3-D Crustal Structure Modelling of the Proterozoic Cuddapah Basin, Its Regional Geodynamic Evolution and Columbia–Rodinia Connection

The Cuddapah Basin located in the southern part of India, is one of the largest Proterozoic basins in the world. This basin witnessed multi-stage growth that involved sedimentation, magmatism and tectonism. This region has been studied geophysically for over nine decades, nevertheless, its subsurface structural configuration, underlying crust–mantle structure and its evolutionary relationship with east Antarctica during the Columbia and Rodinia supercontinental assembly periods, remain an enigma. In the present study, we provide a 3-D crustal structural configuration of this basin based on gravity modelling along 13 east–west and 3 north–south profiles, utilising density and subsurface depth constraints from crustal seismic studies. We have delineated the presence of a 15–20 km anomalously thick, high density differentiated magmatic layer above the Moho, which varies widely from 32 km below the Eastern Ghats Belt to about 50 km below the Nellore Schist Belt. It is invariably shallower at 37–38 km north of 14°45'N, compared with 43 to 46 km south of it, indicating possible tilting of the basin from north to south. Importantly, we also notice a high order translational movement of the crustal column across the Nallamalai Basin as revealed by conspicuous change in the Moho trend, which correlates with the absence of Eastern Ghats Belt for a distance of about 400 km south of Ongole, mimicking the translational movement. We infer that the Napier complex of east Antarctica, may form the missing part of Eastern Ghats Belt of India's east coast. Similarly, a high gravity over the Nellore Schist Belt region would possibly indicate presence of remanent slab material (or magmatic material) underneath which may be related to erstwhile supra-subduction during the Rodinia supercontinental assembly period. The present study further suggests that the Iswarakuppam dome, located in northern part of the Nallamalai Basin, contains a thin veneer of Cumbum shale (5.10 km/s) followed by a high velocity (Vp: 5.70 km/s) sequences containing Bairenkonda quartzites and possibly mantle derived mafic rocks that may be correlatable with the sills of southwestern part of the Cuddapah Basin. This domal structure probably formed due to underthrusting of the western Cuddapah Basin and the collision of Nellore Schist Belt/Eastern Ghats Belt terrains after the cessation of supra-subduction below eastern Dharwar Craton at the end of the Rodinia period (around 950 Ma).