Building Blocks and Crustal Architecture of Indian Peninsular Shield: Cratons and Fold Belts and their Interaction Based on Geophysical Data Integrated with Geological Information

The geoid corrected Bouguer anomaly map of India presents linear gravity highs along (I) the Aravalli-Delh Fold Belt (ADFB), (II) the Satpura Fold Belt (SFB), (III) the Eastern Ghat Fold Belt (EGFB), (IV) the Shear Zone (SZ) between the Western and the Eastern Dharwar Cratons (WDC and EDC) (v) the Transition Zone (TZ) between Dharwar Craton and the Southern Granulite Terrain(SGT) and (vi) the Cauvery Shear Zone (CSZ) between Moyar-Bhavani and Palghat-Cauvery Shear Zones (MBSZ and PCSZ) in SGT. Gravity highs over the relatively younger Proterozoic fold belts are accompanied by gravity lows over adjoining older Archaean cratons, which are primarly attributed to granite plutons and/or crustal thickening. The gravity high observed over the ADFB is related to domal shaped high density and high velocity body in lower crust under Delhi Supergroup of rocks (1 45 Ga) whch is suggestive of an extensional regime giving rise to rift basins/Aulacogens. The accompanying gravity low towards the west is related to K-Granite plutons and intrusives( 0 8-0 75 Ga) associated with back arc type basins, which along with subsurface extent and shape of exposed ophiolites and granulite rocks in gravity model suggest compression and thrusting from west to east and presumed subduction from east to west during Meso-Neoproterozoic period in this section. Geophysical signatures of paired gravity anomaly with gravity high related to high density, high velocity and high conductivity lower crustal mafic rocks in upper crust under the SFB and large wavelength gravity low to low density and low velocity rocks in upper mantle and calc-Alkaline magmatism of Bhandara craton suggest N-S directed compression and subduction across central part of SFB during Mesoproterozoic period related to Sausar orogeny. Opposite dipping reflectors associated with low density and low conductivity over the Central Indian Shear (CIS) along southern margin of the SFB with different densities and conductivities of rocks on either side of it suggest to it to be a suture. Based on gravity anomalies, extension of the CIS and two phases of granulite rocks related to pre and post Sausar orogeny (1 6 and 1 0 Ga) in central part of the SFB are traced towards east associated with Singhbhum orogeny and towards west upto west coast of India separating Bundelkhand Craton towards north and Dharwar-Bhandara-Singhbhum Cratons towards south. The contemporary Vindhyan Basin of Mesoproterozoic period along the ADFB and the SFB with shelf type of sediments may represent a peripheral foreland basin. There is also a case of extension and compression across the ADFB and the SFB during Paleoproterozoic period related to Aravalli and Mahakoschal orogeny, respectively. This suggest cyclic E-W and N-S directed compression across the ADFB and the SFB, respectively during Paleo and Meso-Proterozoic periods. The gravity highs related to them join together in NW India to form an arcute shaped collision zone indicating resultant stress in NE-SW direction, during these periods. Such cyclic orogeny has been reported from several places in the world such as Appalachan-Caledonian system and in case of several other natural processes.

Seismic section across Dharwar craton provide domal and bowl shaped reflectors in crust under the WDC and the EDC and east ward dipping reflectors below Moho under the EGFB, which suggest extension and compression, respectively supported by paired gravity anomalies. Schist belts of the WDC and the EDC containing riftogenic volcano sedimentary sequences with bimodal volcanics of Neoarchean (3 0-2 7 Ga) and Paleoproterozoic (2 6-2 5 Ga) periods, respectively also suggest extension across the WDC and the EDC during these periods. Regional gravity lows of schist belts of WDC suggest low density crustal rocks under them indicating a marginal or intra arc basin set up for them in plate tectonics paradigm with vertical stretch and transpressive zones to form ischolar_mains due to horizontal flow instead of orogenic belts. Similarly, westward verging thrusts and thrusted high density and high velocity lower crustal rocks under the SZ between the WDC and the EDC and along the EGFB (1 1 Ga) and K-Granite plutons of EDC of 2 6-2 5 Ga suggest west to east directed compression and subduction across SZ and EGFB during these periods. Further south, multi-Disciplinary geophysical investigations along a geotransect across the SGT and modelling of gravity field provide linear gravity highs over the TZ and the CSZ being related to high density intrusives of mafic lower crustal rocks supported from high seismic velocity and high conductivity and adjoining gravity lows towards south of them, are caused by crustal thickening upto 45-46 km. These observations along with opposite dipping thrusts, the MBSZ and the PCSZ and exposed lower crustal rocks of metamorphic ages 2 6-2 5 Ga north of the MBSZ and in the CSZ indicate NS directed compression during this period with the CSZ as central core complex of collision tectonics. Airborne magnetic anomalies indicate that the MBSZ and the PCSZ are connected to NE-SW oriented Mettur shear and Gangavalli shear in the SGT, respectively in their eastern parts extending to east coast of India, forming an arcuate shaped collision zone during 2 6-2 5 Ga. This compressional phase across SGT may be related to contemporary compression across Dharwar Craton towards north with collision between the WDC, the EDC and the SGT during 2 6-2 5 Ga in NW-SE direction with W-E component between the WDC and the EDC and N-S component between Dharwar Craton and the SGT.