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The Thousand Lake Fault: Earthquake Geology of a Long Recurrence Normal Fault at the Eastern Edge of the Basin and Range

Nathan A. Toke, David W. Marchetti, Christopher M. Bailey, Robert Biek, Joseph Phillips, Hanna Bartram, & Clayton Forster

Published August 15, 2018, SCEC Contribution #8791, 2018 SCEC Annual Meeting Poster #223

The Thousand Lake fault (TLF) is 50-kilometer-long normal fault and a prominent structural boundary between the Basin and Range and the Colorado Plateau in southern Utah. Many of the most active faults in the Basin and Range (those with slip rates > 0.5 mm/a) are located along the margins of the physiographic province such as the Wasatch, East Cache, Great Salt Lake, and Oquirrh faults in northern and central Utah. However, faulting along this margin in southern Utah becomes distributed across an array of less active faults. Determining the relative rates of activity within this array is important for understanding how deformation has been accommodated across the Basin and Range and for characterizing the earthquake hazard. The easternmost fault in this array is the TLF, which trends north-south and dips steeply to the west. Here, we present a preliminary assessment of the TLF earthquake geology based upon bedrock mapping, cross sections, geomorphology, and paleoseismic investigation. Geologic mapping and cross sections reveal that the cumulative throw across the TLF is defined by a Tertiary volcanic tableland which has been displaced by 700-1500 m in the central portion of the fault zone. This indicates that the slip rate has ranged from 0.05-0.15 mm/a since Basin and Range initiation during the mid-Miocene. Surface age dating of terraces along the nearby Fremont River, which flows to the east, cross-cutting the central TLF, indicates that the river is incising at ~0.5 mm/a since about 200 kya, clearly outpacing the fault. In 2018 we conducted a paleoseismic investigation across a ~4m high fault scarp developed within an abandoned alluvial fan surface near Bicknell, Utah. Helium-3 exposure age dating of the large andesite boulders (> 1 m in diameter) which are strewn across the surface indicate that the fan was abandoned during the Late Pleistocene (from 81-246 kya). Paleoseismic trenching revealed good evidence for one ground-rupturing earthquake that displaces a stage III soil carbonate horizon by 0.8-1.2 m. These data indicate that the late Pleistocene slip rate could be less than 0.05 mm/a and that the hazards-relevant recurrence rate along the TFL is likely within the range of 50,000 to 125,000 years per event.

Key Words
Paleoseismology, Basin and Range, Normal Faulting

Toke, N. A., Marchetti, D. W., Bailey, C. M., Biek, R., Phillips, J., Bartram, H., & Forster, C. (2018, 08). The Thousand Lake Fault: Earthquake Geology of a Long Recurrence Normal Fault at the Eastern Edge of the Basin and Range. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology