Fabric development during shear deformation in the Main Central Thrust Zone, NW-Himalaya, India

Quartz microfabrics and associated microstructures have been studied on a crustal shear zone—the Main Central Thrust (MCT) of the Himalaya. Sampling has been done along six traverses across the MCT zone in the Kumaun and Garhwal sectors of the Indian Himalaya. The MCT is a moderately north-dipping shear zone formed as a result of the southward emplacement of a part of the deeply rooted crust (that now constitutes the Central Crystalline Zone of the Higher Himalaya) over the less metamorphosed sedimentary belt of the Lesser Himalaya. On the basis of quartz c- and a-axis fabric patterns, supported by the relevant microstructures within the MCT zone, two major kinematic domains have been distinguished. A noncoaxial deformation domain is indicated by the intensely deformed rocks in the vicinity of the MCT plane. This domain includes ductilely deformed and fine-grained mylonitic rocks which contain a strong stretching lineation and are composed of low-grade mineral assemblages (muscovite, chlorite and quartz). These rocks are characterized by highly asymmetric structures/microstructures and quartz c- and a-axis fabrics that indicate a top-to-the-south sense that is compatible with south-directed thrusting for the MCT zone. An apparently coaxial deformation domain, on the other hand, is indicated by the rocks occurring in a rather narrow belt fringing, and structurally above, the noncoaxial deformation domain. The rocks are highly feldspathic and coarse-grained gneisses and do not possess any common lineation trend and the effects of simple shear deformation are weak. The quartz c-axis fabrics are symmetrical with respect to foliation and lineation. Moreover, these rocks contain conjugate and mutually interfering shear bands, feldspar/quartz porphyroclasts with long axes parallel to the macrosopic foliation and the related structures/microstructures, suggesting deformation under an approximate coaxial strain path.On moving towards the MCT, the quartz c- and a-axis fabrics become progressively stronger. The c-axis fabric gradually changes from random to orthorhombic and then to monoclinic. In addition, the coaxial strain path gradually changes to the noncoaxial strain path. All this progressive evolution of quartz fabrics suggests more activation of the basal, rhomb and a slip systems at all structural levels across the MCT.     

Magnetic fabric evolution in ductile shear zones: examples in metagranites of the Aar Massif (Swiss Central Alps)

The Aar Massif forms part of the polycyclic basement of the External Crystalline Massifs in central Switzerland. Strong heterogeneous Alpine deformation produced a network of broad, anastomosing shear zones, with deformation strongly localized in mylonitic domains. This study investigates the combined effects of high‐strain deformation and synkinematic metamorphism on magnetic fabric evolution in Tertiary shear zones of the Aar granite and Grimsel granodiorite. In transects across several mesoscale shear zones with large strain gradients, magnetic fabric orientations are in excellent agreement with principal strain orientations determined from outcrop fabrics and strain markers. However, the magnitude and shape of the magnetic anisotropy do not change systematically with increasing finite strain, likely as a result of recrystallization and metamorphism. The overall pattern of steeply dipping fabrics is consistent with the main shortening stage of regional Alpine kinematics, while some mylonite structures reflect a local component of dextral shearing.     

Palaeozoic late collisional strike-slip deformations in Tianshan and Altay, Eastern Xinjiang, NW China

ABSTRACT In Central Asia, thrusts and shear zones resulting from Palaeozoic accretional events were reworked by E–W-trending ductile strike-slip faults during late Palaeozoic–early Mesozoic time. In the Tianshan range, microstructures and quartz C-axis fabrics show a main dextral shearing associated with sinistral localized shear zones. The relationship between these conjugate structures indicates a NNW–SSE-trending bulk shortening. In the Chinese Altay mountains, the existence of δ-type microstructures in an important sinistral mylonitic zone infers high rates of deformation. This shear zone is bordered by a late dextral ductile fault synchronous with a granite emplacement. Field evidence and datings from the literature provide chronological constraints. In the late Carboniferous, the sinistral mylonitic deformation took place in the Erqishi–Irtysh shear zone in the northeastern part of Xinjiang and in Kazakhstan. During the Early Permian, a regional dextral event occurred in the Tianshan range and under the whole of northern Xinjiang.     

The reliability of asymmetric c-axis fabrics of quartz to determine sense of vorticity

Asymmetric c-axis fabrics of quartz are commonly used to determine sense of vorticity in ductile shear zones. This method seems to work if the fabric pattern resembles a model fabric proposed by Lister and Hobbs (1980). Usually, however, c-axis fabrics are rather vague. The reliability of such vague fabrics was tested in a major shear zone with known sense of vorticity. Only 62% of the c-axis fabrics predict the correct sense. Great care should therefore be taken in applying this method to determine sense of vorticity.     

P–T conditions of decompression of the Limpopo high-grade terrane: record from shear zones

The Southern Marginal Zone of the late Archean Limpopo Belt of southern Africa is an example of a high‐grade gneiss terrane in which both upper and lower crustal deformational processes can be studied. This marginal zone consists of large thrust sheets of complexly folded low‐strain gneisses, bound by an imbricate system of kilometre‐wide deep crustal shear zones characterized by the presence of high‐strain gneisses (‘primary straight gneisses’). These shear zones developed during the decompression stage of this high‐grade terrane. Low‐ and high‐strain gneisses both contain similar reaction textures that formed under different kinematic conditions during decompression. Evidence for the early M1/D1 metamorphic phase (> 2690 Ma) is rarely preserved in low‐strain gneisses as a uniform orientation of relict Al‐rich orthopyroxene in the matrix and quartz and plagioclase inclusions in the cores of early (M1) Mg‐rich garnet porphyroblasts. This rare fabric formed at > 820 °C and > 7.5 kbar. The retrograde M2/D2 metamorphic fabric (2630–2670 Ma) is well developed in high‐strain gneisses from deep crustal shear zones and is microscopically recognized by the presence of reaction textures that formed synkinematically during shear deformation: M2 sigmoid‐shaped reaction textures with oriented cordierite–orthopyroxene symplectites formed after the early M1 Mg‐rich garnet porphyroblasts, and syn‐decompression M2 pencil‐shaped garnet with oriented inclusions of sillimanite and quartz formed after cordierite under conditions of near‐isobaric cooling at 750–630 °C and 6–5 kbar. The symplectites and pencil‐shaped garnet are oriented parallel to the shear fabric and in the stretching direction. Low‐strain gneisses from thrust sheets show similar M2 decompression cooling and near‐isobaric cooling reaction textures that formed within the same P–T range, but under low‐strain conditions, as shown by their pseudo‐idioblastic shapes that reflect the contours of completely replaced M1 garnet and randomly oriented cordierite–orthopyroxene symplectites. The presence of similar reaction textures reflecting low‐strain conditions in gneisses from thrust sheets and high‐strain conditions in primary straight gneisses suggests that most of the strain during decompression was partitioned into the bounding shear zones. A younger M3/D3 mylonitic fabric (< 2637 Ma) in unhydrated mylonites is characterized by brittle deformation of garnet porphyroclasts and ductile deformation of the quartz–plagioclase–biotite matrix developed at < 600 °C, as the result of post‐decompression shearing under epidote–amphibolite facies conditions.     

Domainal and fabric heterogeneities in the Cap de Creus quartz mylonites

A characteristic domainal configuration is reported for both micro-structures and c-axis fabrics in the Cap de Creus pure quartz mylonites as displayed in 50 samples from the centres of different shear zones. Three types of domains are found a, b and c. Each domain has a distinct c-axis orientation pattern. These three fabric elements, also labelled a, b and c make up the total fabric. c-axis fabrics are symmetric or asymmetric with respect to the main mylonitic foliation depending on the presence or absence of the b domain and its fabric element. The boundaries of the domains are parallel to the main mylonitic foliation. Two domain types, a and b display an internal foliation defined by preferred grain boundary alignment parallel to the direction of optical orientation within the domain. The internal foliations are oblique to the main mylonitic foliation in two different senses giving the sample a herring-bone appearance. These internal foliations are shown to be related to extensional crenulations. Domains are not produced by host-controlled recrystallization. The fabric elements and corresponding domains are the expression of kinematic heterogeneities on the scale of the thin section.     

辽南金州拆离断层带中花岗质岩石的变形:显微构造、组构与年代学分析

花岗岩(脉)在中下地壳韧性剪切带中普遍发育,如何正确鉴别剪切带中剪切前、剪切期及剪切后花岗岩(脉)以及正确理解剪切过程中构造变形与岩浆作用之间的关系一直是一个重要课题。本文以辽南金州拆离断层带为研究对象,选取中部地壳伸展作用过程中具有不同变形表现的花岗岩(脉)开展宏观-微观构造观察、石英EBSD组构分析及锆石LA-ICP-MS年代学测试等工作,从而进一步丰富构造-岩浆关系判别准则。剪切前花岗岩(脉)多变形强烈且具有后期固态变形叠加在早期高温岩浆组构之上的特点,而剪切期的花岗岩由于侵位的时间不同,岩石的变形程度也会不同。剪切晚期侵入的岩脉遭受了较弱的晶内塑性变形,而剪切早期的岩脉可以显示岩浆流动或结晶后高温至中温固态变形。从组构特点上看,剪切前和剪切期花岗质岩石石英c轴组构大多表现为中高温组构叠加有低温组构的特点。剪切后的花岗质岩石仅发生微弱的晶内变形或未变形而显示低温或无规律的组构特征。对五个典型的样品进行年代学测试,其结果符合相应的期次划分类型。应用宏观构造、显微构造与组构分析,结合年代学测试综合分析,对于辽南变质核杂岩构造-岩浆活动性进行了精细划分,包括134~130Ma初始伸展阶段,130~115Ma峰期伸展与强烈岩浆活动阶段,以及115Ma前后伸展作用结束。     

Strain path partitioning within thrust sheets: microstructural and petrofabric evidence from the Moine Thrust zone at Loch Eriboll,northwest Scotland

Quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the Loch Eriboll area of the Moine Thrust zone and are used to interpret the detailed processes involved in fabric evolution. The intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude. A correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry.Two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the Moine Thrust. Within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate, mutually interfering shear bands, globular low strain detrital quartz grains whose c axes are aligned sub-parallel to the principal finite shortening direction (Z) and quartz c axis fabrics which are symmetric (both in terms of skeletal outline and intensity distribution) with respect to mylonitic foliation and lineation. Non-coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation.Tectonic models offering possible explanations for the presence of kinematic (strain path) domains within thrust sheets are considered.     

Transverse lineation and large-scale structures related to Alpine obduction in Corsica

In Alpine Corsica, the major tectonic event during the late Cretaceous was the thrusting to the west of an ophiolitic nappe and its sedimentary cover upon the Variscan basement and its Mesozoic cover. A detailed field survey shows that the basal contact of the nappe corresponds to a pluri-kilometric scale shear zone. Thus gneissified basement slices have been tectonically emplaced in the ophiolitic nappe. The thrusting was responsible for small scale structures: foliation, lineation and folds, initiated in a HP/LT metamorphic context. The deformation analysis shows that the finite strain ellipsoid lies in the constriction field close to that for plane strain. Moreover occurrences of rotational criteria in the XZ planes (sigmoidal micas, asymmetric pressure shadows, quartz C-axes fabrics) are in agreement with shear from east to west. All structural data from microscopic to kilometric scales, of which the most widespread is a transverse stretching lineation, can be interpreted by a simple shear model involving ductile synmetamorphic deformation. At the plate tectonic scale the ophiolitic obduction is due to intraoceanic subduction blocked by underthrusting of continental crust beneath oceanic lithosphere.     

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.     

Advective heat transfer and fabric development in a shallow crustal intrusive granite — the case of Proterozoic Vellaturu granite,south India

Syntectonic plutons emplaced in shallow crust often contain intermediate-to low-temperature deformation microstructures but lack a high-temperature, subsolidus deformation fabric, although the relict magmatic fabric is preserved. The Proterozoic Vellaturu granite emplaced at the eastern margin of the northern Nallamalai fold belt, south India during the late phase of regional deformation has a common occurrence of intermediate-to low-temperature deformation fabric, superimposed over magmatic fabric with an internally complex pattern. But high-T subsolidus deformation microstructure and fabric are absent in this pluton. The main crystal plastic deformation and fluid enhanced reaction softening was concentrated along the margin of the granite body. Resulting granite mylonites show Y-maximum c-axis fabric in completely recrystallized quartz ribbonds, dynamic recrystallization of perthites, and myrmekite indicative of fabric development under intermediate temperature (∼ 500–400°C). The weakly-deformed interior shows myrmekite, feldspar microfracturing and limited bulging recrystallization of quartz. The abundance of prism subgrain boundaries is indicative of continuing deformation through low-temperature (∼ 300°C). The relative rates of cooling influenced by advective heat transfer and deformation of the pluton seem to control the overall subsolidus fabric development. The rapid advective heat transfer from the interior in the early stages of subsolidus cooling was followed by slow cooling through intermediate temperature window as a well-developed phyllosilicate rich mylonitic skin around the granite body slowed down conductive heat loss. Low-T crystal plastic deformation of quartz was effected at a late stage of cooling and deformation of the shallow crustal granite body emplaced within the greenschist facies Nallamlai rocks.     

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.     

辽吉裂谷带北缘连山关韧性剪切带的构造属性和运动学分析

韧性剪切带在岩石圈地壳中广泛存在,蕴含应力、应变和温压等环境参数,是构造解析、流变学和成因机制研究的重要对象.辽吉裂谷带位于辽宁东部-吉林南部,是华北克拉通重要的古元古代活动带之一.连山关岩体地处辽吉裂谷带北缘,经历复杂变质变形作用,岩体南缘发育NWW向右行走滑韧性剪切带.糜棱岩显微结构观测分析揭示,剪切带内糜棱岩以S、SL构造岩为主,总体呈压扁型应变.运动学涡度值为0.91~0.97,均大于0.75,指示简单剪切为主的变形特征.糜棱岩中云母显示塑性拉长,石英动态重结晶明显,以膨凸重结晶作用为主.EBSD分析结果表明,石英发育中低温菱面组构,对应变形温度450~550℃,暗示糜棱岩形成于低绿片岩相-低角闪岩相.结合前人研究成果,我们认为连山关韧性剪切带可能起源于早元古代晚期.连山关岩体先后经历早期隆起造成的伸展-滑脱作用和晚期与上覆辽河群共同经历的南北向挤压,从而在岩体南缘形成陡倾的右行韧性剪切带.      

藏南冈底斯岩浆带中段曲水韧性剪切带的变形特征及其年代学约束

藏南曲水地区鸡公-色甫韧性剪切带系统的运动学和动力学研究,不仅对造山带构造研究具有重要理论意义,而且对青藏高原中-南部区域地质研究具有推进作用.作为藏南冈底斯岩浆带曲水岩基中的重要断裂构造,曲水韧性剪切带经历了新生代以来的大规模构造变形,其中走滑剪切作用最为显著,整体表现为右行走滑为主.通过对曲水剪切带中的构造片岩、初糜棱岩、糜棱岩以及长英质脉体等野外观测并结合室内镜下薄片以及石英EBSD(electron backscatter diffraction)组构分析,认为曲水剪切带主要经历了一期韧性变形事件.根据石英-长石变形矿物对并结合石英EBSD组构分析得出,构造变形发生的温度大约为500~550 ℃,高绿片岩相到角闪岩相.剪切带内普遍发育有不对称的褶皱、构造透镜体、σ碎斑、S-C组构和石香肠等变形组构,显示出右行走滑的特征.对研究区34组糜棱面理和9组拉伸线理进行极射赤平投影,糜棱面理的优选方位约为355°∠70°,拉伸线理产状约为95°∠8°.此外,在糜棱岩中发育两类长英质脉体,根据野外分布特征、显微组构、锆石成因学以及岩石地球化学特征综合研究认为,两类长英质脉体为同构造剪切脉体,其年代学可以对剪切带的形成时限起到很好的制约.通过对两套长英质脉体分别进行LA-ICP-MS锆石U-Pb定年,得到了38.67±0.88 Ma和35.05±0.29 Ma两组加权平均年龄,表明了曲水剪切带韧性变形发生于35~38 Ma的始新世末期(普利亚本期).这一年龄值处于印度-亚洲大陆晚碰撞期,因此曲水韧性剪切带右行走滑事件可能是印度板块持续向北俯冲,构造应力在欧亚板块边缘释放引起的陆内构造响应.      

Deformed dike swarms as an implication of transpression deformation in Western Arabian shield,Wadi Fatima,Saudi Arabia

Utilization of satellite images and field observations of dike swarms in pre-Fatima basement show that these dikes are older than the overlaying Fatima Formation. Dikes digitization and orientation analysis on satellite images show that the prevailing trend of the dikes is ENE-WSW. The granitic rocks of pre-Fatima basement and its hosted dikes expose evidences of completely a different deformation regime from the overlaying Fatima Formation. These evidences include shearing, dextral shear indicators, isoclinal folds, deflection and rotation of crystals, mineral elongation, and mylonitic and gneissose textures. Strain analysis results of using Fry method on quartz and feldspar grains support the presence of deformation in these ENE-WSW dikes. These results gave a strain ratio of 2.1:1.3:1, which suggest an amount of 40% stretching in the ENE-WSW direction parallel to the dike walls, and an amount of 30% shortening in the NNW-SSE direction. Mesoscopic and microscopic scale structures confirm the existence of dextral ductile-brittle shearing followed the emplacement of the dikes and before the pure shear deformation that caused the thrusting and folding of Fatima Formation. This ductile-brittle deformation is correlated with the dextral transpression that formed the Fatima Shear Zone (FSZ).     

舒兰韧性剪切带应变分析及石英动态重结晶颗粒分形特征与流变参数估算

舒兰北东向韧性剪切带位于佳木斯-伊通断裂带(佳-伊断裂带)中南段, 剪切带内糜棱岩具有明显左行走滑特征, 片麻理产状近NNE向.糜棱岩中长石有限应变Flinn图解判别岩石类型为L-S型构造岩, 属拉长型应变.石英C轴EBSD组构分析表明, 石英组构以中低温菱面为主, 滑移系为{0001} < 110>.剪切带内糜棱岩的剪应变为0.44, 不同方法计算所得运动学涡度值均大于0.95, 指示剪切变形以简单剪切为主.综合矿物变形温度计、石英C轴EBSD组构、石英的粒度-频数图及Kruhl温度计综合估计该韧性剪切带变形机制以位错蠕变机制为主, 变质相为低绿片岩相, 发生韧性变形和糜棱岩化温度范围在400~500 ℃之间.糜棱岩内石英动态重结晶新晶粒边界普遍具有锯齿状或港湾状结构, 利用分形方法对其重结晶新晶边界研究表明, 这些晶粒边界具有自相似性, 表现出分形特征, 分形维数值为1.195~1.220.根据石英重结晶粒径估算差应力值为24.35~27.59 MPa, 代表了舒兰韧性剪切带糜棱岩化作用过程的差异应力下限.使用不同实验方法估算、比较和分析了该剪切带古应变速率, 认为该速率应为10-12.00~10-13.18 s-1, 与区域性应变速率10-13.00~10-15.00 s-1对比, 说明舒兰韧性剪切带的应变速率与世界上大多数韧性剪切带中的糜棱岩应变速率一致, 是缓慢变形的结果, 其形成可能与早白垩世伊泽纳崎板块向欧亚大陆俯冲发生转向有关.      

HT-fabrics in a garnet-bearing quartzite from Western Portugal: geodynamic implications for the Iberian Variscan Belt

ABSTRACT The study of a garnet-bearing quartzite from a major suture zone in Iberia reports an unusual high-T fabric. Quartz c -axis patterns were plotted using shaped garnet as reference axis for the finite stretch ( X -axis). The pole figures are characterized by a dominant single maximum around X together with other point maxima along the XY plane (mylonitic foliation). These patterns suggest that dominant < c > prism slip and subordinated < a > prism slip operated during quartz plastic deformation in agreement with P–T conditions for syntectonic garnet growth (4–5 kbar and 700 ± 50 °C) and, pre-dating the well-known (late) Variscan D1 event (<6 kb and 600 ± 30 °C). The geotectonic framework suggests that the fabrics were formed along the western shear margin of the Ossa-Morena Zone during the early stages of the Variscan orogeny.     

贺兰山北段古元古代结晶基底变形特征及其区域构造意义

贺兰山北段结晶基底中保留有不同程度的韧性变形剪切带.通过详细的野外考察和室内显微构造研究,明确贺兰山北段的古元古代基底经历了4期韧性剪切变形:(1)早期顺层剪切带表现出中下部地壳层次的变形样式,运动学特征一致反映了近南北向的伸展;(2)麻粒岩相变质的糜棱片麻岩剪切带为南北向挤压的产物,导致经历高温高压变质的孔兹岩系从下地壳向中部地壳抬升;(3)高级糜棱岩(低角闪岩相-高绿片岩相)剪切带涉及的2次伸展运动(北西-南东向伸展和北东-南西向伸展)使得基底进一步向中部地壳抬升,可能发生在形成孔兹岩系的同一造山运动的晚期伸展垮塌过程中;(4)北东-近东西向左行逆冲绿片岩相糜棱岩剪切带则将结晶基底抬升到中上部地壳层次,其运动学特征与高级糜棱岩剪切带明显不同,可能是另一造山运动的产物.贺兰山北段与大青山-乌拉山地区有相似的韧性剪切带和构造变形,表明华北克拉通西部北缘存在一致的近东西走向的古元古代碰撞造山运动以及随后另一造山运动的改造.     

Small-scale ductile shear zones as transposed rheologically weak domains: A numerical modeling investigation and practical application

We test the hypothesis that small ductile shear zones are developed from initial rheologically weak domains. We regard weak domains as ellipsoidal inhomogeneities and apply Eshelby's formalism extended for power-law viscous materials to investigate the kinematics and finite strain evolution of the partitioned flow fields in weak domains. We show that, under an imposed bulk flow field, weak domains, regardless of their initial shapes and orientations, deform into zone-like features at relatively small bulk strains and the deformation paths inside weak domains have all characteristics expected in ductile shear zones. We apply our model to the Cap de Creus area Spain, where abundant small ductile shear zones exist. To relate the fabrics inside these shear zones with the regional deformation, we take a multi-scale approach. We assume that the area contained many weak domains which were randomly shaped and oriented initially and were deformed into shear zones eventually in response to the regional flow. We constrain the regional flow field by the fabric patterns across the area and compute numerically the partitioned flow fields in individual weak domains. The latter are related to fabrics inside shear zones. Our model reproduces first-order features of field-observed fabrics. Although the deformation path of each shear zone was close to simple shearing, the deformation of the whole belt was not. Our approach also resolves the strain compatibility problem for a finite-sized shear zone embedded in a far less deformed country rock.     

中蒙边界早白垩世不对称花岗岩穹隆的伸展时限、剪切作用类型和区域构造意义

东北亚大陆于晚中生代时期发生了大规模地壳伸展,发育变质核杂岩和不对称花岗岩穹隆,其伸展剪切机制一直是构造研究的重要内容之一。中蒙边界东南段沿北东向展布了罕乌拉、纳兰和宝德尔等3个不对称花岗岩伸展穹隆,主体均为晚中生代花岗岩侵入体,岩体西北缘发育韧性剪切(糜棱岩)带,并被后期高角度正断层所围限,整体为穹隆状。根据罕乌拉穹隆韧性剪切带内强变形中粗粒钾长花岗岩(133±1 Ma)和弱变形细粒花岗岩(128±2 Ma)的构造关系及其锆石U-Pb年龄,推测该穹隆内岩体可能为同伸展岩体,韧性伸展时间在133 Ma之后并持续至128 Ma或更晚,与同区其他穹隆发育时限相同。笔者用R_f/ф方法测量了3个穹隆剪切带内糜棱岩中长石的有限应变轴率,利用Hsu图解获得其应变类型为平面压扁应变(k=0.5)。用长石极莫尔圆法、刚性颗粒网法和C轴石英组构法估算了韧性剪切的长期变形过程,得到糜棱岩的平均运动学涡度值为0.68～0.74,表明这些穹隆的韧性剪切作用主要是纯剪切和简单剪切分量几乎相等的一般剪切作用。石英斜向条带法测得的韧性变形后期的运动学涡度值为0.87～0.99,平均值为0.93...