Abstract

Extensional reactivation of thrust faults north of the Uinta Mountains in Utah and Wyoming is accompanied by Quaternary surface faulting, tilting, and monoclinal folding. The 40-km-long Bear River fault zone consists of down-to- the-west, right-stepping, en echelon scarps, each about 3.0 to 3.5 km in length, striking N20°W to N20°E. Scarp-derived colluvial deposits record two Holocene surface ruptures with net vertical tectonic displacements ranging from <1 to >5 m per event (West, 1989, in press). Radiocarbon ages indicate surface faulting events occurred at about 4.6 ka and 2.4 ka. The length of surface rupture and net slip per event in the Bear River fault zone imply a seismic potential comparable to the Lost River fault, Hebgen Lake fault, and the Wasatch fault zone.

A late Quaternary normal scarp is also coincident with the leading edge of the Absaroka thrust fault 7.2 km west of the Bear River fault zone. Scarp-derived colluvial deposits record one surface faulting event over a length of at least 5.0 km. The age of surface rupture is coeval with the Bear River fault zone, ∼2.4 ka. Normal faults displacing Pleistocene surfaces/deposits and tectonic tilt indicate that the Hogsback thrust east of the Bear River fault zone was reactivated in a normal sense but now may be inactive due to subsequent development of the Bear River fault zone. Cumulative normal throw of 200± m on the Hogsback fault may be responsible for postulated tectonic separation of the Bear River and Green River drainage basins <600 ka.

Neotectonic deformation results from east-west extension superposed on the Hogsback and Absaroka thrust plates. Extensional reactivation of thrust faults caused propagation of "new" normal faults over stress points, particularly at the Hogsback ramp to flat transition, and at the thrust leading edges. The ages of surface rupture and cumulative throws indicate normal reactivation of the Hogsback and Absaroka thrusts, and propagation of the intervening Bear River fault zone proceeded from east to west opposite initial development of the thrusts in Late Cretaceous to Paleocene time. Normally reactivated thrust faults and propagation of listric normal faults accompanied by surface rupture have significant implications for the tectonic/seismogenic development of the eastern Basin and Range transition zone.

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