Reconstructing the Strain History of the Southern Gulf of California Oblique Rift

We report progress on a project to investigate the evolution of the Gulf of California - Salton Trough oblique-divergent plate boundary through the construction of a series of ArcGIS paleotectonic maps. The maps will track plate boundary deformation in 2-million-year increments from 14 Ma to 2 Ma and in 1-million-year increments from 2 Ma to present. The reconstructed region lies between the stable Sierra Madre Occidental to the east and active strike-slip faults in the borderland west of the Baja California peninsula in the west, and from the mouth of the Gulf of California in the south to the southern San Andreas fault system just north of the Salton Sea; latitude 28.4° N near the Midriff islands divides the northern and southern reconstructions. The reconstructions in this poster will complement those in the northern Gulf (see abstract in this session). In the southern Gulf of California, the reconstructions are being performed by assigning spreading and slip rates, and direction of motion, to tectonic blocks (footwall blocks) defined largely by known active and inactive faults, seismic data, and bathymetry. Interspersed basins and oceanic crust are then closed accordingly over time. From 2-Ma to the present, the fault systems in the southern Gulf include the gulf-axis system of the primary plate boundary, the gulf-margin fault system adjacent to and on the Baja California peninsula, and the borderland fault system west of the peninsula. A GPS study by Plattner et al. (2007) indicated that ~92% of the modern-day Pacific-North American strain is occurring between the Baja California microplate and North America; accordingly we partition the strain at 0 and 1 Ma with 46 - 45 mm/yr on the primary plate boundary in the gulf (from the Alarcon to Guaymas spreading centers), ~1 mm/yr on the gulf-margin system, and ~4 mm/yr on the borderland faults to the west. At and before 2 Ma, strain in the southern Gulf of California was more complex. Our model for reconstructions shows that before oceanic spreading initiated at any given rift domain, the gulf-margin system near Baja California had higher rates of faulting, and the plate boundary was wider because faulting was active on the east side of the gulf and on mainland Mexico. We use 2.4 Ma and 6 Ma for the time of initiation of the Alarcon and Guaymas spreading centers, respectively, while the Farallon, Pescador, and Carmen spreading centers began at ~2 - 1 Ma. Therefore, the 2 Ma reconstruction has oceanic spreading at the N East Pacific Rise, Alarcon, and Guaymas spreading centers with a wide transtensional fault zone between Alarcon and Guaymas, and a nascent passive margin on the east side of the Gulf. At 4 and 6 Ma, the East Pacific Rise (Maria Magdalena rise) and Guaymas spreading center were active, and connected by a longer transtensional zone between them.