Detailed re-examination of existing magnetic anomaly data reveals the fine structure of variations in spreading rates and directions at the Juan de Fuca and Explorer Ridges during the last 10 million years. A geometrical model using these variations delineates the theoretical history of the interactions between the lithospheric plates involved. These interactions demonstrate the independent movement of the Juan de Fuca and Explorer plates and the development of the Sovanco Fracture Zone. The latter was apparently initiated E–W at 7 Ma, rotated clockwise to 120° and may have been the site of up to 50 km of crustal shortening. The model demonstrates that subduction rates at the Canadian continental margin declined from 5 cm/yr to a present 1.5 cm/yr and that recent relative movements are compatible with the N–S compression observed from earthquakes. It also suggests that the existence of both E–W and NE trending faults in the downgoing lithosphere beneath Vancouver Island, shows that a triple junction remained static near the northern end of Vancouver Island from 10–4 Ma, and predicts a buried northern edge of subducted material striking NE in this area.

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