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Otton (1995) has proposed an interesting detachment model of the western frontal fault of the Canyon Range. However, his model fails to completely address the effect of Mesozoic tectonics on surface geology. Tertiary sediment records provide little evidence of a low-angle normal fault illustrated in the Otton cross-section. High-angle normal faulting and rotation provide better explanations of the Tertiary outcrop geology of the Canyon Range.

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-wall 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 hypothesized Sevier Desert detachment, which is not exposed in outcrop and is inferred to exist primarily on the basis of seismic reflection data. The footwall of the Cave Canyon detachment is characterized by a 200-m-thick granite cataclasite, which exhibits a clear decrease in grain size and increase in microfracture density as the fault surface is approached. Undulatory extinction in quartz and feldspar and abundant quartz deformation lamellae at distances more than 200 m from the fault surface are interpreted as related to cooling of the Miocene granite rather than to normal faulting. Although mylonitic textures have previously been described in the granite, we found no evidence for mylonitization in the footwall rocks. The hanging wall of the detachment is characterized by 9 m of deformed, partially dolomitized limestone, with a 2-m-thick carbonate mylonite at the contact. Deformation features include dynamic recrystallization, grain-size reduction, development of twinning with a strong preferred orientation, some grain-size layering, and undulatory extinction close to the fault. Static recrystallization 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 deformation (ARCO Hole-in-the-Rock No. 1, ARCO Meadow Federal No. 1, and Argonaut Energy Federal No. 1 wells). Fossils and ooids are undeformed in Paleozoic carbonate rocks within 3 m below the contact, and sandstone and conglomerate with rounded clasts lacking more than background levels of microfracturing are found in samples within 3 m above the contact. These features contrast markedly with those of the Cave Canyon detachment, which was active at a considerably shallower and cooler level in the crust (~5 km and [is less than] 300 [degrees] C) than is implied for Paleozoic rocks beneath the Sevier Desert, once hanging-wall rocks are restored along the hypothesized detachment (9-14 km and 280-425 [degrees] C at the locations studied). The very different character of the two surfaces reinforces our earlier suggestion that beneath much of the Sevier Desert basin, the base of the Tertiary section is an unconformity rather than a low-angle normal fault. Keywords: brittle rock deformation, fault zones, normal faults, Sevier Desert, Utah.

It is true that the Sevier Desert reflection is just a hypothesisto be examined. The study gathered data for fault-related deformation insamples obtained from two boreholes across this feature. No data were gatheredconcerning cataclasis in the hanging-wall block and for ductile deformation in thefootwall. They will certainly alter the geologic interpretation of Sevier Desert.However, basic geologic data were not disregarded in interpreting the geology ofSevier Desert based on the new data gathered, nor are there any data that showthe need for a major low-angle normal fault.

Most tectonic models for the Sevier Desert basin, west-central Utah, envision it as the result of large-magnitude, normal-sense slip on a regional detachment fault. That interpretation, based principally on seismic reflection data, has helped shape views on the tectonics of the northeastern Great Basin area and, in a larger sense, the historical development of ideas about low-angle normal faulting. In recent years, however, several researchers have suggested, based on rock-mechanical, field, and subsurface evidence, that the hypothesized detachment fault does not exist and that the basin must have another explanation. Even among proponents of the detachment model, opinion has differed regarding the hypothesized extensional fault's total displacement, estimates for which vary widely; the timing of detachment slip; and whether the hypothesized fault represents a "new" Tertiary extensional structure or an extensional reactivation of the Mesozoic Pavant thrust. A comprehensive reinterpretation of available subsurface data for the basin, including several previously unpublished seismic profiles, suggests: (1) that slip on the hypothesized detachment must have ceased by the Miocene in the southern Sevier Desert; (2) that estimates of large-magnitude offset on the hypothesized detachment are essentially unconstrained by structural data and need to be reevaluated; and (3) that models that view the hypothesized detachment as a Tertiary extensional reactivation of a Mesozoic thrust are likely incorrect. The newly available seismic data demonstrate that reflections from the Pavant thrust do indeed closely align with reflections from the Paleozoic/Tertiary contact in many parts of the northern Sevier Desert basin; however, to the south these same thrust fault reflections are directly traceable to a position well above the down dip projection of the presumed detachment within the Paleozoic Cricket Mountains block. Erosional truncation of the thrust faults and the absence in the south of other reflections aligned with the Paleozoic/Tertiary contact preclude extensional back-sliding on a Mesozoic thrust fault. These interpretations, if correct, are incompatible with the detachment hypothesis and necessitate alternative explanations for the basin's origins. [ABSTRACT FROM AUTHOR]