PROPOSED MEGA-LANDSLIDE ORIGIN FOR THE EAST TRAVERSE MOUNTAINS, UTAH – EVIDENCE FROM PEBBLE DIKES
The ETM are centered on a swarm of 22 recently discovered pebble dikes that are enveloped by a large halo of argillically altered volcanic rocks. The composition of unusual igneous clasts in the ETM pebble dikes matches an unusual porphyry molybdenum deposit 16 km away in the high Wasatch Mountains. Many of the pebble dikes contain traces of euhedral molybdenite, both green and yellow titanite, uranothorite, allanite, and scheelite in QSP-altered (quartz-sericite-pyrite) breccia clasts. The QSP-altered leucocratic phase of the Little Cottonwood stock contains the same unusual minerals. U-Pb ages of zircon for both igneous clasts in the pebble dikes and the QSP-altered stock are indistinguishable. If landslide movement is restored, a coherent tight group of pebble dikes is situated over intensely altered and pyritized portions of the Little Cottonwood stock (Sharp, 1958; Crittenden, 1965).
Additional evidence for the slide event consists of the quartzite and carbonate rocks (Paleozoic Oquirrh Group?) of the ETM being thoroughly brecciated (80% of clasts < 1”), with local randomly oriented slickenlines and slickensides; it has been mined with dozers for 20 years with no blasting needed. The shear zone at the base of the landslide is partially preserved and is marked by ultracataclasite grading to pseudotachylyte.
The timing of this event is unknown. Naeser et al. (1983) reported a fission-track age of 6.5 ± 0.5 Ma for a rhyolitic tuff deposited in a lacustrine environment at nearby Jordan Narrows. A conglomerate with abundant Oquirrh clasts overlies the tuff and may be related to this landslide event. Mega-landslides are probably more common in the Basin and Range province than previously thought.