THRUST FAULTING IN THE NORTHERNMOST FORELAND ZONE OF THE CAPE FOLD BELT, FORT BEAUFORT, EASTERN CAPE, SOUTH AFRICA

Geological structures at two localities, some 20 km apart, along the R67 National Road between Grahamstown and Fort Beaufort, show that thrust faulting and folding have affected rocks of the Karoo Supergroup at both localities. At the northern locality, which is only 10 km south of Fort Beaufort, a prominent backthrust displaces shales and mudstones in hanging wall strata southwards over footwall rocks containing a series of forethrusts with a ramp/flat configuration, as well as imbricate faults, all dipping south. The backthrust developed in response to southward movement of hanging wall rocks, creating a gentle fold above a proposed footwall ramp. At the southern locality, a complex arrangement of fore and backthrusts in a structural triangle zone, composed mainly of sandstone beds, formed as a result of duplexing. The duplex is truncated by a backthrust on its northern side, the latter forming part of a series of backthrusts into the foreland. The development of structures at both localities is interpreted as coeval because the fold axial planes and strike orientation of thrusts have the same structural orientations. A compressional model is invoked to explain the structural harmony displayed by the thrust faulting and folding. Structures at the study sites are linked to a proposed sole thrust/detachment fault rooted in Cape Supergroup rocks farther south, where thrust faulting is known to be present in these rocks. The presence of backthrusts just south of Fort Beaufort suggests that the deformation front for the Cape Fold Belt, including further fore-thrusting, occurs to the north of the northern locality studied.