RELATING GEOLOGIC STRUCTURE TO GEODETIC MEASUREMENTS WITHIN THE SANTA CRUZ MOUNTAINS, NORTHERN CALIFORNIA, THROUGH OFF-FAULT PLASTIC FAILURE

Geologic and geodetic measurements of deformation record the behavior of fault zones in Earth’s crust to far-field loading on vastly different timescales. We hypothesize that differences between geologically and geodetically measured crustal displacements may reflect the constitutive laws in operation over these different timescales. While geodetic measurements capture interseismic elastic bending in response to far-field loading, geologic observations of deformation record the accrued effect of the relaxation of these stresses by yielding. This may include both frictional slip along plate-boundary structures, as well as off-fault plastic yielding of Earth’s crust.

Our study is focused in the Santa Cruz Mountains, where seismic hazards potentially impact >7 million people living in the San Francisco Bay Area. The Santa Cruz Mountains host a restraining bend in the San Andreas Fault, which serves to sufficiently elevate stresses to induce off-fault plastic strains that are geologically resolvable. The extensive prior work in this area, as well as our own augmentation of this dataset, allow us to quantify these strains. We utilize the low temperature apatite (U-Th)/He system to image trends in inferred exhumation associated with the advection of crust through the restraining bend. We observe recently reset apatite (U-Th)/He ages (1.7Ma) near the beginning of the bend and adjacent to the San Andreas fault, with ages steadily increasing moving northward in the direction of the advection of crust through the bend. We couple these measurements with a 3D geologic model, which we retrodeform to elucidate geospatial trends in young (<4Ma) deformation off of the main San Andreas fault trace. Irrecoverable strains imparted through off-fault plastic failure may serve to accommodate a fraction of plate motion and reduce the resultant frictional slip on the San Andreas fault. Thus, irrecoverable off-fault failure may be essential in reconciling fault zone behavior observed over geodetic and geologic timescales.