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

George A. McMechan,Xiaoxian Zeng,E. John W. Jones

doi:10.1016/j.margeo.2015.03.008

George A. McMechan, Xiaoxian Zeng + Show 1 more

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https://doi.org/10.1016/j.margeo.2015.03.008

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Abstract

Wide-angle seismic profiles reveal anomalously thick crust with a high-velocity (> 7.3 km s− 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 3 km water layer and 1 km of sediments, the basement extends to 16–20 km 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 7.3–7.5 km s− 1 material beneath thick, lower-velocity volcanics suggests that magmatic underplating of the crust has occurred. A prominent change in velocity gradient 10–12 km 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 7.0–7.2 km s− 1 basement above the underplated zone extending into water depths of almost 5 km. 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.

Highlights

The Atlantic contains many elevated platforms more than 300 km in extent which are believed to have resulted from copious outpourings of basic volcanics
Variations in seismic velocity with depth beneath the crest of the Sierra Leone Rise and its environs are compared in Figs. 9 and 10 with those observed under normal oceanic crust (White et al, 1992), stretched continental crust (Christensen and Mooney, 1995) and other volcanic elevations
It is 2–3 times thicker than the crust of the Sierra Leone Basin (Lines D–F; Figs. 3, 8) but includes a section, approximately 7 km thick, with velocities greater than 7.3 km s−1. This zone lies beneath basement with velocities of 7.0–7.3 km s−1, values which are appreciably higher than the 6.26 km s−1 characterizing oceanic Layer 3 in the central and northern part of the Sierra Leone Basin (Lines E, F, Fig. 8) suggesting that 7.0–7.3 km s−1 basement is the intrusive foundation of early-formed anomalous crust on the Sierra Leone Rise

Summary

Introduction

The Atlantic contains many elevated platforms more than 300 km in extent which are believed to have resulted from copious outpourings of basic volcanics Whether these large igneous provinces have a common structure and whether they trace out plate motions over mantle hot-spots or lie above hot-lines that follow the upwelling limbs of Langmuir-like convection cells in the mantle, or are related to ridge tectonics over anomalous mantle are questions that have catalysed both vigorous debate and field observations (Morgan, 1972; Bonatti and Harrison, 1976; Anderson et al, 1992; Coffin and Eldholm, 1994; Ito et al, 2003; Fairhead and Wilson, 2005; Koppers, 2011; Ernst, 2014). Deep reflection profiles across the Cameroon Volcanic Line just north of the Equator (Meyers and Rosendahl, 1991; Meyers et al, 1998) have revealed normal oceanic crust that has been elevated about 2 km above the level of the surrounding basins

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