T169 Integrated Geologic and Chronometric Insights into Tectonic Processes at Plate Boundaries

Tuesday, 24 September 2024
8:00 AM - 12:00 PM
304C

OPTIMIZING PSEUDOTACHYLYTE 40Ar/39Ar AGES TO INFER THE TIMING OF UTAH’S ANCIENT GRAVITY SLIDES

Paper number: T169-7
Program: Technical Sessions
Time: 9:25 AM – 9:40 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-400882

© Copyright 2024 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.