Utilizing global gravity data to delineate magmatic underplating and its implications: A case study from Proterozoic Vindhyan basin, south of Bundelkhand craton, India

Available globally gridded topography and free-air gravity anomaly are used to compute the Bouguer gravity anomaly over the southern Bundelkhand region and its boundary with the Vindhyan basin. The Bouguer anomaly map displays a large E-W trending gravity high in the central region of the anomaly map, which is majorly covered by the sedimentary Vindhyan rocks, south of the Bundelkhand craton and Deccan trap outcrops, southwest of the craton. The existence of this high in 30 km upward continued regional Bouguer gravity anomaly and the corresponding residual gravity map indicates the large depth as well as spatial extent of the high-density source giving the gravity high. The deep crustal source yielding the high gravity anomaly is backed by the depth estimates obtained for three interfaces (~30.2 km, ~11.9 km, ~2.7 km) from the radially averaged power spectrum analysis. Moho topography obtained as the result of 3D inversion of the Bouguer gravity data using the Parker-Oldenburg iterative algorithm exhibits a shallow Moho of ~32 km below the region covered by the Vindhyan rocks, giving the high gravity signatures. A 2D forward model is developed along the AA’ profile using density and thickness constraints from prior studies, along with the depth estimates obtained from the radially averaged power spectrum analysis. The resulting crustal model exhibits a thick high-density layer above the Moho interface, being the thickest beneath the region covered by the Vindhyan basin rocks underlain by the Bijawar rocks and Bundelkhand basement rocks. Correlating the Moho depths obtained from the inversion with the forward model, it is observed that the shallow Moho below the Vindhyan rocks in the inverted Moho topography is depicting the top surface of the high-density layer modelled over the Moho beneath the region. This high-density layer is theorized to be magmatic underplating arising from crustal extension induced by subduction-led extension tectonics involving the Bundelkhand cratonic block in the Proterozoic times. The presence of the underplated layer below the Vindhyan basin can be correlated with the proposed initiation of the Central Indian Tectonic Zone (CITZ) within the Paleo-Mesoproterozoic period. This points to the tentative formation mechanism of the Vindhyan basin, that is rifting, with the crustal thinning being compensated by the magmatic emplacement above the Moho. This probably resulted due to the onset of oblique collision between the northern and southern Indian blocks along the CITZ at around ~2 Ga, up to ~1Ga, which is said to be the closure age of the Upper Vindhyan rocks. Thus, the obtained results and inferences from the present study deciphers the Moho topography and the underplated high-density layer below the region covered by Vindhyan rocks, south of the Bundelkhand craton, providing a preliminary understanding of the crustal structure beneath the study area and paving the way to undertake further studies to comprehend the implications of the geodynamics of the region with respect to supercontinent reconstructions.