Quantifying the role of off-fault plastic failure in reconciling differing geodetic and geologic slip rates along the San Andreas fault in the Santa Cruz Mountains, CA using thermochronology and finite element modeling

Geologic and geodetic measurements of deformation record the response of Earth's crust to 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 timescales. In this view, geodetic measurements capture interseismic elastic bending in response to far-field loading, while geologic observations of deformation record the accrued effect of the relaxation of these stresses by yielding.

We constrain deformation surrounding a restraining bend along the San Andreas Fault (SAF) within the Santa Cruz Mountains, CA, and use finite-element modeling to understand contrasting patterns and rates of deformation over geodetic and geologic timescales. Here, geodetic measurements imply a SAF strike-slip rate of 16.8 mm/yr, versus the geologically-derived fault slip rate of 10 mm/yr. We complement these geologic inferences of strike-slip rates using the apatite (U-Th)/He system to quantify deformation and inferred exhumation associated with the advection of crust through the restraining bend. We observe recently reset ages (1.7Ma) near the beginning of the bend and adjacent to the SAF, with ages increasing moving northward in the direction of the advection of crust through the bend. These data suggest a SAF slip rate of 11 mm/yr, similar to the geologically derived rate.

Next, we use a finite element model of the restraining bend by considering a frictional contact surface embedded in an elastoplastic medium to interpret geologic and geodetic observations in a unified framework. Model results show that, under certain constitutive rules and boundary conditions, crust is advected and uplifted along the restraining bend, consistent with thermochronologic, topographic, and geomorphic observations. Additionally, velocities along the modeled fault slow within around the restraining bend as the surrounding crust plastically deforms. We contrast these patterns of long-term deformation with those expected over geodetic time-scales, by calculating velocities for a purely elastic medium at the end of each model simulation. We find that irrecoverable off-fault failure may be essential in reconciling fault zone behavior observed over geodetic and geologic timescales.