1 Tectonic Controls on Cenozoic Volcanism in the Gulf of California-Salton Trough Region and the Adjacent Southwestern North American Plate Boundary

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 - 10:00 AM
LMH, Deli

The Comondú Arc-Gulf of California Rift Transition, Baja California Sur, Mexico

Paper number: 1-7
Program: Technical Sessions
Time: 9:35 AM – 9:50 AM

Authors

Abstract

We show that tectonic setting has a significant impact on eruptive styles and magma compositions on both the Comondú arc and the transition to the Gulf of California rift. The Comondú arc is an extensional continental arc that was active between 24 and 12 Ma in the region where the Gulf of California would eventually open; continental rifting initiated at 12 Ma, when subduction ended and the Pacific-North American plate boundary jumped inboard, to occupy the thinned and thermally-weakened Comondú arc axis.

New geologic mapping, petrography, 40Ar/39Ar geochronology and whole rock geochemical data from Baja California Sur (Islas Monserrat and Danzante, Bahía Concepción, La Trinidad and Santa Rosalía) show that the arc and rift volcanic rocks are dramatically different from one another in both their field and geochemical characteristics. Arc volcanic rocks are dominated by andesites, mainly lavas and flow breccias, with lesser block-and-ash-flow tuffs, and abundant, large hypabyssal intrusions. In contrast, rift volcanic rocks are bimodal, with trachydacite to rhyolite welded ignimbrites, and alkalic and basalt lavas, and the hypabyssal intrusions are small and few. These field criteria also predict almost perfectly the geochemical and petrologic characteristics. Arc samples show the hallmarks of slab-derived fluid enrichments and differentiation at higher pressures than would be expected of rift magmas. This includes early saturation of Fe-Ti oxides (low FeOt and TiO2), and high Sr and Sr/Y, as well as higher Al2O3 at a given SiO2 or MgO content which indicates delayed saturation of plagioclase. In contrast, rift samples are shifted to higher FeOt, showing delayed Fe-Ti oxide saturation, and lack arc-like fluid inputs.

The best section through the arc-rift transition we have found to date lies on the Concepción Peninsula. This section is laterally-extensive (30 km), thick (>2 km), pristine, and superbly exposed. Extension accompanied both arc magmatism (20-12 Ma) and rift magmatism (<12 Ma). New Nd and Sr isotopic data, together with trace element data (Sr, Y, La, Nb), show that rift samples plot in the East Pacific Rise field, while arc samples show evidence for greater fluid enrichment and differentiation at greater depths. Interestingly, the youngest arc sample and oldest rift sample, which overlap in age, are transitional between arc and rift samples on the plots.

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

doi: 10.1130/abs/2026CD-410836

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