Tectonic Rotations in the Cascade Mountains of Southern Washington

Paleomagnetic directions from 34 widely distributed sampling sites in the Oligocene Ohanapecosh Formation in the Cascade Mountains of southern Washington are well grouped with a mean and confidence interval of: Dec. = 27°, Inc. = 64°, α95 = 4°. When compared to the expected direction, computed from the Oligocene reference pole for stable North America, a clockwise rotation of 31° ± 12° is apparently present. This result is not significantly different from other studies within the Cascades and Coast Range of southern Washington and suggests that the two Ranges have rotated as a single unit during late Eocene to mid-Oligocene time. Comparison with the results from rocks of similar age in Oregon suggests that at least two crustal blocks are involved in Pacific Northwest tectonics