ZIRCON GEOCHRONOLOGY OF THE BASAL LAYER OF THE SEVIER GRAVITY SLIDE, MARYSVALE VOLCANIC FIELD, UTAH, USA

The Sevier gravity slide (SGS) in southwest Utah is one of the largest terrestrial landslides on the planet. This newly discovered gravity slide represents collapse of part of the south flank of the Marysvale volcanic field, coeval with Oligocene igneous activity. The slide is approximately 1,300 km³ in volume and covers an area of ~2,500 km² and consists of andesitic lava flows, lahars and regional ash-flow tuffs that slid south ~40km over the Oligocene land surface. The base of the slide consists of a thin layer of brecciated volcanic rocks that originated from the catastrophic emplacement. This basal layer ranges from millimeters to several tens of meters in thickness and locally was injected into the upper plate as clastic dikes. Two samples of the basal layer of the SGS were collected (Rock Creek and South Prospect Creek) to determine a maximum depositional age (MDA) of the slide and to understand the ages of both the allochthonous rocks and the rocks that were overrun. Zircons were separated using traditional gravitational and magnetic techniques and dated at the University of Arizona Laserchron Center. The basal layer at Rock Creek contains zircons (n=57) ranging in age from 25-2801 Ma with an MDA of 25.30 ± 0.41 Ma. The basal layer at South Prospect Creek contains zircons (n=294) ranging in age from 25-2636 Ma with an MDA of 25.73 ± 0.33 Ma. As these two MDAs overlap, both basal layers are correlative and indicate that the SGS emplacement occurred ~25 Ma. Precambrian zircons were found in both samples, indicating that some dated zircons are xenocrysts that were incorporated from ancient eruption events. If the youngest fraction of zircons formed immediately prior to slide emplacement, the SGS is as much as 2 Ma older than the larger Markagunt gravity slide to the west. Moreover, these data in addition to field relationships indicate that the SGS emplacement was coeval with the eruption of the Antimony Tuff (25.11 ± 0.06 Ma), which may have contributed to the initiation of the slide.