24 Undergraduate Research Posters

This poster session highlights geoscience research conducted and presented by undergraduate students. Abstracts must be written by students (but may include non-student co-authors). Research topics may include any area of the earth sciences.

Thursday, 23 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Structural analysis of the southern Tehuantepec Isthmus, and its tectonic implications, Oaxaca, Mexico.

Paper number: 24-5
Program: Technical Sessions
Time: 8:15 AM – 8:15 AM

Authors

Abstract

The central-southern Tehuantepec Isthmus in Oaxaca, Mexico, is a tectonically complex region situated at the boundary of the Maya, Cuicateco, and Xolapa terranes. These terranes are organized in NW-SE trending belts separated by lithospheric-scale faults. The intricate fault patterns have disrupted geological contacts, complicating both regional mapping and the reconstruction of overprinting tectonic events. This study documents the deformation of these lithological units through new geological mapping 1500 km2 and a 60km structural cross-section extending from Salina Cruz (SW) to Nizanda (NE).

Our results identify three distinct boundaries associated with sinistral shear: Southern Boundary: Juxtaposes Eocene-Miocene granitic rocks against a sequence of pillow lavas and volcaniclastic sediments (tentatively the Tithonian-Barremian Chivillas Formation).Central Boundary: Places the Chivillas Formation in contact with Upper Cretaceous meta-limestones and Northern Boundary: Delimits the meta-limestones from Upper Cretaceous-Paleocene volcaniclastic rocks of the Chontal Arc.

Deformation varies by unit: the Chivillas Formation exhibits low-grade metamorphism and bedding-parallel shear with NE-directed transport; the meta-limestones show intense deformation, including calci-mylonites and isoclinal folding; and the Chontal Arc sequences feature muscovite development and NE-vergent thrusting. These sequences are truncated by the N30° oriented Salina Cruz Fault, a sinistral structure that displaced footwall folds. This fault was subsequently overprinted by NW-SE brittle-ductile sinistral faults (e.g., Maravillas Fault) and E-W sinistral faults later reactivated as normal faults. These systems facilitated Miocene granitic intrusions and the formation of limestone roof-pendants. Our findings suggest that shortening structures relate to the final stages of the Mexican Orogeny, which in this area only caused localized deformation, as opposite to folding involving large volumes of rocks (e. g. most of the sierra Madre Oriental), while the superimposed sinestral strike-slip systems seem to have assisted the translation of the Chortis Block, mostly during the Neogene.

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

doi: 10.1130/abs/2026CD-410865

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