Assessment of (U-Th)/He thermochronometry: The low-temperature history of the San Jacinto mountains, California

Abstract

1 (U-Th)/He ages have been measured on igneous apatites from the SanJacinto mountains, a high region at the junction of the Peninsular and Transverse Ranges, to investigate the potential of this technique for thermochronometry of slowly cooled rocks. Helium ages from 79 to l 7 Ma are younger than ages obtained by other dating techniques, including apatite fission-track counting, and are consistent with laboratory experiments that indicate this system has a uniquely low closure temperature. Helium ages are strongly correlated with elevation and record the latest low-temperature thermal evolution of the range. They suggest relative tectonic quiescence in the latest Cretaceous through mid-Tertiary, and provide no evidence for rapid unroofing of the block during this period. Nor do they obviously require a large degree of uplift associated with convergence between the Transverse and Peninsular ranges in the last few million years. Helium ages document modest westward tilting of the block (~7 °)and a significant vertical component of motion on the block's bounding faults after helium retention began. This work suggests that apatite helium ages record low-temperature tectonic and thermal histories that are not apparent from other dating techniques.

Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from 40Ar/39Ar and (U‐Th)/He thermochronology

“…[34] (U-Th)/He thermochronology is a recently redeveloped technique that provides age information in the low-temperature range of thermochronologic systems [Zeitler et al, 1987;Lippolt et al, 1994;Wolf et al, 1996;House et al, 1997;Warnock et al, 1997;Wolf et al, 1997]. Helium diffusion in most fluorapatite appears to be a thermally activated process with a nominal closure temperature (for a cooling rate of 10°C/m.y.)…”

Exhumation processes

“…Low-temperature thermochronometers have deeper closure depths, and thus provide a more averaged estimate of the erosion rate. For instance, the apatite ( U -T h ) / H e thermochronometer (Lippolt et al 1994;Wolff et al 1997) has a very low closure temperature (c. 75°C), which implies closure depths of 2-3 km, assuming slow erosion rates. This depth will be even shallower in areas of fast erosion because of the upward advection of heat caused by erosion.…”

Transient thermal regimes in the Sierra Nevada and Baja California extinct outer arcs following the cessation of Farallon subduction

“…These two batholitic belts were part of the outer arc during the Farallon subduction and have recently been decoupled from each other with the inland jump of the southern part of the transform boundary at 5 Ma and capture of the Salinian and the Baja blocks by the PAC [Atwater, 1970;Spencer and Normark, 1989;Bohannon and Parsons, 1995]. Studies indicate that past thermal regime, based on fission track thermochronometry and (U-Th)/He methods (Dumitru [1990] and House et al [1997] for SN and George and Dokka [1994] and Wolf et al [1997] for the northern PB), and the present thermal regime, based on direct heat flow measurements, indicate a low-temperature history in these outer arc regions. Present low heat flow in these regions has been discussed by Blackwell [1971] and Roy et al [1972] as thermal evidence of the recent Farallon subduction, uniquely supporting the paleomagnetic reconstruction of the PacificNorth American plate interactions proposed by Atwater [1970].…”