Comparisons between the rift systems of East Africa, Earth and Beta Regio, Venus

The rift systems of southern East Africa and Beta Regio, Venus are similar in a number of ways. The rifted East African and Venusian lithospheres have effective elastic thicknesses of ∼ 30 km, suggesting that both lithospheres maintain significant flexural strength during rifting. Both rift systems have maximum fault segment lengths of ∼ 100 km. The effective elastic thickness and maximum fault segment length of both rifts are greater than those seen in many other active extensional regions on Earth, despite the high surface temperatures on Venus. We suggest that the southern East African and Venusian elastic thicknesses and maximum fault segment lengths are due to stronger lithosphere. The rift systems differ in the maximum width of their half graben. East African half grabens are up to ∼ 50 km wide, whilst those on Venus are up to ∼ 150 km wide. To support the topography associated with such half grabens requires shear stresses to act on the bounding faults. In East Africa the greater elastic thickness (compared to most other terrestrial extensional regions) means that wide half grabens can form without requiring the shear stresses acting on the bounding faults to be greater than the ∼ 1–10 MPa (10–100 bars) stress drop typically seen in earthquakes. However, on Venus the absence of sediment infill, greater widths and larger effective topographic steps of the half grabens require shear stresses of up to ∼ 80 MPa (800 bars) to act on the bounding faults. This difference is significant; Venusian faults must be stronger than those on Earth.