Abstract

The complicated structure of British Columbia is the cumulative result of no less than six major orogenies since the Proterozoic. Using local names to avoid unintended identification with orogenies elsewhere, these are: (1) East Kootenay, pre-Lower Cambrian; (2) Cariboo, post-Ordovician - pre-Mississippian; (3) Cassiar, post-Upper Permian - pre-Upper Triassic; (4) Coast Range, continuing from pre-Middle Jurassic to post-Lower Cretaceous and perhaps later; (5) Rocky Mountain, Paleocene; and finally, (6) Puget orogeny, extending into modern time. The effects of East Kootenay orogeny are preserved in Purcell strata that were greatly uplifted and mildly folded. The effects of the succeeding Cariboo orogeny are much more widespread. Eugeosynclinal strata of Proterozoic and Lower Paleozoic age from the 49th Parallel to Alaska were intensely folded, regionally metamorphosed, and extensively granitized. Later deformations have not obliterated but only modified these ancient structures. Folding, faulting, and widespread ultramafic intrusion occurred during the Cassiar orogeny. A complicated sequence of folding, regional faulting, syntectonic sedimentation, and profound granitic intrusion marked Coast Range orogeny. The dominant NNW, structural trend was modified by 2 prominent salients convex eastward. These resemble "drag-folds" oriented to conform with a counter-clockwise rotation of the entire continental margin around the N. Pacific basin. As time went on Coast Range tectonism migrated eastward, merging insensibly into Rocky Mountain orogeny that culminated in Paleocene time. Profound crustal shortening was accomplished in the Rocky Mountain region by folding and thrust faulting. Puget orogeny includes intermittent crustal disturbances that have continued into historic time. It is expressed by several stages of locally intense folding; fairly extensive granitic intrusion; successive uplifts and tilting in the Rockies, Cascades, and Coast Mountains; and by Upper Tertiary to Recent vulcanism.

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