PSEUDOTACHYLYTE--CATACLASITE IN THE DAMAGE ZONE LOCATED NORTH OF BOX ELDER PEAK, WASATCH MOUNTAINS, UTAH, ASSOCIATED WITH THE 57 KM2 TRAVERSE MOUNTAIN LANDSLIDE

The East Traverse Mountain (ETM) Landslide, now covering ~57 km², slid catastrophically 16 km from the upper reaches of the Little Cottonwood stock to the Salt Lake valley ~6.1 Ma where it now forms the ETM. However, a small portion of the landslide was buttressed and preserved by Box Elder Peak during the sliding event. It is unknown how far this portion of the slide block moved, perhaps less than 1 km, but it covers 0.5 km² and is now located in the Wasatch Mountains directly north of Box Elder Peak. High elevation exposures of the slide damage zone reveal many sliding surfaces, as evidenced by pseudotachylyte veins, slickenlines, and anastomosing shear fractures. These findings support the interpretation of a catastrophically fast strain rate such as the ETM Landslide.

The damage zone is composed of broken Little Cottonwood Stock (LCS) and Mississippian Doughnut Formation (Fm). Within the Doughnut Fm the damage zone is brecciated and does not retain bedding. In the LCS, the damage zone is expressed as granodiorite that is penetrated by closely spaced (few cm) shear fractures locally intruded by pseudotachylyte/cataclasite. The pseudotachylyte veins range from a few cm thick to over a meter thick. No pseudotachylyte has been found in the Doughnut Formation. This is interpreted as the result of a lack of biotite which is crucial to lowering the melting point of the rock. Biotite (and sericite in some places) is abundant in the roof and margins of the LCS. These two units, when combined, form an unstable damage zone over 100 m thick with Miocene and Quaternary movement.

The presumption is that the Doughnut was overlain by a thick section of Oquirrh Formation (similar to the adjacent Box Elder Peak and the ETM). In places where the Doughnut Formation served as the roof-zone of the granitic intrusion, greisen veins formed in the LCS along the roof zone, shortly after it was emplaced. Several km of displacement along the Wasatch Fault has created dramatic relief that places this weakened block in an unstable position. The roof-parallel pyrite-sericite veins, and associated alteration, may have weakened the LCS so that the potential energy from the 2 km of relief between the peaks of the Wasatch Range and the rift valley floor created by the Wasatch Fault could have initiated a mega-landslide ~6.1 Ma.