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

Miocene Extension within the Comondú Arc, Gulf of California: Isla Monserrat, Parque Nacional Bahía de Loreto, Baja California Sur (Mexico)

Paper number: 1-4
Program: Technical Sessions
Time: 8:50 AM – 9:05 AM

Authors

Abstract

Isla Monserrat is an uninhabited, roadless desert island ~20 km offshore the Baja California peninsula in the Gulf of California (Mexico). Previous geologic mapping of the island (1:250,000-scale, Servicio Geológico Mexicano, 2000) divided it into two units: (1) Pliocene to Quaternary limestones uplifted up to 200 m asl, which rest in angular unconformity upon (2) undated, undifferentiated volcanic rocks, which make up 90% of the island; this presentation focuses on the latter. Our mapping of these volcanic rocks began with drone surveys that yielded a high-resolution orthomosaic and a digital elevation model (DEM) base map for the entire island. The volcanic rocks form a ~N-S-striking, steeply E-dipping section that includes two units of unknown age (from west to east): Unit (1) ultrawelded hornblende dacite ignimbrite; and Unit (2) andesitic volcanic debris flow deposits interbedded with lavas and cut by hypabyssal intrusions, ranging from trachyandesite to trachybasalt. Unit (2) is overlain with angular unconformity by moderately-dipping, nonwelded, bedded biotite-hornblende trachydacite crystal vitric tuffs, with an 40Ar/39Ar hornblende age of 15.849 ± 0.062 Ma. This angular unconformity thus demonstrates that the onset of intra-arc extension occurred prior to the cessation of subduction at ca. 12 Ma.

Major- and trace-element geochemistry on 27 samples from the three units show characteristics of a subduction-related volcanic arc, including greater fluid enrichment and higher pressures of fractionation than would be expected of rift magmas. The rocks lack an Fe-enrichment trend, and FeOt and TiO2 indicate early saturation in Fe-Ti oxides. High Sr and Sr/Y, as well as elevated Al2O3 at a given SiO2, indicate delayed saturation of plagioclase. These are all hallmarks of arc volcanism.

Volcanic rocks on Isla Monserrat are cut by a series of NNW-SSE detachment faults that dip ~10-30°W, in addition to high-angle normal faults with similar strikes. We infer that these faults are strands of a master detachment fault at depth that tilted the Early Miocene arc volcanic section.

The lithologies, compositions and age of the volcanic rocks on Isla Monserrat contrasts markedly with younger (<12 Ma) continental rift volcanic rocks elsewhere in the Gulf of California region (e.g. Busby et al., this meeting). Instead, they closely resemble the 25-12 Ma Comondú arc volcanic rocks exposed for >400 km along the southern Baja California peninsula.

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

doi: 10.1130/abs/2026CD-410942

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