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Brief Report for South Granite Mountains fault system, Crooks Mountain section (Class B) No. 779a

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citation for this record: Machette, M.N., compiler, 1999, Fault number 779a, South Granite Mountains fault system, Crooks Mountain section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website,, accessed 09/20/2014 01:54 AM.

Synopsis General: This 125-km-long, west-northwest trending, north-dipping fault system is located along the north margin of a low chain of anomalous west-northwest-trending mountain ranges within the Wyoming Basin province of central Wyoming. Two episodes of movement have been documented on the fault system. The first was near the end of the Eocene when the Granite Mountains (to the north) were uplifted at least 3 km, and during the Pliocene to Quaternary when the fault system was reactivated in the opposite sense (down-to-the-north) resulting in subsidence of the previously uplifted Precambrian-cored Sweetwater Arch. Part of this subsidence was accommodated by the Split Rock syncline [778], which lies north of the South Granite Mountains fault system. There has been a thorough reconnaissance of the fault system and detailed paleoseismic investigations at two locations along its middle portion. This study revealed clear evidence for Quaternary deformation on the three central faults (sections) of the system, but Quaternary deformation has not been proven for the distal sections (Class B structures). Pleistocene to Holocene displacement was found in the Green Mountain and Ferris Mountains areas, and minor Quaternary displacement was found in the Muddy Gap area. However, all five sections are considered to be of potential Quaternary age because of the prevalence of lineaments, springs, alignment of vegetation and fault scarps. Trenching has shown that a displacement of about 0.5 m (net vertical) is typical of the average surface-rupturing event on the Ferris Mountains section of the fault system. In addition, using reported average to maximum displacement ratios for historic faulting events, they proposed that the active (late Quaternary) sections of the South Granite Mountains fault system might have a maximum surface faulting event of 1-1.5 m displacement.

Sections: This fault has 5 sections. Geomatrix Consultants (1988 #2980) defined five segments (herein considered as sections) for the South Granite Mountains fault system. From west to east, these are the Crooks Mountain [779a], Green Mountains [779b], Muddy Gap [779c], Ferris Mountains [779d] and Seminoe Mountains [779e] sections. Quaternary movement (Class A structures) has been documented in the Green Mountain area, along the Ferris Mountains, and in the Muddy Gap area. As such, only these three sections are described in detail; the Crooks Mountain (779a) and Seminoe Mountains [779e] sections are considered to be of Class B (potential Quaternary) structures, pending further investigations.
County(s) and State(s) FREMONT COUNTY, WYOMING
Physiographic province(s) WYOMING BASIN
Length (km) This section is 34 km of a total fault length of 133 km.
Average strike N73°W (for section) versus N72°W (for whole fault)
Sense of movement Normal
Dip Direction N
Historic earthquake
Most recent prehistoric deformation Quaternary (<1.6 Ma)
Slip-rate category Less than 0.2 mm/yr
Date and Compiler(s) 1999
Michael N. Machette, U.S. Geological Survey