PROPAGATION OF RIFTING ALONG THE ARABIA-SOMALIA PLATE BOUNDARY - INTOAFAR

It is generally accepted that the Aden ridge has propagated westward f rom similar to 58 degrees E to the western tip of the Gulf of Aden/Tad joura, at the edge of Afar. Here, we use new tectonic and geochronolog ical data to examine the geometry and kinematics of deformation relate d to the penetration of that ridge on dry land in the Republic of Djib outi. We show that it veers northward, forming a narrow zone of dense faulting along thp northeastern edge of the Afar depression. The zone includes two volcanic rifts (Asal-Ghoubbet and Manda Inakir), connecte d to one another and to the submarine part of the ridge by transfer zo nes. Both rifts are composite, divided into two or three disconnected, parallel, NW-SE striking subrifts, all of which appear to have propag ated northwestward. In Asal-Ghoubbet as in Manda Inakir, the subrifts appear to have formed in succession, through north directed jumps from subrifts more farther south. At present, the northernmost subrifts (M anda and Dirko Koma) of the Manda Inakir rift, form the current tip of the northward propagating Arabia-Somalia plate boundary in Afar. We a ccount for most observations by a mechanical model similar to that pre viously inferred for the Gulf of Aden, in which propagation is governe d by the intensity and direction of the minimum horizontal principal s tress, sigma 3. We interpret the northward propagation on land, almost orthogonal to that in the gulf, to be related to necking of the Centr al Afar lithosphere where it is thinnest. Such necking may be a conseq uence of differential magmatic thickening, greater in the center of th e Afar depression where the Ethiopian hot spot enhanced profuse basalt ic effusion and underplating than along the edges of the depression. T he model explains why the Aden ridge foregoes its WSW propagation dire ction, constant from similar to 58 degrees E to Asal-Ghoubbet. At a sm aller scale, individual rifts and subrifts keep opening perpendicular to the Arabia-Somalia (or Danakil-Somalia) motion vector and propagate northwestward. Concurrently, such lithospheric cracks are forced to j ump northward, such that the plate boundary remains inside the regiona l N-S necking zone. Changes of obliquity between the directions of ove rall and local propagation may account for different segmentation patt erns, a small angle promoting long, en echelon subrifts, and a high-an gle, smaller, nested, ''subrifts within subrifts.'' The propagation me chanism is thus similar, whether in oceanic or continental lithosphere , the principal change being thp overall propagation path here governe d by thickness changes rather than by the geometry in map view as prev iously inferred for the rest of the Aden ridge. Finally, because the s ame mechanism has led rifting along the Red Sea to propagate southward and jump to the western edge of Afar, the Arabia-Somalia and Arabia-N ubia plate boundaries tips have missed each other and keep overlapping further, leading to strain transfer by large-scale bookshelf faulting .