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The western margin of the Cordillera and adjacent offshore areas of western Canada exhibit pronounced tectonic activity through a variety of plate tectonic interactions, includingocean-ridge spreading, transform faulting, and subduction. Interactions occur among three principal plates or plate systems: the Pacific Plate, the America Plate and the intervening small Juan de Fuca Plate system.

The en échelon Juan de Fuca spreading ridge system is the accretionary boundary between the Pacific and Juan de Fuca plates. The ridge system spreads at rates of between 40 and 60 mm/a and is fragmented and complex. Detailed surveys show that the ridge morphology and tectonics are extremely variable. The Juan de Fuca Plate system east of the ridge is equally complex and has apparently responded to varying resistance at the subduction zone along the continental margin by plate breakup, ridge jumping and re-orientation.

The Pacific-America interaction, which extends along the continental margin northwards from Queen Charlotte Sound, is predominantly right-lateral transform at a rate of 50 to 60 mm/a. However, there is evidence for a small component of convergence along the Queen Charlotte Islands that causes underthrusting. To the north, off Dixon Entrance, there appears to be pure transcurrent motion. Farther north in southeast Alaska and the western Yukon, the plate boundary becomes more complex and through studies of seismicity, it seems likely that several faults in the region transfer strike-slip motion along the Fairweather system into thrust motion in the Chugach-Saint Elias system and the Aleutian Trench.

Convergence beneath the western margin

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