22 Evolution of Cordilleran-type orogenic systems (Posters)

This session seeks contributions that connect surface, crustal, and mantle processes associated with Cordilleran systems, specifically addressing geodynamics, kinematics, magmatism, uplift, erosion, basin subsidence, and sediment dispersal in ancient and modern settings.

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

Tectonic control on type and volume of volcanic structures in Southwestern Mexico City

Paper number: 22-5
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The tectonic evolution of southwestern Mexico City, in the central part of the Transmexica Volcanic Belt (TMVB), controlled the transition from polygenetic to monogenetic volcanism. Around 3 Ma, polygenetic activity was concentrated along N65°W faults, where elevated stress sustained fracture opening and enhanced crustal permeability. This structural configuration allowed magma to accumulate in large reservoirs, enabling the construction of major polygenetic volcanoes such as Ajusco (Santo Tomás dome, 2.8 km³) and Los Picachos (47 km³), as well as other large domes of the Sierra de las Cruces. These volumes reflect the capacity of fault-controlled reservoirs to sustain long-lived eruptions. In Ajusco, collapse was triggered by activity of a NW–SE normal fault, marking the end of this intense phase.

After a volcanic hiatus of ~1.27 Ma, volcanism renewed again around 1.6 Ma ago with the formation of a new N65°W-trending graben southwest of Ajusco volcano. Faulting provided new outlets for magma migration into the graben center, where intermediate-aged domes were emplaced. Compared to the earlier phase, stress magnitudes were lower, resulting in smaller domes with volumes between 0.005 and 0.18 km³. These reduced volumes highlight diminished reservoir connectivity and shorter-lived magma supply, marking a transitional stage between polygenetic and monogenetic volcanism.

At ~27 ka B.P., volcanism migrated westward and northward, surrounding older and intermediate domes within pre-existing grabens. Two new grabens formed contemporaneously, one parallel to the earlier N65°W structure. This younger graben displaced older faults, indicating a transtensional regime. These conditions favored rapid ascent of small magma batches, producing monogenetic scoria cones with volumes typically <0.12 km³, fissures, and dikes.

This spatio-temporal progression demonstrates that the shift from polygenetic to monogenetic volcanism was driven by tectonic regime evolution: from compressional–transtensional conditions sustaining large-volume reservoirs, to extensional and transtensional faulting fragmenting magma supply that promoted isolated eruptions.

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

doi: 10.1130/abs/2026CD-410925

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