Abstract

On the basis of new petrological, radiometric, and other geotectonic data, we rationalize the character and onset of Neogene-Quaternary volcanism in the northern Cordilleran volcanic province of British Columbia. From 20 Ma to present, volcanism across the northern Cordillera is attributable to incipient rifting of the continental margin of northwestern North America. Our tectonic model is consistent with the observed petrology of these volcanic rocks; magmatism is dominantly alkaline and chemically bimodal. The model also explains the temporal distribution of volcanic activity. The majority of magmatism falls in two distinct pulses (8–4 Ma and 2–0 Ma), and the mean rate of magma production was higher from 7 to 5 Ma than from 2 to 0 Ma (250 km3/m.y. vs. 100 km3/m.y.). The first pulse of magmatism correlates with a period of net extension along the Pacific–North American plate margin. The second pulse (2–0 Ma) resulted from local domains of extension during a period of net compression between the Pacific and North American plates.

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