Wrangellia, the disjointed oceanic plateau accreted to northwestern North America in the late Cretaceous, shows how continental crust is formed from two different sources: the upper crust from subduction-related processes and the lower crust from an accreted oceanic plateau. Using results from seismic reflection exploration from the Lithoprobe project in Canada (Varsek 1993), Wrangellia can be detected laterally at depth into central British Colombia (Fig. 3). Fraser fault, separating the accreted oceanic plateau from the Precambrian craton, is located in the eastern margin of Wrangellia. The upper crust has been erupted by the Coast Range batholith, a complex of felsic plutons characterized as a subduction-zone geochemical signature and juvenile isotope. In addition, the Garibaldi arc system has been erupted on the surface of Wrangellia. With the emplacement of the Coast Range batholith and the eruption of arcs, the upper crust of Wrangellia has become more felsic and appears to be evolving into more typical upper-continental crust.
Growth by Plate Collisions
In addition to the Wrangellia collision described earlier, remnants of other oceanic plateaus and juvenile oceanic arcs are important. Most of these terranes began to evolve into continental crust at or not long before the time when they accreted to the continent. Subduction-related processes related to collisionally thickened crust result in irreconcilable element enrichment. Similar mechanisms have been proposed for Archean continents \cite{Percival_1989} .