Analyse quantitative du rifting et de la relaxation thermique de la partie occidentale de la marge transformante nord-africaine:le rif externe (maroc)

Abstract

We propose here a summary of the kinematics of the central and western External Rif, from the the rifting to recent tectonic events. This will be approached quantitatively with the help of certain physical mecanisme which may could have affected this part of the North African transform margin (the thermal dilatation and contraction of the lithosphere and the flexure of the African margin during the collision). The geometry and densities of the lithosphere obtained from the subsidence modelling, are compared to gravimetric and seismic refraction data. In the External Rif, one of the aspects of Wernicke’s model (1985), the asymmetry of rifting, seems to be confirmedby the sedimentological analysis and by subsidence modelling. However, the subsidence modelling indicates the existence of an adiabatic fusion of the upper mantle, not taken into account by the simple shear model. The uplift of the margin towards the end of rifting, would not be only due to thermal dilatation. Field data indicate important uncoformities on the African margin. This uplift is locally attributed to the mid-oceanic ridge push effect in the Atlantic and tothe N-African transform trough.The subsidence modelling also explains the actual thinness of the crust by the high thinning factors during the Triassic-Jurassic phases of rifting. This quantitative approach confirmthe existence of an aborted intra-continental basin in the southwest and a growing mid-oceanic ridge in the north-east. The oceanisation appears later in the north-african basin (Callovian) than in Central Atlantic (Bajocian). The post-rift subsidence of theexternal Rif does not differ from that of a normal passive margin. © 1995 Taylor and Francis Group, LLC.