The mean direction of remanent magnetism for 44 sampling sites from Oligo-Miocene lava flows in northern California points about 12° east of the expected Oligo-Miocene geomagnetic field direction for the area. Our paleomagnetic data and other data indicate that the Cascade Range has rotated clockwise since the middle Tertiary. Similar, but larger, clockwise rotations have been documented in previous studies throughout the Coast Ranges. Two mechanisms are suggested to account for the differential rotation that has occurred within the Coast and Cascade Ranges. First, the Coast Ranges are rotated and then accreted to a curved continental margin during the Eocene, leaving the Washington Coast Range relatively unrotated at the end of the Eocene. Secondly, during post- Eocene rotation, the thick crystalline crust of the Klamath Mountains prohibited the southern end of the Cascade Range from rotating as rapidly as the northern end, producing an oroclinal bend in the range
