Widespread high-pressure Cretaceous metamorphism in the Coast Plutonic Complex of southeast Alaska and British Columbia and its southeast extension, the Cascades crystalline core, is commonly attributed to thrust loading during superterrane collision. However, terrane boundaries within the Cascades core are intruded by relatively shallow mid-Cretaceous plutons, and crustal loading of ∼2-5 kbar postdates these plutons. These observations are not consistent with proposed collisional models, and we suggest that loading occurred by structural and/or plutonic processes operating within a magmatic arc. Such loading may be an important process in arc tectonics, although it has not been widely reported.

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