A fault runs through it: Modeling the influence of rock strength and grain-size distribution in a fault-damaged landscape

We explore two ways in which the mechanical properties of rock potentially influence fluvial incision and sediment transport within a watershed: rock erodibility is inversely proportional to rock cohesion, and fracture spacing influences the initial grain sizes produced upon erosion. Fault-weakened zones show these effects well because of the sharp strength gradients associated with localized shear abrasion. A natural example of fault erosion is used to motivate our calibration of a generalized landscape evolution model. Numerical experiments are used to study the sensitivity of river erosion and transport processes to variable degrees of rock weakening. In the experiments, rapid erosion and transport of fault gouge steers surface runoff, causing high-order channels to become confined within the structure of weak zones when the relative degree of rock weakening exceeds 1 order of magnitude. Erosion of adjacent, intact bedrock produces relatively coarser grained gravels that accumulate in the low relief of the eroded weak zone. The thickness and residence time of sediments stored there depends on the relief of the valley, which in these models depends on the degree of rock weakening. The frequency with which the weak zone is armored by bed load increases with greater weakening, causing the bed load to control local channel slope. Conversely, small tributaries feeding into the weak zone are predominantly detachment limited. Our results indicate that mechanical heterogeneity can exert strong controls on rates and patterns of erosion and should be considered in future landscape evolution studies to better understand the role of heterogeneity in structuring landscapes.