34-5 Why the San José del Cabo Fault is younger than the Los Barriles Fault, Southern Baja California Sur, Mexico

Session: Recent Advances in the Gulf of California Oblique Rift: Offshore and Onshore Studies // Avances Recientes en el Rift Oblicuo del Golfo de California: Estudios Marinos y Terrestres (Posters)

Poster Booth No.: 5

Presenting Author:

Authors:

Walde, Michel Alain¹, Schwennicke, Tobias², Plata Hernández, Elvia³, Cota Ramírez, Daniel⁴

(1) Departement Umweltwissenschaften, Universität Basel, Basel, Basel-Stadt, Switzerland, (2) Departamento Académico de Ciencias de la Tierra, Universidad Autónoma de Baja California Sur, La Paz, Baja California Sur, Mexico, (3) Departamento Académico de Ciencias de la Tierra, Universidad Autónoma de Baja California Sur, La Paz, Baja California Sur, Mexico, (4) Departamento Académico de Ciencias Marinas y Costeras, Universidad Autónoma de Baja California Sur, La Paz, Baja California Sur, Mexico,

Abstract:

The Los Barriles Basin, located near the southern tip of the Baja California peninsula, forms part of the Gulf of California Extensional Province and records Miocene–Pliocene crustal extension associated with Gulf opening. Contrary to previous interpretations that depict a single, continuously and oddly curved fault system, our study recognizes this area as comprising two distinct structural domains: The Los Barriles Basin and the San Jose del Cabo Basin. The basin is a west-dipping half-graben bounded by the Los Barriles fault system along its western margin, whereas its southern boundary, is defined by the San José del Cabo fault. Our study integrates new stratigraphic, structural, and geomorphological data from the southern portion of the Los Barriles Basin aimed at constraining the relative timing and tectonic roles of these two fault systems.

Detailed field-based geologic mapping and stratigraphic analysis allow recognition of sin-rift and post-rift sequences. The sin-rift sequence comprises the Los Barriles Formation, dominated by coarse- to fine-grained conglomerates deposited in high-gradient alluvial fans adjacent to the marginal fault, and its lateral eastward interfingering with the Trinidad Formation, composed of sandy to muddy marine deposits of probable late Miocene age. These units represent a tectonically controlled deep-water Gilbert-type fan delta system. Westward-dipping beds with dips up to 30° reflect rapid subsidence and rotation of the basin during and shortly after late Miocene extension, attributed to activity along the Los Barriles fault system.

Five fault segments with NW–SE trends and relay ramp geometries are identified within the Los Barriles fault system, indicating sustained activity from the late Miocene into the Pliocene. In contrast, the general geometry of the faults suggests that the San José del Cabo fault cuts the southernmost part of the basin which caused basin uplift, erosion, and river incision. Post-rift fluvial deposits of the El Chorro Formation (probable Pleistocene) unconformably overlie and locally cover the Los Barriles fault system, demonstrating its prior inactivity. The distribution of the El Chorro Formation differs markedly between the two basins: within the uplifted and dissected Los Barriles Basin, it is preserved as terrace remnants in valleys, while in the topographically lower San José del Cabo basin, it forms extensive, blanket-like deposits that broadly cover older units.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

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