Shear strength and shear bands of anisotropic sand

Acta Geotechnica - Due to sedimentation, irregular particles of sand arrange anisotropically in nature. Anisotropy diversifies the internal friction angles between relative research planes at...     

Shear bands and related extensional structures in a mylonitized quartz dyke

Chiefly monomineralic quartz dykes in the Canigó massif (Eastern Pyrenees) are affected by mylonitic deformation. Deformation took place under low-temperature conditions and produced incipient to well-developed mylonitic foliation. In the deformed quartz, grainsize reduction was mainly achieved by brittle failure of crystals along two conjugate sets of cracks. Two sets of conjugate shear bands are also present even in samples with poorly developed mylonitic foliation. Cracks and shear bands are consistent with an extension parallel to foliation. Both are simultaneously present in all the studied samples. Hydrolytic weakening may have aided the development of ductile structures.     

The role of discrete heterogeneities and linear fabrics in the formation of crenulations

The fabric elements in pelitic rocks of Brioverian age in South Finistere (France) consist of three main components; planar, linear and discrete, corresponding to schistosity, elongate mineral alignment and porphyroblast distribution respectively. Each component plays a role in the initiation, propagation and evolution of crenulations which deform the planar anisotropy.Strain associated with the initiation of crenulation is heterogeneous on the thin section scale, producing spatially associated, contemporaneous microshears and kink-structures. The influence of porphyroblasts becomes increasingly evident as crenulation develops and microshearing becomes the dominant mode of deformation. Linear fabric appears to influence the orientation of crenulation structures and slip directions on microshear planes.     

Effects of mechanical layering on hydraulic fracturing in shale gas reservoirs based on numerical models

Generally, induced hydraulic fractures are generated by fluid overpressure and are used to increase reservoir permeability through forming interconnected fracture systems. However, in heterogeneous and anisotropic rocks, many hydraulic fractures may become arrested or offset at layer contacts under certain conditions and do not form vertically connected fracture networks. Mechanical layering is an important factor causing anisotropy in sedimentary layers. Hence, in this study, with a shale gas reservoir case study in the Longmaxi Formation in the southeastern Chongqing region, Sichuan Basin, we present results from several numerical models to gain quantitative insights into the effects of mechanical layering on hydraulic fracturing. Results showed that the fractured area caused by hydraulic fracturing indicated a linear relationship with the neighboring layer’s Young’s modulus. An increase of the neighboring layer’s Young’s modulus resulted in better hydraulic fracturing effects. In addition, the contact between two neighboring layers is regarded as a zone with thickness and mechanical properties, which also influences the effects of hydraulic fracturing in reservoirs. The initial hydraulic fracture was unable to propagate into neighboring layers under a relatively low contact’s Young’s modulus. When associated local tensile stresses exceeded the rock strength, hydraulic fractures propagated into neighboring layers. Moreover, with the contact’s Young’s modulus becoming higher, the fractured area increased rapidly first, then slowly and finally became stable.     

地理信息系统（GIS）中的数据分层和应用

大千世界万物众生,具有物以类聚、人以群分的客观规律,因而促使对客观对象的科学研究皆可采用分门别类的方法去进行,这一理念同样可以延伸到地图制图及地理信息系统等以地物、地图为研究对象的学科中。由此衍生的数据分层是地理信息系统的关键词之一,科学有效的数据分层,是数据管理、GIS地图编制、制图综合、专题制图、空间分析的前提,国内、外著名的CAD、GIS软件都充分利用了层概念和其技术。在讨论层概念的基础上,利用两个在地学界应用比较广泛的和著名的GIS软件即我国具有自主版权的MapGIS和来自美国的ArcGIS,仔细研究和分析了层概念在软件中的应用;经研究进一步表明,数据分层是GIS技术得以进行空间数据管理、表达及进行空间分析的基础;在此基础上,对主题层、数据层、图层进行了概念上的界定,以此帮助对GIS技术的理解和应用。     

Physico-chemical models of the internal structure of partially differentiated Titan

We analyze models of the internal structure of Titan, a large icy satellite of the Saturn system. Calculations are carried out using information on the mass, mean density, moment of inertia, orbital parameters, and elastic properties of the satellite obtained by the Cassini–Huygens mission, as well as geochemical data on the composition of chondrite materials, equations of state of water and ices I, III, V, VI, and VII, and thermodynamic models for conductive heat transfer in the outer icy crust and of global convection in the interior zones of the satellite. The analysis of the models shows that models of partially differentiated Titan are most consistent; they include an outer water–ice shell, an intermediate ice–rock mantle, and an inner rock–iron core. It is shown that for the models of this type the maximum thickness of the water–ice shell is 460–470 km; it can be composed of an outer conductive crust of Ih ice 80–110 km thick and a subsurface water ocean 200–300 km deep. The maximum radius of the central rock–iron core of Titan can reach ~1300 km. The thickness of Titan’s ice–rock mantle does not exceed 2100 km at a density of 1.22–2.64 g/cm³. The model of partially differentiated Titan is feasible in the moment of inertia range of 0.312 < I/MR ² < ~0.350.     

Origins and implications of soil layering

Layering is common in soils, due to a variety of pedologic and geologic processes, and has important consequences for the interpretation of soils and landscapes. Layering can derive from original sedimentary layering; depositional upbuilding; episodic surface erosion, deposition, and stability; soil production by weathering; vertical or lateral translocation; bioturbation; and various combinations of these. Complex and polygenetic models incorporate both geogenic and pedogenic processes, and allow for physical and biological processes, as well as both vertical and horizontal movements. We review these conceptual frameworks and synthesize them into a vertical contrast model (VCM) for interpreting layered surficial materials. The VCM incorporates a variety of geologic and pedologic processes which may create, destroy, enhance, or obscure vertical contrasts. The model is illustrated via application to sites in the Ouachita Mountains, USA, and northwest Saxonian Lowlands, Germany. The examples illustrate the importance of a comprehensive pedogeomorphic interpretation of layering, since neither standard stratigraphic or top–down pedogenetic principles necessarily apply. The examples also show that the same process can, sometimes contemporaneously, both create and destroy vertical contrasts.     

Silica‐rich layering at Blanche Point,South Australia

The chemistry and mineralogy of much of the Late Eocene Blanche Point Formation of South Australia show that biogenic and volcanogenic products were the only significant contributor to the sedimentary record. Intermittent volcanic activity followed by dissolution of the resultant silicic ash and small scale migration of silicon with reprecipitation as the oxide, provides the simplest and most likely explanation for the repetitive nature of the silicification. Seemingly, this was controlled by local silica concentrations which in turn were apparently controlled by the biota. Changes in circulation patterns and/or water depth may have initiated the environmental variations recorded in the formation.     

地表基质综合调查：内涵、分层、填图与支撑目标

地表基质综合调查评价是支撑自然资源管理和服务宜林宜草、宜耕宜荒的基础性工作。关于地表基质综合调查的理论基础、目标定位、科学内涵、填图方法及分类分层等是目前探索的焦点与热点。文章在近年来承德生态文明示范区地表基质综合调查与多尺度填图的基础上,结合全国不同典型地区地表基质调查研究试点工作,进一步厘清了地表基质的科学内涵与支撑服务目标,建立了地表基质第三级分类方案和填图单元及命名方式,提出了地表基质调查中生产层、生态层和生活层的深度范围,最后从支撑塞罕坝地区植树造林和坝上地区狼毒草生态防治两个案例揭示了地表基质对植被群落的约束作用。研究成果可为深入理解地表基质的科学意义和开展全国范围的地表基质调查、监测、评价、区划提供基础参考。     

Anorthosite-adamellite-troctolite layering in the Barth Island structure of the Nain complex, Labrador

The Barth Island structure is an oval, 9 by 6 km body of layered igneous rocks. The central part contains a rhythmically layered troctolite-adamellite sequence. The sequence is repeated outward, but appears overturned, strongly sheared and reduced in thickness. Outward follows an adamellite-anorthosite sequence in which the effects of shear gradually diminish. In both sequences cryptic layering is evidenced by gradual change in mineral compositions. Isopleth maps based on the compositions of plagioclase, orthopyroxene and olivine show a circular high of An, En and Fo percentages in troctolite, gradually decreasing inward to a low in adamellitic rock. Between the highs in troctolite and the surrounding anorthosite occurs a low-temperature trough where both sequences grade into adamellitic rock. The structure may represent a deformed portion of pre-existing igneous layering. An alternative explanation would involve mixing of successive magma batches of gradually more sodic and ferrous composition.     

Ductile Shearing in Attur Shear Zone and its Relation with Moyar Shear Zone, South India

In the eastern part of southern Peninsular India, the charnockitic hills of the Madras block are cut across by the E-W trending Attur shear zone (ASZ) which is characterised by a thick (1 to 1.5 km) phyllonite zone, showing intense mylonitisation due to ductile shearing. Steeply plunging (70°–80°) stretching lineation on steeply dipping mylonitic foliation within this zone indicates a relative vertical upliftment of the adjacent blocks. A dextral shearing event from west to east is envisaged from the kinematic analysis of shear sense indicators such as S-C fabric, asymmetric folds, asymmetric augens and asymmetric porphyroclasts. Simultaneous development of these features, related to vertical and horizontal movements may be explained by the mechanism of transpressional deformation. The Attur shear zone may be correlated with the Moyar shear zone based on distinct lithological and structural similarities.     

A model for the development of crenulations in shear zones with applications from the southern appalachian piedmont

Two sets of crenulations are associated with a major, Alleghanian, dextral shear zone which deformed stratigraphic and structural boundaries in the eastern Piedmont of South Carolina. Both sets of crenulation planes are oblique to the boundaries of the shear zone. The morphologies and orientations of the crenulation sets and their spatial distributions indicate that they are related to slip along foliation planes, and that they serve to compensate for displacement components of foliation slip normal to the overall movement direction in the shear zone. The crenulations act to maintain the initial thickness of the shear zone. Our evaluation of the recent literature on shear zones suggests that crenulations related to foliation slip are abundant and constitute a reliable sense of shear indicator.     

Zoning and layering in diorites of the scottish caledonian appinite suite

Dioritic plugs (< 1 km across) are common associates of the late Caledonian, post-tectonic granites of the Scottish Highlands. These contain a very wide range of rock types from ultramafic through mesocratic diorites to anorthositic and granitic. These rocks form steeply dipping, wall-parallel layers and zones within the plugs. Outer layers are shown to form first, inner layers later. Mafic zones are composed of the minerals seen as phenocrysts in chilled margins and have the chemical characteristics of cumulates. The inner margins of the mafic zones reflect the onset of plagioclase crystallization. Core zones also show inward variations in mineralogy and texture which define vertical, wall-parallel cylinders. These variations are ascribed to fractional crystallization accompanying accretion of rock onto the pipe walls. Cores in different intrusions may be mafic, mesocratic, or leucocratic, which indicates vertical zonation in the bodies. The mafic cores additionally show that recharge with less evolved magma occurred in some cases. The few larger (> 1 km diameter) intrusions show examples of layering and lamination dipping at low to moderate angles. These indicate the beginning of a change from wall-dominated to floor-dominated crystallization as intrusion diameter increases. A comparison is made with the nature and origin of layering in other intrusions.     

Igneous layering through oscillatory nucleation and crystal settling in well-mixed magmas

Oscillatory or competitive nucleation about a binary (or perhaps pseudo binary) eutectic and ensuing crystal growth and settling is a commonly suggested means of producing layering in magmatic systems. A quantitative model is presented of this, outwardly, relatively simple process of crystal nucleation, growth, and settling in an otherwise initially crystal-free magma. Avrami-style kinetics of crystallization in an always wellmixed body, buried in conductive wall rock, are coupled to a Stokes-like formulation of crystal settling in magma whose viscosity depends on temperature and crystallinity. Two dimensionless numbers (Se, the settling number and Av, the Avrami or kinetic number) govern all the results. Av and Se measure the relative importance of crystallization time and settling time, respectively, relative to the overall cooling time. For any value of Av, which increases strongly with the maximum nucleation and growth rates and cooling time, layering is possible only over a range or window of values of Se. Both above and below this window a single layer (crystalline below, vitric above) forms, and within this window the number of layers increases systematically with increasing Av and Se. Grain size within any single layer generally coarsens upward. Because the characteristic settling and cooling times both depend on body thickness, the lower limit of the settling window is also dependent on sheet thickness. Within the confines of this model and for nucleation and growth rates set by those observed in natural systems, layering is unlikely in sheet-like magmas thinner than about 100 m. When the body is not always well-mixed and crystallization is within inward-propagating solidification fronts, it is expected that this minimum body thickness will increase.     

Crystal aging and the formation of fine-scale igneous layering

Summary Fine-scale layering in igneous intrusions shows parallels with experimentally produced banding observed in crystallizing salt solutions in which recent advances have demonstrated the importance of crystal aging on the development of banding. These experiments, as well as numerical models of fine-scale layering development during crystal aging, reproduce features observed in fine scale layering that are not readily explained by periodic nucleation and crystal growth models. These include the phenomenon of lost segments, in which the development of a layer may be reversed such that the layer disappears over time but does so without affecting the overall spacing of later-developing layers, and the development of doublet layers, which results when crystals at the margins of developing layers preferentially grow at the expense of those in the center. While these results suggest that crystal aging is the dominant process in the development of fine-scale layering, crystal aging may enhance the textural and modal features of igneous layering initiated by a variety of other crystal settling, nucleation or growth processes.

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Kristall-Alterung und die Bildung von feiner magmatischer Schichtung

Zusammenfassung Feine Schichtung in magmatischen Intrusionen zeigt Parallelen mit experimentell erzeugter Bänderung, wie sie in kristallisierenden Salzlösungen beobachtet werden kann. An solchen Lösungen haben jüngste Arbeiten gezeigt, daß Kristall-Alterung bei der Ausbildung einer Bänderung eine wichtige Rolle spielt. Diese Experimente reproduzieren ebenso wie numerische Modelle, die die Ausbildung einer feinen Schichtung während der Kristall-Alterung simulieren, Merkmale, die in feinen Schichten auftreten, über nicht leicht mit periodischer Keimbildung und Kristallwachstumsmodellen erklärt werden können. Diese Merkmale umfassen das Phänomen der verlorenen Segmente, bei dem die Ausbildung einer Lage in der Weise umgekehrt werden kann, daß die Lage mit der Zeit verschwindet, ohne dabei den Gesamtabstand der später gebildeten Schichten zu beeinflussen, oder die Ausbildung von Lagenpaaren. Diese erfolgt, wenn Kristalle an den Rändern der sich entwickelnden Lagen bevorzugt auf Kosten jener im Zentrum wachsen. Während diese Ergebnisse nahelegen, daß Kristall-Alterung bei der Ausbildung von feiner Schichtung der vorherrschende Prozeß ist, kann Kristall-Alterung auch die texturellen und modalen Eigenschaften einer magmatischen Schichtung verstärken, die durch eine Reihe anderer Prozesse wie Kristallsetzung, Keimbildung oder Wachstum eingeleitet wurden.

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With 8 Figures     

Biomass layering and metabolism in mats of the macroalgaUlva lactuca L.

Studies of photosynthesis and respiration in the sheet-like green macroalgaUlva lactuca L. have typically made use of cut discs or individual thalli. Because this species accumulates in layered mats in the field, its structure has the potential to significantly alter metabolic rates compared to those measured for a single layer. The effect of biomass layering on photosynthesis and respiration inU. lactuca was assessed by incubating stacks of 1, 2, 4, and 8 thalli across a range of surface irradiance and in the dark. The photosynthesis-irradiance curve for a single thallus was hyperbolic in shape with maximum photosynthetic rates of 16–24.5 mg O₂ g^?1 dry weight h^?1, a photosynthetic efficiency of 0.18 mg O₂ g^?1 dry weight h^?1 (μE m^?2 s^?1)^?1, and dark respiration rates of 0.95–1.2 mg O₂ g^?1 dry weight h^?1. Weight-specific photosynthesis and respiration rates decreased as the amount of layering increased, with rates approximately twice as high in a single layer as for 8 layers. A simple model revealed that the reduced photosynthetic rates were due to attenuation of light through the stack. Each layer ofUlva reduced the light available to the next layer by approximately 55%, and variability in this absorption was directly related to thallus chlorophyll content. Model runs suggested that acclimation of the thalli by changes in chlorophyll content has the potential to reduce some, but not all of the effect of layering. Reduced respiration rates were atributed to the depletion of oxygen from the interstices between the layers. Results were incorporated into a model of anUlva mat, which predicted decreasing rates of weight-specific total mat production with increasing mat thickness as additional layers are added to the bottom of the mat that neither produce nor consume significant amounts of oxygen. The model predicted area-weighted mat production to increase up to a thickness of 10 layers, after which the rate was constant with increasing mat thickness for the same reason. SinceUlva has a tendency to accumulate in layers under natural conditions, these results improve upon metabolic measurements made with single thalli and will be useful for calculations of macroalgal production as well as in ecosystem models in whichUlva is an important primary producer.     

High-field-strength elements in Javanese arc basalts and chemical layering in the mantle wedge

Summary Quaternary basalts from the Java-Bali sector of the Sunda arc show the following characteristics: As the vertical distance increases between the seismic Benioff zone and the respective eruptive centre, high-field-strength elements (HFSE's) increase in abundance and Zr/Nb and Hf/Nb decrease in value. If subduction processes do not affect the abundance and ratios of HFSE's, both of these trends are consistent with a progressively enriched mantle wedge with depth, independent of any metasomatism by the northward subducting Indian Ocean plate. Furthermore, the volcanic rocks display negative trends between Nb/U-Zr/Nb, Nb/U-⁸⁷Sr/⁸⁶Sr, and K/Nb-K and positive ones between Zr/Nb-⁸⁷Sr/⁸⁶Sr and Ce/Pb-Nb/U. Also, Nb-depletion (Nb/Nb*), as measured on mantle normalized incompatible-trace-element diagrams, decreases in size with increasing distance to the Benioff zone. These trends are in accord with a model in which the slab-derived metasomatic component-which is low in HFSE abundance-makes a greater chemical imprint on shallower, depleted mantle than on deeper, enrichedmantle. The nature of the chemical layering within the mantle magma source beneath Java implies that this source may be part of the lithospheric mantle.

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HFSE-Elemente in Basalten des Java-Bogens und chemischer Lagenbau im Mantelkeil

Zusammenfassung Quartäre Basalte des Java-Bali Sektors des Sunda-Bogens zeigen die folgenden Charakteristika: Mit zunehmender Vertikaldistanz zwischen der seismischen Benioff Zone und den betreffenden Eruptivzentren nehmen die Elemente hoher Feldstärke (HFSE's) zu, während die Zr/Nb und Hf/Nb Verhältnisse abnehmen. Sofern Subduktionsprozesse die Häufigkeit und die Verhältnisse der HFSE's nicht beeinflussen, sind beide Trends in Übereinstimmung mit einem mit der Tiefe zunehmend angereicherten Mantelkeil, unabhängig von Metasomatose durch die nach Norden abtauchende indische Ozeanplatte. Darüber hinaus zeigen die vulkanischen Gesteine negative Trends zwischen Nb/U-Zr/Nb, Nb/U-⁸⁷Sr/⁸⁶Sr, and K/Nb-K, sowie positive Trends zwischen Zr/Nb-⁸⁷Sr/⁸⁶Sr und Ce/Pb-Nb/U. Auch die Nb-Verarmung (Nb/Nb*) wie she auf Mantelnormalisierten Diagrammen der inkompatiblen Spurenelemente dargestellt wird, nimmt mit zunehmender Distanz von der Benioff-Zone zu. Diese Trends sind in Übereinstim-mung mit einem Modell, in deco die auf die abtauchende kruste zurückzuführende metasomatische Komponente - mit niedrigen HFSE Gehalten — einen größeren chemischen Einfluß auf den seichten, verarmten Mantel als auf den tiefen, angereicherten Mantel ausübt. Die Art des chemischen Lagenbaus in der Mantel-Magmaquelle unterhalb Java weist darauf hin, daß these Quelle Teil des lithosphdrischen Mantels sein kann.

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With 9 Figures     

A flow mechanism for layering in a heterogeneous porous medium

The flow of a particle fluid mixture through a heterogeneous permeable material is studied with a view to obtaining characteristic fingerprints for the structures that such flows may exhibit. This flow has a variable density and behaves as a compressible fluid, with the compressibility derived from the variations of the local field variables. Both pressure-sensitive or speed-fluctuation-dependent rheologies are studied in the steady state and a speed-fluctuation-dependent material is studied in a time-dependent analysis. The salient findings are that for a pressure-sensitive material structures normal to the mean flow direction are obtained. For the speed-fluctuation-dependent rheology, the pressure structures are normal to the mean flow, whereas the density structures are aligned with the flow (fingering).Electronic Supplementary Material Electronic Supplementary Material is available in the online version of this article at .     

鄂西白垩纪远安盆地的变形带构造

鄂西白垩纪远安盆地中大量发育变形带及其密切相关的断层,主要分为两期,早期为带有正断性质的北北东向组变形带及简单的正断层,形成在同沉积或早成岩阶段,而晚期为带走滑性质的北-南向组和北东向组变形带及复活断层,形成在白垩纪以后.根据实测的共轭变形带和断层滑动数据,古应力分析的结果表明,早期为北西西-南东东向拉张的伸展状态,而...     

The Skaergaard trough layering: sedimentation in a convecting magma chamber

The upper parts of the floor cumulates of the Skaergaard Intrusion, East Greenland, contain abundant features known as troughs. The troughs are gently plunging synformal structures comprising stacks of crescentic modally graded layers with a sharply defined mafic base that grades upward into plagioclase-rich material. The origin of the troughs and layering is contentious, attributed variously to deposition of mineral grains by magmatic currents descending from the nearby walls, or to in situ development by localised recrystallisation during gravitationally-driven compaction. They are characterised by outcrop-scale features such as mineral lineations parallel to the trough axis, evidence of erosion and layer truncation associated with migration of the trough axis, and disruption of layering by syn-magmatic slumping. A detailed microstructural study of the modal trough layers, using electron backscatter diffraction together with geochemical mapping, demonstrates that these rocks do not record evidence for deformation by either dislocation creep or dissolution–reprecipitation. Instead, the troughs are characterised by the alignment of euhedral plagioclase crystals with unmodified primary igneous compositional zoning. We argue that the lineations and foliations are, therefore, a consequence of grain alignment during magmatic flow. Post-accumulation amplification of the modal layering occurred as a result of differential migration of an unmixed immiscible interstitial liquid, with upwards migration of the Si-rich conjugate into the plagioclase-rich upper part of the layers, whereas the Fe-rich immiscible conjugate remained in the mafic base. Both field and microstructure evidence support the origin of the troughs as the sites of repeated deposition from crystal-rich currents descending from the nearby chamber walls.