KEY STEPS IN DEVELOPMENT OF THE CONCEPT OF A METAMORPHIC CORE COMPLEX: A VIEW FROM THE “ARIZONA SCHOOL” OF THOUGHT

Development of the conceptual entity known as a metamorphic core complex arose in the 1970's as geologists working in different parts of the North American Cordillera realized that certain, dome-shaped mountains shared a similar assemblage of rocks and structures. Peter Coney first recognized these mountains ranges as something special and applied the core-complex term to the many similar mountain ranges discovered from Canada to Mexico. His interest in these mountains had been kindled by students of Peter Misch and by Richard Armstrong, who documented some unusually metamorphosed Paleozoic rocks in eastern Nevada. On a field trip to the Catalina and Rincon Mountains of southern Arizona, Peter (Coney) was immediately impressed by how similar the exposed rocks and structures were to those in eastern Nevada. Together with George Davis and others at the University of Arizona, Peter developed the idea that all these mountains represented a new kind of structural feature. Work by Tom Anderson in Mexico, Rehrig and Reynolds in Arizona and the Pacific Northwest, Greg Davis and his students in California, and Peter's Canadian colleagues greatly increased the number of known core complexes.

A key step was determining that mylonitic fabrics and uplift of the ranges were mid-Tertiary, not older. These mid-Tertiary ages came, in rapid succession, from the South Mountains and Catalina Mountains of Arizona and the Raft River – Grouse Creek mountains of northern Utah. Holdouts for older ages for the mylonites gradually acquiesced as U-Pb dates accumulated. The next step was in constraining the kinematic significance of the characteristic core-complex mylonites and associated brittle detachment fault. George Davis and his students documented that the mylonitic deformation was formed by stretching parallel to the lineation, as opposed to thrusting (the prevailing view). The next breakthrough came on a GSA Penrose field trip to the Palm Springs area, where within 30 seconds of exiting the buses at the first stop, Gordon Lister, Carol Simpson, and their European colleagues stunned the Cordilleran crowd, literally, by being able to interpret the sense of shear in the mylonitic rocks. In the field! Quickly! This led to documentation of a normal-sense shear for the mylonites, which could be tied to extreme upper-plate extension documented in Nevada by John Proffitt and by Ernie Anderson.