Extensional Collapse along the Sevier Desert Reflection, Northern Sevier Desert Basin, Western United States: Comment and Reply

Anders, Mark H.; Christie-Blick, Nicholas; Wills, Stewart; Coogan, James C.; DeCelles, Peter G.

Comment and reply on the Sevier Desert reflection debate.
Comment abstract: Coogan and DeCelles (1996) provided a welcome addition to the debate on the Sevier Desert reflection. The evidence and arguments presented on the nature of this subsurface feature merit particular scrutiny, as they bear directly on a first order issue in tectonics: the mechanical paradox of low-angle normal faults. Field geologists have argued that in some cases such faults must have moved at dips of 20° or less; tectonophysicists maintain that such interpretations are inconsistent with our present knowledge of rock mechanics, and seismologists have yet to record a single earthquake that can be related unequivocally to slip on a low-angle normal fault. If, as Coogan and DeCelles (1996) and others have argued, the seismically imaged Sevier Desert reflection of west-central Utah is a rooted detachment fault with as much as 39 km of top-to-the-west slip, the seismic-reflection geometry effectively requires normal-sense slip on a surface dipping 11°. We believe, however, that geometry can also support alternative interpretations.
Reply abstract: In their comparison of two seismic reflection profiles across the western margin of the Sevier Desert basin, Anders et al. fall victims to a classic pitfall of seismic interpretation—the misinterpretation of multiples as primary stratal reflections. Their error lies in ignoring the acquisition parameters of the two data sets and overlooking fundamental characteristics of long-path multiples. The industry profiles in their are published in Mitchell and McDonald (1987) for detailed inspection. We focus on two attributes of long-path multiples to document the error in Anders et al.’s analysis: the effects of fold on multiple identification and attenuation, and the periodicity of multiples.


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