Extension and subsidence adjacent to a "weak" continental transform: An example from the Rukwa rift, East Africa

The Rukwa rift is a 300-km-long half-graben basin that lies along the central part of a 1000-km-long continental transform in the East African Rift. The basin contains Carboniferous to Quaternary sedimentary strata ∼11 km thick. Within the basin, maximum extension has been directed nearly normal to the border fault, even though this border fault has been dominated by dextral strike slip. However, extension and subsidence in the Rukwa rift are not explained by releasing-bend or oblique-extension fault geometries. We interpret the orientation of the extension as a local stress rotation symptomatic of a low- friction, "weak" strike-slip border fault. Structural and morphologic features suggest that an along-strike contrast in the frictional strength of the border fault controls the location of this extension. Subsidence and extension normal to the border fault in the Rukwa rift occur along a weak transform fault segment, as an along-strike accommodation of drag and local shortening against an adjacent strong segment. Thus, three-dimensional strain is accommodated differently along the strike of the border fault as a function of the frictional strength of the fault zone.