共查询到20条相似文献，搜索用时 15 毫秒
1.
《Journal of Structural Geology》2002,24(6-7):1087-1099
This paper investigates the geometry, microstructure, and c-axis fabrics of an outcrop scale, micaceous quartzite fold produced under greenschist facies metamorphic conditions in the Moeda quartzite, Quadrilátero Ferrı́fero granite–greenstone terrain, southeastern Brazil. The fold limbs show development of opposed S–C fabrics and asymmetric quartz c-axis fabrics compatible with flexural slip along the fold surface. Towards the fold hinge, there is an increasing presence of oblique shear bands (here named S-bands) which gradually change to crenulations within the hinge zone. The oblique S-bands are interpreted to have formed through connection of several S-planes, increasing accommodation of antithetical shear along these S-planes and offset of the initial C-planes at intermediate stages of folding. This mechanism represents a kinematic inversion in the role played by the two sets of foliations in S–C structures. Our observations support flexural slip for early stages of folding. However, with progressive closure of the fold, the flexural slip mechanism involves increasing contributions from oblique shear on the S-bands, thus approximating an intermediate situation between flexural slip and passive folding (shear parallel to the axial plane). 相似文献
2.
《Geomechanics and Geoengineering》2013,8(4):259-273
An important design parameter in cement-grouted soil nailed structures is the shear strength at the interface between the grouted nail and the surrounding soil. Both field and laboratory pull-out tests are normally used to investigate this interface shear strength. However, these tests have some limitations. In this study, direct shear box tests are adopted to investigate the interface shear strength behaviour between a completely decomposed granite (CDG) soil and a cement grout plate. Tests were carried out in a large direct shear test apparatus over a range of constant normal stress, soil moisture content, and soil–cement grout interface surface waviness. The laboratory test procedures are briefly described and the main test results are presented, followed by a discussion of the shear behaviour of the soil–cement grout interface. The interface shear behaviour is compared with the shear strength behaviour of the same soil tested under comparable conditions. It is shown that the shear stress–displacement behaviour of the soil–cement grout interface is similar to that of the soil alone. The test results indicate that the interface shear strength of the CDG and cement grout material depends on the normal stress level, the soil moisture content, and the interface surface waviness. 相似文献
3.
John Menzies Jürgen M. Reitner 《Proceedings of the Geologists' Association. Geologists' Association》2019,130(2):196-209
Tills from an exposure in Wildschönau Valley, northern Austria were examined using microsedimentological techniques. The tills exhibit a range of microstructures indicative of soft sediment deformation within temperate subglacial bed conditions. The tills can be subdivided at the macroscale into a lower grey and upper red till both of which exhibit some sedimentological variations; however, at the micro-level the tills appear essentially identical. The microstructures in the tills are illustrative of structures developed during deformation both during and following their emplacement. Of note are the microshears within these tills that are demonstrative of changes in applied stress. Both low (<25°) and high angle (>25°) microshears were mapped and their fabric data analyzed. The microshears show a change in stress levels ascending through successive till units. The changes in stress are demonstrative of spatially and temporally changing rheological conditions undergone by the subglacial tills during deformation, ongoing deposition/ emplacement and stress localization. These findings indicate that microstructures reveal local deformation conditions in tills and a more detailed micro-history of paleo-stress. 相似文献
4.
The northerly dipping Sha’it–Nugrus shear zone (SNSZ) is the boundary separating the Central Eastern Desert from the South Eastern Desert of Egypt. The hangingwall of this shear zone is composed of low-grade metavolcanics and ophiolitic nappes of the Central Eastern Desert, while the footwall consists of South Eastern Desert high-grade metapsammitic gneisses (Migif-Hafafit gneissic complex). The SNSZ is about 700 m thick and represents the shear foliated lower parts of the hangingwall and upper parts of the footwall. A significant part of the SNSZ has been truncated by a later normal fault along Wadi Sha’it, however the SNSZ is well-preserved along Wadi Nugrus. Features of the SNSZ include shear-related schistosity (termed Ss), mylonite zones, sheared syn-kinematic granitoid intrusions, diverse metasomatism and metamorphic effects (higher T overprinting of hangingwall lithologies and retrogression of footwall lithologies). Shear-sense indicators clearly show top-to-N or NW displacement sense. SNSZ structures overprint arc collision related nappe structures (～680 Ma) and are therefore post-arc collision. SNSZ syn-kinematic intrusives have been dated at ～600 Ma. The SNSZ is deformed (regionally and locally folded and thrust dissected) during later NE–SW compressive tectonism. The SNSZ had an originally approximately E–W strike, low-angle N-dip and a normal shear sense, making this an example of a low-angle normal ductile shear (LANF) or detachment fault. The steep NE dip of Ss foliations and low-pitching slip lineations along Wadi Nugrus are due to NW–SE folding of the SNSZ, and do not indicate a sinistral strike-slip shear zone. The normal shear sense activity is responsible for juxtaposing the low-grade Central Eastern Desert lithologies against South Eastern Desert gneisses. A displacement of 15–30 km is estimated on the SNSZ, which is comparable to LANF displacements in the Basin and Range province of the western USA. Frictional resistance along this shear was probably reduced by high magmatic fluid pressure and hydrothermal fluid pressure. The vastness and diversity of the hydrothermal activity along this shear zone is a characteristic of other LANFs in the Eastern Desert, e.g. at Gabal El-Sibai, and may be Gabal Meatiq. The SNSZ formed during the Neoproterozoic extensional tectonic phase of Eastern Desert that began ～600 Ma, and followed arc collision and NW-ward ejection of nappes. 相似文献
5.
Acta Geotechnica - Many shallow foundations are constructed within the soil layer above the groundwater table, where the soil remains unsaturated, and the failure of shallow foundation is mostly... 相似文献
6.
This paper presents a series of cyclic 2D direct shear tests on sand–rough material interfaces under constant normal load (CNL) and constant normal stiffness (CNS) conditions. The aim of these tests is to describe the behavior of the soil–pile contact subjected to a large number of cycles due to environmental or anthropic loadings. These cycles (typically 104 or less due to an early rupture) are small (10, 20 and 40 kPa in terms of shear stress). A new interpretation of the direct shear tests is proposed. The sample of soil is schematically composed of a sheared interface and of a buffer under oedometric load. The problem of sand leakage between the shear box and the rough plate, classical phenomenon in this type of test, is focused. The effect of initial density, position of “center of cycles” in stress plane (mean cyclic variables) and cyclic amplitude is investigated. The cycles are defined by the initial mean cyclic normal stress, the level of initial mean cyclic stress ratio and the normalized cyclic amplitude. Under CNL condition, either dilation or contraction is exhibited, in agreement with the characteristic state developed by Luong (International symposium on soils under cyclic and transient loading, Swansea, 7–11 January, pp 315–324, 1980). The influence of a prescribed normal stiffness is especially considered. It can be highlighted that CNS cyclic paths are always contractive. This contraction results in a drop of mean cyclic normal stress often called degradation of friction. 相似文献
7.
《Journal of Structural Geology》1999,21(8-9):923-932
In analyzing deformation in rocks, it is important to ensure that the solutions obtained satisfy strain compatibility. This creates a challenge to understanding patterns of strain associated with shear zones, in which measured strain may appear incompatible with the strain in the shear zone walls. Flattening strains are common in natural shear zones with locally straight and parallel boundaries: to satisfy compatibility conditions such strains require volume loss across the shear zone or deviations from plane strain, with or without discontinuities between the shear zone and the wall rock. In the case of shear zones for which there is no evidence of volume loss or discontinuities along the shear zone walls, problems of strain compatibility may be resolved if individual shear zones are linked together in an appropriate fashion. Shear zones commonly occur in anastomosing arrays, and simple configurations of such arrays and the strains associated with them are examined. It is shown that local transpression with strain compatibility can be accounted for in this way. Quite complex local strain patterns can develop in simple arrays. 相似文献
8.
The microstructures of cm-scale displacement faults offsetting unlithified sequences of finely interbedded sands, silts and clays from outcrops in Denmark have been examined. A variety of shear band types are recognised based on their grain-scale deformation mechanism and internal structure. Shear bands in a Jurassic sequence exposed along the coastline of Bornholm are characterised by intense cataclasis of both sand and clay layers. This deformation mechanism is accompanied by extensive grain scale mixing along discrete shear bands to give a fault rock composition that reflects the relative amount of sand and clay within the faulted sequence. In contrast, shear bands at Nr. Lyngby and Jensgaard, both on the Jutland coast, are characterised by granular flow within the sand units. Grain scale mixing is subdued at these locations so that layers maintain their integrity across the shear band to form a layered internal structure of sand, silt and clay smears. In some instances, particularly at Nr. Lyngby, clays have deformed in a brittle manner so that they do not contribute material to the shear band, which is then comprised exclusively of coarser-grained components. The different deformation mechanisms and internal structures of shear bands are thought to be controlled by burial depth at the time of faulting. 相似文献
9.
Sun Meng Cao Junnan Cao Jingjing Zhang Shuai Chen Yunmin Bate Bate 《Acta Geotechnica》2022,17(7):2633-2649
Bioremediation is widely used to improve ground soil by introducing calcium carbonate (CaCO3). Shear wave velocity (Vs) is usually adopted to evaluate effect but the microscopic mechanism is unclear. The discrete element method (DEM), a promising tool for simulating the behaviors of cohesive and noncohesive materials, was used in this study to simulate Vs evolution and wave propagation path of sand reinforced by calcite precipitates. Two basic calcite precipitate forms are proposed for representing individual calcite precipitation (CaCO3-P) and calcite aggregation (CaCO3-C). Contact cementation between adjacent sand grain pairs was the primary association pattern for calcite precipitates at a low calcite content. At a higher calcite content, the preferential shear wave propagation pathway is the clusters cemented by CaCO3-C. With calcite content increasing from 0 to 9%, the coordination number and average contact force increased. Vs increased from 169.73 to 2132.64 m/s but had high variability due to the spatial distribution. The results suggest that the calibrated DEM model can elucidate the microscopic mechanisms and evaluate the enhancement effect of microorganism-reinforced soil.
相似文献10.
The small strain shear stiffness G0 of the soil is of interest and importance in both theory and practice. It is expected that for granular materials G0 would slightly increases with over-consolidation ratio (OCR). However, laboratory tests indicate that G0 may decrease with increasing OCR, especially for loose specimens, which is counterintuitive. To explore the underlying mechanism, discrete element method (DEM) is used to investigate the effect of OCR on G0. The DEM simulations successfully capture the laboratory observations. The analyses at the particulate level reveal that the decrease in small strain stiffness is mainly due to the decreases in coordination number and the uniformity of contact force distribution during unloading process. 相似文献
11.
Simonetti Matteo Carosi Rodolfo Montomoli Chiara Langone Antonio D’Addario Enrico Mammoliti Elisa 《International Journal of Earth Sciences》2018,107(6):2163-2189
International Journal of Earth Sciences - In the Western Alps, a steeply dipping km-scale shear zone (the Ferriere-Mollières shear zone) cross-cuts Variscan migmatites in the... 相似文献
12.
不同类型的非饱和土在自然界中广泛存在。由于不同类型土的颗粒级配和矿物组成成分的不同,其强度特性存在较大的差异。为了建立一种适用于不同类型非饱和土的强度预测统一模型,本研究系统地分析了土体类型对非饱和土强度演化趋势的影响,在综合考虑两种常用模型优越性的基础上,提出了一种改进的非饱和土抗剪强度模型,同时,利用现有常用的强度模型,对不同类型土在广吸力范围内的强度特性进行了预测,并对拟合结果进行了对比分析。研究结果表明:(1)在广吸力范围内,不同类型土的强度演化规律可分为两类,山峰型和增长稳定型;(2)改进模型可以较为准确地预测不同类型土在广吸力范围内的强度特性;(3)明确了改进模型中各参数的物理意义,并初步给出了改进模型在应用于不同类型土时各参数取值的建议范围。 相似文献
13.
Yuan Tang Junlai Liu My-Dung Tran Zhijie Song Wenbin Wu Zhaochong Zhang Zhidan Zhao Wen Chen 《International Journal of Earth Sciences》2013,102(3):605-626
As the boundary between the Indochina and the South China blocks, the Ailao Shan-Red River (ASRR) shear zone underwent a sinistral strike-slip shearing which is characterized by ductile deformation structures along the Ailao Shan range. The timing issue of left-lateral shearing along the ASRR shear zone is of first-order importance in constraining the nature and regional significance of the shear zone. It has been, therefore, focused on by many previous studies, but debates still exist on the age of initiation and termination of shearing along the shear zone. In this paper, we dated 5 samples of granitic plutons (dykes) along the Ailao Shan shear zone. Zircon U–Pb ages of four sheared or partly sheared granitic rocks give ages of 30.9 ± 0.7, 36.6 ± 0.1, 25.9 ± 1.0 and 27.2 ± 0.2 Ma, respectively. An undeformed granitic dyke intruding mylonitic foliation gives crystallization age of 21.8 ± 1 Ma. The Th/U ratios of zircon grains from these rocks fall into two populations (0.17–1.01 and 0.07–0.08), reflecting magmatic and metamorphic origins of the zircons. Detailed structural and microstructural analysis reveals that the granitic intrusions are ascribed to pre-, syn- and post-shearing magmatisms. The zircon U–Pb ages of these granites provide constraints on timing of the initiation (later than 31 Ma from pre-shearing granitic plutons, but earlier than 27 Ma from syn-shearing granitic dykes) and termination (ca. 21 Ma from the post-shearing granitic dykes) of strong ductile left-lateral shearing, which is consistent with previous results on the Diancang Shan and Day Nui Con Voi massifs in the literature. We also conclude that the left-lateral shearing along the ASRR shear zone is the result of southeastward extrusion of the Indochina block during the Indian–Eurasian plate collision. Furthermore, the left-lateral shearing was accompanied by the ridge jump, postdating the opening, of the South China Sea. 相似文献
14.
In order to address the question of the processes involved during shear zone nucleation, we present a petro-structural analysis of millimetre-scale shear zones within the Roffna rhyolite (Suretta nappe, Eastern central Alps). Field and microscopic evidences show that ductile deformation is localized along discrete fractures that represent the initial stage of shear zone nucleation. During incipient brittle deformation, a syn-kinematic metamorphic assemblage of white mica + biotite + epidote + quartz precipitated at ca. 8.5 ± 1 kbar and 480 ± 50 °C that represent the metamorphic peak conditions of the nappe stacking in the continental accretionary wedge during Tertiary Alpine subduction. The brittle to ductile transition is characterized by the formation of two types of small quartz grains. The Qtz-IIa type is produced by sub-grain rotation. The Qtz-IIb type has a distinct CPO such that the orientation of c-axis is perpendicular to the shear fracture and basal and rhombhoedric slip systems are activated. These Qtz-IIb grains can either be formed by recrystallization of Qtz-IIa or by precipitation from a fluid phase. The shear zone widening stage is characterized by a switch to diffusion creep and grain boundary sliding deformation mechanisms. During the progressive evolution from brittle nucleation to ductile widening of the shear zone, fluid–rock interactions play a critical role, through chemical mass-transfer, metasomatic reactions and switch in deformation mechanisms. 相似文献
15.
《Journal of Structural Geology》2001,23(6-7):1015-1030
The Malpica–Lamego Line (MLL) is a deformation zone in the Variscan belt of NW Iberia (NW Spain and N Portugal) that runs parallel to the chain for at least 275 km, bearing I-type granodiorite plutons along most of its length. The MLL affects previous structures by which high pressure and ophiolitic rocks were exhumed and emplaced on the Iberian plate during earlier deformation phases. Correlation and reconstruction of the stratigraphy of these sheets or tectonic units at both sides of the shear zone allows a preliminary estimate of the accumulated vertical and horizontal offsets after the tectonic activity of the fault. The value of the separations, of crustal-scale proportions, reaches a maximum 15 km of vertical offset that decreases gradually to the south. The structural record found in the rocks indicates a strike-slip regime that, in general, does not fit the geometry of the offsets. We suggest that the MLL went through two different stages during the same orogenic cycle: a first dip-slip episode, a reverse faulting event, overprinted by a later strike-slip reactivation. 相似文献
16.
《Russian Geology and Geophysics》2014,55(2):237-251
The authors analyze the geodynamic settings of large fields of spodumene pegmatites hosting Li and complex (Li, Cs, Ta, Be, and Sn) deposits of rare metals within the Central Asian Fold Belt. Most of the studied fields show a considerable time gap (from few tens of Myr to hundreds of Myr) between the spodumene pegmatites and the associated granites, which are usually considered parental. This evidence necessitates recognition of an independent pegmatite stage in the magmatic history of some pegmatite-bearing structures in Central Asia. The Precambrian–Late Mesozoic interval is marked by a close relationship between the large fields of spodumene pegmatites and extension settings of continental lithosphere. They occur either as (1) zones of long-lived deep faults bordering on trough (rift) structures experiencing the tectonic-magmatic activity or as (2) postcollisional zones of shearing and pull-apart dislocations. Thus, large fields of spodumene pegmatites might serve as indicators of continental-lithosphere extension. Important factors favoring the formation of rare-metal pegmatites both in collision zones and continental-rift settings are the presence of thick mature crust dissected by long-lived, deeply penetrating (down to the upper mantle) fault zones. They ease the effect of deep sources of energy and substance on crustal chambers of granite and pegmatite formation. 相似文献
17.
《Journal of Structural Geology》1999,21(8-9):975-984
By measuring S spacing, C spacing and the S–C angle (α) in deformed rocks, this paper investigates the geometry of previously published examples of S–C and S–C-like structures on a scale range between micrometres and several hundred kilometres. The results indicate that common S–C fabrics of thin-section, hand-specimen and outcrop scale, and conjugate fault/mylonite zones of map scale define a simple function Cspacing=2Sspacing, which depicts a scale-invariant geometry over ten orders of magnitude. Logarithmic plots of cumulative frequency suggest that the S–C fractal set (D=0.13) is restricted to the scale range between 600–800 μm and 1 km where genuine S–C structures, characterized by antithetic shear on the S planes, can be formed. Below 600–800 μm, grain scale processes seem to influence the development of S–C structures. Above the upper limit (1 km), only S–C-like structures with duplex kinematics (synthetic shear on S planes) occur. The S–C and S–C–C′ fractals are envisaged as self-similar structures where the foliations work as both S or C planes, depending on which scale is considered. 相似文献
18.
Acta Geotechnica - Previously, in situ tests have been conducted in cold regions since infrastructures such as pipelines have been actively built on frozen ground. However, the engineering... 相似文献
19.
Summary ?The Betroka sinistral shear belt is a major geotectonic unit in the Precambrian of southern Madagascar. It consists of migmatitic
paragneiss commonly interlayered with phlogopite-bearing diopsidite, phlogopite-humite-diopside-spinel marble, sillimanite-garnet
quartzite and syn-tectonic S-type leucogranite. H?gbomite occurs sporadically in the migmatitic paragneiss in patches of magnetite
with hercynite, and at the border of magnetite where it is in contact with hercynite, rare ilmenite, rutile and cordierite,
which contains a network of chlorite, pyrophyllite and rare corundum/diaspore. XMg = Mg/(Mg + Fe) decreases as follows: Crd > Bt > Chl > H?g > Hc. The textural relations suggest the following h?gbomite-participating
reactions:
Ti-bearing hercynite ↠ hercynite + h?gbomite (intergrown/exsolution lamellae)
ilmenite + cordierite ↠ hercynite + h?gbomite + rutile + chlorite/pyrophyllite
h?gbomite ↠ hercynite + ilmenite + corundum
The chemical composition of h?gbomite varies substantially from grain to grain in individual samples and from sample to sample,
this variation being highly dependent on the associated minerals. There is a weak zoning from core to rim in individual grains
intergrown with hercynite and also in grains at the margin of hercynite, but this zoning is overprinted by zones formed at
grain rims depending on the surrounding phases. In contact with hercynite, h?gbomite has FeO (total Fe as FeO) 27.1–28.5 wt.%,
and MgO 4.5–5.8 wt.%, and in contact with magnetite FeO 24.9–26.5 wt.%, and MgO 6.0–8.5 wt.% and the core contents are within
these values. TiO27.5–4.0 wt.% and Al2O362.0–59.0 wt.% show zonations with increase from core to rim. Estimated P-T conditions are 6.0 ± 1.0 kbar and 700 ± 100 °C
reached during a peak metamorphic stage of the Pan-African orogeny. However, the presence of diaspore with exsolved hercynite-magnetite
indicates extreme retrograde metamorphism in the decompressional central part of this shear belt of southern Madagascar.
Received January 15, 1999;/revised version accepted July 6, 1999 相似文献
Zusammenfassung ?H?gbomit in migmatitischem Paragneis von Vohidava in der Betroka Scherzone im südlichen Pr?kambrium von Madagaskar Die sinistrale Betroka Scherzone ist eine ausgepr?gte tektonische Einheit des Pr?kambriums in Süd Madagaskar. Sie besteht aus migmatitischem Paragneis, in dem Phlogopit-führender Diopsidit, Phlogopit-Humit-Diopsid-Spinell-Marmor, Sillimanit-Granat-Quarzit und syntektonischer S-Typ Granit eingelagert sind. Im migmatitischen Paragneis kommen sporadisch H?gbomit/Hercynit Nester im Magnetit vor und am Magnetitrand findet sich H?gbomit im Kontakt mit Hercynit und Cordierit, der mit einem Netzwerk aus Chlorit/Pyrophyllit gefüllt ist, sowie sporadisch mit Korund/Diaspor, Ilmenit und Rutil. XMg = Mg/(Mg + Fe) nimmt in folgender Reihung ab: Crd > Bt > Chl > H?g > Hc. Aus den texturellen Beziehungen werden folgende H?gbomit-partizipierende Reaktionen abgeleitet: Ti-führender Hercynit ↠ Hercynit + H?gbomit (verwachsen/Entmischungslamellen) H?gbomit ↠ Hercynit + Ilmenit + Korund Die chemische Zusammensetzung von H?gbomit variiert betr?chtlich von Korn zu Korn in einer Probe und von Probe zu Probe; wobei die Variation von den Kontaktmineralen abh?ngt. H?gbomit im Hercynit hat eine schwache Zonierung von Kern zum Rand. Im H?gbomit am Hercynitrand ist die Kern-Rand-Zonierung durch die von den Kontaktmineralen abh?ngige Randzusammensetzung überpr?gt. Im Kontakt zum Hercynit hat H?gbomit 27,1–28,5 Gew.% FeO (total Fe als FeO) und 4,5–5,8 Gew.% MgO und im Kontakt zu Magnetit 24,9–26,5 Gew.% FeO und 6,0–8,5 Gew.% MgO, die Kernzusammensetzung liegt zwischen den beiden Randwerten. TiO2nimmt vom Kern zu den R?ndern von 7,5 bis 4,0 Gew.% ab und Al2O3von 62,0 bis 59,0 Gew.%. Die P-T Bedingungen des Metamorphose-Peaks w?hrend der Pan-Afrikanischen Orogenese erreichten 6,0 ± 1,0 Kbar und 700 ± 100 °C. Die sp?te Bildung von Diaspor und die Hercynit-Magnetit-Entmischung weisen auf eine tiefgreifende retrograde Metamorphose im Dekompressions-Zentralbereich der Betroka-Scherzone im südlichen Madagaskar hin.
Received January 15, 1999;/revised version accepted July 6, 1999 相似文献
20.
Use of scrap tyres in isolation systems for seismic damping, requires a knowledge of the engineering properties of sand–rubber mixtures (SRM). The primary objective of this study is to assess the influence of granulated rubber and tyre chips size and the gradation of sand on the strength behaviour of SRM by carrying out large-scale direct shear tests. A large direct shear test has been carried out on SRM considering different granulated rubber and tyre chip sizes and compositions. The following properties were investigated to know the effect of granulated rubber on dry sand; peak shear stress, cohesion, friction angle, secant modulus and volumetric strain. From the experiments, it was determined that the major factors influencing the above-mentioned properties were granulated rubber and tyre chip sizes, percentage of rubber in SRM and the normal stress applied. It was observed that the peak strength was significantly increased with increasing granulated rubber size up to rubber size VI (passing 12.5 mm and retained on 9.5 mm), and by adding granulated rubber up to 30%. This study shows that granulated rubber size VI gives maximum shear strength values at 30% rubber content. It was also found that more uniformly graded sand gives an improved value of shear strength with the inclusion of granulated rubber when compared to poorly graded sand. 相似文献