The Paleozoic stratigraphic-structural belts of the Cordilleran foldbelt from Alaska to Nevada and California can be explained by a succession of marginal ocean basins opening and closing behind volcanic arcs. Cambrian through Devonian rocks of the foldbelt are divided into (1) a carbonate rock and quartzite belt (miogeosyncline), a continental shelf and upper continental slope deposit, (2) a graptolitic shale and chert belt (inner part of eugeosyncline), a marginal ocean-basin deposit developed behind a volcanic arc, and (3) a vulcanic rock and graywacke belt (outer part of eugeosyncline), a volcanic arc deposit.
The volcanic rock and graywacke belt had a long history of volcanic island development, which began in northern California no later than the Late Ordovician Epoch and in southeastern Alaska and western Washington during the Precambrian or Cambrian Period. Initially, rocks of this belt formed a primitive crust on oceanic basement and through a long process of volcanism, erosion and sedimentation, metamorphism, and plutonism developed a crust of more continental character. In this interpretation, the outer eugeosynclinal rocks are not continental fragments of Asia or parts of oceanic features of unknown provenance that coll'ded with North America. Instead, they are parautochthonous deposits of volcanic island chains and interarc basins developed outward from the North American continental margin as it faced a proto-Pacific basin. Deep-sea pelagic deposits, condensed sections of graptolitic shale and chert, apparently accumulated on basaltic basement in newly formed marginal ocean basins behind the volcanic chains as they migrated from the continent. In the Late Devonian and Mississippian, these marginal ocean basins closed, perhaps by collision with the frontal volcanic arcs, and the deep-sea pelagic sediments were thrust eastward onto the continental shelf, uplifted, and eroded to provide thick wedges of coarse clastic sediments.
In the Pennsylvanian and Permian Periods, the collapsed earlier Paleozoic marginal ocean basins were rifted, and a new system of basins, floored apparently by new crust, developed behind segments of the older volcanic arc complexes. This late Paleozoic basin in the latitude of Nevada was closing during the Late Permian and Early Triassic, and its rocks were again thrust eastward as during the Late Devonian and Mississippian orogeny. This succession of migrations of Paleozoic volcanic arc complexes away from the North American continent and creation of new basins, presumably with oceanic crust behind the frontal arcs, is closely analogous with the tectonic history of the western Pacific, where most of the world's marginal seas are now concentrated.