Mr Eka Saputra1, Dr Chris Fergusson1, Professor Anthony Dosseto1, Professor Colin Murray-Wallace1, Professor Allen Nutman1, Amy Dougherty1
1School of Earth, Atmospheric, and Life Sciences, University of Wollongong, Wollongong, Australia
Plate motions result in the Bird’s Head Peninsula of northwestern New Guinea undergoing oblique convergence from the northeast driven by westward motion of the Pacific-Caroline Plate and northward motion of the Australian Plate. The Bird’s Head Peninsula is part of a northwestern extension of the Australian Plate and has a basement of a Paleozoic to early Mesozoic orogenic belt containing mid Paleozoic accretionary turbidite successions and Devonian to Triassic granites. This basement is overlain by a Mesozoic passive margin succession with rifting in the Jurassic. Subsequent development reflects complex Cenozoic tectonics with island arcs colliding with the northern margin of New Guinea. The Kemum High, the northern mountainous spine of Bird’s Head, was caused by major uplift in the Pleistocene. Further to the northeast the hilly terrain around Manokwari has been more recently uplifted. The northern and northeastern part of Bird’s Head are cut by two major strike-slip faults, the Sorong and Ransiki faults respectively.
The present-day tectonics reflects the oblique plate convergence with partitioning of plate motion into normal convergence across the Manokwari Trough/New Guinea Trench and sinistral strike-slip along the Sorong and Yapen faults. Abundant earthquakes immediately north of Bird’s Head have first motion solutions that indicate south-dipping, east-southeast trending, thrust faults and are consistent with underthrusting along the Manokwari Trough. On land much of the Sorong Fault is aseismic although offshore to the east the Yapen Fault is associated with earthquakes that have first motion solutions indicating east-southeast-trending, sinistral strike-slip movement. A cluster of earthquakes is associated with the junction of the north-northwest trending Ransiki Fault and the east-southeast trending Yapen Fault. First motion solutions, aftershocks and earthquake damage, including fissures, cracks, faults, sand flows, and subsidence, are indicative of dextral movement on the Ransiki Fault and sinistral movement on the Yapen Fault.
In the Manokwari region in the northeastern part of the Bird’s Head north-northeast convergence is reflected in Pleistocene contractional deformation and uplift of Pliocene to Pleistocene shallow marine sediments and coral reefs. Earlier uplift of parts of the Kemum High is indicated by provenance (rock types and detrital zircon ages) of conglomeratic/sandy units in the Pliocene to lower Pleistocene Befoor Formation. The Miocene Maruni Limestone and Pliocene to lower Pleistocene Befoor Formation are affected by east-southeast trending, upright to steeply inclined, folds that decrease in interlimb angle from north to south reflecting buttressing against the uplifted basement rocks of the Kemum High. The deformation occurred within the Pleistocene as shown by an angular unconformity with weakly deformed overlying uplifted coral reefs of the Pleistocene Manokwari Limestone. Upraised coral reefs have U series coral ages of 65–130 ka at elevations up to 100 m asl. Fissures filled with coral debris (coral gravel dykes) are indicative of paleoseismicity and occur in the Befoor Formation and also in Manokwari Limestone. They are near-vertical with east-southeast and north-northeast trends consistent with stress directions shown by the first motion solutions of earthquakes.
Chris Fergusson (Honorary Fellow, University of Wollongong) is presenting on behalf of Eka Saputra. Eka is a geologist in the Indonesian geological survey and has been undertaking a PhD on the geology and tectonics of the northeastern Bird’s Head Peninsula in western New Guinea.