Date of Award:

1995

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

James P. McCalpin

Abstract

The Greys River fault is the easternmost active normal fault associated with Basin and Range extension in the lntermountain seismic belt. It is a north south trending normal fault, located in the Sevier-age fold and thrust belt of western Wyoming , and bounds the west side of the Wyoming Range . The fault is at least 54 km long , and juxtaposes Permo-Pennsylvanian Wells Formation against various Triassic formations . Throw ranges from 100-1000 m. Seismic reflection data suggest that the Greys River fault is a listric normal fault that soles into the Darby thrust at depth. The fault offsets late Quaternary deposits on the west flank of the Wyoming Range. These deposits were mapped and differentiated on the basis of provenance and weathering characteristics. Four sets of moraines were identified in the study area. The moraines segregated into a local four-fold relative age sequence based on topographic position and relative dating parameters. The four-fold relative age sequence was correlated to Blackwelder's original Rocky Mountain glacial chronology. The most extensive glacial deposits were correlated to the late Pinedale glacial advance. Alluvial deposits associated with the Pinedale glacial cycle also contain evidence for late Quaternary faulting on the Greys River fault.

Maximum vertical surface displacements of late Quaternary deposits range from 8.6 m in Blind Bull Creek to 8.3 m in Box Canyon, 25 kilometers to the south. Between these points, vertical surface offset ranges from 3 to 8.2 meters. Three trenches were excavated in a late Pinedale alluvial terrace complex in Sheep Creek across a 7.5-meter scarp, a 3.1-meter scarp, and a 7.5-meter scarp-graben. The trenches revealed evidence for two paleoseismic displacements, the ages of which were constrained by eight radiocarbon dates. The most recent event involved a maximum displacement of 4.5 meters and occurred between 2110 +/- 60 and 2910 +/- 60 14C years BP. The earlier event was associated with a maximum observed displacement of 3.2 meters and occurred between 5080 +/- 60 and 5310 +/- 60 14C years BP. The moment magnitudes for these events, calculated from displacement and surface rupture length , range from Mw = 6.9 to 7.4 for both events. Scarp heights indicate an uplift rate of 1.11 mm / year. If the late Pinedale alluvium is 14,000 years old, then it appears no events occurred between 14,000 yr BP and 5310 14C yr BP, and two events have occurred between 5310 14C yr BP and 2110 14C yr BP. Such irregular recurrence interval is typical for normal faults in the Basin and Range and makes long-term prediction difficult.

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