Cordilleran metamorphic core complexes: Cenozoic extensional relics of Mesozoic compression

Cordilleran metamorphic core complexes form a belt of uplifted metamorphic rock that extends from southern Canada to northwestern Mexico just west of, or astride, the foreland thrust belt of the North American Cordillera. During the past several years the age and tectonic significance of the core complexes have been a topic of considerable controversy. Some geologists view the complexes as an uplifted erogenic core zone that formed behind the thrust belts mainly during Mesozoic regional compression. An opposing view is that they are mainly Tertiary in age and of extensional origin. We support a model that unifies these seemingly inconsistent views by suggesting that Mesozoic crustal telescoping resulted in an overthickened plateaulike crustal welt along the Cordilleran hinterland. During Cenozoic time this gravitationally unstable mass spread laterally, resulting in deep-seated crustal extension. The extension was aided by a thermal pulse of Cenozoic magmatism that reduced crustal viscosity and by a lowering of intraplate convergent stress fields due to changing plate kinematics. The chief advantage of the model is that it reconciles opposing views as to the age and tectonic significance of the complexes and places them in a more comprehensible setting amid Mesozoic-Cenozoic Cordilleran thermotectonic history.