Observation of relative timing of deformation, metamorphism, and plutonism in a high-temperature-low-pressure metamorphic belt in the eastern Chugach Mountains of Alaska leads to a model of ridge subduction followed by plate reorganization to account for the abnormally high geothermal gradients in the fore arc. Between 56 and 53 Ma, a change in the direction of the Kula-Farallon spreading halted the previous southward migration of the Kula-Farallon-North American triple junction. This forced the triple junction to migrate back north along the plate margin, enlarging the slab window beneath the accretionary margin. The expanded slab window produced a large-scale thermal manifestation now recognized as the Chugach metamorphic complex. The accretionary complex responded to plate reorganization by orogen-parallel extension associated with oblique subduction of the Kula plate. Contraction began again following passage of the triple junction and subduction of the Farallon plate.