THE THOUSAND LAKE FAULT: A LONG RECURRENCE NORMAL FAULT THAT HAS SLOWED DOWN AT THE EASTERN EDGE OF THE BASIN AND RANGE

The Thousand Lake fault (TLF) is a west-dipping normal fault demarcating the easternmost boundary of the transition zone between the Basin and Range and the Colorado Plateau for 50 - 100 km in southern Utah. Faulting within this part of the transition zone is distributed across an array of five range bounding faults. Determining the relative rates of activity of faults like the TLF is important for understanding how deformation is accommodated across the eastern Basin and Range and characterizing earthquake hazard. This study assesses the TLF slip rate based upon cumulative bedrock displacement and from a paleoseismic investigation on an alluvial fan that was abandoned between 130 ka and 52 ka. Geologic maps and profile analysis demonstrate that cumulative throw across the TLF is defined by a Tertiary volcanic tableland which is displaced up to 1500-2500 m near the fault’s center. This indicates a long-term slip rate of 0.1-0.3 mm/a since Basin and Range initiation (mid-Miocene). Surface age dating of terraces along the Fremont River, which flows to the east and cuts across the central TLF, indicates that the river is incising at ~0.5 mm/a over the last 200 ka and clearly outpaces the fault’s recent activity. In 2018 we dug a paleoseismic trench (30 m long and 2.5 m deep) across a 4 m fault scarp developed within a fan surface near Bicknell, Utah (38.3332, -111.5262). The investigation revealed evidence for two earthquakes. The penultimate event (PE) ruptured into what now is a stage III soil carbonate horizon and resulted in a doubling of the thickness of the soil carbonate on the hanging wall. The most recent event (MRE) is expressed as a colluvial wedge which sits stratigraphically above the stage III soil and grades into a younger soil carbonate (stage I-II). Preliminary OSL analyses indicate the PE occurred between 68.2 +/- 16.1 ka and 47.4 +/- 25.0 ka and the MRE occurred after 47.4 +/- 25.0 ka. Displacement from the two events is ~2 m. These data allow us to infer that the central TLF ruptures in M6.7-M7.1 earthquakes that have a mean recurrence of 25 ka to 42 ka. The cumulative 4 m fault displacement across the fan may be attributed to 1-2 buried events. Age constraints for the 2 m displacement and the inferred 4 m fan displacement indicate that the recent slip rate can be no faster than 0.05 mm/a and may be as slow as 0.02 mm/a.