Anomalous subsidence on the rifted volcanic margin of Pakistan: No influence from Deccan plume

The role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern Arabian Sea is investigated using subsidence analysis of a drill site located on the seismically defined Somnath volcanic ridge. The ridge has experienced >4 km of subsidence since 65 Ma and lies within oceanic lithosphere. We estimate crustal thickness to be 9.5-11.5 km. Curiously <400 m of the thermal subsidence Occurred prior to 37 Ma, when Subsidence rates would normally be at a maximum. We reject the hypothesis that this was caused by increasing plume dynamic support after continental break-Lip because the size of the thermal anomalies required are unrealistic (>600 degrees C, especially considering the rapid northward drift of India relative to the Deccan-Reunion hotspot. We suggest that this reflects very slow lithospheric growth, possibly caused by Vigorous asthenospheric convection lasting > 28 m.y., and induced by the steep continent-ocean boundary. Post-rift slow subsidence is also recognized on volcanic margins in the NE Atlantic and SE Newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental bleak-up.Crown Copyright (D 2008 Published by Elsevier B.V All rights reserved.