We use apatite and zircon (U-Th)/He thermochronometry to evaluate the timing, magnitude, and spatial pattern of Miocene strain within the Beaver Dam Mountains, Tule Springs Hills, and Mormon Mountains of southwestern Utah and southeastern Nevada (USA). The region is host to three major low-angle structures, the Castle Cliffs, Tule Springs, and Mormon Peak detachments, the origin and role of which in regional extension are vigorously debated. We analyzed 36 samples collected from Precambrian basement gneisses and Paleozoic to Jurassic siltstones and sandstones exposed in the footwalls of these detachments. Zircon He ages from the footwall of the Castle Cliffs detachment record rapid footwall exhumation ca. 18–17 Ma. At structurally higher positions, apparent ages become progressively older, defining a zircon He partial retention zone. Paleodepth reconstructions of the data using published cross sections suggest 180 °C, or greater, of cooling or 6.8–8.2 km of total exhumation, yielding a maximum of ∼13 km of extension across this detachment. In contrast, zircon and apatite ages from the footwall of the Mormon Peak record rapid exhumation at 14–13 Ma and 5.8–7.1 km of vertical exhumation. Using a range of restored fault dips (20°–28°) for the Mormon Peak detachment, the thermochronology data record 10.9–19.5 km horizontal extension. Data from the Tule Springs detachment also show a similar timing of exhumation and indicate that there has been 5.0–6.8 km of vertical exhumation and a minimum of ∼5 km of extension. The results demonstrate that extension initiated in the east along the Castle Cliffs detachment and migrated westward with time. Although our data indicate that existing extension estimates across this system of detachment faults are too high (40 km versus 54 km), the pattern of cooling ages and protracted cooling history recorded in these ranges are inconsistent with rootless gravity slide interpretations and low-magnitude extension models.
- Received 3 June 2014.
- Revision received 17 November 2014.
- Accepted 5 February 2015.
- © 2015 Geological Society of America