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
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.