The Cascades crystalline core forms part of the Cretaceous magmatic belt of western North America and exposes a crustal section composed of primarily tonalitic plutons that intruded siliciclastic metasediments of an arc-derived accretional system, and local meta-basalt/chert sequences. This study is the first attempt to correlate the well-understood intrusive and P-T-t history of the metasedimentary and plutonic terrane with the kinematics and tectonic boundary conditions by rigorous analysis of structures documented in the Tonga formation exposed at the western edge of the core. The Tonga formation comprises pelite-psammite metasediments, which increase from greenschist (~300-350 °C) to amphibolite grade (~500-600 °C) from south to north. This metamorphic gradient is inverted relative to a major westward-verging and downward-facing fold system that dominates the internal architecture of the formation and implies that the initial regional metamorphic signature was established prior to the early fold generation. Subsequent co-axial fold superposition is seen as a consequence of the persistent accretional W-vergent thrusting in the foreland of the magmatic arc.