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

This study provides an empirical calibration of the apatite (U-Th)/He thermochronometer using the thermal structure derived from an extensive apatite fission-track study of an exhumed, normal-fault–bounded crustal block in the White Mountains in the western Basin and Range province. This fault block has been tilted ∼25° to the east during extension, exposing a continuous section of rocks previously buried to ∼7 km. Apatites 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 paleodepth. The ages exhibit a well-defined exhumed apatite He partial retention zone over a pre-extensional temperature range of ∼40–80 °C and are completely reset above 80 °C, as calibrated from the apatite fission-track data. This pattern is in good agreement with He diffusion behavior predicted by laboratory experiments. The (U-Th)/He and fission-track methods yield concordant estimates for the timing of the onset of extensional faulting in the White Mountains ca. 12 Ma. Given the partially overlapping temperature-sensitivity windows, the (U-Th)/He and fission-track thermochronometers are highly complementary and may be used together to reconstruct thermal histories over the temperature window of ∼40–110 °C.