Seismic evidence for crustal underplating beneath a large igneous province: The Sierra Leone Rise, equatorial Atlantic

Abstract

Wide-angle seismic profiles reveal anomalously thick crust with a high-velocity (>7.3kms -1) zone under the Sierra Leone Rise, a major mid-plate elevation in the Atlantic lying between the Cape Verde platform and the Cameroon Volcanic Line. A profile recorded over the crest using an ocean-bottom seismometer and surface sonobuoys shows that beneath a 3km water layer and 1km of sediments, the basement extends to 16-20km below sea level. Most velocity-depth values fall outside the expected range for Mesozoic-early Cenozoic ocean floor and stretched continental crust. The detection of 7.3-7.5kms -1 material beneath thick, lower-velocity volcanics suggests that magmatic underplating of the crust has occurred. A prominent change in velocity gradient 10-12km below sea level may mark the transition to underplated material emplaced during the late Cretaceous-early Cenozoic. A pronounced change in Moho depth lies on the line of a long offset fracture zone extending from the African margin, implying underplating was influenced by a pre-existing discontinuity in the lithosphere. Other seismic lines show 7.0-7.2kms -1 basement above the underplated zone extending into water depths of almost 5km. This is probably the intrusive foundation of early-formed crust over a mantle hot-spot. It is suggested that the development of the Sierra Leone Rise is distinct from other Atlantic hot-spot features to which it has been linked because of its setting in a region of intense lithospheric shear.