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

The east-west trending Gulf of Aden rift, between Arabia and Somalia, is sp reading along a N35 degreesE direction. It propagates westward inside the A frican continent, toward the Afar triple junction, oblique to the spreading direction. These combined propagation and oblique rifting generate a speci al faulting pattern associated with an irregular distribution of volcanism. We analyse this pattern using a structural map derived from the analysis o f multibeam bathymetric data and backscattering images from the 1995 TADJOU RADEN cruise in the western Gulf of Aden, The rift valley is bounded by faults trending from N110 degreesE to N130 de greesE in the eastern part of the surveyed area where the valley trends N90 degreesE and by large faults in the central-western part where the valley is oriented N70 degreesE. The direction of extension deduced from the fault histogram is N37 degreesE +/- 10 degrees over the whole area, whatever the rift trend. Therefore, this bend of the plate boundary does not result fro m a change in the stretching direction. This bending increases the rifting obliquity westward and therefore modifies the faulting pattern. Faults disp lay two distinct patterns, either en-echelon on the walls, indicating a rig ht-lateral component of shear, or purely normal and parallel to the rift tr end 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 direct ion deduced from fault directions. Backscattering images show that recent l ava 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 valle y. This distribution is interpreted as a westward propagation of the volcan ism, and as the direction of deformation migration. The thickness of the upper brittle layer of the lithosphere: is estimated f rom the width if the inner graben is inside the rift. It indicates a deepen ing of the brittle/ductile transition from east (6 km) to west (16 km), The se 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 degreesE and 45 degrees 20 'E, characterised by the lack of recent volcanism. The nature of this m echanical transition is discussed in terms of propagation of an spreading c entre into a continental or transitional, Afar-type lithosphere. (C) 2001 E lsevier Science B.V. All rights reserved.