Paper No. 169-6
Presentation Time: 9:25 AM
OPTIMIZING PSEUDOTACHYLYTE 40Ar/39Ar AGES TO INFER THE TIMING OF UTAH’S ANCIENT GRAVITY SLIDES
The Oligocene-Miocene Marysvale volcanic field in central Utah represents continental arc magmatism associated with the rollback of the Farallon Plate slab and the associated ignimbrite flare-up in Nevada. The field straddles the leading edge of the Sevier fold and thrust belt. Volcanic activity produced predominantly calc-alkaline andesites and dacites from about 30 to 20 Ma, and transitioned to bimodal basaltic and rhyolitic volcanism about 20 Ma. Stratovolcanoes were constructed atop the clay-rich Brian Head Formation. In contrast to the eastward-directed Sevier thrust faults, three large-scale, low-angle (<3°) southward-directed gravitational collapses (gravity slides) have been identified within this volcanic field, with runout distances exceeding 20 miles. These collapses involved a chaotic mix of older lahars, lava flows, and ash-flow tuffs with total thicknesses of over 6000 feet. During the slide movements, high frictional heating and overpressured fluids within the allochthonous block generated pseudotachylyte—a form of glass. This obsidian-like pseudotachylyte is found on shear planes, in associated emanating dikes, and within fractures 1-3 cm wide, emphasizing the dynamic faulting and high-energy conditions during gravity slide events. This study introduces new high-precision 40Ar/39Ar eruption ages for the ash-flow tuffs within the slide masses, the gravity slide-generated pseudotachylyte, and post-slide emplacement ash-flow tuffs. Dating pseudotachylyte from the Marysvale volcanic field is challenging due to its low potassium content, leading to large uncertainties. To address this, 40Ar/39Ar ages, associated uncertainties, and stratigraphic positioning were integrated into a Bayesian stacked bed model to probabilistically refine the age of the glass and reduce overall uncertainty. This refined age serves as a proxy for the timing of the gravity slide. Through this innovative geochronologic and modeling approach, we use cm-scale gravity slide-generated glass to determine the timing of mega-scale landscape changes, which occurred approximately 23-25 million years ago. This research underscores the interplay between fault dynamics, plate tectonic processes, and large-scale volcanic and gravitational events in shaping the geological evolution of the Marysvale volcanic field.