8 Cenozoic Cordilleran Tectonics, Magmatism, and Crustal Evolution: Perspectives from Mexico and Western North America (Posters)

Tectonic reconstructions of the southwestern North American plate boundary provide an excellent framework for understanding Cenozoic volcanism (arc, rift, slab window, seafloor) in this largely extensional and transtensional setting. Geologic, geochemical and geophysical submissions encouraged.

Wednesday, 22 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

First Reports of Pseudotachylyte (Frictional Melt) at the Base of a Massive Landslide Breccia in the Miocene Comondú Group, Sierra La Giganta, Baja California Sur, Mexico

Paper number: 8-5
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Pseudotachylytes (PsT), or friction melts, form during high-temperature geological events like earthquakes, impacts, and large-scale landslides. They result from shearing and subsequent quenching, typically appearing as fine-grained, black, glassy material. PsT usually forms planar layers or veinlets but can also create micro-breccias and injections. This study presents the first descriptions of landslide-related pseudotachylyte layers in Mexico. These layers are found intermittently at the base and other levels of the Los Reyes-Tarabillas debris-avalanche deposit, an early Miocene heterolithic breccia in the lower Comondú Group, Baja California Sur.

The PsT layers are generally two to five centimeters thick and extend a few meters laterally. They are opaque, glassy to devitrified, and appear black, grey, or green when altered. Texturally, they show micro flow-banding, contorted laminae, vesicles, folding, and centimeter-scale injections. Contacts with the host rock are relatively sharp. In thin section, the textures vary with the degree of melting, from cataclasite to aphanitic, with schlieren-like glass. Fresh samples resemble a gauge or ultramylonite but with an ultra-fine, isotropic matrix. They contain abundant small broken and rotated crystals (<100 um), but scarce microlites. The average crystallinity is 65%, with 35% amorphous material. Crystalline phases include anorthite, albite, smaller amounts of quartz and tridymite. The bulk chemistry of the PsT is typically andesitic (58-60 wt.% SiO2), slightly more mafic than the overall deposit chemistry (andesitic to dacitic, 60-64 wt.% SiO2). With low alkalis and high alumina. Trace elements show enrichment in Ba, Th, and U, and negative anomalies for Pb, P, and Ti. Rare Earth Elements display a subtle Eu anomaly.

Previous attempts to date these layers using Ar/Ar (Holliday et al., 2022; Rivera et al., 2025) have been useful. We are currently performing similar dating to better constrain the age of the event. Dating of underlying and overlying units provides a time range of 20.5 to 23.9 Ma (U/Pb; Aranda-Gómez et al., 2025), with a juvenile block yielding a closer age of 21.3 Ma. Our findings confirm the presence of pseudotachylyte within the massive Los Reyes-Tarabillas debris-avalanche deposit. The formation of these PsT is attributed to high temperatures, high frictional rates, and the presence of fluids during the downslope movement of the large mass (ca. 200 m thick) from an unknown volcanic collapse source. PAPIIT AG101523

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-411037

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