CoachIn the simple-shear rift model the lithosphere is pulled apart along a low-angle shear zone that cuts through it from the surface to its base. The plate above the shear zone, the upper plate, loses its lower part; the plate below, the lower plate, is pulled out from under it. The thinning of the crust and the thinning of the mantle are offset sideways, and the two conjugate margins that form when the continent breaks are asymmetric.
