In mountainous areas, landslides regulate temporal variations in sediment production and may suppress simple linkages between topographic development and tectonic forcing. Rates and mechanisms of mass wasting depend on lithology, bedrock structure, and climatic and tectonic setting. These factors tend to vary significantly in active tectonic regions, thus clouding our ability to predict how landsliding modulates topographic development over human and geological time scales.
Here, we use a novel DEM-based technique to document the distribution of large landslides in the Oregon Coast Range (OCR) and quantify how they affect topographic relief. We developed an automated algorithm that exploits the distinctive topographic signature (specifically the relationship between curvature and gradient) of large landslides to map their distribution within the gently folded Tyee Formation (Eocene deltaic-submarine ramp sediments). In contrast to steep and highly dissected terrain frequently identified as characteristic of the OCR (which exhibits steep, planar side slopes and highly curved, low-gradient ridge tops and valleys), terrain prone to large landslides tends to have low values of both drainage density and curvature and gradient values that cluster between 0.16 and 0.44. The distribution of failur-dominated terrain in our 10,000 km2 study area is influenced by systematic variations in sedimentary facies and bedrock structure. The fraction of terrain altered by large landslides (>0.1 km2) varies from 5% in the sand-rich (delta-slope and proximal ramp facies) southern section of our study area to ∼25% in the north (distal ramp facies), coincident with an increase in the thickness of siltstone beds and a decrease in the sandstone:siltstone ratio. Local relief declines progressively northward, suggesting that deep-seated landsliding is sensitive to the thickness and frequency of low-shear-strength siltstone beds and may serve to limit topographic development in the OCR. Structural controls are superimposed on facies-related variations as deep-seated landslides are frequently found on slopes whose downslope aspect corresponds to the bedrock dip direction. For 1516 strike and dip measurements in our study area, we calculated the fraction of proximal terrain (<2.5 km) altered by deep-seated landsliding. In the sand-rich southern region, the proportion of proximal slid-dominated terrain increases modestly with bedrock dip. In the silt-rich northern region, terrain altered by deep-seated landsliding is pervasive, and an increase in dip from 0° to 16° corresponds to a change in the fraction of slid-prone terrain from 10% to 28%.
Our technique for mapping large landslides has utility for hazard analysis and land management. Over million-year time scales, the progradational character of the Tyee Formation suggests that continued uplift and exhumation of the OCR should result in a southward propagation of slid-prone, silt-rich distal facies. As a result, deep-seated landsliding will become increasingly prominent, and topographic relief in the central and southern OCR will progressively decline. Whereas spatial variability in climatic or tectonic forcing is often invoked to explain systematic variations in topographic development, our results emphasize the importance of structural and intraformation lithologic controls on landsliding. As such, analyses linking surface processes, climate, tectonics, and landscapes should be couched in the context of diverse geologic and topographic data.