Structural controls on the continent-ocean transition in the northern Gulfof California

In the Gulf of California the Pacific-North America prate boundary changes character from an oceanic-type spreading center and transform fault system (to the south) to a region of diffuse continental deformation (to the north ). The presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric, heat flow, gravity, or seismic data w hich indicate significant differences north and south of latitude similar t o 30 degrees N. We suggest instead that north of similar to 30 degrees N a continent-ocean transition begins which we name the Wagner Transition Zone (WTZ). Diffuse deformation characterizes the WTZ where slip occurs along re activated north to NNW striking normal faults developed during late Miocene or Pliocene ENE directed extension. Transtensional deformation varies from ENE directed extension along dip-slip faults in the west to dextral shear along the coast to dextral-oblique slip along inferred north to NNW strikin g faults submerged in the northern gulf. By accounting for rotational and e xtensional plate motion deformation in northeastern Baja California, vector constraints require that submerged structures accommodate similar to 30 mm /yr of slip in a direction slightly clockwise of the relative plate motion direction. The juxtaposition of the discrete spreading center system in the central gulf with the diffuse WTZ appears to have been a stable configurat ion since 4-6 Ma, perhaps controlling the evolution of spreading center jum ps between Upper and Lower Tiburon and Delfin basins due to the juxtapositi on of kinematically partitioned structural domains. Different histories of prerift extension and subduction-related are magmatism along the length of the gulf, partly related to the migration of the Rivera triple junction, ma y explain the location of the continent-ocean transition.