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AUSTRALIA/Tasman Orogenic Belt 237 The submarine features of the divergent margins have a certain bilateral symmetry about the meridian halfway across Australia: complexes of marginal plateaus and abyssal plains are backed by a broad shelf in the north, and narrow margins expand into long appendages (Naturaliste Plateau and Tasmania–South Tasman Rise) in the south This pattern extends to the arrangement of seafloor-spreading magnetic anomalies in that the azimuth of the anomalies off the west coast is reflected in that off the east coast, while the set to the south is crossed by the line of symmetry The age of onset of spreading follows another pattern, with anticlockwise propagation from 156 Ma in the north-west through 132 Ma in the west, and 99 Ma in the south, and as back-arc spreading at 83.5 Ma, 68 Ma, and 64 Ma in the south-east, 62 Ma in the north-east, and Ma off eastern New Guinea Present Motion Figure 27 shows the velocities (azimuth and magnitude) from a few years of Global Positioning System (GPS) measurements and from the NUVEL-1A global-plate-motion model averaged over several million years For example, the GPS estimate in western Victoria of 58.3 mm year at 022 approximates the NUVEL-1A estimate of 61 mm year at 019 , and that near Darwin of 67.2 mm year at 034.9 matches the NUVEL-1A estimate of 71 mm year at 033 Most of this motion comes from seafloor spreading in the Southeast Indian Ocean at a full rate of 60 mm year from an almost stationary Antarctica The stations on mainland New Guinea, including one near the plate boundary, record an azimuth of ca 050 , which locates them on the south-western (Australian) side of the plate boundary On the other side, the Pacific Plate is moving at a velocity of 66 mm year at 289 , almost at a right angle to Australia’s motion, confirming the idea expressed by Alfred Wegener that the Pacific’s westerly motion has planed off the obtruding anvil of New Guinea According to NUVEL-1A, the Eurasian Plate is moving slowly south-eastwards GPS measurements on either side of the Flores and Wetar thrusts show that the southern Banda arc is virtually accreted to the Australian plate margin and that most of the convergence seems to be accommodated by left-lateral slip at the plate boundary, suggesting that the Timor Trough is now inactive as a thrust See Also Antarctic Australia: Proterozoic; Tasman Orogenic Belt Gondwanaland and Gondwana New Zealand Oceania (Including Fiji, PNG and Solomons) Palaeoclimates Pangaea Plate Tectonics Tectonics: Mountain Building and Orogeny Further Reading BMR Palaeogeographic Group (1990) Evolution of a Con tinent Canberra: Bureau of Mineral Resources, Geology and Geophysics Veevers JJ (ed.) (1984) Phanerozoic Earth History of Australia Oxford: Clarendon Press Veevers JJ (ed.) (2000) Billion Year Earth History of Aus tralia and Neighbours in Gondwanaland Sydney: GEMOC Press Veevers JJ (2001) Atlas of Billion Year Earth History of Australia and Neighbours in Gondwanaland Sydney: GEMOC Press Tasman Orogenic Belt D R Gray, University of Melbourne, Melbourne, VIC, Australia D A Foster, University of Florida, Gainesville, FL, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction The eastern part of Australia formed along the margin of Gondwana during the Palaeozoic due to the accretion of oceanic platform and basinal sequences (Figure 1) A region of eastern Australia over 1000 km wide now consists of three distinct orogenic belts (the Delamerian, Lachlan–Thomson, and New England orogens), which are collectively referred to as the Tasman Orogenic Belt (Figures and 2) This deformed and metamorphosed tract of rocks was once part of a major orogenic system that extended some 20 000 km along the Gondwanan margin, incorporating parts of the Andes in South America (see Andes), the Cape Fold Belt in southern Africa, and the Ross Orogen in Antarctica (see Antarctic) (Figure 1) In eastern Australia accretion occurred in a stepwise fashion, with an eastward younging from the Cambrian to the Triassic, reflected
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