Regional-scale, high-resolution measurements of hilltop curvature reveal tectonic, climatic, and lithologic controls on hillslope morphology

Abstract

Climate, tectonics, lithology, and biology are encoded within the morphology of landforms. Hillslopes record uplift and erosion rate through hilltop curvature, in which sharper, more convex hilltops correspond with more rapid erosion rates. However, past hilltop curvature studies that map uplift and erosion rates have been limited to small spatial scales largely due to relatively slow speeds of curvature measurement techniques. This lack of regional-scale observations has made deconvolving the relative contributions of tectonics, climate, and lithology to hillslope morphology a challenge. Here, we used high performance computing and continuous wavelet transforms of topography to rapidly map hilltop curvature in the steep and dissected Oregon Coast Range (OCR) and the adjacent gentler Cascadia Forearc Lowland (CFL) in western Oregon, amounting to ∼43,000 km² of 1 m lidar data. We additionally compared mapped hilltop curvature to published erosion rates derived from cosmogenic ¹⁰Be, including 11 newly sampled watersheds. We observed that hilltops are systematically sharper in the OCR than in the CFL, and we noted a linear relationship between catchment-averaged erosion rate and hilltop curvature, consistent with previous observations and theory that erosion rate scales linearly with hilltop curvature in soil-mantled landscapes. The boundary between the OCR and CFL, as demarcated by hilltop curvature, is often abrupt and occurs across mapped structures that separate disparate baselevels but where lithology and mean annual precipitation remain constant. Thus, while we observed significant variability in hilltop curvature that results from secondary lithologic and climatic controls, our results demonstrate that hillslope morphology in western Oregon is set primarily by uplift via tectonically-controlled baselevel lowering rates. These regional interpretations additionally highlight the computational advantages of the wavelet transform for rapidly quantifying hilltop curvature.