Calibration of the apatite (U-Th)/He thermochronometer on an exhumed faultblock, White Mountains, California

This study provides an empirical calibration of the apatite (U-Th)/He therm ochronometer using the thermal structure derived from an extensive apatite fission-track study of an exhumed, normal-fault-hounded crustal block in th e White Mountains in the western Basin and Range province. This fault block has been tilted similar to 25 degrees to the east during extension, exposi ng a continuos section of rocks previously buried to similar to7 km. Apatit es yield (U-Th)/He apparent ages of ca. 50-55 Ma at shallow pre-extensional crustal levels that decrease systematically to ca, 12 Ma at >4.5 km paleod epth. The ages exhibit a well-defined exhumed apatite He partial retention zone over a pre-extensional temperature range of similar to 40-80 degreesC and are completely reset above 80 degreesC, as calibrated from the apatite fission-track data. This pattern is in good agreement with He diffusion beh avior predicted by laboratory experiments. The (U-Th)/He and fission-track methods yield concordant estimates for the timing of the onset of extension al faulting in the White Mountains ca. 12 Ma. Given the partially overlappi ng temperature-sensitivity windows, the (U-Th)/He and fission-track thermoc hronometers are highly complementary and may be used together to reconstruc t thermal histories over the temperature window of similar to 40-110 degree sC.