COULD THE MARGINAL GRABENS ALONG THE WESTERN AFAR MARGIN (SOUTHERNMOST RED SEA RIFT) REPRESENT THE BREAKAWAY ZONE OF A DETACHMENT FAULT SYSTEM?

Segmented grabens, separated by complexly faulted topographic highs and volcanic centers, mark the western Afar margin in the southernmost Red Sea rift. The right-stepping system of marginal grabens, clearly visible on digital topography data, extend from around 10º30'N to 14º30'N. Longitudinally, the graben systems follow a narrow region between 39º30'E and 40º00'E. Contrasting topographies are exhibited east and west of the marginal grabens. To the west the dissected Ethiopian-Eritrean plateau, at an elevation of 3000-3500 m, displays a gentle westward tilt that is reflected in a westward flowing drainage system. The region east of the grabens is dominated by an eastward sloping surface that drops from ~1500 m to well below sea level (-146 m). Late Oligocene–Miocene to Present extensional tectonism has affected the region east of the grabens, while the plateau to the west is very little affected by this phase of deformation. Clustering of seismic events along longitude 40ºE closely follows the trend of the marginal grabens. Seismic activity along the western Afar margin drops significantly south of ~10º30'N latitude, coinciding with the southern termination of the marginal grabens. The few focal plane solutions along the marginal grabens indicate a dominantly dip-slip motion perpendicular to the strike of the marginal grabens. The western Afar margin is unique among the rift margins of the Afar triple junction in displaying marginal grabens accompanied by a high level of seismicity. The marginal grabens are interpreted to mark the breakaway zone of an east verging detachment fault system that may have been present during the initial phase of continental breakup.