UC Santa Cruz

California’s Franciscan mélange is well-known for hosting persistent, slow-moving earthflows, a landslide style favored by the unit’s significant local variability in shear strength (e.g. Medley, 2001; Kim et al., 2004; Roering et al., 2015). Big Sur’s mélange, however, is unusual: landslide mapping there reveals both recent (e.g. the 2017 Mud Creek landslide) and prehistoric rotational landslides on slopes that are unusually steep for this lithology. Because rotational landslides and earthflows pose markedly different hazards, understanding what drives the shift in failure style is critical for hazard assessment. Here we test the hypothesis that the rate of base-level lowering governs the transition between earthflow and rotational failure. We synthesize landslide mapping and analysis of high-resolution topography across two Franciscan mélange settings with contrasting rates of base-level lowering: the South Fork Eel River canyon, where river incision drives base-level lowering at ~1 mm/yr, and the southern Big Sur coast, where wave erosion drives local base-level lowering at ~2 cm/yr. We then use 2-D numerical modeling to test whether rapid coastal retreat alone can drive a transition from earthflow to rotational failure in a Franciscan mélange hillslope.Landslide mapping shows that landslide activity in Big Sur, as defined by InSAR-derived deformation rates, is concentrated along the actively eroding coastline, and that the two regions differ systematically in failure style according to overall hillslope angle. Big Sur’s steeper slopes are dominated by rotational landslides, while the Eel River region’s shallower slopes are dominated by earthflows. Numerical modeling shows that progressive lateral erosion can produce rotational failure on hillslopes that were previously deforming as earthflows, provided there is some degree of vertical heterogeneity in the weathering profile. Taken together, these results indicate that changes in the rate of base-level lowering can drive wholesale shifts in landslide failure style and hazard, likely mediated by long-term coupling between base level and hillslope weathering.