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

Petrophysical Fingerprints of Welding Degree in the Aguajito Ignimbrite, Baja California Sur, Mexico

Paper number: 8-2
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Aguajito Caldera is a key volcanic center in northeastern Baja California Sur and part of the Aguajito–Reforma–Tres Vírgenes volcanic system, an area of recognized geothermal potential along the Gulf of California margin. Its ignimbrites provide a natural laboratory to evaluate how welding controls the physical behavior of volcanic deposits. Although welding is commonly inferred from pumice textures (fiamme development), these descriptors alone do not uniquely constrain physical and mechanical properties, because pore connectivity, microcrack damage, and densification can vary substantially within the same lithofacies association. Here we quantify welding in the Aguajito ignimbrite as a measurable, multi-parameter continuum by integrating pore structure, gas transport, elastic response, and strength, providing quantitative relationships to objectively partition welding domains that may be ambiguous in outcrop.

We integrate gas permeability, bulk (ρb) and grain density (ρr), total (ϕt) and connected porosity, uniaxial compressive strength (UCS), indirect tensile strength, and dry ultrasonic P- and S-wave velocities (Vp and Vs) measured at 1 MHz on specimens collected along a stratigraphic profile. Bulk density (∼ 1.50–2.40 g/cm³) and total porosity (~2–39%) define a continuous welding-driven densification pathway. A key outcome is that welding does not merely reduce pore volume; it reorganizes pore geometry, progressively converting equant vesicles into flattened, crack-like voids and tightening connectivity, which disproportionately controls permeability and stiffness. Consistent with this microstructural pathway, permeability drops from ~25 mD to <0.01 mD while UCS increases from ~2 MPa to >100 MPa, recording a transition from a compliant, connected-porosity framework to a load-bearing welded fabric.

Elastic properties evolve coherently with this structural transformation. At high porosity (ϕt ~25–39%), low velocities (Vp ~1.8–2.5 km/s; Vs ~1.0–1.5 km/s) and elevated Vp/Vs (~1.70–2.00) indicate strong sensitivity to connected pores and crack-like compliance. With progressive welding (ϕt ~12–25%), velocities rise (Vp ~2.8–3.5 km/s; Vs ~1.6–2.0 km/s) and Vp/Vs shifts toward ~1.65–1.90 as pore collapse stiffens the framework. The densest material (ϕt ~1–12%) reaches high velocities (Vp ~3.5–5.0 km/s; Vs ~2.4–3.4 km/s) and lower Vp/Vs (~1.45–1.60). By treating welding as a quantifiable process grounded in pore geometry and connectivity, we provide a framework to interpret ignimbrite architecture and anticipate permeability–strength contrasts relevant to volcanic and geothermal systems.

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

doi: 10.1130/abs/2026CD-410888

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