ALTERNATIVES TO MESOZOIC PLATE CONFIGURATIONS AND TERRANE DISPLACEMENTS WITHIN THE PACIFIC BASIN AND ALONG WESTERN NORTH AMERICA

We have explored several alternatives to plate boundary configurations for the Pacific (PA), Farallon (FA), Kula (KU) and North America (NA) plates since 180 Ma. A key element in our preferred paleogeography and kinematic history is the assumption that the KU and PA plates shared a common spreading ridge in the earliest history of the PA basin (beginning at 180 Ma). It is further assumed that the Japanese M-sequence magnetic lineations in the Pigafetta Basin of the western Pacific record this spreading history. Analyses of PA-FA, PA-KU, and KU-FA relative motions suggest KU-FA convergence during much of the Late Jurassic and Early Cretaceous. Our preferred paleogeography places KU-FA oceanic arcs near the western margin of NA implying that the KU plate occupied most of the north PA and was the dominant plate interacting with NA during this time. Major aspects of this model include: 1) the existence of offshore oceanic island arcs capable of thousands of kilometers of subduction; 2) a clockwise "pinwheel" style of KU motion capable of the displacement of terranes from low paleolatitudes in the western PA and deliver them to subduction zones along western NA; and 3) provide sinistral-oblique displacements along NA from 180 to ~95 Ma. Late Cretaceous interactions would involve the dismemberment and accretion of portions of the oceanic arcs accompanied by highly-oblique dextral displacements along NA. An example of this scenario is applied to Middle Jurassic basalt and radiolarian chert of the eastern San Juan Islands, Washington. These rocks show strong similarities in age, stratigraphy and geochemistry to rocks drilled during ODP leg 129. We thus infer that rocks in the eastern San Juans originated as the KU equivalent to the Pigafetta Basin, displaced with the KU "pinwheel" and brought to a subduction zone along western NA.