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°, D=54.5°, a95=17.4°, k=20.3) and a middle Oligocene direction (I=5.3°, D=68.4°, a95=12.8°, k=52.4) have been obtained from the Palau Islands. Only one reliable result, a middle Oligocene direction (I=15.1°, D=66.1°, a95=11.1°, 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°, D=28.1°, a95=8.4°, k=64.6), and a lower Eocene result (I=−12.0°, D=43.0°, a95=12.5°, k=16.0). Two lower Eocene results from Chichijima (I=9.5°, D=105.3°, a95=12.6°, k=6.1) and Anijima (I=−0.8°, D=91.7°, a95=16.9°, 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° clockwise and moved northward around 20° 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.

Paleomagnetic data from 231 samples from 31 sites in rocks of Upper Jurassic to Miocene age in Sarawak (Malaysian Borneo) reveal a trend of increasing counterclockwise (CCW) declination deflection with age. Six sites in Tertiary hypabyssal intrusions show 8° to 52° of CCW deflection. The intrusion deflected 52° CCW was K‐Ar dated at 26 m.y. (Upper Oligocene), while one deflected 22° CCW gives a 17 m. y. age (Lower Miocene). Three sites in the Upper Eocene to Miocene(?) Silantek Formation show an average 40° of CCW deflection. Prefolding directions, showing 90° of CCW deflection, are isolated in 4 sites (including two positive fold tests) in Upper Jurassic and Cretaceous rocks of the Bau Limestone and Pedawan Formations. A postfolding, Cenozoic remagnetization with an average of 60° of CCW deflection is found in five Bau Limestone sites. Three sites in the Upper Jurassic Kedadom Formation show an average of 50° of CCW deflection. CCW declination deflections found in Mesozoic and Cenozoic rocks as far as 400 km east and 150 km south of Sarawak, in Kalimantan (Indonesian Borneo), also fit the trend of deflection versus age. On the basis of the regional consistency of declination deflection versus age, along with geologic evidence the data are considered to be evidence of a regional (rather than a local block or distributed shear) rotation. The domain of CCW rotation extends into West Malaysia, suggesting that West Borneo and the Malay Peninsula may have been a stable block during the latest Cretaceous and Cenozoic. West Malaysia and Borneo may have had different histories in the rest of the Mesozoic. The data imply up to 108° CCW rotation of Borneo with respect to stable Eurasia, sometime during the Cretaceous and Cenozoic. Cenozoic rotation may also have occurred between Indochina and Borneo. The sense of rotation shown by the data does not support the “propagating extrusion tectonics” model for Cenozoic Southeast Asia.

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