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

The history of motion of the Philippine Sea Plate is poorly known beca use it is isolated from the oceanic ridge system. Interpretation of pa laeomagnetic results from the plate has been controversial because dec lination data have been obtained only from the eastern margin where su bduction-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 site s within the Sorong Fault system. These sites record southward movemen t during the Eocene and northward movement during the Neogene. Sites w ithin the Sorong Fault system record both counterclockwise and clockwi se rotations interpreted as the result of Neogene block movements at t he southern boundary of the Philippine Sea Plate. North of the Sorong Fault, all sites record clockwise declinations. Neogene rocks have sma ll deflections consistent with rotation about the present-day Eurasia- Philippine Sea Plate pole. Oligocene-middle Eocene rocks show consiste nt clockwise declination deflections of similar to 40 degrees. Declina tions of lower Eocene rocks indicate similar to 90 degrees of clockwis e 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 pol es which satisfy all the palaeomagnetic data from the Philippine Sea P late and permit its reconstruction.