Processes and rates of terrane amalgamation: the Sorong Fault Zone, eastern Indonesia 

The Sorong Fault Zone is a major left-lateral fault system separating Australia from the Philippine Sea Plate and the Molucca Sea Plate. The fault zone juxtaposes Mesozoic-Tertiary continental and arc/ophiolitic rocks. Continental crust was derived from the Australian margin. Crust of Philippine Sea Plate origin has a basement of ophiolitic and/or arc origin.

SRTM DEM of eastern Indonesia

SRTM DEM of eastern Indonesia

Fragments of Australian and Philippine Sea Plate origin have a common stratigraphic history after the early Miocene. The geology of the region indicates that arc-continent collision between the Philippine Sea and Australia occurred at ~25 Ma and led to creation of the left-lateral Sorong Fault Zone. Subsequent Neogene convergence between East Asia and the Philippine Sea Plate occurred by subduction to produce the Halmahera arc. Arc activity started earliest in the south, in Obi, and ceased earliest in the south. Neogene movement of Australia northward has occurred without subduction although accompanied by movements of small fragments and local ‘collisions’. The Australian-Philippine Sea plate boundary has been a strike-slip zone since the early Miocene. This implies northward movement of the plate boundary in the Sorong Fault Zone region at a similar rate to that of Australia.

All Cretaceous-Neogene rocks of Philippine Sea Plate origin in the Halmahera region record shallow inclinations and formed at low latitudes. Our data indicate southward translation of this part of the plate between ~50 and ~25 Ma and northward translation during the Neogene. Geologically composite islands immediately adjacent to the Sorong Fault Zone include continental and arc/ophiolitic rocks. Palaeomagnetism indicates latitudinal shifts similar to the Philippine Sea Plate and both counter-clockwise and clockwise rotations interpreted as block movements within the left-lateral Sorong Fault Zone. In the region north of the Sorong Fault Zone we interpret declination shifts to indicate long-term clockwise rotation although rotation was discontinuous.

Upper Neogene rocks record small clockwise declination deflections suggesting rotation rates of approximately 1�/Ma consistent with amounts that would be expected from angular velocities and rotation poles calculated for the Philippine Sea Plate for the interval 0-~5 Ma. Rocks of ages between ~20-25 Ma and ~38-42 Ma show clear clockwise declination deflections of ~40�. We interpret results from Lower Miocene-Upper Eocene rocks as indicating that (1) approximately 40� of clockwise rotation occurred after ~20-25 Ma and (2) no significant rotation took place between ~20-25 and ~38-42 Ma. The very large area over which consistent declination shifts are observed in sites of post-Eocene age indicate that this region has behaved as a single block since the end of the Eocene. We assume rigid block behaviour since the beginning of the Eocene. Lower Eocene sites record declinations of ~270� interpreted as indicating ~45� of clockwise rotation between ~50 and 38-42 Ma. Cretaceous rocks record primary magnetisations wi northward declinations and shallow inclinations. We interpret these results to indicate large clockwise rotations between the Late Cretaceous and the Early Eocene of the order of ~90�.

The new palaeomagnetic data from eastern Indonesia provide a means to determine the history of motion of the Philippine Sea Plate in the interval 0-~50 Ma. These data support previous models indicating large clockwise rotations but indicate more complex latitudinal translations than previously suggested. An important constraint in locating possible rotation poles is the need to satisfy the condition that all palaeomagnetic investigations of the plate north of Halmahera report northward motions for the period ~50-25 Ma whereas the Halmahera region moved south during most of this interval. An additional geological condition is evidence of a major regional unconformity at the beginning of the Miocene representing collision of a Philippine Sea Plate volcanic arc with the Australian margin. Thus, rotation poles must satisfy the requirement that the Halmahera-Waigeo block should be on or near the Australian margin at ~25 Ma. This provides an important limit for pole positions. Using the new data, earlier palaeomagnetic data, and this geological condition we can estimate quite accurately the positions of rotation poles for the plate with respect to magnetic north. We describe the motion of the plate using the Philippine Sea Plate-Eurasia pole of Seno et al. (1987) for the interval 0-5 Ma. Two poles best fit the palaeomagnetic declination and inclination data for the period 5-50 Ma: for the interval 5-25 Ma 15�N 160�E 1.75�/Ma, for the interval 25-50 Ma 10�N 150�E 2�/Ma.

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