CONSTRAINING THE AGE OF THE SEVIER GRAVITY SLIDE (UTAH) THROUGH ZIRCON AND FELDSPAR GEOCHRONOLOGY

The late Oligocene-Miocene Marysvale volcanic field in central Utah experienced three sequential south-directed, mega-scale collapse events. These slides cover 1000s of km² and involve 100s of km³ of allochthonous volcanic rocks. The Sevier gravity slide is the oldest and easternmost of the three. Here we present U/Pb dates determined via LA-ICPMS on zircon crystals separated from the upper plate Mount Dutton Formation and its Kingston Canyon Tuff member. Each sample was collected within 10 m of the slide base east of Blind Spring Mountain near Panguitch, Utah. The Mount Dutton Formation here consists of >500 m of gray and brown, crystal-poor, pyroxene-bearing, andesitic to dacitic volcanic breccia conglomerate and minor sandstone. The Kingston Canyon Tuff member is a densely welded crystal-lithic tuff with a basal vitrophyre; zircon crystals were extracted from the welded tuff. The Mount Dutton zircon U/Pb age spectrum primarily spans from 25-35 Ma, with a maximum depositional age of ~26 Ma. The Kingston Canyon Tuff has a similar ~26 Ma age peak, but is bimodal with a secondary age peak of ~34 Ma. We interpret that the younger peak is reflective of the eruption age of the tuff, and that the older peak likely represents the age of the entrained lithic clasts. We use the new U/Pb data with ⁴⁰Ar/³⁹Ar eruption ages for the Kingston Canyon Tuff and overlying Osiris and Antimony Tuffs to constrain the timing of the Sevier gravity slide to between 25 and 26 Ma. This temporal information is key to understanding the tectonic provenance of the slide basal layer and timing of volcanic buildup in the Marysvale volcanic field.