LATE TERTIARY TECTONIC EVOLUTION OF THE SEA-FLOOR SPREADING SYSTEM OFF THE COAST OF CALIFORNIA BETWEEN THE MENDOCINO AND MURRAY FRACTURE-ZONES

A forward modeling program of two-plate spreading on a sphere was utilized to reconstruct the late Tertiary tectonic evolutionary history of the seafloor spreading system off the coast of California between the Mendocino and Murray fracture zones. This analytical technique was applied to a unique set of magnetic anomaly data consisting principally of a new digital data set: the original Raff-Mason data from one of the most significant marine geophysical surveys: the 1955-1956 USCGS Pioneer survey. All the data, critical for the development of seafloor spreading and plate tectonic theory, were digitized (32-degrees to 52-degrees-N); however, only the southern half was analyzed in this study. Investigation of the Raff-Mason data, supplemented by additional magnetic anomaly data, indicates that the primary mechanism for reorganization of this part of the Pacific-Farallon ridge, as the spreading center migrated toward and arrived at the North American trench, was rift propagation. The model presented here, containing a total of 21 propagation episodes from 36 Ma to 19.8 Ma (initiation time of the last propagator), shows generally excellent agreement with the isochrons identified from the magnetic anomaly pattern, although minor exceptions exist locally in complex areas. A step-by-step evolution of the model is presented in a series of reconstructions documenting the history of spreading between the Pacific plate and five smaller plates created from the breakup of the large ancestral Farallon plate. These plates include the Vancouver plate north of the Pioneer, the Farallon plate existing between the Murray and Pioneer transforms prior to the major reorganization at chron 10 (30.33 Ma), and the Arguello, Monterey, and Reyes microplates with northwest-southeast spreading existing after chron 10.