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.
This 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.
We employ the Abaqus finite element software package to mechanically model and quantify the effects of this off-fault plastic deformation in the production of deformation and topography in the Santa Cruz Mountains. The Santa Cruz Mountains juxtapose compliant and rigid crust in the vicinity of asymmetric topography and deformation surrounding the Santa Cruz Mountains restraining bend. This juxtaposition appears to strongly modulate the topography and deformation within the range. While topographic advection and fault interaction may influence the distribution of topography and deformation observed surrounding the bend, we propose that contrasts in integrated strength of the crust may fundamentally control the localization of plastic deformation and the subsequent development of asymmetric crustal displacement fields and topographic distributions we observe. In actively deforming regions, the juxtaposition of weak and strong crustal units may localize deformation and plastic strain in relatively weak parcels of crust and ultimately lead to a reduction in frictional slip along the San Andreas fault.