Relay and accommodation zones in the Dobe and Hanle grabens, central Afar, Ethiopia and Djibouti

Abstract

The right-stepping Dobe and Hanle grabens display a variety of structures that serve to transfer extensional displacement. These structures range from relay zones between overlapping fault segments to accommodation zones between interacting rift segments. The study reveals the presence of three examples of displacement transfer structures: a breached relay zone, a large-scale accommodation zone that is partially breached, and a composite zone that combines elements of both. All three examples exhibit common structural elements. First, dipping ramps develop between horizontal horst blocks and graben floors. Second, these ramps are cut by numerous faults, most of which are antithetic to the ramps and the graben boundary faults. The antithetic faults bound elongate blocks that are rotated into the grabens. Third, crosscutting faults partially or completely link the en echelon or overlapping graben boundary faults. The identification of precursory structures (mode I fractures) at the leading edge of the Dobe–Hanle accommodation zone breaching faults suggest that the breaching process may be continuing. The spatial alignment from north to south of the crosscutting faults, open fractures and lineaments indicates that the breaching process is progressing from the Dobe graben towards the Hanle graben.