Tertiary Normal Faulting in the Canyon Range, Eastern Sevier Desert

The contact between pre-Mesozoic and Tertiary rocks in the western Canyon Range, west-central Utah, has been interpreted as a large, low-angle normal fault that marks the breakaway zone of the hypothesized, basin-forming Sevier Desert detachment. Recent fieldwork suggests that the contact may in fact be depositional along much or all of its length. Deformational fabric in the supposed footwall likely traces to the Mesozoic Sevier orogeny rather than to Tertiary detachment faulting. Kinematic indicators at the range front are not generally consistent with low-angle normal-fault motion; instead, well-exposed high-angle faults are the dominant range-bounding structures. The Tertiary conglomerates of the western Canyon Range foothills, previously viewed as an evolving syntectonic deposit related to detachment faulting, are here reinterpreted as three distinct units that reflect different periods and tectonic settings. The pattern in these conglomerates, and in fault-offset gravity-slide deposits that mantle the western foothills, is consistent with block faulting and rotation along several generations of high-angle structures. Local seismic-reflection data lend qualitative support to this interpretation, and underscore the need to consider alternative working hypotheses for evolution of the Sevier Desert basin.