Abstract

Extensional collapse after crustal shortening is a common process in mountain belts. The resulting overprinting of structures can make structural reconstruction quite challenging, especially where both extensional and shortening structures are geometrically complex. We address this challenge in a segment of the Sevier thrust belt located in the Basin and Range province of southwestern Utah. The Wah Wah Mountains are unusual because all known thrusts in this part of the Sevier belt are exposed in a single range, and therefore it is unnecessary to correlate thrusts between ranges in order to assemble a complete cross section across the thrust belt. However, the thrusts are overprinted by a three-dimensional array of extensional faults that make conventional two-dimensional cross-section restoration methods inadequate. On the basis of new mapping in the Blawn Mountain–Rose Spring Canyon area and published maps of adjacent areas, we re construct the geometry and kinematics for part of the Sevier thrust stack, which we here refer to as the “Wah Wah thrust system,” by iterative restoration and balancing of a grid of three-dimensional cross sections. We removed the effects of Tertiary extensional faulting and then restored the thrust geometry. The results of the final restoration indicate that the structure of the Wah Wah thrust system is more complex than previously recognized and comprises a roof thrust above a stack of two major and four minor footwall imbricates. Minimum total shortening across the thrust system is 38 km. The internal evolution of the thrust system appears to have been mainly backward-breaking, in contrast to the overall regional forward-breaking sequence of the Sevier thrust belt along strike. Cross-section restorations indicate that the roof thrust accumulated at least 15 km of slip after emplacement of all footwall imbricates. We speculate that bleeding off of pore pressure from the décollement by fluid discharge along ramp thrusts caused incremental strengthening and abandonment of the frontal area of the décollement and therefore led to development of the backward-breaking thrust sequence.

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