Oblique rifting and segmentation of the NE Gulf of Aden passive margin

Abstract

The Gulf of Aden is a young, obliquely opening, oceanic basin where tectonic structures can easily be followed and correlated from the passive margins to the active mid‐oceanic ridge. It is an ideal laboratory for studies of continental lithosphere breakup from rifting to spreading. The northeastern margin of the Gulf of Aden offers the opportunity to study on land the deformation associated with oblique rifting over a wide area encompassing two segments of the passive margin, on either side of the Socotra fracture zone, exhibiting distinct morphologic, stratigraphic, and structural features. The western segment is characterized by an elevated rift shoulder and large grabens filled with thick synrift series, whereas the eastern segment exhibits low elevation and is devoid of major extensional structures and typical synrift deposits. Though the morphostructural features of the margin segments are different, the stress field analysis provides coherent results all along the margin. Four directions of extension have been recognized and are considered to be representative of two tensional stress fields with permutations of the horizontal principal stresses σ2 and σ3. The two dominant directions of extension, N150°E and N20°E, are perpendicular to the mean trend of the Gulf of Aden (N75°E) and parallel to its opening direction (N20°E–N30°E), respectively. Unlike another study in the western part of the gulf, our data suggest that the N150°E extension stage is older than the N20°E extension stage. These conflicting chronologies, which are nowhere unambiguously established, suggest that the two extensions coexisted during the rifting. On‐land data are compared with offshore data and are interpreted with reference to oblique rifting. The passive margin segmentation represents a local accommodation of the extensional deformation in a homogeneous regional stress field, which reveals the asymmetry of the rifting process. The first‐order segmentation of the Sheba Ridge is inherited from the prior segmentation of the passive margin.