CONSTRAINING THE AGES OF THE MARKAGUNT AND SEVIER MEGA GRAVITY SLIDES, UTAH

The early Miocene Markagunt Gravity Slide and late Oligocene Sevier Gravity Slide are two large-volume landslides located in southwestern Utah that originated within the Marysvale Volcanic Field. Volumetrically, the slides are comparable to the Heart Mountain Gravity Slide in Wyoming, previously considered to be the largest terrestrial landslide. Landslides of this scale are difficult to interpret as emplacement is often structured similarly to tectonic features. The Markagunt and Sevier Gravity Slides are interpreted as separate events, but the timing of each is poorly constrained. Both landslides generated pseudotachylyte and several different volcanic tuffs bracket each of the landslides. Pseudotachylyte (frictionite) is generated through friction induced melting at the basal surface of a landslide or fault, and has been discovered in landslides globally. We present a new ⁴⁰Ar/³⁹Ar date for the one-centimeter thick layer of pseudotachylyte found at the base Markagunt slide, along with five new ages for tuffs bracketing the two slides. Two volcanic tuffs emplaced on top of the Sevier slide suggests that the slide event is older than ~25 Ma. The Markagunt Gravity Slide, west of the Sevier slide, is proposed to be about twice as large (5000 km²) and slightly younger. Chemical analyses of the calc-alkaline tuffs help us to link the eruptive products to their source within the Marysvale Volcanic Field, which may allow us to determine a relationship between the slides and the calderas to the north. The ⁴⁰Ar/³⁹Ar ages presented in this work prompt new questions regarding the location of the Markagunt slide boundary proposed by Hacker et al. (2014).