Crustal structure variations along the conjugate central Atlantic continental margins: A comparison of new and existing models from wide-angle and reflection seismic data

The Central Atlantic is a key to understand rifting of magma-poor margins. It allows studying the mechanisms of opening by mapping the extend of continental and oceanic domains and the presence of exhumed upper mantle or volcanic underplate in the ocean-continent transition zone. The NW-Moroccan – Nova Scotia conjugate margin pair formed during initial rifting of the Atlantic and is one of the oldest passive margin pairs in the world opening about 180-200 Ma ago during the early Jurassic. In this study the Central Atlantic conjugate margins are compared, with regard to volcanism, crustal thickness and geometry, and seismic velocities in the transition zone. Continental crustal thickness is about 37 km along both margins, except for the region of the Precambrian Reguibat Ridge. While profiles on the US Atlantic margin are characterised by thick layers of magmatic underplate, no such underplate was imaged along the African continental margin. In the north, two wide-angle seismic profiles acquired in exactly conjugate positions show that the crustal geometry of the unthinned continental crust and the necking zone are nearly symmetric. A region including seismic velocities too high to be explained by either continental or oceanic crust is imaged along the Canadian side, corresponding on the African side to an oceanic crust with slightly elevated velocities. These might result from asymmetric spreading creating amagmatic oceanic crust on the Canadian side and magmatic crust including pockets of serpentinite on the Moroccan margin. After isochron M25 a large-scale plate reorganisation might than have led to an increase in spreading velocity and the production of thin magmatic crust on both sides.