Long-term exhumation rates exceed paleoseismic slip rates in the central Santa Monica Mountains, Los Angeles County, California

摘要

Where short-term (10(2)-10(4) yr) tectonic deformation rates derived from paleoseismology and tectonic geomorphology differ from long-term (10(5)-10(6) yr) rates derived from geologic and thermochronometric methods, opportunities arise to assess the causes of spatio-temporal variations in fault slip rates. Such discrepancies are germane to understanding seismic hazard and fault interactions in regions such as southern California, USA, that are underlain by complex fault systems prone to large earthquakes on blind thrust faults. We present thermochronometric results from two transects in the hanging wall of the Malibu Coast fault in the Santa Monica Mountains, on which limited evidence for recent earthquakes has been found. Apatite (U-Th-Sm)/He ages from these transects range from late Miocene to Pleistocene but are dominantly Pliocene. Zircon (U-Th)/He ages from Cretaceous-Paleocene strata are partially reset. Inverse thermal modeling of these data indicates that the most deeply buried samples reached peak temperatures of similar to 150 degrees C (similar to 5 km depth) at 5-7 Ma, and were then exhumed at rates of similar to 1 mm/yr. These long-term exhumation rates are faster than short-term fault slip rates on which current hazard assessments are based.