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Two-phase Neogene extension in the northwestern Basin and Range recorded in a single thermochronology sample

¹ U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
² Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA
³ Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA

Correspondence: *E-mail: jcolgan{at}usgs.gov.

We use a combination of apatite ⁴He/³He, (U-Th)/He, and fission-track thermochronology to date slip on the Surprise Valley fault in northeastern California by analyzing a single sample from the Warner Range in the footwall of the fault. This sample, a granitic clast from a conglomerate, yielded a fission-track age of 11.6 ± 2.8 Ma and a (U-Th)/He age of 3.02 ± 0.52 Ma. Geologic relationships indicate that this sample was buried to a depth of ~3.3 km prior to exhumation during slip on the Surprise Valley fault. Fission-track age and length data indicate that the sample was fully reset (>120 °C) prior to exhumation, which began sometime after 14 Ma. A single aliquot of nine apatite grains was step-heated for ⁴He/³He analysis; modeling of the resulting ⁴He distribution indicates that cooling from >80 °C to ~20 °C occurred between 3 and 1 Ma. Interconsistent time-temperature (t-T) solutions to the combined ⁴He/³He, (U-Th)/He, and fission-track data require two distinct periods of cooling, consistent with non-continuous slip on the Surprise Valley fault. Early cooling and fault slip took place between 14 and 8 Ma, followed by more recent fault slip ca. 3 Ma. This timing is consistent with both local geologic relationships and with the regional timing of faulting along the western margin of the Basin and Range Province. These data demonstrate the resolving power of combined fission-track, (U-Th)/He, and ⁴He/³He thermochronometric data to extract low-temperature t-T information from a single sample close to Earth's surface.

Key Words: ⁴He/³He thermochronology • (U-Th)/He dating • fission-track analysis • Surprise Valley fault