Modes of Extensional Faulting Controlled By Surface Processes

Abstract:

We investigate the feedbacks between surface processes and extensional faulting by coupling a 2D thermo-mechanical rifting model with a landscape evolution law. Long-term climate-tectonic feedbacks are essential in shaping the dynamics of convergent plate boundaries, yet the nature and importance of these feedbacks is not well characterized in extensional settings. Focusing on the evolution of a single rift-bounding normal fault, we show that erosion and deposition can significantly enhance fault life span—i.e., the amount of horizontal extension a fault can accommodate before a new fault breaks in intact lithosphere. In simulations with very slow erosion rates, a typical 15 km-thick brittle layer extends via a succession of crosscutting short-lived faults (2–5 km heave). By contrast, erosion rates on par with or faster than the regional extension rate promote the sequential growth of long-lived faults (heave >10 km). We complement our numerical simulations with a simple force balance model in which we isolate the effect of topography growth on fault evolution. We show that in faulted layers thicker than 10–15 km, fault life span is weakly sensitive to lithospheric thickness and strength, but can be highly sensitive to changes in erosion and deposition rates within geologically relevant bounds. We therefore propose that the major range-bounding normal faults observed in many continental rifts owe their large offsets to erosional and depositional processes rather than to the mechanical properties of the lithosphere.