2369, EAST GREAT SALT LAKE FAULT ZONE

Structure number: 2369

Comments: Hecker's (1993) fault number 6-8.

Structure name: East Great Salt Lake fault zone.

Comments:

Synopsis: Zone of Holocene faulting beneath Great Salt Lake identified from seismic reflection data. The fault may take a right step and connect with the Oquirrh fault zone (2398) to the south.

Date of Compilation: 10/99 and 9/00

Compiler and affiliation: Bill D. Black (Utah Geological Survey), Suzanne Hecker (U.S. Geological Survey), and Gary E. Christenson (Utah Geological Survey).

State: Utah.

County: Box Elder, Davis, Weber.

1° x 2° sheet: Brigham City and Tooele.

Province: Basin and Range.

Geologic setting: Generally north-trending normal faults beneath Great Salt Lake identified from seismic reflection data. Subsidiary faulting is common in the hanging wall west of the main fault in the southern Great Salt Lake.

Number of sections: 3

Comments: Dinter and Pechmann (1999a, 1999b, in press) indicate the active East Great Salt Lake fault trace west of Antelope Island shows a 2-kilometer left step, suggesting the fault may form two north-northwest-trending sections south of Promontory Point: a 35-kilometer-long Antelope Island section and a 30-kilometer-long Fremont Island section. A right step west of Promontory Point indicates that a northern Promontory section probably exists north of the Fremont Island section, although no high-resolution seismic profiles exist for the Promontory section. High-resolution seismic profiles show a sharp westward bend in the southern end of the fault indicating a step-over to the Oquirrh fault zone.

Length: End to end (km): 103

Cumulative trace (km): 138

Average strike (azimuth): N29°W

2369a, PROMONTORY SECTION

Section number: 2369a

Section name: Promontory section.

Comments: Hecker's (1993) fault number 6-8.

Reliability of location: Poor

Comments: The location of the Promontory section in the northern Great Salt Lake basin is less accurate than sections to the south because it is based on less detailed reflection data (Pechmann and others,1987). Mapping is from Mikulich and Smith (1974) and Viveiros (1986).

Sense of movement: N.

Comments:

Dip: No data.

Comments: A steeply west-dipping fault is evident on seismic reflection profiles but dips are not reported. Interpretation of reflection data suggests the fault may flatten with depth (Smith and Bruhn, 1984) and merge into a horizontal detachment at a depth of about 6 kilometers (Viveiros, 1986). However, the evidence is equivocal.

Dip direction: W.

Geomorphic expression: Subaqueous.

Age of faulted deposits: High-resolution seismic reflection profiles have not been obtained for the Promontory section, but Quaternary deposits are displaced and we presume the age of faulted deposits is similar to that on the southern sections.

Paleoseismology studies: None

Timing of most recent paleoevent: (2) Latest Quaternary (<15 ka).

Comments: Because high-resolution seismic work has not been performed on the Promontory section, much less is known than about sections to the south. The timing of the most recent paleoevent is assumed to be similar to that on southern sections (middle to late Holocene). Restricted basins adjacent to the Promontory section existed prior to deposition of a 11.6 ky calcareous clay that overlies mirabilite salt deposits (Eardley, 1962; Mikulich and Smith, 1974). Deformation is evident in deeper near-surface sediments which pre-date salt deposition (Mikulich and Smith, 1974).

Recurrence interval: No data.

Comments:

Slip rate: Unknown, probably (C) 0.2-1 mm/yr, similar to other sections.

Comments:

Length: End to end (km): 49

Cumulative trace (km): 54

Average strike (azimuth): N39°W

2369b, FREMONT ISLAND SECTION

Section number: 2369b

Section name: Fremont Island section.

Comments: Hecker's (1993) fault number 6-8.

Reliability of location: Good

Comments: The mapped traces are from high-resolution seismic reflection lines located using GPS by Dinter and Pechmann (1999a, 1999b, in press; unpublished mapping, 1:95,000 scale).

Sense of movement: N

Comments:

Dip: No data.

Dip direction: W.

Geomorphic expression: Subaqueous. Evidence of repeated displacements in post-Bonneville time (Dinter and Pechmann, 1999a, 1999b, in press). No scarps are preserved in the lakebed west of Fremont Island where the fault is buried beneath horizontal lake-bottom sediments, indicating that movement on this part of the fault may predate movement on the Antelope Island section to the south. Diapiric domes and lake-bottom piercement structures, identified from seismic data, developed at least partly from flowage of the salt deposits during construction of the Southern Pacific Causeway in 1957-1959 (Mikulich and Smith, 1974).

Age of faulted deposits: Holocene.

Paleoseismology studies: None. Cores collected in August 2000 by Dinter and Pechmann (in press) may yield dates and improve the geologic interpretation.

Timing of most recent paleoevent: (2) Latest Quaternary (<15 ka).

Recurrence interval: 3.8-5.6 ky (<9.2-9.3 ka).

Comments: Dinter and Pechmann (in press) indicate three events since 9.2-9.3 ka. Segment lengths are comparable to those of the Wasatch fault zone and suggest comparable earthquake magnitudes. This recurrence interval is preliminary pending dating of deposits obtained from cores collected in August 2000.

Slip rate: (C) 0.2-1 mm/yr

Comments: A preliminary vertical slip rate of 0.67+0.06 millimeters/year and a fault-parallel slip rate of 0.9+0.2 millimeters/year are estimated by Dinter and Pechmann (in press). Slip rates are preliminary pending dating of deposits obtained from cores in August 2000.

Length: End to end (km): 30

Cumulative trace (km): 45

Average strike (azimuth): N32°W

2369c, ANTELOPE ISLAND SECTION

Section number: 2369c

Section name: Antelope Island section.

Comments: Hecker's (1993) fault number 6-8.

Reliability of location: Good

Comments: The mapped traces are from high-resolution seismic reflection lines located using GPS by Dinter and Pechmann (1999a, 199b, in press; unpublished mapping, 1:89,700 scale).

Sense of movement: N

Comments:

Dip: No data.

Dip direction: W.

Geomorphic expression: Subaqueous. Evidence of repeated displacements in Holocene time (Dinter and Pechmann,1999a, 1999b, in press). A 3-meter-high scarp is preserved at the lakebed on the Antelope Island section, indicating a relatively recent displacement.

Age of faulted deposits: Holocene

Paleoseismology studies: None. Cores collected in August 2000 by Dinter and Pechmann (in press) may yield dates and improve the geologic interpretation.

Timing of most recent paleoevent: (2) Latest Quaternary (<15 ka).

Comments: High-resolution seismic data show stratigraphic and structural anomalies, including auxiliary faults and tectonically produced angular unconformities and on-lap surfaces, that indicate three surface-faulting earthquakes on the Antelope Island section since the disappearance of Lake Bonneville (Dinter and Pechmann, in press). A 1.5-km-long zone of ten or more en-echelon fractures, beneath the lake west of Antelope Island, appears unmodified by coastal processes and to have slight down-to-the-west displacement on aerial photos, and may date from the latest Holocene (Currey, 1980).

Recurrence interval: 3.8-5.6 ky (<9.2-9.3 ka).

Slip rate: (C) 0.2-1 mm/yr

Comments: A preliminary vertical slip rate of 0.67±0.06 millimeters/year and a fault-parallel slip rate of 0.9±0.2 millimeters/year are estimated by Dinter and Pechmann (in press). Slip rates are preliminary pending dating of deposits obtained from cores in August 2000.

Length: End to end (km): 35

Cumulative trace (km): 39

Average strike (azimuth): N11°W

REFERENCES

Currey, D.R., 1980, Coastal geomorphology of Great Salt Lake and vicinity, in Gwynn, J.W., editor, Great Salt Lake-A scientific, historical, and economic overview: Utah Geological and Mineral Survey Bulletin 116, p. 69-82.

Dinter, D.A., and Pechmann, J.C., 1999a, Sublacustrine paleoseismology-Evidence for recent earthquakes on the East Great Salt Lake fault, Utah: Association of Engineering Geologists Program with Abstracts, 42^nd Annual Meeting, p. 62-63.

Dinter, D.A., and Pechmann, J.C., 1999b, Multiple Holocene earthquakes on the East Great Salt Lake fault, Utah-Evidence from high-resolution seismic reflection data [abs.]: EOS, Transactions of the American Geophysical Union, v. 80, no. 46 (supplement), p. F734.

Dinter, D.A., and Pechmann, J.C., in press, Late Quaternary slip rates and recurrence intervals of large earthquakes on the East Great Salt fault, Utah-Estimates from high-resolution seismic reflection data [abs.]: Geological Society of America, Abstracts with Programs, 2000 Annual Meeting, v. 32.

Eardley, A.J., 1962, Glauber’s salt bed west of Promontory Point, Great Salt Lake: Utah Geological and Mineral Survey Special Studies 1, 12 p.

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.

Mikulich, M.J., and Smith, R.B., 1974, Seismic-reflection and aeromagnetic surveys of Great Salt Lake, Utah: Geological Society of America Bulletin, v. 85, no. 6, p. 991-1002.

Pechmann, J.C., Nash, W.P., Viveiros, J.J., and Smith, R.B., 1987, Slip rate and earthquake potential on the East Great Salt Lake fault, Utah [abs.]: EOS, Transactions of the American Geophysical Union, v. 68, p. 1369.

Smith, R.B., and Bruhn, R.L., 1984, Intraplate extensional tectonics of the western U.S. Cordillera-Inferences on structural style from seismic-reflection data, regional tectonics, and thermal-mechanical models of brittle-ductile deformation: Journal of Geophysical Research, v. 89, no. B7, p. 5733-5762.

Viveiros, J.J., 1986, Cenozoic tectonics of Great Salt Lake from seismic-reflection data: Salt Lake City, University of Utah, M.S. thesis, 81 p.