Oblique subduction, rotating crustal blocks, and the active tectonics of the Pacific Northwest

Ray Wells

U.S. Geological Survey

5:30 pm social hour

Oregon has been slowly rotating clockwise over geologic time. Coast Range basalt of the Siletz River Volcanics, about 50 million years in age, are rotated about 75°, whereas younger flows of the Columbia River Basalt (16-12 million years in age) exposed in the Coast Range are rotated about 20°. Rotations are largest along the coast and decrease inland. Northeast-directed subduction of the Juan de Fuca Plate beneath the continent drags the leading edge of Oregon northward, producing a clockwise shear in the continental plate. Ray Wells will present current understanding of active tectonics in the Pacific Northwest and the role that crustal rotation plays in the Cascade volcanoes and in shallow crustal faults in the Northwest urban corridor.