Tectonic evolution of the central U.S. Cordillera: A synthesis of the C1 and C2 Continent-Ocean Transects
The evolution of the North American continent and adjacent ocean basins in the central Cordillera of the western United States in Phanerozoic time was governed by three sequential tectonic regimes. The first included the creation of a passive margin during the latest Proterozoic to Early Cambrian (Stewart, 1976) and the removal of an unknown amount of sialic crust from the western margin of the continent. The second regime maintained a passive continental margin of western North America from Middle Cambrian to Triassic time, but permitted collisions of outboard terranes with the sialic margin in Mississippian and Permian-Triassic time (Speed, 1982; Dickinson and others, 1983). Since Triassic time, western North America, adjacent oceanic plates, and intervening microplates and other tectonic packets have existed in a regime of active margin tectonics (Hamilton, 1969; Coney and others, 1980; Saleeby, 1983; Saleeby and Busby-Spera, 1992) driven mainly by eastward subduction of oceanic lithosphere. This third and currently operating regime has been marked by diverse phenomena including subduction of oceanic lithosphere below the continent and phases of highly oblique convergence and suture-zone or intra-arc spreading, ridge-trench collision, growth of a continental arc, major foreland contraction and extension, and the accretion of displaced terranes to the sialic edge. Corridors C1 and C2 of the Ocean-Continent Transect Program traverse all essential elements of the transition from Pacific plate oceanic crust to cratonal North America that have resulted from these three tectonic regimes. Figure l shows the locations of the corridors in relation to the major tectonic elements and Figure 2 shows the corridors in relation to regional morphotectonic domains. Corridor C1 begins offshore at the Mendocino triple junction; it crosses accretionary terranes of the California Coast Ranges, southern Klamath Mountains-northern Sierra Nevada and northwestern Nevada, traverses the deformed foreland, including the Sevier and Idaho-Wyoming foreland thrust belts, and ends in the central U.S. craton. Corridor C2 begins in the Pacific plate; it crosses east through an inactive Paleogene-early Neogene trench complex, the San Andreas fault, accretionary terranes similar to those of Cl, and the deformed foreland, and ends in disrupted craton of the Colorado plateau. Subduction-related continental volcanic-plutonic arcs of the modem Cascades and Mesozoic Sierra Nevada are traversed by the two corridors as well as the modem Basin and Range extensional province and its precursor, the mid-Tertiary extensional belt. Corridor C1 also crosses the southernmost edge of the late Cenozoic high lava plains of southern Oregon-Idaho and northernmost Nevada-Califomia. The objective of this chapter is an overview of critical phenomena and products of Phanerozoic ocean-continent interactions in the regions traversed by transects C1 and C2. Several alternative interpretations to those advocated along various segments of the C1 and C2 displays are also reviewed. Systematic coverage of the transects and more extensive reference lists are provided with the graphic displays (Saleeby and others, 1986; Blake and others, 1987).
© 1994 Geological Society of America. Manuscript received by the Society December 16, 1992.