Backscattering and geophysical features of volcanic ridges offshore Santa Rosalia, Baja California Sur, Gulf of California, Mexico

Volcanic ridges formed by series of volcanic edifices are identified in the central part of the Gulf of California, between Isla Tortuga and La Reforma Caldera–Santa Rosalı́a region. Isla Tortuga is part of the 40-km-long Tortuga Volcanic Ridge (TVR) that trends almost perpendicular to the spreading center of the Guaymas Basin. The Rosalı́a Volcanic Ridge (RVR), older than TVR, is characterized by volcanic structures oriented towards 310°, following a fracture zone extension and the peninsular slope. It is interpreted that most of the aligned submarine volcanic edifices are developed on continental crust while Isla Tortuga lies on oceanic-like crust of the Guaymas Basin. From a complete Bouguer anomaly map, it is observed that the alignments of gravity highs trending 310° and 290° support the volcanic and subvolcanic origin of the bathymetric highs. Volcanic curvilinear structures, lava flows and mounds were identified from backscattering images around Isla Tortuga and over a 400-m high (Vı́rgenes High), where the TVR and the RVR intersect. A refraction/wide-angle seismic profile crossing perpendicular to the Vı́rgenes High, together with gravity and magnetic data indicate the presence of shallow intrusive bodies presumably of basaltic or andesitic composition. It is inferred that most volcanic edifices along the ridges have similar internal structures. gest that the growth of different segments of the ridges have a volcano-tectonic origin. The older RVR lies along the extension of a fracture zone and it probably is associated with Pliocene NE–SW extension.