Lithosphere thinning and welding by Reunion Hotspot: impingement timing, transitional state and dynamics of Deccan Volcanic Province. 

Abstract

During the late Cretaceous period of 130-65 Ma, Indian subcontinent experienced three major mantle plume events that affected the western Indian shield significantly. Two alkaline complexes in the Deccan Volcanic Province: Sarnu-Dandali Alkaline Complex (SDAC) and Mer-Mundwara Alkaline Complex (MMAC) served as prominent witnesses to those recurrent impingement episodes. Despite their chronological and spatial proximity, they demonstrate distinct evolutionary signatures which are ascertained through multidisciplinary approaches encompassing structural geology, magma chamber dynamics, and seismic velocity structure. We report significantly high erupted magma volume,depth of dike emplacement, radial and tangential stress at the magma chambers in MMAC than those in SDAC. Using inversion results of surface wave dispersion data, while we propose Moho depth of ~50 km in both the complexes, the Lithosphere Asthenosphere boundary (LAB) is speculated at a depth of ~70 km beneath SDAC, and ~140 km beneath MMAC. An anomalous high velocity anomaly lies immediately beneath the LAB at SDAC, while it is absent beneath MMAC. Amalgamting the evidences, we propose that, a) SDAC and MMAC clearly delineate the boundary of lithospheric thinning induced by the Reunion plume at 65±2 Ma, where SDAC lies within the thinned zone, and MMAC lies just outside of it; b) the contrasting geometry and geology are the results of disposition of the fracture pattern and volume of the magmatic outpour; c) high velocity body beneath SDAC at LAB depth is remnant of late stage magmatic underplating caused by the Reunion Plume. We conjecture that terminal underplating was caused due to the rapid drift of the Indian plate by dragging the remnants of the plume material during post-Deccan period.