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21.
Erwan Hallot Philippe Davy Jean de Bremond d'Ars Bernard Auvray Hervé Martin Henri Van Damme 《Journal of Volcanology and Geothermal Research》1996,71(1):31-44
Injection of Newtonian crystal-free magmas into a partially crystallised host which may exhibit non-Newtonian properties produces magmatic structures such as pipes, syn-plutonic dikes or dendritic structures. Field relationships between the structure and the host rock commonly indicate what the rheological contrasts during the injection were. The manner in which a magma deforms in response to injection is mainly linked to crystal content and strain rate (i.e., injection rate). Three kinds of behaviour can be distinguished: (1) Newtonian at low crystal contents; (2) Non-Newtonian at intermediate (40–60%) crystal contents, or at high crystal contents if the strain rate is small; and (3) brittle failure at high crystal content or strain rates.Petrologic observations indicate that injection can take place when the host magma still behaves as a fluid. To investigate the physics of the injection process we review the results of injection experiments in non-Newtonian fluids. These experiments were performed to study viscous fingering in 2-D Hele Shaw cells. They provide the first step to establishing the main non-Newtonian effects during the formation of interfacial instabilities arising when a Newtonian fluid is injected into a more viscous fluid or paste. The qualitative comparison of the morphological features of the interfaces between the fluids in the experiments with those in nature suggests that, in magmas, irregularities of the interfaces (dikes and dendrites) result from non-Newtonian properties of the host. We conclude that fluid-like deformation, rather than brittle behaviour of the host, during injection is likely to produce the general features observed on the field. Cooling effects might be responsible for the widespread phenomenon of fragmentation. We emphasise that the main effect of non-Newtonian properties in partially crystallised magmas is to generate strongly heterogeneous media producing discontinuities which could explain the main morphological features of syn-plutonic injection structures. 相似文献
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Between 1990 and 1995 a series of bed sediment, suspended sediment and fresh floodplain samples were collected within the Seine River Basin, in France, to evaluate variations in trace element geochemistry. Average background trace element levels for the basin were determined from the collection and subsequent analyses of bed sediment samples from small rural watersheds and from a prehistoric (5000 BP) site in Paris. Concentrations are relatively low, and similar to those observed for fine‐grained bed sediments from unaffected areas in the United States and Canada. However, the concentrations are somewhat higher than the reference levels presently adopted by French water authorities for areas north of the Seine Basin, which have similar bedrock lithologies. Downstream trace element variations were monitored in 1994 and 1995 using fresh surficial floodplain samples that were collected either as dried deposits a few days after peak discharge, or immediately after peak discharge (under ≤30 cm of water). Chemical comparisons between fresh floodplain deposits, and actual suspended sediments collected during flood events, indicate that, with some caveats, the former can be used as surrogates for the latter. The floodplain sediment chemical data indicate that within the Seine Basin, from the relatively unaffected headwaters through heavily affected urban streams, trace element concentrations vary by as much as three orders of magnitude. These trace element changes appear to be the result of both increases in population as well as concomitant increases in industrial activity. (This article is a US government work and is in the public domain in the United States.) 相似文献
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To investigate how magma rises through the brittle upper crust in a context of compressional tectonics, we have performed experiments on scaled physical models. Powdered silica (having a cohesion of 300 Pa and an angle of internal friction of 38°) was used to represent brittle crust. A vegetable oil (with a Newtonian viscosity of 10?2 Pa·s at 50 °C) was used to represent magma. A moving piston shortened the models in a box, while oil was injected steadily at the base. On cooling to room temperature, the oil solidified. The resulting intrusions were thin sills, dykes and laccoliths. Their shapes and emplacement modes depended on the ratio R between rates of shortening and injection. From shapes and orientations of intrusions, we infer that hydraulic fracturing was one mechanism of emplacement. Unconsolidated intrusions strongly influenced thrust formation. On the basis of our experiments, we suggest that magmas in orogenic belts can rise along thrust faults. 相似文献
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Study of the spatial and historical distribution of sediment inorganic contamination in the Toulon bay (France) 总被引：4，自引：0，他引：4
Tessier E Garnier C Mullot JU Lenoble V Arnaud M Raynaud M Mounier S 《Marine pollution bulletin》2011,62(10):2075-2086
This study presents trace elements levels in surface and deep sediments of the Toulon bay (SE France) subjected to anthropogenic inputs (navy base, harbors, etc.). The studied elements (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) are defined as priority contaminants in aquatic systems. Fifty-five points scattered on the entire bay were sampled, allowing the determination of contaminants distribution with a high resolution. Several approaches were used to assess the degree of contamination and the potential toxicity of the Toulon bay sediments: comparison to the French legislation, surface-weighted average metal concentrations, enrichment factors (EF), geoaccumulation indices (Igeo), trace element stock calculation and comparison to sediment quality guidelines. A principal component analysis was performed to reveal common behavior of the studied contaminants. Results demonstrated the very high contamination of the small bay, especially in Hg (EF up to 1500), Cu, Pb and Zn, with export to the large bay further governed by hydrodynamics. 相似文献
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The analysis of flow at the pore scale in porous media has been facilitated with the use of microtomography. A powerful tool for quantifying the fluid structure using these tomographic 3D reconstructions is skeletonisation, but the significant disadvantage of this method is its sensitivity to noise, resulting in artefacts in the skeleton. A pre-processing of the 3D image is therefore required, but no method has yet proven to completely solve this problem. By developing a new procedure that, by construction, directly identifies the voxels and only those that are responsible for topological artefacts in the skeleton, we are able to remove all artefacts, and furthermore can prove that we do so by modifying a minimal amount of voxels in the segmented 3D image (i.e. the tomographic image in which each voxel has been assigned to either the porous or the solid phase). This is possible by identifying the three fundamental types of artefacts that can arise in a 3D skeleton, and dealing with each appropriately. Application to a microtomographic image of a sintered glass powder is presented. Impact of the different processing methods on the flow within its porosity is measured through the computed permeability deviations. 相似文献
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Erwan Le Guerrou Philip A. Allen Andrea Cozzi James L. Etienne Mark Fanning 《地学学报》2006,18(2):147-153
Sedimentary rocks deposited during the Ediacaran period (～630–542 Ma) contain carbonates whose carbon isotopic ratios show a marked negative excursion consisting of a precipitous drop from +5‰ to ?12‰, followed by a sub‐linear recovery to positive δ13C values. Isotopic ages (U/Pb) and thermal subsidence modelling are combined to constrain the excursion in time and indicate an onset at ～600 Ma, and duration of recovery of approximately 50 Myr. The excursion is widely recognized in Oman and has potential correlatives in Ediacaran strata elsewhere, and may thus represent a characteristic feature of the Ediacaran period. The amplitude of this carbon isotope excursion far exceeds those of other Neoproterozoic anomalies. The isotopic trend of negative excursion and long‐term recovery spanned at least one short‐lived glacial episode (at 580 Ma), but appears unrelated to glaciation, which indicates that negative anomalies in the Neoproterozoic marine carbon isotope record are not directly or uniquely linked to ice ages. 相似文献
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A 1D analytical framework is implemented in a narrow convergent estuary that is 78 km in length (the Guadiana, Southern Iberia) to evaluate the tidal dynamics along the channel, including the effects of neap-spring amplitude variations at the mouth. The close match between the observations (damping from the mouth to ～ 30 km, shoaling upstream) and outputs from semi-closed channel solutions indicates that the M2 tide is reflected at the estuary head. The model is used to determine the contribution of reflection to the dynamics of the propagating wave. This contribution is mainly confined to the upper one third of the estuary. The relatively constant mean wave height along the channel (<?10% variations) partly results from reflection effects that also modify significantly the wave celerity and the phase difference between tidal velocity and elevation (contradicting the definition of an “ideal” estuary). Furthermore, from the mouth to ～ 50 km, the variable friction experienced by the incident wave at neap and spring tides produces wave shoaling and damping, respectively. As a result, the wave celerity is largest at neap tide along this lower reach, although the mean water level is highest in spring. Overall, the presented analytical framework is useful for describing the main tidal properties along estuaries considering various forcings (amplitude, period) at the estuary mouth and the proposed method could be applicable to other estuaries with small tidal amplitude to depth ratio and negligible river discharge. 相似文献
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A nested workflow of multiple‐point geostatistics (MPG) and sequential Gaussian simulation (SGS) was tested on a study area of 6 km2 located about 20 km northwest of Quebec City, Canada. In order to assess its geological and hydrogeological parameter heterogeneity and to provide tools to evaluate uncertainties in aquifer management, direct and indirect field measurements are used as inputs in the geostatistical simulations to reproduce large and small‐scale heterogeneities. To do so, the lithological information is first associated to equivalent hydrogeological facies (hydrofacies) according to hydraulic properties measured at several wells. Then, heterogeneous hydrofacies (HF) realizations are generated using a prior geological model as training image (TI) with the MPG algorithm. The hydraulic conductivity (K) heterogeneity modeling within each HF is finally computed using SGS algorithm. Different K models are integrated in a finite‐element hydrogeological model to calculate multiple transport simulations. Different scenarios exhibit variations in mass transport path and dispersion associated with the large‐ and small‐scale heterogeneity respectively. Three‐dimensional maps showing the probability of overpassing different thresholds are presented as examples of management tools. 相似文献