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The AAPG/Datapages Combined Publications Database
Wyoming Geological Association
Abstract
Evidence for Right-Oblique-Slip on a Northern Segment of the Big Trails
Fault
System, Southern Big Horn Mountains, Wyoming
Abstract
The Big Trails fault
is traditionally defined as a northeast-trending, basement-controlled, high angle
fault
extending nearly 53 miles along the crest of the southern Big Horn Mountains. However, the authors have observed several features along this
fault
typical for strike-slip or oblique-slip
fault
models. The
fault
begins at the intersection of the Owl Creek and the Casper arch thrusts on the south and terminates on the east-west trending Tensleep
fault
to the north.
In an area mapped by the authors, the main fault
trace is one eighth to a half mile wide, and characterized in map view by multiple braided
fault
strands. Variable displacement and reversals of displacement occur on the main
fault
trace and on subsidiary faults. Horizontal to near horizontal slickensides were measured along the
fault
; plunge of the striations ranged from 10 to 55° in a predominantly northerly direction. "Horsetail"
fault
splays mapped in the west
fault
block indicate drag from right oblique-slip. Horst and graben features identified in the
fault
trace correlate with confining and releasing bends. The trace of the main
fault
was near vertical in profile. Vertical offset, i.e., stratigraphic separation, which is down to the west on the
fault
, ranges from almost zero on the north end to a maximum of more than 4500 feet near the middle of the
fault
. Horizontal offset in the form of right-slip is estimated at 2 to 3 miles.
The Big Trails fault
is apparently basement controlled and coincident with zones of weakness. These zones are defined by northeast trending quartz diorite, mafic, and amphibolite dikes of early Proterozoic and Archean age. We suggest that the
fault
may continue north of the Tensleep
fault
and connect with the Crazy Woman "tear"
fault
. The Crazy Woman
fault
forms the southern boundary of the Sisters Hill segment of the large east verging thrust feature along the eastern flank of the Big Horn Mountains, west of the town of Buffalo. The Big Trails
fault
is a result of a northeast-southwest oriented horizontal compressional event in the Rocky Mountain foreland during the Laramide orogeny.
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