Abstract

The orientation of the convergent margin of the Juan de Fuca subduction zone changes from north-south off Oregon to northwest-southeast off British Columbia. Because of this change, the effective ratio of overriding plate velocity to roll-back velocity of the subducting plate increases by at least 20% from Oregon to British Columbia. This appears to alter the tectonic regime from slightly extensional in Oregon to slightly compressional in Washington and British Columbia and thus may account for the lesser amount of volcanism at the northern end of the Cascade volcanic chain. The variation in margin geometry may also explain other tectonic features such as initiation of the Nootka fault zone and the Quaternary change of the volcanic pattern in British Columbia.

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