2-5 Chronology of faults in Tembabichi region affecting the Comondú sequence in Baja California Sur

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

Presenting Author:

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Abstract:

The tectonic setting of the Late Oligocene–Middle Miocene Comondú Group in Baja California Sur remains a topic of debate. While traditionally interpreted as a stable forearc sequence accumulated before  the Rifting of the Gulf of California, recent models suggest it records the early stages of continental rupture. Here, we present structural mapping and kinematic analysis from the Tembabichi area prior to 12.5 Ma.

We document a complex progressive deformation history defined by four distinct fault systems, temporally constrained by cross-cutting relationships with magmatic intrusions. The earliest deformation phase is characterized by NE-SW-striking, NW-dipping normal faults and subordinate E-W dextral-normal faults. We interpret these structures not as isolated features, but as intra-basin faults related to a major, large-offset master fault located to the east (in the present-day offshore Gulf). This inferred basin-bounding structure controls the abrupt contact between the Lower Comondú Group and the Cretaceous granitoids from Punta Botella to Santa Catalina Island.

Field observations reveal that these early faults are intruded by pyroclastic dikes dated at 28.05 ± 0.18 Ma. This cross-cutting relationship provides physical evidence of syn-sedimentary extension during the Oligocene, geodynamically linking the southern Baja California microplate to the broad Basin and Range extensional province.

This early diffuse extension was subsequently overprinted by strain localization along NW-SE-striking, NE-dipping normal faults, which control the modern topography of the Main Gulf Escarpment. Finally, the superposition of a younger NNE-striking left-lateral system suggests that the overall fault pattern is primarily accomplished by strain partitioning within a developing transtensional shear zone.

We propose that the documented Oligo-Miocene distributed faulting and magmatism acted as a crucial lithospheric pre-conditioning mechanism. This early internal deformation thermally and mechanically weakened the lithosphere, creating a rheological heterogeneity that subsequently led to the localization of the narrow, oblique rift axis established after ~12.5 Ma.

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

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