Present-day geodynamics in the bend of the western and central Alps as constrained by earthquake analysis   总被引：3，自引：0，他引：3  

Bastien Delacou  Christian Sue  Jean-Daniel Champagnac  Martin Burkhard 《Geophysical Journal International》2004,158(2):753-774

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Aspects of the mineralogy of alnöitic breccia,Malaita, Solomon Islands; comparison with continental kimberlites     

J. B. Dawson  J. S. Delaney  J. V. SmitH 《Contributions to Mineralogy and Petrology》1978,67(2):189-193

Megacrysts of titanian pyrope, high-Ti bronzite, sub-calcic diopside to augite, and phlogopite in an alnöitic breccia, Malaita, Solomon Islands, have compositional features which distinguish them from silicates in kimberlites. Particularly important is the high content of Al₂O₃ in the Malaita silicates, which may reflect the higher Al₂O₃ content of alnöite than kimberlite. The phlogopite megacrysts fall into two populations, one richer in Mg and Cr, and lower in Ti, than the other. The pyroxenes may also fall into two groups characterized by high and low Mg, Cr and Ni. Two stages of crystallization may be responsible.  相似文献   

Primary ionization coefficients for dry and moist air; secondary ionization coefficients for a water surface     

R. R. Stout  G. A. Dawson 《Pure and Applied Geophysics》1978,116(1):159-166

Summary A comparison has been made between the Townsend primary ionization coefficient, , for dry air and for air with humidities typical of those in the atmosphere. is defined as the number of new electrons produced by an electron per centimeter of drift in a field. A range of field/pressure ratios,E/p ₀, of 40 to 100 V (cm torr)^–1 was employed. The variation of with humidity is very small.Over the range ofE/p ₀ from 50 to 100 V (cm torr)^–1, the secondary ionization coefficient, , of a water surface has been found from sparking potential data to be typically 2×10^–4. represents the fraction of primary ionizing collisions that ultimately result in the production of additional electrons at the surface.  相似文献   

Geology of the Olduvai Gorge: A study of sedimentation in a semi-arid basin: R.L. Hay. Univ. California Press,London, 1976, 203 pp. £ 15.75     

J.B. Dawson 《Sedimentary Geology》1978,21(1):68-70

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A faecal sterol survey in the Clyde Estuary     

R. M. Goodfellow  J. Cardoso  G. Eglinton  J. P. Dawson  G. A. Best 《Marine pollution bulletin》1977,8(12):272-276

5β-Stanols, and in particular coprostanol, which is one of the principal sterols of human sewage, show promise as chemical tracers of faecal pollution in estuaries. In the present survey, coprostanol has been measured gas chromatographically in the waters and sediments of the Clyde Estuary. Thus, the bottom sediments contained 0.1–14 ppm of coprostanol (5β-cholestan-3β-ol) together with the corresponding 24-methyl- and 24-ethyl-5β-stanols, while in the seawater samples, the coprostanol concentrations closely paralled the faecal coliform counts. Overall, the amounts of coprostanol correlate well with the location and amounts of the known inputs of sewage from outfalls and dumping.  相似文献   

The MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths in kimberlite     

J.Barry Dawson  Joseph V. Smith 《Geochimica et cosmochimica acta》1977,41(2):309-323

Within the ‘glimmerite’ nodules occurring within kimberlite pipes we recognize the MARID suite consisting of varying proportions of mica, amphibole, rutile, ilmenite and diopside. Banding of some specimens is interpreted as cumulate layering. All specimens were deformed either before incorporation into the host kimberlite or during intrusion. Compared with minerals in peridotite xenoliths, the MARID ones are lower in Al₂O₃ and Cr₂O₃, but richer in total iron. The MARID micas, amphiboles, diopsides, ilmenites and probably rutiles contain substantial Fe₂O₃ indicative of oxidizing conditions. The amphibole is potassic richterite. Micas of the megacryst suite in kimberlite have less total iron and Fe₂O₃ than micas of the MARID suite. We suggest that the rocks of the MARID suite crystallized under oxidizing conditions from a magma, chemically similar to kimberlite, within the higher parts of the upper mantle: the presence of amphibole restricts the depth to less than ~ 100 km. A xenolith containing olivine and orthopyroxene as well as minerals similar to but not the same compositionally as MARID-types is interpreted as a metasomite, possibly representing wall-rock of a magma body from which MARID-suite rocks crystallized.  相似文献   

The stratigraphy of flandrian relative sea-level changes at a site in tayside,Scotland     

D. E. Smith  A. G. Dawson  R. A. Culungford  D. D. Harkness 《地球表面变化过程与地形》1985,10(1):17-25

Peat deposits in a gully at the inland margin of the Firth of Tay carselands contain a transgressive wedge of estuarine sediments laid down at the culmination of the Main Postglacial Transgression. In previous work, the sequence of deposits in the gully and their age have been examined at one location. The present study both extends and evaluates that previous work, employing further field work and laboratory analyses. The detailed stratigraphy of the deposits is identified, whilst from a borehole close to the limit of the transgressive deposit, diatom analysis and geochemical analyses, involving the determination of Al, Mg, C, and δ ¹³C_PDB are outlined. Eight ¹⁴C assays from peat at the transgressive and regressive contacts in four boreholes along the wedge of estuarine sediments are described. Whilst the results of diatom analysis are inconclusive, the geochemical analyses indicate that the sediment accumulated broadly continuously without major changes in the depositional environment. It is concluded that the Main Postglacial Transgression in the gully was at first rapid, then slowed and culminated between 6240 ± 80 and 6030 ± 80 (or 6170 ± 90) radiocarbon years, B.P. It is subsequently maintained that the geochemical analyses described here may be of value in determining the continuity of transgressive deposits, whilst the practice of obtaining multiple radiocarbon assays at such locations should enable assessment of age to be refined. This may in turn assist in testing the hypothesis of shoreline diachroneity in glacioisostatically affected areas.  相似文献   

Environmental reconstructions of a late devensian terrace sequence. Some preliminary findings     

Martin Dawson 《地球表面变化过程与地形》1985,10(3):237-246

Facies analysis of Severn Main terrace sediments at Eardington, Shropshire is instructive in determining aspects of the sedimentary environment at a ‘proximal’ location in the terrace. Evaluation of the structural and directional properties of the gravels indicates that the terrace is a composite feature comprising two units. The lower unit is notable for the widespread occurrence of large sandy facies, a near absence of massive gravel facies, and the presence of large, locally derived, lithoclasts in excess of 1 m in diameter. Facies associations observed indicate the existence of in-channel bars and large channels with bedforms at the dune-plane bed transition. The directional properties of the unit indicate flows parallel to the main valley axis and although the unit is unlike previously reported examples it is interpreted as being the product of a proximal low sinuosity environment. The upper unit is composed mainly of multistorey, coarse, massive gravel units with limited interbedded sand facies. Fabric analysis indicates progradation of the unit from the west. The facies sequence is similar to those encompassed by facies models proposed for proximal alluvial fan deposits and therefore, on structural and directional grounds, the upper unit is interpreted as an alluvial fan deposit which prograded into the main Severn valley from the adjacent Mor Brook tributary. The implications of the composite nature of the terrace at this point are considered both in terms of terrace correlation and palaeohyrological estimation.  相似文献   

Seabird mortality in the north Irish Sea and firth of clyde early in 1974     

C. Lloyd  J.A. Bogan  W.R.P. Bourne  P. Dawson  J.L.F. Parslow  A.G. Stewart 《Marine pollution bulletin》1974,5(9):136-140

Heavy mortality of guillemots in the Irish Sea was recorded in 1969 and was the subject of a detailed enquiry. Organochlorine contamination was thought to be a contributory factor, although this could not be conclusively established. These persistent pollutants remain in the environment and further mortalities might be expected if they were indeed responsible. In January, 1974 there was a further heavy loss of seabirds in the area and, this time, investigators were prepared and the incident was carefully monitored.  相似文献   

Enrichment of the continental lithosphere by OIB melts: Isotopic evidence from the volcanic province of northern Tanzania   总被引：1，自引：0，他引：1  

Cassi Paslick  Alex Halliday  Dodie James  J. Barry Dawson 《Earth and Planetary Science Letters》1995,130(1-4):109-126

Alkali basalts and nephelinites from the southern end of the East African Rift (EAR) in northern Tanzania have incompatible trace element compositions that are similar to those of ocean island basalts (OIB). They define a considerable range of Sr, Nd and Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr= 0.7035−0.7058,ε_Nd = −5to+3, and²⁰⁶Pb/²⁰⁴Pb= 17.5−21.3), each of which partially overlaps the range found in OIB. However, they occupy a unique position in combined Nd, Sr and Pb isotopic compositional space. Nearly all of the lavas have radiogenic Pb, similar to HIMU with high time-integrated²³⁸U/²⁰⁴Pb coupled with unradiogenic Nd (+2 to −5) and radiogenic Sr (>0.704), similar to EMI. This combination has not been observed in OIB and provides evidence that these magmas predominantly acquired their Sr, Nd and Pb in the subcontinental lithospheric mantle rather than in the convecting asthenosphere. These data contrast with compositions for lavas from farther north in the EAR. The Pb isotopic compositions of basalts along the EAR are increasingly radiogenic from north to south, indicating a fundamental change to sources with higher time-integratedU/Pb, closer to the older cratons in the south. An ancient underplated OIB melt component, isolated for about 2 Ga as enriched lithospheric mantle and then remelted, could generate both the trace element and isotopic data measured in the Tanzanian samples. Whereas the radiogenic Pb in Tanzanian lavas requires a source with high time-integratedU/Pb, most continental basalts that are thought to have interacted with the continental lithospheric mantle have unradiogenic Pb, requiring a source with a history of lowU/Pb. Such lowU/Pb is readily accomplished with the addition of subduction-derived components, since the lower averageU/Pb of arc basalts (0.15) relative to OIB (0.36) probably reflects addition of Pb from subducted oceanic crust. If the subcontinental lithosphere is normally characterized by low time-integratedU/Pb it would appear that subduction magmatism is more important than OIB additions in supplying the Pb inventory of the lithospheric mantle. However,U/Pb ratios of xenoliths derived from the continental lithospheric mantle suggest that both processes may be important. This apparent discrepancy could be because xenoliths are not volumetrically representative of the subcontinental lithospheric mantle, or, more likely, that continental lithospheric mantle components in basalts are normally only identified as such when the isotopic ratios are dissimilar from MORB or OIB. Lithospheric enrichment from subaccreted OIB components appears to be more significant than generally recognized.  相似文献