evidence that the OWL is not (contrary to expectations) itself a boundary between oceanic and continental crust. The lack of results so far suggests that the broader context of the OWL should be considered. Following are some elements of that broader context, which may – or may not – relate in some way to the OWL. Plate tectonics The broadest and fullest context of the OWL is the global system of plate tectonics, driven by convective flows in the Earth's mantle. The primary story on the western margin of North America is the accretion, subduction, obduction, and translation of plates, micro-plates, terranes, and crustal blocks between the converging Pacific and North American plates. The principal tectonic plate in this region (Washington, Oregon, Idaho) is the North American plate, consisting of a craton of ancient, relatively stable continental crust and various additional parts that have been accreted; this is essentially the whole of the North American continent. The interaction of the North American plate with various other plates, terranes, etc., along its western margin is the primary engine of geology in this region. Since the breakup of the Pangaea supercontinent in the Jurassic (about 250 million years ago) the main tectonic story here has been the North American Plate's subduction of the Farallon Plate (see below) and its remaining fragments (such as the Kula, Juan de Fuca, Gorda, and Explorer plates). As the North American plate overrides the last of each remnant it comes into contact with the Pacific Plate, generally forming a transform fault, such as the Queen Charlotte Fault running north of Vancouver Island, and the San Andreas Fault on the coast of California. Between these is the Cascadia subduction zone, the last portion of a subduction zone that once stretched from Central America to Alaska. This has not been a steady process. 50 Ma (million years) ago there was a change in the direction of motion of the Pacific plate (as recorded in the bend in the Hawaiian-Emperor seamount chain). This had repercussions on all the adjoining plates, and may have had something to do with initiation of the Straight Creek Fault, and the end of the Laramide orogeny (the uplift of the Rocky Mountains). This event may have set the stage for the OWL, as much of the crust in which it is expressed was formed around that epoch (the early Eocene); this may be when the