Effect of depleted continental lithosphere counterflow and inherited crustal weakness on rifting of the continental lithosphere: General results

Abstract

Some non‐volcanic rifted continental margins differ from the predictions of uniform lithospheric extension in two significant ways, namely: 1) large tracts of continental mantle lithosphere are exposed at the seafloor or are inferred to have been emplaced beneath the outer regions of the margin; 2) rifting evolves from distributed extension with multiple rifts to terminal focused rifting. These characteristics are explained by depth‐dependent extension. We investigate this type of deformation using 2D upper‐mantle‐scale dynamical finite element models of rifting that include: 1) lithospheric counterflow, the gravitationally driven lateral flow of depleted lower lithosphere during rifting, as a mechanism for underplating and exhuming continental mantle, and; 2) inherited crustal weak zones that cause initially distributed syn‐rift extension. Depleted lower lithosphere (compositionally less dense than sublithospheric mantle by 50–80 kg m−3, with a moderately ‘dry’ olivine rheology) efficiently flows laterally thereby exhuming zones of continental mantle lithosphere that are respectively up to 90 and 140 km wide. Inherited crustal weak zones that are offset from the main central rift axis can be reactivated as offset rift basins. Extension in these basins depends on their proximity to the central rift and the ease with which crust decouples from mantle: narrow (<50 km) basins when crust does not decouple, and longer‐lived wider basins for moderate decoupling. For weak crust that readily decouples during depth‐dependent extension, the offset rift basins may remain active throughout rifting, giving rise to highly allochthonous crustal terranes that are translated toward and over the central rift axis.