TESTING THE HYPOTHESIS OF A MEGA-SCALE LANDSLIDE ORIGIN FOR THE EAST TRAVERSE MOUNTAINS, UTAH - EVIDENCE FROM MINERALS

The East Traverse Mountains (ETM) form a topographic salient on Utah’s Wasatch fault zone. We propose the ETM is a mega-scale landslide block, 57 km² in size, that slid 16 km westward from the upper reaches of the Oligocene Little Cottonwood stock (LCS) to its current location. One avenue of testing this hypothesis is to examine the alteration and unique accessory minerals of pebble dikes from the top (ETM) and bottom (LCS) of this porphyry Mo-W mineralized magmatic system.

The Little Cottonwood stock contains a younger central phase, the White Pine (WP), about 3 km², which is slightly less mafic than normal LCS. It hosts a sub-economic, broadly-distributed porphyry molybdenum deposit. This phase was intruded, fractured, and altered roughly 26 Mya with quartz, sericite, and pyrite (QSP). Within the mineralized system, a small (0.10 km²) intrusion is exposed, the Red Pine Porphyry. It is intensely altered, contains molybdenite and fluorite, with up to 1 cm cubes of pyrite being most diagnostic.

Heavy mineral separates from the WP contain pyrite, molybdenite, uranothorite, green Fe-poor titanite as well as yellow Fe-rich titanite, zircon, and allanite. This same unique suite, along with WP clasts containing QSP veins, is found 16 km away in 22 pebble dikes exposed in the ETM. They penetrate Oligocene volcanic rocks deposited on Paleozoic sedimentary rocks. The timing of failure is poorly constrained but may have been in the Miocene, based on the fission-track age of 6.5 ± 0.5 Ma of a volcanic ash horizon (Naeser et al., 1983) in disturbed lacustrine sediments overlain by cobble conglomerate at the toe of the landslide block.

We take this as evidence that the top of the mineralized system with the pebble dikes slid westward separating it from the lower part of the system. From stratigraphic and structural considerations, we infer that the pebble dikes extended 1-5 km below the paleosurface. We have not found any clasts of the Red Pine porphyry in the pebble dikes of the ETM. Apparently, the roots of the pebble dikes did not extend deep enough to reach the Red Pine porphyry. The unique assemble of minerals contained within the WP and pebble dikes clasts give petrographic evidence that the East Traverse Mountain block was once stratigraphically above the White Pine of the LCS.