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

• Published in: Marine geology Vol. 365; pp. 52 - 60
• Main Authors: Jones, E. John W., McMechan, George A., Zeng, Xiaoxian
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.07.2015
• Subjects: Atlantic Ocean; Basements; Crustal underplating; Crusts; Foundations; Hot-spots; Large igneous province; Magmatic underplating; Mantle; Marine; Marine geology; Sea level; Sediments; Seismic structure; Shear; AS, Equatorial Atlantic; Cameroon; ASE, Atlantic, Sierra Leone Rise; Africa; Crustal underplating; Magmatic underplating; Large igneous province; Seismic structure; Hot-spots; Atlantic Ocean
• ISSN: 0025-3227; 1872-6151
• DOI: 10.1016/j.margeo.2015.03.008

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. •Seismic data show that the Sierra Leone Rise has an anomalously thick (>16km) crust.•High seismic velocities (>7.3kms−1) at depth suggest magmatic underplating.•The seismic structure differs from other mid-plate elevations in the central Atlantic.•The seismic structure is distinct probably because the Sierra Leone Rise formed in a region of intense lithospheric shear.•Underplating may have been promoted by a high-density barrier to rising melt.