NEW PSEDOTACHYLYTE OCCURRENCES FOUND ON THE LITTLE COTTONWOOD STOCK -- PRODUCED FROM A MEGA-SCALE LANDSLIDE EVENT

Pseudotachylyte found in sheared fault gouge on the dip slope of the Little Cottonwood Stock has been known for many years. Previous work attributed its occurrence to movement along the Wasatch Fault.

We report new occurrences of pseudotachylyte on the Little Cottonwood Stock in Dry Creek Canyon above Alpine, Utah. The first occurrence is 1.5 km up the canyon and continues intermittently with ultracataclasite for another 1.7 km. These outcrops are not associated with the nearby Wasatch and Deer Creek faults. The outcrops are parallel to the inferred southeastern edge of the glide path of a mega-scale slide block – the East Traverse Mountains. The pseudotachylyte and ultracataclasite occurrences are part of the disrupted fault gouge, 50 to 200 meters thick, which sit atop the inferred upper surface of the pluton. The pluton lid, made of Oquirrh sedimentary rocks and volcanic cover (~1 km thick), slid towards the Salt Lake Basin during the cataclysmic Miocene event. Pseudotachylyte was created as a result of frictional heating of the slide surface. The pluton lid may have traveled a total of 16 km during this event and then came to rest on Miocene lake sediments or in a Miocene lake. The exact cause and timing of this event is unknown, but may ultimately be related to inflation and creation of the Little Cottonwood Stock about 31 to 24 million years ago. This cataclysmic event was also coupled with, and caused by, uplift along the Wasatch Fault. The uplift and slide mechanism is a modified version of what Hacker et al proposed in 2014. After the event, continued isostatic crustal adjustments, due to the low-density pluton, followed by glaciation and erosion of the stock have caused the pseudotachylyte and glide surface to be tilted significantly.

The initial trigger for the event is unknown, but may have been due to seismicity along the Wasatch Fault. New conodont ages on the limestone slide block and new zircon U-Pb ages on the intrusion supplement and support this model.