TOPOGRAPHIC RESPONSE TO ACTIVE DEFORMATION IN THE SANTA CRUZ MOUNTAINS, CALIFORNIA

Metrics of landscape steepness have been used as a reconnaissance tool to identify locations of rapid modern tectonic rock uplift rates. Our previous work in the Carrizo Plain, California, indicates that various landscape metrics correlate systematically with tectonic rates along a ~4.5-km-long landform that is underlain by a poorly consolidated, relatively mechanically homogeneous substrate. To determine if such relationships are applicable to larger mountain ranges, we have focused our attention on the Santa Cruz Mountains, which was the location of the 1989 Loma Prieta Earthquake. Several lines of geologic and geomorphic evidence indicate that the topography of this range results from deformation of material as it is moved into and through a restraining bend that creates a zone of active uplift. To constrain the kinematics of the range, we used low-temperature thermochronometry (paired apatite fission track and (U-Th)/He analyses) to infer the exhumation history of the Sierra Azul Block, and cosmogenically derived estimates of averaged erosion rates in basins draining the La Honda Block, between the San Andreas and San Gregorio Faults. The low-temperature thermochronometric measurements in the Sierra Azul image a pulse of exhumation around ca. 4-6 Ma, which apparently resulted from rapid deformation in this range. Since that time, exhumation across the range appears to have been relatively steady and slow. In contrast, cosmogenically derived erosion rates within the La Honda Block decrease systematically with distance from the Santa Cruz Mountains restraining bend. However, topographic metrics showed little evidence for systematic variation along the range. Instead, landscape steepness appears primarily controlled by the exposure of particular rock types within catchments of this range. We conclude that in simple landscapes constructed in unconsolidated, relatively homogeneous substrates such as those we have studied in the Carrizo Plain, various metrics inferred from topographic data may provide faithful proxies for vertical tectonic deformation rates. However, the exposure of different lithologies in complex composite geologic features such as an entire mountain range may leave an imprint on the topography that is at least as important as tectonic uplift.