Metamorphic core complexes evolved in the western Cordillera in early to middle Tertiary time as a response to profound regional extension and thermal incursion; the deformation was punctuated by an unusually thorough tectonic denudation. In southern Arizona examples, the nonconformity between crystalline basement and layered cover rocks played an important mechanical and physical-chemical role during the deformation. The basement stretched and necked in a manner akin to megaboudinage, while simultaneously parts of the layered cover flowed passively during metamorphism and were plated tectonically to crystalline rocks along the unconformity and along ductile normal growth faults. Topographic basins created by regional pinch-and-swell were filled by lower to middle Tertiary sediments and by volcanics that poured out of the rifted basement. Physical-chemical conditions were such that the base of the Phanerozoic section became a vast heat sink as well as a zone of remarkably high fluid pore pressure. Thus, décollement zones, located near the base of the Phanerozoic section, are marked by sharp thermal gradients, wholesale denudation, and a great variety of mechanical and chemical chaos.