Bathymetric and structural insights into the Islas Marías Archipelago, Mexico, and surrounding areas

The Islas Marías Archipelago is located south of the Gulf of California. This study presents a structural geologic analysis of María Madre Island, coupled with a detailed bathymetric survey of the surrounding area. Our goal is to delineate the deformation geometry and identify new morphostructural features in the region. Bedding tilting coincided with normal faulting of the lower member of the Ojo de Buey sequence, likely at the end of the Pliocene or the beginning of the Pleistocene, associated with the early stage of the Gulf of California rifting. Seismic reflection profiles from previous studies indicate that the West Ranges are within a thinned continental crust, structurally controlled by the María, Magdalena, and South Magdalena faults. Furthermore, a detailed bathymetric survey and orthoimages from the islands revealed structural lineaments in both oceanic and continental crust. In the continental crust, these lineaments have a preferred orientation between 050° to 090°, similar to the faulting trend on María Madre Island. However, they are oblique to those in the oceanic crust and mainland Mexico. We propose that María Madre Island may have undergone a 50° clockwise rotation. If we apply a counterclockwise rotation of this magnitude to the faults and lineaments in the continental crust of the archipelago and the region west of it, their preferred orientation becomes parallel to the Tamayo Fracture Zone in the oceanic crust, the San Blas fault within the continental shield and, the Tepic-Zacolaco rift extending eastward into mainland Mexico. This suggests that rotation affected the continental crust, either west of Puerto Vallarta, in the archipelago, or the West Ranges, while the oceanic crust remained relatively unaffected. We propose the term “Islas Marías Block” to designate a fragment of continental crust encompassing the archipelago and the West Ranges, which share a common deformation style.