Rigid-Block Rotation: An Explanation for Extensional Deformation In the South Dobe Graben, Central Afar, Ethiopia

The graben- and horst-dominated terrain of central Afar contains arrays of closely spaced normal faults located in areas of displacement transfer between major graben bounding faults that are identified as relay zones, accommodation zones or composite zones where elements of both are encountered. In the south Dobe composite zone, a major ramp between the Dobe and Hanle grabens displays an overall dip of 8-10º to the SE. Elongate fault blocks that are variably rotated to the NE and bounded by faults antithetic to the graben-bounding faults characterize the ramp. The width and tilt of the fault blocks in the south Dobe composite zone show a systematic variation: widths, which range from 22m to 1173m, generally increase away from the graben-bounding faults; whereas dips, which range from 4º to 25º, increase towards the graben-bounding faults. In other words, there is an inverse relation between fault block width and dip. The rotated fault blocks are massive and display consistent dips signifying no internal deformation. The absence of lateral displacement along an old drainage network that runs across four of the rotated fault blocks indicates pure dip-slip movement on the antithetic faults. Analysis of the data suggests that deformation in the south Dobe zone was accommodated primarily by rigid-block rotation of discrete fault blocks between antithetic faults. Fault-block widths are a function of the wedge-shaped geometry of the hanging wall of the major bounding fault, with the thinner part of the wedge adjacent to the graben-bounding fault having more closely spaced antithetic faults. A better understanding of the mechanism of formation of the south Dobe composite zone may help to decipher the much bigger but similar faulting pattern manifest along the enigmatic Mak'arrasou fault zone bordering the southwest Danakil horst.