THE THOUSAND LAKE FAULT: NORMAL FAULTING AND LANDSCAPE EVOLUTION AT THE COLORADO PLATEAU MARGIN, UTAH

The Thousand Lake Fault (TLF) system forms the easternmost fault in Utah’s High Plateaus region at the western edge of the Colorado Plateau. Understanding the kinematic history of the TLF is critical for understanding the landscape evolution and uplift of the High Plateaus/Colorado Plateau transition zone. The TLF is a steeply dipping, listric normal fault that cuts through the entire upper crustal section and offsets Tertiary volcanic rocks as well as Paleogene (?) to Jurassic sedimentary units. Kinematic evidence indicates down-to-the-west normal slip on the TLF with a total displacement of 800-1000 m. TLF displacement decreases to the northeast as throw is transferred westward to the Paradise Fault system. Volcanic rocks on the adjacent Fish Lake Plateau are tilted to the southeast as a consequence of slip and rotation in the TLF hanging wall.

A spatially extensive Quaternary debris flow boulder fan/apron is offset by 100-150 m across the TLF. Cosmogenic ³He exposure age dating of abundant pyroxenes in exposed andesite boulders on the displaced hanging wall block will constrain the timing and rate of Quaternary slip on the TLF. Locally, dilational clastic dikes cut the Jurassic Navajo sandstone in the footwall immediately below the TLF. Dikes are filled with matrix-supported sediment containing abundant Oligocene volcanic clasts sourced from the hanging wall; we interpret these dikes to be the result of seismic rupture at the surface.

The Rabbit Valley Salient (RVS) is an enigmatic feature that extends westward from the flank of Thousand Lake Mtn. into the Fremont River valley. The distinctive topography of the RVS is underlain by a chaotic array of strongly deformed Paleogene to Neogene sedimentary and volcanic strata that are extensively folded and faulted. We hypothesis that the RVS is a 3 to 4 km³ catastrophic mass movement deposit sourced from the uppermost flanks of Thousand Lake Mtn., perhaps due to seismic activity on the TLF. Given the large exposures of Jurassic strata currently exposed in the TLF footwall and the absence of Jurassic strata or blocks of Jurassic strata in the RVS deposit, we suspect this mass movement occurred prior to the exposure of Jurassic strata. Continued slip on the TLF eventually exposed the Jurassic strata. Dissected Quaternary debris flow fans overly deformed strata in the RVS.