The crustal structure across the transform continental margin off Ghana, eastern equatorial Atlantic

Abstract

Forward modeling of a suite of wide‐angle seismic lines across the transform continental margin at the eastern end of the Romanche fracture zone off Ghana has shown the transition from continental to oceanic crust to be confined to a narrow, 6 to 11‐km‐wide zone located at the foot of the steep continental slope. The structure of the adjacent oceanic and continental crusts has also been resolved. These results are confirmed and enhanced by gravity and magnetic models. The crust of the ocean‐continent transition zone is characterized by high velocities (5.8–7.3 km s−1), a high density (3.10 Mg m−3), and high magnetizations (1.1–1.25 A m−1). This rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin. The oceanic crust within 70 km of the margin is abnormally thin compared to normal Atlantic oceanic crust and shows an average thickness of just 4.4 km. We suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded, on at least three sides, by cold continental lithosphere. The preferred seismic model also shows a new fracture zone, 30 km southeast of the ocean‐continent transition, which is characterized by low velocities in the upper crust, a lens‐shaped layer with velocities of 7.2–7.4 km s−1 at the base of the crust, and a crustal thickness of just 3.4 km. The continental crust appears largely unaffected by the proximity of the adjacent oceanic crust. There is no evidence for underplating of the continental crust adjacent to the ocean‐continent transition zone.