Fault damage as a primary control on patterns and rates of erosion in alpine rivers: examples from the Southern Alps, New Zealand

Abstract:

Fault damage has a quantifiable influence on the rates and patterns of fluvial erosion. Tectonic stresses can exceed the critical yield of bedrock and cause brittle failure response, generating a heterogeneous distribution of strength and fracture density in the form of fault damage zones within an intact bedrock host formation. Field observations of fault structures in the Southern Alps of New Zealand demonstrate that fluvial processes of erosion and transport are highly sensitive to a significant local increase in erodibility attributed to rock disaggregation and a comparatively smaller critical discharge required to transport fine grained fault gouge and cataclasites. In order to provide quantitative data for a problem that has remained largely conceptual for over a century, we combine models of landscape evolution and rock damage to investigate the influence of fault damage on the rates and patterns of landscape development and adjustment. Model results suggest that high order rivers will align with exposed fault structures due to the relatively rapid erosion and transport of damaged rock if strength is reduced by a factor of 30 relative to surrounding intact bedrock. Observations of NZ fault damage zones indicate that strength reduction by a factor of 1000 is characteristic of cataclasis, far exceeding the threshold factor for channel entrainment. Fault erosion also increases the response rate of landscapes to base level perturbations by an order of magnitude or greater. Further, the relief of structurally aligned valleys creates capacity for substantial transient sediment storage deep within tectonically active landscapes. Sensitivity analysis in a coupled landscape evolution-geodynamics model suggests that fluvial erosion plays an important role in the development of topographic stress and ultimately in the partitioning of strain within active orogens, due primarily to a strong positive feedback response between tectonic strain and fluvial erosion, driven by rock damage and concentrated in fault structures.