- Email: [email protected]
Proterozoic Rifting in the Pranhita-Godavari Valley: Implication on India-Antarctica Linkage
592
RODINIA, GONDWANA AND ASIA
Devonian to Visean) and may be Trianosphaera sicarius Deflandre (Upper famenian to Visean). Thus, the melange may have an age ranging between Early and Middle Carboniferous and was probably structured by a southward deformation during this time. A late normal deformation led to the formation of normal faults reworking former structures. In Xinjiang province, eastern Tianshan suffered two main stages of northward deformation according to our new structural, chronological and field work data. First, the subduction of the North Tianshan oceanic crust under Ordovician Central Tianshan arc led to the accretion of Junggar type North Tianshan block, during Ordovician to Middle Silurian. This collision was responsible for the Early Silurian starting closure of the South Tianshan paleo-ocean. Accretion of Tarim to Central Tianshan continued until Devonian and induced the formation of large thrusts and nappes of ophiolitic melanges and gneiss in the present South Tianshan unit. Later, the Carboniferous volcanic arc on North Tianshan block, with a supposed continental basement, was formed during the subduction of the huge Junggar paleo-ocean from Late Devonian to Late Carboniferous. Cross-sections carried out in Bogeda arc show that calc-alkaline volcanic rocks are affected by a northward deformation, characterised by north-verging overturned folds and south dipping ductile-brittle thrusts in a low greenschist facies metamorphism in an upper structural level (Fig. lc). During Carboniferous, the north-eastern margin of Junggar block collided with Siberian-Outer Mongolian block inducing a southward structuration along the suture zone. Relics of the older oceanic crust were hauled on Junggar flyschs (Fig. Id). Undeformed Middle Permian sandstones lie unconformably on the schistosed Carboniferous volcanics and indicate the end of accretional events in eastern Xinjiang. Finally, the whole region was affected by two stages of Late Permian strike-slip shearing reworking former thrust structures (Laurent-Charvet et al., 2000).
References Berzin, N., Coleman, R.G., Dobretsov, N.L., Zonenshain, L.P., Xiao, X.C. and Chang, E.Z. (1994) Geodynamic map of the western part of the Paleoasian ocean. Russ. Geol. Geophys., v. 35, pp. 5-22. Coleman, R.G. (1989) Continental growth of northwest China. Tectonics, V. 8, pp. 621-635. Dobretsov, N.L., Coleman, R.G., Liou, J.G. and Maruyama, S. (1987) Blueschist belts in Asia and possible periodicity of blueschist facies metamorphism. Ofioliti, v. 12, pp. 445-456. Gao, J., He, G.Q., Li, M.S., Xiao, X.C., Tang, YQ., Wang, J. and Zhao. M. (1995) The mineralogy, petrology, metamorphic PTDt trajectory and exhumation mechanism of blueschists, south Tianshan, northwestern China. Tectonophys., v. 250, pp. 151-168. Hao, J. and Liou, X. (1993) Ophiolite melange time and tectonic evolution model in the South Tianshan area. Sci. Geol. Sinica, v. 28, pp. 93-95. Laurent-Charvet, S., Charvet, J. and Shu, L.S. (2000) Late Palaeozoic strike-slip faults around Junggar basin, Xinjiang, NW China. In: Chaves, H. (Ed.), 31th Inter. Geol. Cong., Rio de Janeiro, CD-ROM. Ma, R.S., Wang, C.Y and Ye, S.E (1993) Tectonic framework and crustal evolution of Eastern Tianshan mountains. Publishing House of Nanjing University, Nanjing, 225p. (in Chinese). Ma, R.S., Ye, S.F., Wang, C.Y. and Liu, G.B. (1990) Framework and evolution in the East Tianshan Orogenic belt. Geosci. Xinjiang, v. 2, pp. 21-36 (in Chinese with English abstract). Natal’in, B.A. and Sengor, A.M.C. (1994) The tectonic setting of the Tien Shan within the Altaid orogenic belt. In: Geol. SOC.Amer. Abstr. Program, Annual Meeting, Seattle, p. A464. Sengor, A.M.C., Natal’in, B.A. and Burtman, V.S. (1993) Evolution of the Altaid tectonic collage and Paleozoic crustal growth in Eurasia. Nature, v. 364, pp. 299-307. Shu, L.S., Shi, Y.S., Lu, H I , Charvet, J. and Laurent-Charvet, S. (1999) Paleozoic terrane tectonics in Northern Tianshan, northwestern China. In: Evenchick, C.A. et al. (Eds.), Terrane Paths 99 CircumPacific Terrane Conf., Canada, pp. 63-65. Shu, L.S., Wang, C.Y. and Ma, R.S. (1996) Granulite relics and pyroxenefacies ductile deformation in the northern boundary of the Southern Tianshan. Sci. Geol. Sinica, v. 31, pp. 63-71 (in Chinese with English abstract). Windley, B.F., Allen, M.B., Zhang, C., Zhao, Z.Y. and Wang, G.R. (1990) Paleozoic accretion and Cenozoic redeformation of the Chinese Tien Shan Range, central Asia. Geology, v. 18, pp. 128-131.
Proterozoic Rifting in the Pranhita-Godavari Valley: Implication on IndiaAntarctica Linkage Asru K. Chaudhuri and Gautam K. Deb Geological Studies Unit, Indian Statistical Institute, 203, B. T. Road, Calcutta - 700 035, India, E-mail: [email protected] The Pranhita-Godavari Valley (PG Valley) is a vast repository of Meso- and Neoproterozoic rocks in Indian Peninsula. It occurs as a NW-SE trending megalineament and can be traced for about 450 km from the NNE-SSW trending Eastern Ghats Belt in the southeast to Wardha in the northwest, where the basin-filling sedimentary rocks are covered by the Cretaceous Deccan volcanics. It has been postulated from geophysical studies that the basin extends further northwestwards under the cover of the volcanics for about 400km, and meets the Narmada-Son rift (Biswas, in press). The Proterozoic outcrops are flanked on either side by linear belts of late Archaean granulites and/or gneissic rocks. Along the southwestern margin of the Valley, the
Proterozoic sedimentaries unconformably overlie the granulites and/or gneisses, whereas along the northeastern margin the sedimentaries are delimited by a boundary fault. The Proterozoic succession is punctuated by a number of unconformities into several genetically related lithologic assemblages, designated as ‘Groups’ or ‘Subgroups’. The rocks of different assemblages exhibit signatures of deposition in widely varying environments, from continental to deep marine. Unconformities, the thickness variation of the unconformitiesbound sequenFes, and highly variable rate of deposition of clastics derived from the gneissic rocks of the basement and fluctuating bathymetry of depositional interface collectively Gondwana Research, V.4, No. 4,2001
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point to oscillatory mode of basin filling through rapid cratonic uplift accompanied by high angle faulting of the basement. 'ILvo regionally extensive unconformities (Sloss, 1984) demarcate the Proterozoic succession from the underlying Archaean basement complex, and the overlying Gondwana rocks. Tho other major unconformities divide the succession into three sequences, the Pakhal sequence, the Penganga sequence, and the Sullavai-Albaka sequence, in the ascending order. Each sequence displays a strong degree of individuality dictated by supply of detrital sediment or lack thereof. The Pakhal and Penganga sequences comprise both siliciclastics and carbonates. Each starts with alluvial conglomerates and sandstones, passes upwards into shallow marine sandstone and finally into limestone and dolomitic limestone deposited in variable bathymetry, from shallow shelf to deep-water slope and base of slope environments. The Penganga sequence terminates with a thick deep-water, basinal shale (Mukhopadhyay and Chaudhuri, in press). The Sullavai-Albaka sequence, by contrast, consists only of siliciclastics, fluvial-eolian red sandstones of the Sullavai Group, and shallow marine sandstones and shale of the Albaka Group. The radiometric date (1330rf:53 Ma; Chaudhuri and Howard, 1985) obtained from glauconitic minerals from the basal part of the Pakhal sequence indicates that Pakhal sedimentation started in early Mesoproterozoic time. Chaudhuri et al. (1999) placed the Penganga sequence in the Mesoproterozoic, and the red sandstones of the Sullavai Group in the early Neoproterozoic mainly on the lithologic - stratigraphic considerations. The stratigraphic architecture represented by alternation of coarse siliciclastics a n d carbonate rocks with major unconformities preceeding the siliciclastic depositional events, points to episodic changes in depositional base level. Scale of the unconformities, generation of vast amount of arkosic detritus, and their deposition as basin margin facies suggest major uplifts of the cratonic blocks, that could involve kilometers of vertical movement consuming time spans of millions of years (cf. Sloss, 1991) and fracturing of the crust to accommodate the detritus in rift basins. The episodic uplift, fracturing and subsidence strongly points to repeated rifting. The occurrence of rhyolites and rhyolitic tuffs interbedded with Pakhal and Penganga rocks indicates episodic partial melting of the crust, generation of felsic magma and rifting (Patranabis Deb, in press). Repeated rifting in the valley is also supported by geophysical investigations (Ramanamurthy and Parthasarathy, 1988; Biswas, in press). In the central and southern part of the outcrop belt, the thickness of the unconformities bound assemblages, as well as of individual formations therein increase towards southeast. Furthermore, the relatively shallow water and coastal facies are gradually replaced by deeper water facies towards southeast pointing to southeasterly paleoslope. The basin was connected
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to a deep marine basin with open marine circulation, and it existed in the areas now occupied by the Eastern Ghats mountain. It is well established that tectonics of a craton is influenced by the events at craton margins, and divergence at the craton margin may be instrumental in generating rift basins far inboard the continent (Sloss, 1988, 1991). The PG Valley rift(s) with typical oscillatory behaviour characterized by abrupt termination of submerging episodes through rapid cratonic uplift with high angle faulting, may be attributed to extensional activities along the Eastern Ghats Mobile Belt, or the SonMahanadi rift. The initiation of the rift accommodating the Pakhal sequence broadly coincides with the time of emplacement of sub-alkalic basalt in the Eastern Ghats at about 1450 Ma attesting to continental rifting (Shaw et al., 1997) and opening of the seaway between India and Antarctica. The PG Valley rift basin was connected to the seaway through the Mesoproterozoic and early part of the Neoproterozoic. The proposed palaeogeography does not favour linkage between India and Antarctica, or the uplift of the Eastern Ghats mountain and its amalgamation with the Indian craton till the early Neoproterozoic. The notion is also supported by the absence of any correlatable Mesoproterozoic history between the Ryner Complex and the Eastern Ghats. The exact timing of the India-Eastern Ghats-Antarctica assembly, however, can only be determined with reliable dating of the rifting events in the PG Valley.
References Biswas, S . K. Regional tectonic framework of Pranhita-Godavari basin. J. Asian Earth Sci. Spl. vol., (in press). Chaudhuri, A.K. and Howard, J.D. (1985) Ramgundam sandstone: a middle Proterozoic shoal bar sequence. 3 . Sed. Petrol., v. 55, pp. 392-397. Chaudhuri, A.K., Mukhopadhyay, J., Patranabis Deb, S., and Chanda, S.K. (1999) The Neoproterozoic successions of Peninsular India. Gondwana Res., v. 2, pp. 213-225. Mukhopadhyay, J. and Chaudhuri, A.K. Proterozoic Penganga group, Pranhita-Godavari Valley, South India: depositional setting and paleogeography of a deep-water cratonic basin succession. J. Asian Earth Sci. Spl. vol., (in press). Patranabis Deb, S . Proterozoic felsic volcanism in the Pranhita Godavari Valley, India: its implication on the origin of the basin. J. Asian Earth Sci. Spl. vol., (in press). Ramanamurthy, B.V. and Parthasarathy, E.V.R. (1988) On the evolution of the Godavari Gondwana Graben, based on LANDSAT Imagery interpretation. J. Geol. SOC.India, v. 32, pp. 417-425. Shaw, R., Arima, M., Kagami, H., Fanning, C. M., Shiraishi, K. and Motoyoshi, Y. (1997) Proterozoic events in the Eastern Ghats Granulite Belt, India: evidence from Rb-Sr, Sm-Nd systematics, and SHRIMP dating. J. Geol., v. 105, pp. 645-656. Sloss, L.L. (1984) Comparative anatomy of cratonic unconformities, AAPG Mem., No. 36, pp. 1-6. Sloss, L.L. (1991) Epilog, in, Interior cratonic Basins. AAPG Mem., NO. 51, pp. 799-805.
RODINIA, GONDWANA AND ASIA
Devonian to Visean) and may be Trianosphaera sicarius Deflandre (Upper famenian to Visean). Thus, the melange may have an age ranging between Early and Middle Carboniferous and was probably structured by a southward deformation during this time. A late normal deformation led to the formation of normal faults reworking former structures. In Xinjiang province, eastern Tianshan suffered two main stages of northward deformation according to our new structural, chronological and field work data. First, the subduction of the North Tianshan oceanic crust under Ordovician Central Tianshan arc led to the accretion of Junggar type North Tianshan block, during Ordovician to Middle Silurian. This collision was responsible for the Early Silurian starting closure of the South Tianshan paleo-ocean. Accretion of Tarim to Central Tianshan continued until Devonian and induced the formation of large thrusts and nappes of ophiolitic melanges and gneiss in the present South Tianshan unit. Later, the Carboniferous volcanic arc on North Tianshan block, with a supposed continental basement, was formed during the subduction of the huge Junggar paleo-ocean from Late Devonian to Late Carboniferous. Cross-sections carried out in Bogeda arc show that calc-alkaline volcanic rocks are affected by a northward deformation, characterised by north-verging overturned folds and south dipping ductile-brittle thrusts in a low greenschist facies metamorphism in an upper structural level (Fig. lc). During Carboniferous, the north-eastern margin of Junggar block collided with Siberian-Outer Mongolian block inducing a southward structuration along the suture zone. Relics of the older oceanic crust were hauled on Junggar flyschs (Fig. Id). Undeformed Middle Permian sandstones lie unconformably on the schistosed Carboniferous volcanics and indicate the end of accretional events in eastern Xinjiang. Finally, the whole region was affected by two stages of Late Permian strike-slip shearing reworking former thrust structures (Laurent-Charvet et al., 2000).
References Berzin, N., Coleman, R.G., Dobretsov, N.L., Zonenshain, L.P., Xiao, X.C. and Chang, E.Z. (1994) Geodynamic map of the western part of the Paleoasian ocean. Russ. Geol. Geophys., v. 35, pp. 5-22. Coleman, R.G. (1989) Continental growth of northwest China. Tectonics, V. 8, pp. 621-635. Dobretsov, N.L., Coleman, R.G., Liou, J.G. and Maruyama, S. (1987) Blueschist belts in Asia and possible periodicity of blueschist facies metamorphism. Ofioliti, v. 12, pp. 445-456. Gao, J., He, G.Q., Li, M.S., Xiao, X.C., Tang, YQ., Wang, J. and Zhao. M. (1995) The mineralogy, petrology, metamorphic PTDt trajectory and exhumation mechanism of blueschists, south Tianshan, northwestern China. Tectonophys., v. 250, pp. 151-168. Hao, J. and Liou, X. (1993) Ophiolite melange time and tectonic evolution model in the South Tianshan area. Sci. Geol. Sinica, v. 28, pp. 93-95. Laurent-Charvet, S., Charvet, J. and Shu, L.S. (2000) Late Palaeozoic strike-slip faults around Junggar basin, Xinjiang, NW China. In: Chaves, H. (Ed.), 31th Inter. Geol. Cong., Rio de Janeiro, CD-ROM. Ma, R.S., Wang, C.Y and Ye, S.E (1993) Tectonic framework and crustal evolution of Eastern Tianshan mountains. Publishing House of Nanjing University, Nanjing, 225p. (in Chinese). Ma, R.S., Ye, S.F., Wang, C.Y. and Liu, G.B. (1990) Framework and evolution in the East Tianshan Orogenic belt. Geosci. Xinjiang, v. 2, pp. 21-36 (in Chinese with English abstract). Natal’in, B.A. and Sengor, A.M.C. (1994) The tectonic setting of the Tien Shan within the Altaid orogenic belt. In: Geol. SOC.Amer. Abstr. Program, Annual Meeting, Seattle, p. A464. Sengor, A.M.C., Natal’in, B.A. and Burtman, V.S. (1993) Evolution of the Altaid tectonic collage and Paleozoic crustal growth in Eurasia. Nature, v. 364, pp. 299-307. Shu, L.S., Shi, Y.S., Lu, H I , Charvet, J. and Laurent-Charvet, S. (1999) Paleozoic terrane tectonics in Northern Tianshan, northwestern China. In: Evenchick, C.A. et al. (Eds.), Terrane Paths 99 CircumPacific Terrane Conf., Canada, pp. 63-65. Shu, L.S., Wang, C.Y. and Ma, R.S. (1996) Granulite relics and pyroxenefacies ductile deformation in the northern boundary of the Southern Tianshan. Sci. Geol. Sinica, v. 31, pp. 63-71 (in Chinese with English abstract). Windley, B.F., Allen, M.B., Zhang, C., Zhao, Z.Y. and Wang, G.R. (1990) Paleozoic accretion and Cenozoic redeformation of the Chinese Tien Shan Range, central Asia. Geology, v. 18, pp. 128-131.
Proterozoic Rifting in the Pranhita-Godavari Valley: Implication on IndiaAntarctica Linkage Asru K. Chaudhuri and Gautam K. Deb Geological Studies Unit, Indian Statistical Institute, 203, B. T. Road, Calcutta - 700 035, India, E-mail: [email protected] The Pranhita-Godavari Valley (PG Valley) is a vast repository of Meso- and Neoproterozoic rocks in Indian Peninsula. It occurs as a NW-SE trending megalineament and can be traced for about 450 km from the NNE-SSW trending Eastern Ghats Belt in the southeast to Wardha in the northwest, where the basin-filling sedimentary rocks are covered by the Cretaceous Deccan volcanics. It has been postulated from geophysical studies that the basin extends further northwestwards under the cover of the volcanics for about 400km, and meets the Narmada-Son rift (Biswas, in press). The Proterozoic outcrops are flanked on either side by linear belts of late Archaean granulites and/or gneissic rocks. Along the southwestern margin of the Valley, the
Proterozoic sedimentaries unconformably overlie the granulites and/or gneisses, whereas along the northeastern margin the sedimentaries are delimited by a boundary fault. The Proterozoic succession is punctuated by a number of unconformities into several genetically related lithologic assemblages, designated as ‘Groups’ or ‘Subgroups’. The rocks of different assemblages exhibit signatures of deposition in widely varying environments, from continental to deep marine. Unconformities, the thickness variation of the unconformitiesbound sequenFes, and highly variable rate of deposition of clastics derived from the gneissic rocks of the basement and fluctuating bathymetry of depositional interface collectively Gondwana Research, V.4, No. 4,2001
RODINIA, GONDWANA AND ASIA
point to oscillatory mode of basin filling through rapid cratonic uplift accompanied by high angle faulting of the basement. 'ILvo regionally extensive unconformities (Sloss, 1984) demarcate the Proterozoic succession from the underlying Archaean basement complex, and the overlying Gondwana rocks. Tho other major unconformities divide the succession into three sequences, the Pakhal sequence, the Penganga sequence, and the Sullavai-Albaka sequence, in the ascending order. Each sequence displays a strong degree of individuality dictated by supply of detrital sediment or lack thereof. The Pakhal and Penganga sequences comprise both siliciclastics and carbonates. Each starts with alluvial conglomerates and sandstones, passes upwards into shallow marine sandstone and finally into limestone and dolomitic limestone deposited in variable bathymetry, from shallow shelf to deep-water slope and base of slope environments. The Penganga sequence terminates with a thick deep-water, basinal shale (Mukhopadhyay and Chaudhuri, in press). The Sullavai-Albaka sequence, by contrast, consists only of siliciclastics, fluvial-eolian red sandstones of the Sullavai Group, and shallow marine sandstones and shale of the Albaka Group. The radiometric date (1330rf:53 Ma; Chaudhuri and Howard, 1985) obtained from glauconitic minerals from the basal part of the Pakhal sequence indicates that Pakhal sedimentation started in early Mesoproterozoic time. Chaudhuri et al. (1999) placed the Penganga sequence in the Mesoproterozoic, and the red sandstones of the Sullavai Group in the early Neoproterozoic mainly on the lithologic - stratigraphic considerations. The stratigraphic architecture represented by alternation of coarse siliciclastics a n d carbonate rocks with major unconformities preceeding the siliciclastic depositional events, points to episodic changes in depositional base level. Scale of the unconformities, generation of vast amount of arkosic detritus, and their deposition as basin margin facies suggest major uplifts of the cratonic blocks, that could involve kilometers of vertical movement consuming time spans of millions of years (cf. Sloss, 1991) and fracturing of the crust to accommodate the detritus in rift basins. The episodic uplift, fracturing and subsidence strongly points to repeated rifting. The occurrence of rhyolites and rhyolitic tuffs interbedded with Pakhal and Penganga rocks indicates episodic partial melting of the crust, generation of felsic magma and rifting (Patranabis Deb, in press). Repeated rifting in the valley is also supported by geophysical investigations (Ramanamurthy and Parthasarathy, 1988; Biswas, in press). In the central and southern part of the outcrop belt, the thickness of the unconformities bound assemblages, as well as of individual formations therein increase towards southeast. Furthermore, the relatively shallow water and coastal facies are gradually replaced by deeper water facies towards southeast pointing to southeasterly paleoslope. The basin was connected
Gondwana Research, V. 4,No. 4,2001
593
to a deep marine basin with open marine circulation, and it existed in the areas now occupied by the Eastern Ghats mountain. It is well established that tectonics of a craton is influenced by the events at craton margins, and divergence at the craton margin may be instrumental in generating rift basins far inboard the continent (Sloss, 1988, 1991). The PG Valley rift(s) with typical oscillatory behaviour characterized by abrupt termination of submerging episodes through rapid cratonic uplift with high angle faulting, may be attributed to extensional activities along the Eastern Ghats Mobile Belt, or the SonMahanadi rift. The initiation of the rift accommodating the Pakhal sequence broadly coincides with the time of emplacement of sub-alkalic basalt in the Eastern Ghats at about 1450 Ma attesting to continental rifting (Shaw et al., 1997) and opening of the seaway between India and Antarctica. The PG Valley rift basin was connected to the seaway through the Mesoproterozoic and early part of the Neoproterozoic. The proposed palaeogeography does not favour linkage between India and Antarctica, or the uplift of the Eastern Ghats mountain and its amalgamation with the Indian craton till the early Neoproterozoic. The notion is also supported by the absence of any correlatable Mesoproterozoic history between the Ryner Complex and the Eastern Ghats. The exact timing of the India-Eastern Ghats-Antarctica assembly, however, can only be determined with reliable dating of the rifting events in the PG Valley.
References Biswas, S . K. Regional tectonic framework of Pranhita-Godavari basin. J. Asian Earth Sci. Spl. vol., (in press). Chaudhuri, A.K. and Howard, J.D. (1985) Ramgundam sandstone: a middle Proterozoic shoal bar sequence. 3 . Sed. Petrol., v. 55, pp. 392-397. Chaudhuri, A.K., Mukhopadhyay, J., Patranabis Deb, S., and Chanda, S.K. (1999) The Neoproterozoic successions of Peninsular India. Gondwana Res., v. 2, pp. 213-225. Mukhopadhyay, J. and Chaudhuri, A.K. Proterozoic Penganga group, Pranhita-Godavari Valley, South India: depositional setting and paleogeography of a deep-water cratonic basin succession. J. Asian Earth Sci. Spl. vol., (in press). Patranabis Deb, S . Proterozoic felsic volcanism in the Pranhita Godavari Valley, India: its implication on the origin of the basin. J. Asian Earth Sci. Spl. vol., (in press). Ramanamurthy, B.V. and Parthasarathy, E.V.R. (1988) On the evolution of the Godavari Gondwana Graben, based on LANDSAT Imagery interpretation. J. Geol. SOC.India, v. 32, pp. 417-425. Shaw, R., Arima, M., Kagami, H., Fanning, C. M., Shiraishi, K. and Motoyoshi, Y. (1997) Proterozoic events in the Eastern Ghats Granulite Belt, India: evidence from Rb-Sr, Sm-Nd systematics, and SHRIMP dating. J. Geol., v. 105, pp. 645-656. Sloss, L.L. (1984) Comparative anatomy of cratonic unconformities, AAPG Mem., No. 36, pp. 1-6. Sloss, L.L. (1991) Epilog, in, Interior cratonic Basins. AAPG Mem., NO. 51, pp. 799-805.