Miocene Rift Volcanism and Tectonic Insights Into the Pacific–North American Plate Boundary: Paleomagnetic Evidence From Sonora, Mexico

We present paleomagnetic evidence from a detailed study of a middle to late Miocene volcanic succession in the southern Sierra Libre region, Sonora, Mexico, providing new insights into the tectonic evolution of the Pacific–North America plate boundary. The volcanic succession comprises basaltic to trachytic lava flows and silicic ignimbrites formed during continental rifting along the proto‐Gulf of California margin. A basal welded ignimbrite is correlated with the Tuff of San Felipe–Hermosillo Ignimbrite (TSF–HI), a key stratigraphic marker across the Gulf of California region. This unit exhibits reverse magnetic polarity and low inclination. TSF–HI is tentatively assigned to subchrons C5An.1r (12.272–12.174 Ma) or C5r.3r (12.049–11.657 Ma) of the geomagnetic polarity timescale. Paleomagnetic vectors from the TSF–HI in the study area are highly stable and indicate a slight counterclockwise vertical‐axis rotation of the southern Sierra Libre, consistent with deformation observed in the Baja California microplate. The tectonic regime reflects transtensional deformation with a subordinate strike‐slip component, decreasing inland from the coast. Magnetostratigraphic and petrologic data support an age of 10.5–12.2 Ma for the volcanic succession, representing synrift volcanism linked to the early tectonic evolution of the Gulf of California. These findings improve our understanding of Miocene continental rifting processes and kinematics along the Pacific–North America plate boundary.