NEW EVIDENCE FOR A MEGA-LANDSLIDE ORIGIN FOR THE EAST TRAVERSE MOUNTAINS, UTAH

The East Traverse Mountains ( are a proposed mega-landslide block 57 km² in size along Utah’s Wasatch fault zone that slid ~16 km from the upper reaches of the Little Cottonwood stock (LCS).

Evidence reported last year for the landslide include newly discovered pebble dike swarms containing clasts of LCS and QSP altered leucocratic phase of the LCS (White Pine) on the ETM block, pseudotachylyte on a basal shear surface, and matching compositions of zircon, titanite, molybdenite, and uranothorite in pebble dikes on the ETM and in LCS.

New mapping indicates that the roof-zone of the LCS pluton is generally hydrothermally altered and contains pyrite and white mica. This likely corresponds to the widespread pyritic and argillic alteration of the displaced lid of the pluton – the ETM. New U-Pb ages of zircon (LA-ICP-MS) show the White Pine and the Red Pine (26-27 Ma) porphyries of Sharp (1958) are distinctly younger than the LCS (29-32 Ma).

The ETM block is partially capped by altered Oligocene volcanic rocks but is composed primarily of Paleozoic Oquirrh . It is highly brecciated and contains fracture-filling uraniferous opal. Lake and/or ground water percolated through the major fractures deposited opal which has a U-Pb age of 6.1+/-0.9 Ma. Thirty meters , a 10 cm ashfall tuff occurs in a lacustrine marlstone. An zircon fission track age of 6.5 ± 0.5 Ma was acquired for this unit (Naeser et al., 1983). Thus, we infer the age of the slide to be about 6 Ma.

Additionally, recent mapping has documented that pseudotachylyte/ultracataclasite within the LCS extends for 4 km defining the remnants of the slide plane. Outcrops include both undeformed LCS and Mississippian Doughnut Formation (Md) near the roof of the pluton but above the slide plane. The Damage Zone is composed of anastomosing shear fractures with slickenlines that penetrate both the LCS and Md. Veins of pseudotachylyte range from a few cm to over a meter thick in brecciated LCS and not within the Md, probably due to a lack of mica that is needed to lower the melting point of the rock and instead slipped on low strength clay-rich units. More recent small landslides have occurred on the slip plane.

Ultimately, the combined effects of pluton-wide alteration and Miocene normal faulting along Utah’s Wasatch Fault zone, may have caused the landslide.