Showing papers on "Gondwana published in 2000"

Tectonic implications of U-Pb zircon ages of the himalayan orogenic belt in nepal

Abstract: Metasedimentary rocks of the Greater Himalaya are traditionally viewed as Indian shield basement that has been thrust southward onto Lesser Himalayan sedimentary rocks during the Cenozoic collision of India and Eurasia. Ages determined from radioactive decay of uranium to lead in zircon grains from Nepal suggest that Greater Himalayan protoliths were shed from the northern end of the East African orogen during the late Proterozoic pan-African orogenic event. These rocks were accreted onto northern Gondwana and intruded by crustal melts during Cambrian-Ordovician time. Our data suggest that the Main Central thrust may have a large amount of pre-Tertiary displacement, that structural restorations placing Greater Himalayan rocks below Lesser Himalayan rocks at the onset of Cenozoic orogenesis are flawed, and that some metamorphism of Greater Himalayan rocks may have occurred during early Paleozoic time.

Episodic Silicic Volcanism in Patagonia and the Antarctic Peninsula: Chronology of Magmatism Associated with the Break-up of Gondwana

Abstract: New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and 40Ar–39Ar data are presented for the Jurassic silicic volcanic rocks and related granitoids of Patagonia and the Antarctic Peninsula. U–Pb is the only reliable method for dating crystallization in these rocks; Rb–Sr is prone to hydrothermal resetting and Ar–Ar is additionally affected by initial excess 40Ar. Volcanism spanned more than 30 My, but three episodes are defined on the basis of peak activity: V1 (188–178 Ma), V2 (172–162 Ma) and V3 (157–153 Ma). The first essentially coincides with the Karoo–Ferrar mafic magmatism of South Africa, Antarctica and Tasmania. The silicic products of V1 are lower-crustal melts that have incorporated upper-crustal material. The geochemistry of V2 and V3 ignimbrites is more characteristic of destructive plate margins, but the presence of inherited zircon still points to a crustal source. The pattern of volcanism corresponds in space and in time to migration away from the Karoo mantle plume towards the proto-Pacific margin of Gondwana during rifting and break-up. The heat required to initiate bulk crustal fusion may have been supplied by the spreading plume-head, but thinning of the crust during continental dispersion would also have facilitated anatexis.

Grenville-age basement provinces in East Antarctica: Evidence for three separate collisional orogens

Abstract: Three Grenville-age provinces can be distinguished in East Antarctica with U-Pb zircon data. The Maud, Rayner, and Wilkes provinces each have a distinctive age signature for late Mesoproterozoic–early Neoproterozoic magmatism and high-grade metamorphism and are correlated with similar rocks in the Namaqua-Natal (Africa), Eastern Ghats (India), and Albany-Fraser (Australia) provinces, respectively. These crustal segments represent three separate collisional orogens. They are separated by regions of intense late Neoproterozoic–Early Cambrian tectonism, consistent with their juxtaposition during the final assembly of Gondwana and indicating that previous models for a single, continuous, Grenville-age mobile belt around the East Antarctic coastline should be discarded.

Age and magmatic history of the Antananarivo Block, central Madagascar, as derived from zircon geochronology and Nd isotopic systematics

Abstract: We report single zircon 207 Pb/ 206 Pb evaporation and SHRIMP ages, combined with whole-rock Nd isotopic systematics for granitoid rocks from the Antananarivo Block (terrane), one of five tectono-metamorphic units making up the Precambrian basement of central and northern Madagascar. Our data reveal three distinct age groups at approximately 560 to 530, approximately 820 to 720, and 2520 to 2500 Ma respectively that reflect major magmatic events and correlate with similar events in various parts of East Africa and Sri Lanka but not in southwestern India. A widespread high-grade metamorphic event at approximately 550 Ma transformed many of the earlier granitoid gneisses into enderbite-charnockite assemblages. This granulite-facies event is common to Madagascar, East Africa, and southernmost India/Sri Lanka and reflects the final amalgamation of East and West Gondwana. Contrary to previous interpretations, there is a distinct lack of Kibaran-Grenvillian magmatism or metamorphism in Madagascar, making it unlikely that the island played a major role in the accretionary history and amalgamation of the supercontinent Rodinia. The widespread and voluminous granitoid magmatism at approximately 824 to 720 Ma remains enigmatic, and the tectonic scenario with which it is associated is difficult to reconstruct due to severe tectonic transposition of most gneisses. The Nd isotopic systematics as well as abundant zircon xenocrysts attest to extensive remelting of Archean and Paleoproterozoic crust. On presently available data the approximately 740 to 820 Ma granitoids are either related to magmatic underplating following plume generation, subcrustal mantle delamination during break-up and dispersal of Rodinia, or to continental arc magmatism related to subduction of the Mozambique ocean. They were emplaced into the ancient crust of central Madagascar as it lay either attached to East Africa or formed a microcontinent within the Mozambique ocean.

Evolution and Structure of the Lachlan Fold Belt (Orogen) of Eastern Australia

Abstract: The Lachlan Fold Belt (Lachlan Orogen) of eastern Australia was part of a Paleozoic convergent plate margin that stretched around the supercontinent of Gondwana from South America to Australia. Lower Paleozoic (545–365 Ma) deep-water, quartz-rich turbidites, calcalkaline volcanic rocks, and voluminous granitic plutons dominate the Lachlan Orogen. These rocks overlie a mafic lower crust of oceanic affinity. Shortening and accretion of the Lachlan occurred through stepwise deformation and metamorphism from Late Ordovician (∼450 Ma) through early Carboniferous times, with dominant events at about 440–430 Ma and 400–380 Ma. The development and accretion of the Lachlan Orogen and other related belts within the Tasmanides added about 2.5 Mkm2 to the surface area of Gondwana. The sedimentary, magmatic, and deformational processes converted an oceanic turbidite fan system into continental crust of normal thickness. The addition of this recycled continental detritus and juvenile material to Australia represents an...

Making ends meet in Gondwana: retracing the transforms of the Indian Ocean and reconnecting continental shear zones

Abstract: Interpretation of the detailed patterns of ocean-floor transforms revealed by satellite altimetry enables the creation of the Indian Ocean to be described quantitatively as four consecutive plate-tectonic regimes separated at 200, 136, 89 and 43 Ma. Each regime is reversed in turn by keeping transform termini coincident and colinear until conjugate points on the margins of pre-existing plates regain their pre-regime integrity. Progressive elimination of the Indian Ocean, demonstrable as a smooth computer animation (http://www.kartoweb.itc.nl/gondwana), leads to a refined re-assembly of the continental fragments of central Gondwana that is substantiated by new geological data. A sequence of Euler interval poles that describes the dispersal of the Gondwana fragments, time-calibrated against available magnetic anomaly data, is given. The model requires a mid-Cretaceous position for India’s southern tip about 1000 km south of Madagascar, prior to India’s rapid northward migration.

Rapid onset of late Paleozoic glaciation on Gondwana: Evidence from Upper Mississippian strata of the Midcontinent, United States

Abstract: Direct evidence of the late Paleozoic glaciation of Gondwana from glacial deposits suggests that geographically extensive continental glaciation began some time in the Namurian (Late Mississippian). However, the timing and characteristics of the onset of glaciation are poorly understood because of a lack of reliable paleontological control and reworking of initial glacial deposits by subsequent glacial advances. Indirect evidence of glaciation preserved in unconformity-bounded, low-latitude ramp sequences in the Illinois basin, United States, suggests that geographically extensive continental glaciation of Gondwana actually began in the late Visean. An abrupt change from carbonate-dominated sequences bounded by disconformities with little evidence of erosion to mixed carbonate-siliciclastic sequences bounded by unconformities with deep incised valleys was likely produced by a three-fold increase in the magnitude of eustatic sea-level fluctuations. The increase in the magnitude of sea-level fluctuations was likely driven by an equally abrupt increase in ice volume and marks the onset of the geographically extensive late Paleozoic glaciation of Gondwana. A possible explanation for the rapid onset of glaciation is the closing of the equatorial seaway between Laurussia and Gondwana. Closing of this seaway would have led to an abrupt change in oceanic and atmospheric circulation patterns that could have initiated major continental glaciation in the Southern Hemisphere.

From Cadomian subduction to Early Palaeozoic rifting: the evolution of Saxo-Thuringia at the margin of Gondwana in the light of single zircon geochronology and basin development (Central European Variscides, Germany)

Abstract: Abstract Saxo-Thuringia is classified as a tectonostratigraphic terrane belonging to the Armorican Terrane Collage (Cadomia). As a former part of the Avalonian-Cadomian Orogenic Belt, it became (after Cadomian orogenic events, rift-related Cambro-Ordovician geodynamic processes and a northward drift within Late Ordovician to Early Silurian times), during Late Devonian to Early Carboniferous continent-continent collision, a part of the Central European Variscides. By making use of single zircon geochronology, geochemistry and basin analysis, geological processes were reconstructed from latest Neoproterozoic to Ordovician time: (1) 660–540 Ma: subduction, back-arc sedimentation and tectonomagmatic activity in a Cadomian continental island-arc setting marginal to Gondwana; (2) 540 Ma: obduction and deformation of the island arc and marginal basins; (3) 540–530 Ma: widespread plutonism related to the obduction-related Cadomian heating event and crustal extension; (4) 530–500 Ma: transform margin regime connected with strike-slip generated formation of Early to Mid-Cambrian pull-apart basins; (5) 500–490 Ma: Late Cambrian uplift and formation of a chemical weathering crust; (6) 490–470 Ma: Ordovician rift setting with related sedimentation regime and intense igneous activity; (7) 440–435 Ma: division from Gondwana and start of northward drift. The West African and the Amazonian Cratons of Gondwana, as well as parts of Brittany, were singled out by a study of inherited and detrital zircons as potential source areas in the hinterland of Saxo-Thuringia.

Geological History of Britain and Ireland

Abstract: Part I: Introduction:. 1. Regional Geological History: Why And How:N. H. Woodcock & R. A. Strachan. 2. Geological Framework Of Britain And Ireland: R. E. Holdsworth, N. H. Woodcock & R. A. Strachan. Part II: The Northern Margin Of The Iapetus Ocean:. 3. Early Earth History And Development Of The Archaean Crust: R. A. Strachan. 4. Proterozoic Sedimentation, Orogenesis And Magmatism On The Laurentian Craton: R. A. Strachan & R. E. Holdsworth. 5. Late Neoproterozoic To Early Ordovician Passive Margin Sedimentation Along The Laurentian Margin: R. A. Strachan & R. E. Holdsworth. 6. The Grampian Orogeny: Mid-Ordovician Arc-Continent Collision Along The Laurentian Margin: R. A. Strachan. 7. Mid-Ordovician To Silurian Sedimentation And Tectonics On The Northern Active Margin Of Iapetus: R. A. Strachan. Part III: The Southern Margin Of The Iapetus Ocean:. 8. Neoproterozoic To Cambrian Accretionary History Of Eastern Avalonia And Armorica: R. A. Strachan. 9. The Cambrian And Earliest Ordovician Quiescent Margin Of Gondwana: N. H. Woodcock. 10. Ordovician Volcanism And Sedimentation On Eastern Avalonia: N. H. Woodcock. 11. Late Ordovician To Silurian Evolution Of Eastern Avalonia During Convergence With Laurentia: N. H. Woodcock. Part IV: The End Of The Iapetus Ocean:. 12. The Caledonian Orogeny: A Multiple Plate Collision: N. H. Woodcock & R. A. Strachan. 13. Devonian Sedimentation And Volcanism On The Old Red Sandstone Continent: N. H. Woodcock. Part V: The Variscan Cycle: Consolidation Of Pangaea:. 14. Carboniferous Sedimentation And Volcanism On The Laurussian Margin: P. D. Guion, P. Gutteridge & S. J. Davies. 15. The Variscan Orogeny: The Welding Of Pangaea: L. N. Warr. Part VI: Post-Variscan Intraplate Setting:. 16. Permian To Late Triassic Post-Orogenic Collapse And Early Atlantic Rifting: A. H. Ruffell & R. G. Shelton. 17. Late Triassic To Jurassic: The Beginning Of The End For Pangaea: S. P. Hesselbo. 18. Early Cretaceous: Rifting And Sedimentation Before The Flood: A. S. Gale. 19. Late Cretaceous To Early Tertiary Deposition Through The Global Highstand: A. S. Gale. 20. Tertiary Events: The North Atlantic Plume And Alpine Pulses: R. Anderton. 21. The Quaternary: The History Of An Ice Age: N. H. Woodcock

Palaeomagnetism and Palaeozoic palaeogeography of Gondwana and European terranes

Abstract: Abstract Neoproterozoic to Late Palaeozoic times saw the break-up of the supercontinent Rodinia, and the subsequent construction of Pangaea. The intervening time period involved major redistribution of continents and continental fragments, and various palaeogeographical models have been proposed for this period. The principal differences between these models are with regard to the drift history of Gondwana, the timing of collision between northern Africa and Laurussia, and formation of Pangaea. Palaeomagnetic evidence provides basically two contrasting models for the Ordovician to Late Devonian apparent polar wander (APW) path for Gondwana involving either rapid north and southward movement of this continent, or gradual northward drift throughout Palaeozoic time. In contrast, palaeobiogeographical models suggest contact between Laurussia and Gondwana as early as mid-Devonian time with the continents basically remaining in this configuration until break-up of Pangaea in the Mesozoic era. This is in conflict, however, with most palaeomagnetic data, which demonstrate that in Late Devonian time, north Africa and the European margin of Laurussia were separated by an ocean of at least 3000 km width. This is also in agreement with the geological record of present-day southern Europe, which argues against any collision of northern Africa with Europe in Devonian time. With regard to formation of Laurussia, however, palaeobiogeographical and palaeomagnetic data are in excellent agreement that by mid-Devonian time the oceanic basins separating Baltica, Laurentia, Gondwana-derived Avalonia and the Armorican Terrane Assemblage (ATA) had all closed. Palaeomagnetic and geological data are also in agreement that the Palaeozoic basement rocks of the European Alpine realm formed an independent microplate, which was situated to the south of Laurussia. In Late Silurian times it was separated by an ocean of c. 1000 km, and by Late Devonian time was approaching the southern Laurussian margin. According to palaeomagnetic data, the northern margin of Gondwana was still further to the south in Late Devonian time, and according to the geological record in southern Europe, the main continent-continent collision of northern Africa with European Laurussia and closure of the intervening ocean occurred in Late Carboniferous times. Location of this suture is situated to the south of the Palaeozoic alpine units (e.g. the Greywacke zone, Carnic Alps, Sardinia and Sicily), but has been obscured by younger deformational events and cannot be precisely positioned. Assessing available evidence and as discussed in the text, it is proposed that the most likely scenario is that the northern margin of Gondwana drifted gradually northwards from Ordovician to Late Carboniferous times when it collided with Laurussia, resulting in formation of Pangaea.

New look at the Siberian connection: No SWEAT

Abstract: The Proterozoic connection between northeastern Siberia and western Laurentia that we proposed in 1978 is strongly supported by several new lines of evidence. New age data and refined structural trends in predrift basement rocks improve the resolution of the fit between the cratons. The mouth of the large river that is inferred to have provided the point source for the lower part of the Mesoproterozoic Belt-Purcell Supergroup in western Laurentia aligns with the Mesoproterozoic Udzha trough of Siberia. The elbow bend in the Udzha trough bypasses the Archean Wyoming Province to link the Belt-Purcell basin with Paleoproterozoic regions in southwest Laurentia having appropriate Nd crustal-residence ages and zircon crystallization ages to have provided sources for much of the sediment. The Grenville and Granite-Rhyolite provinces of southwest Laurentia provide sources for detrital zircons and felsic volcanic fragments in the east-derived Mesoproterozoic Mayamkan Formation of Siberia. The ages of mafic sills in the Sette-Daban region of Siberia overlap those in southwest Laurentia. Ediacara occur in off-shelf environments on both margins. The two margins have very similar latest Neoproterozoic–earliest Cambrian rift-drift signatures, including a breakup unconformity and Tommotian shelf assemblages that record the onset of thermally driven subsidence. Two possible submarine volcanoes with archeocyathan caps may confirm the establishment of Early Cambrian seafloor spreading. The Siberian–west Laurentian connection provides better correlations among prerift terranes than does the southwest United States–East Antarctic connection (SWEAT), and is more compatible with the overall geologic history of Laurentia and Gondwana.

Continental rift to back‐arc basin: Jurassic–Cretaceous stratigraphical and structural evolution of the Larsen Basin, Antarctic Peninsula

Abstract: The Larsen Basin developed in Jurassic times as a result of continental rifting during the early stages of Gondwana break‐up. Lower‐?Upper Jurassic non‐marine sedimentary and volcanic rocks constitute a syn‐rift megasequence recording initial largely amagmatic extension and subsequent widespread extension‐related silicic volcanism. A succeeding, Kimmeridgian–early Berriasian transgressive megasequence, consisting largely of anoxic‐dysoxic hemipelagic mudstones, is thought to have been deposited during a thermal subsidence phase when relative magmatic quiescence and peak Jurassic eustatic sea levels served to maximize sediment starvation. The fragmentary record for late Berriasian–Barremian times suggests that a ?regressive megasequence may have developed in the earlier part of this period, recording increased sediment yield to the Larsen Basin from the increasingly emergent Antarctic Peninsula arc. Subsequently, strata in the southern, but not the northern, part of the basin underwent relatively intense eastward‐verging deformation, possibly during the formation of a retro‐arc fold‐thrust belt. Where exposed, the lower part of the succeeding Aptian–Eocene megasequence consists of a deep‐marine clastic wedge deposited along the fault‐bounded western basin margin during a phase of arc uplift and related differential subsidence. Following partial basin inversion in Late Cretaceous times, regression took place as reduced basinal subsidence rates allowed shallow marine facies to prograde basinward.

Marie Byrd Land, West Antarctica: Evolution of Gondwana's Pacific margin constrained by zircon U-Pb geochronology and feldspar common-Pb isotopic compositions

Abstract: The Paleozoic and Mesozoic development and subsequent fragmentation of Gondwana's Pacific margin are recorded in igneous and metamorphic rocks that crop out in Marie Byrd Land, West Antarctica, recognized on geologic and paleomagnetic grounds to compose a discrete crustal block. Widespread metaluminous granitoids dated by the zircon U-Pb method as middle to late Paleozoic show that convergence-related magmatism dominated the early evolution of this margin. Dates for granodiorites, monzogranites, and granites from the Ruppert and Hobbs coasts of western Marie Byrd Land reveal a prolonged period of subduction-related calc-alkaline magmatism between at least 320 ± 3 Ma (age of the oldest granodiorite dated) and 110 ± 1 Ma (the age of the youngest I-type granitoid in the area). The latter, known as the Mount Prince granite, is intruded by swarms of mafic and intermediate dikes believed to record the onset of rifting that led to separation of the New Zealand microcontinent. The dikes have been dated by zircon U-Pb as 101 ± 1 Ma. Thus, the regime along the Ruppert and Hobbs coasts had shifted from subduction-related to rift-related magmatism within an ∼9 m.y. period. In the Kohler Range and the Pine Island Bay areas of eastern Marie Byrd Land, the calc-alkaline magmatism did not terminate until 96 ± 1 Ma, based on U-Pb dating of zircons from one granitoid sample, or 94 ± 3 Ma based on zircons from another. This evidence requires that subduction shut off from west to east, as suggested previously on the basis of geophysical models. No continental separation occurred to the east of Marie Byrd Land. The margins of the Thurston Island and Antarctic Peninsula crustal blocks went directly from convergent to inactive, except at the northernmost tip of the peninsula, where the South Shetlands Island block is actively separating. With their zircon U-Pb ages clustering around 100 ± 2 Ma, dike-free anorogenic syenites and quartz syenites along the Ruppert and Hobbs coasts show that the transition to extensional magmatism was rapid in the west. This is also reflected by the fact that from the onset of rifting at 101 ± 1 Ma to formation of oceanic crust between Marie Byrd Land and greater New Zealand (Campbell Plateau, Chatham Rise, North Island, and South Island) prior to chron 33o ca. 81 Ma required only 20 m.y. For comparison, this is only two-thirds of the ∼30 m.y. it took for the Central Atlantic to open after initial rift-related magmatism. The swiftness of the separation between Marie Byrd Land and greater New Zealand demonstrated by our data is consistent with ridge-trench interaction rather than a mantle plume as the primary cause of the breakup, as is the west to east diachroneity in the cessation of subduction. Exposures of host rocks to the erosion-resistant plutons are scarce in mostly snow- and ice-covered Marie Byrd Land. The occurrence in the zircons of widely separated granitoids of discordant U-Pb patterns we attribute to inheritance (the best-constrained upper concordia intercepts are as high as 1576 ± 55 Ma). This suggests either that stretched Precambrian basement underlies most of Marie Byrd Land, or that clastic sedimentary sequences with Precambrian detrital zircons underlie much of the margin.

The eastern Palmer Land shear zone: a new terrane accretion model for the Mesozoic development of the Antarctic Peninsula

Abstract: A major ductile fault zone, the eastern Palmer Land shear zone, has been identified east of the spine of the southern Antarctic Peninsula. This shear zone separates newly identified geological domains, and indicates that during Late Jurassic terrane accretion and collision, two and possibly three separate terranes collided, resulting in the Palmer Land orogeny. The orogeny is best developed in eastern Palmer Land and eastern Ellsworth Land. There, shallow-marine sedimentary rocks of the Latady Formation, and a metamorphic and igneous basement complex of possible Lower Palaeozoic to pre-Early Jurassic age, are thrust and folded. This forms an arcuate, east-directed, foreland, fold and thrust belt up to 100 km wide and 750 km long, parallel to the axis of the Antarctic Peninsula. The newly identified Antarctic Peninsula domains include: (1) a parautochthonous Eastern Domain that represents part of the margin of the Gondwana continent, comparable to the Western Province of New Zealand, the Ross Province superterrane of Marie Byrd Land, the Eastern Series of south-central Chile, the Pampa de Agnia and Tepuel rocks of north Patagonia, and the Cordillera Darwin rocks of Tierra del Fuego, (2) a suspect Central Domain that represents an allochthonous, microcontinental, magmatic arc terrane, comparable to the Median Tectonic Zone of New Zealand, the Amundsen Province superterrane of Marie Byrd Land, and Coastal Cordillera of north Chile and (3) a suspect Western Domain, with strong similarities to the Eastern Province of New Zealand, Western Series of south-central Chile, and Chonos metamorphic complex of north Patagonia, that represents either a subduction–accretion complex to the Central Domain, or another separate crustal fragment. Although an allochthonous terrane hypothesis for the Antarctic Peninsula remains to be fully tested, this has much in common with models for the New Zealand and South American parts of the Pacific margin of Gondwana. The identification of a potential allochthonous terrane–continent collision zone allows us to define the edge of the Gondwana continent in the Antarctic Peninsula sector of the supercontinent margin, which has implications for Mesozoic reconstructions of Gondwana.

Distribution and biogeography of non-marine cretaceous turtles

Abstract: During the Cretaceous, non-marine turtles show strong patterns of provincialism, mirroring the pattern of land masses resulting from the breakup of Pangea since the Jurassic. These patterns are a result of several factors, of which vicariancy and ecological controls on the distribution of groups of turtles are considered the most significant. The large scale patterns, such as the dominance of pleurodires in the southern land masses, including Africa, South America, and India, and the dominance of Cryptodires in the northern land masses cannot be strictly attributed to vicariancy because exceptions to both distributional patterns are present. The pleurodires in Europe and North America during the Late Cretaceous may reflect the removal of the barriers that prevented the terrestrial faunal interchange between the northern and southern continents. Two groups of cryptodires that occurred in the southern continents during the Cretaceous, the Meiolaniidae and Otwayemys , seem to reflect a widespread distribution of the very primitive cryptodires which were diverse prior to the breakup of Pangea in the Early or possibly Middle Jurassic. In Laurasia, three regions of turtle diversity can be identified, the North American region, the European region, and the Asian region. In the Early Cretaceous, North American region is dominated by members of the Paracryptodira, and the Asian region is dominated by members of the Eucryptodira. Europe includes taxa from both groups. In the Late Cretaceous, Eucryptodires become increasingly more abundant and diverse in North America. The Baenidae which are not found outside North America appears to be truly endemic to this region. Two groups of “modern,” non-marine cryptodires or Chelomacryptodira, the Testudinoidea and the Trionychoidea, appear to have an Asian origin. Both have their earliest record in the Neocomian of Japan.

Was the Late Triassic orogeny in Turkey caused by the collision of an oceanic plateau

Abstract: A belt of Late Triassic deformation and metamorphism (Cimmeride Orogeny) extends east-west for 1100 km in northern Turkey. It is proposed that this was caused by the collision and partial accretion of an Early-Middle Triassic oceanic plateau with the southern continental margin of Laurasia. The upper part of this oceanic plateau is recognized as a thick Lower-Middle Triassic metabasite-marble-phyllite complex, named the Niltifer Unit, which covers an area of 120 000 km 2 with an estimated volume of mafic rocks of 2 x 105 km 3. The mafic sequence, which has thin stratigraphic intercalations of hemipelagic limestone and shale, shows consistent within-plate geochemical signatures. The Niliifer Unit has undergone a high-pressure greenschist facies metamorphism, but also includes tectonic slices of eclogite and blueschist with latest Triassic isotopic ages, produced during the attempted subduction of the plateau. The short period for the orogeny (< 15 Ma; Norian-Hettangian) is further evidence for the oceanic plateau origin of the Cimmeride Orogeny. The accretion of the Niltifer Plateau produced strong uplift and compressional deformation in the hanging wall. A large and thick clastic wedge, fed from the granitic basement of the Laurasia, represented by a thick Upper Triassic arkosic sandstone sequence in northwest Turkey, engulfed the subduction zone and the Niltifer Plateau. An east-west trending belt of latest Triassic deformation and regional metamorphism extends for over 1100 km in northern Turkey. The Early Mesozoic deformation (but not the regional metamorphism) was known previously ($eng6r 1979; Bergougnan & Fourquin 1982) and was referred to as the Cimmeride deformation ($eng6r et al. 1984). The Cimmeride deformation was ascribed to the closure of the Palaeotethys ocean following the collision of a Cimmerian continental sliver with the southern margin of Laurasia ($eng6r 1979; Seng6r et al. 1984). Here, an alternative explanation, involving the collision and partial accretion of an oceanic plateau to the southern margin of Laurasia, is proposed for the origin of the latest Triassic deformation and metamorphism in northern Turkey. A tectonic map of Turkey and the surrounding region is shown in Fig. 1. During the Palaeozoic and Mesozoic, the various continental blocks that make up present-day Turkey were situated on the continental margins of the Tethys Ocean. The Pontides, which comprise the Strandja, istanbul and Sakarya Zones, show Laurasian stratigraphic affinities, while the Anatolide-Tauride Block and the Klr~ehir Massif are tectonically and stratigraphically related to Gondwana ($eng6r & Yllmaz 1981; Okay et aL 1996; Okay & Ttiystiz 2000). The istanbul Zone is a continental fragment, which was translated south from the Odessa Shelf with the Cretaceous opening of the oceanic West Black Sea Basin (Fig. 1; Okay et al. 1994). Its stratigraphy is similar to that of the Scythian and Moesian platforms, with a fully developed Palaeozoic sedimentary sequence unconformably overlain by Triassic and younger sedimentary rocks (Haas 1968; Dean et al. 1997; G6rtir et al. 1997). In the Istanbul Zone, a weak latest Triassic deformation is marked by an unconformity between the Norian siliciclastic turbidites and the overlying Upper Cretaceous carbonates. The Strandja Zone consists of a Late Hercynian metamorphic and granitic basement unconformably overlain by Lower Triassic-Middle Jurassic sedimentary rocks (Chatalov 1988; Okay et al. 1996). The Anatol ide-Tauride Block and the Klr~ehir Massif are also devoid of Triassic metamorphism, and of any significant Triassic deformation. Several well studied Lower Mesozoic stratigraphic sections in the Taurides, including those in the Bornova Flysch Zone (Erdo~an et al. 1990.). and in the central Taurides (Gutnic et al. 1979; Ozgti11997), show a continuous transition between Triassic and Jurassic with no evidence of an intervening deformation phase. The pre-Jurassic thrusting, described by Monod & Akay (1984) from a small locality in the central Taurides, is as yet of unknown significance. Late Triassic deformation and regional metamorphism in Turkey are predominantly found in the Sakarya Zone, which will form the main subject of this paper. From: BOZKURT, E., WINCHESTER, J. A. & PIPER, J. D. A. (eds) Tectonics and Magmatism in Turkey and the Surrounding Area. Geological Society, London, Special Publications, 173, 25-41.1-86239-064-9/00/$15.00 (C) The Geological Society of London 2000.

Deducing the ancestry of terranes: SHRIMP evidence for South America–derived Gondwana fragments in central Europe

Abstract: We present here an example of how the sensitive high-resolution ion microprobe (SHRIMP) zircon dating method can provide a terrane-specific geochronological fingerprint for a rock and thus help to reveal major tectonic boundaries within orogens. This method, applied to inherited zircons in a ca. 580 Ma metagranitoid rock from the eastern Bohemian Massif, has provided, for the first time in the central European Variscan basement, unequivocal evidence for Mesoproterozoic and late Paleoproterozoic geologic events ca. 1.2 Ga, 1.5 Ga, and 1.65–1.8 Ga. The recognition of such zircon ages has important consequences because it implies that parts of the Precambrian section of Variscan central Europe were originally derived from a Grenvillian cratonic province, as opposed to the common assumption of an African connection. A comparison with previously published SHRIMP data suggests, however, that these Mesoproterozoic and late Paleoproterozoic zircon ages may be restricted to the Moravo-Silesian unit in the eastern Variscides, whereas the Saxothuringian and Moldanubian zones appear to contain a typical north African (i.e., Neoproterozoic plus Eburnian) inherited-zircon age spectrum. This finding supports new tectonic concepts, according to which Variscan Europe is composed of a number of completely unrelated terranes with extremely different paleogeographic origins. The Moravo-Silesian unit can be best interpreted as a peri-Gondwana terrane, which was situated in the realm of the Amazonian cratonic province by the late Precambrian, comparable to the Avalonian terranes of North America and the United Kingdom.

Weddell triple junction: The principal focus of Ferrar and Karoo magmatism during initial breakup of Gondwana

Abstract: Middle Jurassic Ferrar tholeiites of Antarctica were emplaced during a short time interval (<1 m.y.) into an active rift system initiated in the Early Jurassic. Ferrar magmas were dispersed into the Antarctic sector of Gondwana from a source in the Weddell Sea region, within the thermal anomaly that also gave rise to the Karoo basaltic rocks of southern Africa. The Golden Gate lavas, part of the Karoo central area low-Ti tholeiites, show geochemical similarities to Ferrar rocks. Tectonic and geochemical relationships of the Ferrar and Karoo low-Ti magmas, which constitute a major part of the Jurassic Gondwana large igneous province, suggest that they were derived from a single source associated with a triple junction in the proto-Weddell Sea region.

The Late Palaeozoic relations between Gondwana and Laurussia

Abstract: Abstract Reconstructions based on biogeography, palaeomagnetism and facies distributions indicate that, in later Palaeozoic time, there were no wide oceans separating the major continents. During the Silurian and Early Devonian time, many oceans became narrower so that only the less mobile animals and plants remained district. There were several continental collisions: the Tornquist Sea (between Baltica and Avalonia) closed in Late Ordovician time, the Iapetus Ocean (between Laurentia and the newly merged continents of Baltica and Avalonia) closed in Silurian time, and the Rheic Ocean (between Avalonia and Gondwana and the separate parts of the Armorican Terrane Assemblage) closed (at least partially) towards the end of Early Devonian time. Each of these closures was reflected by migrations of non-marine plants and animals as well as by contemporary deformation. New maps, based on palaeomagnetic and faunal data, indicate that Gondwana was close to Laurussia during the Devonian and Carboniferous periods, with fragments of Bohemia and other parts of the Armorican Terrane Assemblage interspersed between. It follows that, after Early Devonian time, the Variscan oceans of central Europe can never have been very wide. The tectonic evolution of Europe during Devonian and Carboniferous time was thus more comparable with the present-day Mediterranean Sea than with the Pacific Ocean.

The Early Palaeozoic geography of Europe

Abstract: In the early Ordovician three main continents are recognized by faunas in Europe, Laurentia (northwest of the closed suture of the Iapetus Ocean), which was tropical; Baltica (north of the general area of the Trans‐European Suture Zone), which was temperate; and Gondwana (south of the TESZ), which was high latitude. As the Ordovician progressed, various terranes separated and drifted away from the Gondwana supercontinent at different times, namely Avalonia (which was probably originally part of Gondwana near South America), Iberia–Armorica, Perunica (Bohemia) and various Alpine fragments. Each terrane developed progressively different faunas as time went by. Avalonia collided with Baltica near the end of Ordovician, confirmed by faunas, tectonics and palaeomagnetism, and subsequently Avalonia–Baltica collided with Laurentia to form Laurussia from the mid‐Silurian to the early Devonian. The Ibero‐Armorica, Perunica and Alpine terranes did not join the European collage until the Devonian. The modern sites of the terrane boundaries bear little direct relationships to the original palaeogeographical boundaries in the Early Palaeozoic.