Structural controls on the continent-ocean transition in the northern Gulf of California

In the Gulf of California the Pacific-North America plate 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 which indicate significant differences north and south of latitude -30øN. We suggest instead that north of-30øN a continent-ocean transition begins which we name the Wagner Transition Zone (WTZ). Diffuse deformation characterizes the WTZ where slip occurs along reactivated north to NNW striking normal faults developed uring 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 o dextral-oblique slip along inferred north to NNW striking faults submerged in the northern gulf. By accounting for rotational and extensional plate motion deformation in northeastern Baja California, vector constraints require that submerged structures accommodate -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 configuration since 4-6 Ma, perhaps controlling the evolution of spreading center jumps between Upper and Lower Tibur6n and Delfin basins due to the juxtaposition of kinematically partitioned structural domains. Different histories of prerift extension and subduction-related arc magmatism along the length of the gulf, partly related to the migration of the Rivera triple junction, may explain the location of the continent-ocean transition.