CENOZOIC MOTION OF THE PHILIPPINE SEA PLATE - PALEOMAGNETIC EVIDENCE FROM EASTERN INDONESIA

The history of motion of the Philippine Sea Plate is poorly known because it is isolated from the oceanic ridge system. Interpretation of palaeomagnetic results from the plate has been controversial because declination data have been obtained only from the eastern margin where subduction-related tectonic processes may have caused local rather than plate-wide rotations. New palaeomagnetic data relevant to the problem have been obtained from 34 sites north of the Sorong Fault and 29 sites within the Sorong Fault system. These sites record southward movement during the Eocene and northward movement during the Neogene. Sites within the Sorong Fault system record both counterclockwise and clockwise rotations interpreted as the result of Neogene block movements at the southern boundary of the Philippine Sea Plate. North of the Sorong Fault, all sites record clockwise declinations. Neogene rocks have small deflections consistent with rotation about the present-day Eurasia-Philippine Sea Plate pole. Oligocene-middle Eocene rocks show consistent clockwise declination deflections of similar to 40 degrees. Declinations of lower Eocene rocks indicate similar to 90 degrees of clockwise rotation. We propose that the entire area north of the Sorong Fault in east Indonesia has always been part of the Philippine Sea Flare and that the whole plate has rotated clockwise in a discontinuous manner by approximately 90 degrees since the early Eocene. The new data from north of the Sorong Fault provide a basis for determining rotation poles which satisfy all the palaeomagnetic data from the Philippine Sea Plate and permit its reconstruction.