FAULT BLOCK, TECTONIC DETACHMENT, OR MEGA-LANDSLIDE? EAST TRAVERSE MOUNTAINS, UTAH

The structure and origin of the east Traverse Mountains (eTM) of central Utah, which trend perpendicular to the Wasatch fault, have been debated for over a century. The eTM have historically been explained as a normal fault-bounded salient or as the hanging wall of a detachment. Our research uncovered multiple lines of evidence that the eTM are not a tectonic feature, but are instead a ~75 to 150 km³ mega-landslide deposit that catastrophically slid 16 km from the upper reaches of the Little Cottonwood stock to its present location in the late Miocene. There is pervasive brecciation of quartzites, limestones, volcanic strata, and dikes in the block that are broken into fragments <10 cm across. Fracture surfaces are locally polished and have slickenlines. Pseudotachylyte and cataclasite near the slide’s inferred base consistently dip shallowly to the southwest parallel to slickenline orientations, and they form a shear zone that was offset by later slip on the steeper Fort Canyon normal fault. A swarm of andesitic dikes and a separate set of Mo-mineralized pebble dikes within the landslide’s inferred source region appear to correlate with similar dike swarms in the eTM based on dike orientations, mineralogy, geochemistry, and age, suggesting they have been offset 16 km to the southwest. Distinctive normal faults and sedimentary strata now found in the eTM are also displaced. Cooling ages (~6 Ma) for the Oligocene granodiorite in the landslide source are anomalously young. Near the western flank of the megabreccia, a distinctive Miocene conglomerate within a succession of fine-grained lacustrine sediments in the Salt Lake Formation appears to be the sedimentary response to the landslide. U-Pb ages of post-slide opal and a pre-slide tuff constrain the age of the slide to be about 6.5 Ma. We propose that, as a result of progressive normal faulting, the mountain front had become over-steepened and gravitationally failed, and the roof of the Little Cottonwood stock slid 16 km to the southwest. Failure may have started in the pluton’s roof, which was hydrothermally altered by multiphase intrusions, and includes the weak and shaley Doughnut Formation. Understanding mega-landslides such as this is crucial for assessing potential hazards in extensional basins and for distinguishing large landslides from tectonic features.