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31.
Natalia M. Levashova Kirill E. Degtyarev Mikhail L. Bazhenov Adam Q. Collins Rob Van der Voo 《Tectonophysics》2003,377(3-4):249-268
In order to test different hypotheses concerning the Paleozoic evolution of the Ural–Mongol belt (UMB) and the amalgamation of Eurasia, we studied Middle Devonian basalts from two localities (11 sites) and Lower Silurian volcanics, redbeds, and intra-formational conglomerates from three localities (20 sites) in the Chingiz Range of East Kazakhstan. The Devonian rocks prove to be heavily overprinted in the late Paleozoic, and a high-temperature, presumably primary, southerly, and down component is isolated at only four sites from a homoclinal section. Most Silurian redbeds are found to be remagnetized in the late Paleozoic; in contrast, a bipolar near-horizontal remanence, isolated from Silurian volcanics, is most probably primary as indicated by positive tilt and conglomerate tests. Analysis of paleomagnetic data from the Chingiz Range shows that southward-pointing directions in Ordovician, Silurian, and Devonian rocks are of normal polarity and hence indicate large-scale rotations after the Middle Devonian. The Chingiz paleomagnetic directions can be compared with Paleozoic data from the North Tien Shan and with the horseshoe-shaped distribution of subduction-related volcanic complexes in Kazakhstan. Both paleomagnetic and geological data support the idea that today's strongly curved volcanic belts of Kazakhstan are an orocline, deformed mostly before mid-Permian time. Despite the determination of nearly a dozen new Paleozoic paleopoles in this study and other recent publications by our team, significant temporal and spatial gaps remain in our knowledge of the paleomagnetic directions during the middle and late Paleozoic. However, the paleomagnetic results from the Chingiz Range and the North Tien Shan indicate that these areas show generally coherent motions with Siberia and Baltica, respectively. 相似文献
32.
Establishing relative and absolute time frameworks for the sedimentary, magmatic, tectonic and gold mineralisation events in the Norseman-Wiluna Belt of the Archean Yilgarn Craton of Western Australia, has long been the main aim of research efforts. Recently published constraints on the timing of sedimentation and absolute granite ages have emphasized the shortcomings of the established rationale used for interpreting the timing of deformation events. In this paper the assumptions underlying this rationale are scrutinized, and it is shown that they are the source of significant misinterpretations. A revised time chart for the deformation events of the belt is established. The first shortening phase to affect the belt, D1, was preceded by an extensional event D1e and accompanied by a change from volcanic-dominated to plutonic-dominated magmatism at approximately 2685–2675 Ma. Later extension (D2e) controlled deposition of the ca 2655 Ma Kurrawang Sequence and was followed by D2, a major shortening event, which folded this sequence. D2 must therefore have started after 2655 Ma—at least 20 Ma later than previously thought and after the voluminous 2670–2655 Ma high-Ca granite intrusion. Younger transcurrent deformation, D3–D4, waned at around 2630 Ma, suggesting that the crustal shortening deformation cycle D2–D4 lasted approximately 20–30 Ma, contemporaneous with low-volume 2650–2630 Ma low-Ca granites and alkaline intrusions. Time constraints on gold deposits suggest a late mineralisation event between 2640–2630 Ma. Thus, D2–D4 deformation cycle and late felsic magmatism define a 20–30 Ma long tectonothermal event, which culminated with gold mineralisation. The finding that D2 folding took place after voluminous high-Ca granite intrusion led to research into the role of competent bodies during folding by means of numerical models. Results suggest that buoyancy-driven doming of pre-tectonic competent bodies trigger growth of antiforms, whereas non-buoyant, competent granite bodies trigger growth of synforms. The conspicuous presence of pre-folding granites in the cores of anticlines may be a result from active buoyancy doming during folding. 相似文献
33.
A record of accelerated erosion in the recent sediments of Blelham Tarn in the English Lake district 总被引:1,自引:0,他引:1
K. D. van der Post F. Oldfield E. Y. Haworth P. R. J. Crooks P. G. Appleby 《Journal of Paleolimnology》1997,18(2):103-120
Two frozen cores from Blelham Tarn were subsampled and measured using mineral magnetic, loss-on-ignition (LOI), radiometric, granulometric and diatom analyses. A detailed chronology was established using varves, radioisotopes and diatoms. This has enabled an accurately dated reconstruction of sedimentation over the past forty years. Despite a large increase in lake productivity, evidence suggests that the observed exponential increase in sedimentation rates can be attributed to erosion within the catchment. The predominant sediment source has been identified as surface soil. A comparison between the trend of accelerated sedimentation and the record of increased sheep stocking density for the area within which the most of the catchment lies, as well as observations of contemporary surface processes within the catchment, both suggest that much of the recent erosion is a direct response to increased pressure from sheep grazing. 相似文献
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36.
The identifiability of parameters in a water quality model of the Biebrza River, Poland 总被引:1,自引:0,他引:1
The identifiability of model parameters of a steady state water quality model of the Biebrza River and the resulting variation in model results was examined by applying the Monte Carlo method which combines calibration, identifiability analysis, uncertainty analysis, and sensitivity analysis. The water quality model simulates the steady state concentration profiles of chloride, phosphate, ammonium, and nitrate as a function of distance along a river. The water quality model with the best combination of parameter values simulates the observed concentrations very well. However, the range of possible modelled concentrations obtained for other more or less equally eligible combinations of parameter values is rather wide. This range in model outcomes reflects possible errors in the model parameters. Discrepancies between the range in model outcomes and the validation data set are only caused by errors in model structure, or (measurement) errors in boundary conditions or input variables. In this sense the validation procedure is a test of model capability, where the effects of calibration errors are filtered out. It is concluded that, despite some slight deviations between model outcome and observations, the model is successful in simulating the spatial pattern of nutrient concentrations in the Biebrza River. 相似文献
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38.
The structure of the mid-Norwegian volcanic Vøring margin at the onset of the Maastrichtian–Paleocene extension phase reflects the cumulative effect of earlier consecutive rifting events. Lateral structural differences present on the margin at that time are a consequence of migration of the location of maximum extension in time between Norway and Greenland. The most important imprints (Moho depth, thermal structure) of these events on the lithosphere are incorporated in a numerical simulation of the final extension phase. We focus on a possible mechanism of formation of the Vøring Marginal High and address the relationship between spatial and temporal evolution of crustal thinning and thickening, uplift of the surface and strength of the lithosphere.It is found that the Vøring Basin formed the strongest part of the margin which explains why the Maastrichtian–Paleocene rift axis was not located here but instead jumped westward with respect to the earlier rift axes locations. The modeling study predicts that local crustal thickening during extension can be expected when large lateral thermal variations are present in the lithosphere at the onset of extension. Negative buoyancy induced by lateral temperature differences increases downwelling adjacent to the rifting zone; convergence of material at the particular part of the margin is mainly taken up by the lower crust. The model shows that during the final phase of extension, the crust in the Vøring Marginal High area was thickened and the surface uplifted. It is likely that this dynamic process and the effects of magmatic intrusions both acted in concert to form the Marginal High. 相似文献
39.
M. Roy-Barman C. Jeandel M. Souhaut M. Rutgers van der Loeff I. Voege N. Leblond R. Freydier 《Earth and Planetary Science Letters》2005,240(3-4):681-693
230Th, 232Th and 234Th were analyzed in sinking particles collected by moored and drifting sediment traps in the NE Atlantic Ocean (POMME experiment) in order to constrain the phase(s) carrying Th isotopes in the water column. It reveals a contrasted behaviour between 234Th and 230Th. 234Th is correlated to the particulate organic carbon suggesting that it is primarily scavenged by organic compounds in the surface waters. 230Thxs is correlated with Mn, Ba and the lithogenic fraction that are enriched in small suspended particles and incorporated in the sinking particulate flux throughout the water column. The lack of correlation between 230Thxs and CaCO3 or biogenic silica (bSi) indicates that CaCO3 and bSi are not responsible for 230Th scavenging in the deep waters of this oceanic region. 230Th is generally correlated with the lithogenic content of the trapped material but this correlation disappears in winter during strong atmospheric dust inputs suggesting that lithogenic matter is not directly responsible for 230Th scavenging in the deep waters or that sufficient time is required to achieve particle–solution equilibration. MnO2 could be the prevalent 230Thxs-bearing phase. The narrow range of Kd_MnO2Th obtained for very contrasted oceanic environments supports a global control of 230Thxs scavenging by MnO2 and raises the possibility that the 230Th–231Pa fractionation is controlled by the amount of colloidal MnO2 in seawater. 相似文献
40.
Mariëtte Wolthers Laurent Charlet Peter R. van der Linde 《Geochimica et cosmochimica acta》2005,69(14):3483-3492
Arsenate, As(V), sorption onto synthetic iron(II) monosulfide, disordered mackinawite (FeS), is fast. As(V) sorption decreases above the point of zero surface charge of FeS and follows the pH-dependent concentration of positively charged surface species. No redox reaction is observed between the As(V) ions and the mineral surface over the time span of the experiments. This observation shows that As(V) dominantly forms an outer-sphere complex at the surface of mackinawite. Arsenite, As(III), sorption is not strongly pH-dependent and can be expressed by a Freundlich isotherm. Sorption is fast, although slower than that of As(V). As(III) also forms an outer-sphere complex at the surface of mackinawite. In agreement with previous spectroscopic studies, complexation at low As(V) and As(III) concentration occurs preferentially at the mono-coordinated sulfide edge sites. The Kd (L g−1) values obtained from linear fits to the isotherm data are ∼9 for As(V) and ∼2 for As(III). Stronger sorption of As(V) than As(III), and thus a higher As(III) mobility, may be reflected in natural anoxic sulfidic waters when disordered mackinawite controls arsenic mobility. 相似文献