Paleoseismology of Utah, Volume 4

Seismotectonics of north-central Utah and southwestern Wyoming

Special Study 82, 1994, 93 pages, 5 plates scale 1:100,000, $15.00

ABSTRACT

Geomorphic evidence of late Quaternary faulting and related tectonic deformation is present north of the Uinta Mountains in Uinta County, Wyoming and Summit County, Utah. A major zone of late Quaternary normal faulting, termed the Bear River fault zone, extends over 25 miles (40 km) from southeast of Evanston, Wyoming to an apparent complex intersection with the North Flank fault of the Uinta Mountains in north-central Utah. The Bear River fault zone consists of well-defined scarps each about 1.9 to 2.2 miles (3.0 to 3.5 km) in length arranged in a right en echelon pattern. Major scarps trend N. 20 W. to N. 20 E. and show consistent, down-to-the-west displacement. Short, down-to-the-east scarps trend N. 15-20 W. and are interpreted to be antithetic faults. Near the south end of the fault zone, scarps in Pleistocene glacial deposits show strong angular discordance (70 ) with the main north-northeast pattern of faulting. Late Quaternary movement is indicated by scarps ranging from <3 to 49+ feet (<1 to 15+ m) high in till, outwash, alluvium, and bedrock of the Eocene-age Wasatch Formation; beheading and reversal of streams and numerous sag ponds.

Neotectonic deformation results from regional cast-west extension superimposed on the Darby-Hogsback and Absaroka thrust plates. Pre-existing thrust faults were reactivated in a normal sense and caused propagation of "new" listric normal faults over stress points, particularly at the transition from thrust ramps to flats in Jurassic salts and Cretaceous marine shales. The Bear River fault zone developed above the Darby-Hogsback ramp and has experienced recurrent, Holocene movement over a length of 21 to 25 miles (34 to 40 km) with net vertical tectonic displacements ranging from less than 3 feet to greater than 16 feet (< 1 to >5 m) per event. Two distinct surface-faulting events are represented by scarps and associated scarp-derived colluvial deposits.

Ages of surface rupture were estimated by radiocarbon dating of tectonically buried and modem soil A-horizons and other organic material exposed in trenches excavated across late Quaternary fault scarps. Calibrated radiometric ages indicate surface-faulting events occurred at 4,620 ± 690 and 2,370 ± 1050 radiocarbon years before present (yr B.P.). Recurrence intervals, based on these ages, range from about 2,250 to over 2,370 years. Interpreted ages of surface-faulting events are not corrected for apparent mean soil residence time; thus, ages of surface-faulting events may be too old by several hundred years.

Surface rupture lengths of 21 to 25 miles (34 to 40 km), vertical tectonic displacements of <3 to >16 feet (51 to >5 m) per event, and slip rates of 0.03 to 0. 11 in/yr (0.8 to 2.7 mm/yr) are comparable to the Wasatch fault, a major earthquake source zone in the Intermountain seismic belt (ISB). Based on empirical relationships for seismogenic normal faults, the Bear River fault zone could have produced paleoearthquakes of surface wave magnitude Ms = 7.5. The mean age of latest surface rupture (2,370 yr B.P.) and minimum apparent recurrence interval (2,250 years) suggest a major earthquake could occur at any time in southwestern Wyoming and north-central Utah.

The Martin Ranch scarp is coincident with the leading edge of the Absaroka thrust about 4.5 miles (7.2 km) to the west of the Bear River fault zone and developed in response to normal reactivation of the pre-existing thrust plane. Related tectonic deformation extending at least 0.6 mile (1 km) south of the Martin Ranch scarp deflected the channel of the Bear River. Scarp-derived colluvial deposits record one surface-faulting event over a length of 3.1 miles (5.0 km). Mean net vertical tectonic displacements for the single event range from 2.8 to 4.9 feet (0.8 to 1.4 m). The age of latest surface rupture is coeval with latest surface rupture in the Bear River fault zone, 2,370 ± 1,050 yr B.P.

Similar ages of movement suggest displacement along the Martin Ranch scarp occurred as a simultaneous response to east-west extension superimposed on pre-existing thrust and ramp-normal faults. Slip rates on the reactivated leading edge of the Absaroka thrust range from 0.02 to 0.03 in/yr (0.5 to 0.7 mm/yr).

Fault scarps displacing Pleistocene geomorphic surfaces and associated outwash/alluvium and regional, eastward tilt of terrace surfaces indicate the leading edge of the Darby-Hogsback thrust was also reactivated but now may be inactive due to development of the Bear River fault zone over the ramp structure to the west. Normal displacements along the leading edge of the Darby-Hogsback fault are believed responsible for apparent separation of the Bear and Green River drainage basins less than 600,000 years ago.

Research in southwestern Wyoming and north-central Utah in comparison with the Hebgen Lake and Borah Peak areas suggests different levels of maturity and tectonic/structural relationships exist with time and location in the ISB/eastern basin-and-range transition zone. Hebgen Lake and Borah Peak represent evidence of mature seismogenesis manifested by imposing fault-bounded mountain blocks and evidence of recurrent normal fault movements with great displacements. The Bear River fault zone and normally reactivated thrust faults represent an early, youthful stage of seismogenesis in a thrust-faulted terrain. Continued tectonic deformation may produce fault-bounded mountain ranges with remnants of thrust plates preserved within the block similar to the Wasatch Range east of Salt Lake City. Major seismogenic faults, which may be "blind" sub-decollement structures in early stages of extension, eventually rupture the surface as 45 to 60 planar faults. The early tectonic relationship between regional extension and normally reactivated leading edges of thrust faults and ramp structures is destroyed with time. The idea that all late Quaternary surface faults in the ISB are steeply dipping and penetrate from ground surface to depths of 7.4 to 9.3 miles (12 to 15 km) may be an oversimplification that is applicable to certain seismically mature areas but cannot be applied unilaterally to all areas in the ISB/eastern basin-and-range transition zone.