Showing papers on "Gondwana published in 1983"

The Separation of Madagascar and Africa

Abstract: Identification of a sequence of east-west trending magnetic anomalies of Mesozoic age in the western Somali Basin helps define the position of Madagascar in the Gondwana reconstruction. The anomalies are symmetric about ancient ridge segments and are flanked to the north and south by the Jurassic magnetic quiet zone. The motion of Madagascar relative to Africa was from the north and began in the middle Jurassic, about the same time as the initial breakup of Gondwanaland. Sea-floor spreading ceased when Madagascar assumed its present position in the Early Cretaceous.

Cenozoic Glaciation in the Southern Hemisphere

Abstract: With the end of glaciation in Gondwana in early Permian time, a prolonged interval began when the Earth was without ice sheets, though not necessarily free of mountain glaciers. Not until sometime during the Tertiary Era did ice sheets build up once more, first in Antarctica and later in the Northern Hemisphere. The fundamental cause of the cooling that led to the formation of the Antarctic Ice Sheet and, eventually, to major glaciation in lower latitudes is generally held to have been the increasing thermal isolation of Antarctica as Australia, and later South America, separated from it, allowing zonal oceanic circulation to develop. Rifting between Australia and Antarctica began between 1 10 and 90 m.y. ago (Cande & Mutter 1982), but not until much later, probably sometime between 30 and 25 m.y. ago, had it progressed far enough for deep circulation to begin as the South Tasman Rise cleared north Victoria Land (Kennett et a1 1974, Kennett 1977, Weissel et aI 1977). However, Antarctica did not reach its present high level of thermal isolation until South America had separated from it, enabling the Antarctic Circumpolar Current to become established (Savin et aI 1975). According to Barker & Burrell (1977, 1982), coherent spreading between the Antarctic Peninsula and Tierra del Fuego started about 29 m.y. ago, and deep circulation began 23.5 ± 2.5 m.y. ago, thereafter increasing in volume. The initial ice fields and ice caps that later expanded to become the

Fragmentation and assembly of the continents, Mid-carboniferous to present

Abstract: Eighteen maps showing the motions of the major continents following the break-up of Wegener's Pangaea in the Early to Mid-Jurassic are presented. Palaeolatitudes are determined palaeomagnetically, palaeolongitudes mainly from sea-floor spreading evidence. The break-up commences with the opening of the southern North Atlantic in the mid-Jurassic and its extension north and south in the Cretaceous and Cenozoic. Concurrently the northern continents move northwards away from the southern continents (Gondwana) and a continuous east-west seaway is formed between them in the Cretaceous. Successive fragmentation of continents then created the Arctic, Indian and Antarctic Oceans. Five maps showing the disposition of land in the Permian are also given. These are based on the palaeomagnetic evidence and the idea of minimizing the motions required to bring the continents into their known Early Jurassic configuration. The Permian maps show Gondwana situated further east than in the Pangaea configuration of Wegener. There are severe problems in constructing palaeogeographical maps for the Triassic and none are presented. Palaeomagnetic results from smaller crustal fragments are also reviewed and the evidence for the former dismemberment of Eurasia and the western part of the North American cordillera are set out. The results indicate that most orogenies are to some degree collisional in nature.

Braidplain deposition of the Upper Triassic Molteno Formation in the main Karoo (Gondwana) Basin, South Africa

Abstract: The Upper Triassic (Carnian?) Molteno Formation in the main Karoo (Gondwana) Basin, South Africa forms a northerly thinning, intracratonic clastic wedge comprising sandstones, shales and coals occurring within thick (up to 140 m) laterally persistent fining-upward sequences. These sequences were deposited by braided streams draining an alluvial plain which may have been built on to the distal slopes of alluvial fan complexes of glacial outwash type. Geometric relations between sequences indicate three phases of tectonic activity. The lowermost fining-upward sequence in the south accumulated against a rising mountain front; cessation of movement and an eastward shift in the main locus of tectonism and sedimentation was followed by renewed uplift and basinwide progradation of the second fining-upward sequence adjacent to a fault-block granite terrain located close to the present south-east coastline of South Africa. This is believed to be the granite at the eastern end of the Falkland Island Plateau, an interpretation consistent with its position on most continental reconstructions and the fracture zone marking its northern scarp face. Faulting is attributed to the first phase of extension prior to continental breakup. The sourceward recession and lack of gross fining-upward trends shown by the uppermost fining-upward sequences is accounted for by limited back-faulting of the still active basin margin. Cessation of activity and further basin margin recession occurred with deposition of the overlying floodplain deposits (Elliot Formation) which were distal equivalents of the braided alluvial plain.

Supposed Permo-Triassic megashear between Laurasia and Gondwana

Abstract: A consensus has emerged that during the Palaeozoic several landmasses progressively collided to form the supercontinent Pangaea, which subsequently split up in the Mesozoic and Tertiary to produce the present array of continents. But whereas there is a high measure of agreement about the general shape of the early Jurassic Pangaea, shortly before the initial Atlantic opening, three very different reconstructions of the end-Palaeozoic supercontinent have recently been proposed. These differences, which involve the relative positions of the northern and southern sectors of Pangaea, known respectively as Laurasia and Gondwana, relate to the interpretation of palaeomagnetic data. The three constructions are here tested with reference to geological and palaeobiogeographic data and arguments.

The significance of the pdaeomagnetism of Jurassic—Cretaceous rocks from South America: predrift movements, hairpins and magnetostratigraphy

Abstract: Summary. In this paper we show that: (1) The positions of the Cretaceous palaeomagnetic poles (PP) for South America and Africa exhibit elongated distributions that are due to rapid movement of these continents from the south pole. (2) The positions of the Middle—late Jurassic virtual geomagnetic poles for South America exhibit an elongated distribution along the meridians 20–200° E; it is suggested that this is due to a rapid shift of South America in Middle—late Jurassic time. (3) The late early—early late Cretaceous sections of the apparent polar wandering paths for South America and Africa are consistent with South Atlantic seafloor spreading data. On the basis of the comparison of the reliable late Palaeozoic—late Cretaceous PPs for South America and Africa, taking into account the restrictions established by geological, palaeontological and seafloor spreading data, it is suggested that minor movements could have occurred within Western Gondwana in middle—late Jurassic time along a narrow zone which later became the South Atlantic divergent boundary. Four ‘hairpins’ are defined in the late Palaeozoic—late Cretaceous section of the apparent polar wandering path for South America; the two youngest of these can be correlated with the origin of the South Atlantic Ocean basin and the onset of the Andean Orogeny, respectively. The magnetostratigraphy for the Serra Geral lava flow sequence suggests that some of these flows were poured out rapidly without significant interruption.

A palaeomagnetic study of Jurassic rocks from the Antarctic Peninsula and its implications

Abstract: Palaeomagnetic measurements have been made on rocks of Jurassic age from the Antarctic Peninsula to determine whether there has been relative movement between the region and East Antarctica. Samples were collected from the Oscar II Coast and the Cole Peninsula. Reliable results have been obtained from Jurassic dykes dated in one area at 174 ± 5 Ma, from an acid pluton of approximately the same age and from lavas of middle Jurassic age. Combining these results a position for the palaeopole was obtained at 48°S 238°E. If a reconstruction of Gondwana consistent with sea-floor spreading, palaeomagnetic and geological data is adopted, such as that of Norton & Sclater (1979), then a well-determined position for the Antarctic Peninsula within the reconstruction is obtained. Space considerations suggest that during the Jurassic the relative positions of the Antarctic Peninsula and the Ellsworth block were not very different from the present day, though palaeomagnetic measurements (Watts & Bramall 1981) indicates that the orientation was different.

Mineral resources of Antarctica

Abstract: Metallic and nonmetallic mineral occurrences are abundant in Antarctica. The most significant known deposits are of iron, copper, and coal. In the Precambrian shield of East Antarctica, for example, iron is present as banded iron-formation and as magnetite in veins, pods, and schist. The largest deposits of iron are in the Prince Charles Mountains, where bodies of banded iron-formation at least as thick as 400 m extend, mostly under the ice for at least 120 km. Widely scattered morainal boulders and outcrops of iron-rich rock suggest that undiscovered iron deposits are also distributed over many other parts of East Antarctica. Gondwana reconstructions suggest that many more mineral deposits occur in Antarctica. However, ice covers nearly 98 percent of the continent, and few of the bedrock areas have even been prospected or geologically, geophysically, or geochemically mapped in detail.

Amazon rift and Pisco-Juruá fault: Their relation to the separation of North America from Gondwana

Abstract: The Amazon fault passes eastward from south of Guayaquil on the Pacific to the Amazon mouth on the Atlantic coast. Along its eastern segment, a rift with pyroxenite intrusions opened up at the start of Paleozoic time; its Early Jurassic reactivation produced more than 100,000 km 3 of diabase sills, intruded into the evaporitic upper Paleozoic simultaneously with North Atlantic rifting. Pliocene reactivation faulted the coastal Tertiary and sheared the subducted slab beneath the Pacific end of the fault. The northeast-trending Pisco-Jurua fault, first described here, cuts across the continent from Pisco on the Pacific to the Guyana-Surinam border on the Atlantic coast. In early Mesozoic time the fault formed the southwestern continuation of the North Atlantic Rift; its Guyanan end opened up as the North Atlantic Rift opened. The separation of North and South America caused northwestern South America to move southwest along the Pisco-Jurua fault, creating the Tacutu graben in the Guyanan Shield, the gently folded Jurua zone across the Amazon Basin, and the Pisco-Abancay deflection in the Andes. Pliocene reactivation at the Pacific end of the fault sheared the subducted slab of the Nazca plate and may have contributed to the formation of the Nazca Ridge.

Age of the Banda Sea, eastern Indonesia

Abstract: The age of the Banda Sea is critical for palaeogeographical reconstructions of eastern Indonesia. Assumed ages for its formation range from Mesozoic to Neogene1–10. However, geophysical evidence shows that the Banda Sea is floored by oceanic crust11,12 with a minimum age of Eocene4. Evidence for a sequence of events involving continental breakup and the initiation of spreading that led to the formation of the Banda Sea should be found in the microcontinents around the margins of the sea. Reinterpretation of the geological history of these microcontinents based on new data reported here from western Irian Jaya9,13 shows that they separated from Gondwana during the Jurassic and strongly supports the suggestion of Bowin et al.4 that the Banda Sea is trapped Mesozoic oceanic crust of Indian Ocean affinities.

Lower Permian Rgtliegend Desert Sedimentation in the North Sea Area

Abstract: Publisher Summary This chapter examines the lower Permian Rotliegend desert sedimentation in the North Sea area. Rotliegend deposition took place in the environment of a Northern Hemisphere Trade Wind desert during the later phases of the major Southern Hemisphere glaciation over Gondwana. Rotliegend deposition was brought to an abrupt end early in the Late Permian as a result of the rapid flooding of the continental basins by the marine waters of the Zechstein Sea. The Rotliegend is divided into two distinct rock-stratigraphic units—the Lower and Upper Rotliegend—on the presence or absence of volcanic rocks associated with the sedimentary sequence. It is found that although the Lower Rotliegend is generally overlain by the Upper, the latter may locally be coeval with the younger parts of the Lower Rotliegend in areas where there was no Early Permian volcanic activity. In the Rotliegend sandstones of many North Sea wells, the relatively narrow spread of bedding attitudes indicates that these sands were deposited largely on dunes of the transverse type.

An Ictidosaur Fossil from North America

Abstract: Teeth of a North American ictidosaur, Pachygenelus milleri, n. sp., found in the Upper Triassic Dockum Group of Texas, indicate that it is very similar to Pachygenelus monus of South Africa and Chaliminia musteloides of South America. The presence of a Gondwana element in the Northern Hemisphere attests to the ease of dispersal of the Late Triassic vertebrates through Pangea. Ictidosaurs are small, highly advanced, carnivorous cynodonts that display a mosaic of reptilian and mammalian features in the masticatory apparatus. They were contemporaneous with early mammals and probably closely related to them.

Kaolinisation and the formation of silicified wood on late Jurassic Gondwana surfaces

Abstract: Summary In some parts of Australia and Africa intense kaolinisation took place on late Jurassic land surfaces, transforming considerable thicknesses of bedrock into masses of pure kaolin and quartz. Overlying these surfaces occur fluviatile, kaolinitic sandstones of late Jurassic or earliest Cretaceous age. They contain an abundance of silicified wood, including whole trunks of trees. The formation of silicified wood has previously been attributed to hot and arid desert environments, but in the examples described here, silicification proceeded under constantly high groundwater levels, requiring humid and probably warm climatic conditions. Silica was liberated by the kaolinisation of silicate minerals. Humic acids, produced during the initial decay of the fossil timber provided the low pH microenvironment for the fixation of SiO2.

Early permian brachiopods and molluscs from gondor formation, Eglinton valley, Southland

Abstract: Deformed poorly preserved fossils from Gondor Formation in the Eglinton valley belong to a zone particularly widespread in the Early Permian of Gondwana, and indicate a mid-Sakmarian age. They include the oldest brachiopods yet known from New Zealand Permian sequences. The brachiopod and molluscan taxa are summarised, and a new genus is proposed for an allied species from east Australia Pustulospiriferina n. gen., type species Punctospirifer etheridgei Armstrong, 1970.

Mélange within subduction–accretion complex rocks of Fredriksen Island, South Orkney Islands

Abstract: Summary. A m´lange on Fredriksen Island, South Orkney Islands, is part of a Mesozoic subduction- accretion complex which formed along the western, Pacific side of Gondwana. It consists of a chaotic arrangement of irregular sized blocks, up to 8 m across, of basic pillow lava, chert, felsite and epiclastic sandstone in a pervasively sheared cataclastic matrix. Inclusions are typically lozenge-shaped and are characteristic of a tectonic m´lange. As the m´lange incorporates both possible ocean floor material that was accreted on to the continental margin and inner slope basin sediments it probably formed beneath the trench-slope basin along a shear zone at a high level in the subduction complex.

Tectonostratigraphic terranes of the frontier circum-Pacific region

Abstract: Many major exploration frontiers around the Pacific are in regions where complex geologic relations reflect plate-tectonic processes, crustal mobility, and accretion of exotic terranes. The destruction of the proto-Pacific ocean (Panthalassa) involved accretion of terranes to cratonal regions such as Gondwana and Laurasia. Terranes in southwestern New Zealand and eastern Antarctica were also probably accreted during the Paleozoic. The southern margin of Siberia, extending into China, underwent a protracted period of accretion from the late Precambrian through the early Mesozoic. Mid-Paleozoic accretion is reflected in the Innuitian foldbelt of the Arctic Ocean, the Black Clastic unit of the northern Rocky Mountains, and the Antler orogeny of the western US cordillera. The Mesozoic breakup of Pangaea and the acceleration of subduction aided in the rifting and dispersal of terranes from equatorial paleolatitudes. Fragments of these terranes now compose much of the continental margins of the Pacific basin, including New Zealand, Indochina, southern China, southeast Siberia, the North American cordillera, and South America. Some terranes are presently being further fragmented by post-accretionary dispersion processes such as strike-slip faulting in western North America and Japan. Although the character and distribution of terranes in the western US are fairly well documented, details are neededmore » for other terranes around the Pacific basin. Interpretation of structure and stratigraphy at depth will be aided by more data on the timing of accretion and the nature of deformation associated with accretion and dispersion. Such data are needed for further define specific exploration targets in the circum-Pacific region.« less

The Position of Indian Tin Occurrences in the Tin-Belts of Gondwana

Abstract: Tin deposits and occurrences show an inhomogeneous distribution in elongate belts on all the continents. When the continents are fitted together in pre-drift position, these belts form a coherent pattern. The delineation of tin-belts in Gondwana is difficult due to the inaccessibility of Antarctica and to the fact that most continents belonging to Gondwana seem to have a relatively deep average level of erosion. This may have caused the removal of pre-existing tin-deposits to a large extent. Still it appears that most of the Indian tin occurrences including the recently discovered tin province of Bastar, M.P., fall on a continuation of the tin-belt in Western Australia. The association of cassiterite with Nb-Ta minerals and with Li-minerals is a characteristic of most of the eastern Indian occurrences as well as of most of the Western Australian deposits.