Rock deformation studies in the Mineral Mountains and Sevier Desert of west-central Utah: Implications for upper crustal low-angle normal faulting

The Cave Canyon detachment, a low-angle normal fault that crops out in the Mineral Mountains, west-central Utah, has been interpreted as a hanging-wal l splay of a much larger structure (the Sevier Desert detachment) that was influential in development of the idea that low-angle normal faults play a role in crustal extension. The Cave Canyon detachment provides expectations for the deformational, features that might be expected along the hypothesi zed Sevier Desert detachment, which is not exposed in outcrop and is inferr ed to exist primarily on the basis of seismic reflection data. The footwall of the Cave Canyon detachment is characterized by a 200-m-thic k granite cataclasite, which exhibits a dear decrease in grain size and inc rease in microfracture density as the fault surface is approached, Undulato ry extinction in quartz and feldspar and abundant quartz deformation lamell ae at distances more than 200 m from the fault surface are interpreted as r elated to cooling of the Miocene granite rather than to normal faulting, Al though mylonitic textures have previously been described in the granite, we found no evidence for mylonitization in the footwall rocks. The hanging wa ll of the detachment is characterized by 9 m of deformed, partially dolomit ized limestone, with a 2-m-thick carbonate mylonite at the con-tact. Deform ation features include dynamic recrystallization, grain-size reduction, dev elopment of twinning with a strong preferred orientation, some grain-size L ayering, and undulatory extinction close to the fault. Static recrystalliza tion overprints fossils and ooids at distances greater than 9 m, Drill cuttings and some core recovered at similar distances above and below the hypothesized Sevier Desert detachment show no evidence for localized d eformation (ARCO Hole-in-the-Rock No. I, ARCO Meadow Federal No. 1, and Arg onaut Energy Federal No. I wells). Fossils and ooids are undeformed in Pale ozoic carbonate rocks within 3 m below the contact, and sandstone and congl omerate with rounded clasts lacking more than background levels of microfra cturing are found in samples within 3 m above the contact, These features c ontrast markedly with those of the Cave Canyon detachment, which was active at a considerably shallower and cooler level in the crust (similar to5 km and < 300 degreesC) than is implied for Paleozoic rocks beneath the Sevier Desert, once hanging-wall rocks are restored along the hypothesized detachm ent (9-14 km and 280-425 degreesC at the locations studied). The very diffe rent character of the two surfaces reinforces our earlier suggestion that b eneath much of the Sevier Desert basin, the base of the Tertiary section is an unconformity rather than a low-angle normal fault.