Apatite fission track results from Neoproterozoic and Lower Cambrian quartzites collected from the Canyon Range in west central Utah reveal a significant early to middle Miocene cooling event (~19--15 Ma). Preextensional temperatures estimated from multicompositional apatite fission track data suggest ~4.5 to >5.6 km of unroofing during the early to middle Miocene, assuming a geothermal gradient of ~25 ¿C/km. The spatial distribution of these preextensional temperatures indicates ~15¿--20¿ of eastward tilting of the Canyon Range during rapid extensional unroofing along a moderately west dipping detachment fault (~35¿--40¿). We interpret this fault to be the breakaway of the Sevier Desert Detachment fault (SDD), the existence of which has been contested. The new thermochronologic data presented in this study provide compelling evidence for the existence of the SDD and thus the general viability of low-angle detachment faulting. The data directly date the onset of extensional faulting along the SDD starting at ~19 Ma and constrain the fault slip rate in the SDD breakaway zone at 2.4-2.1 mm/yr between ~19 and 15 Ma. An early Miocene apatite fission track age obtained from a Proterozoic clast from the Tertiary Oak City Formation confirms that these conglomerates were deposited in a synextensional basin in the hanging wall of the SDD. The timing of tectonic unroofing of the Canyon Range in response to faulting along the SDD appears to be synchronous with large-magnitude extension along the Snake Range d¿collement and with early extension along the Cave Canyon detachment exposed in the Mineral Mountains, pointing to widespread east-west extension in the eastern Great Basin in the early and middle Miocene. ¿ 2001 American Geophysical Union