The construction and destruction of continents, cratons and supercontinents on the globe involved multiple events of plume, rifting, subduction-accretion, and collision through the long history of evolution of our planet. Archean cratons preserve the records of continent building in the Early Earth and understanding their lithospheric architecture is fundamental to building geodynamic models. The Dharwar craton in southern India is among the major cratons on the globe. Recent geological and geochronological studies indicate that the craton formed through the assembly of a number of micro-blocks, although the terrane boundaries and lithospheric structure have not been well-defined. Here, satellite gravity data is analyzed, covering the entire Dharwar craton and its surrounding regions, with a view to understand the regional lithospheric architecture and the geodynamic evolution. Present results on the residual gravity field, based on finite element approach, can clearly demarcate the boundaries of the major geotectonic units in the Dharwar craton and surrounding regions. The presence of a conspicuous residual gravity low (55 mGal) is identified that runs north–south for about 700 km, paralleling the western margin of the Dharwar craton. This gravity low passes through the Chitradurga schist belt and covers the Koyna seismogenic region in the north to Coimbatore in the south. The gravity patterns observed over this region, might suggest a deep-seated paleo- rift zone, which is termed here as ‘Koyna-Coimbatore rift’. The prominent gravity low associated with this rift structure, can be explained by the presence of a 70 km thick low velocity/low density zone (LVZ) in subcrustal lithospheric mantle, suggesting asthenospheric flow into the upper mantle. The Western Dharwar Craton (WDC) might extend further north encompassing the Koyna-Warna seismic zone. Another major finding from this study is delineation of the Central Dharwar Cratonic (CDC) block between the Chitradurga shear zone and the Kolar greenstone belt, which is characterized by a prominent residual gravity high zone (+20 mGal), coinciding with the exposures of the Closepet granitoids and their equivalents, possibly marking a collisional suture between two micro- blocks. The 21⁄2D residual gravity modeling across an E-W profile, which cuts all the major geotectonic features of WDC, CDC and Eastern Dharwar Craton (EDC), reveals popping-up of the CDC terrane, below which Moho has upwarped to a depth of 36 km and the lithosphere is considerably thinner at about 120 km, compared to 48 km and 175 km respectively in the WDC. This feature corresponds to litho- spheric destruction of almost 100–120 km below the Dharwar craton, with magma underplating. On the eastern side of the craton, the positive residual gravity contours that correspond to the Eastern Ghats Belt rocks, take a sharp turn into the adjacent offshore region northeast of Ongole, and continue further south, until the granulite terrane north of Chennai, indicating the absence of the Eastern Ghats Belt rocks east of Cuddapah Basin. Present gravity model is able to demarcate three collisional boundaries between (i) WDC and CDC, (ii) CDC and Cuddapah Basin, and (iii) Nellore Schist Belt and adjoining east coast terrane. The extensive lithospheric destruction with removal of nearly 100 km of the cratonic root is correlated to prolonged subduction during Paleo-Neoproterozoic, together with mantle plumes at different times playing a subordinate role, thus indicating combined mechanical, thermal and chemical erosion.
Fig. Lithospheric density model along Profile AA’ as deduced from the present study. Depths to the Moho and LAB are found to be thicker at around 48 km and 175 km respectively in the Western Dharwar Craton (WDC), whereas these estimates are shallower at around 36 km and 100–120 km in the Central Dharwar Craton (CDC) and Eastern Dharwar Craton (EDC). Abbreviation: CB-Cuddapah Basin, NSB-Nellore Schist Belt, ECSB-East Coast Sedimentary Basin, UC-Upper Crust, MC-Mid Crust, LC-Lower Crust, LVZ-Low Velocity Zone, LAB- Lithospheric-Asthenospheric Boundary.
A. Vasanthi , M. Santosh
Lithospheric architecture and geodynamics of the Archean Dharwar craton and surrounding terranes: New insights from satellite gravity investigation