"Compressive" folding, thrusting, and transcurrent fault movement have occurred during late Tertiary and Recent times in parts of southeastern Alaska and southern Yukon west of a line joining the Denali, Queen Charlotte, and San Andreas fault systems. East of this line "compressive" deformation and transcurrent faulting ceased in the Canadian Cordillera by late Eocene time or earlier, and was followed by a period of crustal relaxation that has continued to the present. The onset of relaxation was accompanied by acid volcanism, block faulting, and high-level intrusion of granitic plutons and dike swarms. Explosive eruptions that characterized early Tertiary volcanism were followed in late Miocene and Pliocene times by quiet outpouring of plateau basalt in central British Columbia and later, during Pleistocene and Recent times, by construction of nearly 150 cinder cones and strata volcanoes. Most of these young volcanic centers are confined to two north-south trending belts and an east–west belt offsetting them. The two north–south belts parallel the direction of post Eocene dike swarms and normal faults, and are considered to have formed in response to east–west regional extension that has persisted in the western Canadian Cordillera since late Eocene time.

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