Extensional fault-related folding, northwestern Red Sea, Egypt

The Duwi and Hamadat areas on the northwestern margin of the Red Sea rift system exhibit superb outcrop examples of kilometric scale, extensional fault-related folds. In this area, the Nakheil and Hamadat faults of the Rift border fault system consist of a series of WNW- and NW-trending segments that link through breached relay ramps. The fault hanging walls are characterised by a series of offset synclines with non-linear axial traces sub-parallel to the faults. The synclines are doubly plunging and, in the immediate hanging wall to the normal faults, the strata dip sub-parallel to the fault. These folds result from along-strike displacement variations on the individual fault segments together with extension-related fault-propagation folding as the faults propagated upwards through a highly anisotropic pre-rift sedimentary section. In the Duwi area, the axial traces of the synclines are offset and bend in the regions of relay ramps. In the Hamadat area, the synclines associated with individual fault segments are separated by transverse hanging-wall anticlines that formed where segment linkage occurred. Numerical simulations of extension-related fault-propagation folding using a trishear model produce geometries that closely match those of the Duwi and Hamadat folds. The dimensions of these hanging-wall synclines and their relationships to fault displacement variations indicate that they are formed by extensional fault-propagation folding rather than by frictional drag along the fault surface.