Exhumation history of the Sierra Azul Block of the Santa Cruz Mountains Revealed Using Low Temperature Thermochronology

Restraining bends along strike-slip faults are important features for transforming lateral plate motion into vertical rock uplift. One of the most archetypal of these bends is the Santa Cruz Mountains restraining bend in the San Andreas Fault in the southern Bay Area, California. The Sierra Azul structural block lies within this restraining bend, and was the site of coseismic and postseismic deformation caused by the M=6.9 1989 Loma Prieta Earthquake. Previous geomorphic studies and Apatite Fission Track (AFT) thermochronology indicate that this restraining bend is fixed to the northeastern side of the fault, and so deformation accrues within the Sierra Azul while crust to the southwest is uplifted as it is translated through the restraining bend by lateral movement along the SAF. In this view, rapid and recent deformation has continuously accrued within the Sierra Azul since 6-8 Ma, and young AFT ages within the core of the range support this general idea. In this contribution, we present the results of additional AFT, as well as new (U-Th)/He ages from the Sierra Azul block. Structurally, the range is composed of a generally east-northeast-vergent fold-and-thrust belt. The deepest exposures found in the range are found adjacent to the SAF, and here, AFT data have been reset within latest Miocene-Pliocene time. However, within the shallower exposures to the east, AFT samples are only partially reset. In contrast, (U-Th)/He samples within rocks throughout the range yield ages between 5-7 Ma, suggesting that exhumation of the entire range occurred rapidly between 5-7 Ma and that limited exhumation has occurred since. Thus, the details of the tectonic development of this restraining bend is apparently more complicated than expected based on previous models of this restraining bend. In addition, despite the spatially uniform exhumation within the range, basin and channel relief varies significantly and systematically between different thrust sheets in the block. Thus, differential erosion of different rock types exposed within the range apparently exerts an important role in controlling the topographic structure of this range that may obscure the imprint of tectonic rates on the landscape if this differential erosion is not considered.