应力幅值比对土-结构接触面非共轴特性影响研究

运用80 t大型三维多功能土工试验机（3DMAS）,进行了不同应力幅值比下粗粒土与结构接触面大型三维循环直剪试验,深入分析了应力控制往返椭圆剪切路径下接触面切向位移、非共轴角和剪切柔度等三维力学特性以及应力幅值比的影响规律。应力控制往返椭圆剪切路径下,接触面产生了明显的x向和y向位移、非共轴角和剪切柔度。x向和y向位移幅值、剪切柔度峰值均随循环剪切的进行逐渐减小而后趋于稳定,表现出演化特性。非共轴角受切向应力幅值及切向应力增量方向共同影响,其最值、稳定值随循环周次基本保持不变;正向剪切时非共轴角最大值（最小值）与反向剪切时最小值（最大值）基本出现在同一位置。接触面剪切柔度与非共轴角随旋转角度的变化趋势相反,剪切柔度的增加会抑制非共轴角的发展,反之亦然,两者对立统一。应力幅值比对接触面切向位移间椭圆关系及其时程变化形式、切向应力位移类椭圆关系等影响较小,主要影响切向位移幅值及其偏移程度、切向位移间椭圆关系的长短轴大小及方向、切向应力位移类椭圆关系的长短轴大小、非共轴角的数值及其随旋转角度的变化形式和最值出现位置、剪切柔度随旋转角度的变化形式和峰值及其产生位置等。应力幅值比越大,切向位移幅值越大、向负向偏移越大,剪切柔度峰值越大,随循环剪切的进行减小的速率越慢。应力幅值比ξ_τ=1和ξ_τ≠1时接触面非共轴角和剪切柔度随旋转角度的发展变化形式差别很大。     

The geometry and kinematics of a suite of conjugate kink bands, southeastern Australia

A conjugate set of subvertical kink bands is exposed in coastal outcrops of well-foliated Ordovician turbidites near Mystery Bay, Australia. All kink bands with widths exceeding 3 cm have complex internal structures including compound and parasitic kinks, stepped kink boundaries, internal crenulations, variable kink angles and prismatic voids. The kink bands are interpreted to result from rotation of short foliation segments between fixed kink planes with subsequent widening and modification by layer-parallel shear external to the kink band. Layer-parallel shear of both sinistral and dextral sense accompanied kinking and indicates a variable stress system during kink band development.Conjugate kink bands are abundant and are used to estimate bulk strain orientations. In general, the dominant kink set of a conjugate pair is inclined at a lower angle to the external foliation than the weaker set and this angular disparity increases with increasing dominance of one set. These observations are at variance with relationships described from experimental bulk pure shear deformation of anisotropic materials. It is suggested that orthogonal constraints in these experiments restrict layer-parallel shear to within a developing kink band and are, therefore, unlike many natural kink systems. Simple shear experiments can produce structures geometrically similar to natural kink bands.     

Fabric development in shear zones: theoretical controls and observed phenomena

It is suggested that the kinematic framework controls the orientation of crystallographic fabrics developed in plastically deformed quartzites. Important directions in this framework are those of the instantaneous stretching axes, and the flow plane and flow direction if these can be uniquely defined. Rotation of the crystal axes takes place at any instant of time dependent on the orientation of the grain relative to the stretching axes. Because of this dependence the skeletal outline^* of a pattern of preferred orientation is sensitive to the closing stages of deformation. Thus fabrics measured in major movement zones cannot be related to early thrust or shear displacements without considering the effects of the geological history subsequent to those events.Nevertheless, asymmetric fabrics in movement zones may allow determination of the shear direction and sense of shear. Asymmetry in the intensity distribution is less susceptible to modification than asymmetry in the fabric skeleton, and may remain as a persistent measure of the sense of shear in mylonites subjected to coaxial deformation after non-coaxial events. However, fabric asymmetry need not always be related to the deformation history, and effects related to the population of initial grain-orientations must be considered, as well as the influence of recrystallization and grain growth.A problem of scale is involved in extrapolating the movement picture inferred from the behaviour of a few hundred crystal grains to larger dimensions. This question is also encountered when trying to specify deformation paths in mesoscopic shear zones. It is difficult to obtain simple shear experimentally because of the role discontinuities play in deformation. In certain cases in natural shear zones the quartz grains may be subjected to a coaxial deformation path while the bulk deformation is progressive simple shear. Caution must therefore be exercised when attempting to use quartz fabrics to infer characteristics of the bulk kinematics or movement picture applicable during deformation.     

Evolution of microstructures in Precambrian shear zones: An example from eastern India

Shear zones are areas of intense deformation in localized zones which can be used as natural laboratories for studying deformation characteristics. Metre to-micro scale structures that develop in response to a progressive simple shear in a shear zone are characterized by a protracted history of deformation and are immensely useful in delineating the history of progressive deformation. To decipher these localized zones of deformation and to establish the continuous non-coaxial character of deformation, detail microstructural studies are very useful. Singhbhum shear zone (SSZ), a regional Precambrian tectonic dislocation zone in eastern India, depicting a top-to-south thrust movement of the hanging wall provides a scope for studying microstructural characteristics developed in response to a progressive shear at mid-crustal level. SSZ is characterized by intense stretching lineation, isoclinal folds, shear planes, superposed schistosity and deformed quartz veins. Quasi-plastic (QP) deformation mechanisms were predominantly active in the SSZ. The overprinting relationship between the earlier and later schistosity with a consistent sense of shear indicates that earlier schistosity is transposed to later schistosity through the intermediate stages of crenulation cleavage during a progressive non-coaxial deformation. The recrystallization of quartz in mylonitic quartzite suggests protracted history of deformation. The analysis of the character of quartz grains of both the porphyroclasts and recrystallized grains suggests that strain was partitioned between the most intensely deformed central part of the shear zone and the shear-related deformation zone outside the central part of the shear zone.     

塔北哈拉哈塘地区剪切断裂系统构造特征及其对油气分布的控制

塔北哈拉哈塘地区发育多组北东、北西向主干断裂组成的剪切断裂系统, 本文在剪切断裂地球物理识别的基础上, 分析了研究区内剪切断裂系统构造特征及其对油气分布的控制.研究区内剪切断裂发育9种典型构造样式.平面上, 整体为共轭剪切断裂, 分支断裂延伸较短, 与主干断裂呈小角度相交; 单条剪切断裂带具有一定的分段性, 不同部位具有不同的构造样式和平面组合.纵向上, 哈拉哈塘地区剪切断裂系统具有分层变形的特征, 可以划分为下、中、上3个构造层.下构造层为寒武系-奥陶系, 为纯剪作用下形成的共轭剪切断裂系统; 中构造层为志留系-二叠系, 为受下构造层控制的张剪作用下形成的断裂; 上构造层为三叠系及其以上地层, 为受中下构造层控制的张剪作用下形成的雁列式分布的正断层.这种分层变形特征主要受控于区域古构造应力场的变化.哈拉哈塘地区剪切断裂系统的构造特征决定了该区油气分布具有"多层系含油、沿断裂富集"的特征.     

Lack of porphyroblast rotation in the Otago schists, New Zealand: implications for crenulation cleavage development, folding and deformation partitioning

Porphyroblasts of garnet and plagioclase in the Otago schists have not rotated relative to geographic coordinates during non-coaxial deformation that post-dates their growth. Inclusion trails in most of the porphyroblasts are oriented near-vertical and near-horizontal, and the strike of near-vertical inclusion trails is consistent over 3000 km². Microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain, and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures. This is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding.
Several crenulation cleavage/fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding. In the light of accumulating evidence that porphyroblasts do not generally rotate, the applicability of these models to deformed rocks is questionable.
Whether or not porphyroblasts rotate depends on how deformation is partitioned. Lack of rotation requires that progressive shearing strain (rotational deformation) be partitioned around rigid heterogeneities, such as porphyroblasts, which occupy zones of progressive shortening or no strain (non-rotational deformation). Therefore, processes operating at the porphyroblast/matrix boundary are important considerations. Five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast: (a) a thin layer of fluid at the porphyroblast boundary; (2) grain-boundary sliding; (3) a locked porphyroblast/matrix boundary; (4) dissolution at the porphyroblast/matrix boundary, and (5) an ellipsoidal porphyroblast/shadow unit.     

共轭伸展褶劈理夹角的定量解析

通过静力和动力学解析,共轭伸展褶劈理面对最大主应力方向的理想夹角为１０９°２８′,与实际观测值一致。共轭伸展褶劈理的取向取决于形成时剪切作用类型,纯剪切时以糜棱面理为对称面,但在一般剪切时则与之不对称。同向褶劈理与糜棱面理间的夹角变化于－ ９°４４′～３５°１６′之间,取决于简单剪切与纯剪切的相对组分。共轭两组常不同等发育。Ｐ方向的同向剪切趋势抑制反向褶劈理的形成。同向伸展褶劈理的扩展是导致低角正断层形成的有效机制     

Mineral fish: their morphological classification,usefulness as shear sense indicators and genesis

Mineral fish are sheared and commonly asymmetric mineral grains or clusters of grains. This work reports 11 sub-types of mineral fish showing a top-to-SE sense of ductile shearing in the Karakoram Metamorphic Complex (KMC). The mineral fish are of three broad geometries: sigmoid, lenticular and parallelogram. Reliable senses of shear are indicated by the overall asymmetry and inclination of mineral fish. On the other hand, the true shear sense is not always indicated by either the orientations of their cleavage planes or those of the individual grains in composite mineral fish. The ranges of local orientations of single sigmoid mineral fish that include the lower values (<23°) in the KMC indicate their extensive ductile shearing. The studied mineral fish were products of a range of deformation mechanisms including homogeneous deformation, simple shear, intra-granular slip, crystal-plastic deformation, fracturing and synthetic shearing. Additionally, some examples might have undergone duplex slips and a few nucleated and grew either prior to or during the top-to-SE shearing.     

The structural evolution of sheath folds: A case study from Cap de Creus

It has long been recognised that within zones of intense non-coaxial deformation, fold hinges may rotate progressively towards the transport direction ultimately resulting in highly curvilinear sheath folds. However, there is a surprising lack of detailed and systematic field analysis of such “evolving” sheath folds. This case study therefore focuses on the sequential development of cm-scale curvilinear folds in the greenschist-facies El Llimac shear zone, Cap de Creus, Spain. This simple shear-dominated dextral shear zone displays superb three dimensional exposures of sheath folds defined by mylonitic quartz bands within phyllonite. Increasing amounts of fold hinge curvature (δ) are marked by hinge segments rotating into sub-parallelism with the mineral lineation (Lm), whilst the acute angle between the axial-planar hinge girdle and foliation (ω) also displays a sequential reduction. Although Lm bisects the noses of sheath folds, it is also clearly folded and wrapped-around the sheath hinges. Lm typically preserves a larger angle (θ) with the fold hinge on the lower limb (L) compared to the upper (U) limb (θL > θU), suggesting that Lm failed to achieve a steady orientation on the lower limb. Adjacent sheath fold hinges forming fold pairs may display the same sense of hinge arcing to define synthetic curvature, or alternatively opposing directions of antithetic curvature. Such patterns reflect original buckle fold geometries coupled with the direction of shearing. The ratio of long/short fold limbs decreases with increasing hinge curvilinearity, indicating sheath folds developed via stretching of the short limb, rather than migrating or rolling hinge models. This study unequivocally demonstrates that both hinges of fold pairs become curvilinear with sheaths closing in the transport direction recording greater hinge-line curvilinearity compared to adjacent return hinges. This may provide a useful guide to bulk shear sense.     

Progressive deformation across a ductile shear zone: an example from the Singhbhum Shear Zone,eastern India

Structural investigations in the Precambrian Singhbhum Shear Zone of eastern India document an intimate relationship between micro- to meso-scale structures and the deformation history. Shear zone rocks are characterized by composite foliation, a well-developed stretching lineation, folds, shear planes, and quartz veins. These structures reflect thrusting of the Proterozoic north Singhbhum hanging wall block over the Archaean south Singhbhum footwall block. Microstructural analysis of multiple foliation and mylonitic rocks within the shear zone helps to define its progressive evolution. During progressive deformation, overprinting of microstructures resulted in incomplete transposition or complete erasing of previously formed structures and mineral assemblages, allowing room for new dynamic equilibrium structures to form. The dominant deformation mechanism was dissolution–recrystallization, with locally important fluid circulation responsible for transformation of the quartzo-feldspathic mass into phyllonite, and quartzites and schists into mylonite. Textural features suggest that the bulk deformation was non-coaxial, evolving from dominant pure shear in the early stage followed by simple shear in a single progressive strain history of the Singhbhum Shear Zone.     

Finite-element modelling of shear zone development in viscoelastic materials and its implications for localisation of partial melting

The development of shear zones initiating on random weaker initial perturbations is modelled numerically for low Deborah number viscoelastic materials, considering the influence of effective viscosity contrast, power law rheology, strain softening, and different imposed bulk deformation geometries, ranging from pure to simple shear. Conjugate shear zones initiate at ∼90° to one another, and rotate with increasing bulk deformation, the basic pattern not being markedly influenced by the vorticity of imposed deformation. The rate of propagation of individual conjugate shear zones is little affected by increased effective viscosity contrast between matrix and inclusion but is promoted by power-law rheology. However, the most marked effect is observed for strain softening behaviour, where rapid propagation produces straighter and narrower shear zones. The localisation of strain is reflected in a correspondingly heterogeneous stress distribution. In particular, mean stress or pressure is higher in the extending, near planar, weaker zones of localised shear. Melting of gneissic or pelitic compositions is pressure dependent. With free water present, increased pressure promotes melting, whereas the opposite is true for water-absent melting. For water-present conditions, a positive feedback could develop between localised shearing, increased pressure and partial melting. This is potentially more effective in concentrating melt in shear zones than shear heating, where melt-related softening has a negative feedback effect.     

伸展褶劈理(C’面理)在递进变形过程中的行为及其对剪切带面理(S面理)的影响

伸展褶劈理（extensional crenulation cleavage,简称C’面理）作为韧性剪切带内常见的构造面理之一,在递进变形过程中是否旋转以及其如何影响剪切带面理（S面理）是尚未解决的构造地质前沿问题。本文统计了不同剪切带中C’面理、S面理和C面理（糜棱岩面理）的夹角。结果发现,从剪切带边缘向中心随着应变逐渐增大,C’面理与剪切平面（C面理）的夹角（α）呈近正态单峰式分布,表明C’面理在递进变形过程中不旋转。在C’面理发育的情况下,S面理与C面理的夹角（β）随着应变的增加逐渐增大,即S面理在C’面理形成后发生反向旋转,因此原用于计算剪应变的公式：tan 失效。强弱互层材料的应变分配结果显示,一般剪切条件下,粘性分层与剪切平面的夹角（β）减小到20°时,强硬层的运动学涡度接近于0,即表现为纯剪切,而软弱层表现为近简单剪切;β小于10°时,强硬层发生反向旋转。由此可见,伸展褶劈理是不均匀岩层在递进变形过程中应变分配（strain partitioning）的结果。随着递进变形的进行,S面理与C面理的夹角逐渐减小,强硬层被分配以更多的纯剪切而布丁化,随后内部产生微型剪切条带并扩展形成C’面理,承担应变分配的简单剪切。至此,随着应变分配的完成,C面理上的剪切作用自行停止,因此C’面理不发生旋转,而S面理在C’面理的制约下开始反向旋转。     

Stibnite microstructures as imprints of fault movement

The natural deformation of stibnite in relation to the kinematic history of a regional fault is resolved using microstructural analysis of massive stibnite along the Biards Fault in the Massif Central (France). Stibnite underwent intra-crystalline deformation associated with a strong linear and planar anisotropy. Lepidoblastic textures, undulatory extinction of oriented grains, growth of antimony between these grains, and a colour change of stibnite from white to black, result from dynamic recrystallisation. A range of asymmetric microstructures indicate non-coaxial deformation and a left-lateral sense of shear, related to the movement of the fault zone. The computed displacement corresponds to the geological offsets measured along the fault. We conclude that stibnite has microstructurally recorded the bulk kinematics of its host fault zone.Editorial Handling: F. Tornos     

韧性剪切带的变形分域现象——以辽东的丹东韧性剪切带为例

丹东韧性剪切带的实例表明:韧性剪切带的持续变形包含了共轴与非共轴两种应变线路或状态,由于分布的不均匀性,导致变形分域现象,形成平面的变形岩石分区。持续变形过程中,又存在应变线路的转换和叠加。微构造是确定变形体制的主要依据。野外观测与应变分析证实,变形分域存在于不同尺度,在总体剪切(非共轴)变形条件下,初始糜棱岩往往表现以共轴变形占优势,而糜棱岩和超糜棱岩则以非共轴变形为主。持续变形导致从共轴向非共轴转变,最后形成后者的主体地位。     

Quartz microstructures and fabrics in the Island of Groix (Brittany,France)

Quartz microstructures and fabrics in the southeastern part of the island of Groix developed during the last stages of the Palaeozoic synmetamorphic deformation. The zonation of the quartz microstructures in map view suggests an upward positive gradient of strain. The plastic flow plane in quartz found in folds with axes aligned parallel to the stretching lineation, is parallel to the axial planes of the folds. The dominant regional sense of shear, as deduced from quartz fabrics, corresponds to the northward displacement of the upper block. This sense of displacement supports the hypothesis made by previous workers that the synmetamorphic deformation in Groix occurred in a N-directed intra-lithospheric thrust rather than in a N-dipping subduction zone. Quartz c-axis patterns argue for the distinction of two synmetamorphic phases with different transport directions. The transition between these two phases is thought to have occurred progressively during the course of the thrusting.     

The use of asymmetric pressure shadows in mylonites to determine the sense of shear

Asymmetric pressure shadows (APS) on both sides of a rigid porphyroclast are commonly observed in mylonites along the Median Tectonic Line (MTL) in Japan. It is one of the most noticeable asymmetric microstructures, showing that the porphyroclasts have rotated during non-coaxial laminar flow in a ductile shear zone. The shadow domains are filled with recrystallized quartz and K-feldspar. Excepting APS, various asymmetric microstructures in the mylonites indicate a sinistral sense of displacement throughout the ductile shear zone along the MTL.Based on the shape analysis of APS in XZ section (parallel to the mylonitic lineation and normal to the mylonitic foliation), the following results were obtained: (1) the relative position of the APS with respect to a porphyroclast is not a reliable criterion for deducing the sense of shear; and (2) the drag angle (β) of the shadow boundaries with respect to the mylonitic foliation in each quartered domain is diagnostic of the sense of shear; when the shearing is sinistral, β in upper right- and lower left-hand side of a porphyroclast is larger than β in upper left- and lower right-hand side, and vice versa for dextral shearing. These results demonstrate that the drag patterns of APS around porphyroclasts in mylonites are highly reliable indicators for the determination of the sense of shear.     

The simulation of fabric development during plastic deformation and its application to quartzite: the influence of deformation history

The effect of deformation history on the development of crystallographic preferred orientation in quartzities has been simulated using a computer program based on the Taylor-Bishop-Hill analysis. Model quartzities with different combinations of glide systems have been subjected to various coaxial and non-coaxial deformation histories. It is possible to obtain information from the fabrics that develop during simple histories; for example, the location of the axis of extension is generally associated with a pole free area on a c-axis plot, and progressive axial shortening, plane strain and axial shortening produce characteristic fabrics. In progressive simple shear the fabric skeleton becomes asymmetric relative to the sense of shear and a-axes preferentially align in the flow plane parallel to the flow direction. However, this example illustrates that the fabric orientation and characteristics are controlled by the kinematic framework and bear only an indirect relationship to the finite strain accumulated to that point in the history.The imprint of the closing stages of deformation limits to some degree the use of crystallographic fabrics as a tool for structural geologists, but in favourable circumstances data can be obtained concerning characteristics of the deformation history, on the scale of the hand-specimen, for the last part of this history.     

Microstructural shear criteria associated with grain-boundary sliding during ductile deformation

Experimental studies on rock analogues are described which establish that certain microstructures (diamond grain structures, tabular grain structures and asymmetric grain-boundary bulges) can be used to determine (i) if deformation was coaxial or non-coaxial and (ii) the sense of shear in zones of non-coaxial deformation. Mechanisms for the development of the structures are described which involve the linked operation of grain-boundary sliding and migration.     

Ductile Thrusting Recorded by the Garnet Isograd from Blueschist-Facies Metapelites of the Ile de Groix, Armorican Massif, France

Mineral assemblages in the blueschist-facies metapelites fromthe Ile de Groix (Armorican Massif, France) permit the distinctionof two main units. The Upper Unit is characterized by: (1) highmodal proportions of garnet; (2) larger grain size; (3) therarity of graphite-bearing layers; (4) a single, although composite,foliation S₁. A Lower Unit is defined by: (1) low modal proportionsof garnet; (2) smaller grain size; (3) an abundance of graphite-bearinglayers; (4) a pervasive crenulation cleavage S₂. In the UpperUnit, coexisting garnet and chloritoid are more magnesian andless manganiferous than in the Lower Unit. The differences inmodal proportions and chemistry of coexisting minerals reflectdifferent P–T conditions. The P–T history of theblueschist-facies metapelites is estimated using a simplifiedpetrogenetic grid in the NFMASH system and thermodynamic calculations,which suggest peak P–T conditions at about P = 16–18kbar, T = 450–500°C and P = 14–16 kbar, T =400–450°C in the Upper and Lower Units, respectively.Peak P–T conditions were followed by a nearly isothermaldecompression for both units at slightly different temperatures(of the order of 50°C). The contact between the two units,i.e. the garnet isograd, is interpreted as a greenschist-faciesductile thrust. Thrusting of the higher-grade unit, i.e. theUpper Unit, over the Lower Unit occurred after the high-pressureevent, i.e. during the exhumation of both units. The observedsuperposition of higher-grade rocks over lower-grade rocks arguesagainst models where the exhumation history is entirely controlledby crustal-scale vertical shortening (i.e. extension). KEY WORDS: Armorican Massif; blueschist facies; Ile de Groix; metapelites; P–T path; garnet isograd     

Progressive development of structures in a ductile shear zone

In the Singhbhum Shear Zone of eastern India successive generations of folds grew in response to a progressive ductile shearing. During this deformation a mylonitic foliation was initiated and was repeatedly transposed. The majority of fold hinges were formed in an arcuate manner at low angles to the Y-axis in an E-W trending subhorizontal position and major segments of the fold hinges were then rotated towards the down-dip northerly plunging X-axis. The striping and intersection lineations were rotated in the same manner. The down-dip mylonitic lineation is a composite structure represented by rotated early lineations and newly superimposed stretching lineations. The consistent asymmetry of the folds, the angular relations between C and S surfaces and the evidence of two-dimensional boudinage indicate that the deformation was non-coaxial, but with a flattening type of strain with λ₁ ⪢ λ₂. The degree of non-coaxiality varied both in space and time. From the progressive development of mesoscopic structures it is concluded that the 2–3 km wide belt of ductile shear gave rise to successive anastomosing shear zones of mesoscopic scale. When a new set of shear lenses was superimposed on already sheared rocks, the preexisting foliation generally lay at a low angle to the lenses. No new folds developed where the acute angle was sympathetic to the sense of shear displacements. Where the acute angle was counter to the sense of shear, the pre-existing foliation, lying in the instantaneous shortening field, was deformed into a set of asymmetric folds.