|Posted on November 14, 2012 at 8:35 PM|
Geology of New Guinea
Hugh L. Davies1 and Joseph O. Espi
Earth Sciences, University of Papua New Guinea, PO Box 414, University NCD, Papua New Guinea.
1currently seconded to the PNG Mineral Resources Authority
New Guinea lies across the northern margin of Australia and has the outline of a giant bird flying westward (Fig.1). It is the second largest island in the world, 2200 km long and up to 750 km wide, and one of the most mountainous with peaks to almost 4900 m above sea level. A central mountain range runs the length of the island and is bounded to the north by lesser mountain ranges and plains, and to the south by a broad plain. The Mamberamo and Sepik rivers drain the north side of the central range, and the Digul and Fly rivers drain the south.
Figure 1. Physiographic map of New Guinea. OJP Ontong Java Plateau, EP Eastern Plateau, PP Papuan Platform. Seafloor topography from Smith, W. H. F., and D. T. Sandwell, Global seafloor topography from satellite altimetry and ship depth soundings, Science 277:1957-1962 (1997); http://topex.ecsd.edu/marine_topo/mar_topo.html.
Beyond the southern plains a broad shallow shelf extends to the Australian coast. Other shorelines are steeper and some are bounded by deep sea trenches (Figs. 2). Small ocean basins lie to the northeast and southeast and a great submarine plateau (Ontong Java Plateau) lies to the extreme northeast, beyond the islands of the Bismarck Archipelago. Smaller submarine plateaus lie south of the bird’s tail.
Politically, the island is divided between the independent state of Papua New Guinea (PNG) in the east and Indonesia in the west, with a boundary that coincides, for the most part, with the 141oE meridian. The western half was known as Irian Jaya and is now known as Papua and Western Irian Jaya (Western Irian Jaya is the bird’s head and neck). The population of PNG is 6 million and is dominantly Melanesian. The population of the Indonesian provinces is 2.1 million and comprises 60-70% indigenous Melanesian and 30-40% migrants from Java, Sulawesi and Ambon.
New Guinea is at the interface between the northward-moving Australian plate and the WNW-moving Pacific Plate (Fig.2). The resultant motion is convergence at a rate of 110 mm/yr on an azimuth close to 070o. Convergence has led to a succession of collisions of the Australian craton with the microcontinents and volcanic islands of the Pacific and with fragments of the craton that had been separated from the craton and then docked again. While the southern half of the island was always part of the Australian continent, the northern part has been built up by successive collisions. In geological terms, the southern part is autochthonous and the northern part allochthonous, being made up of accreted terranes.
Convergence of the Pacific and Australian plates has caused the development of a number of microplates; these are bounded offshore by spreading ridges, deep sea trenches, and transform faults and onshore by thrust, extensional and strike-slip faults, and folds. Earthquakes are located on the microplate boundaries (Fig. 3) and volcanic activity is associated with the deep sea trenches and spreading ridges.
Figure 2. Geological map of New Guinea. AB Aru Basin; AFB Aure fold belt; B Bougainville; BB Bintuni Basin; BK Biak; BT Bismarck Sea Transform; C Cyclops Mountains; CB Cenderawasih Bay; FR Finisterre Range; G Gauttier Range; GR Grasberg Mine; KT Kilinailau Trench; L Lihir Island (mine); LFB Lengguru Fold Belt; M Manus; MB Manus Basin; MI Misool; MT Manus Trench; MU Mussau; NB New Britain; OT Ok Tedi; P Porgera; PT Pocklington Trough; R Rabaul; SB Salawati Basin; SF Sorong Fault; ST Seram Trench; T Timor Trough; TT Trobriand Trough; W Wau; WA Waipona Basin; WB Woodlark Basin; WM Wamena; WN Wandamen Peninsula; WO Waigeo; WT Weyland Thrust; Y Yapen.
Figure 3: Earthquakes stronger than M 5 in the period 1963-2004. Focal depths as follows: Red <50km, yellow <100 km, green <200 km, blue <300 km, purple <400 km, brown <500 km, grey >500 km. Map by Emile Okal.
In northwestern New Guinea the oblique convergence between the Australian and Pacific plates is accommodated in three ways: by left-lateral strike-slip motion on the fault systems that connect the Bismarck Transform in the east with the Sorong Fault in the west; by subduction at the New Guinea Trench; and by transpressional folding and faulting in the fold belt and in the Mamberamo Basin.
Measurements by Stevens et al. (2002) showed that the bird’s head is moving WSW at a rate of 93 mm/yr. This is almost the same motion as the Pacific Plate (110 mm/yr) and suggests that the western part of the New Guinea Trench is locked intermittently. This motion has caused the opening of Cendrawasih Bay, the development of Waipona Basin, and the development of the Lengguru Fold Belt.
The lithosphere of the Caroline Sea is subducted at the New Guinea Trench. Seismic tomography shows the subducted slab to dip at a shallow angle and to extend beneath the island of New Guinea to near the line of the south coast (Tregoning and Gorbatov, 2004). If this interpretation is correct then the igneous activity in the Papuan Basin fold belt, including the Grasberg and Ok Tedi intrusive rocks, can be seen as slab-related, rather than related to slab break-off as was suggested by Cloos et al. (2005). A shallow-dipping slab that is partly coupled to the upper plate also would explain the transfer of convergent motion for 400 km from the line of the New Guinea Trench to the southern front of the fold belt.
In northeastern New Guinea, collision between the Finisterre and Sarawaged ranges and the Bismarck volcanic arc causes uplift of the north coast of the Huon Peninsula at (averaged) rates of 1-3 mm/yr (Chappell, 1974, Chappell, J., 1974, Geological Society of America Bulletin 85:553-570).. Study of raised coral terraces on the peninsula yielded a high-quality record of fluctuations in sea level during the Late Quaternary. The same convergence causes the Finisterre mountain mass to ride southward and results in down-warping of the northern end of the Papuan peninsula, which is subsiding at a rate of 5 mm/yr.
In eastern New Guinea, active sea floor spreading in the Woodlark Basin is advancing westward and causes north-south extension of the mainland and adjacent islands. One result is the emergence in the Pliocene of domes and half-domes of metamorphic rocks by low-angle extensional faulting in the islands and on the mainland (Abers, et al., 2002, Nature 418:862-865). Another is the opening of small rift basins offshore. Spreading within the last 1.2 Ma has caused the separation of Misima Island from a position adjacent to Woodlark (Muyua) Island (Taylor, B., et al., 2002, Nature 374:534-537).
Oil is produced from the Salawati Basin and a large volume of gas is to be developed in the Bintuni Basin beneath Bintuni Bay. In PNG oil and gas are produced from structures in the fold belt. Copper and gold are produced from major mines at Grasberg and Ok Tedi, and gold from Porgera and Lihir Island. Gold at Hidden Valley, near Wau, will start production in 2009, Gold and massive base metal sulfides associated with seafloor hydrothermal activity in the eastern Bismarck Sea are at advanced exploration stage.
Cloos, M., B. Sapiie, A. Quarles van Ufford, R.J. Weiland, P.Q. Warren & T.P. McMahon. 2005. Collision delamination in New Guinea: The geotectonics of subducting slab breakoff. Geological Society of America Special Paper 400, 51p.
Davies, H.L. 1990. Structure and evolution of the border region of New Guinea, in G.J. and Z. Carmen, eds., Petroleum Exploration in Papua New Guinea: Proceedings of the First Papua New Guinea Petroleum Convention, Port Moresby, p. 245-269.
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Parris, K. 1996. Central Range Irian Jaya Geology Compilation 1:500,000 scale geological map. P.T. Freeport Indonesia.
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Quarles van Ufford, A., & M. Cloos. 2005. Cenozoic tectonics of New Guinea. American Association of Petroleum Geologists Bulletin 89:119-140.
Stevens, C.W., R. McCaffrey, Y. Bock, J.F. Genrich, M. Pubellier, & C. Surabaya. 2002. Evidence for block rotations and basal shear in the world’s fastest slipping continental shear zone in NW New Guinea. American Geophysical Union Geodynamics Series 30:87-99
Tregoning, P., & A. Gorbatov. 2004. Evidence for active subduction at the New Guinea Trench. Geophysical Research Letters 31, L13608, doi:10.1029/2004GL020190, 2004
Categories: Sejarah Papua