PALEOMAGNETIC DATA FROM THE PHILIPPINE SEA PLATE AND THEIR TECTONIC SIGNIFICANCE

New paleomagnetic data from Guam and Saipan have been obtained and are used, in conjunction with previously published results from around the Philippine Sea plate, to interpret the origin and history of the Philippine Sea plate. A lower Miocene direction (I = 3.5-degrees, D = 54.5-degrees, a95 = 17.4-degrees, k = 20.3) and a middle Oligocene direction (I = 5.3-degrees, D = 68.4-degrees, a95 = 12.8-degrees, k = 52.4) have been obtained from the Palau Islands. Only one reliable result, a middle Oligocene direction (I = 15.1-degrees, D = 66.1-degrees, a95 = 11.1-degrees, k = 20.1), has been obtained from Guam. Many characteristic remanent magnetic directions have been obtained from the lower Eocene Facpi Formation; however, the results are too scattered (k = 4.5) to be considered reliable. Two reliable directions have been obtained from Saipan: a middle Miocene result (I = 30.7-degrees, D = 28.1-degrees, a95 = 8.4-degrees, k = 64.6), and a lower Eocene result (I = -12.0-degrees, D = 43.0-degrees, a95 = 12.5-degrees, k = 16.0). Two lower Eocene results from Chichijima (I = 9.5-degrees, D = 105.3-degrees, a95 = 12.6-degrees, k = 6.1) and Anijima (I = 0.8-degrees, D = 91.7-degrees, a95 = 16.9-degrees, k = 4.9) are the reliable results from the Bonin islands. There are two end-member interpretations for these data: (1) small scale local rotation of blocks along the plate margin, or (2) rotation of the Philippine Sea plate as a whole. To distinguish between these interpretations, we reconstructed the East Philippine Sea province back to its prerifting configuration. The rotation caused by deformation can be estimated by comparing the paleomagnetism of Guam, Saipan, and the Bonin islands to Palau. Prior to reconstruction, the older declination values are scattered. Closing the marginal basins reduces the declination scatter, and all of the islands show increasing clockwise deflection with time. This suggests that the Philippine Sea plate has rotated up to 80-degrees clockwise and moved northward around 20-degrees since the Eocene, and deformational rotation is a second-order feature. The finite pole of rotation of the Philippine Sea plate relative to Eurasia must be somewhere near the southeast comer of the plate, because this is the only location that is compatible with both the change in orientation and paleolatitude for the plate. These data cannot distinguish between a backarc origin or trapped crust origin for the West Philippine Sea province. The existing models require major modification to be consistent with the paleomagnetic data.