Showing papers on "Gondwana published in 1994"

Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period

Abstract: The end Ordovician glaciation is distinct among Phanerozoic glaciations in that CO 2 , levels were generally high, yet major continental ice sheets accumulated on the Gondwana supercontinent. New oxygen isotopic data indicate substantial changes in sea-water temperatures and ice volume coinciding with glacio-eustatic changes in sea level reflecting the growth and decay of the Gondwana ice cap. Major glaciation was apparently confined to the Hirnantian and was 0.5-1 m.y. long, rather than the 35 m.y. of earlier estimates. Carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep-water circulation and then back again. We believe that the changes in the carbon cycle effected a reduction in P CO 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world.

Paleozoic Laurentia-Gondwana interaction and the origin of the Appalachian-Andean mountain system

Abstract: Laurentia, the rift-bounded Precambrian nucleus of North America, may have broken out from a Neoproterozoic supercontinent between East and West Gondwana. Several lines of evidence suggest that the Appalachian margin of Laurentia subsequently collided with the proto-Andean margin of the amalgamated Gondwana supercontinent in different relative positions during early and mid-Paleozoic time, in route to final docking against northwest Africa to complete the assembly of Pangea. Hence the Appalachian and Andean orogens may have originated as a single mountain system. The overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the Paleozoic Era. Laurentia-Gondwana collisions may help to explain contemporaneous unconformities in the Paleozoic sedimentary cover of the Laurentian, Gondwanan, and Baltic cratons.

Cambrian Orogenic Belt in East Antarctica and Sri Lanka: Implications for Gondwana Assembly

Abstract: Ion microprobe U-Pb dating of zircons from the Lutzow-Holm Complex (LHC) and the Yamato-Belgica Complex (YBC), East Antarctica indicate high-grade regional metamorphism and associated folding of LHC occurred between $$521 \pm 9$$ and $$553 \pm 6 Ma$$. This shows, for the first time, the existence of a Cambrian orogenic belt within the East Antarctic Shield. Many zircons from the LHC contain cores that record inherited ages from ~2900 to ~1500 Ma. Components of ~1000 Ma zircon have been identified in three locations. This may indicate the maximum age of the deposition of LHC. One metasedimentary gneiss from the YBC records a well-defined age at about 600 Ma, whereas another yields a dispersion of ages interpreted as the result of varied radiogenic Pb loss in ~1000 Ma zircons at about 500-600 Ma. These gneisses have inherited ages of up to ~2500 Ma. Our work enables an improved fit to the once contiguous fragments of Gondwana. The Highland/Southwestern Complex (HSWC) of Sri Lanka has remarkable petrological...

Sedimentary history of the Tethyan basin in the Tibetan Himalayas

Abstract: After an epicontinental phase, the sedimentary rocks in the Tibetan Himalayas document a complete Wilson cycle of the Neo-Tethyan (Tethys Ill) evolution between the Gondwana supercontinent and its northward drifting margin (Lhasa block) from the Late Permian to the Eocene. During the Triassic rift stage, the basin was filled with a huge, clastic-dominated sediment wedge with up to > 5 000 m of flysch in the northern zone. Widespread deltaic clastics and shallow-water carbonates of late Norian to earliest Jurassic age in the southern zone mark, in conjunction with decreasing tectonic subsidence, the transition to the drift stage. Some 4 500 m of Jurassic and Early Cretaceous shallow-water carbonates and siliciclastics accumulated on the Tethyan Indian passive margin. Deepening-upward sequences with condensed beds at their tops alternate with repeated progradational packages of shelf sediments. Extensive abyssal sediments with basaltic volcanics in the northern deep-water zone reflect continued ocean spreading and thermal subsidence. Paleomagnetic data, gained separately for the northern Indian plate and the Lhasa block, indicate that the Neo-Tethys reached its maximum width about 110 Ma ago with a spreading rate of 4.8 cm/year, before it commenced to close again. During the remnant basin stage in the Late Cretaceous and Paleogene, a shallowing-upward megasequence, capped by a carbonate platform, developed in the southern inner shelf realm. In the northern slope/basin plain zone, turbidites and chaotic sediments, derived from both the acretionary wedge and the steepening slope of the passive margin, accumulated. The depositional center of the remnant basin shifted southward as a result of flexural subsidence and southward overthrusting. The sediments from the Triassic to the Paleogene are tentatively subdivided into five mega-sequences, which are controlled mainly by regional tectonics. Climatic influence (e.g., carbonate deposition), due to northward plate motion, is partially subdued by terrigenous input and/or increased water depth. During the Oligocene and Miocene, crustal shortening led to rapid uplift and the deposition of fluvial molasse in limited basins.

Tectonic framework of the Precambrian of Madagascar and its Gondwana connections: a review and reappraisal

Abstract: The Precambrian of Madagascar is divided into two sectors by the north-west trending sinistral Ranotsara shear zone, which continues in the Mozambique belt, probably as the Surma shear zone, and in Southern India as the Achankovil shear zone. South of Ranotsara six north-south trending tectonic belts are recognized that consist largely of granulite and high amphibolite facies paragneisses, phlogopite diopsidites, concordant granites and granulites. North of Ranotsara the central-northern segment is traversed by a north-trending axial 100–150 km wide dextral shear zone of probable Pan-African age, which was metamorphosed under granulite and high amphibolite facies conditions and which has reworked older basement. This shear zone continues across southern India as the Palghat-Cauvery shear zone. Major stratiform basic -ultrabasic complexes occur in the axial zone and in the basement to the west. Well preserved low grade continental margin-type sediments (quartzites, mica schists and stromatolitic marbles) of Kibaran age are present in western Madagascar. Two partly greenschist grade sedimentary groups lie unconformably on high grade basement in north-east Madagascar. Isotopic age data suggest the presence in Madagascar of Archaean, Early and Mid-Proterozoic crustal material that was extensively reworked in Pan-African times.

Uranium‐lead zircon ages from the Median Tectonic Zone, New Zealand

Abstract: The Median Tectonic Zone (MTZ) of New Zealand is a generally north trending belt of Mesozoic subduction‐related I‐type plutonic, volcanic, and sedimentary rocks in South Island and Stewart Island that separates Permian strata of the Eastern Province Brook Street Terrane from lower to mid‐Paleozoic Gondwana margin assemblages of the Western Province. High‐precision isotope dilution U/Pb ages of zircons from 30 rocks are reported. Pre‐digestion leaching of zircon in hydrofluoric acid yielded significantly more concordant residues by removing common Pb and dissolving more soluble high‐U domains that have been more affected by relatively recent Pb loss. The results show that MTZ magmatism ranges in age from at least Early Triassic to Early Cretaceous (247–131 Ma), with a pronounced gap in the Middle Jurassic. Triassic plutons tend to occur on the eastern side of the MTZ, and they intrude volcanic/sedimentary sequences of the MTZ in Nelson and eastern Fiordland. These sequences are in turn intruded by...

Laurentia, Australia, and Antarctica as a Late Proterozoic supercontinent: Constraints from isotopic mapping

Abstract: The reconstruction of Laurentia, Australia, and Antarctica into a Proterozoic supercontinent is evaluated by analyzing the fit of Precambrian provinces defined by isotopic and geochronologic mapping. The analysis is complicated by allochthonous segments of the Antarctic and eastern Australian margins. Removal of the allochthonous provinces produces a closer fit of the continents; there is a match of Early Proterozoic basement between southwestern Laurentia and the only exposure of craton known from the paleo-Pacific margin of Antarctica. In addition, western Laurentia is brought closer to the Australian Gawler block, consistent with provenance interpretations of the Belt Supergroup. Removal of the allochthonous provinces by right-lateral translation relative to the Antarctic craton margin places them in a pre-750 Ma position where they could be southwestward extensions of the Yavapai-Mazatzal and Grenville provinces of southern Laurentia. This modified reconstruction leads to a prediction of extensive Archean basement in Antarctica between the South Pole and Victoria Land, a prediction partly borne out by Archean rocks in the Miller Range of the Transantarctic Mountains; it also predicts the presence of 1.4Ga rapakivi granites in the Transantarctic Mountains basement. This configuration implies assembly of the Australia-Antarctica Gondwana margin by terrane accretion following, or accompanied by, left-lateral translation. This is compatible with a tectonic regime of clockwise rotation of Laurentia relative to Australia and Antarctica after rifting. Thus, the proposed supercontinent, with some modifications, has potential for explaining several aspects of the pattern of Precambrian provinces in the three continents.

The Falkland Islands: A key element in Gondwana paleogeography

Abstract: Stratigraphic and sedimentological fieldwork on the Falkland Islands confirms that they were originally part of South Africa. East and West Falkland have contrasting geological and deformation histories and are separated by the Falkland Sound fault, which marks a significant deformation edge to the Cape Fold Belt. The degree of structural deformation and metamorphism in West Falkland is substantially less than that of East Falkland, adjustments to differential shortening between the two islands being reflected by structures developed along the Falkland Sound fault. Prior to fragmentation of Gondwana, the Falkland Platform and Maurice Ewing Bank were positioned in the Natal Valley, adjacent to the east coast of South Africa. This predrift reconstruction shows that the Falkland Islands have subsequently been rotated by 180° relative to South Africa and complements recent paleomagnetic data. There was a complete overlap of the Maurice Ewing Bank and Falkland Platform to the exclusion of the Falkland Plateau Basin which is a small oceanic basin formed during the Jurassic fragmentation of Gondwana. Extension in this basin was accommodated by movements along the Aghulas Falkland Fracture Zone and the Gastre Fault System. Predrift restoration of these faults shows that Patagonia was positioned closer to both Africa and Antarctica. The Falkland Plateau Basin was an integral component of the West Gondwana Jurassic microplate system and provides an invaluable insight into the evolution of its western margin. At some time prior to the main opening of the South Atlantic, the Falkland Platform was rotated clockwise by 120° against the Agulhas Falkland Fracture Zone, such that the Falkland Islands were translated some 300 km to the south. The driving force for this rotation is not known but could possibly have been initiated by periodic stress buildups in the Falkland Plateau Basin as the Gastre Fault System, within continental crust, became unable to accommodate the continuous dextral shear in the Agulhas Falkland Fracture Zone. The Falkland Islands represent an outboard extension of the Eastern Cape and are significant for understanding the paleogeography of both the Cape and Karoo supergroups. East Falkland is closely similar to South Africa in its geology, while West Falkland shows significant contrasts with both the Devonian and Permian successions being more proximal in character but still demonstrate the eastward continuation of the Cape and Karoo basins.

Tectonics of the southern central Andes : structure and evolution of an active continental margin

Abstract: Structure and State of the Lithosphere.- The Lithospheric Structure of the Central Andes (20 - 26 S) as Inferred from Interpretation of Regional Gravity.- Variation in the Crustal Structure of the Southern Central Andes Deduced from Seismic Refraction Investigations.- Crustal High Conductivity Zones in the Southern Central Andes.- Geothermal Structure of the Central Andean Crust - Implications for Heat Transport and Rheology.- Large- and Fine-Scale Geochemical Variations Along the Andean Arc of Northern Chile (17.5 -22 S).- Partial Melting in the Lower Crust: New Constraints on Crustal Contamination Processes in the Central Andes.- State of In Situ Stress in Northern Chile and in Northwestern Argentina.- Large Events, Seismic Gaps, and Stress Diffusion in Central Chile.- Geological Evolution.- Tectonic Development of the North Chilean Andes in Relation to Plate Convergence and Magmatism Since the Jurassic.- K-Ar and Fission Track Mineral Age Determinations of Igneous Rocks Related to Multiple Magmatic Arc Systems Along the 23 S Latitude of Chile and NW Argentina.- Geothermal and Tectonic Evolution of the Eastern Cordillera and the Subandean Ranges of Southern Bolivia.- Sedimentary and Structural Evolution of the Salar de Atacama Depression.- The Purilactis Group of Northern Chile: Boundary Between Arc and Backarc from Late Cretaceous to Eocene.- Early Jurassic to Early Cretaceous Magmatism in the Coastal Cordillera and the Central Depression of North Chile.- Sediment Accumulation and Subsidence History in the Mesozoic Marginal Basin of Northern Chile.- Jurassic Cretaceous Palaeogeographic Evolution of the Chilean Andes at 23 -24 S Latitude and 34 -35 S Latitude: A Comparative Analysis.- The Southern Andes Between 39 and 44 S Latitude: The Geological Signature of a Transpressive Tectonic Regime Related to a Magmatic Arc.- Terranes of Southern Gondwanaland and Their Control in the Andean Structure (30 -33 S Latitude).- Some Isotopic and Geochemical Constraints on the Origin and Evolution of the Central Andean Basement (19 -24 S).- The Late Carboniferous to Triassic Volcanic Belt in Northern Chile.- Geodynamic Evolution of the Early Palaeozoic Continental Margin of Gondwana in the Southern Central Andes of Northwestern Argentina and Northern Chile.- The Nitrate Deposits of Chile.- Petrochemical Factors Governing the Metallogeny of the Bolivian Tin Belt.- Maps of the Central Andean Segment Between 20 and 26 S.- Comments on the Geological and Geophysical Maps.- Enclosure.- 1 Geological Map.- 2 Bouguer Anomaly and Epicenters.- 3 Residual Gravity Anomaly (each at the scale 1:1 000 000 covering the area 20 -26 latitude and 63 -71 30'W longitude).

Early Permian to Middle Jurassic rifting and sedimentation in East Africa and Madagascar

Abstract: Pre-drift sediments of Madagascar (Early Permian-Middle Jurassic) have been studied palynologically. These studies resulted in a more precise dating of individual stratigraphic units and the recognition of minor and major breaks in the succession. Palynostratigraphic and physical evidence of unconformities have been used to subdivide the pre-drift sediments into depositional cycles and to infer rifting events. A comparison with equivalent strata of Mozambique, Tanzania, Kenya, Ethiopia and Somalia shows a general correspondence and provides additional information for the construction of a generalized framework for the East African/Madagascan region which demonstrates the relationships between rifting and sedimentation and elucidates the prehistory of the break-up of Gondwana into a western and eastern fragment during the Middle Jurassic. The predrift succession of East Africa/Madagascar can be subdivided into two major sequences, a Karoo sequence (cycles 1–5, Asselian-early Smithian) and a transitional sequence (cycles 6–9, Ladinian-early Bajocian). The two sequences are separeted by a late Scythian to Anisian hiatus which indicates extensive uplift and erosion before a major rifting event initiated the resumption of sedimentation in the Ladinian. This Middle Triassic event marks the transition from the intracratonic rift stage to the pericratonic basin stage and correlates with a significant event in the Pangaean history, the transition from final coalescence to initial dispersal. The onset of the southward drift of Madagascar is believed to have occurred about 60 Ma later near the Aalenian-Bajocian boundary, contemporaneous with or immediately after the deposition of syntectonic sediments of cycle 9.

Cooling history of the northern Ford Ranges, Marie Byrd Land, West Antarctica

Abstract: Thermochronologic data from the Fosdick, Phillips and Chester mountains of Marie Byrd Land, West Antarctica, have been obtained through U-Pb analysis of monazite, 40Ar/39Ar analysis of hornblende, muscovite, biotite and K-feldspar, and apatite fission track methods. These data were collected to test the hypothesis that high-grade metamorphic rocks in the Fosdick Mountains occupy the footwall of a Cordilleran-style metamorphic core complex, exhumed during the breakup of this sector of Gondwana in early Late Cretaceous time. High-grade metamorphism of rocks exposed in the Fosdick Mountains was followed by rapid cooling starting at ∼105 Ma, during the transition from convergence to extension in the adjacent continental margin of Gondwana. Monazite, hornblende, muscovite, biotite, and K-feldspar from the Fosdick Mountains record rapid cooling (70±30°C/m.y.) from peak metamorphic conditions of 725°–780°C at 4.3–5.6 kbar to below 165°C between 105 and 94 Ma. Subsequent slow cooling was apparently punctuated by a short period of accelerated cooling through the apatite partial annealing zone (∼110°–60°C) between ∼80 and 75 Ma. Cooling rates decreased to an average of ∼l°C/m.y. after 70 Ma. Cooling ages become progressively older to the south; metamorphic grade decreases in concert with the increasing cooling ages. The southernmost samples, from the Chester Mountains, probably cooled to below K-feldspar closure temperature (∼165°C) before inferred reheating associated with metamorphism in the Fosdick Mountains. North of the Fosdick Mountains, Devonian Ford granodiorite in the Phillips Mountains was below K-feldspar closure temperature by early Cretaceous time. Byrd Coast granite intrusions in the eastern Phillips Mountains and east of the Fosdick and Chester mountains were emplaced between 100 and 105 Ma, and these plutons cooled very rapidly (>100°C/m.y.) to below biotite closure temperature, consistent with their epizonal character. The relationship of these granitoids to metamorphic rocks in the Fosdick Mountains is uncertain. We hypothesized the following sequence of events during the transition from convergence to extension along the Pacific margin of Gondwana. Voluminous intrusion into the lower and middle crust led to increased heat flow and high-tem-perature, low-to moderate-pressure metamorphism, forming the Fosdick metamorphic complex (FMC) exposed in the Fosdick Mountains. Decrease in strength due to intrusion and partial melting resulted in large-scale flow, probably driven by extension-related differential stresses. This deformation ended before the onset of rapid cooling of the FMC at ∼105 Ma. Cooling rates determined for the FMC can be modeled by decreasing the heat flux into the crust and exhuming the complex at a rate of 1.5 mm/yr. The decrease in cooling rate between closure of K-feldspar 40Ar/39Ar at ∼94 Ma and cooling into the apatite fission track partial annealing zone by ∼80 Ma is interpreted to indicate that exhumation was at least a two-stage process. Our observations indicate that the Fosdick and Chester mountains are part of a coherent block that was tilted ∼20° to the south during the exhumation of the FMC, probably by movement along an east trending, north dipping, normal fault between the Fosdick and Phillips mountains. The Fosdick Mountains are not a Cordilleran-style metamorphic core complex, but the FMC provides a record of middle-crustal processes related to the rifting of New Zealand from Gondwana in the Late Cretaceous.

Precambrian Scotland as a Laurentia-Gondwana link: Origin and significance of cratonic promontories

Abstract: Refinement of the supercontinental reconstruction for the latest Precambrian that places the Labrador-Greenland promontory of Laurentia within the Arica embayment along the margin of the Gondwana craton juxtaposes the Rockall microcontinent and northwestern British Isles with the continental margin of Peru. The conjugate cratonic margin to the northwest Caledonian foreland, the Hebridean shield, may have been Amazonia. Possible South American correlatives of the Precambrian Moinian and Dalradian complexes of the Scottish Highlands can be identified in the reconstruction; notably, there are possible equivalents of the "older granites" within the Moinian and of Dalradian glacial deposits. Therefore, the well-studied Precambrian rocks of the Scottish Highlands provide critical tests for the suggested reconstruction. The latest Precambrian setting of the Arequipa massif with respect to the Gondwana craton margin and the Paleozoic intracratonic basin of Peru-Bolivia bears a striking similarity to the early Mesozoic setting of the Rockall Plateau and northwestern British Isles with respect to the European continental margin and the North Sea-Western Approaches graben system. In each case rifting appears to have been controlled by the youngest, and presumably weakest, lithosphere, but the extremity of the Labrador-Scotland-Greenland promontory was detached during final continental separation in Vendian to earliest Cambrian time to form the Arequipa massif and in Jurassic time to form the Hebridean shield. The promontory may have played an important tectonic role through at least 1 b.y. of Earth history including influence on the development of the Andean Cordillera. Features of this type likely have played a major role in the development of orogenic belts of all ages.

Gondwana and Cathaysian blocks, palaeotethys sutures and cenozoic tectonics in South-east Asia

Abstract: The Triassic Indosinian Orogeny followed extinction of the Palaeotethys Ocean resulting in suturing of Gondwana affinity and Cathaysian blocks.

East Avalonia, the third partner in the Caledonian collisions: evidence from deep seismic reflection data

Abstract: Further evidence for the existence of the terrane East Avalonia (Cadomia) in north-west Europe, its boundaries and its role in the Caledonian collisional processes comes from studies of deep seismic reflection data at sea and on land. Various sutures are found in the north-east and the north-west, and a generally poor reflectivity dominates in the major part. Further details of reflectivity patterns support the idea that a huge terrane which split from the northern rim of Gondwana moved northward and collided with the merging plates Laurentia-Baltica. Its docking features are analysed.

Gamilaroi Terrane: A Devonian rifted intra-oceanic island-arc assemblage, NSW, Australia

Abstract: The Gamilaroi terrane comprises part of the New England orogen of eastern Australia and contains Devonian strata preserving evidence of their development in a variety of arc-related environments Radiolarian studies provide new and more precise constraints on stratigraphy and together with geochemistry allow development of a new model for evolution of the terrane A diverse suite of volcaniclastic sedimentary rocks intercalated with regionally extensive extrusive and high level intrusive meta-felsic igneous rocks of low-K and calc-alkaline volcanic affinity is present These rocks constitute the oldest in-situ volcanic rocks in the Gamilaroi terrane and are similar to those found in modern intra-oceanic volcanic island-arc settings Basaltic intrusions and lavas occur at higher stratigraphic levels Major and trace element compositions, together with relatively flat chondrite-normalized REE distributions, resemble those from rocks formed in modern intra-oceanic island-arc rift zones The close spatial association of these rock types together with an absence of continent-derived detritus is consistent with interpretation of development of the Gamilaroi terrane in an intra-oceanic island arc which experienced local rifting This interpretation is a departure from previous models for the New England orogen, which interpret the Gamilaroi terrane as a forearc basin that developed above a long-lived (Cambrian to Permian), west-dipping subduction zone fringing the continental margin of Gondwana By analogy with modern arc-continent collision zones in which crust between colliding continents and island arcs is preferentially subducted beneath the approaching arc, we suggest that oceanic crust intervening between Gamilaroi terrane and Gondwana was subducted eastwards under the western margin of the Gamilaroi terrane arc During a latest Devonian collision event the Gamilaroi terrane was obducted onto the Gondwana margin This resulted in subduction flip and subsequent development of an east-facing continental margin arc system on top of the Gamilaroi terrane

Was Scotland a Vendian RRR junction

Abstract: In Early Palaeozoic time northern Scotland occupied a 120° promontory between the Appalachian and Greenland Iapetus-facing margins of Laurentia. It is proposed that this originated at a triple rift (RRR) jundion between Laurentia and two other continents, possibly Baltica and West Gondwana. The third arm is marked by trans-Caledonoid extensional lineaments that controlled Vendian sedimentation and MORB-type volcanicity in the Grampians. The model accounts for the huge scale of the SE-prograding Moine-Dalradian clastic wedge as compared with Neoproterozoic sequences elsewhere on the E Laurentian margin where an extensional regime persisted throughout the Grenville-Caledonian interval. Evidence currently cited for Precambrian orogeny in the Highlands needs to be reappraised from that viewpoint. Major stretching events are inferred at c . 750 and 600 Ma; anorogenic ‘older granite’ magmatism was associated with the latter, which culminated in the opening of Iapetus.

Paleozoic ophiolitic assemblages within the southern New England orogen of eastern Australia: Implications for growth of the Gondwana margin

Abstract: Several ophiolitic assemblages occur in the southern New England orogen. The development of these rocks and their relations to the rest of the orogen have major implications for the tectonic evolution of eastern Gondwana. A major, narrow but elongate belt of Early Cambrian supra‐subduction zone ophiolite crops out along and near the Peel‐Manning Fault System and is juxtaposed against younger arc and subduction complex terranes. No pre‐Permian links with the rest of the New England orogen have been established for this terrane. It may represent portions of Lachlan Fold Belt basement which underlies younger, westward overthrust New England terranes, and has been diapirically emplaced at higher crustal levels as serpentinite‐matrix melange. Middle to Late Devonian ophiolitic rocks in the Yarras Complex comprise basement to the Birpai subterrane and represent a crustal cross section through a rifted island arc. Correlatives of this terrane also occur within the more extensive Gamilaroi terrane to the west of which deeper crustal levels are not exposed. The various components of serpentinite‐matrix melange in the Ngamba terrane at Port Macquarie superficially appear to represent a dismembered ophiolite association. However, the various components of the melange exhibit a wide range of ages, metamorphism, and tectonic affinities, rendering a genetically related origin unlikely. This terrane includes fragments of ocean floor accreted into a Late Devonian subduction complex, which was later affected by Early Carboniferous forearc serpentinite diapirism and high Mg series magmatism. Zircon inheritance in Triassic dikes, which intrude the melange attest to the development or later emplacement of this forearc region over an older Lachlan Fold Belt basement. Ultramafic rocks of the Bundjalung terrane in the east of the New England region probably formed at deep levels in an intraoceanic island arc and are intruded by boninitic dikes. The tectonic development of the NEO was significantly more complicated than has been suggested in earlier published models. Periodic accretion of island arc systems, some of which are now represented by supra‐subduction zone ophiolites, to the eastern margin of Gondwana suggests multiple phases of subduction with the possibility of polarity reversals throughout the history of accretion. Lateral accretion was not the only means by which Gondwana continental crust grew, and there was considerable postaccretion continentward overthrusting of younger terranes. Copyright 1994 by the American Geophysical Union.

Neoproterozoic sinistral displacement along the Darling Mobile Belt, Western Australia, during Gondwanaland assembly

Abstract: Evidence is provided for Neoproterozoic sinistral displacement along the Darling Mobile Belt between Greater India and Precambrian terranes of Western Australia. A dilatational bend in the shear zone is thought to have controlled emplacement of alkali granites, anorthosite and mafic intrusives of the Leeuwin Complex which was deformed during progressive deformation. Faults formed in this event extend across the Albany Mobile Belt, and control the formation of the Stirling Basin on the southern margin of the Yilgarn Craton. The regional extent of this event and the rotation of Mesoproterozoic trends in the Albany Mobile Belt through 90° suggests that the Darling Mobile Belt may be part of a shear system of regional importance during assembly of proto-East Gondwanaland. Much attention is being focused on the assembly of Gondwanaland and the relative displacements between Archaean cratons along Proterozoic mobile belts (e.g. Powell 1993; Unrug 1994; Yoshida 1994). Southwestern Australia is a key area within proto-East Gondwanaland as, in reconstructions such as de Witt et al. 1988 and Powell et al. 1988, it constitutes the junction between the Archaean Yilgarn Craton of Western Australia, India/Greater India, and the East Antarctic Shield (Fig. 1). This paper examines the timing and nature of displacements along the Darling Mobile Belt (Glikson & Lambert 1973; Mathur & Shaw 1982) on the western margin of the Yilgarn Craton and contemporaneous structures within the Albany Mobile Belt and southern Yilgarn Craton (Fig. 2). The Darling Mobile Belt. The Darling Mobile Belt, referred to by Myers (1990) as the

Late paleozoic stratigraphy and petrography of the thini chu group (manang, central nepal): sedimentary record of gondwana glaciation and rifting of neotethys

Abstract: In the Manang area (north Annapurna Range; Nepal Himalaya), the Permo-Carboniferous succession is 1000 to 1500 m thick. Crinoidal biocalcarenites (Tilicho Lake Fm.) pass upward to alternating black shales and sharp-based white quartzose sandstones (Thini Chu Group). Detailed stratigraphic analysis of this unit allowed us to recognize and establish 5 new formations and 8 new members. The Marsyandi Fm., of Visean age, records an increase of subarkosic terrigenous detritus during the initial stage of Neotethyan rifting, and is capped by two sequences "Syringotbyris beds") characterized by transgressive sandstones rich in Serpukhovian brachiopods. The overlying black shales with subordinate quartzarenites (Col Noir Shale) are followed in the more proximal Bangba section by diamictites yielding dolostone rock fragments, documenting the first advance of glacial ice on rift shoulders, actively uplifted since the Bashkirian-Moscovian (Bangba Fm.). Next, glaciomarine to transgressive shelfal deposits are enriched first in igneous detritus and then in arenaceous rock fragments (Braga Fm.). Mafic to felsic magmatism during the climax of rifting was thus followed by active erosion of sedimentary successions, probably around the Carboniferous/Permian boundary. A major Early Permian transgression, coinciding with ameliorating climates at the end of the Gondwana glaciation, was followed by mainly estuarine chert-bearing quartzose pebbly sandstones capped by richly bioclastic shelfal deposits (Puchenpra Fm.). This second major transgression, associated with quartzose sandstones documenting subdued rift reliefs, is dated as Bolorian at Bangba, as Murgabian-Midian at Col Noirand as Djulfian at Tilicho. The base of the condensed outer shelf/upper slope carbonates capping the Thini Chu Group ("topmost biocalcarenites") is also strongly heterochronous, being dated as Bolorian to Kubergandian-Murgabian at Bangba and as Djulfian-Dorashamian at Col Noir and Tilicho. Thermal subsidence associated with the opening of Neotethys thus began as early as the Early Permian.

Discovery of a crystalline basement and Early Ordovician marine transgression in the Karakorum mountain range, Pakistan

Abstract: A granite pluton intruding low-grade quartzites and migmatites lies north of the Karakorum axial batholith of Cretaceous to Miocene age. The pluton is covered by transgressive litharenite and slate that contain a fairly rich assemblage of acritarchs of Arenigian age (Early Ordovician), belonging to the Peri-Gondwana biogeographic province. This is the first time that crystalline basement has been found in Karakorum and acritarchs in central Asia. These discoveries add new evidence that the Karakorum microplate, together with the Helmand block of central Afghanistan and the Lhasa block of Tibet, belongs to a Cimmerian domain at the northern fringe of Gondwana.