A methodology for regionalizing 3‐D effective porosity at watershed scale in crystalline aquifers          下载免费PDF全文

Benoît Dewandel  Yvan Caballero  Jérôme Perrin  Alexandre Boisson  Fabrice Dazin  Sylvain Ferrant  Subash Chandra  Jean‐Christophe Maréchal 《水文研究》2017,31(12):2277-2295

An innovative approach for regionalizing the 3‐D effective porosity field is presented and applied to two large, overexploited, and deeply weathered crystalline aquifers located in southern India. The method derives from earlier work on regionalizing a 2‐D effective porosity field in that part of an aquifer where the water table fluctuates, which is now extended over the entire aquifer using a 3‐D approach. A method based on geological and geophysical surveys has also been developed for mapping the weathering profile layers (saprolite and fractured layers). The method for regionalizing 3‐D effective porosity combines water table fluctuation and groundwater budget techniques at various cell sizes with the use of satellite‐based data (for groundwater abstraction), the structure of the weathering profile, and geostatistical techniques. The approach is presented in detail for the Kudaliar watershed (983 km²) and tested on the 730 km² Anantapur watershed. At watershed scale, the effective porosity of the aquifer ranges from 0.5% to 2% in Kudaliar and between 0.3% and 1% in Anantapur, which agrees with earlier works. Results show that (a) depending on the geology and on the structure of the weathering profile, the vertical distribution of effective porosity can be very different and that the fractured layers in crystalline aquifers are not necessarily characterized by a rapid decrease in effective porosity and (b) that the lateral variations in effective porosity can be larger than the vertical ones. These variations suggest that within a same weathering profile, the density of open fractures and/or degree of weathering in the fractured zone may significantly vary from a place to another. The proposed method provides information on the spatial distribution of effective porosity that is of prime interest in terms of flux and contaminant transport in crystalline aquifers. Implications for mapping groundwater storage and scarcity are also discussed, which should help in improving groundwater resource management strategies.  相似文献   

Numerical simulation of the response of a reinforced wall to a high speed train passage          下载免费PDF全文

A. Corfdir  E. Bourgeois  J.‐B. Payeur 《国际地质力学数值与分析法杂志》2017,41(11):1285-1303

The dynamic response of a mechanically stabilized earth wall to the passing of a high‐speed train is modelled using the finite element method. A three‐dimensional analysis is carried out, using a specific framework that allows performing the analysis with a moderate computational effort. In the first place, a so‐called multiphase approach is used to take into account the reinforcing strips. The moving load is taken into account by performing the calculation in a mobile referential using the properties of symmetry of the train cars and a simplifying assumption of periodicity for the whole train. We also assume a steady state. A partial validation of the approach is obtained by means of a comparison with an analytical solution. The quick increase in displacements induced by the train passing when the speed comes close to the celerity of Rayleigh waves clearly appears in the results. The vertical displacements, vertical stresses in the backfill, tensile forces in the strips and the influence of the stiffness of the soil are discussed. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Understory and small trees contribute importantly to stemflow of a lower montane cloud forest          下载免费PDF全文

Teresa Margarita González‐Martínez  Guadalupe Williams‐Linera  Friso Holwerda 《水文研究》2017,31(5):1174-1183

Stemflow (Sf) measurements in tropical rain and montane forests dominated by large trees rarely include the understory and small trees. In this study, contributions of lower (1‐ to 2‐m height) and upper (>2‐m height and <5‐cm diameter at breast height [DBH]) woody understory, small trees (5 < DBH < 10 cm), and canopy trees (>10‐cm DBH) to Sf per unit ground area (Sf_a) of a Mexican lower montane cloud forest were quantified for 32 days with rainfall (P) during the 2014 wet season. Rainfall, stemflow yield (Sf_y), vegetation height, density, and basal area were measured. Subsequently, stemflow funneling ratios (SFRs) were calculated, and three common methods to scale up Sf_y from individual trees to the stand level (tree‐Sf_y correlation, P‐Sf_y correlation, and mean‐Sf_y extrapolation) were used to calculate Sf_a. Understory woody plants, small trees, and upper canopy trees represented 96%, 2%, and 2%, respectively, of the total density. Upper canopy trees had the lowest SFRs (1.6 ± 0.5 Standard Error (SE) on average), although the lower understory had the highest (36.1 ± 6.4). Small trees and upper understory presented similar SFRs (22.9 ± 5.4 and 20.2 ± 3.9, respectively). Different Sf scaling methods generally yielded similar results. Overall Sf_a during the study period was 22.7 mm (4.5% of rainfall), to which the understory contributed 70.1% (15.9 mm), small trees 10.6% (2.4 mm), and upper canopy trees 19.3% (4.4 mm). Our results strongly suggest that for humid tropical forests with dense understory of woody plants and small trees, Sf of these groups should be measured to avoid an underestimation of overall Sf at the stand level.  相似文献   

δ18O and δ13C in fossil shells of Radix sp. from the sediment succession of a dammed palaeo‐lake in the Yarlung Tsangpo valley,Tibet, China     

Hai‐Ping Hu  Jin‐Liang Feng  Feng Chen 《Boreas: An International Journal of Quaternary Research》2017,46(3):412-427

A series of confirmed and suspected dammed palaeo‐lake sedimentary successions is scattered within the middle Yarlung Tsangpo valley in Tibet. However, the chronology, the genesis of the dam and its location, the water level of the dammed lake, the process of dam failure and the spatiotemporal relationships between the sedimentary successions remain controversial. Here, we focus on one sedimentary succession of the suspected dammed palaeo‐lake at Xigazê. We measured the grain‐size distribution, magnetic susceptibility, organic and inorganic carbon content, and δ¹³C_org and δ¹⁵N_total ratios of the sediments. In addition, we measured the δ¹⁸O_shell and δ¹³C_shell values of modern and fossil Radix sp. shells, and the δ¹⁸O_water and δ¹³C_DIC values of the ambient water with different hydrological regimes. The results indicate that the δ¹⁸O_shell values of modern Radix sp. and the δ¹⁸O_water of the ambient water body significantly depend on its hydrological status. In addition, a strong positive relationship was observed between δ¹⁸O_shell values of modern Radix sp. shells and the δ¹⁸O_water of the ambient water on the Tibetan Plateau. According to this correlation, the δ¹⁸O_water values of the palaeo‐water body are reconstructed using the δ¹⁸O_shell values of Radix sp. fossil shells in the Xigazê section. Further, based on the δ¹⁸O_shell values of fossil Radix sp., the reconstructed δ¹⁸O_water of the palaeo‐water body and the specific habitats of Radix sp., we infer that the sedimentary succession in the Xigazê broad valley was mainly formed within the backwater terminal zone of a dammed palaeo‐lake and that the elevation of the water level of the lake was approximately 3811 m a.s.l. AMS ¹⁴C dating indicates that the deposits of the dammed palaeo‐lake were formed at about 33–22 cal. ka BP. Finally, the presence of Radix sp. fossil shells within the Xigazê section suggests that Radix sp. survived the late Last Glacial Period on the Tibetan Plateau.  相似文献   

Insights into the complexity of crustal differentiation: K2O‐poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt,South Africa          下载免费PDF全文

G. Nicoli  G. Stevens  J‐F. Moyen  A. Vezinet  M. Mayne 《Journal of Metamorphic Geology》2017,35(9):999-1022

Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well‐exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm‐ to m‐scale, K₂O‐poor garnet‐bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), also known as future leucosome; (3a) Re‐equilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H₂O, K₂O), which triggers the co‐precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt‐dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction, and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H₂O and K₂O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K₂O‐poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling, is <5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis.  相似文献   

Mid‐Weichselian interstadial in Kolari,western Finnish Lapland     

Veli‐Pekka Salonen  Julien Moreau  Outi Hyttinen  Kari O. Eskola 《Boreas: An International Journal of Quaternary Research》2014,43(3):627-638

Finnish Lapland is known as an area where numerous sites with sediments from Pleistocene glacial and interglacial periods occur. Recent sedimentological observations and dating call for reinterpretation of the record, which shows a complicated Mid‐Weichselian ice‐sheet evolution within the ice‐divide zone. Here, a large, previously unstudied section from a former Hannukainen iron mine was investigated sedimentologically and dated with optically stimulated luminescence (OSL). Ten sedimentary units were identified displaying a variety of depositional environments (glacial, glaciolacustrine, fluvial and aeolian). They are all – except for the lowermost, deeply weathered till – interpreted to be of Mid‐ or Late Weichselian/Holocene age. Five OSL samples from fluvial sediments give ages ranging from 55 to 35 ka, indicating two MIS 3 ice‐free intervals of unknown duration. The Mid‐Weichselian interstadial was interrupted by a re‐advance event, which occurred later than 35 ka and caused glaciotectonic deformation, folding and stacking of older sediments. This new evidence emphasizes the importance of the Kolari area when unravelling the complex Late Pleistocene glacial history of northern Finland and adjacent regions.  相似文献   

Zircon fission‐track ages from Newfoundland—A proxy for high geothermal gradients and exhumation before opening of the Central Atlantic Ocean     

Arne P. Willner  Stuart N. Thomson  Johannes Glodny  Hans‐Joachim Massonne  Rolf L. Romer  Cees R. van Staal  Alexandre Zagorevski 《地学学报》2019,31(1):1-10

Following Appalachian orogenesis, metamorphic rocks in central Newfoundland were exhumed and reburied under Tournaisian strata. New zircon fission‐track (ZFT) ages of metamorphic rocks below the Tournaisian unconformity yield post‐depositionally reset ages of 212–235 Ma indicating regional fluid‐absent reheating to at least ≥220°C. Post‐Tournaisian sedimentary thicknesses in surrounding basins show that burial alone cannot explain such temperatures, thus requiring that palaeo‐geothermal gradients increased to ≥30–40°C/km before final late Triassic accelerated cooling. We attribute these elevated palaeo‐geothermal gradients to localized thermal blanketing by insulating sediments overlying radiogenic high‐heat‐producing granitoids. Late Triassic rifting and magmatism before break up of Pangaea likely also contributed to elevated heat flow, as well as uplift, triggering late Triassic accelerated cooling and exhumation. Thermochronological ages of 240–200 Ma are seen throughout Atlantic Canada, and record rifting and basaltic magmatism on the conjugate margins of the Central Atlantic Ocean preceding the onset of oceanic spreading at ~190 Ma.  相似文献   

Modeling the 2D behavior of dry‐stone retaining walls by a fully discrete element method          下载免费PDF全文

James J. Oetomo  Eric Vincens  Fabian Dedecker  Jean‐Claude Morel 《国际地质力学数值与分析法杂志》2016,40(7):1099-1120

The dry‐stone retaining walls (DSRW) have been tipped as a promising solution for sustainable development. However, before recently, their behavior is relatively obscure. In this study, discrete element method (DEM) approach was applied to simulate the plane strain failure of these walls. A commercial DEM package (PFC2D™) was used throughout this study. The authors used a fully discrete approach; thus, both the wall and the backfill were modeled as discrete elements. The methodology for obtaining the micromechanical parameters was discussed in detail; this includes the three mechanical sub‐systems of DSRWs: wall, backfill and interface. The models were loaded progressively until failure, and then the results were compared with the full‐scale experimental results where the walls were loaded, respectively, with hydrostatic load and backfill. Despite its complexity and its intensive calculation time, DEM model can then be used to validate a more simplified approach. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Unique achondrite Northwest Africa 11042: Exploring the melting and breakup of the L chondrite parent body     

Zoltn Vci  Carl B. Agee  Munir Humayun  Karen Ziegler  Yemane Asmerom  Victor Polyak  Henner Busemann  Daniela Krietsch  Matthew Heizler  Matthew E. Sanborn  Qing‐Zhu Yin 《Meteoritics & planetary science》2020,55(3):622-648

Northwest Africa (NWA) 11042 is a heavily shocked achondrite with medium‐grained cumulate textures. Its olivine and pyroxene compositions, oxygen isotopic composition, and chromium isotopic composition are consistent with L chondrites. Sm‐Nd dating of its primary phases shows a crystallization age of 4100 ± 160 Ma. Ar‐Ar dating of its shocked mineral maskelynite reveals an age of 484.0 ± 1.5 Ma. This age coincides roughly with the breakup event of the L chondrite parent body evident in the shock ages of many L chondrites and the terrestrial record of fossil L chondritic chromite. NWA 11042 shows large depletions in siderophile elements (<0.01×CI) suggestive of a complex igneous history involving extraction of a Fe‐Ni‐S liquid on the L chondrite parent body. Due to its relatively young crystallization age, the heat source for such an igneous process is most likely impact. Because its mineralogy, petrology, and O isotopes are similar to the ungrouped achondrite NWA 4284 (this work), the two meteorites are likely paired and derived from the same parent body.  相似文献   

Segmentation and kinematics of the North America‐Caribbean plate boundary offshore Hispaniola          下载免费PDF全文

Sylvie Leroy  Nadine Ellouz‐Zimmermann  Jordane Corbeau  Frédérique Rolandone  Bernard Mercier de Lépinay  Bertrand Meyer  Roberte Momplaisir  José‐Luis Granja Bruña  Anne Battani  Céline Baurion  Evgueni Burov  Valérie Clouard  Rémy Deschamps  Christian Gorini  Youri Hamon  Manfred Lafosse  Jottin Leonel  Laetitia Le Pourhiet  Pilar Llanes Estrada  Nicolas Loget  Francis Lucazeau  Daniel Pillot  Jeffrey Poort  Kevin R. Tankoo  José‐Luis Cuevas  José‐Fernando Alcaide  Claude Jean Poix  Alfonso Muñoz‐Martin  Serge Mitton  Yamil Rodriguez  Julien Schmitz  Leonardo Seeber  Andres Carbo‐Gorosabel  Santiago Muñoz 《地学学报》2015,27(6):467-478

We explored the submarine portions of the Enriquillo–Plantain Garden Fault zone (EPGFZ) and the Septentrional–Oriente Fault zone (SOFZ) along the Northern Caribbean plate boundary using high‐resolution multibeam echo‐sounding and shallow seismic reflection. The bathymetric data shed light on poorly documented or previously unknown submarine fault zones running over 200 km between Haiti and Jamaica (EPGFZ) and 300 km between the Dominican Republic and Cuba (SOFZ). The primary plate‐boundary structures are a series of strike‐slip fault segments associated with pressure ridges, restraining bends, step overs and dogleg offsets indicating very active tectonics. Several distinct segments 50–100 km long cut across pre‐existing structures inherited from former tectonic regimes or bypass recent morphologies formed under the current strike‐slip regime. Along the most recent trace of the SOFZ, we measured a strike‐slip offset of 16.5 km, which indicates steady activity for the past ~1.8 Ma if its current GPS‐derived motion of 9.8 ± 2 mm a^?1 has remained stable during the entire Quaternary.  相似文献