A Unified Model of Neoarchean-Proterozoic Convergence and Rifting of Indian Cratons: Geophysical Constraints

摘要

Neoarchean and Proterozoic sutures and collision zones are identified in the Indian Peninsular Shield based on high seismic velocity; gravity highs and high conductivity in the upper crust due to thrusting while sub- ducted side are demarcated based on geophysical signatures of crustal thickening and back arc type basins. Some of them appear to form triple junctions. The Bouguer anomaly map of the south Indian shield when transformed to apparent density map through harmonic inversion, provided high density linear zones coin- ciding with the shear zone and the transition zone-the Moyar Bhavani Shear Zone (MBSZ) between the Eastern Dharwar Craton (EDC) and the Western Dharwar Craton (WDC) and the Dharwar cratons and the Southern Granulite Terrain (SGT), respectively. It is supported by high seismic velocity and high conductiv- ity suggesting them to be caused by high grade granulite rocks related to Neoarchean-Paleoproterozoic su- tures and collision zones. These investigations also suggest thick crust (~40 - 50 km) under the WDC and the SGT forming crustal root of 50 - 52 km in the south western part and thin crust of 31 - 32 km under the EDC indicating direction of convergence and subduction as E-W and N-S between the EDC and the WDC and Dharwar cratons and the SGT, respectively. It gave rise to contemporary lower crustal granulite rocks in the northern part of the SGT and Cauvery shear zone (CSZ) as collision related central core complex of various deep seated intrusive rocks of Paleo-Mesoproterozoic period. The second case belonging to Meso-proterozoic period is related to the collision of the Bundelkhand craton and the Bhandara-Bastar craton (BBC) and the Dharwar craton (DC) in Central India along the Satpura Mobile Belt (SMB) and the BBC and the DC along the Godavari Proterozoic Belt due to N-S and NE-SW convergences, respectively. This process has given rise to lower crustal granulite rocks of high density, high velocity and high conductivity along the SMB and the GPB. An upper mantle conductor delineated south of the western part of the SMB under Dec- can Volcanic Province and a regional gravity gradient almost sub parallel to it indicate an interface with flu- ids separating rocks of different densities that appears to demarcate the trace of the Proterozoic subduction and suture related to the SMB collision zone during Mesoproterozoic period. High reflectivity of the lower crust along seismic profiles across the SMB indicate an extensional phase prior to this convergence. The SMB is connected to the Aravalli Delhi Mobile Belt (ADMB) in the western part that is another collision zone of Meso-proterozoic period, forming an arcuate shaped collision zone between the Bundelkhand craton and Rajasthan block with E-W convergence. There are indications of a prior phase of convergence during Paleo-Proterozoic period followed by rifting during Paleo-Meso-proterozoic period (~1.9 - 1.6 Ga) along the SMB, the ADMB and the GPB that gave rise to large scale contemporary intrusive in these sections. The contemporary Mahakoshal-Bijawar and Pakhal group of rocks of Paleo Proterozoic period (~1.9 - 1.6 Ga) were deposited over the rifted platform of the Bundelkhand craton along the SMB and cratons along the GPB, respectively during the extensional phase as suggested above based on high reflectivity of the lower crust. It is followed by deposition of the Vindhyan sediments of Meso-Neoproterozoic period (~1.6 - 0.7 Ga) along the SMB and the ADMB as foreland basins during Meso-Neoproterozoic convergence. Simultaneous N-S and E-W directed convergences in the two cases, viz., the SMB and the ADMB that are connected forming an arcuate shaped collision zone suggest NE-SW directed primary stress direction similar to the GPB that is supported by NW-SE oriented large lineaments in Bundelkhand craton and Peninsular shield. The Eastern Ghat Mobile Belt (EGMB) also shows signatures of E-W or NE-SW directed Mesoproterozoic (~1.5 - 1.0 Ga) convergence wi