Cordilleran Metamorphic Core Complexes -- From Arizona to Southern Canada

Cordilleran metamorphic core complexes occur in a sinuous belt that lies west of the Cordilleran fold and thrust belt from Canada to California. It then continues southeastward through the Basin and Range country of Arizona, where it lies athwart the northeast edge of the fold and thrust belt across Arizona, before continuing south into Mexico (Figure 1). This curious geo­ graphic distribution, in addition to the relatively recent recognition of the young age of metamorphic fabrics in many areas , has attracted attention to these complexes and led to a variety of hypotheses for their origin. The Shuswap complex in Canada is the largest and longest recognized metamorphic core complex and is considered the type example (Coney 1980). Only during the past two decades has an awareness of the complexes in the US blossomed. This awareness reached its fullest expression in the recent Geolog­ ical Society of America memoir devoted to discussion of these metamorphic complexes (Crittenden et aI1980). Because of the richness of that source , this review focuses on ideas and interpretations rather than descriptive details, but some examples must be given to demonstrate both variability and common features. The investigation of each complex has tended to follow a similar historical pattern. Prior to 1960 most were regarded as exposures of pre-Phanerozoic crystalline basement or granitic intrusive bodies, and thus not structurally active parts of the Mesozoic orogen. In the 1960s, field work and recon­ naissance K-Ar geochronometry drew attention to the complexes as sites of Mesozoic and Cenozoic deformation and metamorphism affecting Phan­ erozoic supracrustal rocks. During the last two decades numerous detailed