共查询到20条相似文献，搜索用时 31 毫秒
1.
Simon K. Hanmer 《Journal of Structural Geology》1982,4(2):151-160
Vein arrays associated with natural kink bands have been, even recently, used by some workers as kinematic markers to distinguish between the migration and the rotation models of kink band mechanics.In an outcrop of schistose meta-agglomerate, conjugate kinks and synthetic sinistral shears are associated with a component of sinistral slip along the foliation. Discordant scalariform vein arrays form in association with the shears, but not the kinks. Tension gash elements of the arrays form by combined layer-parallel slip and layer-parallel extension along the rotated schistosity. They are only kinematically significant at the scale of the length of the rotated segment anisotropy.Applying this analysis to kink bands elsewhere, it is concluded that discordant vein arrays are not diagnostic of the rotational kink band model, or of high resolved shear stress on kink boundaries. 相似文献
2.
3.
In rocks possessing a strong planar fabric, shear bands of constant shear sense and oriented at an oblique angle to the foliation are considered by many authors to be characteristic of a non-coaxial bulk deformation history, whereas conjugate shear bands are considered to indicate coaxial shortening. However, in two areas where bulk deformation history appears to be non-coaxial (Cap Corse, Corsica and Ile de Groix, Brittany), conjugate shear bands are observed. In order to investigate this problem, experiments were performed by bulk simple shearing using Plasticine as a rock analogue. When slip between layers of the model is permitted, shear bands of normal-fault geometry form with both the same and opposite shear sense as the bulk simple shearing at approximately the same angle with the layering (40°) irrespective of layer orientation in the undeformed state (for initial orientations of 50, 30 and 15°). Shear bands are initially formed within individual layers and may propagate across layer interfaces when further movement along these is inhibited. The existence of conjugate shear bands in Corsica and Ile de Groix is therefore not incompatible with a model of bulk simple shearing for these two regions. In field studies, one should perhaps exercise care in using shear bands to determine the kind of motion or the sense of bulk shearing. 相似文献
4.
P. Stock 《Journal of Structural Geology》1992,14(10)
A numerical strain model based on infinitesimal strain theory is presented which simulates the progressive deformation and rotation of a foliated microlithon. It can be shown that an antithetical rotation of the microlithon's foliation with respect to the bulk sense of shear is a geometric consequence of specified strain conditions. The application of the model to asymmetric shear band structures described from the Gurktal Nappe, Eastern Alps, reveals that even the internal foliation of these structures must have been rotated antithetically. This corresponds with rare field observations and supports the suitability of asymmetric shear band structures as indicators for the local sense of shear. 相似文献
5.
Schistose mylonitic rocks in the central part of the Alpine Fault (AF) at Tatare Stream, New Zealand are cut by pervasive extensional (C′) shear bands in a well-understood and young, natural ductile shear zone. The C′ shears cross-cut the pre-existing (Mesozoic—aged) foliation, displacing it ductilely synthetic to late Cenozoic motion on the AF. Using a transect approach, we evaluated changes in geometrical properties of the mm–cm-spaced C′ shear bands across a conspicuous finite strain gradient that intensifies towards the AF. Precise C′ attitudes, C′-foliation dihedral angles, and C′–S intersections were calculated from multiple sectional observations at both outcrop and thin-section scales. Based on these data the direction of ductile shearing in the Alpine mylonite zone during shear band activity is inferred to have trended >20° clockwise (down-dip) of the coeval Pacific-Australia plate motion, indicating some partitioning of oblique-slip motion to yield an excess of “dip-slip” relative to plate motion azimuth, or some up-dip ductile extrusion of the shear zone as a result of transpression, or both. Constant attitude of the mylonitic foliation across the finite strain gradient indicates this planar fabric element was parallel to the shear zone boundary (SZB). Across all examined parts of the shear zone, the mean dihedral angle between the C′ shears and the mylonitic foliation (S) remains a constant 30 ± 1° (1σ). The aggregated slip accommodated on the C′ shear bands contributed only a small bulk shear strain across the shear zone (γ = 0.6–0.8). Uniformity of per-shear slip on C′ shears with progression into the mylonite zone across the strain gradient leads us to infer that these shears exhibited a strain-hardening rheology, such that they locked up at a finite shear strain (inside C′ bands) of 12–15. Shear band boudins and foliation boudins both record extension parallel to the SZB, as do the occurrence of extensional shear band sets that have conjugate senses of slip. We infer that shear bands nucleated on planes of maximum instantaneous shear strain rate in a shear zone with Wk < 0.8, and perhaps even as low as <0.5. The C′ shear bands near the AF formed in a thinning/stretching shear zone, which had monoclinic symmetry, where the direction of shear-zone stretching was parallel to the shearing direction. 相似文献
6.
One of the rules of thumb of structural geology is that drag folds, or minor asymmetric folds, reflect the sense of layer-parallel shear during folding of an area. According to this rule, right-lateral, layer-parallel shear is accompanied by clockwise rotation of marker surfaces and left-lateral by counterclockwise rotation. By using this rule of thumb, one is supposed to be able to examine small asymmetric folds in an outcrop and to infer the direction of axes of major folds relative to the position of the outcrop. Such inferences, however, can be misleading. Theoretical and experimental analyses of elastic multilayers show that symmetric sinusoidal folds first develop in the multilayers, if the rheological and dimensional properties favor the development of sinusoidal folds rather than kink folds, and that the folded layers will then behave much as passive markers during layerparallel shear and thus will follow the rule of thumb of drag folding. The analyses indicate, however, that multilayers whose properties favor the development of kink folds can produce monoclinal kink folds with a sense of asymmetry opposite to that predicted by the rule of thumb. Therefore, the asymmetry of folds can be an ambiguous indicator of the sense of shear.The reason for the ambiguity is that asymmetry is a result of two processes that can produce diametrically opposed results. The deformation of foliation surfaces and axial planes in a passive manner is the pure or end-member form of one process. The result of the passive deformation of fold forms is the drag fold in which the steepness of limbs and the tilt of axial planes relative to nonfolded layering are in accord with the rule of thumb.The end-member form of a second process, however, produces the opposite geometric relationships. This process involves yielding and buckling instabilities of layers with contact strength and can result in monoclinal kink bands. Right-lateral, layer-parallel shear stress produces left-lateral monoclinal kink bands and left-lateral shear stress produces right-lateral monoclinal kink bands. Actual folds do not behave as either of these ideal end members, and it is for this reason that the interpretation of the sense of layer-parallel shear stress relative to the asymmetry of folds can be ambiguous.Kink folding of a multilayer with contact strength theoretically is a result of both buckling and yielding instabilities. The theory indicates that inclination of the direction of maximum compression to layering favors either left-lateral or right-lateral kinking, and that one can predict conditions under which monoclinal kink bands will develop in elastic or elastic—plastic layers. Further, the first criterion of kink and sinusoidal folding developed in Part IV remains valid if we replace the contact shear strength with the difference between the shear strength and the initial layer-parallel shear stress.Kink folds theoretically can initiate only in layers inclined at angles less than
to the direction of maximum compression. Here φ is the angle of internal friction of contacts. For higher angles of layering, slippage is stable so that the result is layer-parallel slippage rather than kink folding.The theory also provides estimates of locking angles of kink bands relative to the direction of maximum compression. The maximum locking angle between layering in a nondilating kink band and the direction of maximum compression is
. The theory indicates that the inclination of the boundaries of kink bands is determined by many factors, including the contact strength between layers, the ratio of principal stresses, the thickening or thinning of layers, that is, the dilitation, within the kink band, and the orientation of the principal stresses relative to layering. If there is no dilitation within the kink band, the minimum inclination of the boundaries of the band is
to the direction of maximum compression, or
to the direction of nonfolded layers. Here α is the angle between the direction of maximum compression and the nonfolded layers. It is positive if clockwise.Analysis of processes in terminal regions of propagating kink bands in multilayers with frictional contact strength indicates that an essential process is dilitation, which decreases the normal stress, thereby allowing slippage and buckling even though slopes of layers are low there. 相似文献
7.
John Starkey 《Contributions to Mineralogy and Petrology》1968,19(2):133-141
A crystallographic explanation for the geometry of kink bands in crystals is proposed and three possible types of kinking are discussed, 1. simple kinking, 2. kinking associated with a phase change and 3. kinking associated with twin formation. The angular relationships at kink band boundaries have been calculated for a number of minerals and the results compared with observations in the literature. It is proposed that the crystallography of a mineral may be the dominant control of kink band geometry rather than external stress conditions as is generally assumed, this is probably particularly true of kink bands formed under natural conditons. 相似文献
8.
This part concerns folding of elastic multilayers subjected to principal initial stresses parallel or normal to layering and to confinement by stiff or rigid boundaries. Both sinusoidal and reverse-kink folds can be produced in multilayers subjected to these conditions, depending primarily upon the conditions of contacts between layers. The initial fold pattern is always sinusoidal under these ideal conditions, but subsequent growth of the initial folds can change the pattern. For example, if contacts between layers cannot resist shear stress or if soft elastic interbeds provide uniform resistance to shear between stiff layers, sinusoidal folds of the Biot wavelength grow most rapidly with increased shortening. Further, the Biot waves become unstable as the folds grow and are transformed into concentric-like folds and finally into chevron folds. Comparison of results of the elementary and the linearized theories of elastic folding indicates that the elementary theory can accurately predict the Biot wavelength if the multilayers contain at least ten layers and if either the soft interbeds are at most about one-fifth as stiff as the stiff layers, or there is zero contact shear strength between layers.Multilayers subjected to the same conditions of loading and confinement as discussed above, can develop kink folds also. The kink fold can be explained in terms of a theory based on three assumptions: each stiff layer folds into the same form; kinking is a buckling phenomenon, and shear stress is required to overcome contact shear strength between layers and to produce slippage locally. The theory indicates that kink forms will tend to develop in multilayers with low but finite contact shear strength relative to the average shear modulus of the multilayer. Also, the larger the initial slopes and number of layers with contact shear strength, the more is the tendency for kink folds rather than sinusoidal folds to develop. The theoretical displacement form of a layer in a kink band is the superposition of a full sine wave, with a wavelength equal to the width of the kink band, and of a linear displacement profile. The resultant form resembles a one-half sine curve but it is significantly different from this curve. The width of the kink band may be greater or less than the Biot wavelength of sinusoidal folding in the multilayer, depending upon the magnitude of the contact shear strength relative to the average shear modulus. For example, in multilayers of homogeneous layers with contact strength, the Biot wavelength is zero so that the width of the kink band in such materials is always greater than the Biot wavelength. In general, the higher the contact strength, the narrower the kink band; for simple frictional contacts, the widths of kink bands decrease with increasing confinement normal to layers. Widths of kink bands theoretically depend upon a host of parameters — initial amplitude of Biot waves, number of layers, shear strength of contacts between layers, and thickness and modulus ratios of stiff-to-soft layers — therefore, widths of kink bands probably cannot be used readily to estimate properties of rocks containing kink bands. All these theoretical predictions are consistent with observations of natural and experimental kink folds of the reverse variety.Chevron folding and kink folding can be distinctly different phenomena according to the theory. Chevron folds typically form at cores of concentric-like folds; they rarely form at intersections of kink bands. In either case, they are similar folds that develop at a late stage in the folding process. Kink folds are more nearly akin to concentric-like folds than to chevron folds because kink folds form early, commonly before the sinusoidal folds are visible. Whereas concentric-like folds develop in response to higher-order effects near boundaries of a multilayer, kink folds typically initiate in response to higher-order shear, as at inflection points near mid-depth in low-amplitude, sinusoidal fold patterns. Chevron folding and kink folding are similar in elastic multilayers in that elastic “yielding” at hinges can produce rather sharp, angular forms. 相似文献
9.
Howard R. Williams 《Tectonophysics》1987,140(2-4):327-331
Differential shear stresses acting along or adjacent to a non-planar fault surface or shear zone may cause uneven acceleration during slip. Alternatively, at the initiating and closing stages of motion of parts of a stick-slip fault, localised shear stresses may be variable. Stress variation of this nature causes zones of relative compression and tension, especially close to the “stick” zones on the fault. In fissile rocks adjacent to the fault, kink bands form in zones of local relative compression, while stratal extension features such as veins, fractures and extensional crenulations might be expected in the corresponding zones of relative tension. Repetitive motion on the fault should therefore cause the development of a suite of kink bands superimposed on each other and on any complementary extensional structures. Field evidence indicates that the extensional structures are not developed to the same extent as the kinks, perhaps due to ductile flow during layer-parallel extension of phyllosilicate rocks.
The advantages of this model are that it does not require bulk shortening of the shear zone relative to the enclosing less strained rocks, nor does it depend on complex stress orientation changes. 相似文献
10.
J.P. Platt 《Journal of Structural Geology》1984,6(4):439-442
Secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone. These are: the orientation of a single secondary cleavage oblique to the shear zone boundaries; conjugate sets with opposite senses of shear, and multiple sets with the same sense of shear. These features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation (S), coaxial stretching along the foliation, and spin. 相似文献
11.
The displacement-distance (d−x) method can be modified to study the geometry and development of contractional kink bands by dividing displacements into cartesian component vectors. Kink bands are idealised as having constant layer lengths, enabling simple trigonometry to be used to determine the displacement of one wall of the kink band relative to the other wall.
In a consideration of several applications of the d−x method for kink bands, it is shown that displacement is transferred between conjugate and overstepping kink bands in a similar way to displacement transfer between conjugate and overstepping faults and extension fractures. The several different models of kink band formation are shown to each have different displacement characteristics. The d−x method can also be used to study the geometry and evolution of folds related to thrust-propagation and ramps, which are often modelled as having kink band geometries. For instance, the d−x method can be used to show how fault-tip and fault-bend folds cause or accommodate thrust displacement variations, and to estimate displacement rates from the amounts of deformation in different syn-thrust sedimentary layers. 相似文献
12.
Microstructural aspects of room-temperature deformation in experimental Westerly granite gouge were studied by a set of velocity stepping rotary-shear experiments at 25 MPa normal stress. The experiments were terminated at: (a) 44 mm, (b) 79 mm, and (c) 387 mm of sliding, all involving variable-amplitude fluctuations in friction. Microstructural attributes of the gouge were studied using scanning (SEM) and scanning transmission electron microscopy (STEM), image processing, and energy dispersive X-ray (EDX) analyses. The gouge was velocity weakening at sliding distances >10 mm as a core of cataclasites along a through-going shear zone developed within a mantle of less deformed gouge in all experiments. Unlike in experiment (a), the cataclasites in experiments (b) and (c) progressively developed a foliation defined by stacks of shear bands. The individual bands showed an asymmetric particle-size grading normal to shearing direction. These microstructures were subsequently disrupted and reworked by high-angle Riedel shears. While the microstructural evolution affected the effective thickness and frictional strength of the gouge, it did not affect its overall velocity dependence behavior. We suggest that the foliation resulted from competing shear localization and frictional slip hardening and that the velocity dependence of natural fault gouge depends upon compositional as well as microstructural evolution of the gouge. 相似文献
13.
H.P. Laubscher 《Tectonophysics》1976,36(4):347-365
Jura folds do not resemble models of continuously distributed buckling; they are never sinusoidal. Rather, they may be approximated by discrete, externally rotated conjugate kink bands with rounded hinges. Observational details are confusing, and good outcrops are limited; however, several types of geometrical adjustments necessary during growth (particularly external rotation) of a variety of elementary kink models may be plausibly correlated with observed features. The suitable way to the modeling of fold growth and an explanation of observations on different scales seems to be through synthesis of different modes of rotation and adjustment mixed in varying degrees to fit individual folds. A dominant role is played by incompetent beds of variable importance giving rise to several types of disharmonic folding. 相似文献
14.
剪切带损伤研究对于理解材料破坏机制和建立剪切带本构模型具有重要意义。为了研究单轴压缩黏性土样的剪切带损伤演化规律,根据纵向应变较高时清晰剪切带位置布置切向测线,对数字图像相关方法获得的结果进行双三次样条插值,从而获得光滑性较好的各种应变场。将土样整体的损伤变量-纵向应变曲线与各条剪切带的损伤演化曲线进行了对比。研究发现：（1）总体上,土样整体的损伤变量演化曲线呈线性,而各条剪切带的损伤变量演化曲线均上凹,这表明随着纵向应变的增加,各条剪切带的损伤发展越来越快;（2）各条剪切带的损伤变量演化曲线的轮廓线呈马尾形,这说明随着剪切带的逐渐发育,各条剪切带的相互影响和作用规律变强;（3）对于含水率较低的土样,平行或共线剪切带的损伤变量变化基本同步,特别是在剪切带充分发育之后,但两条剪切带的距离应较小;在一定时期,共轭或交叉剪切带中剪切带的损伤占优,这与剪切带的相互竞争有关,但若两条剪切带达到独立发展的程度,则二者的损伤变量变化可以同步;对于含水率较高的土样,各条平行或共轭剪切带的损伤变量变化基本同步,剪切带的相互作用不明显。 相似文献
15.
《Comptes Rendus Geoscience》2019,351(5):395-405
We aimed to determine variations in stress regimes during the youngest Variscan deformations in the northern part of the Bohemian Massif. For this purpose, we calculated the orientation of the principal stress and strain axes for kink folds observed in the metamorphic envelope of the Karkonosze Granite, using two methods: 1) the traditional method, incorporating structural diagrams (for conjugate kink folds only), and 2) butterfly diagram analysis. The use of both methods enabled us to determine the stress regime, based not only on conjugate but also on monoclinal kink bands. The obtained results prove that butterfly diagram analysis, when applied to monoclinal kink folds, yields reliable results, especially when calibrated using the internal friction angle (Ф) calculated for the conjugate structures.We identified two generations of kink folds: 1) an older one, developed under sublatitudinal shortening and most probably related to the Early Carboniferous terminal stages of the northwest-directed thrusting of the metamorphic units, and 2) a younger one; produced by north-south Variscan Carboniferous compression, and the emplacement and subsequent doming of the Karkonosze Granite. This is the first study on brittle-ductile structures observed commonly in the metamorphic units of the Bohemian Massif, showing their relation to the granitoid intrusion and complementing the tectonic models that usually omit kink folds. 相似文献
16.
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
17.
《Tectonophysics》1999,301(1-2):21-34
In order to clarify deformation mechanisms and behaviours of quartz in a low-temperature regime in the earth's crust, microstructural analyses, particularly on kink bands have been carried out for quartz veins moderately deformed under subgreenschist conditions. Both the dominance of subbasal deformation lamellae and geometry of kink bands suggest that basal (0001) slip was the sole active slip system in the deformed quartz. On a morphological basis, kink bands in the quartz were classified into two types: type I is characterized by conjugate and narrow bands with angular hinge zones, and type II by a wide monoclinal band. Dynamic analyses using deformation lamellae and kink bands have revealed that type I kink bands were formed in grains with basal plane (sub-)parallel to the compression axis, whereas type II kink bands were formed in grains with basal planes inclined to it. Using a numerical model of kinking of elastic multilayers modified after Honea and Johnson (Tectonophysics 30, 197–239, 1976), changes of the level of yielding stress for kinking and the width of kink bands as a function of the angle θ between the slip plane and the compression axis have been examined. The theory predicts that type I kink bands were formed at a higher stress level than type II kink bands, and hence occurrence of type I kink bands suggests that a significant strain hardening occurred in the deformed quartz veins. The theory also well explains the fact that the width of type I kink bands (θ=0 to 10°) is narrower by an order of magnitude than type II kink bands (θ=10 to 80°). 相似文献
18.
C.J.L Wilson 《Journal of Structural Geology》1984,6(3):303-319
Thin sheets of composite ice-mica have been deformed in order to simulate the development of cleavages in quartz-mica rocks. A strong initial mica preferred orientation was variably oriented to the shortening direction. Deformation parallel to the foliation results in a crenulation type cleavage developing from shear bands initiated after a component of pure shear. Deformation oblique to the foliation produces a differentiated cleavage and involves a large component of shear strain subparallel to the original anisotropy. The strain is accommodated by intra- and intercrystalline processes that produce extensive grain elongation and rearrangement of the ductile matrix, thereby forming ice vs mica rich regions. On the other hand, there is no drastic morphological change when a sample is shortened perpendicular to an original foliation: that is, where the micas lie in the plane of no shear strain. Instead, the mica fabric is strengthened and the grains in the ductile matrix are flattened.Two models are presented for the initiation, propagation and evolution of the observed crenulation versus differentiated cleavage types. These depend on mica stacking and orientation relative to the transverse properties of the sample and also on the direction of anisotropy to the XY plane of the bulk strain ellipsoid. The models invoke shear on planes of high shear strain and rotation of the shear bands and rigid mica grains into a direction approximately parallel to the bulk extension direction. 相似文献
19.
The analysis of three cataclastic band sets from Provence (France) reveals that the band density, their conjugate angles, their ratio of shear displacement to compaction, and the amount of cataclasis within the bands differ and can be expressed as functions of tectonic setting and petrophysical properties. We identify (1) a dense and closely spaced network of shear-enhanced (reverse) compaction bands; (2) a regularly spaced less dense network of reverse compactional shear bands; and (3) a localized network of normal shear bands. The field data show that strain localization is favored in an extensional regime and is characterized by shear bands with a large shear to compaction ratio and a small conjugate band angle. In contrast, distributed strain is favored in a contractional regime and is characterized by compactional bands with a low ratio of shear to compaction and a large conjugate band angle. To explain the mechanical origin of this strain localization, we quantify the yield strength and the stress evolution in extensional and contractional regimes in a frictional porous granular material. We propose a model of strain localization in porous sands as a function of tectonic stresses, burial depth, material properties, strain hardening and fluid pressure. Our model suggests that stress reduction, inherent to extensional regime, favors strain localization as shear bands, whereas stress increase during contraction favors development of compactional bands. 相似文献
20.
Three main sets of deformation bands are identified in the Lower Pleistocene carbonate grainstones of Favignana Island (Italy). A bedding-parallel set is interpreted to contain compaction bands, based on the lack of evidence for shear. The other two sets are oriented at a high-angle to bedding, forming a conjugate pair comprised of compactive strike-slip shear bands. In this study, we focus on the compactive shear bands documenting their development, as well as analyzing their dimensional parameters and scaling relationships.Single compactive shear bands are thin, tabular zones with porosity less than the surrounding host rocks, and have thicknesses and displacements on the order of a few mm. The growth process for these structures involves localizing further deformation within zones of closely-spaced compactive shear bands and, possibly, along continuous slip surfaces within fault rocks overprinting older zones of bands. During growth, single bands, zones of bands and faults can interact and link, producing larger structures. The transitions from one growth step to another, which are controlled by changes in the deformation behavior (i.e. banding vs. faulting), are recorded by different values of the dimensional parameters for the structures (i.e. length, thickness and displacement). These transitions are also reflected by the ratios and distributions of the dimensional parameters. Considering the lesser porosity values of the structures with respect to the host rock, the results of this contribution could be helpful for mapping, assessing, and simulating carbonate grainstone reservoirs with similar structures. 相似文献