GEOCHRONOLOGY OF A GIGANTIC LANDSLIDE ALONG THE WASATCH FRONT, UTAH

Geochronological data provide critical constraints on the timing and sequence of events that formed the east Traverse Mountains (eTM) mega-landslide that separates Salt Lake and Utah Valleys. Zircon ages from pebble dikes in the eTM and their presumed sources in the White/Red Pine phase of the adjacent Oligocene Little Cottonwood stock show that the dike sets are likely offset equivalents of one another. The geochemistry and U-Pb ages of zircon xenocrysts in andesitic dikes in Albion Basin and in the eTM allow their correlation and indicate the same direction and magnitude of offset as the pebble dikes. Geochemistry, mineralogy, and LA-ICP-MS U-Pb ages for zircons in a fallout tuff underlying a distinctive conglomerate in the Salt Lake Formation related to the landslide deposit allowed us to correlate the tuff with a distinctive rhyolitic ignimbrite from the Snake River Plain with ⁴⁰Ar/³⁹Ar sanidine age of 6.62 ± 0.07 Ma (Morgan and McIntosh, 2005). This, combined with U-Pb ages of 6.1 ± 0.9 Ma from U-bearing opal formed within brecciated volcanic rocks in the slide, tightly constrains the timing of this catastrophic event to the latest Miocene. These geochronological results support a model in which extensional faulting initiated a massive landslide that unroofed the Little Cottonwood stock and transported ~100 km³ of fractured Oquirrh quartzites, Oligocene volcanic rocks and dikes, and capping sedimentary rocks 16 km southwest. The integration of geochronology with field mapping establishes a temporal framework for understanding the role of large-scale landslides in the evolution of extensional fault systems and the associated geohazards