BEDROCK FRACTURE DENSITY CONTROLS ON SOIL PRODUCTION, TRANSPORT, AND BEDROCK EXPOSURE IN STEEP LANDSCAPES

The architecture of a hillslope is strongly dictated by bedrock fractures, which typically influence fluid flow through fresh bedrock, hillslope susceptibility to mass wasting, and the grainsize of hillslope sediment. Although these changes affect weathering, soil formation, and the mobility of hillslope sediment, bedrock fracture spacing controls on hillslope morphology and erosion rate remain challenging to quantify at the landscape scale. Here, we compare the San Gabriel Mountains (SGM) and Northern San Jacinto Mountains (NSJM) in southern California; two steep landscapes with similar climate and lithology, but with different tectonic setting and bedrock fracture density. In each landscape, we measure bedrock fracture density by tracing fractures on centimeter-resolution orthophotos (n = 50) derived from 3D structure-from-motion models of bedrock cliffs. Each model is georeferenced to airborne lidar point clouds, allowing us to quantify spatial trends in bedrock fracture spacing at the landscape scale. We pair measurements of bedrock fracture spacing with detrital in-situ cosmogenic ¹⁰Be-derived erosion rates, catchment mean hillslope angle measurements, and high-resolution mapping that distinguishes bare-bedrock and soil-mantled hillslopes. NSJM cliffs have a mean bedrock fracture spacing of 0.45 m m^-2, and SGM cliffs have a mean fracture spacing of 1.8 m m^-2. For similar mean hillslope angles (35-46°), NSJM catchments erode at rates of 0.1-0.6 m kyr^-1, compared to 0.2-2.2 m kyr^-1 in the SGM. Higher erosion rates in the SGM require a 3x higher soil transport efficiency, reflecting an indirect control of bedrock fracture density on the size of sediment armoring hillslopes. For a given erosion rate, ~2x more bare-bedrock cliffs are exposed in the NSJM than the SGM, indicating that wider bedrock fracture spacing reduces soil production efficiency and supports steeper, rockier hillslopes. The contrast in erosion rate and soil production for similar hillslope morphology in the SGM and NSJM implies a link between tectonic setting, bedrock fracture density, and hillslope erodibility, such that bedrock fracturing may reduce the relief of tectonically active mountain ranges. Contrarily, sparsely fractured bedrock may support large cliffs and high relief long after tectonic activity has ceased.