Breakup of a supercontinent between 625 Ma and 555 Ma: new evidence and implications for continental histories   总被引：1，自引：0，他引：1  

Gerard C. Bond  Peter A. Nickeson  Michelle A. Kominz 《Earth and Planetary Science Letters》1984,70(2):325-345

A method developed recently for constructing tectonic subsidence curves in early Paleozoic miogeoclines has produced new evidence for the breakup of a late Proterozoic supercontinent. Tectonic subsidence analyses in miogeoclines of eastern and western North America, northwestern Argentina, the Middle East and northwestern Australia limit the timing of the continental breakup to between 625 and 555 Ma. These results refine the implications of a much broader range of radiometric ages of rift-related igneous rocks and biostratigraphic ages of the transition from active extension to passive subsidence in miogeoclines.

The recognition of the timing and extent of rifting has led to testable hypotheses for latest Proterozoic and early Paleozoic continental histories. Breakup and onset of drift along an extensive system of continental fractures within a relatively short period of time would generate a large amount of young ocean floor, thereby reducing the volume of the global ocean basin and causing a sea level rise. Maximum reduction of ocean basin volume would postdate the time of breakup, probably by about 70 m.y., placing the transgressive peak at a time not older then about 510–520 Ma. That age agrees well with the time of maximum flooding on the continents close to the end of the Cambrian. Restriction of the breakup to between 625 and 555 Ma reduces the time gap between an essentially intact late Proterozoic supercontinent and the oldest reliable paleomagnetic reconstruction of the dispersed continents at about 560 Ma. A continental reconstruction produced by rotating Laurentia and Baltica into Gondwana a minimum distance from the 560 Ma position is consistent with limited geologic data. However, that reconstruction places Laurentia and Baltica in low latitudes which is difficult to reconcile with the absence of evaporites in syn-rift complexes in both continents.  相似文献   

Large tectonic rotation of part of New Zealand in the last 5 Ma     

I.C. Wright  R.I. Walcott 《Earth and Planetary Science Letters》1986,80(3-4):348-352

Detailed paleomagnetic data from the Wairoa Syncline, a middle Miocene to the present forearc basin on the East Coast of the North Island, New Zealand, show that the rate of clockwise rotation for the last 5 Ma has been 7–8°/Ma of which less than 1.5°/Ma can be explained by apparent polar wander due to motion of the Australian or Pacific plates. This rotation is similar to a present-day rate of 7°/Ma determined from geodetic data. Between 5 and 20 Ma ago the rate of tectonic rotation is poorly determined and may be between 0° and 2°/Ma.

The change in the rate of rotation of the Wairoa Syncline around 5 Ma is probably related to a markedly different tectonic style in the New Zealand region within the last 5 Ma, associated with a change in position of the Euler poles of rotation for the Pacific-Australian plates.  相似文献   

Observations of multiple core reflections of the PnKP and SnKP type and regional variations at the base of the mantle     

Cedric Wright 《Earth and Planetary Science Letters》1973,19(4):453-460

Seismological results interpreted as evidence for large inhomogeneities near the base of the Earth's mantle below Hawaii have recently been published. It is possible to place constraints on the magnitude of such heterogeneities by identifying seismic phases multiply reflected within the Earth's core. The value of such a simple technique is illustrated by using array recordings of P and S5KP waves that have traversed the bottom of the mantle beneath Hawaii to show that there is no clear evidence for the unusual physical properties attributed to this region of the Earth. Identification of the phase S7KP is also reported.  相似文献   

Magmas,mineralization and sea-floor spreading     

J. B. Wright  Patricia Mc Curry 《International Journal of Earth Sciences》1973,62(1):116-125

Relationships between granite bodies and mineralization in Nigeria and southwest England are cited as examples to support the thesis that ore solutions associated with igneous bodies do not necessarily develop by magmatic crystallization processes, but may form as independent by-products of magma generation. Similar conclusions have been reached for the metal provinces of western North America (Noble, 1970). Some hydrothermal solutions develop by expulsion of connate brines, mobilized by the passage of hot magma through sediments. Others may develop at deeper levels.Long-lived geochemical culminations in the deep crust or upper mantle (Schuiling, 1967) provide a possible source of reactivated metalliferous emanations in both nonorogenic and orogenic environments. An additional source of metals for ore deposits associated with island arcs and continental margin tectonism and igneous activity, may be provided by the mechanism of sea-floor spreading.The bulk of ocean floor sediments is scraped off and acreted onto non-descending crustal plates at subduction zones²). However, if the lowermost layers of these sediments are sufficiently compacted, they may be carried down into the mantle on the descending oceanic plate.They contain considerable amounts of heavy metals, which could be remobilized to reappear as ore deposits in island arcs and mountain chains of Andean type.In this way, the major episode of late Mesozoic underflow of the northeastern Pacific floor beneath western North America may have been responsible for much of the contemporaneous mineralization of that region. If this view is correct, much of the rich mineralization in the Andes may have a similar origin.Differences in amount and composition of sea floor sediment consumed, could account for regional changes in metal provinces along the tectonic grain. Remobilization of metals at different depths along Benioff zones could explain regional changes across the tectonic grain.

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Zusammenfassung Beziehungen zwischen Granitkörpern und Mineralisation in Nigeria und Südwest-England werden als Beispiele angeführt, um die These zu stützen, daß Erzlösungen in Verbindung mit Magmatit-Körpern sich nicht notwendigerweise von magmatischen Kristallisationsprozessen ableiten, sondern sich als unabhängige Nebenprodukte bei der Magmenerzeugung bilden können.Ähnliche Schlußfolgerungen sind für die Erzprovinzen des westlichen Nordamerika gezogen worden (Noble, 1970). Einige hydrothermale Lösungen entwickeln sich durch Ausstoß von connaten Laugen, mobilisiert durch das Durchströmen heißer Magma durch Sedimente. Andere mögen sich in tieferem Niveau entwickeln.Langandauernde geochemische Kulminationen in der tiefen Erdkruste oder im oberen Mantel (Schuiling, 1967) schaffen eine mögliche Quelle von reaktivierten, metallhaltigen Emanationen sowohl unter nichtorogenen als auch unter orogenen Bedingungen. Eine zusätzliche Quelle von Metallen für Erzlagerstätten, die an Inselbögen und Kontinentalrand-Tektonik sowie magmatische Aktivität gebunden sind, mag durch den Mechanismus des Sea Floor Spreading hervorgerufen werden.Die Masse der Ozeanboden-Sedimente wird abgetragen und auf nicht absteigenden Krusten-Plateaus tieferer Zonen angehäuft¹). Wenn jedoch die untersten Schichten ausreichend verdichtet sind, können sie in den Erdmantel auf dem sinkenden Ozean-Plateau hinabgelangen. Sie enthalten beträchtliche Mengen von Schwermetallen, die als Erzlagerstätten remobilisiert in Inselbögen und Gebirgsketten vom Anden-Typus wieder in Erscheinung treten können.Auf diese Weise mag die größere Episode der spät-mesozoischen Unterströmung des nordöstlichen Pazifikbodens unter das westliche Nordamerika für den Hauptanteil der gleichzeitigen Mineralisation dieser Region verantwortlich gewesen sein. Falls dieser Gesichtspunkt richtig ist, mögen viele der reichen Mineralisationen in den Anden einen ähnlichen Ursprung haben.Unterschiede in Menge und Zusammensetzung des aufgezehrten Maaresbodensediments könnten als regionale Veränderungen in Metallprovinzen entlang der tektonischen Struktur gedeutet werden. Remobilisierung von Metallen in verschiedenen Tiefen entlang der Benioff-Zonen könnten regionale Veränderungen quer zur tektonischen Richtung erklären.

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Résumé Les auteurs prennent comme exemple les rapports existant entre les corps granitiques et la minéralisation en Nigérie et dans le sud-ouest de l'Angleterre pour soutenir la thèse que les solutions minéralisantes associées avec les corps magmatiques, ne se développent pas nécessairement à partir de processus de cristallisation magmatiques, mais peuvent se former comme sous-produits indépendants de la génération du magma. Semblables conclusions ont été tirées pour les provinces métalliques de l'ouest de l'Amérique du Nord (Noble, 1970). Quelques solutions hydrothermales se développent par l'expulsion de saumures connées, mobilisées par le passage du magma chaud à travers des sédiments. D'autres peuvent se développer à des niveaux plus profonds.Des culminations géochimiques de longue durée dans la croûte terrestre ou dans le manteau supérieur (Schuiling, 1967) fournissent une source possible d'émanations métallifères réactivées, dans des conditions aussi bien non-orogéniques qu'orogéniques. Une source additionelle de métaux pour les gisements qui, comme l'activité magmatique, sont associés avec les guirlandes d'îles et avec la tectonique propre à la bordure continentale, peut être fournie par le mécanisme de l'expansion du fond des mers.La masse des sédiments du fond océanique est érodée et accumulée sur des plateaux crustaux, non en voie d'affaissements de zones plus profondes³).Quand cependant les couches plus inférieures de ces sédements sont suffisamment compactées, elles peuvent être entraînées dans le manteau sur le plateau océanique en voie d'affaissement.Elles contiennent des quantités considéreables de métaux lourds qui, remobilisés sous la forme de gisements de minerais, peuvent apparaître dans les guirlandes d'îles et les chaînes de montagnes de type andin.De cette façon, l'épisode majeur de la subduction, survenue au Mésozoïque supérieur, du fond du Pacifique NE sous l'ouest de l'Amérique du Nord peut avoir été responsable de la majeure partie de la minéralisation de cette région. Si cette opinion est juste, beaucoup de riches minéralisations rencontrées dans les Andes pourraient avoir une origine similaire.Des différences dans la quantité et la composition du sédiment océanique absorbé pourraient expliquer les changements régionaux dans les provinces métalliques tout le long de la structure tectonique. La remobilisation des métaux à des profondeurs différentes le long des zones Benioff pourrait expliquer les changements régionaux survenant transversalement à la direction tectonique.

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Introduction     

H.E. Wright  M.L. Heinselman 《Quaternary Research》1973,3(3):319-328

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Origin of the younger granites of northern Nigeria     

J. B. Wright 《Contributions to Mineralogy and Petrology》1970,29(1):89-90

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Formation of the Green Tuff,Pantelleria     

J. A. Wolff  J. V. Wright 《Bulletin of Volcanology》1981,44(4):681-690

The strongly peralkaline Green Tuff, Pantelleria, is an example of a thin, densely welded air-fall tuff which mantles an area of at least 85 km². Offshore the tuff is correlated with the Y-6 ash layer in the central Mediterranean Sea, and the total volume of the eruption is estimated at 7 km³ D.R.E. New petrological data suggests that the tuff was erupted from a zoned magma chamber containing a cooler, more fractionated upper zone relative to be bulk of the magma. Analysis of the distribution of accessory lithic fragments in terms of existing models of eruption dynamics indicates emplacement by a plinian-type eruption. It is shown that, due to the low viscosity of pantelleritic ejecta, dense welding can occur at moderate tephra accumulation rates and a rate of the order of 1 cm/minute is suggested for the Green Tuff; this yields an estimate for the eruption duration of rather less than one day. It is predicted that welded tuff should be formed during large plinian eruptions of pantelleritic magma, and therefore that welded airfall tuffs should be common in areas of peralkaline volcanism.  相似文献   

The dali ash,island of rhodes,greece: a problem in interpreting submarine volcanigenic sediments     

J. V. Wright  E. Mutti 《Bulletin of Volcanology》1981,44(2):153-167

In the southern part of Rhodes, Greece, rhyolitic subaqueous pyroclastic deposits are interbedded with Tertiary, deep water, marine sediments. The lowermost and best exposed of these deposits — the Dali Ash — is described here. The deposit has been previously described as a deep water welded subaqueous ignimbrite. This paper shows that there is no evidence of welding, and texture previously reported were misidentified. The Dali Ash consists of a lower massive unit (5 m thick), overlain by a sequence of ash-turbidites (2.5 m thick). The lower unit was deposited by a high concentration turbidity current and the ash-turbidites by dilute turbidity currents. Foraminifera are dispersed throughout the deposit and indicate that all the sedimentary gravity flows were cold water/particulate systems. A palaeomagnetic study also suggests they were deposited cold. The Dali Ash can be interpreted as the lateral equivalent of a subaerial pumiceous pyroclastic flow deposit (ignimbrite). The ash-turbidites then may be redeposited slumps off the submarine slope of the lower massive unit, or, may represent later, smaller pyroclastic flows in the eruption. Other alternatives for the origin of the Dali Ash are fully discussed to show the problems in interpreting submarine volcanigenic sediments. It is possible that the deposits are not even a primary eruptive product and are remobilized pyroclastic debris, slumped, for example, off the sides of a shallow marine rhyolitic tuff ring.  相似文献   

Characterization and quantification of inorganic constituents of tropical peats and organic-rich deposits from Tasek Bera (Peninsular Malaysia): implications for coals     

Raphael A. J. Wüst  Colin R. Ward  R. Marc Bustin  Michelle I. Hawke 《International Journal of Coal Geology》2002,49(4)

Since the Carboniferous, tropical latitudes have been the site of formation of many economic coal deposits, most of which have a restricted range of mineralogical composition as a result of their depositional environment, climatic conditions, and diagenesis. Mineralogical and microscopic investigations of tropical peats from Tasek Bera, Peninsular Malaysia, were performed in order to better understand some of these factors controlling the nature, distribution and association of inorganic matter in peat-forming environments. Distribution and nature of the inorganic fraction of peat deposits give insight into the weathering conditions and detrital input into the mire system. Because the inorganic composition of peat deposits is determined by plant communities, height of water table, and climate, the results of the quantitative and qualitative analysis can be used to reconstruct palaeoclimatic conditions.Tasek Bera is a peat-accumulating basin in humid tropical Malaysia with organic deposits of low- to high-ash yield and thus representative of many ancient peat-forming environments. Clay minerals dominate the mineralogical composition of the peat and organic-rich sediments, while quartz and clays dominate the underlying siliciclastic deposits. Kaolinite is the most abundant clay mineral in the organic deposits with minor amounts of illite and vermiculite. Particle size analyses indicate that >50% of the inorganic detrital fraction is <2 μm. Most detrital quartz grains range in size from fine silt to fine sand. The fine sand fraction accounts for a maximum of 5 wt.% of the inorganic constituents. In addition, abundant biogenic and non-biogenic, Al- and Si-rich amorphous matter occur. In the ombrotrophic (low-nutrient) environment, biogenic inorganic material contributes up to >75% of the ash constituents. As a consequence, the vegetational communities make an important contribution to the inorganic and overall ash composition of peats and coals. The ash content of the often inundated peat consists on average of 10% opaline silica from diatoms and sponge spicules, while the ash of the top deposits may have up to 50% biogenic silica. Hence, Al- and Si-hydroxides and the opaline silica from diatoms and sponges represent a large repository of Al and Si, which may form the basis of mineral transformation, neoformation and alteration processes during coalification of the peat deposits. As a result, most coal deposits from paleotropical environments are anticipated to have little to no biogenic inorganic material but high amounts of secondary clays, such as kaolinite (detrital kaolinite, resilisified kaolinite, or desilisified gibbsite) or illite, and various amounts of detrital and authigenetic quartz.  相似文献   

The structure of the crust and upper mantle to depths of 320 km beneath the Kaapvaal craton, from P wave arrivals generated by regional earthquakes and mining-induced tremors     

C. Wright  M. T. O. Kwadiba  E. M. Kgaswane  R. E. Simon   《Journal of African Earth Sciences》2002,35(4)

Travel times from earthquakes recorded at two seismic networks were used to derive an average P wavespeed model for the crust and upper mantle to depths of 320 km below southern Africa. The simplest model (BPI1) has a Moho depth of 34 km, and an uppermost mantle wavespeed of 8.04 km/s, below which the seismic wavespeeds have low positive gradients. Wavespeed gradients decrease slightly around 150 km depth to give a ‘knee’ in the wavespeed-depth model, and the wavespeed reaches 8.72 km/s at a depth of 320 km. Between the Moho and depths of 270 km, the seismic wavespeeds lie above those of reference model IASP91 of Kennett [Research School of Earth Sciences, Australian National University, Canberra, Australia (1991)] and below the southern African model of Zhao et al. [Journal of Geophysical Research 104 (1999) 4783]. At depths near 300 km all three models have similar wavespeeds. The mantle P wavespeeds for southern Africa of Qiu et al. [Geophysical Journal International 127 (1996) 563] lie close to BPI1 at depths between 40 and 140 km, but become lower at greater depths. The seismic wavespeeds in the upper mantle of model BPI1 agree satisfactorily with those estimated from peridotite xenoliths in kimberlites from within the Kaapvaal craton.The crustal thickness of 34 km of model BPI1 is systematically lower than the average thickness of 41 km computed over the same region from receiver functions. This discrepancy can be partly explained by an alternative model (BPI2) in which there is a crust–mantle transition zone between depths of 35 and 47 km, below which seismic wavespeed increases to 8.23 km/s. A low-wavespeed layer is then required at depths between 65 and 125 km.  相似文献