LACK OF REGIONAL PATTERNS IN UNROOFING RATES ACROSS CALIFORNIA: A NEGATIVE TEST OF THE FARALLON SLAB SEGMENTATION HYPOTHESIS

Two low-temperature thermochronologic methods, apatite fission track, and (U-Th)/He dating, are commonly used to determine rock cooling ages and infer bedrock uplift rates. We are using a compilation of more than five hundred published data points from the mountainous regions of California in an attempt to decipher regional denudation patterns. Almost all these studies were carried out in apatites extracted from Mesozoic arc-related rocks from the once continuous California magmatic arc. The arc has since been dismembered by subsequent tectonism. Although previous studies have been used to determine the uplift history of small-scale regions, no such study was aimed at investigating the uplift rates across California. The predicted result from this exercise was to be able to identify distinct exhumation patterns at regional scale. The majority of rock cooling ages in the analyzed rocks range between 41-75 Ma. Since most low temperature chronologic data predate the late Cenozoic displacements along the San Andreas and related faults, these segments had to be reconstructed to their pre-San Andreas locations. A recent hypothesis (Saleeby et al. 2003) suggests that the subducting Farallon slab was segmented beneath California during the Laramide (~75-50 Ma) orogeny: the slab beneath the central and northern parts of the California arc remained relatively steeply dipping, whereas the southern part (in the Mojave region) was underlain by a very shallow slab and an underthrusted forearc. A corollary of this hypothesis is that the Mojave region underwent rapid uplift during this period, whereas the Sierra Nevada proper experienced a more limited and slower uplift. The low temperature thermochronology data is used here to test this hypothesis. Our preliminary analysis argues against a major discrepancy between exhumation histories of the southern vs. central California arc rocks: neither the absolute ages nor the denudation rates are different between the regions of postulated different Laramide tectonic histories.