Mountain-building under extension

Abstract

A mechanism is presented which explains how intra-continental rifting can cause large topographic uplift. The effect is sufficient to account for the uplift of rift flanks and the very high and strongly localized uplift of the Rwenzori horst in the Western Branch of the East African Rift System. We propose that the uplift is generated by crustal bending, which is caused by a misfit of the lateral tensile stress between the upper and middle crust. The misfit is a function of different yield mechanisms when the upper crust breaks whereas the middle crust flows. Two independent numerical schemes confirm the suggested uplift mechanism. Both models—a 2 and 2.5 D elastoplastic lattice-particle model and a multilayer beam model—were used to calculate the surface topography as a result of lateral uniaxial extension. Using the fault geometry of the Rwenzori area, we find that the amount of topographic uplift is controlled by the viscosity and elasticity of the crust. The extreme uplift of the Rwenzori horst is—at least to some extent—a function of its considerably high elastic stiffness. The stiffness unites the two rifts that bound the Rwenzori horst and leads to an extremely high topography and a high Moho uplift in the center of the two rifts where the Rwenzori mountains sit.