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531.
532.
Isabel Abad Gabriel Gutirrez-Alonso Fernando Nieto Igor Gertner Alex Becker Ana Cabero 《Tectonophysics》2003,365(1-4):103
Geological mapping and structural analysis of the Talas Ala Tau (Tien Shan, Kyrgyz Republic) have revealed a complex structure composed of folds with axial-plane cleavage and thrust faults verging towards the NE. The main structures of the range correspond to minor Tertiary and Carboniferous–Permian deformation superimposed on the main deformation event that took place during the Baikalian orogeny. The pervasive axial-plane cleavage diminishes in penetrativity from the hinterland to the foreland in both the Uzunakhmat and Karagoin sheets. The main thrusts developed phyllonitic shear-related rocks on the hangingwall immediately above the thrust planes. A crystal-chemical study of the phyllosilicates growth during the Baikalian deformation event along a cross-section revealed changes in the crystallinity, composition and lattice parameters of them. The phyllosilicates present in the Talas Ala Tau rocks were crystallized in very low-grade metamorphic conditions, that is below 300 °C, as indicated by their Kübler Index (KI), which decreases from SW towards the NE. Detailed TEM study of the phyllosilicates reveals a clear textural difference at the lattice level between samples with higher or lower KI parameters. There is also a clear difference in crystal-chemical parameters (KI and b) and composition between the phyllosilicates growth in relation to the axial-plane cleavage and the ones belonging to the thrust-related phyllonites. The first ones are more affected by the ferrimuscovitic vector than the phyllosilicates of phyllonites, closer to the theoretical phengitic component. Huge ranges of values of phengitic content of micas at sample level are interpreted as the result of a decompression path from at least 8 kbar. We propose a subduction geodynamic environment for the regional deformation and the origin of the phyllosilicates, as they are similar to those obtained in more recent accretionary complexes. 相似文献
533.
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,εNd = −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
534.
Determination of a shallow velocity–depth model from seismic refraction data by coherence inversion1
Seismic refractions have different applications in seismic prospecting. The travel- times of refracted waves can be observed as first breaks on shot records and used for field static calculation. A new method for constructing a near-surface model from refraction events is described. It does not require event picking on prestack records and is not based on any approximation of arrival times. It consists of the maximization of the semblance coherence measure computed using shot gathers in a time window along refraction traveltimes. Time curves are generated by ray tracing through the model. The initial model for the inversion was constructed by the intercept-time method. Apparent velocities and intercept times were taken from a refraction stacked section. Such a section can be obtained by appling linea moveout corrections to common-shot records. The technique is tested successfully on synthetic and real data. An important application of the proposed method for solving the statics problem is demonstrated. 相似文献
535.
536.
High-silica rhyolites of the Sierra La Primavera, a late Pleistocene center near Guadalajara, are extremely Sr-poor (0.3–1.3 ppm), yet (with one exception) values of 87Sr/86Sri are relatively low at 0.7041–0.7048. Values of 143Nd/144Nd for all the rhyolites are (within errors) identical to a basalt at 0.5129. These surprisingly primitive values, along with feldspar 18O of +6.6, are consistent with an origin by fractional crystallization of mantle-derived basalt. However, absence of the large volume of associated intermediate rocks that would be expected if the 40 km3 of erupted rhyolite were produced mainly by fractional crystallization suggests alternative processes involving partial melting of Mesozoic or Tertiary mafic intrusive rocks (or lower-crustal metamorphic equivalents). The latter interpretation is preferred, especially in light of comparative data for other North American, Cenozoic, high-silica rhyolites. Isotopic compositions correlate with basement age, but generally lie between values for associated basalts and the underlying crust. Nearly all can be interpreted as containing both a young mantle-derived component and a crustal component, probably derived by partial melting at intermediate to deep levels of the crust. No matter what the proportions of mantle- and crust-derived material in parental magmas, the extremely low concentrations of Sr and Ba in the high-silica rhyolites require extensive fractional crystallization of feldspar-rich assemblages after parental liquids attain rhyolitic compositions.At La Primavera, contamination by shallow roof rocks probably led to the 0.708 87Sr/86Sri ratio of the earliest postcaldera lava dome, which is thought to have erupted through the same vent as the caldera-forming pyroclastic flows. Contamination associated with collapse apparently affected only a small volume of magma in contact with brecciated wall rocks close to the vent, as nearby lavas that erupted during the same episode about 95 ky ago are unaffected. No identifiable lowering of 18O took place on caldera collapse. Rhyolitic lavas that erupted 75, 60, and 30 ky ago document postcaldera chemical recovery of the chamber to progressively more evolved compositions in its upper reaches, but show little variation in 18O, 87Sr/ 86Sri, or 143Nd/144Nd with time, suggesting that the bulk of the rhyolitic magma within the chamber was isolated from significant wall-rock contamination. Most of the small range of 87Sr/86Sri among the rhyolites can be attributed to pre-eruptive, in situ decay of 87Rb, resulting in a measurable secular increase of 87Sr/86Sr in these Sr-poor magmas. The 87Sr/86Sri of the youngest rhyolite, however, is somewhat lower than predicted, suggesting that the silicic magma chamber was at times open to interaction with more-mafic magmas from below. 相似文献
537.
David Petruska Jeff Geyer Rick Macon Michael Craig Alex Ran Neil Schulz 《Ocean Engineering》2005,32(7):767-782
The Mad dog project will use a polyester mooring system on the drilling and production truss spar. This will be the first use of a permanent polyester mooring system on a Floating Production System (FPS) outside of Brazil and the first time polyester has been used on a spar. As such, there were many challenges, which include:
- • Regulatory approval.
- • Designing a mooring system which is dominated by current loadings.
- • Largest polyester rope break load ever required.
- • Rope design qualification and testing.
- • Quality control and assurance.
- • Inspection, Maintenance, Repair and Retirement (IMRR) of such a mooring system.
538.
The objective of this study was to determine whether there was significant spatial and temporal variation in macroalgae epiphytic on pneumatophores of the Grey Mangrove, Avicennia marina (Forsk.) Vierh., in the Clyde River, located 280 km south of Sydney, Australia. Three estuarine sites in the Clyde River were surveyed seasonally on four occasions over a two-year period, and algal distribution and abundance assessed in respect to temporal, inter-site, intertidal (from front to back of mangrove stand) and vertical (from bottom to top of pneumatophores) variation. Sediment and water characteristics, including nutrient levels, were also assessed in order to examine all variables of potential influence on algal distribution and abundance. The results indicated that intertidal position within sites, and vertical height along the length of the pneumatophore, were the greatest influence on algal frequency and biomass. Individual species dominated in different intertidal and vertical zones. These observations, together with the identification of three species of macroalgae that fulfil the criteria for bioindicators/biomonitors of environmental impacts are discussed. 相似文献
539.
We analyze the possibility of determining the masses of outer planetary satellites from their mutual gravitational perturbations via ground-based observations. Such a technique has been applied in (Emelyanov, 2005b) to determine the mass of the Jovian satellite Himalia. In this paper, we use the least-squares method to compute the errors of satellite masses inferred from simulated observations. We analyze several of the most suitable variants of groups of outer satellites of planets with maximum mutual attraction. We found that the mass of the Satumian satellite Phoebe (S9) can be refined by continuing observations of the satellite S25 Mundilfari until 2027. We show that the masses of other known outer planetary satellites cannot be determined from ground-based observations. 相似文献
540.
Humberto Marotta Luiz Bento Francisco de A. Esteves Alex Enrich-Prast 《Estuaries and Coasts》2009,32(4):654-660
The purpose of this research was to assess the effects of dredging performed in a marginal wetland colonized by aquatic macrophytes
on eutrophication of the adjacent shallow tropical lake (Imboassica Lake, Brazil). The river mouth of the Imboassica River
that drains into Imboassica Lake had been densely colonized by aquatic vegetation dominated by Typha domingensis (Pers.) when it was dredged. Total and dissolved nitrogen and phosphorus concentrations were measured monthly over 13 years
at four stations in the Imboassica river-lake system. Dredging activities reduced phosphorus and nitrogen retention at the
river mouth and subsequently increased these nutrient stocks in the lake waters. Nutrient retention by non-dredged wetland
was estimated to be ca. 1,200 kg year−1 (87.3 g m−2 year−1) for nitrogen and 60 kg year−1 (4.5 g m−2 year−1) for phosphorus. Our whole-lake approach suggested that dredging might intensify rather than mitigate eutrophication in shallow
tropical lakes when the removal of aquatic macrophytes is coupled to the persistence of anthropogenic nutrient inputs from
the watershed. 相似文献