PACIFIC-FARALLON RELATIVE MOTION 42-59 MA DETERMINED FROM MAGNETIC AND TECTONIC DATA FROM THE SOUTHERN AUSTRAL ISLANDS

We have used recently collected magnetic profiles and high resolution multibeam bathymetry data near the southern Austral Islands in French Polynesia to constrain the relative motion of the Pacific and Farallon plates between chron 25 and chron 18 (42-59 Ma). A change in plate mo tion at about chron 21 (50 Ma), which reoriented spreading direction i n the area by 10-12 degrees clockwise was apparently accommodated by a southward propagating rift which transferred 160 km of Pacific crust to the Farallon plate, the outer pseudofault of which is preserved as the Adventure Trough. These new constraints significantly revise previ ous estimates of Pacific-Farallon relative motion at the crucial time period encompassing the bend in the Hawaiian-Emperor chain, believed t o represent an abrupt change in the absolute motion of the Pacific pla te. Disrupted lithosphere at the Adventure Trough has apparently been a preferred site for later midplate volcanic activity.