38-4 Through the Melting Glass: A Comparison of Pseudotachylyte Geochemistry from Two Sites in the Marysvale Gravity Slide Complex, Utah.

Session: Petrology and Geochemistry (Posters)

Poster Booth No.: 4

Presenting Author:

Alexander Smith

Authors:

Smith, Alexander Stone¹, Rivera, Tiffany², Braunagel, Michael³, Hacker, David B.⁴, Biek, Robert F.⁵, Malone, David H.⁶, Griffith, William Ashley⁷

(1) Geological Sciences, University of Missouri, Columbia, MO, USA, (2) University of Missouri, Columbia, MO, , (3) Swenson College of Science and Engineering, University of Minnesota Duluth, Duluth, Minnesota, USA, (4) Kent State University, Seville, OH, , (5) Fort Bragg, CA, , (6) Illinois State University, Normal, IL, , (7) Ohio State University, Columbus, OH, ,

Abstract:

Pseudotachylyte (PST) is a glass that is generated during slip events where friction produces enough heat to melt the rock along the slide plane. Despite the simplicity of the concept, the extent of homogenization between the upper and lower plates of the host rock is unclear, and the presence of relict grains, devitrification, and complex flow regimes obscures the order in which these minerals melt during slide movement or high-velocity impacts (Kirkpatrick et al, 2012; Spray, 2010).

The Marysvale Gravity Slide Complex (MGSC) of southwest Utah reveals well-preserved PST within the Sevier and Markagunt gravity slides along subsidiary slide planes (Braunagel et al., 2023). To understand the interaction between the melt phase and the constituent minerals in PST production, we analyzed PST from two locations, Sandy Wash and Hellhole, in the 23 Ma Markagunt gravity slide using electron microprobe analysis (EMPA) on glass and plagioclase grains. 

At Sandy Wash, the upper plate is Mt. Dutton lahar and lower plate Bear Valley volcanic arenite; at Hellhole, the upper plate is Mt. Dutton lahar and lower plate Mt. Dutton basaltic andesite. Petrographic observations indicate a difference in the cooling regime between the two sites. Sandy Wash has plagioclase microlites, displays skeletal overgrowths on relict plagioclase, and is set within a homogenous brown glass. In contrast, the Hellhole site has no microlites or overgrowths, and the glass is fractured and contains many finely comminuted grains and xenoliths.

The plagioclase grains vary in K2O, CaO, and Na2O systematically across both sites, reflecting the solid solution of felspar, and the median content of minor FeO and MgO are consistent between the sites. The glass geochemistry is distinct. Sandy Wash demonstrates higher FeO and MgO and lower SiO2 content despite an abundance of quartz in the lower plate. Moreover, a comparison of the glass content as a function of distance from relict plagioclase grains in the Hellhole PST indicates there is no change in glass chemistry from proximal to distal locations, even when compared to randomly selected locations throughout the PST sample. Despite the textural differences the glasses are remarkably compositionally homogenous within each PST sample. This suggests that homogenization of the glass occurs primarily during comminution of the host rock than during post-melt sliding.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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