EVOLUTION OF THE BOUNDARY BETWEEN THE PHILIPPINE SEA PLATE AND AUSTRALIA - PALEOMAGNETIC EVIDENCE FROM EASTERN INDONESIA

The boundary between the Philippine Sea and Australian plates is the l eft-lateral Sorong Fault system of eastern Indonesia, Until recently, modelling this boundary for the period before about 5 Ma was difficult ; the Tertiary motion of the Philippine Sea Plate was uncertain and pa laeomagnetic data from areas adjacent to the fault were lacking. Recen t geological and palaeomagnetic studies of the area north of the Soron g Fault have elucidated the Tertiary motion history of the Philippine Sea Plate, providing a reference for examining movements within the fa ult system, We report new palaeomagnetic data from within the Sorong F ault Zone, from the islands of Taliabu and Obi. Taliabu is part of the Sula Platform and is considered to be derived from Australia. Pelagic limestones from the Upper Cretaceous Tanamu Formation of Taliabu yiel ded a direction of D = 329.1 degrees, I = -34.9 degrees implying count er-clockwise rotation and a formation latitude of 19 +/- 5 degrees S. Sula and Misool are postulated to be part of a single microcontinent w hich had a different Late Cretaceous-mid-Tertiary movement history fro m Australia. The Sula Platform was transported to its present position by movement along the Sorong Fault system in the Late Miocene. Obi in cludes rocks of Philippine Sea and Australian origin; all the new site s are in rocks of Philippine Sea Plate origin. Since the Early Neogene the Philippine Sea Plate, which includes all islands north of the Sor ong Fault, has rotated 40 degrees clockwise and moved 10-15 degrees no rthwards. Philippine Sea Plate rocks within the Sorong Fault Zone reco rd similar latitude shifts, but different rotations. In north Obi, the Upper Oligocene Anggai River Formation and the Middle Miocene Woi For mation record similar to 60 degrees and similar to 30 degrees counter- clockwise rotations, respectively, The sense of rotation is consistent with motion within a left-lateral fault system, with the Philippine S ea and Australian plates providing the shear couple. In contrast, the Woi Formation in southeast Obi records 15-20 degrees clockwise rotatio n; this area is separated from the zone of counter-clockwise movement in north Obi by a strand of the Sorong Fault. Are volcaniclastic rocks from the Upper Cretaceous Leleobasso Formation of northwest Obi have a primary magnetisation with a mean direction of D = 357.1 degrees, I = -21.9 degrees. These rocks formed at similar to 11 degrees N or simi lar to 11 degrees S, depending on the interpreted rotation history, an d indicate a Pacific rather than Indian Ocean origin. A volcanic are a t the southern edge of the Philippine Sea Plate collided with eastern New Guinea at similar to 25 Ma. The Philippine Sea-Australia plate bou ndary then changed from subduction to strike-slip, as the Philippine S ea Plate began its Neogene rotation, initiating the Sorong Fault syste m. We suggest that many of the are fragments in the New Guinea orogeni c belt originated in the southern Philippine Sea Plate are which has s ubsequently been dismembered by strike-slip faulting.