THE EVOLUTION OF THE COASTAL ZONE OF THE PACIFIC - NORTH AMERICA PLATE BOUNDARY IN CENTRAL AND NORTHERN CALIFORNIA: INSIGHTS FROM GEOMORPHIC, GEOLOGIC AND THERMOCHRONOLOGIC RECORDS

Since its initial recognition in the 1890s, the San Andreas Fault has been the type example of a transform plate boundary, playing a primary role in the development of models for strike-slip fault evolution and kinematics. Our understanding is best developed in subaerially exposed segments of the main strand of the San Andreas fault in central and southern California. The more-elusive, variably offshore faults of the plate boundary – the San Gregorio-Hosgri fault and the San Andreas fault in central and northern California, respectively – are less accessible and thus have been historically less well studied and understood. However, efforts by multiple government, industry and academic research groups over the past decade, are yielding complementary datasets encompassing geophysics, geodesy, geology, geomorphology, and thermochronology which provide fodder for a more holistic understanding of the plate boundary.

Onshore in the Santa Lucia range, fluvial geomorphology, marine terrace surveys and low-temperature thermochronology show that rapid, high-amplitude exhumation is observed within a narrow fault-bound sliver on the northeast side of the San Gregorio-Hosgri fault. Offshore, the USGS California Seafloor Mapping Program is engaged in a massive effort to compile and interpret new marine geological and geophysical datasets to understand both the kinematics and seismic hazards associated with the San Gregorio-Hosgri fault. Together, these results highlight the importance of the “Big Sur Bend” in controlling and focusing vertical strain related to plate convergence. Quantification of basin-average erosion rates using cosmogenic radionuclides illustrates the complexity of the geomorphic response to convergence in time and space.

In northern California, the coastal segment of the San Andreas fault exhibits similar geometric and plate kinematic complexities to the San Gregorio-Hosgri fault. Here, too, significant new onshore (marine terrace mapping, fluvial geomorphology, basin average erosion rates) and offshore (marine geology and geophysics) provides the framework for more integrated models of strike-slip fault kinematics. Combined with existing data, our ongoing low-temperature thermochronologic work should make an important contribution to these kinematic models.