EXTENSIONAL REACTIVATION OF THRUST FAULTS ACCOMPANIED BY COSEISMIC SURFACE RUPTURE, SOUTHWESTERN WYOMING AND NORTH-CENTRAL UTAH

Authors
Citation
Mw. West, EXTENSIONAL REACTIVATION OF THRUST FAULTS ACCOMPANIED BY COSEISMIC SURFACE RUPTURE, SOUTHWESTERN WYOMING AND NORTH-CENTRAL UTAH, Geological Society of America bulletin, 105(9), 1993, pp. 1137-1150
Citations number
51
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geology
ISSN journal
0016-7606
Volume
105
Issue
9
Year of publication
1993
Pages
1137 - 1150
Database
ISI
SICI code
0016-7606(1993)105:9<1137:EROTFA>2.0.ZU;2-H
Abstract
Extensional reactivation of thrust faults north of the Uinta Mountains in Utah and Wyoming is accompanied by Quaternary surface faulting, ti lting, and monoclinal folding. The 40-km-long Bear River fault zone co nsists of down-to-the-west, right-stepping, en echelon scarps, each ab out 3.0 to 3.5 km in length, striking N20-degrees-W to N20-degrees-E. Scarp-derived colluvial deposits record two Holocene surface ruptures with net vertical tectonic displacements ranging from <1 to >5 m per e vent (West, 1989, in press). Radiocarbon ages indicate surface faultin g events occurred at about 4.6 ka and 2.4 ka. The length of surface ru pture and net slip per event in the Bear River fault zone imply a seis mic potential comparable to the Lost River fault, Hebgen Lake fault, a nd the Wasatch fault zone. A late Quaternary normal scarp is also coin cident 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 o f surface rupture is coeval with the Bear River fault zone, approximat ely 2.4 ka. Normal faults displacing Pleistocene surfaces/deposits and tectonic tilt indicate that the Hogsback thrust east of the Bear Rive r fault zone was reactivated in a normal sense but now may be inactive due to subsequent development of the Bear River fault zone. Cumulativ e normal throw of 200+/- m on the Hogsback fault may be responsible fo r postulated tectonk separation of the Bear River and Green River drai nage basins <600 ka. Neotectonic deformation results from east-west ex tension superposed on the Hogsback and Absaroka thrust plates. Extensi onal reactivation of thrust faults caused propagation of ''new'' norma l faults over stress points, particularly at the Hogsback ramp to flat transition, and at the thrust leading edges. The ages of surface rupt ure and cumulative throws indicate normal reactivation of the Hogsback and Absaroka thrusts, and propagation of the intervening Bear River f ault 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/seismo genic development of the eastern Basin and Range transition zone.