Abstract

Geologic evidence developed from surface exposures demonstrates that the western frontal fault of the Canyon Range is a major structure representing the eastern breakaway zone of the Sevier Desert detachment. The western frontal fault of the Canyon Range east of Delta, Utah, is a north-trending, west-dipping, low-angle (18 °–24 ° ) normal fault that juxtaposes syntectonic conglomerates and large-rock avalanches of the Miocene Oak City Formation against footwall rocks that have been ductilely, cataclastically, and brittlely deformed. The Oak City strata dip variably to the east; dips are shallow in eastern outcrops (5 ° –15 ° ), steeper in central outcrops (20 ° –60 ° ), and shallow in the western outcrops (10 ° –20 ° ). In eastern exposures, the shallow dips of the Oak City Formation (5 ° –15 ° ) and the acute angle of 20 ° –35 ° between the western frontal fault and Oak City units limit past rotation of the fault to ≤15 ° . The systematic variation in the dip of Oak City units from east to west suggests that the western frontal fault probably flattens beneath present Oak City outcrops, linking the fault to the shallow reflector seen in the COCORP (Consortium for Continental Reflection Profiling) Utah line 1 seismic line beneath the westernmost outcrops of the Oak City Formation. Upper plate movement is constrained by Oak City clast compositions and rock-avalanche lithologies, fault geometry, and lower plate reconstruction to a maximum of 6–7 km since Oak City syntectonic deposition at ∼12–13 Ma. Maximum upper plate displacement for the entire Sevier Desert detachment cannot be determined from these breakaway-zone exposures.

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