Metamorphic core complexes of the North American Cordillera: Summary

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Abstract
To attempt to summarize the series of papers that constitute this volume in a few pages, much less a few paragraphs, would require temerity indeed. Nevertheless, I will take this opportunity to point out a few of the geologic relations described that appear to be of unusual interest for someone attempting to unravel the origin of these fascinating geologic terranes. SALIENT CONCLUSIONS Clearly the most important conclusion regarding these complexes, particularly those south of the Snake River Plain to which the bulk of this volume is devoted, is that they are startlingly young and that they show consistent evidence of horizontal extension and vertical attenuation. It is primarily the latter processes that have operated to create the array of features by which these structures are characterized (Davis, this volume). In the core, ductile deformation took place at depths and temperatures sufficient to cause quartz to behave plastically producing extensive foliated and lineated mylonitic fabrics. In the cover, horizontal extension and vertical attenuation was accommodated by low-angle, younger-over-older faults and by listric normal faults that commonly affected rocks of Tertiary age. In some areas, however, evidence suggests that these two processes operated together or sequentially over a considerable period of time, in places beginning in the Mesozoic. TECTONIC SETTING As Coney (this volume) and many others have pointed out, most of the characteristic metamorphic core complexes lie in the midst of the Cordilleran miogeocline between the zone of imbricate thrusts that define the leading edge of the Cordilleran thrust and fold. . .

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