RAPID PLIOCENE EXHUMATION OF THE SANTA MONICA MOUNTAINS, CALIFORNIA, FROM APATITE AND ZIRCON (U-TH)/HE THERMOCHRONOLOGY

Large earthquakes on blind or unrecognized thrust faults are of significant concern in southern California, where numerous individual mountain ranges are underlain by active faults. Some of the most hazardous thrust faults in Southern California are located in the northwestern Transverse Ranges, while the southwestern region, including the Santa Monica Mountains, 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 and zircon (U-Th)/He thermochronometry (~70° and 180°C, respectively). As a result, these older rocks, now exposed in the southern Santa Monica Mountains, record exhumation associated with fault slip and associated structural deformation of the range. Apatite (U-Th)/He ages near Las Flores Canyon range from 2.3 to 6.2 Ma, while a transect near Zuma Canyon yields ages from 4.5 to 7.7 Ma. Zircon (U-Th)/He ages from Cretaceous sandstones along these transects are partially reset, as revealed by thermochronometric ages that are substantially younger than depositional ages. Thermal modeling of the cooling history of these transects using QTQt suggests progressive burial from Cretaceous to late Miocene time, consistent with the stratigraphic record preserved in the range. Exhumation of the range began in late Miocene to early Pliocene time, and appears to continue to present at rates of 1 mm/yr. This exhumation rate implies slip rates on the fault that underlies the range of 2-3 mm/yr, depending on assumptions of fault dip. The results from this study are consistent with geodetic models of high slip rates and indicate increased seismic hazard in the Santa Monica Mountains.