PRELIMINARY INVESTIGATION OF THE THOUSAND LAKES FAULT FROM THE MID MIOCENE TO LATE PLEISTOCENE: AN APPROACH FOR CHARACTERIZING LOW SLIP RATE NORMAL FAULTS USING GEOMORPHOLOGY AND PALEOSEISMOLOGY

Characterizing the hazard and significance of low slip rate normal faults, like those within the transition zone between the Basin and Range and Colorado Plateau, is challenging because the tectonic signal of uplift is similar to erosion rates. An example of this challenge is presented by the Thousand Lakes fault (TLF) which extends for 50 km along the easternmost boundary of the transition zone in Utah. The TLF trends north-south and dips to the west. Fault zone width ranges from 0.5-6 km. Throw across the fault is measured by the displacement of a Tertiary volcanic tableland which outcrops on the footwall at ~3400 m-asl on both Boulder and Thousand Lakes Mountain and is observed in the hanging wall as beds that dip shallowly toward the fault. The maximum displacement along the TLF is about 1500 m near Bicknell, Utah. Displacements taper by up to 50% within 15 km to the north and south. Assuming that extension began between 10 and 13 Ma, we estimate that the long term slip rate for the central TLF ranges from 0.05 to 0.15 mm/yr.

From 2015 to 2017, the Utah Valley University field camp mapped the fault from ~1 km south of the Fremont River water gap to ~2 km northeast of Bicknell. Over this 6 km length we identified three outcrops preserving evidence for paleoearthquakes with surface displacements of ~1.5 m. These earthquakes occurred after the formation of three geomorphic surfaces with local relief ranging from 3-50 m above the nearby hanging wall elevation. The 50 m surface corresponds to a regionally mapped terrace of the Freemont that has been dated to 51-69 ka by other investigators and is progressively warped upward within the footwall from ~40 m above the active channel (significantly downstream) to the 50 m measurement at the TLF water gap. Given a 10 m displacement we infer that 0.15 mm/yr is a good estimate of the slip rate since ~60 ka. Furthermore, given the slip per event data, a mean recurrence interval of ~10 ka would account for the displacement of this terrace. We are creating a GIS to reconstruct four prehistoric profiles of the Freemont River spanning 0-120 ka. These reconstructions will be used to estimate the slip rate and test for rate variability. Additionally, samples have been collected for radiocarbon analyses from two colluvial wedges. These ages will provide preliminary data for establishing a paleoseismic record on the central TLF.