The upper Eocene to lower Oligocene Goble Volcanic series of southwest Washington is a thick sequence of areal to submarine basaltic to andesitic flows, pyroclastics, and minor sediments. Major element geochemical analyses suggest that these rocks may represent early magmatism of the Cascade arc. Paleomagnetic results from 37 sites indicate that the direction of remanent magnetization in the Goble Volcanics has a declination of 18.5°, an inclination of 57.5°, and a circle of 95% confidence (∝ 95)of 4.33°. The expected upper Eocene direction in the sampling area is D = 353.5°; Ī = 61.5°. Thus the Goble Volcanics block appears to have rotated approximately 25° in a clockwise direction relative to the North American interior since the late Eocene. Comparison of this result with those of Cox, Simpson, and Plumley for the Oregon Coast Range, which show an apparently greater degree of rotation, suggests that the Goble Volcanics are not part of the Coast Range block. This also is supported by geochemical and geophysical differences between the provinces.
Simpson (1977) has proposed two models to explain the Coast Range rotation. Model I assumes that the block extends to the Olympic Mountains and rotated seaward around its northern end in response to extension from behind. Model II assumes the block did not extend as far as the Olympic Mountains, and that it rotated around a pivot point at its southern end. Both models have major problems, which are compounded by results from the Goble Volcanics.
Two possible models for rotation of the Goble Volcanics as part of an independent block have been examined. These are: (1) the ball-bearing model , in which an equant block rotates between right-lateral faults in a large continental shear zone; and (2) the Fitch model , wherein an equant block rotates between a subduction zone and a transcurrent fault pair formed in response to oblique subduction. A preferred but very tentative model proposed in this thesis to explain rotation in both the Goble and Coast Range blocks is a revised model I. It assumes a break in the Coast Range block near the Columbia River. The Coast Range would then rotate around a pivot point in the Tillamook highland area. The Goble Volcanics, which would lie northeast of the rotating block, might then rotate independently in a ball-bearing fashion between right-lateral strike-slip faults trending northwest-southeast. These faults, including the Brothers, Eugene - Denio, Vale and Portland fault may have formed in response to basin and range extension. In particular, the Portland fault which may extend through the Coast Range block, and possible faults to the north parallel to the Olympic - Wallowa lineament may have been responsible for the rotation of the Goble Volcanics
