THE SKIPJACK ISLAND FAULT ZONE - AN ACTIVE TRANSCURRENT STRUCTURE WITHIN THE UPPER PLATE OF THE CASCADIA SUBDUCTION COMPLEX

The Skipjack Island fault zone has been mapped in the San Juan Islands between Vancouver Island, Canada, and the Washington State mainland, USA. A decade ago, interpretation of multibeam sonar seafloor imagery revealed that Skipjack Island, an east–west striking sedimentary bedrock outcrop, was a fault-controlled structural feature. A major fault separates Skipjack Island from a deformed sedimentary bedrock outcrop on the seafloor to the north. Recently the Skipjack Island fault zone’s morphology and extent has been explored both to the west and east of the island using seismic reflection profiling and sediment coring. The character of the Skipjack Island fault zone is well defined locally by the interpreted seismic profiles, which show active faults that displace sediments deposited since the Last Glacial Maximum. The central part of the fault zone, near Skipjack Island, appears as a near-vertical structure that has been subjected to left-lateral motion as evidenced by a bedrock exposure on the seabed north of the island where folded strata bend eastward against the fault, the result of drag from fault motion. Interpretation of recent geophysical data suggests an extension of the Skipjack Island fault zone further to the east where it either cuts through or transitions into a thrust fault and fold belt. The Skipjack Island fault zone is interpreted to be the northern boundary of the San Juan Archipelago with the Devil’s Mountain fault zone being the southern boundary. Both of these fault zones represent the longest continuous faults of the San Juan Archipelago, are generally oriented east–west, and are actively deforming the seafloor and producing a clockwise rotation of the major islands within the Archipelago. The Skipjack Island fault zone may represent the northern boundary of the Cascadia forearc where the northward propagating Sierra Nevada tectonostratigraphic terrane is impinging upon stable North America.