Hillslope erosion by bedrock landslides and diffusion of unconsolidated sediment

Hillslopes overwhelmingly dominate all landscapes on Earth and, therefore, are fundamental to geomorphology. Hillslopes respond to tectonic and climatic forcing by producing either rapidly eroding threshold landscapes dominated by steep bedrock slopes and deep-seated landslides or soil-mantled landscapes dominated by diffusive hillslope processes. Although these two characteristic hillslope styles have been identified throughout the world, the physical mechanics of how these landscapes evolve remain poorly understood. In order to better constrain the factors that govern the rates and styles of hillslope erosion, we conduct two end-member studies to assess how hillslopes evolve in both bedrock-dominated and sediment-mantled landscapes. The first study examines bedrock hillslopes in Fiordland and the Southern Alps of New Zealand. Comparison of landslide distributions from both regions reveals order-of-magnitude differences in the size and frequency of bedrock landslides. In order to better constrain the influence of bedrock properties in governing rates and patterns of landslides, we present a new method for quantifying bedrock fracture densities within the shallow subsurface based on seismic refraction surveys. We show that, in the Southern Alps, tectonic forces have pervasively fractured intrinsically weak rock to the verge of instability, such that frequent landslides can potentially extend to great depths. In Fiordland, the strong intact rock remains stable despite tectonic fracturing and therefore, bedrock failure generally occurs only when geomorphic fracturing further reduces the rock-mass strength, thus limiting the depths of bedrock landslides to within this geomorphically fractured zone. The second study examines diffusive hillslope processes acting on fluvial terrace risers in New Zealand and Idaho. We show that the linear diffusion model does not accurately describe the downslope transport of unconsolidated sediment and, therefore, sediment-flux appears nonlinearly related to surface slopes. We implement an alternative method for assessing relative differences in riser degradation based on geometric scaling relationships. We show that, in both regions, two-fold differences exist between equator- and pole-facing risers Moreover, a two-fold difference exists for risers of the same orientation between Idaho and New Zealand. Similar geometric trends in both regions, however, reveal that terrace riser degradation is governed by a widely applicable relationship between sediment flux and slope.