STRIKE-SLIP FAULTS IN A RIFT AREA - A TRANSECT IN THE AFAR TRIANGLE, EAST-AFRICA

The Afar Triangle, a diffuse triple junction where the Red Sea, Ethiopian and Gulf of Aden rifts converge, is examined along an E-W cross section in order to recognize traces of strike-slip faulting summarily known from earlier studies. Both field evidences from slickensides and airphotograph or satellite image data indicate that strike-slip faults, although less numerous than normal ones, occur throughout this area. These faults mainly strike parallel or at small angles relative to rifting axes, rather than transversal to them as would be expected if they were transforms. Strike slip subparallel to rifts is explained through lateral displacement between the major lithospheric plates around the junction or, subordinately, by a domino fault mechanism in zones of diffuse transform deformation. Faults at small angles with the rift axes often constitute conjugate systems suggesting along-axis compression, which is considered to be frequently induced by the lateral intraplate shift mentioned above. In other cases, this compression may develop in intervals between mantle plumes wedging up along a rift, or at the head of a propagating rift. The main lateral displacements among the boundary lithospheric plates during the last million of years are supposed to have been sinistral. This does not challenge the notion of mainly divergent plate movements, but adds to this divergence an anticlockwise shift of the plates around the junction.