Propagation of an oblique spreading centre: the western Gulf of Aden

Abstract

The east–west trending Gulf of Aden rift, between Arabia and Somalia, is spreading along a N35°E direction. It propagates westward inside the African continent, toward the Afar triple junction, oblique to the spreading direction. These combined propagation and oblique rifting generate a special faulting pattern associated with an irregular distribution of volcanism. We analyse this pattern using a structural map derived from the analysis of multibeam bathymetric data and backscattering images from the 1995 TADJOURADEN cruise in the western Gulf of Aden.The rift valley is bounded by faults trending from N110°E to N130°E in the eastern part of the surveyed area where the valley trends N90°E and by large faults in the central–western part where the valley is oriented N70°E. The direction of extension deduced from the fault histogram is N37°E±10° over the whole area, whatever the rift trend. Therefore, this bend of the plate boundary does not result from a change in the stretching direction. This bending increases the rifting obliquity westward and therefore modifies the faulting pattern. Faults display two distinct patterns, either en-échelon on the walls, indicating a right-lateral component of shear, or purely normal and parallel to the rift trend inside the valley.Volcanoes are mostly located in the eastern part of the rift where they are aligned into swarms trending roughly perpendicular to the extension direction deduced from fault directions. Backscattering images show that recent lava flows fill the axial valley of the eastern part; whereas in the central and western parts, volcanoes are scattered in the middle part of the valley. This distribution is interpreted as a westward propagation of the volcanism, and as the direction of deformation migration.The thickness of the upper brittle layer of the lithosphere is estimated from the width if the inner graben is inside the rift. It indicates a deepening of the brittle/ductile transition from east (6km) to west (16km). These values are typical of oceanic and continental systems, respectively, and consistent with the depth of earthquakes in the western part. This points out the existence of a transitional area between 44°E and 45°20′E, characterised by the lack of recent volcanism. The nature of this mechanical transition is discussed in terms of propagation of an spreading centre into a continental or transitional, Afar-type lithosphere.