UC Santa Cruz

The spatial pattern of uplift and long-term exhumation of the Santa Lucia range in central California is defined and the major structures responsible for this deformation are elucidated using low-temperature thermochronometry (44 apatite and 39 zircon (U-Th)/He cooling ages), geomorphic metrics of erosion, deformed late Quaternary marine terraces, geologic constraints, and a basin-subsidence analysis. Thermochronometers indicate rapid late Miocene cooling along the entire 90 km-long southwest flank of the range; a ~6 Ma onset of rapid exhumation is most consistent with available constraints. Exhumation rates are greatest NE of the San Gregorio-Hosgri fault (SGHF), decrease away from it, and are low across its trace to the SW. Deformed marine terraces also indicate a strong fault control on uplift; terrace elevations are 3–5 times higher directly NE of the SGHF and decay to baseline values over a ~5 km-wide zone; elevations drop rapidly across the fault to the SW. A geomorphic analysis using range-wide normalized channel steepness and river-profile plots of χ indicates higher rates of uplift NE of the SGHF, even when controlled for changes in lithology. Together, these data indicate that the SGHF has focused uplift and exhumation along its NE side since the late Miocene, has continued to do so in the late Quaternary, and is the primary driver of high topography and relief in the Santa Lucia range. A basin analysis indicates a cyclic pattern of uplift and subsidence over the last 80 m.y. that may be related to the emplacement of underplated schist during late Cretaceous crustal restructuring.

A multi-pulse method for single-collector ICP-MS laser ablation systems is presented that interrogates isotopic variation as a function of sample depth. The method resolves U-Pb ages in zircon with ~0.55 µm depth resolution and ~6% 2σ uncertainty. Metrics of radiation damage, crystal-lattice distortion, and ablation depth indicate that crystal structure exerts a fundamental control on elemental fractionation and must be matched between standards and unknowns for ultimate age precision.