PROPAGATION OF RIFTING ALONG THE ARABIA-SOMALIA PLATE BOUNDARY - THE GULFS OF ADEN AND TADJOURA

The localization and propagation of rifting between Arabia and Somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern Gulf of Aden and the Gulf of Tadjoura, using bathymetric, magnetic, seismological, and structural e vidence. Large-scale, southwestward propagation of the Aden ridge, mar kedly oblique td the Arabia-Somalia relative motion vector, began abou t 30 Myr ago between the Error and Sharbithat ridges. It was an episod ic process, with stages of rapid propagation, mostly at rates >10 cm/y r, interrupted by million year pauses on transverse discontinuities co inciding with rheological boundaries between different crustal provinc es of the Arabia-Somalia plate. The longest pause was at the Shukra-El Sheik discontinuity (approximate to 45 degrees E), where the ridge ti p, stalled for approximate to 13 Myr, between approximate to 17 and ap proximate to 4 Ma. West of that discontinuity, rifting and spreading t ook place at an azimuth (approximate to N25 degrees 10 degrees E) and rate (1.2+/-0.3 cm/yr) different from those of the global Arabia-Somal ia motion vector (approximate to N39 degrees E, approximate to 1.73 cm /yr), implying an additional component of movement (N65 degrees+/-10 d egrees E, 0.7+/-0.2 cm/yr) due to rotation of the Danakil microplate. At Shukra-El Sheik, the typical oceanic ridge gives way to a narrow, W SW trending axial trough, resembling a large fissure across a shallow shelf. This trough is composed of about eight rift segments, which res ult from normal faulting and fissuring along N110 degrees-N130 degrees E trends. All the segments step to the left southwestward, mostly thr ough oblique transfer zones with en echelon normal faults. Only two se gments show clear, significant overlap. There is one clear transform, the Maskali fault, between the Obock and Tadjoura segments. The latter segment, which encroaches on land, is composed of two parallel subrif ts (Iboli, Ambabbo) that propagated northwestward and formed in succes sion. The most recent, southwestern subrift (Ambabbo) represents the c urrent tip of the Aden ridge. We propose a mechanical model in which t he large-scale propagation of the ridge followed a WSW trending zone o f maximum tensile stress, while the small-scale propagation of its;NW trending segments was dictated by the orientation of that stress, Obli que propagation was a consequence of passive lithospheric necking of t he Arabia-Somalia plate along its narrow section, in map view, between Socotra and the kink of the Red Sea-Ethiopian rift, above the Afar pl ume. Individual ridge segments oriented roughly perpendicular to plate motion, like lithospheric cracks, were forced to jump southward becau se of confinement within the necking zone. Self-sustaining plate-scale necking may explain why the Aden ridge did not connect with the Red S ea through Bab El Mandeb but continued straight into Afar.