Bounds on fault strength based on simulation of "rider block" structures emerging from brittle lithosphere extension

Continental and oceanic core complexes are the most recently recognized major tectonic features on Earth. They represent extension focused on normal faults with tens of kilometers offset. In some core complexes, kilometer-scale "rider blocks" are cut off from the hanging wall and passively transported on top of the large offset normal fault. Previous numerical simulations of lithospheric extension produced the large offset, core complex-forming, normal faults only when the faults were weaker than a given threshold. We consider extensional faulting of Mohr-Coulomb layers in high-resolution simulations to refine the conditions that can lead to rider block formation and estimate the size of rider blocks formed. Presenting a guiding analytic theory based on Andersonian fault mechanics as well, we show that only a narrow range of fault weakening, relative to intact surrounding rock, allows for a consecutive series of rider blocks to emerge in a core complex-like geometry. Furthermore, rider blocks develop when the dominant form of weakening is by reduction of fault cohesion while faults that weaken primarily by friction reduction do not form distinct rider blocks.