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

Mechanical and microstructural characterization of deformed volcaniclastic rocks from the Lower Comondú Group, Baja California Sur, Mexico

Paper number: 8-4
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The mechanical behavior of volcaniclastic rocks remains poorly understood, particularly in weakly lithified and porous rock masses. In this study, we selected a volcaniclastic succession from the Lower Comondú Group in Baja California Sur. Two poorly consolidated lithologies predominated in this member: pumice-bearing tuff and heterogeneous sandstone. Previous and new mapping show evidence of complex deformation affecting the succession, including folding, inverse faulting, and clastic injection features. The mechanical response was tested in uniaxial and triaxial experiments conducted on selected cylindrical rock specimens. The analysis was integrated with petrographic analyses and micro-CT image analyses, which allowed for assessing the microstructural evolution under compressive stress. The experiments were performed in the Laboratorio de Física de Rocas (LAFIR) at the Instituto de Geociencias, UNAM, using a servo-hydraulic MTS 815 system. Test conditions included confining stress from 0 to 30 MPa and cyclic loading. Tuff samples exhibit unconfined compressive strength (UCS) values between 16.5–17.5 MPa and elastic moduli of 0.8 GPa–1 GPa, while sandstones range from 13.2–18.8 MPa and 0.78–1.6 GPa, respectively. Poisson’s ratios vary between 0.1 and 0.3. Uniaxial cyclic loading experiments revealed progressive fracture development, stiffness degradation, and dilatancy consistent with brittle deformation behavior. During progressive deformation, a more plastic response is inferred. The triaxial results indicate that tuff samples exhibit peak strengths of ~8.8 MPa, with an elastic modulus of around 2.5 GPa, and high dilatancy associated with plasticity. In general, the values obtained place these rocks in the lower-middle of the weak-rock category of engineering rock-mass classifications. The results also indicate that porosity, mineral composition, and pumice-rich textures strongly control the brittle–ductile transition. Micro-CT image analysis confirmed internal damage zones aligned with observed mechanical weakening. We hypothesize that overpressure during deformation may have reduced effective stress to the point that deformation is accommodated by grain rearrangement, cement degradation, and diffuse compaction rather than brittle fracturing, leading to rapid strength loss and mechanically unstable behavior. These results improve the geomechanical characterization of volcanosedimentary successions in compressive tectonic settings and provide new insights into the tectonic deformation conditions of this zone in Baja California.

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

doi: 10.1130/abs/2026CD-411038

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