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

A Remarkable Example of Volcanic Debris-Avalanche Deposits in Rocks of the Lower Comondú Group (Early Miocene), Southern Sierra La Giganta, Baja California Sur, Mexico.

Paper number: 1-5
Program: Technical Sessions
Time: 9:05 AM – 9:20 AM

Authors

Abstract

Volcanic debris-avalanches are some of the largest and most hazardous geological phenomena. They have been recognized extensively in young volcanic terrains and occur due to the catastrophic collapse of volcanic edifices or piles. In the southern sector of the Sierra La Giganta, southeast Baja California peninsula, a thick succession of fluvial and transitional marine deposits are overlain by Miocene volcaniclastic rocks, felsic tuffs and breccias, of the Lower Comondú Group. Within this succession it stands out the presence of thick andesitic breccias, a feature already reported in other locations, but lacking detailed characterization. This succession represents a depocenter that predates the opening of the Gulf of California.

Here, we describe the Los Reyes-Tarabillas volcanic breccia, intermittently exposed for at least 30 km in NW and EW transects. It consists of a thick (up to 150 m), heterolithologic, massive diamictic rock. The breccia exhibits diagnostic features of a volcanic debris avalanche deposit, such as elongated sheared blocks, with pervasive jigsaw-fit fractures, flames, domains corresponding to incorporated soft and rigid substrate, monolithic blocks, and clast-supported zones. It contains exotic blocks up to 30 m long, and syn- and post-depositional faults throughout. It exposes marginal facies (hyperconcentrated flow type) and sandy breccias. Clastic injections and dikes provide evidence of catastrophic emplacement, producing ultracataclasites and frictional melts at the base.

Breccias source areas are still unknown, the region to the west is covered by younger rocks, and to the east by the sea. It is inferred that the breccia represents more than one collapse event, with explosive eruptions and intervening sedimentation, although repetition of units cannot be ruled out. Its multiple interactions with the substrate drove ductile deformation of soft sediment through friction and loading. Evidence such as blocks with bread-crust texture, prismatic jointing, and alteration halos, points to a possible juvenile component. Its relative age is between 20 and 23 Ma (U/Pb, Aranda-Gómez et al., 2025); however, direct dating of a dacite clast resulted in 21.31±0.44 Ma.

We envisage the existence of large volcanic complexes that collapsed catastrophically, generating these voluminous breccias. The implications of its displacement paleodirection, correlation, volume, transport, emplacement mechanisms, and localized friction, as well as the complex relationship of the Sierra Madre Occidental with the Comondú Group will be discussed. PAPIIT AG101523.

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

doi: 10.1130/abs/2026CD-411010

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