STRIKE-SLIP FAULTING IN THE CORE OF THE CENTRAL RANGE OF NEW GUINEA: AFTERMATH OF COLLISIONAL DELAMINATION

Structural analysis in the Gunung Bijih (Ertsberg) Mining District near Puncak Jaya, the highest point in the Central Range of New Guinea (4884 m), has revealed abundant evidence for strike-slip faulting during the latest stage of collisional orogenesis. This faulting played a major role in localizing igneous intrusion and giant copper-gold mineralization at 3 Ma (Grasberg and related orebodies). The Derewo fault zone to the north of the mining district is another major E-W trending strike-slip fault in the highlands. Regional seismicity and GPS studies indicate the Central Range of west New Guinea is tectonically inactive. The scarcity of earthquakes beneath areas with elevations higher than 1000 m is notable. Current movements are localized along the northern edge of the island and west of the Central Range along the left-lateral Tarera-Aiduna strike-slip fault zone that connects to the obliquely convergent Seram Trough. Sometime between 3 Ma and the time of Pleistocene glaciation, strike-slip motion ceased in the western highlands. We believe rupture of the northern end of the Australian plate, collisional delamination, started at about 8 Ma and propagated more than 1000 km eastward by 3 Ma. This caused a short-lived, but major melting event in the asthenosphere that upwelled as the plate separated. Batholithic-scale magma chambers were present in the lower crust from about 7 to 3 Ma. During this time, the core of the collision generated mountain belt was a zone of profound weakness and plate tectonic motions were localized within it. Since then, the upper ~20 km of upwelled asthenosphere has cooled forming new lithospheric mantle. The "healing" of the lithosphere beneath the western Central Range caused plate motions to become concentrated at weaknesses along the northern coast of the island.