Regional Structure and Kinematic History of the Sevier Fold-and-Thrust Belt, Central Utah: Discussion

DeCelles and Coogan (2006) summarize the structure and evolution of the Late Jurassic to Paleocene Sevier orogenic belt in central Utah, inferring at least 220 km of crustal shortening on the basis of a regional balanced cross section. Difficulties arise in an otherwise excellent synthesis in their treatment of the Sevier Desert basin, where 47 km of normal slip is interpreted on a Cenozoic detachment in spite of mounting evidence that no such structure exists. DeCelles and Coogan acknowledge the possibility that the prominent seismic reflection marking the Paleozoic-Cenozoic contact may correspond with an unconformity rather than a fault. However, they dismiss the inconvenient absence of evidence for deformation along the interpreted detachment as “local and equivocal,” ignore a comprehensive re-evaluation of subsurface geophysical and geological data by Wills et al. (2005), and claim incorrectly that the construction of a balanced cross section “implicitly demonstrates the geometric and kinematic validity of the detachment interpretation for the …. reflection” (p. 844). Balanced cross sections are only as good as the observational constraints and assumptions upon which they are based. If the detachment that is a central assumption of DeCelles and Coogan's cross section is, in fact, not a fault, a better section can be drawn