19 Tectonic Evolution and Competing Models for the Opening of the Gulf of California: From Subduction to Oblique Rifting

This symposium will explore major controversies in the tectonic evolution of the Gulf of California, highlighting the timing, kinematics, and driving mechanisms behind its transition from subduction to oblique continental rifting and transform faulting.

Thursday, 23 April 2026
8:00 AM - 12:00 PM
LMH, Fiesta Terrace Salon

KEYNOTE: Progressive Localization of Dextral Shear in the Late Miocene Gulf of California – A Hybrid Tectonic Model

Paper number: 19-6
Program: Technical Sessions
Time: 9:11 AM – 9:41 AM

Authors

Abstract

Decades of research in northwest Mexico underpins tectonic models that interpret the evolution of Late Miocene (~12-6 Ma) dextral-oblique strain which culminated in ~6-Ma localization of the Pacific-North America (PAC-NAM) plate boundary into the Gulf of California. These models include “strain partitioning” (Stock and Hodges, 1989), “distributed transtension” (Fletcher et al., 2007), and “protracted Basin and Range extension” (Ferrari et al., 2018). Here, we present evidence for a fourth, hybrid tectonic model – “progressive localization” – in which major dextral-oblique fault systems and related sedimentary basins initiated ca. 9-8 Ma to form the ~50-100-km-wide Gulf of California shear zone (GCSZ). Onset of dextral transtension in the GCSZ broadly coincided with a clockwise change in the azimuth of PAC-NAM relative motion that began ca. Chron 4n.2 (7.8 Ma). The earliest phase of progressive localization is recorded by development of significant dextral-oblique structures and syn-tectonic sedimentary basins now preserved on the rifted margins of Sonora and Baja California, where geologic mapping, geochronology, thermochronology, basin analysis, and paleomagnetism constrain the timing, displacement, and kinematics of plate boundary faults. Late Miocene transtension in the GCSZ drove subsidence and formed terrestrial basins along the narrow oblique-divergent plate boundary. Basins were inundated by marine waters at ~6.3 Ma in a geologically instantaneous (± 0.1 Myr), regional, marine-flooding event along the northern ~1000 km of the Gulf of California from Santa Rosalía to southern California. Similar offset of 12.5-Ma and 6.1-Ma regional ignimbrites that correlate across the northern Gulf require that dextral motion accelerated when the PAC-NAM plate boundary localized into the core of the GCSZ ca. 6 Ma, coeval with marine flooding, and has continued at modern rates until today. Our animated tectonic reconstruction of the Gulf of California-Salton Trough (GCAST) region integrates local geologic and marine-geophysical datasets, cross-Gulf geologic tie points, global plate circuit constraints, GPS rates, and regional gravity data to illustrate the evolving plate boundary since ~12 Ma. Total Baja-NAM relative motion is ~10-15% greater across the southern Gulf due to greater distance from the Euler pole. Our results illustrate how latest Miocene (9-6 Ma) development of the GCSZ facilitated progressive localization of the dextral-oblique PAC-NAM plate boundary into the future Gulf of California and acted as a catalyst for subsequent lithospheric rupture and seafloor spreading.

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

doi: 10.1130/abs/2026CD-410878

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