Paleomagnetism and tectonic significance of the Goble Volcanic Series, southwestern Washington

The mean direction of remanent magnetism of the upper Eocene-Oligocene Goble Volcanic Series, located on the western flank of the Cascades in southwestern Washington, points about 25° east of the expected middle Tertiary geomagnetic field direction for the area. This deflection is in the same sense as discrepant paleomagnetic directions reported for Tertiary rocks from the Oregon Coast Range, but seems to be somewhat less in magnitude. From their major-element chemistry and their structural location, it appears that the Goble flows may be an early arc assemblage that was ancestral to the present Cascade magmatic arc, although origin as an oceanic basalt assemblage akin to the Yachats basalts and the Siletz River and Tillamook Volcanic Series of the Oregon Coast Range cannot be ruled out. Whatever its origin, the Goble block has been rotated about a pivot located somewhere nearby, either as part of the Coast Range or as an independent microcontinental block. *Present address: Amoco Production Company, Denver, Colorado 80225