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 Cordilleran Coast Plutonic Complex comprises the roots of a middle Cretaceous to Paleogene magmatic arc and orogenic belt that extends from the Yukon Territory to Washington State. Exceptional rock exposure and mineral preservation have made the Cascades crystalline core, the southernmost exposure of the Coast Plutonic Complex, a laboratory for understanding mid-crustal processes in contractional magmatic arcs. Perhaps surprisingly, after decades of study, fundamental tectonic models for the Late Cretaceous evolution of the core remain in question. This study evaluates tectonic models using phase equilibrium modeling, thermobarometry, and high-precision geochronology to constrain present crustal structure and the magnitudes, rates, and lateral preservation of Late Cretaceous regional metamorphism across the Nason terrane, Wenatchee block, Cascades crystalline core. Garnet Sm-Nd ages of 88–86 Ma restrict the last regional metamorphism to no less than 3 m.y. after emplacement of the 96–91 Ma Mount Stuart batholith. These ages and petrologic data reflecting only negligible to moderate pressure increases (0–3.6 kbar) during garnet growth indicate a time lag between a fairly rapid pressure increase (up to 0.5 kbar/m.y.) subsequent to low-pressure contact metamorphism associated with emplacement of the Mount Stuart batholith and temperature increases during initial garnet growth. This thermal relaxation signature supports a thrust-loading model for post–Mount Stuart regional metamorphism. The lateral extent and magnitude of regional metamorphism across the Nason terrane and Mount Stuart domain offer additional support for a tectonic loading model for regional metamorphism. Peak recorded pressures decrease from >8 kbar in the northeast of the terrane to 5 kbar southwest of the Mount Stuart batholith, compatible with loading by a tapered thrust sheet, followed by exhumation and shortening after peak metamorphism. The lack of structural evidence for thrusting and steepening of the paleobarometric gradient across the Nason terrane north of the batholith suggest that strain-partitioned folding was dominant at the exposed crustal level during and after the last regional metamorphism. Thus, a tectonic model compatible with metamorphic P-T data may include a decoupling of upper-crustal and mid-crustal shortening accommodation mechanisms during Late Cretaceous regional metamorphism.

Abstract The mid-Cretaceous San Juan Islands–northwest Cascades thrust system is made up of six or more nappes that are a few kilometers or less thick, up to one hundred kilometers in breadth, and that were derived from previously accreted Paleozoic and Mesozoic terranes. This field trip addresses many questions regarding the tectonic evolution of this structural complex, including the homeland of the terranes and the process of post-accretionary dispersal that brought them together, how thrusting in the San Juan Islands might have been related to coeval orogenic activity in the neighboring Coast Plutonic Complex, and the origin of blueschist metamorphism in the thrust system relative to subduction and nappe emplacement. The geology of this trip has many counterparts in other outboard regions of the Cordillera, but some aspects of the tectonic processes, as we understand them to date, seem to be unique.