Structure and development of the Midocean Ridge Plate Boundary in the Gulf of Aden: Evidence from GLORIA Side Scan Sonar

A GLORIA side scan sonar survey along the crest of the midocean ridge in the Gulf of Aden has provided information on the small‐scale morphology of its spreading centers and fracture zones. Spreading center offsets increase in length to the east. This is thought to result from changes in the behavior of the mantle with increasing distance from the Afar hot spot. A 300‐km‐long spreading center in the western Gulf of Aden is oriented obliquely to the spreading direction. Nested within the obliquely oriented median valley is an en echelon arrangement of a number of smaller, elongated basins, each oriented approximately normal to the spreading direction. Close to the axis of spreading, volcanic and closely spaced tectonic structures run approximately normal to the spreading direction. Higher on the walls of the median valley, more‐widely‐spaced normal faults have formed subparallel to the valley. In the western Gulf of Aden there are no indications of sonar targets that might represent transform faults parallel to the NNE spreading direction. In the east, spreading center offsets of 15–25 km are made at fracture zones which are best described as narrow zones of highly oblique spreading. Offsets of greater than 25 km are achieved on single transform faults, and the associated tectonic fabric has a spreading direction trend. The boundaries between crusts of different ages along inactive transform fault extensions lie along the bases of fracture zone scarps. These scarps are situated in the older crust and face the younger. This is the opposite of the situation predicted on the basis of simple plate cooling models. At some places in the Gulf of Aden the fracture zone topography is very subdued, and this is attributed to the failure of the two slabs of lithosphere at the end of the transform fault to couple completely before the younger lithosphere on the nontransform side of the spreading center completes its ascent of the median valley wall. There is evidence of a change of spreading direction 3.5 m.y. ago, from about 040° to 022°. It is argued that the small‐scale morphology of the transform fault/spreading center intersections can be understood in terms of the transfer of shear fracture on a transform fault to tensile fracture on a spreading center propagating away from the intersection. It is concluded that the transform fault must be weaker than the spreading center.