LATE NEOGENE RELATIVE MOTIONS OF THE PACIFIC AND NORTH-AMERICA PLATES

In this paper a new set of finite rotations describing the relative motion of the Pacific and North American plates during the last 10 m.y., incorporating recently published studies of the Pacific-Antarctic, Antarctic-Africa, and Africa-North America plate boundaries is presented. These finite rotations show that changes have occurred in Pacific-western North America motion at 2.48 Ma and between 3.40 and 3.9 Ma, resulting in increased compression along the Pacific-North America plate boundary. The most significant change in relative motion was of the latter age. During this change the predicted motion of the Pacific plate along the California coast changes from transform to transpressive, due to a clockwise rotation of the relative convergence vector by 12-degrees. This timing of the onset of transpression agrees well with a variety of geologic data along the California plate boundary, including the onset of compressive deformation in onshore and offshore sedimentary basins, formation of reverse faults and anticlines (which are parallel to strike-slip faults of the previous, more westerly directed azimuth of relative motion), a change in the orientation of the San Andreas fault, and formation of a set of new, more northerly trending strike-slip faults. In this model this change in relative motion is caused by a change in the absolute motion of the Pacific plate, due to the detachment of a slab beneath the Fiji Basin between 3.4 and 3.9 Ma. The detachment of this slab and the resultant change in overall Pacific plate torque resulted in a noncollisional "orogeny" along the California plate boundary. This study shows that minor adjustments in the motion of large oceanic plates, such as the Pacific plate, can have profound consequences on the preserved geologic record.