MULTIPLE MIOCENE BASIN-AND-RANGE EXTENSIONAL EPISODES IN THE DEEP CREEK RANGE, WESTERN UTAH

Apatite (U-Th)/He and fission-track thermochronology data from the northern Deep Creek Range of western Utah record two distinct Miocene exhumational events at ~16 Ma and ~11.4 Ma. Both events involved slip on north-south trending normal faults that bound the east side of the range. Apatite fission-track age versus structural relief plots are very steep, and indicate rapid cooling through ~100°C at ~16 Ma. The ~16 Ma event was accommodated by the Reilly Canyon Fault, which forms the northern segment of the ~150 km long Snake Range – Deep Creek fault system. This entire fault system slipped a minimum of 12-15 km at ~17 Ma. The Reilly Canyon fault is well exposed in the southern Deep Creek Range, where it dips ~25° to the east. Field evidence suggests that it formed at an originally steep angle (~55°) and rotated ~30° while accommodating normal displacement. Apatite (U-Th)/He ages from the deepest structural levels are invariant at ~11.4 Ma and indicate rapid cooling through ~70° at that time. At higher structural levels, ages steadily increase. This pattern is interpreted to represent an exhumed He partial retention zone. The highest structural elevation sample at the crest of the range is 17.7 Ma, within uncertainty to the fission track age for the sample. Total slip during the 11.4 Ma event was < 3 km, much less than the 16 Ma event. Reconstruction of isotherms from the He data are not consistent with exhumation along the low-angle Reilly Canyon fault and instead suggest the presence of a younger, cross-cutting, unexposed high-angle normal fault subparallel to the Reilly Canyon Fault. This fault has been previously suggested based on geomorphologic evidence, but this is the first quantitative evidence for its existence. The low temperature thermochronology from the Deep Creek Range therefore records the evolution of a rotational normal fault system, where the initial fault slipped and rotated ~30° before becoming inactive. A younger, more-favorably oriented high-angle fault then cross-cut the original fault, and continued accommodating regional extension. Low temperature thermochronology is therefore important not only in constraining regional spatial and temporal patterns of deformation, but also in understanding the mechanics of normal fault systems.