PALEOMAGNETISM OF THE NORTH CASCADES: DISPLACEMENT AND DEFORMATION IN SPACE AND TIME

Paleomagnetic studies of orogens such as the North Cascades can provide insights into the history of large-scale displacement of terranes, and deformation that produce large tilts or vertical-axis rotations. A number of rocks ranging in age from Jurassic to Eocene in age from the North Cascades and San Juan Islands reveal clear differences in their magnetization. These differences are largely a function of the interpreted age of magnetization. For example, rocks with Cretaceous magnetizations such as the Mount Stuart batholith and Beckler Peak stock (Housen et al., 2003) and remagnetized rocks from the San Juan Islands (Burmester et al., 2000) have remanence directions that are significantly different than the expected (North America) directions. The discordance in directions is most likely due to ca. 3000 km of post 90 Ma latitudinal translation of N. Cascades terranes; however certain complex tilt scenarios can also be invoked to explain these data without large translation. By contrast, rocks with Eocene magnetizations such as the Cooper Mountain pluton, mafic dikes adjacent to Beckler Peak and Mount Stuart rocks, and new results from the Granite Falls stock and Squire Creek pluton have remanence directions that are less than five degrees from the expected Eocene direction for North America. The concordance of these directions indicates no significant post-Eocene tilt, translation, or rotation has affected these units. An interesting comparison, first suggested by Beske et al., 1973, can also be made with similar-aged rocks from the S-Central Cascades and Coast Ranges. Eocene rocks from these areas all have magnetizations with directions that indicate significant rotations of these units. The boundary between the Eocene units with large rotations, and those in the N. Cascades with little or no rotation, may correspond to the Olympic-Wallowa Lineament (OWL). New results from Granite Falls, Squire Creek, and the Barlow Pass Volcanics will be presented that bear on this observation.