Emplacement Age of the Sevier Gravity Slide, Utah, USA

Abstract. The Marysvale volcanic field in southwestern Utah hosts three large volume gravity slides: the Sevier (SGS), the Markagunt (MGS), and the Black Mountains (BGS). The gravity slides are composed of lahar deposits, lava flows, and ash-flow tuffs erupted from former stratovolcanoes and other vents during the Oligocene and Miocene. The ash-flow tuffs are prime targets for dating to constrain the age of the gravity slides because some ash-flow tuffs are deformed within the slides, whereas others are undeformed and cap the slides. Furthermore, the gravity slides produced pseudotachylyte during slide motion, a direct indicator for the timing of each slide. This work provides new ⁴⁰Ar/³⁹Ar dates for several ash-flow tuffs and pseudotachylyte for the SGS, along with U/Pb zircon dates for one deformed tuff and overlying alluvium at the slide plane. Results show that the slide was emplaced at 25.25 ± 0.05 Ma and was immediately followed by the eruption of the Antimony Tuff at 25.19 ± 0.02 Ma. The model presented here suggests that the intrusion of magma related to the Antimony Tuff acted as a triggering mechanism for the slide, and that slide movement itself led to decompression melting and eruption of the Antimony Tuff. This sequence of events occurred on a geologically rapid timescale and may have been virtually instantaneous.