Sevier/Toroweap Fault Zone

997, SEVIER/TOROWEAP FAULT ZONE

Structure number: 997 Comments: Hecker's (1993) fault number 10-1. Structure name: Sevier/Toroweap fault zone. Comments: This fault is named the Sevier fault in Utah and the Toroweap fault in Arizona. In Utah, Hecker (1993) describes the Sevier fault as consisting of a northern (2355, Sevier) and southern (997, Sevier/Toroweap) portion, which are separated by a 50-kilometer unfaulted gap. Based on this gap and differences in displacement style and age of most recent movement, the northern fault appears to be a separate simple fault and not a section of the Sevier/Toroweap fault zone. Synopsis: Moderately to poorly understood zone of late Pleistocene faulting extending south from Panguitch to the Utah-Arizona border. Date of compilation: 10/99 (Utah) and 4/3/97 (Arizona). Compiler and affiliation: Bill D. Black (Utah Geological Survey) and Suzanne Hecker (U.S. Geological Survey)(Utah); Phillip A. Pearthree (Arizona Geological Survey)(Arizona). State: Utah; Arizona. County: Kane, Garfield; Mohave, Coconino. 1° x 2° sheet: Cedar City, Grand Canyon, Williams. Province: Colorado Plateaus. Geologic setting: Left-lateral oblique slip fault zone near the western edge of the Colorado Plateaus in south-central Utah. The fault shows substantial Cenozoic offset; total normal displacement increases northward near the Utah border. Number of sections: 4 Comments: The Toroweap fault in Arizona is divided into three sections (Pearthree, 1998); the Sevier fault in Utah had not been subdivided (Hecker, 1993). The faults have been combined here, and the section in Utah named the Sevier section. The boundary is taken at a left step in the fault near Mt. Carmel Junction, about 20 kilometers north of the Utah-Arizona border. Thus, the northern part of the Northern Toroweap section in Arizona crosses into Utah. No detailed studies have been completed in Utah, so little is known whether this section boundary is a likely segment boundary. Length: End to end (km): 167 Cumulative trace (km): 214 Average strike (azimuth): N14oE

997a, SEVIER SECTION

Structure number: 997a Structure name: Sevier section. Comments: Hecker's (1993) fault number 10-1. Reliability of location: Good. Comments: Mapped or discussed by Cashion (1961), Carpenter and others (1967), Anderson and Miller (1979), and Anderson and Christenson (1989). Mapping from Anderson and Christenson (1989). Sense of movement: NS. Comments: Dip: No data. Comments: Dip direction: W. Geomorphic expression: Striations with a southerly component of rake indicate that the southern Sevier fault has left-lateral oblique slip. Southeast of Panguitch, 200 meters of offset in the 560 ka Red Canyon volcanic flow suggests faulting during late Pleistocene time. Quaternary basalts at Black Mountain, northeast of Glendale, are possibly faulted up to 23 meters (Cashion, 1961), but the amount of displacement is difficult to determine due to uncertainties in distinguishing scarps related to landslides, pre-flow topography, and faults. Near Alton, differential erosion of indurated Quaternary deposits and relatively weak Cretaceous rocks formed a southeast-facing fault scarp. An unfilled sediment trap formed against a small (<5-meter high), 2-kilometer-long, west- (uphill-) facing scarp on the main trace suggests an age of only a few thousand years, but the scarp is believed to be related to gravity-driven subsidence rather than surface faulting because ridgelines cut by the scarp show little or no tectonic throw. Hecker (1993) also notes it seems unlikely that 5 meters of seismogenic surface displacement on a major block-bounding fault would be restricted to such a short segment. East of Panguitch, where the Sevier/Toroweap fault dies out to the north in bedrock, Anderson and Miller (1979) and Carpenter and others (1967) show a number of dashed Quaternary faults. An aerial-photo study by Anderson and Christenson (1989) in this area indicates that the fault traces are expressed only as aligned drainages in bedrock, and that no through-going scarps occur in the irregular mass of Quaternary deposits. The north end of the fault is expressed as a youthful range front, and forms a possible en echelon left step shown as questionable Quaternary fault on Hecker (1993). Age of faulted deposits: Quaternary. Paleoseismology studies: None Timing of most recent paleoevent: (3) Late Quaternary (<130 ka). Comments: Range-front steepness and linearity along the fault suggest recent movement, although in places these are likely fault-line characteristics resulting from juxtaposition of resistant and nonresistant rocks. A small closed basin adjacent to a left step at the south end of the section is consistent with dilation due to left-lateral slip, and indicates that subsidence and fault activity (due either to surface-faulting earthquakes or low-level seismicity/aseismic creep) are likely late Pleistocene in age. The amount of offset in the Red Canyon flows also supports a late Pleistocene age. Recurrence interval: No data. Comments: Slip rate: (C) 0.2-1 mm/yr. Comments: Offset of 200 m in the 560 ka Red Canyon flow yields a vertical slip rate of 0.36 millimeters/year. Length: End to end (km): 88 Cumulative trace (km): 103 Average strike (azimuth): N19oE

997b, NORTHERN TOROWEAP SECTION

Structure number: 997b Structure name: Northern Toroweap section. Comments: Hecker's (1993) fault number 10-1. Reliability of location: Good. Comments: Section in Utah mapped or discussed by Anderson and Miller (1979) and Anderson and Christenson (1989). Mapping in Utah from Anderson and Christenson (1989). Sense of movement: NS. Comments: Dip: No data. Comments: Dip direction: W. Geomorphic expression: In Utah, the section is expressed as low scarps in Mesozoic bedrock buried in much of the area by sand dunes. In Arizona, faulting is expressed as low to moderately high, west-facing escarpments in Paleozoic bedrock, low fault scarps on undated but probable Pleistocene basalts, and low, gentle fault scarps on upper(?) Pleistocene alluvium. An alluvial fault scarp profiled south of Pipe Springs, Arizona is about 3.5 meters high and has a maximum slope angle of 7.5 degrees, suggesting a late Pleistocene time of most recent paleoevent (Pearthree, 1998). Age of faulted deposits: Paleozoic, Mesozoic, late(?) Pleistocene. Paleoseismology studies: None Timing of most recent paleoevent: (3) Late Quaternary (<130 ka). Comments: A small closed basin adjacent to a left step at the northern end of the section is consistent with dilation due to left-lateral slip, and indicates that subsidence and fault activity (due either to surface-faulting earthquakes or low-level seismicity/aseismic creep) are likely late Pleistocene in age. This is supported by the estimated age of faulted alluvium and a single fault scarp profile in Arizona (Pearthree, 1998). Recurrence interval: No data. Comments: Slip rate: (D) <0.2 mm/yr. Comments: A rough long-term slip rate estimate of 0.01-0.04 millimeters/year is based on 2 meters of vertical displacement of late Pleistocene (~50-150 ka) alluvium in Arizona (Pearthree,1998). Length: End to end (km): 81 Cumulative trace (km): 112 Average strike (azimuth): N10oE

REFERENCES

Anderson, R.E., and Christenson, G.E., 1989, Quaternary faults, folds, and selected volcanic features in the Cedar City 1° x 2° quadrangle, Utah: Utah Geological and Mineral Survey Miscellaneous Publication 89-6, 29 p., scale 1:250,000.

Anderson, L.W., and Miller, D.G., 1979, Quaternary fault map of Utah: Long Beach, California, Fugro, Inc., 35 p., scale 1:500,000.

Carpenter, C.H., Robinson, G.B., and Bjorklund, L.J., 1967, Ground-water conditions and geologic reconnaissance of the upper Sevier River Basin, Utah: U.S. Geological Survey Water-Supply Paper 1836, 91 p.

Cashion, W.B., 1961, Geology and fuel resources of the Orderville-Glendale area, Kane County, Utah: U.S. Geological Survey Coal Investigations Map C-49, scale 1:62,500.

Hecker, Suzanne, 1993, Quaternary tectonics of Utah with emphasis on earthquake- hazard characterization: Utah Geological Survey Bulletin 127, 2 plates, scale 1:500,000, 257 p.

Pearthree, P.A., compiler, 1998, Quaternary fault data and map for Arizona: Arizona Geological Survey Open-File Report 98-24, scale 1:750,000, 122 p.