Extensional basin geometry and the elastic lithosphere

Abstract

Although the morphology and dimensions of continental rift basins vary considerably worldwide, one aspect is similar; tectonically active rifts are bordered on one or both sides by relatively long (tens of kilometres) normal fault systems (termed border faults) that largely control basin morphology. We compile data constraining the geometry of border faults within the tectonically active East African Rift system, and evaluate these results with respect to variations in thickness of the elastic lithosphere. Border–fault lengths greater than 75 km occur in regions with deep crustal seismicity and relatively high estimates of effective elastic thickness ( T e derived from forward and inverse models of gravity and topography data ( T e > 25 km). Most East African border faults cross–cut pre–existing structures and basement foliations, although segments of the longest faults (greater than 80 km) reactivate Precambrian shear zones or structural fabrics. From observations in East Africa, comparisons with data from the Aegean and Baikal Rifts, and considerations of the rheology of continental lithosphere, we propose that the elastic lithosphere determines the length, width and style of faults within East Africa, and perhaps other continental rifts.