Cordilleran metamorphic core complexes, the subject of this volume, are a group of generally domal or archlike, isolated uplifts of anomalously deformed, metamorphic and plutonic rocks overlain by a tectonically detached and distended unmetamorphosed cover. The features are scattered in a sinuous string along the axis of the eastern two-thirds of the North American Cordillera from southern Canada to northwestern Mexico. To date, more than 25 of them are known, and it is significant that more than half of them have been recognized only since 1970. They are, without question, the newest and most controversial addition to the recognized architecture of the eastern two-thirds of the Cordillera since the discovery, in the early 1960s, of the Tertiary calderas and their associated vast ignimbrite sheets.

The first hint of the existence and potential significance of these complexes was certainly from the early work of Peter Misch and his army of students who invaded the Great Basin in the 1950s. Misch (1960) discovered and emphasized scattered occurrences of a major subhorizontal dislocation plane or “decollement,” as it was called, separating the unmetamorphosed Paleozoic miogeoclinal cover from a generally metamorphosed substratum. He found this relation repeatedly in many ranges scattered along, and west of, the Utah-Nevada border almost 200 km west of the already known low-angle, east-verging Mesozoic thrust faults in central Utah. What is most puzzling about this regional context is that the thrust faults in central Utah are typical “older-on-younger” faults similar to those found in foreland thrust belts throughout the. . .