Tectonic Evolution of the Southern Part of Aravalli Mountain Belt and its Environs: Possible Causes and Time Constraints

Gondwana Research, b! 3, No. 2, p p . 175-187. 02000 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Tectonic Evolution of the Southern Part of Aravalli Mountain Belt and its Environs: Possible Causes and Time Constraints Manish A. Mamtanil, R.V. Karanth2,S.S. Merh2and R.O. Greiling3 Department of Geology and Geophysics, Indian lnstitute of Technology, Kharagpur-721302, West Bengal, India. E-mail: [email protected] Faculty of Science, M.S. University of Baroda, Vadodara-390002, Gujarat, India Geologisch-PalaontologischesInstitut, Ruprecht-Karls-Universitat Heidelberg, INF-234, D-69120 Heidelberg, Germany

’

(Manuscript received June 16,1999; accepted November 3,1999)

Abstract Structural studies on Proterozoic rocks belonging to the Lunavada Group, Southern Aravalli Mountain Bclt (SAMB), India, have shown that they underwent three episodes of deformation which have led to the formation of various regional scale interference patterns. Whilst the northern parts of the SAMB underwent brittle-ductile deformation, the southern portion underwent more ductile deformation. On the basis of structural as well as metamorphic studies it has been established earlier that the region was subjected to uplift orogenesis during its evolutionary history. In the prescnt paper an attempt is made to visualize the possible causes that led to deformation of the SAMB, the structural geology of which has been established by the authors, and to constraint the timing of these events on the basis of already available geochronological data. A “worhng-hypothesis” is proposed according to which it is suggested that deformation of the SAMB is a result of the accretion of the three protocontinents viz. Aravalli, Dharwar and Singhbhum during the Mesoproterozoic. It is envisaged that the accretion of Aravalli and Singhbhum Protocontinents occurred between 1600 and 1400 Ma along the NE-SW trending Son Suture and this event led to development of NE-SW trending structures in the SAMB. Suturing of Aravalli and Dharwar Protocontinents between 1400 and 935 Ma along the E-W Narmada Suture was responsible for the E-W to NW-SE trending D, structures of the SAMB. It is postulated that the Satpura orogeny which resulted in deformation of rocks in Satpura mountain range lying to the south of Narmada Suture was cocval with the accretion of Aravalli and Dharwar Protocontinents.

Key words : Protocontinents, Aravalli Mountain Belt, deformation, orogeny, Satpura.

Introduction The Precambrian rocks of the Aravalli Mountain Belt (AMB), northwestern India, have been subjected to two major orogenic events during the Proterozoic period Aravalli and Delhi orogenies. Causes which led to these orogenies, however, remain largely unclear. To the south of AMB lies the Satpura mountain range (Fig. l),the rocks of which deformed during the Satpura orogeny in the Mesoproterozoic times (Naqvi and Rogers, 1987). The southern tip of the Aravalli Mountain Belt referred here as the Southern Aravalli Mountain Belt (SAMB) which lies in the western Indian states of southern Rajasthan, Gujarat and partly in Madhya Pradesh has proximity to the Satpura range (Fig. 1). It is likely that the Satpura

orogeny played an important role in the tectonics of the SAMB and in the present paper this tectonic aspect of the SAMB is discussed in the regional context. It is now well accepted that, during the Mesoproterozoic, the Indian continental crust grew by the accretion of several cratons which form three major protocontinents viz. Aravalli, Dharwar and Singhbhum (Naqvi et al., 1974; Rogers, 1986; Radhakrishna and Naqvi, 1986; Naqvi and Rogers, 1987). The accretion of Aravalli-Dharwar, Aravalli-Singhbhum and Dharwar-Singhbhum Protocontinents occurred along the Y-shaped Narmada, Son and Godavari lineaments respectively (Fig. 2). It is envisaged that the impact of this accretionary event must have affected the southern part of Aravalli Protocontinent too and the various structural and

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rock

metamorphic features of the area investigated point to the results of this impact.

Regional Geological Framework The Aravalli Mountain Belt (AMB) in northwestern India contains two major Proterozoic fold belts viz. Aravalli Fold Belt (AFB) and Delhi Fold Belt (DFB) which comprise the Aravalli Supergroup and Delhi Supergroup respectively. The two Proterozoic fold belts rest over an Archean basement which has been referred to as Banded Gneissic Complex (BGC), the Bhilwara Supergroup (Gupta et al., 1980, 1992, 1995) or Mewar Gneissic Complex (Roy and Kroner, 1996). The divergent views on the question of nomenclature of the basement ‘ensemble’, arise out of lack of unanimity on the basement-cover relationship and consequent differences in the distribution

Fig. 1. Generalized geological map of India. A=Ahmedabad, BA=Bangalore, BO=Bombay (Mumbai), C=Calcutta, D=Delhi, Ga.R=Ganga River, Go.R= Godavari River, HY=Hyderabad, Ma.R=Mahanadi River, N=Nagpur, Na.R=Narmada River, So.R=Son River, SAMB= Southern Aravalli Mountain Belt (after Naqvi a n d Rogers, 1987).

of lithologies on map. The Archean basement includes composite gneiss-granite-amphibolite-metasediment assemblages which evolved during the second billion of Earth’s history (Gopalan et al., 1990; Roy and Kroner, 1994; Wiedenbeck et al., 1996). The tectonic evolution of the AMB and the orogenic belts therein have been attributed to the Proterozoic Wilson cycles (Sinha-Roy, 1988), ensialic orogenesis (Roy, 1990; Sharma, 1995) or due to inversion tectonics (Verma and Greiling, 1995). The concept of protoplate tectonics has been envisaged by Sychanthavong and Desai (1977), Sychanthavong and Merh (1981,1985) and Sychanthavong (1990) to explain the deformational patterns observed in the southern parts of the DFB. Considerable research has been carried out on the structural evolution of central and the northern parts of the AMB and detailed information is available from the studies of a number of previous workers (e.g. Gondwana Research, V. 3, No. 2, 2000

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Fig. 2. Tectonic map of India showing the distribution of Archean Proterozoic regions and the three Protocontinents viz. Dharwar, Aravalli and Singhbhum whicli merge along the Y-shaped NarmadaSon-Godavari lineament (Proterozoic suture) (simplified from Naqvi et al., 1974).

Biswal, 1988; Das, 1988; Gangopadhyay, 1972; Naha and Halyburton, 1974; Naha et al., 1984, 1988; Ray, 1974; Roy, 1978, 1995; Roy and Jain, 1974; Roy and Nagori, 1990; Royet al., 1971,1980,1981; Sinha-Royet al., 1998; Sychanthavong, 1990; Sychanthavong and Desai, 1977; Sychanthavong and Merh,1981). On the basis of these earlier studies, it has been established that the rocks of the Aravalli and Delhi Supergroups have undergone four and three episodes of deformation respectively. Table 1 summarizes the general trends of various fold episodes observed in the Aravalli and Delhi Supergroup in the central and northern parts of the AMB. In contrast to the widely studied northern and central parts of the AMB the southern parts have not been investigated in detail. Consequently, the structural evolution of the southern parts of AMB (referred here for the purpose of description as SAMB, i.e., Southern Aravalli Mountain Belt) has Gondwana Research, V. 3, No.2, 2000

remained a mystery. The SAMB occupies an area of more than 30,000 km2 in the western part of India (Fig. 3). Two younger groups of the Aravalli Supergroup viz. Lunavada and Champaner Groups (Gupta et al., 1980, 1992, 1995) comprise most of this region. An area of Table 1. Age and trend of various deformation events in the northern and central parts of the Aravalli Mountain Belt (based on Sychanthavong, 1990). AFB is Aravalli Fold Belt and DFB is Delhi Fold Belt. Major Deformation

Aravalli Folding First Delhi Folding Second Delhi Folding Third Delhi Folding

Folding in AFB

Folding in DFB

AF, AF2

AF, AF4

DF, DF, DF,

Trends of folds E-W NNE-SSW NNE-SSW E-W to WNW-ESE

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approximately 3500 km2 is occupied by granite and gneiss (Figs. 3 and 4). The granite (Godhra Granite) has been dated as 955ri: 20 Ma old (Gopalan et al., 1979). These granitic rocks show an intrusive relationship with the rocks of the Lunavada and Champaner Groups. The gneisses of the SAMB have not yet been classified. Although, Gupta and Mukherjee (1938) considered these gneisses to be younger to the BGC, the possibility of some of the gneisses being the basement rocks cannot be ruled out. This indicates the need for a detailed research on the southern parts of the Aravallis. Moreover, due to proximity of the SAMB to the present-day E-W trending Narmada lineament, a major structural feature considered to be a Proterozoic Suture zone along which the Aravalli and Dharwar Protocontinents accreted (Naqvi et al., 1974; Naqvi and Rogers, 1987), it is vital to understand the structural geology and tectonic evolution of the SAMB. To this end, structural investigations were carried out in a 900 km2area of the Proterozoic Lunavada Group which

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occurs around Lunavada and Santrampur, Gujarat (western India). This group comprises sequence of metapelitic and quartzitic layers with bands of calcsilicates. Structural history of the rocks has been described elsewhere (Mamtani, 1998; Mamtani et al., 1998; 1999a, b). In the present paper, the deformational history of the rocks is briefly summarized with major emphasis being on the likely regional causes that led to the evolution of the SAMB. We would like to mention here that the Jharol Group that comprises metasediments similar to the Lunavada Group has been classified as a deep sea facies sequence (Roy et al., 1981; Roy, 1991). It is quite possible that the Lunavada Group might also represent a similar sequence. However, the stratigraphic position of the Lunavada Group to this date remains unresolved (Merh, 1995; Sinha-Roy et al., 1998). The same is also true for the Champaner Group. Roy (1988) has stated that the deformation pattern observed in the Champaners is simple compared to rocks

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rocks)

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Fig. 3. Generalized geological map of the Aravalli Mountain Belt. Box shows the location of Fig. 4. SAMB is Southern Aravalli Mountain Belt. Inset: AMB is Aravalli Mountain Belt (prepared on the basis of GSI maps of 1969, 1980, 1992).

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India

Gujarat

Q

-1Alluvium ~

~

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Deccan Traps W M G o d h r a Granite and Gneiss m C h a m p a n e r Group F .T .L. u. n. a v a d a Group

......

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I R a k h a b d e v Ultramafic Suite

ARAVALLI SUPERGROUP

Fig. 4. Lithostratigraphic map of the SAMB (after Gupta et al., 1980, 1992, 1995). L is Lunavada and G is Godhra.

of the Aravalli or Delhi Supergroup and hence could be younger to the Delhi Supergroup. However, discussion about the stratigraphic position of these groups is beyond the scope of the present paper. Here we have restricted our interpretations to the ages of deformation and related these to various regional tectonic events that are the likely causes of deformation during the Mesoproterozoic.

Deformational History Field studies, structural mapping and structural analysis of field data have revealed that the rocks around Lunavada and Santrampur have undergone three episodes of deformation - D,, D, and D,. The polydeformed nature of the rocks is clearly seen on the geological map of the area (Fig. 5) and tectonic map of the SAMB (Fig. 6) both of which show presence of regional scale superposed folds (Mamtani, 1998; Mamtani et al., 1998; 1999b). The D, and D, folds are coaxial, both having NE-SW trending fold axes which have resulted in the formation of a TypeI11 interference pattern (Ramsay and Huber, 1987) that is clearly shown on the map-scale in the region lying to Gondwana Research, V. 3,No. 2,2000

the south of Lunavada and Santrampur (Fig. 5). A stretching lineation (mineral lineation) plunging gently to the NNW has developed on S,. This mineral lineation is deformed by D, kinks, thus indicating their development during D,. The high angle relationship between the mineral lineation and D, fold axis implies that shearing occurred along with shortening during D,. This fact is also supported by the fact that degree of overturning of D,-D, folds in the Lunavada region increases from N to S; in the N (around Ditwas) folds are upright with axial plane fractures and cleavages being near vertical or steeply dipping whereas the axial planes dip towards the NW in the southern parts with a vergence d u e SE. Microscopically, crenulation cleavages (S, foliations) developed in the schistose rocks on account of microfolding of S, during D, (Mamtani and Karanth, 1996a, 1997, 1999a). D, that followed D, resulted in development of open folds with the axis trending between E-W to NW-SE. Except for some mesoscopic scale northwesterly plunging fold axis lineation and WNW trending D, kinks, the D, folds superposed on regional scale limbs of large scale D,-D, folds. On account of this

W I S H A. MAMTANI ET AL.

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superimposition, macroscopic Type-I interference pattern (Ramsay and Huber, 1987) has developed in the Lunavada region and this is well reflected in the central parts of the geological map (Fig. 5). This superposition has brought about a variation in direction of plunge of the D,-D, fold axes besides deforming the mineral lineation on S, mentioned above. The orientations of this deformation

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Fig. 5. Geological map of the study area

have also been worked out on the basis of stereographic analysis and Anisotropy of Magnetic Susceptibility ( A M S ) studies (Mamtani et al., 1999b). The structural details of the study area are presented in Figs. 7, 8 and 9. For the purposes of stereographic analysis, the study area was divided into a number of structural domains. The ndiagrams obtained from the analysis of structural data Gondwana Research, V. 3, No.2,2000

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Fig. 6. Tectonic map of the SAMB drawn from satellite imagery (Path31, Row-51 and Path-32, Row-52 of Indian Remote Sensing Satellite). Axial traces of various folds are also shown. W

from the various domains are shown in figure 8. The scarcity of data points is due to lack of exposures in the field. Despite this, the data set provides definite maxima in most of the domains which have helped in determining the orientations of fold axis in each domain. A comparison of the structural history of parts of southern Aravalli region (presented above) with that of the central and northern Aravalli rocks indicates a certain degree of similarity especially with the fold history of the South Delhi fold belt studied by Sychanthavong (1990) and rocks of the Aravalli Fold Belt studied by Naha and Halyburton (1974) and Naha et al. (1984). Table 1shows the general trends of various fold episodes in the Aravalli Fold Belt (AFB) and Delhi Fold Belt (DFB). It is observed that there is a similarity in trends of the three fold events of Lunavada region with AF,-AF, folds in the AFB and DF,-DF, folds in the DFB. Petrographic studies of the schistose rocks indicate a variation in metamorphic grade from greenschist (in northern part) to lower amphibolite facies (in southern part) in different parts of the study area. Schists in the northern parts (i.e., between Kadana and Ditwas) are chlorite schists. These grade into biotite schists to the south of Kadana and become garnet-biotite schists to the south Gondwana Research, X 3, No. 2,2000

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Fig. 7. Structural map of the study area. M.R is Mahi river and PR is Panam river.

of Lunavada and Santrampur. Moreover, as shown in Fig. 6, the northern parts comprise a highly tectonized zone characterized by closely spaced NE-SW trending lineaments and fractures. In contrast, the southern part of the region is characterized by megascopic folds and unlike the northern part, it is marked by absence of closely spaced fractures. The metapelites here belong to the "garnet-grade". All these evidences imply that the northern part of the region underwent a more brittle-ductile deformation and comprises upper (cooler) crust while the southern part of the region represents a deeper (hotter) crust which deformed in a more ductile manner. These evidences along with geothermobarometric data and microthermometry have led Bakker and Mamtani (1999) and Mamtani et al. (1999c, d) to suggest that the rocks of the SAMB have undergone exhumation. The regional metamorphism that accompanied D,-D, is superimposed by a thermal event related to emplacement of the Godhra Granite. This has given rise to considerable static recrystallization and grain coarsening (annealing),

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N

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Fig. 8. Map showing the distribution of structural domains and n-diagrams (stereograms) for each domain are also shown. Domain I: n = 9 So, S,, S;, Contours = 10, IS, 20, 25, 30, 35% per unit area. Domain 11: n = 46 So, S,, S,; Contours = 2, 9, 13, 16, 23, 30% per unit area. Domain 111: n = 20 So; Contours = 5, 13, 22, 30, 38, 47% per unit area. Domain Iv: n = 16 So, S,; Contours = 6, 13, 19, 25, 31, 38% per unit area. DomainV n = 19 So, S,, S;, Contours = 5, 13, 21, 29, 37,45% per unit area. DomainVI: n = 18 So, S,; Contours = 6,10, 15, 19,24,29% per unit area. DomainVII: n = 67 So, S,; Contours = 1,9, 16, 23, 30, 38% per unit area. Domain VIII: n = 11 S,, S,; Contours = 9, 12, 15, 18, 21, 24% per unit area. Domain I X n = 26 So, S,; Contours = 4, 7, 10, 13, 17, 20% per unit area. Domain X n = 36 So, S,; Contours = 3, 7, 12, 17, 21, 26% per unit area. Domain XI: n = 38 So, S,; Contours = 3, 4, 6, 8, 10, 11%per unit area. Stereogram for Domain XI1 was not prepared due to absence of sufficient data from this part of the study area. However, in this domain, there is presence of recumbent D, folds at the banks of river Mahi and horizontal bedding planes are also recorded at a few localities (see Fig. 7). On account of superposition of the D, on D,-D, folds, the n - diagram for Domain VIII shows a spread of maxima in the E-W direction as well as N-S direction due to which a definite n-girdle could not be drawn.

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fl

fl

FI (Plunging) FZ (Plunging)

73140'

73145'

73:w

Fig. 9. Map showing orientations of statistically determined fold axis and axial planes on the basis of structural analysis.

especially in the rocks close to the granite contact, due to heat supplied by the granite (Mamtani and Karanth, 1996b; Mamtani et al., 1999d).

Regional Tectonic Scenario-Possible Causes and Timing Having described the deformational history of a part of the SAMB, we now attempt to extend the structural interpretations further south with a view to understand better the overall Proterozoic tectonics of the Indian shield especially in context of the postulation that the Indian continental crust grew through the welding of three major Protocontinents viz. Aravalli, Dharwar and Singhbhum during the Mesoproterozoic. Although the conclusions arrived at and discussed below may appear to have a certain degree of speculative interpretation, in absence of detailed published geochronological and tectonic data, the events described best explain the regional Proterozoic Gondwana Research, V. 3 , No. 2,2000

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tectonics of the SAMB and deserve serious consideration and deliberation. Radhakrishna and Naqvi (1986) stated that from 35002000 Ma (a total period of 1500 Ma) the greater part of Peninsular India witnessed successive E-W compression events which affected and produced identical fold patterns in the older and younger greenstone belts, in the gneisses and rocks of the Eastern Ghats belt and in Cuddapah basin (Fig. 1). In all these belts, fold axes trend broadly in N-S direction and are cut by the E-W striking Satpura structures which indicate that during Satpura orogeny the compression direction changed from E-W to N-S. In the Aravalli Protocontinent, N-S trending folds (>2000 Ma old) which pre-date the E-W (AF,) folding are not recognizable. On the contrary, in light of inferences of Sychanthavong (1990), the shortening direction in the AMB from 2500-2060 Ma was N-S which gave rise to EWAF, folds (see Table l for trends). However, after 2060 Ma, the shortening directions in the Aravalli Protocontinent changed continually - first to NW-SE to give rise to NE-SW trending D,-D, folds in the SAMB and then to N-S once again so as to form the E-W trending D, folds in SAMB. The E-W Narmada Suture shows a broad parallelism to the trend of the D, structures recorded in rocks of the Lunavada region. This trend is also parallel to the Satpura orogen. Moreover, the elongation of the granite body lying to the west of Lunavada rocks is NWSE (Fig. 4) which is also the trend of few D3 axial plane fractures and axial traces (Figs. 5 and 6). The contact of Godhra Granite with the western part of Lunavada Group also trends NW-SE (Fig. 4). To the south occur gneissic rocks and the contact of these gneisses with the rocks of Lunavada Group has E-W orientation, a trend which is parallel not only to many of the D, axial traces (Fig. 6) but also to the Narmada Suture along which the Aravalli and Dharwar Protocontinents accreted and also parallel to the Satpura trend. This implies that either during the intrusion of Godhra Granite the D, shortening was active or the intrusion occurred along pre-existing regional structural elements which developed during D,. Moreover, the Champaner Group lying to the south of Lunavada Group in the SAMB is reported to have open to tight E-W trending folds (Merh, 1995) and absence of superposed folds, unlike that in the Lunavada region. This would imply that this late E-W folding of the Champaners developed due to the compressive stresses which led to D, deformation in the Lunavada region. On the basis of the above features, we are inclined to infer a coeval relationship between accretion of the Aravalli and Dharwar Protocontinents along Narmada Suture during the Mesoproterozoic, deformation of the SAMB, esp. the third deformation and the development of E-W structures in the Satpura region.

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In light of the above inferences, it is worthwhile to discuss the timing of the accretion of protocontinents and deformation of the SAMB. As mentioned earlier, the accretionary event occurred during the Mesoproterozoic i.e., between 1600 Ma and 900 Ma (ref. cit.). It was discussed above that the Godhra Granite occurring in the SAMB which is a late D, to post-D, intrusion has been dated as 955+ 20 Ma old (Gopalan et a1.,1979). This not only puts a constraint on the age of D, but also on the welding of the two protocontinents mentioned above. Thus, the accretion event can be considered to be at least older than 935 Ma. The problem of the upper age limit of the event however still remains unresolved. Geochronological data from the Satpura region is scarce. Naqvi and Rogers (1987) have stated that the Satpura orogeny occurred sometime during the Mesoproterozoic and also that the time relationship between Vindhyan sedimentation and Satpura orogeny is uncertain. Adding to the confusion is the fact that the age of Vindhyans is not yet fully understood. Whilst Misra (1969) has suggested that the Vindhyans have an age of 1400-900 Ma (also see Naqvi and Rogers, 1987, p. 129), Crawford and Compston (1970) put a younger age limit at 550 Ma (also see Naqvi and Rogers, 1987, p. 186). Moreover, Azmi (1998) has suggested an early Cambrian age for the Lower Vindhyans. However, this has led to considerable debate and Banerjee and Mazumdar (1999) suggest that the major part of Lower Vindhyans belongs t o the Mesoproterozoic. There is evidence of some deformation in the southern part of the Vindhyans which is believed by Naqvi and Rogers (1987, p. 129) to reflect the Satpura orogeny, thus implying that the latter could be younger than 1400 Ma. Considering all the above evidence, we envisage that the suturing of Dharwar and Aravalli

Protocontinents, D, deformation in SAMB and Satpura orogeny all occurred between 1400 Ma and 935 Ma. The problem of the age of D,-D, folds in the SAMB is more complex. As mentioned earlier, the D,-D, folds in the region trend NE-SW, and therefore, the shortening direction must have been NW-SE. From Fig. 2, it is observed that the accretion of Aravalli-Singhbhum Protocontinents which occurred along a NE-SW trending Son lineament (Suture) could have generated NW-SE directed stresses which resulted in NE-SW trending D,D, folds. It is envisaged that although accretion of the three Indian protocontinents occurred during the Mesoproterozoic, it might not have been a single and simultaneous event and there is a strong possibility of a time-gap between the accretion of Singhbhum-Aravalli Protocontinents and Aravalli-Dharwar Protocontinents; the former occurred sometime in the earliest part of the Mesoproterozoic, thus giving rise to NE-SW trending D,D, folds in the SAMB and this was followed by accretion of Dharwar-Aravalli Protocontinents which resulted in EW to NW-SE D, folds. The Singhbhum Protocontinent comprises two major units viz. Singhbhum orogenic belt and Chotanagpur belt and it has been suggested by Sarkar (1982; also see Naqvi and Rogers, 1987, p. 147) that the Singhbhum orogenic belt formed by collision between Singhbhum and Chotanagpur blocks around 1600 Ma. Therefore, it is logical to interpret that the collision and suturing of Singhbhum Protocontinent (as a single unit) with Aravalli Protocontinent along NE-SW Son Suture occurred after 1600 Ma, possibly between 1600 and 1400 Ma to give rise to NE-SW trending D,-D, structures in the SAMB. Based on the above discussion, the existing knowledge of tectonic events in the SAMB can be represented as in Tables 2 and 3.

Table 2. Chronological sequence of regional tectonic events in the vicinity of the Aravalli Protocontinent that have a bearing on the deformation of the SAMB during the Mesoproterozoic. Renional Tectonic Event 1. Collision of Singhbhum and Chotanagpur Orogenic Belts 2. Singhbhum Protocontinent becomes a single unit 3. Formation of Son Suture by collision of Singhbhum and Aravalli Protocontinents 4. Formation of Narmada Suture by collision of Aravalli and Dhanvar Protocontinents 5. Satpura Orogeny 6. Deformation of southern parts of Vindhyans 7. Intrusion of Godhra Granite in the SAMB

1 1

Timinrr -1600 Ma 1600-1400 Ma

1400-935 Ma 9551t 20 Ma

Table 3. Synthesis of deformation events and their ages inferred in rocks of the Southern AravaIli Mountain Belt (SAMB) Deformation Event

Fold Axis trend in Lunavada Group

D, and D, (coaxial)

NE-SW Varies between E-W and NW-SE

Fold axis trend in Champaner Group

Varies between E-W and WNW-ESE

Probable Causes

Possible timing of deformation

Accretion of Aravalli and Singhbhum Protocontinents

1600-1400Ma

Accretion of Aravalli and Dhanvar Protocontinents

1400-935 Ma

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TECTONIC EVOLUTION OF SOUTHERN ARAVALLI

Concluding Remarks The present study provides information about the structural aspects of a part of the SAMB which could be considered as a base for further studies on the region and its neighbouring areas. The following conclusions are drawn regarding the structural geology of the Lunavada area and regional tectonics of the SAMB: 1. Rocks of the Lunavada region have undergone three episodes of deformation - D,, D, and D, and the superposition of these three events gave rise to regional scale interference patterns. D, and D, resulted in development of NE-SW trending folds and Type-I11 interference pattern on regional scale. D, resulted in folds having axial trends varying between E-W and NWSE; the variations in the orientations being on account of variations in local shortening directions in different parts of the area. These folds superposed on km scale limbs of D,-D, folds resulted in the formation of TypeI interference pattern on a regional scale. 2. There is general similarity in trends of the D,, D, and D, folds of Lunavada region to the trends of AF,, AF,, AF, in the Aravalli Fold Belt and also to DF,, DF, and DF, in the Delhi Fold Belt. 3. Regional metamorphism in rocks of Lunavada region progressed upto lower amphibolite facies. The metapelites in the northern, central and southern parts of the Lunavada region are chlorite schists, biotite schists and garnet-biotite schists respectively. This increase in grade of metamorphism from north to south indicates exhumation which occurred during the tectonic evolution of the SAMB. 4. An important aspect is the presence of lower grade rocks (chlorite schists) in Kadana-Ditwas region adjacent to the higher grade rocks of the Banswara region lying to the northeast of Kadana-Ditwas area. Sharma (1988) has shown that in the Banswara region, the rocks have metamorphosed upto amphibolite facies with the grade of metamorphism increasing from northwest to southeast (see Fig. 5 in Sharma, 1988). To the south of Banswara the grade of metamorphism first decreases (around Ditwas and Kadana) and then again increases further south as shown above. This implies that there might exist a tectonic boundary between Ditwas and Banswara region. 5. The process of accretion of three protocontinents Aravalli, Singhbhum and Dharwar along the Y-shaped Narmada-Son-Godavari lineament during the Mesoproterozoic must have played an important role in the tectonic evolution of the SAMB. 6. Welding of Aravalli and Singhbhum Protocontinents probably occurred between 1600 and 1400 Ma and led to D,-D, events in the SAMB. Gondzuana Research, I.!3, No.2,2000

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7. The accretion of Aravalli and Dharwar Protocontinents appears to be responsible for D, in the SAMB and this event occurred between 1400 and 935 Ma. 8. The Satpura orogeny occurred simultaneously with D, deformation in the SAMB and was also a result of the suturing of the Aravalli and Dharwar Protocontinents. We are fully aware that unraveling the complexities of a tectonic problem of a magnitude such as the present one has its limitations and some postulations and explanations fall within the realm of speculation; several questions may remain unanswered. Our attempt, therefore, to interpret the possible tectonic scenario that led to the complex structural evolution of the SAMB and its surroundings has to be accordingly viewed. As stated earlier, there is lack of published data on the southern Aravallis and most researchers have so far concentrated to understand the tectonics of the central and northern parts of the AMB. Geochronological studies on rocks of the region are also scarce. As far as the similarities of trends are concerned it appears that D, deformation of the Lunavada region, Narmada suture, structures in the northern Satpura region and those in southern parts of Vindhyans might have occurred due to the same regional deformation which we believe might be related to the accretion of Dharwar and Aravalli Protocontinents during the Mesoproterozoic. Similarly, the D,-D, deformation of the region could be correlated to the accretion of Singhbhum and Aravalli Protocontinents. However, the precise timing of the events still remains to be a part of the puzzle that needs special attention. The time constraints proposed in the present paper should therefore be considered as “tentative” and the whole concept should be used as a “working model” or a “working-hypothesis” which could be proved or disproved on the basis of detailed further work. This certainly calls for carrying out detailed tectonic and geochronological studies on rocks of the SAMB and its environs.

Acknowledgments The present paper is an outcome of M.A.M’s doctoral and post-doctoral research. The doctoral work was financially supported during various stages by M.S. University Research Scholarship, fieldwork grant from the Association of Geoscientists for International Development (A.G.I.D, Brazil), Senior Research Fellowship from the Council of Scientific and Industrial Research (CSIR, New Delhi; No. 9/114/(92)/96/EMR-I) and German Academic Exchange Service, (DAAD, Bonn; No. A/97/00792) all awarded to M.A.M. The post-doctoral research on the Southern Aravallis was supported by the Extended-SRF awarded to M.A.M by the CSIR (No. 9/114(107)/99/

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EMR-I). Reviews by Prof. A.B. Roy and Dr. T.K. Biswal

were found useful.

References Azmi, R.J. (1998) Discovery of Lower Cambrian small shelly fossils and brachipods from the Lower Vindhyan of Son Valley, Central India. J. Geol. SOC.India, v. 52, pp. 381-390. Bakker, R.J. and Mamtani, M.A. (1999) Fluid inclusions as postpeak metamorphic process indicators in the southern Aravalli mountain belt (India). Contrib. Mineral. Petrol. (accepted for publication). Banerjee, D.M. and Mazumdar, A. (1999) On the late Neoproterozoic-early Cambrian transition events in parts of East Gondwanaland. J. Gond. Res., v. 2, pp. 199-211. Biswal, T.K. (1988) Polyphase deformation in Delhi rocks, southeast of Amirgad, Banaskantha district of Gujarat. In: Roy, A.B. (Ed.), Precambrian of the Aravalli mountain, Rajasthan, India. Geol. SOC.India, Mem. No. 7, pp. 267-277. Crawford, A.R. and Compston, W. (1970) The age ofvindhyan system of peninsular India. J. Geol. SOC.London, v. 125, pp. 351-371. Das, A.R. (1988) Geometry of the superposed deformation in the Delhi Supergroup of rocks, north of Jaipur, Rajasthan, In: Roy, A.B. (Ed.), Precambrian of the Aravalli mountain, Rajasthan, India. Geol. SOC.India, Mem. No. 7, pp. 247-266. Gangopadhyay, P.K. (1972) Structure and tectonics of Alwar region in northeastern Rajasthan, India, with special reference to Precambrian stratigraphy. Proceedings of International Geological Congress, Section 1, pp. 118-125. Gopalan, K., Macdougall, J.D., Roy, A.B. and Murali, A.V. (1990) Sm-Nd evidence for 3 . 3 Ga old rocks in Rajasthan, northwestern India. Precamb. Res., v. 48, pp. 287-297. Gopalan, K., Trivedi, J.R., Merh, S.S., Patel, PP and Patel, S.G. (1979) Rb-Sr age of Godhra and related granites, Gujarat (India). Proc. Ind. Acad. Sci. (Earth Planet. Sci.), v. 88A, pp. 7-17. Gupta, B.C. and Mukherjee, P.N. (1938) The geology of Gujarat and South Rajputana. Rec. Geol. Surv. India, v. 73, pp. 163-208. Gupta, S.N., Arora, YK.,Mathur, R.K., Iqbaluddin, Balmiki Prasad, Sahai, T.N. and Sharma, S.B. (1980) Lithostratigraphic map of Aravalli region, southern Rajasthan and northeastern Gujarat. Geol. Surv. India Publication, Hyderabad. Gupta, S.N., Mathur, R.K. a n d Arora, Y.K. (1992) Lithostratigraphy of Proterozoic rocks of Rajasthan and Gujarat - A review. Rec. Geol. Surv. India, v. 115, pp. 63-85. Gupta, S.N., Arora, Y.K.,Mathur, R.K., Iqbaluddin, Balmiki Prasad, Sahai, T.N. and Sharma, S.B. (1995) Geologicalmap of the Precambrians of the Aravalli region, southern Rajasthan and northeastern Gujarat, India. Geol. Surv. India Publication. Iqbaluddin. (1989) Geology of Kadana Reservoir area, Panchmahals district, Gujarat and Banswara and Dungarpur districts, Rajasthan. Geol. Surv. India, Mem. No. 121, 84 p. Mamtani, M.A. (1998) Deformational mechanisms of the Lunavada Pre-Cambrian rocks, Panchmahal district, Gujarat. Unpub. Ph.D. thesis, M.S. University of Baroda, 268 p.

Mamtani, M.A. and Karanth, R.V. (1996a) Microstructural evidence for the formation of crenulation cleavage in rocks. Curr. Sci., v. 71, pp. 237-240. Mamtani, M.A. and Karanth, R.V. (1996b) Effect of heat on crystal size distributions of quartz. Curr. Sci., v. 70, pp. 396-399. Mamtani, M.A. and Karanth, R.V. (1997) Syntectonic growth of porphyroblasts over crenulation cleavages - an example from the Precambrian rocks of the Lunavada Group, Gujarat. J. Geol. SOC. India, v. 50, pp. 171-178. Mamtani, M.A., Karanth, R.V., Merh, S.S. and Greiling, R.O. (1998) Regional scale superposed folding in the Precambrian rocks of the southern Aravalli mountain belt, India. (Abstract) Symposium TSK-7, Freiberger Forschungsheft, v. C471, pp. 141-142. Mamtani, M.A., Karanth, R.V. and Greiling, R.O. (1999a) Are crenulation cleavage zones mylonites on the microscale? J. Struct. Geol., v. 21, pp. 711-718. Mamtani, M.A., Greiling, R.O., Karanth, R.V. and Merh, S.S. (1999b) Orogenic deformation and its relationship to A M S fabric - an example from the southern margin of the Aravalli Mountain Belt, India. In: Radhakrishna, T. and Piper, J.D. (Eds.), The Indian subcontinent a n d Gondwana: a palaeomagnetic and rock magnetic perspective, Geol. SOC. India, Mem. No. 44, pp. 9-24. Mamtani, M.A., Greiling, R.O., Karanth, R.V. and Merh, S.S. ( 1 9 9 9 ~ )Uplift orogenesis - an example from proterozoic rocks of the Southern Aravalli Mountain Belt, India. (Ab s t r a c t ) “Exh um a t i o n of Met a m o r p hi c Te rr a n e s ” (Conference to be held at Rennes, France) (in press). Mamtani, M.A., Merh, S.S., Karanth, R.V. and Greiling, R.O. (1999d) Time relationship between metamorphism and deformation in the Proterozoic rocks of Lunavada region, Southern Aravalli Mountain Belt (India) - a microstructural study. J. Asian Earth Sci. (revised version submitted). Merh, S.S. (1995) Geology of Gujarat. Geol. SOC.India Publ., Bangalore, 224 p. Misra, R.C. (1969) The Vindhyan system. 56‘h Indian Sci. Congress Proc., Part 2, pp. 111-142. Naha, K. and Halyburton, R.V. (1974) Late stress system from conjugate folds and kink bands in the main Raialo syncline, Udaipur district, Rajasthan, India. Bull. Geol. SOC.h e r . , v. 85, pp. 251-256. Naha, K., Mukhopadhyay, D.K., Mohanty, R., Mitra, S.K. and Biswal, T.K. (1984) Significance of contrast in the early stages of the structural history of the Delhi and Pre-Delhi rock groups in the Proterozoic of Rajasthan, western India. Tectonophys., v. 105, pp. 193-206. Naha, K., Mukhopadhyay, D.K. and Mohanty, R. (1988) Structural evolution of the rocks of the Delhi Group around Khetri, northeastern Rajasthan. In: Roy, A.B. (Ed.), Precambrian of the Aravalli mountain, Rajasthan, India. Geol. SOC.India, Mem. No. 7, pp. 207-245. Naqvi, S.M. and Rogers, J.J.W. (1987) Precambrian geology of India. Oxford University Press, Inc. 223 p. Naqvi, S.M., Divakar Rao, V. and Narian, H. (1974) The Protocontinental growth of the Indian shield and the antiquity of its rift valleys. Precamb. Res., v. 1, pp. 345-398. Radhakrishna, B.P. and Naqvi, S.M. (1986) Precambrian continental crust of India and its evolution. J. Geol., v. 94, pp. 145-166. Gondwana Research, V. 3,No. 2,2000

TECTONIC EVOLUTION OF SOUTHERN ARAVALLI

Ramsay, J.G. and Huber, M.I. (1987) The techniques of modern structural geology, Folds and Fractures. Academic Press, London, v. 2, 391 p. Ray, S.K. (1974) Structural history of the Saladipura pyritepyrrhotite deposit and associated rocks, Khetri copper belt, Rajasthan. J. Geol. SOC.India, v. 15, pp. 227-238. Rogers, J.J.W. (1986) The Dharwar craton and the assembly of Peninsular India. J. Geol., v. 94, pp. 129-143. Roy, A.B. (1978) Modification of flexural folds by flattening process: examples from the Aravalli rocks of Zawar mine area, Rajasthan. J. Geol. SOC.India, v. 29, pp. 21-25. Roy, A.B. (1988) Stratigraphic and tectonic framework of the Aravalli mountain range. In: Roy, A.B. (Ed.), Precambrian of the Aravalli mountain, Rajasthan, India. Geol. SOC.India, Mem. No. 7, pp. 3-31. Roy, A.B. (1990) Evolution of the Precambrian crust of the Aravalli Mountain Range. In: Naqvi, S.M. (Ed.), Precambrian continental crust and its economic resources, developments in Precambrian Geology, Elsevier, Amsterdam, v. 8, pp. 327-347. Roy, A.B. (1991) Evolution of Early Proterozoic Aravalli Depositional Basin. In: Tandon, S.K., Pant, C.C. and Casshyap, S.M. (Eds.), Sedimentary Basins of India: Tectonic Context, pp. 1-11. Roy, A.B. (1995) Geometry and evolution of superposed folding in the Zawar lead-zinc mineralised belt, Rajasthan. Proc. Ind. Acad. Sci., v. 104, pp. 349-371. Roy, A.B. and Jain, A.K. (1974) Polyphase deformation in the Pb-Zn bearing Precambrian rocks of Zawarmala, Udaipur District, southern Rajasthan. Quat. J. Geol. Min. Metall. SOC. India, v. 46, pp. 81-86. Roy, A.B. and Kroner, A. (1996) Single zircon evaporation ages constraining the growth of the Archean Aravalli craton, northwestern Indian shield. Geol. Mag., v. 133, pp. 333-342. Roy, A.B. and Nagori, D.K. (1990) Influences of differential basement mobility on contrasting structural styles in the cover rocks: an example from early Precambrian rocks East of Udaipur. Proc. Ind. Acad. Sci. (Earth and Planet. Sci.), v. 99, pp. 291-308. Roy, A.B., Paliwal, B.S. and Goel, O.P. (1971) Superposed folding in the Aravalli rocks of the area around Udaipur, Rajasthan. J. Geol. SOC.India, v. 12, pp. 342-348. Roy, A.B., Paliwal, B.S. and Goel, O.P. (1981) Superposed folding in the Aravalli rocks of the type area around Udaipur, Rajasthan. J. Geol. SOC.India, v. 12, pp. 342-348. Roy, A.B., Nagori, D.K., Golani, P.R., Dhakkar, S.P. and Chaudhary, R. (1980) Structural geometry of rock phosphate bearing Aravalli rocks around Jhamarkotra mine area,

Gondwana Research, V. 3, No. 2,2000

187

Udaipur district, Rajasthan. Ind. J. Earth Sci., v. 7, pp. 191-202. Sarkar, A.N. (1982) Precambrian tectonic evolution of eastern India; A model of converging microplates. Tectonophys., v. 86, pp. 363-397. Sharma, R.S. (1988) Patterns of metamorphism in the Precambrian rocks of the Aravalli Mountain Belt. In: Roy, A.B. (Ed.), Precambrian of the Aravalli mountain, Rajasthan, India. Geol. SOC.India, Mem. No. 7, pp. 33-75. Sharma, R.S. (1995) An evolutionary model for the Precambrian crust of Rajasthan: some petrological and geochronological considerations. In: Sinha-Roy, S. and Gupta, K.R. (Eds.), Continental crust of northwestern and central India, Geol. SOC.India, Mem. No. 31, pp. 91-115. Sinha-Roy, S. (1988) Proterozoic Wilson cycles in Rajasthan. In: Roy, A.B. (Ed.), Precambrian of the Aravalli mountain, Rajasthan, India, Geol. SOC.India, Mem. No. 7, pp. 95-107. Sinha-Roy, S., Malhotra, G. and Mohanty, M. (1998) Geology of Rajasthan. Geol. SOC.India, Bangalore, 278 p. Sychanthavong, S.P.H. (1990) Destructive plate margin tectonics in the evolution of the Delhi Precambrian fold belt and base metal deposits, western India. In: Sychanthavong, S.P.H. (Ed.), Crustal evolution and orogeny, Oxford and IBH Publ. Co. Pvt. Ltd., New Delhi, pp. 101-158. Sychanthavong, S.P.H. and Desai, S.D. (1977) Proto-plate tectonics controlling the Precambrian deformations and metallogenetic epochs in northwestern peninsular India. Mineral Sci. Engg., v. 9, pp. 218-236. Sychanthavong, S.P.H. and Merh, S.S. (1981) Protoplate tectonics: The energetic model for t h e structural, metamorphic and igneous evolution of the Precambrian rocks, northwestern peninsular India, Proc. SYMPET, Sp. Publ. Geol. Surv. India, pp. 419-457. Sychanthavong, S.P.H. and Merh, S.S. (1985) Role of protoplate movement in the geological evolution of the Precambrian terrain of northwest peninsular India. Proc. Symposium on megastructures and plate tectonics and their role as a guide to ore mineralizations, Geol. Min. Met. SOC.India, v. 53, pp. 124-146. Verma, PK. and Greiling, R.O. (1995) Tectonic evolution of the Aravalli orogen (NW India) : an inverted Proterozoic rift basin? Geol. Rundsch., v. 84, pp. 683-696. Wiedenbeck, M., Goswami, J.N. a n d Roy, A.B. (1994) Stabilization of the Aravalli craton of northwestern India at 2.5 Ga: an ion microprobe study. Chem. Geol., v. 129, pp. 325-340.