halfgraben

A halfgraben is a geological feature characterized by a single major fault bounding one side of a subsiding basin. In contrast to a full graben, which is dropped down between two parallel faults, a halfgraben has a distinct asymmetry. The basin is tilted, with the downthrown side adjacent to the major fault, and the footwall block on the opposite side is either uplifted or remains at a higher elevation.

The formation of halfgrabens is typically associated with extensional tectonic regimes. As the Earth's crust is pulled apart, normal faults develop. When these faults are listric, meaning they curve and flatten with depth, they can lead to the development of halfgrabens. The overlying rock mass slides down the curved fault plane, creating a tilted block and a subsiding basin.

These features are common in rift zones, such as the East African Rift System, and in passive continental margins. Sediments often accumulate within the halfgraben basin, preserved by the subsidence. The asymmetrical nature and tilting of the beds within a halfgraben can provide valuable information about the direction and magnitude of the extensional forces that formed it. The hanging wall block moves down relative to the footwall block along the main bounding fault.