Evidence for Rapid Post-Pliocene Exhumation of the Santa Monica Mountains, California, from Apatite (U-Th)/He Thermochronometry

Abstract:

Potential losses related to large earthquakes on blind or previously unrecognized thrust faults is of significant concern to southern California, where numerous individual mountain ranges are underlain by active faults. Some of the most hazardous thrust fault systems in Southern California are associated with high-slip-rate faults in the northern portion of the western Transverse Ranges, while the southern region is generally considered to be less seismically active. Determining slip rates on faults bounding the Santa Monica Mountains has been challenging, in part because many of the faults that underlie the range have submarine surface traces. Existing geologic studies predict that these faults slip relatively slowly; however, recent GPS models predict a band of relatively fast contraction on faults that lie beneath the Santa Monica Mountains (Marshall et al., 2013). These geodetic models suggest unrecognized hazard associated with shortening and vertical uplift of this range.

Late Cenozoic strata in the central Santa Monica Mountains are of sufficient thickness to bury Cretaceous and Paleocene strata above the closure temperature for apatite (U-Th)/He thermochronometry (~70°C). As a result, these older rocks, now exposed in the southern Santa Monica Mountains, may record exhumation associated with fault slip and associated structural deformation of the range. Preliminary apatite (U-Th)/He ages near Las Flores Canyon span from 3.5 to 6.5 Ma, and are the youngest apatite (U-Th)/He ages we are aware of in southern California outside of the transpressional San Andreas system. When plotted as depth beneath the base of the marine Modelo Formation, an inflection in age/depth gradient at 4 Ma is inferred to reflect the onset of fault motion and is consistent with the late Miocene age of the Modelo Formation. Based on average geothermal gradients for the Ventura and Los Angeles basins and an assumed thrust fault dip of 20°, observed apparent exhumation rates are consistent with ~ 2 mm/yr dip slip motion on a thrust fault beneath the Santa Monica Mountains. The results from this study are consistent with geodetic models of high slip rates beneath the Santa Monica Mountains and thus refine our understanding of seismic hazard in southern California.