Extensional basin geometry and the elastic lithosphere

Although the morphology and dimensions of continental rift basins vary cons iderably worldwide, one aspect is similar; tectonically active rifts are bo rdered 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 tect onically active East African Rift system, and evaluate these results with r espect to variations in thickness of the elastic lithosphere. Border-fault lengths greater than 75 km occur in regions with deep crustal seismicity an d relatively high estimates of effective elastic thickness (T-e) derived fr om forward and inverse models of gravity and topography data (T-e > 25 km). Most East African border faults cross-cut pre-existing structures and base ment foliations, although segments of the longest faults (greater than 80 k m) reactivate Precambrian shear zones or structural fabrics. From observati ons 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 faul ts within East Africa, and perhaps other continental rifts.