Structure of the Irian Jaya Mobile BElt, Irian Jaya, Indonesia

Abstract

Image interpretation and field investigation over the Central Ranges of Irian Jaya has provided a structural overview of the inaccessible and largely unmapped Irian Jaya Mobile Belt (IJMB) in the region bounded by longitudes 136°30′E and 141°00′E. Throughout much of the southern (para‐autochthonous) part of the belt, structural facing directions are visible on imagery through selective illumination of hogbacks and cuestas formed on thick‐bedded Mesozoic siliciclastics and extensive Late Cretaceous‐Neogene platform carbonate sequences. North of the regional Derewo Fault Zone, the Derewo Metamorphics are regarded in part as correlatives of the “metamorphosed Om Beds” in western Papua New Guinea and display a regional structural grain parallel to the orogen. The limited extent of spectrally anomalous features associated with ultramafic rocks indicate that ophiolites are confined to the southern part of the “Irian Jaya Ophiolite Belt”, which may be largely composed of metamorphic rocks. Interpreted macroscopic and megascopic structures suggest the presence of seven discrete structural domains in the IJMB. From north to south these are (1) North Coast Basin region, consisting of Paleogene‐Neogene volcanics and sediments overlain by a Pliocene‐Pleistocene successor basin sequence; (2) an allochthonous terrane composed of ophiolites and high‐grade metamorphic rocks; (3) the Derewo metamorphic assemblage, displaying polyphase deformation; (4) a complex marginal zone within Mesozoic‐Paleogene miogeoclinal sediments which includes steep duplex structures and remnant klippen; (5) a 40 to 50‐km‐wide partly inverted synclinorium composed of miogeoclinal sediments; (6) a regional south vergent overturned anticlinorium formed by incompetent Paleozoic sediments; and (7) a foreland thrust domain involving both Mesozoic‐Cenozoic miogeoclinal cover and a deformed Neogene foreland molasse basin sequence. The regional disposition of interpreted structures in the Irian Jaya Mobile Belt and reference to published mapping in western Irian Jaya and western Papua New Guinea strongly indicates polyphase deformation. Late Oligocene‐Miocene docking of metamorphics, island arc assemblages and ophiolites produced tectonically stacked E‐W trending structures thrust on to the north margin of the Australian continent with its miogeoclinal cover. Late Miocene‐Pliocene collision with the Melanesian Arc brought the accreted Australian margin into contact with the westward moving Pacific Plate and instituted a regime of oblique transpressional convergence. Resulting structures include E‐W sinistral wrenching and NW thrusts along lateral E‐W ramps.