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

We use a combination of apatite He-4/He-3, (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 similar to 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 degrees C) prior to exhumation, which began sometime after 14 Ma. A single aliquot of nine apatite grains was step-heated for He-4/He-3 analysis; modeling of the resulting He-4 distribution indicates that cooling from > 80 degrees C to similar to 20 degrees C occurred between 3 and 1 Ma. Interconsistent time-temperature (t-T) solutions to the combined He-4/He-3, (U-Th)/He, and fission-track data require two distinct periods of cooling, consistent with noncontinuous 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 He-4/He-3 thermochronometric data to extract low-temperature t-T information from a single sample close to Earth's surface.