Spatial and temporal variation in deformation magnitude adjacent to the San Andreas Fault system, Santa Cruz Mountains, California

The Santa Cruz Mountains of California record a lengthy history of deformation, including both slip on the dextral San Andreas Fault (SAF) system and distributed off-fault strain. A restraining bend in the SAF complicates regional kinematics. Following previous workers, we here assume the bend is fixed relative to the crust NE of the SAF and that crust SW of the fault is advected through the bend. This implies differences in timing and style of deformation on opposite sides of the SAF. We present two new datasets quantifying spatial and temporal variations in strain magnitude, focusing on the SW side of the SAF to allow us to consider the effects of the restraining bend on the deformation record. First, we quantify spatial variation in deformation magnitude using the geometry of the base of the Miocene/Pliocene Purisima Formation in both the Santa Cruz region (southern Santa Cruz Mountains) and in the La Honda basin (northern Santa Cruz Mountains), as reconstructed from outcrop elevations, well data, and bedding orientation measurements. This dataset allows us to evaluate the effects of deformation on rocks transported through the restraining bend. In the Santa Cruz region south of the Zayante Fault, the folded base of the Purisima Formation has a half-wavelength to amplitude ratio about ten times larger than in the more deformed La Honda region. We interpret the Santa Cruz region folding to record modest deformation related to regional plate boundary transpression, whereas the La Honda region folding also includes deformation accumulated during transport of the rocks through the restraining bend. Second, we quantified both spatial and temporal variations in deformation magnitude throughout the region based on shortening measurements of several major unconformities along numerous cross sections. To estimate strain magnitude recorded by older surfaces, we progressively subtract shortening magnitude of young markers from older markers. Because uncertainties grow for older surfaces, this method is most reliable for younger surfaces. The results demonstrate that strain magnitude recorded by several units of different age is largest within about 5 km of the SAF, providing evidence of long-term deformation partitioning near this major structure. This pattern is apparent but less pronounced near secondary structures like the Zayante and Butano Faults. Assuming deformation occurred during time periods marked by unconformities, estimates of relative strain rate variation between different fault-bounded crustal blocks allow us to consider factors influencing regional deformation partitioning away from major structures. Depth to basement may to be a significant factor in some cases, as shallow granitic basement of the Ben Lomond block correlates with strain rates between two and ten times lower than other blocks with thicker stratigraphic cover, depending on the time period considered. Finally, temporal variation in the data for all crustal blocks suggests regional strain rates have varied by a factor of between two and five throughout the Tertiary. Conservative estimates of deformation timing and spatial extent suggest these rocks record on average 0.5-1.0 mm/yr of approximately SAF-perpendicular shortening throughout the Neogene.