Three-dimensional strains in fold-hinge zones

Experiments have been carried out to study the orientation and values of the three principal extensions during folding, and to investigate the nature of the deformation paths of the strain ellipsoids at the fold hinges. Single layers embedded in a matrix were deformed by plane strain pure shear with the layers obliquely inclined to the axes of the bulk strain. The strains on the outer and inner arcs of the fold hinges were measured by means of grids on the layer surfaces. The orientation and values of three principal strains during folding depend on the layer orientation and the degree of deformation, and may be different on the outer and the inner arcs. The deformation paths in the outer arc are generally in the constriction field and those in the inner arc lie in the flattening field. The deformation paths have been computed by analyzing the manner by which components of layer shortening, tangential longitudinal strain and elongation parallel to the fold-hinge line combine to give the finite strain.     

Single-layer folding in marble and limestone: An experimental study

Folding experiments have been carried out on single-layers of Carrara marble and Solnhofen limestone at a confining pressure of 275 bars, temperature of 400°C, and strain rates of 5.5×10⁻⁷ to 8.2×10⁻⁷. The marble and limestone layers were embedded in a rock-salt matrix and in a matrix of a mixture of 60% fine-grained halite and 40% fine-grained calcite, respectively, and deformed to different percentages of bulk shortening. Aspect ratios of the layers varied between 11.25 and 15. The stress-strain relationship reveals that strain increased with a very small increment in compressive stresses, once folding was initiated.With progressive deformation the bulk strain is compensated by folding along one fold hinge. The resulting folds are concentric and a combination of class 1a, 1b and 3 type. The changes in the arc length, layer thickness, limb dip and wavelength with progressive folding in marble layers, are discussed.The microstructure and texture of the folded marble and limestone layers have been investigated optically and by means of an X-ray texture goniometer. The inner fold arc exhibits a strong preferred orientation, whereas in the outer fold are the preferred orientation is poorly developed. Differences in the fabric in medium-grained marble and fine-grained limestone layers have been attributed to the difference in mechanism of deformation.     

Volumetric strains in neutral surface folding

Neutral surface folding is a significant contributor to fold development. This mechanism produces contrasting strains in the inner and outer arcs of the folded layer that arise from principal stress orientations that are approximately parallel and perpendicular to the layer. We demonstrate that such stress patterns imply significant gradients of mean stress across the folding layer, being more tensional on the outer arc and more compressive in the inner arc. This could pump fluids towards the outer arc during folding and result in heterogeneous volume changes. We conclude that the neutral surface folding model should be adapted to accommodate volumetric strains, in order to explain dilatational structures (e.g. open fractures, veining) on the extrados and volume-loss structures (e.g. pressure solution seams, stylolitic cleavages) on the intrados. This dilatation has economic implications as it allows prediction of sites of mineralization and zones of secondary permeability in fold-related hydrocarbon traps.     

低级变质沉积岩区压溶作用在褶皱变形中的意义

拉卡兰褶皱带中,发育于Ballarat-Bandigo冲断带中的低级变质砂、泥岩的宏观构造以间离劈理和人字形褶皱为特征,而且劈理在褶皱中呈扇形发育。劈理和褶皱的几何关系分析显示:劈理和褶皱的形成为压溶作用、压扁作用、弯曲作用和被动旋转共同作用的结果,而褶皱砂、泥岩中变形构造则以与压溶作用和再沉淀过程有关的显微构造为其典型特征。Fry法进行的全岩应变测量显示,褶皱砂岩的内部应变相当低(X/Z=1.40—1.83),褶皱应变格局给出变形机制的信息包括:缩短过程中的压扁作用和压溶作用、褶皱过程中由弯滑导致的层平行剪应变、以及褶皱后期发育阶段内弧区强烈的压溶作用。宏观构造、显散构造以及应变特征多方面信息证明:低级变质的沉积岩在褶皱变形过程中,压溶作用为一重要的变形机制。应变分解显示在30％—50％的总地壳水平缩短量下,弯曲导致的缩短最为14％—36％,压扁导致的缩短量为3％—14％,压溶导致的缩短量为8％—26％,而且压溶作用主要发生在褶皱内弧区。     

中下部地壳拆离断层带演化中的褶皱作用:以辽南变质核杂岩为例

作为变质核杂岩构造的重要组成部分,拆离断层带内广泛发育的褶皱构造与其寄主岩石一样记录了中下地壳拆离作用过程。选取辽南变质核杂岩金州拆离断层带内褶皱构造作为研究对象,基于叶理与褶皱构造关系分析,划分了褶皱期次与阶段性;通过形态组构分析、结晶学组构分析及石英古温度计等技术方法的应用,初步分析了拆离断层内褶皱的形成机制,为辽南地区拆离作用过程提供约束。根据褶皱形成与拆离作用的时间关系,将拆离带内褶皱分为拆离前褶皱、拆离同期褶皱和拆离后褶皱;拆离作用同期的褶皱按时间早晚分为早期(a1)阶段、中期(a2)阶段、晚期(a3)阶段。不同阶段褶皱的野外形态、叶理与褶皱关系等方面的差异,以及形态组构与结晶学组构的特征,为判断和恢复褶皱的形成机制提供了佐证,揭示出拆离断层带褶皱是在纵弯压扁和顺层流变的共同作用下递进剪切变形的产物。在拆离作用过程中, a1阶段和a2阶段褶皱以纵弯、压扁褶皱作用为主,a3阶段褶皱以弯滑作用为主。褶皱作用记录了拆离断层一定温度范围内(主要集中在380~500 ℃)的变形特征,拆离作用从早期到晚期的演化整体处于相对稳定的应变状态下。对金州拆离断层带而言,在区域NW-SE向伸展过程中,还伴随着NE-SW向微弱的收缩。     

川东南构造变形特征及其对页岩气保存的影响

川东南地区构造变形复杂,二叠系和志留系含有丰富的页岩气资源。依据岩性和地震资料的解释,寒武系膏盐层与中-下三叠统膏盐层对构造变形有重要控制作用,多数断层沿膏盐层滑脱。研究区包括盆内和盆缘两部分,盆内发育形态对称的盖层滑脱式褶皱;盆缘发育基底卷入式褶皱冲断构造,分为山前推覆带和山前转换带,前者发育高陡的三角楔构造,后者由冲断带、褶皱带和斜坡带组成。构造演化分析表明:晚侏罗世齐岳山断层开始发育,盆内地层发生挠曲变形;白垩纪盆缘形成三角楔构造,盆内主要构造和断裂已经发育;新生代齐岳山褶皱隆升,盆内寒武系之上沉积盖层褶皱形成多个背斜和向斜。山前推覆带构造高陡、变形强烈,页岩气保存条件差;山前转换带构造变形程度适中,其褶皱带背斜完整,页岩气保存条件好;盆内中-下三叠统膏盐层封盖性好,埋深适中的背斜为页岩气有利勘探目标区。      

Buckling stability of a viscous single-layer system,oblique to the principal compression

A new theory is developed for single-layer buckling, where the layer is not parallel to the principal stresses. The model chosen consists of a single layer with Newtonian viscosity η embedded in an infinite matrix of viscosity η₁. The layer lies at an angle θ to the bulk principal compressive stress in the embedding medium. It is deformed in equal-area plane strain, with the direction of no strain and the third principal bulk stress, parallel to the layer; hence the obliqueness to the principal stresses is only in two dimensions. It is shown that stress refraction is a necessary condition for this system, and an expression is derived for its value in terms of η, η₁ and θ. Buckling stability equations are completely developed which satisfy the Navier-Stokes equilibrium equations for the buckling layer, and the condition of stress continuity at the layer-embedding medium interface. The dominant wavelength of the buckles is shown to be independent of θ, but the stress required increases with θ.The results of this work have an important bearing on natural folds, since there is no evidence that rock layers are initially parallel to the stresses which fold them, an assumption made in former buckling theories. It is suggested that refraction of stresses and the resulting incremental strains gives rise to the finite structure of cleavage refraction so common in deformed rocks, and that the progressive development of folds in layers oblique to the principal bulk stresses gives rise to asymmetry.     

库车褶皱冲断带前缘盐层厚度对滑脱褶皱构造特征及演化的影响

库车褶皱冲断带前缘发育一系列滑脱褶皱,虽然卷入变形的新生代地层及底部滑脱层(古近系盐层)相同,但滑脱褶皱的构造特征及演化存在显著差异。文中结合野外地质调查结果以及钻井资料和高品质二维地震反射剖面解析,以南喀背斜和米斯坎塔克背斜为例,估算出盐层初始厚度,并讨论其对于滑脱褶皱样式及其演化过程的影响。结果表明,南喀背斜和米斯坎塔克背斜下伏盐层初始厚度不同,估算出前者厚度介于0.1~0.5 km,主要为0.1~0.3 km,而后者却大约为1.0 km。与此同时,南喀背斜和米斯坎塔克背斜均表现出分段差异变形特征。南喀背斜为低缓的滑脱褶皱,其东段隐伏地下,变形方式为褶皱作用;而西段出露地表,背斜核部发育隐伏的逆冲断层,变形方式为褶皱作用和断层作用。背斜西段平均隆升速率大于东段,导致西段隆升出露地表。米斯坎塔克背斜表现为大规模滑脱褶皱,根据变形特征的不同可以分为3段,东段背斜倾向北,盐岩在其核部及北翼下方聚集加厚;而中-西段背斜倾向南,其中中段背斜核部位置盐岩聚集加厚,两翼下伏盐岩减薄甚至形成盐焊接。而在西段背斜呈箱状,两翼下方盐岩厚度至少为1.0 km。笔者总结出库车褶皱冲断带前缘发育的7种滑脱褶皱变形样式,通过构造分析得出,研究区滑脱褶皱的变形主要受盐层厚度、构造缩短量及盐岩流动变形共同控制,其中盐层厚度起主导作用,控制了滑脱褶皱的发育位置,并影响了滑脱褶皱的变形样式。研究结果将为其他褶皱冲断带中滑脱褶皱的相关研究提供重要参考,特别是在缺少高品质地震资料,或者构造变形强烈、地震资料品质较差的地区。     

Layer shortening and fold-shape development in the buckling of single layers

The progressive development of folds by buckling in single isolated viscous layers compressed parallel to the layering and embedded in a less viscous host is examined in several ways; by use of experiments, an analogue model to simulate simultaneous buckling and flattening and by an application of finite-element analysis.The appearance of folds with a characteristic wavelength in an initially flat layer occurs in the experiments for viscosity ratios (μ_layer/μ_host = μ₁/μ₂) of between 11 and 100; progressive fold development after the initial folds have appeared is similar in the experiments and in the finite-element models. Except for the finite-element model for μ₁/μ₂ = 1,000 layer-parallel shortening occurs in the early stages of folding and a stage is reached where little further changes in arc length occur. The amount of layer-parallel shortening increases with decreasing viscosity contrast, and becomes relatively unimportant after the folds have attained limb dips of about 15°–25°.Thickness variations with dip are only significant here for the finite-element model with μ₁/μ₂ = 10, and in experiments for μ₁/μ₂ = 5 where the layer is initially in the form of a moderate-amplitude sine wave. The variations range from a parallel to a near-similar fold geometry, and in general depend on the viscosity contrast, the degree of shortening and the initial wavelength/thickness ratio. They are very similar to the variations predicted by the analogue model of combined buckling and flattening. The difference between the thickness/dip variations in a fold produced by buckling at low viscosity contrast and one produced by flattening a parallel fold is marked at high limb dips and very slight at low limb dips.Many natural folds in isolated rock layers or veins show thickness/dip relationships expected for a flattened parallel fold, and some show relationships expected for buckling at low viscosity contrasts. Studies of the wavelength/thickness ratios in natural folds have suggested that competence contrast is often low. Many folds in isolated rock layers or veins whose geometry may vary between parallel and almost similar, and may be indistinguishable from those of flattened parallel folds, have probably developed by a process of buckling at low viscosity contrasts.     

鄂西渝东区构造裂缝发育特征及力学机制

针对鄂西渝东区隔档式褶皱发育的构造特点,结合区域构造演化史,分别以隔档式褶皱和正弦曲线几何形态的褶皱建立模型,用有限元法模拟了褶皱不同构造部位的应力场分布特征。褶皱外侧为张应力集中区,内侧为压应力集中区,且应力矢量与地层产状平行,二者以中部既无明显挤压、亦无明显拉张的中和面为分界。通过野外露头和岩心观察,总结了鄂西渝东区构造裂缝发育的三种形式:具多层介质的非能干层中发育顺层裂缝;向斜内侧、背斜外侧的能干层中发育高角度剪切缝;背斜外侧、内侧具多层介质的非能干层中发育顺层裂缝与高角度缝共生的网状缝。这种特征与模拟结果呈现较高程度的一致性,可根据应力场模拟成果和裂缝发育地层的能干性进行合理解释。复向斜中的隆起区可作为页岩气勘探的重要目标。     

Single layer folding in simple shear

Despite the common occurrence of simple shear deformation, laboratory and numerical simulations of folding have so far been almost exclusively in pure shear. Here we present a series of finite-element simulations of single layer folding in simple shear up to high shear strains (γ ≤ 4, and up to 75% shortening of the folding layer). In the simulations we vary the viscosity contrast between layer and its surroundings (25–100), the stress exponent (1 or 3) and the kinematics of deformation (pure- versus simple shear). In simple shear fold trains do not show a clear asymmetry, axial planes form perpendicular to the developing fold train and rotate along with the fold train. Differences in geometries between folds formed in simple and pure shear folds are thus difficult to distinguish visually, with simple shear folds slightly more irregular and with more variable axial plane orientation than in pure shear. Asymmetric refraction of an axial planar cleavage is a clearer indication of folding in simple shear. The main effect of an increase in stress exponent is an increase in effective viscosity contrast, with only a secondary effect on fold geometry. Naturally folded aplite dykes in a granodiorite are found in a shear zone in Roses, NE Spain. Comparison of the folded dykes with our numerical simulations indicates a viscosity contrast of around 25 and a stress exponent of 3. The natural folds confirm that at this moderate viscosity contrast, a significant amount of shortening (20–30%) is achieved by layer thickening instead of folding.     

库车坳陷克拉苏-依奇克里克构造带的构造演化

野外露头上的小尺度构造记录着整个褶皱逆冲带运动学演化进程。通过对库车褶皱逆冲带坎亚肯褶皱发育的小尺度构造的特征和相互叠加、切割关系的分析,认为克拉苏-依奇克里克构造带在新生代的构造挤压变形中经历了三个阶段,即平行层面的挤压缩短作用阶段、弯滑与弯流褶皱作用阶段和逆冲断层改造作用阶段。同时,对不同阶段出现的断层滑动数据进行反演和古应力恢复的结果表明,整个变形过程,最大主应力方向σ1(即构造缩短方向)集中分布在NNW—SSE,没有发生大的变化,说明三个阶段的构造变形是在同一稳定应力场下发生的,从而进一步证明所有的变形是同一次构造挤压下的渐进式序列变形,而不是多期构造挤压变形的结果。此次构造挤压变形主要发生在康村沉积期(12～5Ma),对于克依构造带的构造圈闭最终定型起着决定性作用。     

褶皱相关断裂构造形成条件与发育机制:燕山中部露头尺度变形研究为例

总结了褶皱相关断裂发育机制的3个构造几何学模型:同心圆褶皱模型、膝折带褶皱模型和弯流褶皱模型。基于燕山中部中、新元古界地层中发育的5个露头尺度褶皱及其中、小型断裂构造的实例剖析,探讨了收缩变形过程中褶皱与断裂构造发育时序与褶皱相关断裂构造的产生机制。研究指出,规模与所在褶皱构造相当或略小的断裂构造当中,既有形成时间早于褶皱变形的断层,也有在褶皱变形过程中调节褶皱不同部位应变差异的褶皱相关断裂构造,而且卷入后期变形的早期断裂可能成为制约褶皱成核位置的影响因素,以及成为枢纽叠覆楔构造的形成方式之一。断层位移-距离曲线特征和断层与褶皱变形几何学、运动学关系分析,可用来判断断层、褶皱变形发生相对时序。认为影响褶皱相关断裂构造发育的机制主要有3种:(1)纵弯滑褶皱作用中,翼部顺层滑动受到限制而无法持续时,将通过断层向上切层的方式予以调节,从而形成翼部或转折端揳入逆冲断裂以及背离向斜和指向背斜逆冲断层;(2)各种因素导致的褶皱曲率变化是褶皱相关断裂产生的重要机制之一,褶皱曲率变化可由褶皱轴面的合并和新生直观反映,轴面合并引起褶皱曲率变化的层位,可能是诱发褶皱相关断裂,如背离向斜和指向背斜逆冲构造开始产生的重要部位;(3)能干性差异和强硬层之间距离较大的岩层组合发生纵弯褶皱变形时,软弱岩系在褶皱核部的聚集和逃逸,是迫使递进收缩的强硬层产生褶皱相关断裂构造的重要机制。     

Structures in the banded iron-formation of the southeastern Bababudan Hills, Karnataka, India

The banded iron-formation in the southeastern Bababudan Hills display a macroscopic synformal bend gently plunging towards WNW. The bedding planes in smaller individual sectors show a cylindrical or conical pattern of folding. The dominant set of minor folds has WNW-ESE trending axial planes and the axes plunge towards WNW at gentle to moderate angles, though there is considerable variation in orientation of both axes and axial planes. A later set of sporadically observed folds has N-S trending axial planes. The macroscopic synformal bend within the study area forms the southeastern corner of a horseshoe shaped regional synformal fold closure which encompasses the entire Bababudan range. The minor folds are buckle folds modified to a varying extent by flattening. In some examples the quartzose layers appear to be more competent than the ferruginous layers; in others the reverse is true. The folds are frequently noncylindrical and the axes show curvature with branching and en echelon patterns. Such patterns are interpreted to be the result of complex linking of progressively growing folds whose initiation is controlled by the presence of original perturbations in the layers. Domes and basins have at places developed as a result of shortening along two perpendicular directions in a constrictional type of strain. Development of folds at different stages of progressive deformation has given rise to nonparallelism of fold axes and axial planes. The axes and axial planes of smaller folds developed on the limbs of a larger fold are often oriented oblique to those of the latter. Progressive deformation has caused rotation and bending of axial planes of earlier formed folds by those developed at later stages of the same deformational episode. Coaxial recumbent to nearly reclined fold locally encountered on the N-S limb of the macroscopic fold may belong to an earlier episode of deformation or to the early stage of the main deformation episode. The E-W to ESE-WNW strike of axial plane of the regional fold system in the Bababudan belt contrasts with the N-S to NNW-SSE strike of axial planes of the main fold system in the Chitradurga and other schist belts of Karnataka.     

Distinguishing syn-cleavage folds from pre-cleavage folds to which cleavage is virtually axial planar: examples from the Cambrian core of the Lower Palaeozoic Anglo-Brabant Deformation Belt (Belgium)

Within the Cambrian Jodoigne Formation in the easternmost part of the Anglo-Brabant Deformation Belt, sub-horizontal to gently plunging folds occur within the limbs of steeply plunging folds. The latter folds are cogenetic with cleavage and are attributed to the Brabantian deformation event. In contrast, although cleavage is also (1) virtually axial planar to the sub-horizontal to gently plunging higher-order folds, shows (2) a well-developed divergent fanning across these folds, (3) an opposing sense of cleavage refraction on opposite fold limbs, and (4) only very small cleavage transection angles, an analysis of the cleavage/bedding intersection lineation suggests that these higher-order folds have a pre-cleavage origin. On the basis of a comparison of structural and sedimentological features these higher-order folds are interpreted as slump folds. The seemingly ‘normal’ cleavage/fold relationship across the slump folds within the limbs of the large steeply plunging folds is due to the very small angle between cleavage and bedding.As such, a ‘normal’ cleavage/fold relationship is no guarantee for a syn-cleavage fold origin. It is not unlikely that also within undeformed, recumbent slump folds, a well-developed compaction fabric, formed parallel to the axial surface of the slump folds, may show fanning and contrasting senses of cleavage refraction on opposite fold limbs.     

Early formed regional antiforms and synforms that fold younger matrix schistosities: their effect on sites of mineral growth

In the metamorphic cores of many orogenic belts, large macroscopic folds in compositional layering also appear to fold one or more pervasive matrix foliations. The latter geometry suggests the folds formed relatively late in the tectonic history, after foliation development. However, microstructural analysis of four examples of such folds suggests this is not the case. The folds formed relatively early in the orogenic history and are the end product of multiple, near orthogonal, overprinting bulk shortening events. Once large macroscopic folds initiate, they may tighten further during successive periods of sub-parallel shortening, folding or reactivation of foliations that develop during intervening periods of near orthogonal shortening. Reactivation of the compositional layering defining the fold limbs causes foliation to be rotated into parallelism with the limbs.Multiple periods of porphyroblast growth accompanied the multiple phases of deformation that postdated the initial development of these folds. Some of these phases of deformation were attended by the development of large numbers of same asymmetry spiral-shaped inclusion trails in porphyroblasts on one limb of the fold and not the other, or larger numbers of opposite asymmetry spirals on the other limb, or similar numbers of the same asymmetry spirals on both limbs. Significantly, the largest disparity in numbers from limb to limb occurred for the first of these cases. For all four regional folds examined, the structural relationships that accompanied these large disparities were identical. In each case the shear sense operating on steeply dipping foliations was opposite to that required to originally develop the fold. Reactivation of the folded compositional layering was not possible for this shear sense. This favoured the development of sites of approximately coaxial shortening early during the deformation history, enhancing microfracture and promoting the growth of porphyroblasts on this limb in comparision to the other. These distributions of inclusion trail geometries from limb to limb cannot be explained by porphyroblast rotation, or folding of pre-existing rotated porphyroblasts within a shear zone, but can be explained by development of the inclusion trails synchronous with successive sub-vertical and sub-horizontal foliations.     

鄂西地区燕山期古构造应力场初步定量研究

鄂西地区主要的构造变形为一系列NNE向褶皱,该区域主要构造形迹已经得到比较系统的研究,但对于发生褶皱时期的古构造应力场则至今尚无较深入的研究,前人仅根据褶皱构造形态特征和断层滑动方向进行过一些古构造应力场的定向研究,导致对齐岳山、利川等燕山期构造演化有了一些不同的观点。本文对燕山期古构造应力场的应力作用方向和作用力大小进行了比较系统的研究。根据研究区地层接触关系,侏罗系与三叠系为同一构造层,而白垩系与该构造层的角度不整合接触关系指示发生构造事件的时间为侏罗纪之后、早白垩世之前;根据野外测量的共轭剪切节理的产状、褶皱两翼地层产状等数据,结合区域整体构造形变特征来判断压缩区,确定了当时最大主压应力轴方向,总体上均为NW SE向;依据野外采集侏罗系、三叠系的石英砂岩标本进行实验测试,在透射电子显微镜下对采集标本进行晶内位错构造的观测统计,使用晶体内自由位错密度与差应力值的经验关系公式,计算出古差应力值的大小,为定量研究该区古构造应力场打下了扎实的基础。     

褶皱的地震波反射响应及其大陆地壳构造地质意义

褶皱是挤压造山带中最普遍的地质构造,但是为什么在地震反射剖面上却极少呈现褶皱呢?本文从一个构造地质学者的角度出发,数值模拟了几种常见褶皱构造的地震波反射响应。结果表明,传统的反射地震技术并不能正确成像闭合褶皱、斜歪或倒转褶皱及二级以上的多级复合褶皱。褶皱陡倾翼的反射消失,而褶皱转折端的反射形成一系列断断续续分布的假水平层理,这些假水平层理在下地壳中极易被错误地解释成由地壳拉张形成的水平构造层理或由底侵作用形成的水平基性岩席。笔者认为,老、中、新褶皱造山带的下地壳中大多数不连续水平反射可能是由褶皱造成的。     

纵弯叠加褶皱地区应力场研究-以川东北地区为例

川东北地区主体构造为北西向大巴山弧形构造带和北东-北东东向川东弧形褶皱带,发育早期北东向纵弯褶皱和晚期北西向纵弯褶皱,两者构成明显的纵-纵复合叠加,形成典型的限制褶皱、横跨褶皱、斜跨褶皱和移褶等。早、晚两期褶皱和共轭“X”节理均反映出早期应力场为北西-南东向水平挤压,晚期应力场为北东-南西向挤压。     

Structural pattern and kinematic framework of deformation in the southern Nallamalai fold–fault belt,Cuddapah district,Andhra Pradesh,Southern India

An association of westerly verging asymmetric folds, easterly dipping cleavages and contractional faults control the pattern and intensity of structures at different scales in the southern Nallamalai fold–fault belt, Cuddapah district of Andhra Pradesh, Southern India. Variation in structural geometry is manifested across the section by the occurrence of relatively low amplitude folds, sometimes only a monocline and by the near absence of contractional faults in the WSW, but tight to isoclinal folds with frequent fold–fault interactions through the central areas towards ENE.The relationships of structural elements in terms of orientation, style, sense of movement and general vergence indicate their development under a progressive contractional deformation. The structures are interpreted to result from a combination of bulk inhomogeneous shortening across the belt and a top-to-west, variable simple shear. Localized developments of crenulation cleavage, rotation of cleavage in the shorter limbs of some mesoscale asymmetric folds and general variation of structural elements in morphology and associations across the belt, indicate partitioning of deformation and a varying degree of non-coaxiality in discrete domains of the bulk deformation.