Crustal structure variations along the NW-African continental margin: A comparison of new and existing models from wide-angle and reflection seismic data

Abstract

Deep seismic data represent a key to understand the geometry and mechanism of continental rifting. The passive continental margin of NW-Africa is one of the oldest on earth, formed during the Upper Triassic–Lower Liassic rifting of the central Atlantic Ocean over 200Ma. We present new and existing wide-angle and reflection seismic data from four study regions along the margin located in the south offshore DAKHLA, on the central continental margin offshore Safi, in the northern Moroccan salt basin, and in the Gulf of Cadiz.The thickness of unthinned continental crust decreases from 36km in the North to about 27km in the South. Crustal thinning takes place over a region of 150km in the north and only 70km in the south. The North Moroccan Basin is underlain by highly thinned continental crust of only 6–8km thickness. The ocean–continent transition zone shows a variable width between 40 and 70km and is characterized by seismic velocities in between those of typical oceanic and thinned continental crust. The neighbouring oceanic crust is characterized by a thickness of 7–8km along the complete margin. Relatively high velocities of up to 7.5km/s have been imaged between magnetic anomalies S1 and M25, and are probably related to changes in the spreading velocities at the time of the Kimmeridgian/Tithonian plate reorganization.Volcanic activity seems to be mostly confined to the region next to the Canary Islands, and is thus not related to the initial opening of the ocean, which was associated to only weak volcanism. Comparison with the conjugate margin off Nova Scotia shows comparable continental crustal structures, but 2–3km thinner oceanic crust on the American side than on the African margin.