La marge sud-marocaine et les premières phases d'ouverture de l'océan Atlantique Central

We here propose a twofold approach to study the continental margins of the Central Atlantic Ocean, with special focus on the South Morocco Margin, in front of Reguibat bloc. This approach is based on plate kinematics reconstructions and geological field studies to map and constrain the geometry of the plate boundary though time and seismic sections (reflection and refraction) to decipher the tectonic and sedimentary history of the margin. Sahabi et al. (2004) recently showed that the opening of the central Atlantic Ocean started 195 my ago (Sinemurian basis) (i.e. 20 my later than what was generally proposed in earlier models). Based on this result and on the analysis of geophysical data (including new seismic lines, a new magnetic anomaly grid and satellite derived gravimetry), our kinematic study evidences several tectonic phases: oceanic accretion started at an extremely slow rate (~0.8 cm/y) during about 30 my (Lias-Dogger). At ~165 my ago (Callovian basis), a change in the relative plate motions occurred, that resulted in a change in basement topography and an acceleration of spreading rate, up to ~ 4.8 cm/y. The BSMA (Blake Spur Magnetic Anomaly) is related to this change. From Magnetic Chron M22 (150 my, Tithonian basis) onwards, the spreading rate slowed down to about 2.6 cm/y until Magnetic Chron M0 (125 my, Barremian-Aptian). On geodynamic side, the Atlantic domain divides into two major domains: whereas Europe and the future North Atlantic, at Permo-Triassic times, know period of distension, central Atlantic Ocean stay in compression during the end of Permian (Alleghanian phase) et seems to register distension only at Triassic times. Margin segment, localized between Moroccan Meseta and Nova Scotia, characterized by a huge Triassic province of evaporates, occupies an intermediate position between these two domains. Besides to this segment clearly inherited of Hercynian orogeny, meridional domain is also segmented with a segmentation wavelength of about 600 km. An analysis in several regions worldwide indicates that such segmentation is observed all around the globe. It seems to be linked here both to tectonic inheritance and to a relatively simple thermal mantle structure. The structural study of the margin is based on reflection and refraction seismic data obtained during the Dakhla cruise (2002), and on a compilation of existing comparable datasets (reflection and refraction data) from the North American margin and the Moroccan Meseta. Margins show in general three major pretty good marked: continental domain, little or no thinned, limited by hinge line from which continental crust thins more and more until pass more or less abruptly to oceanic domain. Transitional domain corresponds to atypical oceanic crust characterized by magnetic anomalies little or no marked, a thin crust (about 5 km) underlain by a low velocity (>7.2 km/s) layer. This study allows us to determinate several key points on margin structuration: 1- The data clearly indicate that crustal thinning is abrupt (except at the Dakhla site), not wider than about 60 km, and limited to the continental slope. In this part, continental crust thins from around 30 km to less than 10 km and we do not observe hardly any extensional structures. 2- At the bottom of the continental slope, the crust is completely thinned and stretched. The margins of the central Atlantic Ocean are also characterized by the occurrence of a Jurassic carbonate platform, which implies sedimentation in shallow waters (close to 0 m), for a long time after continental break-up (more than 50 my). Platform edge coincides with limit between these two structural zones. Based on reflexion and refraction seismic profiles, the structural characteristics of the margin are comparable to those of the Liguro-Provençal basin, the margins of which are ten times younger. The model we propose for the evolution of these margins includes three stages. First, extensional deformation, affected all the crust, is accommodated by sparse tilted blocks, grabens or basins. The second phase corresponds to the phase of crustal thinning, leading to the formation of continental slope. It is lower continental crust which seems to register thinning. During the third phase, break-up occurs at the foot of the continental slope, leading to the formation of proto-oceanic crust, probably partly composed of lower continental crust and mantle material. Our study shows that margin formation models that imply crustal volume conservation (e.g. McKenzie, 1978), cannot explain the actual geological observations. Non-conservative models (crustal transformation, crustal erosion, etc.) seem to be necessary to explain crustal thinning. The Dakhla margin differs a little bit of this scheme because of the asymmetry of its conjugated margins, and so, thinning appears to be on American side. The Precambrian cratonic zone of Reguibat, localized between two orogenic segments, seems to be unaffected by the break-up process. The lower crust does not have the same evolution than the neighbouring Appalachians and Mauritanides domains, undermining the importance of the tectonic heritage on the morphology.