C. McA. Powell

Late Paleozoic glacial episodes in Gondwanaland reflected in transgressive-regressive depositional sequences in Euramerica

The Late Paleozoic glaciation of Gondwanaland comprised two short episodes, in the Famennian (I) and Visean (II) confined to Brazil and adjacent northwest Africa, and a long episode that started in the Namurian (IIIA) of eastern Australia and Bolivia/Argentina, expanded to cover much of Gondwanaland in the Stephanian/Asselian (IIIB), and collapsed in the early Sakmarian (IIIC). Dropstones in eastern Australia indicate that small ice centers lingered to the Kazanian. Across the belt of low latitudes north of Gondwanaland, short-ranging fossils in widespread shallow-marine and paralic deposits indicate synchronous deposition of transgressive-regressive sequences in different parts of Euramerica. These sequences correlate with glacial events in Gondwanaland at three levels: (a) four major regressions in Euramerica, in the Famennian (1), Visean (2), Namurian (3), Stephanian (4), and the Tastubian transgression that preceded the Sterlitamakian regression (5), also recorded in Gondwanaland, correlate with glacial episodes I, II, and IIIA, IIIB, and IIIC; (b) the time-interval of cyclothemic deposition in Euramerica (Brigantian or latest Visean to Sterlitamakian) correlates with that of glacial episode III; and (c) the dominant period of the Euramerican cyclothems, as estimated from the Middle and Late Pennsylvanian deposits of the mid-continent of North America, and of the thickest known Gondwanaland glacigenic sediment (the earliest Permian Lyons Group of Western Australia) is 0.4 Ma, equivalent in turn to the long orbital-eccentricity period of the Quaternary ice age, and the dominant period of fluctuation of the late Miocene Antarctic ice cap. The three levels of correlations confirm Wanless and Shepard9s (1936) hypothesis that the Late Paleozoic cyclothems are controlled largely by sea-level fluctuations related to the Gondwanaland glaciation.

Plate tectonics and the Himalayas

Abstract The Himalayas, commonly taken as the type example of continent-continent collision, have developed in two stages. The first stage involves convergence of the northward-drifting Indian subcontinent with a proto-Tibetan landmass during Late Cretaceous and Palaeocene, with collision before Middle Eocene. The second stage involves formation of a fundamental crustal fracture within the Indian block during Late Eocene and Oligocene, and underthrusting of the Indian subcontinent along this fracture from Miocene to Recent. The present elevated Himalayan mountain chain is not a direct result of the continent-continent collision, but of uplift during underthrusting along the deep crustal fracture.

An outline of the palaeogeographic evolution of the Australasian region since the beginning of the Neoproterozoic

Abstract In the last 1000 million years, Australia has been part of two supercontinents: Palaeozoic Gondwanaland and Neoproterozoic Rodinia. Neoproterozoic Australia was covered by shallow epicontinental seas, and, in the late Neoproterozoic, by low-latitude glaciers. The breakup of Rodinia along the Tasman Line occurred at the end of the Sturtian glaciation (760 Ma) giving rise to the Palaeo-Pacific Ocean. Gondwanaland formed in the Early Cambrian, at the same time as the Tarim block broke away from northwestern Australia. Westward subduction of the Palaeo-Pacific Ocean along the eastern margin of Australia–Antarctica commenced during the Early Cambrian in northern Victoria Land and in the Middle Cambrian in South Australia, and culminated to the Late Cambrian–Early Ordovician Ross–Delamerian Mountains. In the Ordovician, the magmatic arc retreated from Australias then-eastern continental margin, forming a marginal sea and offshore island arc. A shallow seaway across Australia in the Late Cambrian and Ordovician gradually gave way to desert-like conditions in Central Australia and the adjacent Canning Basin by Silurian time. The Silurian to mid-Devonian was an interval of rapidly changing palaeogeography in eastern Australia with deep volcanogenic troughs formed in a dextral transtensional tectonic setting. Widespread deformation in the Tasman orogenic zone in the Middle Devonian to Early Carboniferous, was accompanied by the development of an Andean-style magmatic arc along the Pacific continental margin of Australia. The most widespread Phanerozoic mountain-building stage in Central Australia occurred in the Late Devonian to mid-Carboniferous, as part of a world-wide Variscan orogenic episode associated with the collision of Gondwanaland with Laurussia to form Pangea. In the late Visean, Australia drifted rapidly southward from previous low latitudes to a near-polar position. Glacial conditions dominated the Late Carboniferous and earliest Permian. Transtensional basins associated with dextral oroclinal shear along the Panthalassan eastern margin of Australia developed in the Late Carboniferous and persisted until the Late Permian, when an Andean-style magmatic arc was re-established. Large foreland basins inboard of the Late Permian to Early Triassic magmatic arc accumulated major coal deposits during Late Permian volcanic phases, but drastic climatic changes at the end of the Permian, possibly caused by global greenhouse conditions, led to red-bed deposition in the Early Triassic. Pangea began to rift in the mid-Triassic, and by the Late Triassic, the Cimmerian blocks, which lay off northwestern Australia throughout the Palaeozoic, had departed the northern margin of Gondwanaland. A new Andean-style continental magmatic arc became established along the Pacific Ocean margin of Australia. Breakup between Australia–Antarctica and the northern part of Greater India commenced ca. 130 Ma, and between Australia and Antarctica around 96 Ma. At the beginning of the Palaeogene, Australia commenced its northward drift towards its present position. Seafloor spreading between Australia and Antarctica was at first slow, but increased to ca. 5 cm per year around 45 Ma. By 35 Ma, the circum-Antarctic current became established, thereby triggering glaciation in Antarctica. Northern Australia reached the tropics by the beginning of the Miocene, and Australia has progressively moved northwards at 7 to 8 cm per year since.

Pre-breakup continental extension in East Gondwanaland and the early opening of the eastern Indian Ocean

Abstract Extension of continental crust by up to 360 km on a north-northeast azimuth in the Great Australian Bight occurred prior to the Middle Cretaceous (96 Ma) onset of seafloor spreading between Australia and Antarctica. This large amount of continental extension is constrained to the Late Jurassic (≈ 160 Ma)-Middle Cretaceous interval, about a pole estimated to lie about 40° southeastward of southeastern Australia. Using bathymetric data combined with seismic and magnetic determinations of the continent-ocean boundaries off Australia, India and Antarctica, we determine a revised fit of East Gondwanaland prior to continental extension, and at various stages thereafter. In the first stage from 160 to 132.5 Ma (≈ M11), India-Australia rotated from Antarctica with continental crustal extension between Australia and Antarctica and largely transform motion between the Coromandel Coast margin of India and the Kron Prins Olav Kyst margin of Antarctica. In the second stage, from 132.5 to 96 Ma, India rotated northwestward away from Australia-Antarctica, producing M10 and younger magnetic anomalies along the western margin of Australia. Continental extension was greatest between Australia and Antarctica during this stage. The newly determined 132.5-96 Ma rotation between India and Australia provides an improved fit between the orientation of observed M10-M0 magnetic anomalies and the southward decrease in spreading rate off the western Australian margin. Together with bathymetry, the new rotation parameters lead to identification of the abandoned 96 Ma spreading ridge along the Lost Dutchmen and Dirck Hartog ridges. The Naturaliste Fracture Zone is found to lie nearly parallel to the Early Cretaceous transform direction between India and Antarctica suggesting that slow seafloor spreading, required between Australia and Antarctica to accommodate the continental extension further east, occurred between Naturaliste Fracture Zone and the M4-M0 anomalies adjacent to the Naturaliste Plateau. The revised fit of India in East Gondwanaland reduces the eastward extent of Greater India to 95° E but does not change its extent as far north as the Cape Range Fracture Zone. The new pole determined for the Early Cretaceous rotation between India and Australia-Antarctica lies within 15° of the southern tip of India, significantly closer than in previous determinations, thereby reducing the amount of seafloor spreading generated between Africa and Antarctica by the India-Antarctica rotation, and thus offering a possible solution to the perceived spreading-rate problems between M11 and M0 in the Somali, Mozambique and Antarctic basins.

Review of seafloor spreading around Australia. I. synthesis of the patterns of spreading

The existing data on Late Mesozoic and Cenozoic seafloor spreading isochrons (reviewed in the companion paper by Veevers & Li) and fracture zone trends provide the basis for 12 reconstructions of the seafloor around Australia that spread during the dispersal of Argo Land, India, Antarctica, Lord Howe Rise/New Zealand and the Papuan Peninsula. The major changes of plate geometry in the Jurassic, Early Cretaceous, mid‐Cretaceous, early Paleocene and early Eocene reflect global events. The pattern of spreading around Australia was determined by two long‐standing (earlier Phanerozoic) factors that operated in a counter‐clockwise direction: (1) penetration from the northwest by the Tethyan divergent ridge; and (2) rotation from the northeast of the Pacific convergent arc and back‐arc. The only new feature of the modern pattern is the deep penetration by the Indian Ocean ridge into eastern Gondwanaland to fragment it into continents in contrast with the pattern up to 160 Ma ago of breaking off micro‐continents.

Paleomagnetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwana

Paleomagnetic data from East Gondwana (Australia, Antarctica, and India) and Laurentia are interpreted to demonstrate that the two continents were juxtaposed in the Rodinia supercontinent by 1050 Ma. They began to separate after 725 Ma, allowing the formation of the Pacific Ocean. The low-latitude Rapitan and Sturtian glaciations occurred during the rifting that led to continental breakup. East Gondwana remained in low latitudes for the rest of the Neoproterozoic, while Laurentia moved to polar latitudes by 580 Ma. During the Vendian, a wide Pacific Ocean separated the two continental land masses. The younger Marinoan, Ice Brook, and Varangian glaciations in the early Vendian preceded a second continental breakup in the late Vendian, causing formation of the eastern margin of Laurentia and rejuvenation of its western margin. Paleomagnetic data indicate that Gondwana was not fully assembled until the end of the Neoproterozoic, possibly as late as Middle Cambrian.

Positions of the East Asian cratons in the Neoproterozoic supercontinent Rodinia

Three major East Asian crustal blocks, the Tarim, North China and South China Blocks, have records of the Neoproterozoic rifting events that broke up the supercontinent Rodinia. A preliminary tectonostratigraphic analysis suggests that the Tarim Block may have been adjacent to the Kimberley region, the South China Block between eastern Australia and Laurentia, and the North China Block adjacent to the northwestern corner of Laurentia and Siberia during the early Neoproterozoic. All three blocks were probably separated from the larger cratons towards the end of the Neoproterozoic but stayed close to the Australian margins of Gondwanaland from Cambrian until Devonian.

The Himalayan Arc: large-scale continental subduction, oroclinal bending and back-arc spreading

Abstract Palaeomagnetic results from the Himalayan Arc and Southern Tibet compared with simulated apparent polar wander paths for the Indian plate show a consistent pattern of rotations of the Himalayan Arc relative to the Indian Shield, varying gradually from 45° clockwise in the northwestern Himalaya to slightly counterclockwise in the Lhasa region. This pattern is consistent with continental underthrusting of Greater India beneath the Tibetan Plateau since the Early Miocene over at least 650 km at the longitude of western Nepal and oroclinal bending since the latest Miocene. Available palaeomagnetic observations support the steady-state model for the formation of the Himalayan Arc, with refinements as follows: (1) collision between Greater Indias northern boundary and southern Asia occurred at equatorial latitudes, with progressive suturing from Palaeocene in the northwestern Himalaya until Early Eocene in the eastern Himalaya; (2) continuing convergence and indentation of Greater India into southern Asia over about 2000 km up to the Early Miocene resulted in southeastward extrusion of Indochina; and (3) Neogene counterclockwise rotational underthrusting of Greater India along the Main Central Thrust, with Pliocene/Quaternary oroclinal bending of the Himalayan Arc.

Magnetostratigraphic record of the Late Miocene onset of the East Asian monsoon, and Pliocene uplift of northern Tibet

Abstract Widespread eolian red clay underlying the Plio–Pleistocene loess–palaeosol succession in northern China has been dated magnetostratigraphically back to 8.35 Ma, indicating that the East Asian monsoon started at about the same time as the Indian monsoon. An initial sedimentation rate of 11 m/Myr increased gradually to 17.5 m/Myr by 6 Ma, and then decreased to 6 m/Myr between 5 Ma and 3.5 Ma. A marked increase in sedimentation rate and grain size beginning between 3.5 Ma and 3.1 Ma indicates that the East Asian winter monsoon strengthened at this time, and intensified further after 2.6 Ma. The temporal coincidence of the stronger winter monsoon and the Pliocene uplift of northwestern Tibet just before the onset of the Northern Hemisphere glaciation indicate that the three events could be causally linked.

South Australian record of a Rodinian epicontinental basin and its mid-neoproterozoic breakup (∼700 Ma) to form the Palaeo-Pacific Ocean

Abstract The Neoproterozoic Adelaidean System and overlying Cambrian rocks are a thick succession of terrestrial and shallow-marine sediments that were deposited on older continental crust near the eastern edge of the exposed Precambrian rocks in Australia. The Adelaidean System records four major tectonic regimes: (1) early Neoproterozoix formation of an epicontinental, partly rifted basin; (2) mid-Neoproterozoic (∼700 Ma) continental breakup and accumulation of a passive margin wedge; (3) latest Neoproterozoic/Early Cambrian renewed rifting, and (4) Middle to Late Cambrian conversion of the passive margin to a convergence zone during the development of the Delamerian Orogen. Deposition of the Adelaidean System commenced by ∼830 Ma, when Australia was contiguous with Laurentia in the Rodinia supercontinent. Sandstone and evaporitic carbonate of an initial epicontinental basin are preserved as the lowest succession of the Adelaide Geosyncline and the Amadeus, georgina, Ngalia, Officer and Savory-Yeneena basins. Mafic volcanics were extruded under NE-SW continental extension during the Willouran, which was overprinted by a more widespread E-W Torrensian extension. Continental breakup occurred around 700 Ma, during accumulation of the Sturtian glaciogenic deposits in rift valleys. The succeeding siltstone, sandstone and carbonate were deposited on a passive continental margin that faced the newly formed Palaeo-Pacific Ocean as Laurentia rotated clockwise away from Australia. A wide Palaeo-Pacific Ocean existed at the time of the younger Ice Brook (Laurentia) and Marinoan (Australian) glaciations. Dextral shear between northern and southern Australia along the Paterson-Petermann Ranges Orogen in the 600-550-Ma interval, broke up the continuuity of the central Australian basins through uplift of the Musgrave Block, and led to renewed rifting, reflecting the latest Neoproterozoic continental breakup that formed the eastern margin of Laurentia. The South Australian passive margin persisted until the Middle Cambrian when convergence along the Pacific Ocean margin led to development of the Delamerian Orogebn. Deposition of the marine siliciclastic Kanmantoo Group commenced in the late Early Cambrian in a rapidlly subsiding trough, for which both extensional (rift-basin) and compressional (foredeep) origins gave been proposed. Reversal of the eastward-dipping Early Cambrian palaeoslope in the Middle Cambrian shed siliciclastic sediments back onto the continent from an initial uplift in advance of Delamerian deformation.