Stratigraphy, structural geology, and tectonics of a young forearc-continent collision, western Central Range, Irian Jaya (western New Guinea), Indonesia
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New Guinea has long been recognized by geologists as the location of geologically recent mountain building. This study combined field mapping, stratigraphic and remote sensing analysis along and near the Gunung Bijih (Ertsberg) mine road and mining district in order to analyze the geologic development of the collisional New Guinea orogen. As a result of the youthfulness and the quality of data, it is possible to constrain distinct parts of orogenic evolution to 1 or 2 m.y. The southern Central Range of New Guinea is located on the northern Australian continental margin. The southern one-third of the Central Range, exposed along the Gunung Bijih mine access road, is a 30-km-wide, north-dipping homocline exposing an apparently 18-km-thick Precambrian or Early Paleozoic to Cenozoic sequence. Following rifting in the early Mesozoic and until the Middle Miocene, the northern Australian continent was a passive margin. The Central Range of Irian Jaya formed when the Australian passive margin was subducted beneath and collided with a north-dipping subduction zone in the Middle Miocene. Litho- and biostratigraphic analysis of the New Guinea Limestone Group in the Gunung Bijih mining district and regional stratigraphic correlation indicates that the first evidence of subaerial exposure and erosion of the orogen is the widespread deposition of siliciclastic, synorogenic strata at ~12 Ma. I name this event the Central Range Orogeny. There is no evidence of an Oligocene orogenic event in the Irian Jaya region as has been described to the east in Papuan New Guinea. Deformation in the Central Range is dominated by ~12 to ~4 Ma southwest verging (210°-220°) contraction and minor east-west wrenching. This deformation is equally accommodated, there is no evidence for strain partitioning in the Central Range. Lithospheric-scale cross sections, incorporating field observations, predict the Central Range Orogeny is divided into a pre-collision and collisional stage. The pre-collision stage is the bulldozing of passive margin sediments in a north dipping subduction zone. The collision stage occurs when buoyant Australian lithosphere can not be subducted. The collision stage results in basement involved deformation and lithospheric delamination of the already subducted Australian plate.