CLOSING PANTHALASSA: EVIDENCE THAT WESTERN NORTH AMERICA OVERRODE AN OCEANIC PLATE DURING THE JURASSIC

About 180 mya, Panthalassa began to close as the proto-Atlantic basins began to form. The mega-ocean had seafloor as old as Permian or Carboniferous. Along California, within a remarkably brief period of about 15 million years, all traces of Triassic and Paleozoic ocean basin and arcs were either swept into accretionary complexes of the Klamath Mountains and Sierra Nevada Foothills, subducted, or carried away. After that time, no pre-Jurassic fragments of oceanic crust were added to the margin. One explanation is that the western margin of North America had migrated toward one of the many spreading ridges in Panthalassa. If so, the leading edge of the continent would have arrived alongside very young oceanic crust. In the Middle Jurassic, California’s subduction zone stepped westward, initiating the formation of the Franciscan accretionary complex. From Middle Jurassic to earliest Late Cretaceous, an interval of about 65 my, only Jurassic ocean basement was incorporated into the Franciscan. Murchey and Blake interpreted “anomalous” eastward-younging directions of Jurassic ocean crustal fragments in the Franciscan Complex as evidence for the California arrival of a spreading ridge during the Middle to Late Jurassic. The remarkable Middle Jurassic ophiolites of the Coast Ranges, which structurally overlie the Franciscan Complex, have been interpreted as having formed at a mid-ocean ridge or in a suprasubduction complex. Although the latter explanation is the more likely based on chemistry, the patterns of younging directions recorded by radiolarian assemblages in accretionary complexes and the ages of the ophiolites themselves suggest that continent-ridge interaction may have had a key role in western plate boundary reorganization and formation of the ophiolites. In addition, one way to close an ocean is to destroy a mid-ocean spreading ridge.