A prominent west-dipping reflection that can be traced in seismic-reflection profiles over an area of 7000 km2 beneath the Sevier Desert basin of west-central Utah is generally referred to as the Sevier Desert detachment and is widely regarded as one of the best examples of an upper-crustal low-angle normal fault. The absence of evidence for fault-related deformation in drill cuttings and core from two industry boreholes that intersect this feature casts doubt on the fault interpretation. The existing interpretation is based mainly on the observation that high-angle normal faulting is restricted largely to Tertiary sedimentary and volcanic rocks above the reflection. An alternative explanation is that the high-angle faults are related to the withdrawal of early deposited lacustrine salt, which even today is as much as 2 km thick. Reevaluation of the seismic data suggests that the Sevier Desert reflection consists of two spatially and genetically distinct segments: a shallow segment here interpreted as an unconformity between Palezoic and Tertiary strata and a fortuitously aligned deeper segment that is traceable to mid- and lower-crustal levels and that appears to represent a thrust fault related to the Cretaceous Sevier orogeny.
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