3D resistivity model reveals a deep-crustal anomaly beneath the pull-apart basins of the northern Gulf of California active rift

Abstract The Gulf of California Rift (GCR) represents a transitional boundary between the North American and Pacific plates, linking continental breakup along the San Andreas fault system with oceanic spreading centers at the East Pacific Rise. This study focuses on the northern GCR, where pull-apart basins such as Wagner and Consag overlay a basement suspected to be a transitional crust composed of stretched continental material, sediments, and intrusive bodies. Rapid sedimentation from the Colorado River, combined with thermal and magmatic processes, has influenced crustal evolution in this region, potentially delaying the onset of a complete continental breakup. We present the first 3D inversion of marine magnetotelluric (MT) data in Mexico, analyzing 13 observation sites across two profiles spanning a 100 × 120 km area. We register electromagnetic fields at periods ranging from 2 to 5000 by 18 days. The resulting 3D resistivity model highlights two prominent conductive anomalies associated with crustal processes. Shallow conductive zones (< 6 km depth) correspond to fault-related seawater infiltration and sedimentary structures. A deeper anomaly (3–5 Ohm-m) between 10 and 20 km depth in the southern Wagner and northern Consag basins is interpreted as caused by saline fluids in the continental crust, likely derived from serpentinized peridotite at the top of the mantle. This conductive anomaly and other geophysical observations, including earthquake hypocenters, seismic velocities, and gravity anomalies, support the hypothesis of ongoing crustal modification through magmatism and sedimentation. We present an electrical resistivity model that adds independent information to the seismic-wave velocity and mass density derived from previous geophysical studies. The resistivity distribution supports the hypothesis of a transitional crust in the northern GCR, whose trans-tensional deformation causes fractures and upwelling of saline fluids from the top of the mantle. Graphical Abstract