THE ROLE OF SURFACE PROCESSES IN STABILIZING BASIN AND RANGE STYLE HALF-GRABEN STRUCTURES (Invited Presentation)

We present new simulations of rifting coupled with a landscape evolution model to quantify the effect of surface processes on the evolution of half-graben structures. Half-grabens consist of a footwall massif and an asymmetric hanging wall basin formed by slip on a normal fault that takes up most of the regional strain. While half-grabens are quasi-ubiquitous features of continental rifts, theoretical considerations often fail to explain how extension of magnitude comparable to the thickness of the brittle upper crust (~10 km) can remain localized on a single fault, as is observed in the Basin and Range province. This is because many processes that accompany fault slip (e.g., crustal flexure) act to defocus deformation by promoting the initiation of new normal faults. We argue that efficient erosion and sedimentation are the missing ingredient to stabilize half-grabens by alleviating the mechanical cost of building topography.

To test this hypothesis, we couple a 2-D long-term tectonic code with a landscape evolution model that incorporates stream power erosion, hillslope diffusion, and sedimentation. We identify a realistic range of landscape evolution parameters by compiling and modeling the morphology of normal fault bounded massifs throughout the Western US. Erodibility, precipitation, and extension rate are then varied systematically within this documented range, along with the strength of the faulted layer, in order to identify key controls on the modes of extensional faulting. For moderate layer strengths and extension rates of ~1 mm/yr, precipitation rates of order 100–1000 mm/yr generate enough surficial mass redistribution to allow half-grabens to accumulate over 10 km of offset and ~1 km of relief. By contrast, precipitation rates lower than ~100 mm/yr and/or decreased rock erodibility lead to greater relief and topographic stresses that turn half-grabens into horsts or full grabens after a few km of extension. We therefore suggest that the hydrological and lithological conditions of the Basin and Range province are both necessary and sufficient to sustain the growth of half-grabens. Conversely, a horst-and-graben style of faulting may be favored by anomalously inefficient surface processes, perhaps related to the exposure of strong basement lithologies.