Mechanisms of Cenozoic tectonic rotation, Pacific Northwest Convergent Margin, U.S.A.

Large clockwise rotations (15–80°) are characteristic of Cenozoic volcanic and sedimentary rocks along the convergent margin of the northwestern United States. Abundant paleomagnetic data from 62–12 m.y. old rocks in forearc, arc, and backarc regions show that rotation increases with age and with proximity to the coast. Paleomagnetic and structural studies both support dextral shear as a significant contributor to tectonic rotation in the Pacific Northwest, with an added contribution from Basin-Range extension. Paleomagnetism of individual Miocene (15-12 Ma) flows of the Columbia River Basalt Group show a well-defined, progressive increase in rotation across the forearc region toward the trench which is most readily explained by shear. This progressive increase in rotation toward the coast is also seen in Oligocene and Eocene rocks, although with steeper gradients, indicating that shear rotation has been important throughout Cenozoic time. The dextral shear may be accommodated by abundant, small-scale strike-slip faults which are acting as Riedel shears. Coast Range faults, usually with well-developed subhorizontal slickensides, occur throughout the stratigraphie section and are most abundant in the oldest rocks. Displacements are small to moderate (10²–10³ m), and both dextral and sinistral faults are common. Fault trends are variable, with N20°–45°W faults showing dextral slip and N60°W-S70°W faults showing sinistral slip. The driving force for dextral shear presumably results from coupling of the overlying plate with the subducting Farallon plate, which has been moving northeast throughout most of Cenozoic time. The increase in rotation from north to south along the coast probably represents the contribution of extension in the Basin- Range region to the rotations in Oregon.