The Coast orogen of western coastal British Columbia and southeastern Alaska is one of the largest batholithic belts in the world. This paper addresses the structure and composition of the crust in the central part of this orogen, as well as the history of its development since the mid-Cretaceous. The core of the orogen consists of two belts of metamorphic and plutonic rocks: the western metamorphic and thick-skinned thrust belt comprising 105–90-Ma plutons and their metamorphic country rocks, and the Coast Plutonic Complex on the east, with large volumes of mainly Paleogene magmatic rocks and their high-temperature gneissic host rocks. These two belts are separated by the Coast shear zone, which forms the western boundary of a Paleogene magmatic arc. This shear zone is subvertical, up to 5 km wide, and has been seismically imaged to extend to and offset the Moho. Lithologic units west of the Coast shear zone record contractional deformation and crustal thickening by thrusting and magma emplacement in the mid-Cretaceous. To the east, the Coast Plutonic Complex records regional contraction that evolves to regional extension and coeval uplift and exhumation after ca. 65 Ma. Igneous activity in the Complex formed a Paleogene batholith and gave rise to high crustal temperatures, abundant migmatite and, as a result, considerable strain localization during deformation. In both belts, during each stage of the orogeny, crustal-scale deformation enabled and assisted magma transport and emplacement. In turn, the presence of magma, as well as its thermal effects in the crust, facilitated the deformation. After 50 Ma, the style of crustal evolution changed to one dominated by periods of extension oriented approximately perpendicular to the orogen. The extension resulted in tilting of large and small crustal blocks as well as intra-plate type magmatic activity across the orogen. Seismic-reflection and refraction studies show that the crust of this orogen is unusually thin, probably due to the periods of orogen-perpendicular stretching. Magmatic activity west of the Coast shear zone in the Late Oligocene and Miocene was related to one period of orogen-parallel transtension along the margin. Small-scale, mafic, mantle-derived volcanic activity continues in the region today. The change from convergence to translation and extension is related to a major plate reorganization in the Pacific that led to a change from subduction of an oceanic plate to northwestward translation of the Pacific plate along the northwest coast of North America. Although it has been proposed that this orogen is the site of major (up to 4000 km) pre-Eocene northward terrane translation, there is little evidence for such large-scale displacement or for the kind of discontinuity in the geological record that such displacement would entail.

The crystalline core of the North Cascades arc records the Cretaceous to Paleogene history of magmatism, deformation, and crustal growth along a segment of the North American Cordillera. The Nd isotopic compositions of granitoid plutons that intrude the Cascades core are a product of their source regions, and they provide probes of the crustal architecture. We present Sm-Nd isotopic data from 96 Ma to 45 Ma plutons and meta-igneous and metasedimentary terranes across the Cascades core. Sm-Nd data from all metamorphic terranes, excluding the much younger ca. 73 Ma Swakane terrane, yield mid-Cretaceous ε Nd values that range from +8.5 to −1.9 and indicate minor involvement of an enriched crustal component. Amphibolites from the Napeequa complex and Chiwaukum Schist yield near-depleted-mantle ε Nd values in the mid-Cretaceous, and ε Nd values from meta-clastic rocks from these terranes (+3.4 to −1.9) have an isotopic character that is intermediate between arc-derived and continental-shelf (miogeocline) sediments, reflecting a mixture of these two sources. Initial ε Nd values of the Swakane Gneiss range from +0.6 to −5.4 and reflect a significant input from the miogeocline. The initial ε Nd values of the Late Cretaceous to Paleogene plutons studied range from +1.5 to +6.3, consistent with geochemical studies that indicate the plutons were generated by mixing of mantle-derived melt and melt derived by anatexis of the underlying terranes. Initial ε Nd values of plutons from the NE part of the Cascades core generally decrease over time, suggesting a greater contribution of melt from evolved crustal sources, which may reflect a change in the physical parameters of melting. The metamorphic terranes of the North Cascades show a close affinity to the Late Triassic to Early Cretaceous arc terranes of the southern Coast Belt. The similarity in isotopic character supports the assumption that the North Cascades terranes formed in a position outboard of the North American craton but in close enough proximity to derive sediments from the miogeocline. Variations in Nd signature are also observed between the northern and southern Coast plutonic complex, and they indicate changes in the sources of crustal melting along the length of the Cretaceous arc.