QUANTITATIVE-ANALYSIS OF RIFTING AND THER MAL RELAXATION IN THE WESTERN PART OF THE NORTH-AFRICAN TRANSFORM MARGIN - EXTERNAL RIF (MOROCCO)

We propose here a summary of the kinematics of the central and western External Rif, from the rifting to recent tectonic events. This will b e approached quantitatively with the help of certain physical mechanis ms which may could have affected this part of the North African transf orm margin (the thermal dilatation and contraction of the lithosphere and the flexure of the African margin during the collision). The geome try and densities of the lithosphere obtained from the subsidence mode lling, are compared to gravimetric and seismic refraction data. In the External Rif, one of the aspects of Wernicke's model (1985), the asym metry of rifting, seems to be confirmed by the sedimentological analys is and by subsidence modelling. However, the subsidence modelling indi cates the existence of an adiabatic fusion of the upper mantle, not ta ken into account by the simple shear model. The uplift of the margin t owards the end of rifting, would not be only due to thermal dilatation . Field data indicate important uncoformities on the African margin. T his uplift is locally attributed to the mid-oceanic ridge push effect in the Atlantic and to the N-African transform trough. The subsidence modelling also explains the actual thinness of the crust by the high t hinning factors during the Triassic-Jurassic phases of rifting. This q uantitative approach confirm the existence of an aborted intra-contine ntal basin in the south-west and a growing mid-oceanic ridge in the no rth-east. The oceanisation appears later in the north-african basin (C allovian) than in Central Atlantic (Bajocian). The post-rift subsidenc e of the external Rif does not differ from that of a normal passive ma rgin.