Tectonic evolution of the Aravalli orogen (NW India): an inverted Proterozoic rift basin?

Geol Rundsch (1995) 84:683-696 © Springer-Verlag 1995

P. K. Verma • R. O. Greiling

Tectonic evolution of the Aravalli orogen (NW India): an inverted Proterozoic rift basin?

Received: 20 December 1994 / Accepted: 18 August 1995

A b s t r a c t The Aravalli mountain range (AMR) in the northwestern part of the Indian Peninsula consists of two main Proterozoic metasedimentary and metaig- neous sequences, the Aravalli and Delhi Supergroups, respectively, which rest over the Archaean gneissic basement. A synthesis and reinterpretation of the available geological, geochronological and geophysical data, including results of own field work and geophysical in- terpretations pertaining to the AMR, indicate its origin as an inverted basin: rifting into granitoid basement be- gan ca. 2 .5Ga ago with Aravalli passive rifting (ca. 2.5-2.0 Ga) and Delhi active rifting (ca. 1.9-t.6 Ga). Associated mafic igneous rocks show both continental and oceanic tholeiitic geochemistry and are comparable with Phanerozoic, rift-related magmatic products. Available data :showed no conclusive evidence f o r oceanic lithoshere and island-arc/active margin magmatic activity in the AMR. Subsequent inversion and orogeny (Delhi orogeny, ca. 1.5-1.4 Ga) lead to complex deformation and metamorphism. Only in the western and central zones has the basement been involved in this mid-Proterozoic (Delhi) deformation, whereas it is unaffected in the eastern part, except for local shear zones mainly along the basement/cover interface.The grade of metamorphism increases from the greenschist facies in the east to the amphibolite facies in the west with local HP assemblages. These latter are explained by rapid burial and exhumation of thin and cool continental lithosphere. Subsequently, during a final, mild

P. K. Verma School of Studies in Geology, Vikram University, IND-456010 Ujjain (M. P.), India

Geologisch-Pal~iontologisches Institut, Ruprecht-Karls Universit~it, D-69120 Heidelberg, Germany

R. O. Greiling (f~) Geologisch-Pal~iontologisches Institut, Ruprecht-Karls Universit~it, D-69120 Heidelberg, Germany Fax: 49 6221 565503

phase of inversion, the Vindhyan basins consisting mainly of sandstones, limestones and shales, flanking the AMR formed which are comparable to foreland basins. The tectonic evolution of the AMR is therefore interpreted as an example of a major inverted continental rift and of a Proterozoic intra-continental 0rogen.

K e y w o r d s P r o t e r o z o i c • Continental rifting • Inversion tectonics • Intra-continental orogeny • Northwest India

Introduction

Phanerozoic orogeny is generally characterized by an evolution from early plate convergence, calc-alkaline magmatism and collision leading to the accretion of continental lithospheric plates and other terranes (e. g. Dewey 1988; Coward 1994a). Ophiolite sequences in orogens, frequently aligned along suture zones, docu- ment the existence of oceanic lithosphere in between terranes prior to collision. In contrast, Archaean and Proterozoic lithospheric evolution may not always fol- low these lines of accretionary orogeny (e. g. Kr6ner 1981; Condie 1992; Windley 1992). Both modern-type "accretionary" orogens with frequent ophiolite rem- nants and "collisional" orogens without clear oceanic precursors can be distinguished (Windley 1992). Gond- wana is particularly known to contain both these orogenic types (see Hunter 1981; Windley 1992; Stern 1994), and a well-exposed example of a collisional orogeny, which probably evolved from a continental rift, namely the Aravalli mountain range (AMR) of NW India, is discussed herein.

The AMR is presently exposed as a horst-like fea- ture located in the northwestern part of India, extends over 600 km in length with a general NE-SW trend and is bounded by Eastern [or Great Boundary Fault (GBF)] and Western Marginal Faults, respectively (Fig. 1). These faults bound the Aravalli range on either side against the generally undeformed Late P r o -

INDIA ~ Delhi

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Fig. 1 Generalized geological map of the Aravalli mountain range (AMR), India. A - A ' and B - B ' are the section lines in Fig. 2 and X - X ' in Fig. 3. (Modified after Roy 1988)

terozoic Vindhyan Supergroup sedimentary sequence (Reddy and Ramakrishna 1989; Verma 199lb 1993).The orogen itself consists of an Archaean basement (known as the Banded Gneissic Complex) and two main basinal sequences, the Aravalli and Delhi Su- pergroups. These latter supracrustals are complexly deformed and metamorphosed together with the partially remobilized basement (Naha et al. 1967; Naha et al. 1984). A generalized lithostratigraphic succession of the AMR is given in Table 1. In this report the term Aravalli (viz. Aravalli basin, Aravalli sequence, etc.) designates the Aravalli Supergroup, whereas the AMR refers to the orogen as a whole in between the Eastern and Western Marginal Faults, respectively, occupied both by the Banded Gneissic Complex (BGC) and the metamorphic rocks of the Aravalli and Delhi Super- groups (Fig. 1).

The present work is an attempt to combine the wealth of the geological, geochronological and geophysical evidence on the tectonic development of the AMR - both published and from field work of the first author - and to discuss the formation and inversion of the Aravalli and Delhi rift basins. This interpretation re-

quires generalization of the facts; nevertheless, this sim- plification gives an indication that the overall character of the basins in the AMR is similar to that of a rifted basin and that inversion tectonics played a vital role in the formation of the AMR, leading to a kind of collisional orogeny (Windley 1992).

Tectonic evolution of the AMR

The tectonic evolution of the AMR is summarized in Tables 1 and 2 and is discussed below, starting with the oldest features.

Evolution of the basement

The oldest rocks (ca. 3.3 Ga) found on the Indian Shield are metasediments associated with tonalitic biotite gneiss and amphibolite from the AMR (Gopalan et al. 1990). The presence of an older crustal component in this area is indicated by the age of detrital zircons obtained from the Aravalli schist (3.5 +0.2 Ga; Vino- gradov et al. 1964) and from the geochemical study of the mafic inclusions within the BGC (Ahmad and Raja- mani 1988). The subsequent evolution of the sialic crust in the AMR is associated with the emplacement of granitic bodies (Choudhary et al. 1984; Roy and KrOner 1995; Wiedenbeck et al. 1995). The emplacement of these extensive granitic batholiths (e. g. Berach, Untala, Gingla granites, etc.) during the period from ca. 3.0 to 2.5 Ga (Table 2) indicates a period of rapid cratonization resulting in the rapid thickening of the crust in this region (Pichamuthu 1970; Naqvi et al. 1974; Radha- krishnan and Naqvi 1986).

A regional metamorphic event partially overprinted the Berach granite into a gneiss which is characterized by the metamorphic mineral assemblage of quartz-K- feldspar-acidic plagioclase-biotite-hornblende indicating upper amphibolite metamorphic facies (Sharma 1988). Upper amphibolite metamorphic facies from other parts of the basement are described by several authors, for example, Kataria and Chaudhari (1988) and Khandelwal and Pandya (1988). In the AMR, on the basis of these metamorphic data, a crustal thickness of at least 20-25 km is indicated for the Archaean period (cf. Roy 1988). A pre-Aravalli Supergroup tectono- metamorphic history is also documented in pebbles and boulders of schists, gneisses, amphibolites and granitic rocks in the 'basal Aravalli conglomerate (Roy et al. 1985; Mohanty and Naha 1986). A general upward increase in the amount of granite-derived sediments in the Aravalli sequence (Roy 1988) also confirms the dominance of granitic rocks in the basement of the AMR.

Therefore, it may be concluded that the basement in the AMR is sialic by nature. The cratonization continued until ca. 2.5 Ga (Table 2) and the Archaean-Prote- rozoic transition in the AMR marks the end of the

Table 1 Generalised lithostratigraphic succession of the Aravalli mountain range, India

Stratigraphy Tectonic events

685

Vindhyan Supergroup (west of AMR)

Unconformity Malani volcanics Erinpura granites Champaner group Sirohi group

Unconformity (ca. 1.6-1.4 Ga) (Delhi orogenic event)

Vindhyan Supergroup (east of AMR)

Delhi Supergroup Ajabgarh group (Slate, phyllite, quartzite, limestone, conglomerate, volcanics) Alwar group (Quartzite, arkose, grit, limestone, conglomerate, mica schist, volcanics) Raialo series (Marble, quartzite, volcanics)

Unconformity (ca. 2.0-1.9 Ga) (Aravalli orogenic event?)

Aravalli Supergroup

Unconformity (ca. 2.5-2.4 Ga) Banded Gneissic Complex (BGC)

Upper Aravalli group (Quartzite, arkose, phyllite, greywacke-phyllite, arenite, dolomite, conglomerate) Lower Aravalli group (Dolomite, quartzite, phyllite, stromatolitic phosphorite, chlorite schist and amphibolite, conglomerate)

Pre-Aravalli granite gneisses, Amphibolites, metasediments and granitic rocks including Untala-, Gingla- and Berach granites (ca. 3.3 Ga)

Opening of western Vindhyan basin

Uplift of AMR

Deformation of Sirohi and Champaner basins

Sedimentation in Sirohi, Champaner and eastern Vindhyan basins

Formation of foreland basins relative to AMR Deformation of Aravalli and Delhi rocks Inversion of rift basins

Thermal subsidence?

Tectonic subsidence?

Opening of Delhi basin Deformation of Aravalli rocks?

Little trace of thermal subsidence

Formation of Aravalli rift basin

Pre-Aravalli deformation and metamorphism

amalgamation of nucleii to form an early continental crust. Further crustal thickening during Proterozoic time involved the formation and inversion of sedimentary basins. During these processes the Archaean crust was reworked in parts of the A M R (Naha and Roy 1983).

Evolution of the cover sequence

The Aravalli Supergroup

The opening of the Aravalli basin (ca. 2.5-2.4 Ga) marks the beginning of the Proterozoic events in the AMR. The Aravalli basin appears to be morphological-

ly similar to East African rifts and its thick sequence of granite-derived sediments, carbonates and basal volcanics are comparable to Assemblage I of Condie (1982, 1989) indicating the basin to be part of a passive continental margin or an interior rift basin (Fig. 3A; Roy 1988, 1990; Roy et al. 1984; Verma 1995). The sedimentation in the Aravalli basin started synchronously with basic magmatism (Mathur et al. 1978) producing mainly gabbros and basalts of continental tholeiitic character (Ahmad and Rajamani 1991). Several occur~ rences of ultramafic plugs along the strike of these igneous rocks represent the deeper cumulate phase and were emplaced as diapiric bodies due to gravitative uplift relative to the Aravalli basin floor (Srivastava 1988). A gradual subsidence of the Aravalli basin floor

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and simultaneous upwarping of surrounding domains is further indicated by a general upward increase in the amount of granite-derived sediments (Roy and Paliwal 1981) suggesting progressive uncovering of granitic crust.

The Delhi Supergroup

The Delhi basin occupies a further system of grabens (or half grabens) and hosts an enormous thickness of volcano-sedimentary rocks (see Singh 1984, 1988). The volcanic rocks which characterize all the major units of the Delhi Supergroup indicate bimodal rhyolite-basalt associations of continental affinity (Singh 1982, 1985; Srivastava 1988). The early sedimentary sequence indicates fluvial and shallow marine environments and a dominance of immature clastics, which is followed by deep facies sequences. There are several indications of synsedimentary dislocations along the marginal faults (Singh 1984).

Unfortunately, there is as yet no unambiguous structural evidence to separate the two potential Aravalli and Delhi deformational episodes. However, it is clear that both Aravalli and Delhi sequences record only a single, regional metamorphic event (Sharma 1988).Because the Delhi event affected the whole of the area, including the Aravalli, it is argued here that also the regional metamorphism in the Aravalli sequence is related to the Delhi event and that the Aravalli phase, if present, is only of minor importance. Therefore, the deformational events and metamorphism are treated here, in want of further evidence, as a single orogenic phase oc- curring after the deposition of the Delhi Supergroup (see "Time frame for the tectonic events in the AMR" and Fig. 3). The Delhi orogeny resulted in the formation of the AMR and represents the main phase of crustal shortening in the region.

Modification of the Basement-Aravalli-Delhi angular relationships

Deformation and tectonism

The Delhi orogeny affected the whole sequence of rocks comprising the AMR, namely the basement (BGC) and the Aravalli and Delhi Supergroup cover sequences, including the associated igneous rocks (He- ron 1953; Naha and Halyburton 1977; Naha et al. 1988; Roy and Das 1985; Sychanthavong and Merh 1985; Sen 1983; Sinha-Roy 1984). The rocks have undergone po- lyphase deformation (e. g. folding, shearing) together with a single phase of regional metamorphism (see Roy 1990 for a review on the subject and for related references). Whereas the first phase of Delhi folding is con- fined to the sequences within the basin, the second phase is considered to be the phase of regional crustal shortening in the AMR, which has also affected a wider area, including the Aravalli rocks and the basement (Roy 1988; Roy and Das 1985; Naha and Halyburton 1977; Naha et al. 1984). Except in the southern part of the AMR, the general structural trend is NE-SW. The culmination of this Delhi orogenic event is marked by the emplacement of granites (Khetri, Seoli, Bairat, Da- dikar and related granites) at ca. 1.5-1.4 Ga (Choudha- ry et al. 1984). With the exception of local migmatiza- tion and upper amphibolite facies metamorphism re- stricted to an area close to the western margin, the Aravalli rocks are characterized by low-grade (greenschist facies) metamorphism. A series of isoclinal, re- clined or inclined folds, several faults and ductile shear zones (mainly along the basement/cover interface, producing wide zones of mylonites, cf. Roy et al. 1985) have been identified as the first phase of deformation in the Aravalli rocks underlying the Delhi sequence. Based on these observations, Roy (1988, 1990) and Sy- chanthavong (1990) suggested a separate Aravalli orogenic phase preceding the main Delhi orogenic event.

Ramsay (1983) pointed out that before orogenic deformation the individual rock layers and rock types pos- sess characteristic angular relationships between each other, and as a result of tectonic strain during orogenesis, these angular relationships are modified. The most commonly observed modification in regions of high strain is a reduction in the angles between initially obli- que elements, because all lines and planes rotate towards the XY principal plane of the finite strain ellip- soid, and towards the greatest finite extension X within this XY principal plane. Such phenomena are well illus- trated in the AMR where depending upon the initial orientation of the planar components and upon the in- tensity and orientation of the finite strains, a gradual modification of initial angular relationships between the primary elements is distinctly noticeable. In the AMR, although the different lithological units comprising the basement (viz. granites, migmatites, high-alumi- nous paragneisses, amphibolites, quartzites, low-Mg marbles, etc.) are identifiable, there is no clear evidence of unconformable relationships between these units. In places, particularly in the axial zone of the AMR, the foliations in the basement are transposed parallel to the schistosity in the Aravalli rocks and the remobilized basement - Aravalli contact is modified into a migmatite front (see Naha and Roy 1983). However, at several other localities the Aravalli rocks lie over the basement with a clear stratigraphic and angular discor- dance (see Roy and Paliwal 1981; Roy et al. 1980; Roy et al. 1981). For example, to the east and southeast of Udaipur (Fig. 1) the basement is not much affected by the younger orogenic movements (Roy 1988) and the angular nature of the basement-Aravalli contact is evident (Roy et al. 1988). Except for a few localities, the original primary contact between the Delhi and the older rocks is completely modified into tectonic contacts. The tectonic fabric acquired during the Delhi orogeny

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Table 2 Available ages of Proterozoic igneous rocks in the Aravalli mountain range and their relationship with tectonic events

Events Time (Ga) Reference

Post-orogenic period (ca. 1.5-0.7 Ga) Mount Abu granite 0.735 Malani rhyolite 0.745 -+ 0.01 Erinpura granite 0.8 +- 0.05 Sendra granite 0.85 Inversion (orogenic) period (ca. 1.6-1.5 Ga) Khetri granite 1.48-0.04 Seoli granite 1.5 Bairat and Dadikar granites 1.6 Rifting period (Aravalli and Delhi Cycles, ca. 2.5-1.9 Ga) Amet granite 1.9 -+ 0.2 Darwal granite 1.9 + 0.08 Nepheline syenite 1.9-1.4 Pre-orogenic period (> ca. 2.5 Ga) (basement) Berach granite 2.5

2.6 + 0.15 Untala and Gingla granites 2.54-2.61 Jagat gneiss 2.83 Biotite gneisses (tonalitic to granitic in composition) within the BGC 3.3

Crawford (1975) Crawford and Compston (1970) Choudhary et al. (1984) Tobisch et al. (1994)

Gopalan et al. (1979) Choudhary et al. (1984) Choudhary et al. (1984)

Choudhary et al. (1984) Choudhary et al. (1984) Crawford (1970)

Crawford (1970) Choudhary et al. (1984) Roy and KrOner (1995) Roy and KrOner (1995)

Gopalan et al. (1983)

is superimposed on the older fabrics. As a result of increasing internal deformation during the Delhi orogeny, the angular relationships between the older structures were modified towards an apparently simple and parallel disposition of a wide range of initial orienta- tions. In the AMR, as a result of these modifications, it is sometimes quite difficult to separate basement and cover especially where the increasing structural paralle- lism is accompanied by metamorphism and local mig- matization.

"Post-orogenic" evolution

Closely following the formation of the AMR, three isolated marginal basins developed along the flanks of the AMR; two relatively minor, the Champaner and Sirohi basins along the southern fringe, and the Vindhyan basin along the eastern fringe (Fig. 1). All these basins are characterized by shallow water sequences of argilla- ceous, arenaceous and calcareous beds (Coulson 1933; Gopinath et al. 1977; Banerjee and Singh 1981). While the sedimentation continued uninterruptedly in the Vindhyan basin, it ceased in the Sirohi and Champaner basins, and these two latter sedimentary sequences were mildly deformed before the onset of widespread acidic igneous activity in the AMR and to the west of it. This igneous phase represents an anorogenic magmatic activity (Roy 1988) and is characterized by several granitic (Erinpura granites and equivalents) and rhyol- itic (Malani volcanics and equivalents) activities. The Vindhyan sequence (Lower and Upper part) to the east of the AMR is a thick sandstone-shale-limestone sequence resting over the Archaean basement (Berach granite and Bundelkhand gneiss). The Lower Vind-

hyan sequence is completely missing from the western part of the AMR where an undeformed, almost flat- lying Upper Vindhyan sequence (also referred to as Marwar Supergroup; Gupta et al. 1980) represented by a thick sequence of intercalated sandstones, limestones and shales rests directly over the Malani rhyolites. The Vindhyan rocks to the west of the AMR are correlated with the upper units of the Upper Vindhyan sequence present to the east of the AMR (Banerjee and Singh 1981). Although the Vindhyan rocks are present on either side of the AMR, there is no evidence that they were ever deposited in the main tract of the AMR (e. g. Murthy and Mishra 1981; Prasad 1984). This probably indicates that during the period represented by the Vindhyan sedimentation, the AMR was a prominent positive landform acting as a source area and barrier between the eastern and western Vindhyan basins. Along the contact with the GBF the Vindhyan rocks have developed several faults parallel with the eastern margin and are folded into a series of open, N-S trend- ing anticlines and synclines (Verma 1991a). In general, however, the sedimentary sequences of the two isolated Vindhyan Supergroup sequences, separated by the AMR, are practically undeformed and are free from any igneous rocks.

Time frame for the tectonic events in the AMR

The available radiometric information for the Protero- zoic magmatic events in the AMR is presented in Ta- ble 2. No radiometric age data are available for the basal metavolcanics of the Aravalli sequence. The Aravalli sequence rests over the Berach granite suggesting thereby that the Aravalli Supergroup is younger than

688

the granite (ca. 2.5 Ga). Considering that the rift basin opened for the Aravalli Supergroup sedimentation, the age of rifting can be assigned to be between ca. 2.0 and 2.5 Ga ago.

The Darwal granite (1.9+0.8 Ga) is intrusive into the Aravalli sequence and the Amet granite (1.9+0.2 Ga), in many places, forms the basement of the Delhi rocks (Heron 1953). No radiometric date is available for the volcanics at the base of the Delhi Su- pergroup. Sinha-Roy (1984) considers that the time of emplacement of nepheline syenite marks the initiation of continental rifting leading to the formation of the Delhi basin. The model Rb-Sr ages of these syenites vary from 1.4 to 1.9 Ga (Crawford 1970). On the basis of the ages of the Darwal and Amet granites and the age of the nepheline syenite, the age of the initiation of the Delhi rift basin over the supracrustals of the Arav- alli basin may be considered as ca. 1.9 Ga. The initiation of the Delhi sedimentation over the Aravalli rocks is constrained as post-l.9 Ga by the age of detrital K- feldspar from the basal conglomerate of the Delhi Su- pergroup (Crawford 1970)

On the basis of age data for the Aravalli rocks and their granitic intrusions, the metamorphism of the Aravalli Supergroup rocks has been tentatively dated as between 2.0 and 1.5 Ga ago (Crawford 1970). The age of the regional metamorphism in the Delhi rocks apparently lies between 1.6 Ga (age of the Bairat granite which intruded the Delhi rocks) and 1.4 Ga (age of the Lower Vindhyans; Crawford and Compston 1970). These overlapping ages for the regional metamorphism of the Aravalli and Delhi Supergroup rocks are consis- tent with the observation of a single metamorphic phase in the AMR during the Delhi orogeny (Sharma 1988; see above, "Deformation and tectonism"). As discussed above, this tectonic event at ca. 1.4-1.6 Ga was responsible for the formation of the AMR. It is impor- tant to note that the time for post-rift thermal re-equilibration and cooling of the lithosphere was ca. 300 Ma before the onset of compression and thrusting (1.9-1.6 Ga ago).

The period between ca. 1.4 and 0.9 Ga is represented by part of the Vindhyan sequence present to the east of the AMR (Verma 1995). The K-Ar dating of the rocks of the Lower Vindhyan and lower part of the Upper Vindhyan sequences indicated an age between 1.4 and 0.91 Ga (Vinogradov et al. 1964). The period between ca. 0.85 and 0.75 Ga is marked by extensive fe!sic magmatism in the AMR and to its west (Srivas- tava 1988). During the period between ca. 0.85-0.75 Ga, while the AMR and the area to the west of it were witnessing widespread magmatic activities, the Vindhyan sedimentation was in progress to the east of the AMR. It was after the period of the Malani rhyolite (ca. 0.75 Ga) when the Vindhyan basin developed to the west of the AMR. It is difficult to say how long the Vindhyan sedimentation continued thereafter, because no age data is available from the upper part of the Vindhyan sequence. However, on the basis of their

general unfossiliferous character, the entire Vindhyan sequence has been assigned a Precambrian age (Heron 1936, 1953; Pascoe 1959).

Geophysical models on the structure of the AMR

Qureshy (1971), while studying the relation of gravity to elevation and rejuvenation of blocks in India, concluded that the positive and low regression coefficients and positive isostatic anomalies found in the AMR suggest that this tectonic element is a horst beneath which heavier materials moved from depths and were incorporated into the crust. The horst nature of the AMR has also been advocated by Verma et al. (1986) who observed the presence of a large linear gravity high of the order of + 130 mgal flanked by two gravity lows of -60 mgal on the western side and of -70 mgal on the eastern side. Over the Vindhyan sedimentary sequence, present on either side of the mountain range, a gravity low of the order of -10 to -20 mgal is indicated. Consid- ering the shape and gradient of gravity lows on either side of the AMR, Verma et al. (1986) postulated a broad central low of nearly -100 mgal amplitude caused by the formation of a root underneath the mountain range and suggested that the postulated central low is at present marked by a gravity high (+ 130 mgal) due to the presence of high-density material (gabbroic composition) lying underneath and along the strike of the AMR. The presence of a large linear gravity high flanked by two gravity lows with sharp gradient throughout its length invariably points towards faulted margins for the AMR (Fig. 2). Reddy and Ramakrishna (1988, 1989) and Mishra et al. (1995) also favoured the nature of the AMR as a horst flanked by regional faults which are associated with block movements. Heat-flow studies suggest that the lithosphere-asthenosphere boundaries may be upwarped under the AMR (Gaur 1982). The surface manifestation of the eastern gradient (-70 regal) is the Great Boundary Fault of Rajas- than (GBF) and of the western gradient (-60 regal) is the Nawalgarh-Desuri-Jharol fault (western marginal fault in Fig. 1). Both these faults separate the rocks of the AMR from that of the Vindhyan Supergroup. The gravity lows observed over the Vindhyan rocks (-10 to -20 mgal) may be ascribed to the presence of low-density sediments overlying granitic basement (Bundelk- hand and Berach granites in the east, and Erinpura and its equivalent granites and rhyolites in the west), because ensialic anatexis and emplacement of granitic batholiths often lead to gravity lows which are further enhanced in amplitude by overlying low-density sediments (Bott 1981; Browne and Fairhead 1983; Kusznir et al. 1987).

The observed central low (Verma et al. 1986), when restored for the situation before the uplift of the AMR, gives an impression on the nature of the basin where sedimentation took place. When considered for the time prior to the deposition of these sedimentary rocks,

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the gravity lows of the order of -10 to -20 regal observed over the Vindhyan plains would yield slightly 0 positive values. Similarly, considering the time prior to 10 - the uplift of the AMR vis-g-vis emplacement of the 0 high-density mass beneath the range, the presently observed gravity lows of the order of -60 to -70 mgal (gra- dients) flanking the present gravity high would repre- g-20 sent margins of the old basin where the sedimentation took place (observed central low of that period =~ -100 mgal; Verma et al. 1986). Thus, reinterpretation of ~ o -4o the available gravity data, for the period prior to the =~ deposition of the Vindhyan sediments and also prior to ~-60 the uplift of the AMR, indicates a situation where a m ° broad central gravity low was flanked by the marginal -8o gravity high with a sharp gradient throughout its length. This indicates a tectonic setting for a rift basin where the central mass goes down along the marginal faults -100 followed by influx of sediments which fill the basin. Summarily, a reinterpretation of the available geophysical data from the AMR indicates the formation of a rift-basin in Proterozoic time. The geophysical data, however, takes into account both the Aravalli and Del- hi basins collectively; thus, these data alone cannot be used to distinguish between the Aravalli or the Delhi sedimentational cycle.

Rift character of the Aravalli and Delhi sedimentary sequences

In the eastern part of the AMR, the Aravalli rocks are separated from the Vindhyan rocks by the eastern marginal fault (the GBF) and continue far westward where they disappear, being either overthrusted by the basement gneiss or primarily overlain by the Delhi rocks (Fig. 1). At many places the Delhi rocks unconformably overlie the Aravalli rocks (Naha and Roy 1983; Roy 1985) suggesting thereby that the Aravalli rocks represent a distinct, older sequence. Moreover, except for the northeastern part of the AMR, the Aravalli rocks are everywhere in contact with the GBF (Verma 1991). Based on these observations, it is considered that the rift basin, demarcated by the eastern marginal fault (as evident from the geophysical considerations), was opened prior to the Aravalli sedimentation. The available evidence (see Table 3 for a summary) clearly indicates that the Aravalli sediments were laid down in a rift basin.

A critical review of the available literature related to the Delhi basin, which formed after the Aravalli basin, indicates that the sedimentary-volcanic association of the Delhi basin matches well with the assemblage II of Condie (1982, 1989) implying thereby that these deposits were again laid down in intracratonic rift basins or aulacogens. The Delhi basin opened up as a system of grabens (or half grabens) with intervening horsts (see Singh 1988). Some of the mafic volcanics at the base of the Delhi sequence (Alwar group) have chemical affinities to continental tholeiites (Singh 1985; Srivastava

Distance (km) 100 200 300

I I

f* '~ , - l" "\... i/"N\ - A. . . . . . / " "'~ / \"

" ' . . . i ):~ "" : \

B ..*--... \ .. , - ", .

"\ \

"\. \ A '~ ' - 4 ~ .\.. \ / . /

,,~, ,~ / / "'~-k,J2.."

marginal fault Easter n ~ l L marginal fault

Fig. 2 Bouguer anomaly profiles across the AMR. See Fig. 1 for the locations of the section lines A-A' and B-B'. Bouguer anomaly data has been adapted from Verma et al. (1986)

1988). However, volcanic rocks from the southwestern margin of the Delhi basin are reported to have oceanic chemical characteristics (Bhattacharya and Mukherjee 1984; Sinha-Roy and Mohanty 1988). The metasedimentary rocks associated with these volcanics include quartzite, marble and mica schists. The nature of these sediments implies a continental source area, probably including a carbonate platform. Such an assemblage of continental sediments with basaltic rocks of both continental and oceanic affinities are typical of major rifts such as the East African Rift System and related rifts in the Red Sea area and of the continent-to-ocean transi- tional zone of modern passive continental margins (e. g. Coleman 1984; White and McKenzie 1989; see also discussion in Green 1992 and Andrdasson 1994). Because there is no geological record of a true ocean basin (see discussion on the interpretation of mafic-ultramafic assemblage in Srivastava 1988; see also Sharma 1988 for other evidence), the available evidence points to a zone of extensive intracontinental rifting and thinning of the lithosphere during Delhi basin formation (see Ta- ble 3).

Rifts in the AMR: active or passive?

Seng6r and Burke (1978) and Baker and Morgan (1981) have defined two end members of the processes responsible for rifting (see also Green 1992). In "active" rifts uplift and volcanism should precede faulting, whereas faulting should precede volcanism in "passive" rifts. Partial melting of the upwelling asthenosphere (Brown and Girdler 1980) can account for rift magmatism while the regional uplift is an isostatic response to the relative low density of the upwelling asthenospheric

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material or the buoyancy of the rifted lithosphere. The end products of active and passive rifting, however, are likely to be very similar; hence, it is not easy to differ- entiate the two processes in geological records, particularly after tectonic overprinting as in the present case. A critical examination of the stratigraphic relations at the base and characteristics of the rift products may help to characterize the active or passive nature of rifting (Tables 1 and 3).

The basement-Aravalli interface is marked by wedges of quartzite and conglomerate (containing poorly sorted subangular to subrounded pebbles and boulders of vein quartz, quartzite and leucogranite em- bedded in a carbonate matrix) and carbonate rock (containing angular fragments of granite and gneiss scattered haphazardly in the carbonate matrix). Field and petrographic studies suggest that these rocks are a type of basal arkose (Roy et al. 1988). The presence of coarse, poorly sorted, angular to subangular basement fragments in the basal part of the cover sequence implies nearby up-dip relief which may be related to fault scarps at the margin of half grabens (e. g. Roberts and Yielding 1994). The basal part of the Aravalli sequence contains both metavolcanic rocks and quartzite with local conglomerate (with chlorite schist or quartzitic matrix). The sequence is fining upwards, grading into quartzite, phyllite and carbonates. The metavolcanics are represented by chlorite schist and amphibolite (metamorphosed lava flows and tufts) with interbeds of phyllite, dolomite and quartzite. These latter sedimentary interbeds point to the fact that volcanism was active simultaneously with a relatively mature rift sedimentation. Furthermore, the volume of the basal volcanics (Mathur et al. 1978) is much less relative to other rifts of the world (Fitton 1983; Green 1983). The available evidence may therefore be interpreted in terms of normal faulting and rifting preceding volcanism. These geological observations indicate passive rifting and sup- port geochemical considerations that the mafic and ultramafic volcanic rocks at the base of the Aravalli sequence derived from decompressional, adiabatic melting at different levels of the lithosphere (Ahmad and Rajamani 1991).

The Delhi sequence is primarily made up of conglomerate, arkose, quartzite and marble with large volumes of bimodal volcanic rocks which occur at the base and at different levels in the sequence. The synsedimentary dislocation along the marginal faults is indicated by thick accumulations of shallow water deposits, preponderance of coarse clastics near the basin margin, rapid variation in sediment thickness, strong overlapping relationship and presence of synsedimentary volcanics including coarse pyroclastics (see Singh 1988). The areal extent of the Delhi basins suggests a wide zone of rifting (Fig. 1) and the sequence indicates epi- sodic dike injection and volcanism, preceding and alter- nating with faulting (Singh 1988). This observation invariably points towards the active nature of the Delhi phase of rifting.

It appears that with the progression of time, the passive Aravalli rift evolved into the active Delhi rift through the upwelling and spreading of large volumes of asthenosphere beneath the older passive rift (e. g. Condie 1989). The transition from passive to active rifting in the AMR is characterized by the deposition of the Raialo series (Table 1) and the emplacement of granitic bodies (the Darwal and Amet granites). The inception of Delhi sedimentation was preceded by a broad crustal upwarp with intervening shallow basins which received the Raialo sediments. With only subor- dinate volcanic rocks, the lower part of the Raialo sediments is dominated by carbonates and has a very low clastic percentage (Singh 1988). The Raialo sequence thus points to a minor relief of the basin flanks and the lack of major river systems to accelerate terrigeneous input. The mature clastic sediments (quartzite) at high- er levels indicate a very slow rate of basin subsidence giving enough time for the sediments to acquire matur- ity through reworking. Closely following or along with this event, there was extensive magmatism and rifting accompanying the onset of the Delhi sedimentation cycle.

Discussion and conclusions

There are several aspects of the regional geology in the AMR that do not fit readily into such models as explained by the present plate tectonics models (see Shar- ma 1988 for a detailed discussion on this topic). In fact, all available evidence points towards intra-continental or collisional orogeny (e. g. Dimroth 1981; Kr6ner 1979, 1981; Van Schmus and Bickford 1981; Windley 1992; Stern 1994) for the evolution of the AMR. The riftogeneous character of both the Aravalli (2.5-2.0 Ga) and Delhi (1.9-1.5 Ga) sequences indicates repetition of two similar sedimentary cycles in the Proterozoic period. The geological records give an indication that the opening of the Delhi basin initiated over the supracrustals of the Aravalli rift basin without any clear phase of deformation and recrystallization in between the Aravalli and Delhi deposition. The closing of the Delhi sedimentation cycle, on the other hand, which affected the entire Aravalli region, is well mani- fested. The inversion of the Aravalli and the Delhi basins during the Delhi orogeny resulted in the AMR.

The presence of an early generation of folding and of associated older intrusive granites in the Aravalli sequence may suggest a separate AravaUi orogenic cycle (see Roy 1990 and references therein). On the other hand, a single phase of regional metamorphism re- corded within the older Aravalli rocks with associated low- to very-low-grade metamorphic rocks (see Sharma 1988) favours a single orogenic cycle for both the Arav- alli and Delhi sequences. Some authors report further metamorphic episodes, but these are of only local importance and not related to the orogeny of the AMR. For example, Naha et al. (1967) suggested that the Dar-

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wal granite represents the granitized Aravalli and Raia- lo sediments prior to Delhi sedimentation. The most likely source of heat leading to the granitization of these sediments is the asthenosphere, which was upwarped at the beginning of Delhi active rifting. In the AMR a general westward increase in the grade of metamorphism, from greenschist facies in the east to the amphibolite facies in the west (Sharma 1988), suggests an east-west gradient, and the structural data (see Roy 1988 and the references therein including several other papers in the same volume) indicate an easterly tectonic transport direction. The intensely metamorphosed Delhi rocks represent the youngest sequence in the AMR apart from the Vindhyan rocks outside the AMR. Metamorphism and burial of the Delhi rocks indicate that either these rocks have been transported up to the present level from the deeper parts of the basin, or a substantial mass of overburden has been removed from over the Delhi rocks, or a combination of these two processes. Blueschist facies in the Delhi fold belt (Sinha-Roy and Mohanty 1988) points to rapid exhumation and low initial temperature. This high pressure metamorphism has been related to modern-type intra- oceanic subduction zones (e. g. Bhattacharya and Muk- herjee 1984, Sinha-Roy 1988, Sinha-Roy and Mohanty 1988; Sychanthavong 1990). However, there is no conclusive evidence of the existence of oceanic lithosphere nor of any clear island-arc or active margin magmatism in the AMR (e. g. lack of calc-alkaline igneous rocks; see Srivastava 1988). As a consequence, an alternative scenario for the high-pressure metamorphism is envis- aged here, based on the fact that the continental lithosphere, which was thinned by Delhi rifting at ca. 1.9 Ga ago, had time to thermally re-equilibrate for ca. 300 Ma, and therefore was relatively cool before being dis- turbed by inversion and orogeny. In the AMR the basement has been involved in the Delhi folding in the western and central zones, whereas it is mostly unaffected by this event in the eastern part. Therefore, the pre-Aravalli crystalline basement did not everywhere behave as a rigid unit during the orogenic process but was incorporated in thrust sheets and anatectic mobili- zation, thereby, in places concealing the original basement-cover relationship and generating polymetamorphic assemblages (e. g. Kr6ner 1981; Coward 1994b).

-~ Fig. 3A-F Diagram (not to scale) along the section line X-X' .(Fig. 1) illustrating the evolutionary trends in the AMR. A Rift- mg and deposition of the Aravalli Supergroup; emplacement of the mafic-ultramafic rocks and granites (Darwal and Amet); fu- ture ("Delhi") faults shown by dashed lines; B further rifting, bimodal volcanism and deposition of the Delhi Supergroup; C thermal maturation and re-equilibration; sedimentaion (Delhi) in progress; D inversion and crustal thickening, deformation, metamorphism; E further compression and magmatism; F present situation

The Vindhyan basin to the east of the AMR evolved as a kind of foreland basin in response to the rising AMR. The first Vindhyan sediments came from distal low-relief domains to the west and were largly fine- grained, giving rise to deepwater marine shales and carbonates exposed in the lowest stratigraphic levels of the Vindhyan successions (Lower Vindhyans). The entire sequence of the Upper Vindhyan sediments of about 3500-4000 m thickness exhibits characters of shallow water deposition suggesting thereby a gradual and stea- dy subsidence of the Vindhyan basin floor and relative uplift of the AMR (Murthy and Mishra 1981; Verma 1995). A major post-orogenic event was the widespread granitic activity in the AMR and to the west of it. Available data from some of these granites suggest that they have 'S' or 'A' type chemistry and that this event is an anorogenic magmatism post-dating the Delhi orogeny (Gangopadhyay and Lahiri 1984; Roy 1988). Das Gupta and Chandra (1978) suggested that the tectonic elements to the west of the AMR evolved during the Delhi orogeny and that the subsequent epeirogenic movements have reactivated these zones of weakness as in the earlier Proterozoic basement. The alignment of the hills of the Malani rhyolite parallel to the AMR suggests that the rhyolite was probably also extruded along such faults. The Vindhyan sediments to the west of the AMR rest on these rhyolites. The undeformed nature of these sediments documents that no major tectonic episode followed thereafter.

In light of the available evidence and within the constraints presented, the sequence of events in descending order of antiquity for the evolution of the AMR may be summarized as follows: A. Evolution of sialic crust through amalgamation of

nucleii during the Archaean era; initiation of rifting along listric normal faults in the Archaean basement for the sedimentation of the Aravalli sequence ca. 2.5-2.0 Ga; deposition of the Aravalli sediments in rift basins (Fig. 3A)

B. Mild tectonism and migration of the extensional regime towards west; opening of the Delhi basin as a series of half grabens ca. 1.9 Ga; deposition of the Delhi sediments in intra-continental linear half grabens (Fig. 3B) Thermal maturation, re-equilibration and subsidence (ca. 1.9-1.6 Ga) with continued sedimentation in the Delhi basin (Fig. 3C) A change from extensional to compressional stress regime ca. 1.6-1.4 Ga; inversion of the Aravalli and Delhi basins and formation of the AMR; thrusting of the basement over the Delhi sediments under strong eastwast-directed compressive stress and formation Of an orogenic wedge; deformation and metamorphism of the Aravalli and Delhi rocks; initiation of sedimentation in the Vindhyan basin to the east of the AMR (Fig. 3D), similar to orogenic foreland basin sedimentation Erosion of the orogenic wedge; progressive reduction of the orogenic load, rapid exhumation and

C.

D.

E.

694

uplift of the AMR; rotat ion of the older listric faults, extension of some of the older faults to deeper levels (?) to serve as a conduit for the ascent of the magma; remobil izat ion of the basement , partial melting and localized migmatizat ion of the Aravall i rocks, emplacement of ultramafic rocks in the Delhi fold cores, emplacement of Er inpura granites, continuing sedimentat ion in the Vindhyan basin to the east of AMR, ca. 0.9-0.8 Ga (Fig. 3E)

F. Relat ive uplift of the AMR. Change in the nature of the western marginal fault, f rom thrust to normal type (?); extrusion of Malani rhyolite and initiation of the Vindhyan sedimentat ion to the west of the A M R , ca. 0.8-0.7 Ga; continuing Vindhyan sedimenta t ion to the east of AMR; a minor phase of post -Vindhyan uplift and compression in the A M R leading to the present geological setting (Fig. 3F)

In conclusion, the present study indicates that the tectonic deve lopment of the A M R is characterized by the format ion and inversion of rift basins in early and mid-Proterozoic times. The Aravall i (ca. 2.5-2.0 Ga) and the Delhi (ca. 1.9-1.6 Ga) sequences were deposited in rift basins. Whereas the Aravall i rift was mainly passive, the Delhi rift can be classified as an active rift. Associated ultramafic/mafic igneous rocks have both continental and oceanic tholeiitic geochemical characteristics. They are comparab le with Phanerozoic, rift- related magmat ic products. A critical examinat ion of available data showed no conclusive evidence for substantial ophiolites/oceanic l i thosphere and intra- oceanic island-arc/active margin igneous rocks being present in the AMR. The A M R evolved as a consequence of the inversion of the Aravall i and the Delhi rift basins during the Delhi orogenesis (ca. 1.6-1.4 Ga), which included basement reactivation, thickening of the crust, deformat ion and me tamorph i sm of sedimentary sequences followed by rapid exhumat ion and uplift of the AMR. Local HP me tamorph i sm may be explained by rapid burial and exhumat ion of thin and cool continental lithosphere. The Late Proterozoic Vindhyan sequences to the east and west of the A M R represent late-orogenic sediments in a foreland basin setting. The A M R can therefore be regarded as an example of a mid- to late-Proterozoic, intra-continental collisional orogen.

Acknowledgements This work was based on several years of the first author's field and laboratory studies supported by Vikram University, Ujjain (India). It is a pleasure to acknowledge the stimulating discussions held with many colleagues and students during the progress of the work. Thanks for reviews and sugges- tions are due to A. B. Roy (Udaipur) and W. Chr. Dullo (Kiel), which helped in the improvement of the manuscript. The German Academic Exchange Service (DAAD) is acknowledged for sup- porting the first author as a post-doctoral fellow at Heidelberg where the work was finalized.

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Documents

Tectonic evolution of the Aravalli orogen (NW India): an inverted Proterozoic rift basin?