Structure of the Alula‐Fartak Fracture Zone, Gulf of Aden

Abstract

The Alula‐Fartak transform fault is associated with a broad, steep‐sided valley running NNE‐SSW across the mouth of the Gulf of Aden. A discussion of the tectonics of the fracture zone is based principally on GLORIA side scan sonar and gravity data. The transform valley is 180 km long and 25 km across, has 3.5 km of relief, and is bounded by transverse ridges. The northern two‐thirds is highly asymmetrical in cross section. The eastern wall dips 38° and appears to be a single scarp exposing a section of oceanic crust 3100 m thick. The western wall is much less steep and accommodates several normal faults oriented at about 15° to its general trend. A very fine, long sonar target running along part of the valley delineates the main active strike‐slip fault. The sonar data also reveal many fine faultlike targets between the main walls, running approximately at right angles to the trend of the valley, which may be anti‐Riedel shears. A 50‐km‐long, 7.5‐km‐wide, and 1.5‐km‐high median ridge runs along the southern third of the transform valley. Gravity data show that the valley is underlain by a significant mass deficiency thought to be a large serpentinite intrusion. The median ridge is suggested to be the surface manifestation of a large serpentinite diapir. The ridges flanking the transform valley are above a level compatible with isostatic equilibrium and are interpreted as flexural features raised in response to the upward loads imposed on the lithosphere by the fracture zone valley and its underlying mass deficiency. Very shallow crust is located close to the ridge‐transform intersections. This is considered to result from static flexure induced by the negative load of the transform valley superimposed on dynamic compensation of the spreading center valley. Evidence of a c. 3.5 m.y. B.P. change in spreading direction suggests the transform has experienced a recent component of opening, accounting for several aspects of its structure.

Geologically current plate motions

“…35b). The 210‐km‐offset Alula‐Fartak transform fault, which has the longest offset of any transform fault along this plate boundary, has been mapped with both GLORIA side‐scan sonar (Tamsett & Searle 1990) and multibeam bathymetry (Leroy et al 2004; d'Acremont et al 2006). The nearby Socotra transform fault at 55°E has also been mapped with multibeam bathymetry (Leroy et al 2004).…”

The Red Sea and Gulf of Aden Basins

“…Oceanic spreading is more advanced than in the Red Sea, the spreading center connects directly to the world system of oceanic plate boundaries via the Sheba Ridge, and the spreading center is highly dissected by well-defined transform faults ( Fig. 1; Laughton, 1966a,b;Girdler and Styles, 1978;Cochran, 1981;Tamsett and Searle, 1990). Late Neogene kinematics of this plate boundary are therefore better constrained than those of the Red Sea (Le Pichon and Francheteau, 1978;DeMets et al, 1990;Jestin et al, 1994).…”

Structure and evolution of the eastern Gulf of Aden conjugate margins from seismic reflection data

“…It may correspond to the limit of influence of the Afar hotspot and to a major change in the rheology of the lithosphere (Manighetti et al 1998; Dauteuil et al 2001; Hébert et al 2001). The Alula‐Fartak fracture zone separates the central part from the eastern one (Tamsett & Searle 1990). Our study area is located between the Alula‐Fartak and the Socotra fracture zones.…”