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22.
Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite,Oman and UAE)
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Mathieu Soret Philippe Agard Benoît Dubacq Alexis Plunder Philippe Yamato 《Journal of Metamorphic Geology》2017,35(9):1051-1080
Metamorphic soles are tectonic slices welded beneath most large‐scale ophiolites. These slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as forming during the first million years of intraoceanic subduction following heat transfer from the incipient mantle wedge towards the top of the subducting plate. This study reappraises the formation of metamorphic soles through detailed field and petrological work on three key sections from the Semail ophiolite (Oman and United Arab Emirates). Based on thermobarometry and thermodynamic modelling, it is shown that metamorphic soles do not record a continuous temperature gradient, as expected from simple heating by the upper plate or by shear heating as proposed in previous studies. The upper, high‐T metamorphic sole is subdivided in at least two units, testifying to the stepwise formation, detachment and accretion of successive slices from the down‐going slab to the mylonitic base of the ophiolite. Estimated peak pressure–temperature conditions through the metamorphic sole, from top to bottom, are 850°C and 1 GPa, 725°C and 0.8 GPa and 530°C and 0.5 GPa. These estimates appear constant within each unit but differing between units by 100–200°C and ~0.2 GPa. Despite being separated by hundreds of kilometres below the Semail ophiolite and having contrasting locations with respect to the ridge axis position, metamorphic soles show no evidence for significant petrological variations along strike. These constraints allow us to refine the tectonic–petrological model for the genesis of metamorphic soles, formed via the stepwise stacking of several homogeneous slivers of oceanic crust and its sedimentary cover. Metamorphic soles result not so much from downward heat transfer (ironing effect) as from progressive metamorphism during strain localization and cooling of the plate interface. The successive thrusts originate from rheological contrasts between the sole, initially the top of the subducting slab, and the peridotite above as the plate interface progressively cools. These findings have implications for the thickness, the scale and the coupling state at the plate interface during the early history of subduction/obduction systems. 相似文献
23.
The development of a regional stratigraphy in Palaeoproterozoic basins within the Tanami region, Northern Australia has been
hindered by the difficulty of discriminating sedimentary units and facies across this isolated and poorly exposed basin. A
regional stratigraphy is important as it provides constraints on sedimentary basin evolution and assists in gold exploration.
Gold is known to be more concentrated in certain rock formations. Based on Nd isotopes and whole rock geochemistry, five main
sedimentary events have been identified in the Tanami region. Some sedimentary units were derived from homogeneous local sources,
whereas others contain evidence of a well-mixed fine-grained remote provenance. Within the basins, major gold-bearing lithologies
are characterised by mafic source indicators: (1) high Cr/Th ratios; (2) low Th/Sc ratios; (3) low (La/Yb)PAAS ratios relative to Post-Archaean Average Shale (Taylor and McLennan 1985); (4) Eu anomalies equal to ∼1; and (5) distinctive ranges in initial ε
Nd values. Potential future exploration target areas have been identified in the Tanami region at the Cashel and Sunline prospects
using these geochemical parameters. 相似文献
24.
Alexis Rigo 《Tectonophysics》2010,480(1-4):109-118
Earthquake precursors are now regularly described but often detected only after a major or moderate seismic event. Presence and influence of fluids in the seismogenic processes are often observed at the time of earthquake studies. Even today, the understanding of the physical processes involved in the source region is a real challenge for seismic hazard assessment. Here, the aftershock sequence of the ML = 5.2, 1996 Saint-Paul-de-Fenouillet (Eastern Pyrenees, France) earthquake is first re-examined with P-wave cross-correlations, resulting in extracting three multiplets and in determining new locations. Multiplets and spatio-temporal distribution analysis of the aftershocks allow for quantifying the hydraulic diffusivity D at a maximum value of 5 m2/s and the permeability K at 10? 15 m2 in the upper Pyrenean crust. Second, a model is established in order to explain the hydrogeochemical transient anomalies, which occurred during the 15 day-period preceding the 1996 earthquake. These anomalies consist on a temporal and spatial sequence of gas emissions in the epicentral area and on chloride and lead concentration variations in a bottled mineral water 25 km north to the main shock epicenter. The proposed model processed in a standard elastic half-space, consists of creep on a low-angle crustal normal-fault, generating volumetric strain field changes over a distance of 25 km from the epicentral area. This model is able to constrain not only the mechanisms and the locations of the geochemical anomalies, but also their timing and probable casual links to the triggering of the impending major event. Also, the active extension proposed here is compatible with seismological observations in the Pyrenees. Thus, the possibility of such creep, which can be considered as a slow-slip event, is discussed in the Pyrenean tectonic and geological context. The model is discussed and compared to previous proposed models on precursor processes of earthquakes, especially concerning the preparation zone concept. Finally, a complete seismic scenario over the period beginning 15 days before the quake and ending 5 days after is proposed and discussed. 相似文献
25.
Pascal Goderniaux Serge Brouyère Alexis Gutierrez Nicole Baran 《Hydrogeology Journal》2010,18(7):1729-1740
For good management of groundwater resources, and to comply with European and national regulations, a detailed understanding of an aquifer’s hydraulic setting is required. In order to better characterize a sandy aquifer that is affected by diffuse pollution (Brévilles spring catchment, Val d’Oise, France), and to quantify the transfer time in the saturated zone, a multi-tracer test involving a new technique, the ‘finite volume point dilution method’, has been performed in natural flow conditions. In November 2005, injections of four different tracers took place in four piezometers involving different locations and depths in the aquifer. Recovery of the tracers was observed at two different places near the aquifer outlet. A particularly long and unusual monitoring exercise (27 months) demonstrated the existence of several different velocities within the sandy layer, which seems to be linked to the decrease of hydraulic conductivity with depth. The new insight and parameter quantification brought by interpretation of these tests contribute to a better characterization of the saturated zone. The particularly long-term monitoring exercise also gives new information to understand and forecast the trend and persistence of groundwater contamination by pesticides in the catchment. 相似文献
26.
27.
Konštantín Rosina Filipe Batista e Silva Pilar Vizcaino Mario Marín Herrera Sérgio Freire Marcello Schiavina 《International Journal of Digital Earth》2020,13(5):602-626
ABSTRACTData on land use and land cover (LULC) are a vital input for policy-relevant research, such as modelling of the human population, socioeconomic activities, transportation, environment, and their interactions. In Europe, CORINE Land Cover has been the only data set covering the entire continent consistently, but with rather limited spatial detail. Other data sets have provided much better detail, but either have covered only a fraction of Europe (e.g. Urban Atlas) or have been thematically restricted (e.g. Copernicus High Resolution Layers). In this study, we processed and combined diverse LULC data to create a harmonised, ready-to-use map covering 41 countries. By doing so, we increased the spatial detail (from 25 to one hectare) and the thematic detail (by seven additional LULC classes) compared to the CORINE Land Cover. Importantly, we decomposed the class ‘Industrial and commercial units’ into ‘Production facilities’, ‘Commercial/service facilities’ and ‘Public facilities’ using machine learning to exploit a large database of points of interest. The overall accuracy of this thematic breakdown was 74%, despite the confusion between the production and commercial land uses, often attributable to noisy training data or mixed land uses. Lessons learnt from this exercise are discussed, and further research direction is proposed. 相似文献
28.
LI Chengcai MAO Jietai Alexis K. H. Lau YUAN Zibing WANG Meihua LIU Xiaoyang 《中国科学D辑(英文版)》2005,48(Z2)
The direct correlation between NASA MODIS aerosol optical depth (AOD) products and the air pollution index (API) in Beijing was found relatively low based on the long-term comparison analysis. The correlation improved to some extent after taking account of the seasonal variation of scale height and the vertical distribution of aerosols. The correlation coefficient further improved significantly after considering the influencing factor of Relative Humidity (RH). This study concluded that satellite remote-sensing could serve as an efficient tool for monitoring the spatial distribution of particulate pollutants on the ground-level, as long as corrections have been made in the two aforementioned processes. Taking advantage of the MODIS information, we analyzed a pollution episode occurring in October 2004 in Beijing. It indicated that satellite remote-sensing could describe the formation process of the ground-level pollution episode in detail, and showed that regional transport and the topography were crucial factors to air quality in Beijing. The annual averaged distribution in the urban area of Beijing and its surroundings could be also obtained from the high-resolution retrieval results, implicating that high-resolution satellite remote-sensing might be potential in monitoring the source distribution of particulate pollutants. 相似文献
29.
Linear sandbanks are located globally in areas where there are strong currents and an abundance of sand. In the recent years,
these sandbanks have become of strategic interest as a potential source of marine aggregates (sand and gravel) and mineral
deposits. They form the seaward boundary of the nearshore zone and therefore are important for the stability of the coastal
system. They also commonly reach the sea surface and thus pose a threat to navigation. Headland-associated linear sandbanks
are a specific type of sandbanks which are located in the lee of coastal topographic features such as headlands and islands.
Interaction between tidal currents and topographic features generate complex three-dimensional circulation patterns that significantly
influence the distribution of sediments in the vicinity of the feature. Field and numerical model investigations of the three-dimensional
flow structure have been undertaken on the Levillain Shoal, a headland-associated linear sandbank present in the lee of Cape
Levillain (Shark Bay, Western Australia). The field data indicated the presence of secondary flows near the tip of the cape
and around the bank which were re-produced in the numerical simulations. Numerical results have shown that residual eddies
are not representative of the sediment transport and that secondary currents enhance the convergence of sediment towards the
sandbank. Maintenance processes have been investigated. Sediment transport paths near the cape and the bank indicate that
the sandbank is part of a sand circulation cell where the sand is circulating around the bank with exchanges between the sandbank
and the headland. 相似文献
30.
Jerry Shannon Mathew E. Hauer Alexis Weaver Sarah Shannon 《The Professional geographer》2018,70(1):84-93
Although general patterns of food insecurity in the United States are known, few studies have attempted to estimate small area food security or account for ongoing socioeconomic changes. Here we address these issues by producing small area estimates of food insecurity in the Atlanta metropolitan area using two methodologies: fixed effects modeling and demographic metabolism. In both cases, we use county-level data from the Current Population Survey to determine the association between food insecurity and demographic predictors. These associations are then applied to tract-level data from the 2009 to 2013 American Community Survey and projected data for 2020 to create small area estimates of food insecurity. We find broad consensus between our two methods. For both time periods, food insecurity is highest in southern sections of the city of Atlanta and its neighboring suburbs. Projections to 2020, however, show that food insecurity rates are projected to increase in outer-ring suburbs east and west of the city while decreasing in the urban core. These results highlight the need to further adapt antihunger efforts for often sprawling suburban communities, where poverty rates are increasing but spatial mismatch combined with poor transit access might hinder access to food assistance. 相似文献