CENOZOIC TECTONIC ROTATIONS OF OREGON AND WASHINGTON

Paleomagnetic field directions from the Eocene Tillamook Volcanic Series of the Oregon Coast Range point 46(DEGREES) clockwise from the expected Eocene field direction. These results establish that the rotated coastal block of Simpson and Cox {1977} extends north to the Oregon-Washington border and that this block has rotated clockwise 46(DEGREES) (+OR-) 13(DEGREES) during the past 44 m.y. The block has undergone no measurable ((+OR-)400 km) north-south translation. A smaller clockwise rotation, 25(DEGREES) (+OR-) 20(DEGREES), is found for late Oligocene and early Miocene rocks of the Oregon western Cascade Range. These results combined with other paleomagnetic studies in the western Cascades suggest that the entire western Cascades from northern California to central Washington have undergone about 27(DEGREES) clockwise rotation since 25 m.y.B.P. Depositional contacts between rocks of the Cascades and the Oregon Coast Range and Klamath Mountains require that the Coast Range and Klamath have undergone the same post 25 m.y. rotation. In southwest Washington paleomagnetic results from the basalt of Pack Sack Lookout support geologic and geochemical data which indicate that the Pack Sack basalt and the Pomona basalt of the Columbia River Basalt Group of the Columbia Plateau are parts of a single 12 m.y. old flow. Paleomagnetic data from the Pomona and Pack Sack basalts are in excellent agreement if the paleomagnetic vectors from the western sites are rotated counterclockwise by 16(DEGREES), indicating that southwest Washington has undergone 16(DEGREES) (+OR-) 5(DEGREES) clockwise rotation relative to the Columbia Plateau since 12 m.y.B.P.;A two phase model is developed to explain the tectonic history and anomalous paleomagnetic directions of the Oregon-Washington region. Phase I of the rotation is proposed to have taken place during the Eocene (50-42 m.y.) and to have accompanied the accretion of the Oregon Coast Range to the continent. Phase II of the rotation is suggested to be associated with post early Miocene extension of the Basin and Range province and to have produced a clockwise quasi-rigid block rotation of the Coast Range, western Cascade Range and Klamath Mountains. Post 12 m.y. rotation of SW Washington was coincident with Phase II rotation of terrane to the south yet may have been associated with shear rotation as well as regional rigid block rotation. The Neogene Cascade Range from central Washington to northern California appears to have been a major tectonic boundary separating two domains, the Basin and Range to the east, an extensional domain characterized by translation and the Coast Range, western Cascade Range and SW Washington to the west, a more rigid domain undergoing clockwise rotation.