Late cenozoic denudation and uplift rates in the Santa Lucia Mountains, California

Apatite (U-Th)/He ages from a vertical transect through the Santa Lucia Mountains, central California Coast Ranges, are used to reconstruct the history of exhumation and of bedrock and surface uplift in this region since ca. 6 Ma. We find a direct correlation between (U-Th)/He ages and elevation, which we interpret to correspond to denudation rates of ∼0.35 mm/yr between 6 and 2 Ma. The onset of bedrock uplift and exhumation ca. 6 Ma followed a change in plate motion ca. 8 Ma. After 2 Ma, denudation rates increased substantially (∼0.9 mm/yr). This is a rare instance in which long-term average bedrock (∼0.85 mm/yr) and surface (∼0.20 mm/yr) uplift can be calculated from denudation rates and stratigraphic data. The post-2 Ma denudation rate is about one order of magnitude higher than independently determined river erosion rates in the area. We suggest that this discrepancy indicates that exhumation of the steep western slopes of this segment of the Coast Ranges has been dominated by mass wasting via landslides, rather than fluvial erosion, at least since ca. 2 Ma. We also show that the bedrock uplift is predominantly tectonic, not isostatic.