On barotropic and baroclinic tides over an arbitrary sloping topography

Some theoretical concepts about the frictionless dynamics of propagation of the barotropic tide over two-dimensional continental slopes of arbitrary shape are developed. A numerical procedure which generalizes the exact solution obtained over a rectilinear sloping topography is given. This technique can be applied to compute the harmonic components of the barotropic tide everywhere over sloping bottom contours of any shape. It permits in particular the avoidance of discontinuities at the boundaries of rectilinear-continental-slope profiles. The barotropic tidal results are used afterwards to calculate the barotropic forcing for the generation of internal tides. Numerical experiments are performed to study the interaction between the tide and some typical sloping topographies. A three-layered model is used for this purpose. Results are compared with those previously obtained over a rectilinear continental slope.     

Transport properties and microstructural characteristics of a thermally cracked mylonite

An experimental study was carried out on a granitic mylonite (La Bresse, France) to analyze the influence of pore microstructure on transport properties. Different crack networks were obtained by a controlled thermal treatment. Microstructures were analyzed by means of gas adsorption and mercury porosimetry. Transport properties have been investigated by measuring gas permeability and electrical conductivity. The dependence of permeability on confining pressure shows an exponential decrease, characteristic of a porosity made of cracks. Correlations between measured parameters have been analyzed by comparing them with relations deduced from theoretical models. Linking the formation factor to the porosity leads to a rather low tortuosity value (about 2.4), characterizing a medium with a well connected porosity. Correlation between permeabilityk and formation factorF leads to a power-law relationk F ^–n wheren2.9, which is consistent with a crack model describing the behavior of the thermally treated rock.     

Magnetic anomalies in the Shikoku Basin: a new interpretation

Kaiko surveys over the Nankai Trough made available new magnetic and structural data for the northern Shikoku Basin. A survey of the oceanic lithosphere subducting below Southwest Japan along the central Nankai Trough revealed the existence of several north-south basement troughs. They are probably transform faults related to a north-south spreading system. We examine the possibility of a late phase of north-south spreading limited to the axial northernmost Shikoku Basin, active between 14 and 12 Ma. If this system was already active before that time, i.e. during the N55° opening of the southeastern basin, then a triple junction should be found between both areas.Based on these data and previous studies we present a new interpretation of magnetic anomalies over the whole basin. From early east-west rifting to late north-south spreading, opening of the Shikoku Basin proceeded through multiple episodes of spreading. The analysis of magnetic anomalies constrains the kinematic evolution of the basin through time and space. Two successive counter-clockwise rotations of the spreading direction are postulated, at anomaly 6 (19 Ma) and at anomaly 5B (14 Ma), involving segmentation and rotation of the spreading ridge.     

Tracking flux melting and melt percolation in supra-subduction peridotites (Josephine ophiolite,USA)

Here, we investigate the scale and nature of melting and melt percolation processes recorded by 17 supra-subduction peridotites collected in a ~70 km² area in the northern portion of the Josephine ophiolite (Western USA). We present major and trace element variations in whole rocks; major elements in olivine, orthopyroxene, clinopyroxene and spinel; and trace elements [including rare earth element (REE)] in clinopyroxene and orthopyroxene. In the Josephine peridotites, compositional variability occurs at different scales. On the one hand, large systematic changes from depleted to fertile peridotites occur on large kilometer scales. Field, petrological and geochemical data can be consistently explained if the Josephine mantle experienced variable degrees of hydrous flux melting (10 to >20–23 %), and we argue that small fractions of subduction-derived fluids (0.015–0.1 wt%) were pervasive in the ~70 km² studied area, and continuously supplied during wedge melting. Fluid localization probably led to increased extent of flux melting in the harzburgitic areas. On the other hand, in single outcrops, sharp transitions from dunite to harzburgite to lherzolite and olivine websterite can be found on meter to centimeter scales. Thus, some fertile samples may reflect limited degrees of refertilization at the outcrop scale. In addition, clinopyroxene and orthopyroxene in ultra-depleted harzburgites (Spinel Cr# > 58) show variable degrees of LREE enrichment, which reflect percolation of and partial re-equilibration with, small fractions of boninite melt. Because the enriched samples also show the highest spinel Cr#, we argue that these enrichments are local features connected to the presence of dunite channels nearby. Lastly, trace element concentrations of pyroxenes in Josephine harzburgites show that they are one of the most depleted harzburgites among worldwide ophiolitic peridotites, indicating particularly high degrees of melting, potentially past the exhaustion of clinopyroxene.     

Duration and synchroneity of the largest negative carbon isotope excursion on Earth: The Shuram/Wonoka anomaly

Carbonate δ¹³C values provide a useful monitor of changes in the global carbon cycle because they can record the burial ratio of organic to carbonate carbon. The most pronounced isotope excursions in the geologic record occur during the Neoproterozoic and have assumed a central role in the interpretation of biogeochemical events preceding the Ediacaran and Cambrian radiations. The most profound negative carbon isotope excursion is best recorded in the Ediacaran-aged Shuram Formation of Oman and has potential equivalents worldwide including the Wonoka Formation of South Australia and other sections in China, India, Siberia, Canada, Scandinavia and Brazil. All these excursions are less well understood than those in the Phanerozoic because of their unusual magnitude, long duration (> 1 Ma) and the difficulty in correlating Neoproterozoic basins to confirm independently that they do indeed record global change in the mixed ocean reservoir. Alternatively, these δ¹³C anomalies could reflect diachronous diagenetic processes. Currently none of these excursion are firmly time constrained and critical to their interpretation is a coherent reproducibility and synchroneity at the global ocean scale. Here we use available strontium isotope record as an independent chronometer to test the timing and synchroneity of the Shuram δ¹³C and its potential equivalents. The use of the ⁸⁶Sr/⁸⁷Sr ratio allows the reconstruction of a coherent, global δ¹³C record calibrated independently against time. The calibrated δ¹³C curve indicates that the Shuram negative anomaly spans several tens of millions of years and reaches values below −10‰. This carbon isotopic anomaly therefore represents a meaningful oceanographic event that fundamentally challenges our understanding of the carbon cycle as defined in the Phanerozoic.     

Three-dimensional numerical modelling of the behaviour of a pile subjected to cyclic lateral loading

This paper presents a finite element simulation of the behaviour of a vertical pile subjected to a cyclic lateral loading. First, the choice of a suitable constitutive model is discussed. The analytical solution of the model equations for a monotonic triaxial compression is given, and the model is compared with monotonic and cyclic triaxial tests on dry Fontainebleau sand. Simulations of tests carried out on a model pile with the geotechnical centrifuge of the French Public Works Research Laboratory (LCPC) were then performed with the model parameters derived from the triaxial tests. Results are in good agreement with experiments.     

MHYDAS‐Erosion: a distributed single‐storm water erosion model for agricultural catchments

In this paper, we present MHYDAS‐Erosion, a dynamic and distributed single‐storm water erosion model developed as a module of the existing hydrological MHYDAS model. As with many catchment erosion models, MHYDAS‐Erosion is able to simulate sediment transport, erosion and deposition by rill and interrill processes. Its originality stems from its capacity to integrate the impact of land management practices (LMP) as key elements controlling the sedimentological connectivity in agricultural catchments. To this end, the water‐sediment pathways are first determined by a specific process‐oriented procedure defined and controlled by the user, which makes the integration of LMP easier. The LMP dynamic behaviours are then integrated into the model as a time‐dependent function of hydrological variables and LMP characteristics. The first version of the model was implemented for vegetative filters and tested using water and sediment discharge measurements at three nested scales of a densely instrumented catchment (Roujan, OMERE Observatory, southern France). The results of discharge and soil loss for simulated rainfall events have been found to acceptably compare with available data. The average R² values for water and sediment discharge are 0·82 and 0·83, respectively. The sensitivity of the model to changes in the proportion of LMP was assessed for a single rain event by considering three scenarios of the Roujan catchment management with vegetative filters: 0% (Scenario 1), 18% (Scenario 2, real case) and 100% (Scenario 3). Compared to Scenario 2 (real case), soil losses decreased for Scenario 3 by 65% on the agricultural plot scale, 62% on the sub‐catchment scale and 45% at the outlet of the catchment and increased for Scenario 1 by 0% on the plot scale, 26% on the sub‐catchment scale and 18% at the outlet of the catchment. Copyright © 2010 John Wiley & Sons, Ltd.     

江西崇义淘锡坑大型钨矿床成矿花岗岩体研究

简介了江西崇义淘锡坑大型钨矿床的地质背景、矿化特征,详细叙述了淘锡坑岩体的形态、分布、矿物成分、化学成分,归纳了最新的同位素年龄及地球化学数据,探讨了淘锡坑岩体作为成矿母岩的成矿作用,并对其整体形态进行了预测,分析了下步的找矿有利地段。     

从陕西旬邑上新生界剖面看黄土高原新近系/第四系界线

新近系 /第四系界线在黄土高原大都以磁性 M/ Ga界线为界进行划分 ,它与岩性分界往往并不一致 ,后者都低或高于前者一小段距离。根据旬邑剖面的岩性、孢粉、古动物、磁化率、粒度及地化资料 ,并参考了其他一些剖面的岩性 ,认为以二者之间的过渡层底为界进行划分比较合理     

Réponse hydrologique d’un bassin semi-aride aux événements pluviométriques et aménagements de versant (bassin du Merguellil,Tunisie centrale)

RÉSUMÉ

L’influence cumulée des aménagements de versants sur les écoulements en zones semi-arides demeure peu étudiée et comprise à l’échelle de grands bassins versants. En Tunisie centrale, nous étudions les variations de la réponse hydrologique du bassin du Merguellil à l’échelle annuelle et pour 114 événements entre 1989 et 2010. Sur cette période, les aménagements de conservation des eaux et des sols ont progressivement drainé de 5% à 30% des surfaces du bassin. L’analyse statistique révèle la forte variabilité des pluies et des débits annuels, mais ne distingue aucune tendance déficitaire. L’approche événementielle démontre que les variations des écoulements annuels sont liées aux apports générés par un nombre très limité (cinq à six par an en moyenne) d’épisodes pluvieux supérieurs à 15 mm. Le nombre très variable de ces évènements ainsi que les écarts importants observés sur leur coefficient de ruissellement (K_R) respectif sont déterminants sur les apports annuels. Pour les événements de plus de 60 mm, les différences d’intensités pluviométriques, de couvert végétal et d’humidité du sol réduisent le ruissellement jusqu’à 80%. Toutefois, les K_R d’averses de moins de 40 mm, sur des sols au couvert végétal et d’humidité semblables, ont également diminué de 45% après l’extension des aménagements de CES à la fin des années 1990.