2 Tectonic setting of Comondú group: a discussion on alternative interpretations

The Comondú group is a key element in the tectonic evolution of the Gulf of California. This session will discuss different intepretations of Comondú group based on geologic and structural information and geochornology

Wednesday, 22 April 2026
10:00 AM - 12:00 PM
LMH, Deli

Large scale soft sediment Early Miocene deformation in a weak volcanosedimentary substrate: Sierra La Giganta, Baja California Sur, México

Paper number: 2-2
Program: Technical Sessions
Time: 10:20 AM – 10:35 AM

Authors

Abstract

The Late Oligocene to Early Miocene volcano-sedimentary succession (i.e., the Comondú Group) exposed south of San Juan de la Costa displays structures formed by the passage and accumulation of a voluminous volcanic debris avalanche (VDA) over a substrate composed of low-grade felsic ignimbrites, interbedded with reworked felsic tuffs, and stratified epiclastic-volcanic deposits. Deformation is complex, non-penetrative (only profoundly modified sediments in the areas where the avalanche passed), and includes folds (sometimes refolded) and faults. VDA passage and deposition also caused the intrusion of thick clastic dikes and the formation of structural domes by the ascent of fluidized materials. We interpret all these structures as products of soft sediment deformation in wet, unlithified or weakly lithified sediments and low-grade ignimbrites. The inferred age of emplacement of the VDA and deformation, based on U-P zircon dating, is between 20.6 and 23.9 Ma. However, not all the deformations that we documented in the study area are directly attributable to the action of the VDA. Other processes caused deformation in the area, among them: 1) gravitational tectonics that formed syn-sedimentary slumps before and after the VDA, 2) syn-depositional normal faulting, 3) reversed density gradient effects triggered by the andesitic VDA, and 4) regional shortening and thrusting (Bonini et al., Tecto-127518).

We call these deformed rocks the Agua de Yepis structural complex (AYC), as they are well exposed around the Agua de Yepis ranch. There is evidence in the AYC of deformation-induced mixing of lithologies and their fluidization and injection, formation of isoclinal folds or refolded folds, and layer rotations >90º. We also recognized the presence of exotic blocks, torn from the VDA substrate by the avalanche and pushed or incorporated into the VDA deposit. Exotic blocks are often rigid and have elongated shapes. There is evidence of ductile deformation around the exotic blocks.

Based on the presence of the AYC, we divided the stratigraphic succession of the Lower Comondú Group (i.e., below the San Juan Tuff) into pre-, syn-, and post-VDA units. The AYC was formed, in geological terms, instantaneously by shear and overload imposed by the VDA deposit (an andesitic, in places dacitic, massive breccia). Deformation continued for an unknown time period due to the overload imposed on parts of the pre-AYC succession by the breccia. PAPIIT AG101523.

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

doi: 10.1130/abs/2026CD-410950

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