Rheology of the lithosphere in the East African Rift System

The rheological properties of the lithosphere in the East African Rift System are estimated from regional heat flow and seismic constraints. Heat flow data are used to infer average, maximum, and minimum geotherms for the Eastern Rift, the Western Rift, and the surrounding shield (having surface heat flow of 106±51, 68±47, and 53±19mW m−2, respectively). Combining the geotherms with brittle and ductile deformation laws for a lithosphere of appropriate structure and composition yields rheological profiles, thickness of brittle layers, and thickness and total strength of the lithosphere. The thickness of the uppermost brittle layer varies from 10 ± 2 km in the Eastern Rift, to 186+−9km in the Western Rift, and 26+ km in the shield; seismicity is confined to the brittle layers. The rheological thickness of the lithosphere in Eastern Rift (23+8−9km), Western Rift, and shield is approximately in the the ratio 1:2.5:5, and matches the elastic flexural thickness. The total resistance to deformation is one order of magnitude larger in the shield than in the rifted regions (∼1012N m−1), where the whole lithosphere is probably in a state of failure.