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.