Quantitative morphometric analysis of lakes using GIS: rectangularity R,ellipticity E,orientation O,and the rectangularity vs. ellipticity index,REi

Quantitative measures of polygon shapes and orientation are important elements of geospatial analysis. These kinds of measures are particularly valuable in the case of lakes, where shape and orientation patterns can help identifying the geomorphological agents behind lake formation and evolution. However, the lack of built-in tools in commercial geographic information system (GIS) software packages designed for this kind of analysis has meant that many researchers often must rely on tools and workarounds that are not always accurate. Here, an easy-to-use method to measure rectangularity R, ellipticity E, and orientation O is developed. In addition, a new rectangularity vs. ellipticity index, REi, is defined. Following a step-by-step process, it is shown how these measures and index can be easily calculated using a combination of GIS built-in functions. The identification of shapes and estimation of orientations performed by this method is applied to the case study of the geometric and oriented lakes of the Llanos de Moxos, in the Bolivian Amazon, where shape and orientation have been the two most important elements studied to infer possible formation mechanisms. It is shown that, thanks to these new indexes, shape and orientation patterns are unveiled, which would have been hard to identify otherwise.     

Small-scale ductile shear zones as transposed rheologically weak domains: A numerical modeling investigation and practical application

We test the hypothesis that small ductile shear zones are developed from initial rheologically weak domains. We regard weak domains as ellipsoidal inhomogeneities and apply Eshelby's formalism extended for power-law viscous materials to investigate the kinematics and finite strain evolution of the partitioned flow fields in weak domains. We show that, under an imposed bulk flow field, weak domains, regardless of their initial shapes and orientations, deform into zone-like features at relatively small bulk strains and the deformation paths inside weak domains have all characteristics expected in ductile shear zones. We apply our model to the Cap de Creus area Spain, where abundant small ductile shear zones exist. To relate the fabrics inside these shear zones with the regional deformation, we take a multi-scale approach. We assume that the area contained many weak domains which were randomly shaped and oriented initially and were deformed into shear zones eventually in response to the regional flow. We constrain the regional flow field by the fabric patterns across the area and compute numerically the partitioned flow fields in individual weak domains. The latter are related to fabrics inside shear zones. Our model reproduces first-order features of field-observed fabrics. Although the deformation path of each shear zone was close to simple shearing, the deformation of the whole belt was not. Our approach also resolves the strain compatibility problem for a finite-sized shear zone embedded in a far less deformed country rock.     

Sensitivity of the surface temperature to changes in total solar irradiance calculated with the WRF model

The temperature sensitivity of the WRF model to changes in Total Solar Irradiance (TSI). The simulations were performed for a region centered over the North Atlantic Ocean, including portions of Eastern North America, Western Europe and Northwest Africa. Four simulations were run with different TSI values. Also, a fifth simulation was performed in which we varied the initial atmospheric conditions, in order to compare the effect on the temperature of both, changes in the TSI and initial atmospheric conditions. Comparing temperature monthly averages we found that changes in TSI and in the initial conditions have a measurable impact on temperature in the region of study. The sensitivity of the model using non-dimensional parameters was also estimated. The numerical experiments show some features that might allow to distinguish between the effects on the temperature due to changes in TSI from those caused by initial conditions. However, TSI changes are of the same order of magnitude than those of disturbances in the initial conditions. We also found that the mean monthly values of temperature over the full grid, did not present significant variations due to changes of either initial conditions or TSI.     

A spectrophotometric measurement of soil cation exchange capacity based on cobaltihexamine chloride absorbance

Among numerous methods for cation exchange capacity (CEC) determination for soils and sediments, the cobaltihexamine chloride method is frequently used due to its ability to measure CEC at soil pH. After exchange with Co(NH₃)₆³⁺ ions, CEC is estimated via the measurement of the Co remaining in solution. The modified method proposed allows a more rapid determination of CEC based on the measurement of the absorbance at 472 nm of the cobaltihexamine chloride solution before and after exchange. This method has been applied to various soil's horizons from four sites, selected to cover a wide range of CEC and pH values. The model obtained allows one to calculate CEC from absorbance at 472 nm with 95% confidence intervals. As CEC is of relevant meaning in agronomical and environmental purposes, and more recently in ecotoxicological studies, this modified method can be proposed as a rapid test for CEC evaluation.     

Albian extrusion evidences of the Triassic salt and clues of the beginning of the Eocene atlasic phase from the example of the Chitana-Ed Djebs structure (N.Tunisia): Implication in the North African Tethyan margin recorded events, comparisons

In the northern part of Tunisia, close to Testour/Slouguia, new observations and updated biostratigraphy make it possible to highlight the relation between the Triassic saliferous mass and the surrounding Mesozoic beds (T. M.). Near the (T. M.) boundary, the formations observed consist dominantly of Triassic evaporites reworked in the Early and Late Albian deep-water sedimentary deposits. Throughout the studied area, Jurassic rocks are absent. We propose to interpret the Chitana-Ed Djebs structure originally emplaced as gravitational stretch masses in a passive margin in the same way as the salt bodies of widespread salt province in the Gulf of Mexico. A reconstructed schematic position of the Chitana-Ed Djebs salt body displays a scenario of setting of the salt mass on a submarine palaeo-slope. Moreover, the starting clues of the paroxysmal event of the Late Mesozoic tectonic inversion clearly fossilized through the discordance of the Middle Eocene–Early Lutetian limestone on the Albian series.     

Geochemistry constraints of Mesozoic–Cenozoic calc-alkaline magmatism in the South Shetland arc, Antarctica

Geochemical data from basalts, basaltic andesites, and andesites of the Mesozoic–Cenozoic (143–44 Ma) from Livingston, Greenwich, Robert, King George, and Ardley Islands of the South Shetland archipelago, Antarctica, are presented. The rocks have variable SiO₂ of approximately 46–61 wt%, Al₂O₃ of 15–26 wt%, and total alkali (K₂O+Na₂O) of 2–6 wt%. Most samples have low Mg#, Cr, and Ni, which indicates that they have undergone significant fractional crystallization from mantle-derived melts. The presence of olivine cumulatic in the samples from Livingston and Robert Islands explains some high MgO, Ni, and Cr values, whereas low Rb, Zr, and Nb values could be related to undifferentiated magmas. N-MORB-normalized trace element patterns show that South Shetland Islands volcanic rocks have a geochemical pattern similar to that found for other island arcs, with enrichment in LILE relative to HFSE and in LREE relative to HREE. The geochemistry pattern and presence of calcic plagioclase, orthopyroxene, Mg-olivine, and titanomagnetite phenocrysts suggest a source related to the subduction process. The geochemical data also suggest magma evolution from the tholeiitic to the calc-alkaline series; some samples show a transitional pattern. Samples from the South Shetland archipelago show moderate LREE/HREE ratios relative to N-MORB and OIB, depletion in Nb relative to Yb, and high Th/Yb ratios. These patterns probably reflect magma derived from a lithospheric mantle source previously modified by fluids and sediments from a subduction zone.

Resumo

Dados geoquímicos de basaltos, andesitos basálticos e andesitos mesozóicos–cenozóicos (143–44 Ma) das ilhas Livingston, Greenwich, Robert, King George e Ardley do Arquipélago Shetland do Sul, Antártica são discutidas neste artigo. As rochas tem conteúdos de SiO₂ variando de 46 a 61%, Al₂O₃ de 15 a 26% e álcalis (K₂O+Na₂O) de 2 a 6%. A maior parte das amostras tem conteúdos baixos de Mg#, Cr e Ni, indicando que sofreram significante cristalização fracionada de fusões derivadas do manto. A presença de fases cumuláticas nas amostras das ilhas Livingston e Robert explicaria os elevados valores de MgO, Ni, Cr, enquanto que baixos valores de Rb, Zr e Nb observados nas amostras destas ilhas poderiam estar relacionados a magmas não diferenciados. Os padrões de elementos-traço normalizados pelo N-MORB mostram que as rochas vulcânicas das Ilhas Shetland do Sul têm padrão geoquímico similar àqueles encontrados em outros arcos de ilhas com enriquecimento em LILE em relação aos HFSE e em ETRL em relação aos ETRP. O padrão geoquímico e a ocorrência de fenocristais de plagioclásio cálcico, ortopiroxênio, olivina magnesiana e titanomagnetita sugerem origem relacionada a processos de subducção. Dados geoquímicos obtidos para as amostras do arquipélago Shetland do Sul sugerem um magma evoluindo de toleítico para cálcico-alcalino, observando-se em algumas amostras um padrão transicional. As amostras do arquipélago Shetland do Sul mostram em relação ao N-MORB e OIB, moderadas razões ETRL/ETRP, empobrecimento em Nb relativo a Yb e elevada razão Th/Yb Estes padrões refletem, provavelmente, magma derivado de uma fonte mantélica litosférica, que foi modificada por fluídos e sedimentos da zona de subducção.     

(Re) deposition of loess in southern Limbourg,the Netherlands 3. Field evidence for conditions of deposition of the middle and upper silt loam complexes,and landscape evolution at Nagelbeek

This paper is about conditions and processes during the accumulation of the Upper Pleniglacial Middle and Upper Silt Loam complexes at Nagelbeek, Limbourg, a Weichselian stratigraphical type locality. The complexes correlate with Hesbayan and Brabantian loesses in Belgium, formed between 28,000 and 12,400 yrs BP. They are characterized by sediment features (cracks, mass deformations, and cut-and-fill features) that are ascribed to a cryogenic regimen. The main Middle Silt Loam subunit contains contraction cracks deformed and further modified by masswasting. Intra-Upper-Pleniglacial pedogenic intervals are inferred from three greyish bands, from a truncated weathered zone surmounting the Middle Silt Loam, and from an aeolian-cumulic palaeosol at the base of the Upper Silt Loam. Latter two zones and an intervening erosional unconformity were cryogenically reworked, less than 22,000 yrs BP, to form a complex stratigraphical marker-zone: the ‘Horizon à Langues de Nagelbeek’. Morphological variants of the marker-zone are described. The geomorphic evolution of the Nagelbeek upland, from Saalian to Holocene times, is summarized in unconformity maps and a time diagram.     

Tectonique transpressive en terre Adélie au Paléoprotérozoı̈que (Est Antarctique)Palaeoproterozoic transpression in Terre Adélie (East Antarctica)

The Palaeoproterozoic units of Terre Adélie show two types of structural domains associated with HT–LP metamorphic conditions: domes and NS–N340° striking vertical shear zones. Shear zones reflect dextral transpressive motions. Domes reflect sub-vertical shortening and principal stretching subparallel to shear zones. They could partly result from longitudinal flow coeval with transpression. Deformations are comparable to those described along the eastern and western boundaries of the Archean Gawler Craton (South-East Australia), which underlines the continuity between these two areas before opening of the Austral Ocean. To cite this article: A. Pelletier et al., C. R. Geoscience 334 (2002) 505–511.     

大别造山带北部的中生代火山岩

大别造山带北部的北淮阳中生代火山喷发岩带形成于后造山阶段的晚侏罗世—早白垩世。火山岩可以划分为２个独立的火山旋回,分别对应于高钾钙碱性系列（ＨＫＣＡ）和钾玄岩系列（ＳＨＯ）,从岩石构造组合和岩石地球化学数据提供的约束条件分析,前者形成于晚侏罗世的陆内挤压环境,造山带是有“山根”的增厚陆壳,而早白垩世钾玄岩系列岩石的出现表征着造山带已发生“去根”作用,北淮阳处于陆壳减薄的拉张环境     

Application du réseau des neurones artificiels à la prévision des débits horaires: Cas du bassin versant de l’Eure,France

RÉSUMÉ

Les modèles pluie–débit sont fortement utilisés dans la gestion des risques hydrologiques et la prévision des crues. Dans cette étude, nous présentons un modèle pluie–débit pour la prévision des débits horaires basé sur la technique des réseaux de neurones artificiels (RNA). Ce modèle a été développé et appliqué sur le bassin versant de l’Eure au Nord-Ouest de la France afin de dépasser les problèmes dus à la non-linéarité de la relation pluie–débit et à l’imprécision des données collectées. La création de ce modèle a nécessité plusieurs étapes pendant lesquelles nous avons pu déterminer les paramètres du modèle permettant la compréhension de la complexité hydrologique et la production des informations nécessaires à la prévision. Elles ont abouti à un modèle de réseau de neurones artificiels capable d’effectuer, en quelques secondes, des prévisions des crues efficaces jusqu’à un horizon de prévision de 48 h. Ces résultats confirment que les modèles RNA peuvent jouer un rôle important dans le domaine de la prévision car capables de modéliser la non-linéarité des relations pluie–débit rencontrées sur certains bassins hydrologiques.

Editor Z.W. Kundzewicz Associate editor E. Gargouri