Crustal structure of the Moroccan margin from the sismar seismic survey

The Moroccan Atlantic margin of Early Jurassic age has been studied by means of deep seismic reflection and OBS refraction profiles during the SISMAR cruise. Previous works in the frame of ODP program (Leg 79) have revealed a typical passive margin formed after a Triassic rifting. Salt deposits of probable Early Jurassic age give rise to diapiric structures in the deep margin. A prominent magnetic anomaly called S1 is generally interpreted as marking the continent-ocean boundary. Oceanwards, a transitional crust forms a 70 km wide band. It is interpreted by Roeser et al. (2002) as the occurrence of thick magmatic layers. Our profiles shot with an airgun source tuned in single bubble mode image numerous deep reflectors within the crust down to the Moho. In the upper margin, continental tilted blocks are rooted on a large detachment fault. Below, the lower crust shows a pattern of reflectors indicating an heterogeneous stretching and structural variations parallel to the margin. In the deepest part of the margin, thin pieces of upper continental crust seem to be simply floating on the top of the stretched ductile lower crust. At the S1 anomaly location, a set of dipping reflectors suggests the presence of magmatic layers. This is also supported by the fact that laterally, where the S1 magnetic anomaly is weak, these reflectors seem also to disappear. At depth below the magmatic body, the Moho, visible beneath the continental crust, seems to rise and vanish. Oceanwards, the top of basement does not correspond to a single clear reflector: we propose that mantle has been exhumed and that serpentinized peridotites form a transitional basement. As observed in the Alps (Müntener and Hermann, 2001), these peridotites may be capped by some lavas reinforcing the blurred aspect of the basement top. Beyond this transitional zone, the basement shows a more regular acoustic facies until the first recognized oceanic magnetic anomalies and can be ascribed to typical oceanic crust. In conclusion, we propose a cross-section of the Moroccan margin off El Jadida based on real observations of crustal reflectors.