the sharp crested folds
are large and the fold form dies out with
depth) (Fig. 3a) and disharmonic folds. The
disharmonic folds are so complex that they
cannot be represented by simple geometric
model. Parallel (concentric) folds (Fig. 3b)
are also seen in the upper Bhubans.
Overturned, recumbent and box folds (Fig.
3c) are studied in the areas bordering
Tripura and Mizoram.
The longitudinal strike faults which are
common in almost all the anticlinal
structures change their attitude along dip.
192 D. R. NANDY, SUJIT DAS GUPTA, KALYAN SARKAR AND ANRUDDHA GANGULY
Faults which are low angle reverse (Fig. 3d)
at higher tectonic level become vertical at
depth and vertical fault at higher tectonic
level changes to low angle normal fault at
depth, the last probably being due to
compressive stress relaxation causing local
set up of tensile stress field.
For showing the planar orientations of
the oblique faults and fractures Statistically,
as drawn on the LANDSAT imagery, rose
diagram was prepared (Nandy, 1981) which
reveals a bimodal pattern ; the major
population falling between N50°E and
N60°E and minor one falling between
N43°W and N60°W. Therefore, these
conjugate set of fractures have their acute
bisectrex orthogonal to the regional fold
axes.
Many of the NW-SE oblique/transverse
faults, viz. Mat, Tuipui, Saitual and Sateek
faults are sinistral, whereas the NE-S’V
oblique faults, viz., Amarpur, Aizwal and
Kaladan faults are dextral in nature (Figs. 1
and 2).
The longitudinal strike faults (Fig 3e) a
common feature, have vertical throw of the
order of 700 m to 1700 m, producing many
drag folds at higher level.
Tectonic Evolution
Tectonically the Surma basin of which
Tripura- Mizoram area is a part, is related to
the eastward subduction of the Indian plate
along the Arakan Yoma suture during
Eocene time and the subsequent
development of the Indo-Burman Orogenic
belt (Nandy, 1982). The west side of the
Orogen was marked by the development of
a narrow restricted molasse basin of the
Tipams to the northeast of the Shillong
plateau and an open bell-shaped Surma
basin south of the Shillong plateau with a
regional palaeoslope to the southwest
(Sarkar et a!., 1977). Mention may be made
here that Tipams of upper Assam and
Tipams of Surma basin are not coeval
(Murthy et al., 1976). While the Tipams
were directly deposited over the Barails in
upper Assam, the Surma sediments were
laid down over the Barails or coeval rocks in
the Surma valley, south of the Shillong
plateau—both after a post Barail hiatus, a
widespread feature throughout the entire
area including the Indian ocean at the close
of Oligocene (Davis et al, 1975).
Surma sediments continued to be laid
down in a shallow paralic basin since
Miocene and were folded and faulted owing
to post-Oligocene eastward subduction and
collision of the Indian plate along the
Arakan Yoma suture zone. The crust was
shortened horizontally and thickened
vertically as the plate continued to converge.
The long arcuate linear folds of the
Surma basin occurring sub-paia[lel to the
suture zone, and the conjugate faults with
acute bisectrix orthogonal to the regional
fold axes clearly indicate their origin
through layer-parallel compression
generated by stress system grossly acting E-
W. This conclusion is in striking contrast to
the contention of some workers (Mitra,
1971; Ganguly, 1975; Ganju, 1975 and
Nandy, 1977) who invoked the concept of
vertical tectonics in explaining the
geological evolution of this orogenic belt.
However, the probable deviation of the
stress axis indicated by the interference
TECTONIC EVOLUTION OF TRIPURA-MIZORAM FOLD BELT 193
pattern of folds might lend credence to the
rotation of the indian plate during its
eastward drift in late Tertiary as inferred
from other tectonic considerations.
Increasing intensity of deformation from
west to east and westward convexity of the
fold belt suggest that the tectonic transport
was directed westerly.
The mesoscopic box fold, overturned
and recumbent folds, which conform the
regional folds are also indicative of their
origin in a compressive stress field.
Absence of igneous activity and gradual
increase in complexity of folding and
intensity of deformation from west to east
corroborates to the origin of the folds
through E-W compressive stress as against
through vertical tectonics. Pattern of
Bouguer gravity anomaly also goes in
favour of the above contention.
Recently Mukhopadhyay (1983, in
press) obtained fault plane solution for three
earthquake events from the present area
indicating thrust faulting having
cornpressional pressure axes oriented at 23°
towards 280°, 6° towards 241° and 25°
towards 295° respectively indicating thereby
that the sediments are under E-W stress field
till to date.
Summary and Conclusion
1) The Surma sediments were deposited in
bell-shaped gradually shal lowing basin over
the post Barail (close of Oligocene)
unconformity extending for 550 km in strike
and about 200 km across in front
of the Indo-Burman Orogen (eastern margin
of the Indian plate).
2) The floor of the basin might have been
formed by sub-flysch Barail Group or rocks
coeval to it.
3) The sediments of the basin yielded by
folding and faulting in a compressive stress
field as an upper crustal decollement or
“Supra Structure”; intensity of the
deformation was maximum in the east near
the zone of subduction and collision due to
eastward drift of the Indian plate during
Mio-Pliocene time.
4) Deposition of sediments and folding went
hand in hand till Pleistocene time as marked
by the deposition of the Dupi Tilas over an
unconformity in the western part of the
basin and the polarity of the basin was from
east to west.
5) The deformation of sediments in the basin
was first initiated by layer-parallel
compressive strain and by shortening, then
by brittle failure through long strike faults
and adjustment along the conjugate shear
fractures with strike slip movements and
lastly by forming decollement structures
which glided over the “infra-structurcs” of
the Barails or their equivalents to form the
present orography.
6) The folded belt is under E-W stress field
even to the present day.
Acknowledgement
The first three authors are thankful to A. N.
Sarkar, Geological Survey of India, for
helpful and stimulating discussion and
suggestion for improvement of the text.
194 D. R. NANDY, SUJIT DAS GUPTA, KALYAN SARKAR AND ANIRUDDHA GANGULY
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