Strain partitioning and deformation mode analysis of the normal faults at Red Mountain, Birmingham, Alabama

In a low-temperature environment, the thin-section scale rock-deformation mode is primarily a function of confining pressure and total strain at geological strain rates. A deformation mode diagram is constructed from published experimental data by plotting the deformation mode on a graph of total strain versus the confining pressure. Four deformation modes are shown on the diagram: extensional fracturing, mesoscopic faulting, incipient faulting, and uniform flow. By determining the total strain and the deformation mode of a naturally deformed sample, the confining pressure and hence the depth at which the rock was deformed can be evaluated. The method is applied to normal faults exposed on the gently dipping southeast limb of the Birmingham anticlinorium in the Red Mountain expressway cut in Birmingham, Alabama. Samples of the Ordovician Chickamauga Limestone within and adjacent to the faults contain brittle structures, including mesoscopic faults and veins, and ductile deformation features including calcite twins, intergranular and transgranular pressure solution, and deformed burrows. During compaction, a vertical shortening of about 45 to 80% in shale is indicated by deformed burrows and relative compaction of shale to burrows, about 6% in limestone by stylolites. The normal faults formed after the Ordovician rocks were consolidated because the faults and associated veins truncate the deformed burrows and stylolites, which truncate the calcite cement. A total strain of 2.0% was caused by mesoscopic faults during normal faulting. A later homogenous deformation, indicated by the calcite twins in veins, cement and fossil fragments, has its major principal shortening strain in the dip direction at a low angle (about 22°) to bedding. The strain magnitude is about 2.6%. By locating the observed data on the deformation mode diagram, it is found that the normal faulting characterized by brittle deformation occurred under low confining pressure (< 18 MPa) at shallow depth (< 800 m), and the homogenous horizontal compression characterized by uniform flow occurred under higher confining pressure (at least 60 MPa) at greater depth (> 2.5 km).     

Superimposed folding and thrusting by two phases of mutually orthogonal or oblique shortening in analogue models

Orogens may suffer more than one phase shortening resulting in superposition of structures of different generations. Superimposition of orthogonal or oblique shortening is studied using sandbox and centrifuge modelling. Results of sand models show that in orthogonal superimposition, the two resulting structural trends are approximately orthogonal to each other. In oblique superimposition, structures trend obliquely to each other in the relatively thin areas of the model (foreland), and mutually orthogonal in areas where the model is thickened during the first phase of shortening (i.e. the hinterland). Thrusts formed during the first shortening phase may be reactivated during the later shortening phase. Spacing of the later phase structures is not as wide as expected, considering they across the pre-existing thickened wedge. Superposition of structures results in formation of type 1 fold interference pattern. Bedding is curved outwards both in the dome and basin structures. Folded layers are dipping and plunging outwards in a dome, while they are dipping and plunging inwards in a basin. In the areas between two adjacent domes or basins (i.e. where an anticline is superimposed by a syncline or a syncline is superimposed by an anticline), bedding is curved inwards, and the anticlines plunge inwards and the synclines outwards. The latter feature could be helpful to determine the age relationship for type 2 fold interference pattern. In tectonic regions where multiple phases of shortening have occurred, the orogenic-scale dome-and-basin and arrowhead-shaped interference patterns are commonly formed, as in the models. However, in some areas, the fold interference pattern might be modified by a later phase of thrusting. Similar to models results, superimposition of two and/or even more deformation phases may not be recorded by structures all over the tectonic area.     

The mantled gneiss domes of Kuopio (Finland): Interfering diapirs

Finite strain data from the Kuopio mantled gneiss domes are described. Synclines located between two domes have flattening-type strains while those situated between more than two domes exhibit constrictional strains. Cleavage trajectory patterns show that cleavage tends to parallel the dome boundaries and encloses characteristic points termed “cleavage triple points”, at junctions of synclines. The stretching lineation is generally steeply plunging in a mean southwest direction. Folds range from inclined to reclined. Their asymmetry is apparently related to irregularities in the cover-basement boundary. Two shear components of the deformation have been deduced from available data. The major one is steeply plunging and is well exhibited by cleavage fans, steeply plunging lineations and finite-strain gradients. The second shear component is horizontal, as deduced from asymmetric folds and other small-scale structures, and is controlled by bosses and dimples in the cover-basement boundaries.The described structural features are explained in terms of interference between progressively inflating neighbouring diapirs. Furthermore, some of these features, such as cleavage triple points, flattening between two domes and constriction between three or more domes, and horizontal shear components controlled by cover-basement boundary irregularities, could be used as criteria of diapirism. A model of progressive dome interference is presented in which each dome inflates and interacts with neighbours. Such a model could possibly be applied to other orogenic situations with closely spaced diapirs, e.g., Archean greenstone belts or granite-rich orogenic belts.     

An alternative interpretation for the formation of doubly plunging folds in sandstone terrains

Folds are marvellous features of mountain terrains, but despite extensive research, many fundamental problems have still not been solved. In terrains of sandstone, fold hinges are rarely straight lines but curved, forming a pattern characterized by doubly plunging, elliptical dome‐and‐basin structures. Such structures are an obvious manifestation of coeval or successive shortening deformation in two orthogonal principal directions in the horizontal plane. Based on an anatomic investigation of the fold pattern of sandstone beds at the rocky beaches of Saint‐Jean‐Port‐Joli (Quebec, Canada), we propose that the doubly plunging folds may result from a transition from a plane deformation to a constrictional deformation due to auxetic effects of quartz‐rich rocks. The sandstone beds possessed potentially such negative values of Poisson's ratio that, when placed under compression in one direction, they become contracted in the transverse direction, producing a series of doubly plunging folds. Further work is needed to approve or disapprove the interpretation.     

Structural evolution of the Tuscan Nappe in the southeastern sector of the Apuan Alps metamorphic dome (Northern Apennines,Italy)

Structural analysis carried out in the Tuscan Nappe (TN) in the southeastern sector of the Apuan Alps highlights a structural evolution much more complex than that proposed so far. The TN has been deformed by structures developed during four deformation phases. The three early phases resulted from a compressive tectonic regime linked to the construction of the Apenninic fold‐and‐thrust‐belt. The fourth phase, instead, is connected with the extensional tectonics, probably related to the collapse of the belt and/or to the opening of the Tyrrhenian Sea. Our structural and field data suggest the following. (1) The first phase is linked to the main crustal shortening and deformation of the Tuscan Nappe in the internal sectors of the belt. (2) The second deformation phase is responsible for the prominent NW–SE‐trending folds recognized in the study area (Mt. Pescaglino and Pescaglia antiforms and Mt. Piglione and Mt. Prana synforms). (3) The direction of shortening related to the third phase is parallel to the main structural trend of the belt. (4) The interference between the third folding phase and the earlier two tectonic phases could be related to the development of the metamorphic domes. The two directions of horizontal shortening induced buckling and vertical growth of the metamorphic domes, enhancing the process of exhumation of the metamorphic rocks. (5) The exhumation of the Tuscan Nappe occurred mostly in a compressive tectonic setting. A new model for the exhumation of the metamorphic dome of the Apuan Alps is proposed. Its tectonic evolution does not fit with the previously suggested core complex model, but is due to compressive tectonics. Copyright © 2004 John Wiley & Sons, Ltd.     

Superposed deformations in the Eastern Alps: strain analysis and microfabrics

Superposition, mainly of two Eoalpine (Cretaceous) deformation events, can be observed in vertical sections and horizontal traverses throughout a decollement zone in the Eastern Alps consisting of crystalline basement and autochthonous and allochthonous cover stacked in several nappes. The first event is an episode with a non-coaxial strain history caused by W-directed thrusting, the second a N-directed shortening by folding and fold imbrication. Strain analysis based both on deformed pebbles and on the preferred orientation of phyllosilicate grains (March theory), microfabric analysis, and observations of the relative timing of deformation and metamorphism, together indicate that the superposition of structures caused by the two events amounts to one continuous, progressive act of deformation. We attribute gradual and consistent variations in intensity and axial ratio of the tectonic strain to a history influenced by: (a) the position of samples in the pile; (b) the relative importance of the strain increments caused by the two events; and (c) rock ductility. We interpret variations of the c-axis fabrics of quartz in the same way, and draw tentative kinematic conclusions for the Eoalpine orogeny in the Eastern Alps.     

软硬相间岩层底板变形研究

引入损伤变量ω理论,分析水平状软硬相间岩层在竖向应力下的应力-应变关系：再引用弹性模量E和岩层厚度L两个变形参数建立软硬相间岩层底板变形参数方程,得出竖向应力随着底板深度的增加而减小的规律：运用数值模拟软件,建立煤层开采模型,模拟软硬相间岩层底板在采动作用下的变形过程。通过数值分析底板在采动作用下的应力-应变关系和位移特征,发现竖向应力作用下软岩受到的水平应力比硬岩大,竖向应变也比硬岩大,而竖向应力和水平应变变化不大,认为在采动影响下软硬相间岩层底板中的软岩具有吸收更多的应变能来保护软岩下硬岩的变形特点。     

Geological evolution of the Afro-Arabian dome

The Afro-Arabian dome includes the elevated continental regions enclosing the Red Sea, Gulf of Aden, and the Ethiopian rift system, and extends northwards as far as Jordan. It is more than an order of magnitude larger than other African uplifts. Both the structures and the igneous rocks of the dome appear to be products of the superimposition of two, perhaps three, semi-independent generating systems, initiated at different times but all still active. A strain pattern dominated by NW-trending basins and rifts first became established early in the Cretaceous. By the end of the Oligocene, much of the extensional strain had been taken up along the Red Sea and Gulf of Aden axes, which subsequently developed into an ocean. Palaeogene “trap” volcanism of mildly alkaline to transitional character was related to this horizontal extension rather than to doming. Further west, the East Sahara swell has a history of intermittent alkaline volcanicity which began in the Mesozoic and was independent of magmatism in the Afro-Arabian dome. Volcanicity specifically related to doming began in the Miocene along a N-S zone of uplift extending from Ethiopia to Syria. This elongated swell forms the northern termination of the East African system of domes and rifts, characterized by episodic vertical uplift but very little extension. Superimposition of epeirogenic uplift upon structures formed by horizontal extension took place in the Neogene. Volcanicity related to vertical tectonics is mildly alkaline in character, whereas transitional and tholeiitic magmas are found along the spreading axes.     

Heterogeneous constrictional deformation in a ductile shear zone resulting from the transposition of a lineation-parallel fold

We use new (micro-)structural, petrofabric, strain and vorticity data to analyze the deformation path in a mesoscopic quartz mylonite zone. The mylonite zone resulted from the complete transposition of a stretching lineation-parallel isoclinal fold. Symmetric cleft-girdle quartz c-axis fabrics were recorded in the middle domain, which occupies the inner limbs of the precursor isoclinal fold, while asymmetric cleft- and crossed-girdle fabrics were observed in the upper and lower domains that represent the outer limbs. Constrictional strain, with increasing k values towards the middle domain, is inferred from petrofabric and 3D strain data. Oblique grain shape fabrics yield vorticity estimates of 0.72–0.90 in the zone. However, in the middle domain, pure shear dominated deformation is suggested by orthorhombic crystallographic fabrics. Strain rate is constant throughout the zone; a strain decrease towards the zone center implies that deformation ceased earlier in the middle domain. The data indicates that fold transposition and subsequent mylonitization started as pure-shear-dominated constrictional deformation and progressively changed to simple-shear-dominated, plane strain. During this flow path the asymmetric quartz c-axis fabrics likely developed by depopulation of cleft-girdle maxima rather than from the synthetic rotation of fabric maxima itself.     

Soft-sediment deformation of sandstone related to the Dwyka glaciation in South Africa

Sandstones of the uppermost Witteberg Group in the Cape fold belt of South Africa exhibit unusual and distinctive soft-sediment deformation structures. These structures include folds, axial planar cleavage and micro-fold lineations. Interfering fold patterns and intersecting sets of lineations are indicative of repeated deformation. The sandstones are immediately overlain by glacial and proglacial sediments of the Carboniferous Dwyka Group, indicating that the deformation was related to glaciation. Possible environments of deformation include: (a) subglacial dragging of unconsolidated material, (b) subaqueous slumping beyond the limit of floating ice, and (c) englacial deformation of material incorporated by freezing into the base of the glacier.     

Regional dome evolution and its control on ore-grade distribution: Insights from 3D implicit modelling of the Navachab gold deposit,Namibia

We introduce a novel approach to analyse and assess the structural framework of ore deposits that fully integrates 3D implicit modelling in data-rich environments with field observations. We apply this approach to the early Palaeozoic Navachab gold deposit which is located in the Damara orogenic belt, Namibia. Compared to traditional modelling methods, 3D implicit modelling reduces user-based modelling bias by generating open or closed surfaces from geochemical, lithological or structural data without manual digitisation and linkage of sections or level plans. Instead, a mathematically defined spatial interpolation is used to generate 3D models that show trends and patterns that are embedded in large drillhole datasets. In our 3D implicit model of the Navachab gold deposit, distinctive high-grade mineralisation trends were identified and directly related to structures observed in the field. The 3D implicit model and field data suggest that auriferous semi-massive sulphide ore shoots formed near the inflection line of the steep limb of a regional scale dome, where shear strain reached peak values during fold amplification. This setting generated efficient conduits and traps for hydrothermal fluids and associated mineralisation that led to the formation of the main ore shoots in the deposit. Both bedding-parallel and highly discordant sets of auriferous quartz-sulphide veins are interpreted to have formed during the later lock-up stage of the regional scale dome. Additionally, pegmatite dykes crosscut and remobilise gold mineralisation at the deposit scale and appear to be related to a younger joint set. We propose that kilometre-scale active folding is an important deformation mechanism that influences the spatial distribution and orientation of mineralisation in ore deposits by forming structures (traps and pathways for fluids) at different preferred sites and orientations. We also propose that areas that experience high shear strain, located along the inflection lines of folds can act as preferred sites for syn-deformational hydrothermal mineralisation and should be targeted for regional scale exploration in fold and thrust belts. Our research also suggests that examination of existing drillhole datasets using 3D implicit modelling is a powerful tool for spatial analysis of mineralisation patterns. When combined with fieldwork, this approach has the potential to improve structural understanding of a variety of ore deposits.     

Neoproterozoic Deformation at a Boundary Zone Between the Nellore-Khammam Schist Belt and Pakhal Basin, SE India: Strain Analysis of Deformed Pebbles

In the Kinnerasani area in southeastern India, the terrain boundary between the Archean Nellore-Khammam Schist Belt and the Proterozoic Pakhal Supergroup overlying the Dharwar-Bastar cratons can be observed. We analyzed the mesoscopic and microscopic structural features of the highly deformed pebbles in the basal conglomerate bed of the Pakhal Supergroup that occurs at the terrain boundary. The results of the analysis of the pebbles suggest that: 1) deformation of pebbles resulted from ductile deformation during peak metamorphism 2) the mode of strain is plane strain to constrictive and maximum elongation located to be vertical and 3) the apparent stretch of the pebbles is up to 300%.In the Nellore-Khammam Schist Belt, quartz grains constituting the quartz layer of the feldspathized gneiss folded by the last-phase deformation also show vertical maximum stretching in constrictive strain. This observation suggests that the deformational features, at least the mode of strain, during the last-phase deformation is comparable to the deformation forming elongated pebbles of the Pakhal conglomerate. The last-phase deformation structures of the Nellore-Khammam Schist Belt are well observed near the terrain boundary. This indicates that the Pakhal deformation overprinted the rocks of the Nellore-Khammam Schist Belt near the boundary, and that their tectonic juxtaposition occurred during or before this deformation period. Because the Pakhal deformation took place during or soon after the peak metamorphism of the Pakhal Supergroup, which is known to be 1000 Ma, and the last metamorphism of the Nellore-Khammam Schist Belt in the Khammam area were reported to be 1100 Ma. The tectonic juxtaposition between the Pakhal Supergroup and Nellore-Khammam Schist Belt was around 10001100 Ma.     

单层褶皱变形过程中最大主应力与水平应变的变化及其影响因素

对地质构造进行定量半定量研究是地质工作者近几十年来一直努力的方向,构造数值模拟是有效的定量研究方法之一。褶皱是一种典型的构造类型,对褶皱构造进行定量半定量研究也一直是构造地质学家们所致力解决的研究课题。本文结合近年来单层褶皱构造数值模拟研究进展,以及笔者近几年来对单层褶皱所进行的数值模拟实验研究,主要论述了单层褶皱变形的影响因素,单层褶皱变形过程中的最大主应力与水平应变的变化及其影响因素等。数值模拟技术为褶皱变形提供了一种新的研究方法,该方法在构造地质领域将有很好的发展前景。     

沧东凹陷始新统孔店组二段深水细粒沉积岩中的软沉积物变形构造*

湖相深水细粒沉积岩中的软沉积物变形构造主要发育在泥岩和泥晶碳酸盐岩为主的地层中,大部分因为发育规模小(镜下尺度),在岩心观察中很容易被忽略掉。本文以沧东凹陷G108-8井孔店组二段(孔二段)细粒沉积岩为研究对象,通过密集的镜下观察和精细岩心描述,识别出同沉积微断裂、液化岩脉、微褶皱变形、微重荷变形和杂乱变形等多种类型的软沉积物变形构造。研究区深水细粒沉积岩中的软沉积物变形构造总体表现为规模小、垂向无明显重力流砂体伴生、原地成因、沉积纹层发育等特点。从应力调节方式的角度考虑,可以将软沉积物变形构造的形成机制划分为上覆应力卸载、侧向应力挤压和层内应力释放等3种类型,不同的形成机制分别是对不同沉积环境变化的响应。通过湖相深水细粒沉积岩软沉积物变形构造的研究,可以恢复古沉积环境变化、预测重力流方位,对深水细粒沉积岩的油气勘探也具有一定的指导意义。     

On the distribution of fluids in folds: A review of controlling factors and processes

Growing macroscale fold structures are one of the most dynamic and complex systems in geology, where during fold growth pore fluid pressure, fluid temperature, and fluid composition (PTX) conditions do not remain static but instead vary continually at all points within the deforming fold such that no two points within the fold will have the same deformation or fluid history. These PTX conditions in turn directly affect fluid storage and mobility as well as rock strength, which has direct feedback on the mechanical development of the fold. In this review, we first outline the research methodologies that have yielded significant insights, and review four examples of well-constrained fold-related fluid systems. We then discuss the fluid-related, fold-related, and mesoscopic deformation processes that are relevant to fold-related fluid systems. Finally, we close by presenting several conceptual models for fold-related fluid system structure, and highlighting key areas for future work.     

北喜马拉雅E-W向伸展变形时限:来自藏南错那洞穹隆Ar-Ar年代学证据

特提斯喜马拉雅带以广泛发育近E-W向和近S-N向断裂以及北喜马拉雅片麻岩穹隆带为典型特征.藏南错那洞穹隆位于特提斯喜马拉带的东部,是近两年新发现并厘定的穹隆构造.该穹隆从外向内主要由3部分组成:上部单元（盖层）、中部单元（滑脱系）和下部单元（核部）,其中滑脱系主要由一套强烈变形的片岩、伟晶岩、花岗岩、大理岩和矽卡岩组成,片岩包括含石榴石云母片岩、含石榴石十字石云母片岩、含蓝晶石石榴石十字石片岩、含矽线石蓝晶石石榴石片岩和云母石英片岩.野外构造变形特征表明滑脱系为一条强烈变形的韧性剪切带,发育大量的鞘褶皱、"Z"形揉褶皱和眼球状构造、石榴石的旋转碎斑、S-C组构和压力影构造.错那洞穹隆记录了4期构造变形:第1期由北向南的逆冲挤压构造、第2期由南向北的韧性伸展构造、第3期近E-W向的韧性伸展构造变形和第4期成穹后的脆性垮塌构造.通过对滑脱系中含石榴石云母片岩的白云母进行Ar-Ar同位素测年,获得坪年龄为14.0±0.2 Ma,等时线年龄为13.7±0.5 Ma,二者基本一致,同时微观构造特征显示石英呈亚颗粒旋转重结晶（SGR）,其韧性变形的温度为450~550℃,该变形温度高于白云母的封闭温度.因此,白云母Ar-Ar坪年龄（14.0±0.2 Ma）代表错那洞穹隆近E-W向伸展变形的时间,也即近S-N向桑日-错那裂谷的活动时间.结合构造变形和年代学特征,认为错那洞穹隆是STDS向北伸展拆离的主导机制叠加后期近E-W向韧性伸展活动的结果.      

Progressive deformation and the rotation of contemporary fold axes in the Ballybofey Nappe,north-west Ireland

Internal regions of orogenic belts may be characterized by an alignment of fold axes with mineral elongation lineations. This relationship is commonly interpreted as representing progressive tightening and rotation towards the shear direction of early buckle folds, the hinges of which were initiated orthogonal to this direction. Detailed structural analysis of lower amphibolite facies Dalradian metasediments of the Ballybofey (fold) Nappe, north-west Ireland, shows that an intense S₃ schistosity is developed axial planar to mesoscopic and minor F₃ folds. In areas of low D₃ strain, F₃ fold axes plunge gently towards the north-east, whereas in regions of greater strain plunges are towards the south-east subparallel to the constant mineral lineation. Minor folds which initiated at angles of 70–80° from the mineral lineation subsequently rotated towards the shear direction in a consistent clockwise sense. Progressive and variable non-coaxial deformation oblique to the original mean F₃ orientation has resulted in a unimodal distribution pattern of fold axes. Analysis of the angular rotation of fold axes enables estimates of the bulk shear strain to be evaluated and models of progressive deformation to be assessed.     

Strain analysis in Jurassic argillites of the Monte Sirino area (Lagonegro Zone,southern Apennines,Italy) and implications for deformation paths in pelitic rocks

Analysis of strain in Jurassic argillites forming part of the folded and thrusted sedimentary succession of the Lagonegro basin (southern Italian Apennines) has been carried out using ellipsoid-shaped reduction spots as strain markers. Most of the determined finite strain ellipsoids are of oblate type and show a peculiar distribution of the maximum extension direction (X), with maxima either subparallel or subperpendicular to the local fold axes. Using the strain matrix method, two different deformation histories have been considered to assist the interpretation of the observed finite strain pattern. A first deformation history involved vertical compaction followed by horizontal shortening (occurring by a combination of true tectonic strain and volume loss), whereby all strain is coaxial and there is no change in the intermediate axis of the strain ellipsoid. By this type of deformation sequence, which produces a deformation path where total strain moves from the oblate to the prolate strain field and back to the oblate field, prolate strain ellipsoids can be generated and may be recorded where tectonic deformation has not been large enough to reverse pretectonic compaction. This type of deformation history may be of local importance within the study area (i.e. it may characterize some fold hinge regions) and, more generally, is probably of limited occurrence in deformed pelitic rocks. A second deformation sequence considered the superposition of pre-tectonic compaction and tectonic strain consisting of initial layer-parallel shortening followed by layer-parallel shear (related to flexural folding). Also in this instance, volume change during tectonic deformation and tectonic plane strain have been assumed. For geologically reasonable amounts of volume loss due to compaction and of initial layer-parallel shortening, this type of deformation history is capable of producing a deformation path entirely lying within the oblate strain field, but still characterized by a changeover, during deformation, of the maximum extension axis (X) from a position parallel to the fold axis to one perpendicular to it. This type of deformation sequence may explain the main strain features observed in the study area, where most of the measured finite strain ellipsoids, determined from the limb regions of flexural folds, display an oblate shape, irrespective of the orientation of their maximum extension direction (X) with respect to the local structural trends. More generally, this type of deformation history provides a mechanism to account for the predominance of oblate strains in deformed pelitic rocks.     

Folding by cataclastic flow at shallow crustal levels in the Canyon Range,Sevier orogenic belt,west-central Utah

Folds form by ductile deformation typically involving continuous flow. In the elastico-frictional regime, such deformation may be accomplished by cataclastic flow involving collective movement on a population of fractures and zones. The Canyon Range (CR) syncline, part of the CR thrust sheet in west-central Utah, developed in this regime. The CR syncline is composed of thick-bedded quartzite units with a small material contrast between layers, limiting limb rotation by flexural slip alone. Thus, fracture populations developed to accommodate fold tightening by limb rotation and thinning, and the formation of transverse zones across the fold. Several generations of fracture and deformation zone (DZ) networks are recognized from mesoscopic and microscopic evidence, and can be related to stages of folding. The net result of the large number of distributed fractures and deformation zones is a continuous deformation that is homogeneous at the scale of the outcrop. All these lines of evidence suggest that large-scale cataclastic flow accommodated folding by allowing rigid mesoscopic blocks to slide along bounding DZs.Along its length, the CR syncline consists of several segments bounded by transverse zones with different mechanisms accommodating fold tightening in adjacent segments. In one segment, fold tightening progressed by limb rotation, and then out-of-the-core thrusting. In contrast, fold tightening in the adjoining segments occurred by rotation and thinning of one limb and possible hinge migration, with the steeply dipping to overturned limb showing progressive thinning of units on a megascopic scale and progressive increase in the thickness and density of deformation zones at all scales.     

纵弯叠加褶皱地区岩石有限应变特征：以川东北地区典型叠加褶皱为例

纵弯叠加褶皱地区岩石有限应变特征是所受历次变形的综合结果.远离叠加区应变型式的伸长方向与褶皱轴向一致而缩短方向与褶皱轴垂直,叠加区应变型式不规则,且应变椭球伸长量和缩短量之差值较小.