首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Blueschist-facies rocks of the central Seward Peninsula cropout over 8000 km2. Protoliths were Lower Paleozoic-Precambrian(?) shallow-water miogeoclinal sediments that were metamorphosed during the Middle Jurassic. Thermobarometric estimates yield ‘peak’ metamorphic conditions of 10–12 kbar at 460 ± 30°C. Crystallization of blueschist-facies minerals was synkinematic with development of a transposition foliation. This foliation is parallel to lithologic contacts and is axial planar to recumbent mesoscopic isoclinal folds. These folds are refolded by larger scale recumbent tight to isoclinal folds. Both fold sets have hinges parallel to a well-developed N—S stretching lineation. Sheath folds are also present. The long axes of the sheath folds also parallel the stretching lineation. This deformation was non-coaxial as indicated by microstructures and quartz c-axis fabrics. Folds nucleated, then rotated into parallelism with the stretching direction. Kinematic indicators show unequivocal top-to-the-north shear sense, compatible with blueschist formation during mid-Jurassic collision between the Brooks Range continental margin and a N-facing island arc (Yukon-Koyukuk). Convergence of these two plates is believed to have been nearly N—S (in present co-ordinates).  相似文献   

2.
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.  相似文献   

3.
Quartz c-axis fabrics have been investigated within a suite of quartz veins and monomineralic layers around a major post-nappe fold hinge (the Wandfluhhorn Fold) in the Bosco area (Swiss-Italian border) within the lower Penninic nappes.Two kinematic domains which are separated by the axial plane trace of the Wandfluhhorn Fold are recognized; on the lower limb the measured quartz c-axis fabric asymmetry indicates a sense of shear in which the overlying layers move to the southwest (i.e. top-to-SW) whereas on the upper limb the shear sense is reversed with the top moving to northeast. The shear direction (N60°E–N80°E), however, is constant in both areas and oblique to an older stretching lineation as well as to the D3 fold hinge. Such a distribution of asymmetric quartz c-axis fabrics and the constant orientation of their interpreted shear direction, which is apparent only from the fabric data and not from field evidence, indicates fabric development pre- or early syn-Wandfluhhorn folding, with subsequent folding and modification of the existing textures and possibly rotation of the initial fold axis.An overall westward-directed shear has been suggested for the whole of the Lepontine Alps. However, this study demonstrates that this simple general pattern has been modified locally by later folding. It also demonstrates that the dominant lineation may be a finite stretching lineation due to more than one phase of deformation and is not necessarily related to any particular transport direction.  相似文献   

4.
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.  相似文献   

5.
A new method for determining the sense of shear in plagioclase-bearing tectonites from the (010) orientation of plagioclase feldspar is presented. The method is based on the asymmetry of the (010) plane with respect to the structural frame (foliation and lineation) and the dominant activity of the (010) slip plane in the high-temperature plasticity of plagioclase feldspar. Using examples from the Zabargad gneisses (Red Sea) the method is applied to plagioclases of An25–An45 and compared with other methods of shear-sense determination (quartz c-axis fabrics and microstructural criteria).  相似文献   

6.
We ask the question whether petrofabric data from anisotropy of magnetic susceptibility (AMS) analysis of deformed quartzites gives information about shape preferred orientation (SPO) or crystallographic preferred orientation (CPO) of quartz. Since quartz is diamagnetic and has a negative magnetic susceptibility, 11 samples of nearly pure quartzites with a negative magnetic susceptibility were chosen for this study. After performing AMS analysis, electron backscatter diffraction (EBSD) analysis was done in thin sections prepared parallel to the K1K3 plane of the AMS ellipsoid. Results show that in all the samples quartz SPO is sub-parallel to the orientation of the magnetic foliation. However, in most samples no clear correspondance is observed between quartz CPO and K1 (magnetic lineation) direction. This is contrary to the parallelism observed between K1 direction and orientation of quartz c-axis in the case of undeformed single quartz crystal. Pole figures of quartz indicate that quartz c-axis tends to be parallel to K1 direction only in the case where intracrystalline deformation of quartz is accommodated by prism <c> slip. It is therefore established that AMS investigation of quartz from deformed rocks gives information of SPO. Thus, it is concluded that petrofabric information of quartzite obtained from AMS is a manifestation of its shape anisotropy and not crystallographic preferred orientation.  相似文献   

7.
Samples of monomineralic quartz veins from the Simplon Fault Zone in southwest Switzerland and north Italy generally have asymmetric, single girdle c-axis patterns similar to textures measured from many other regions. Several samples have characteristically different textures, however, with a strong single c-axis maximum near the intermediate specimen axis Y (the direction within the foliation perpendicular to the lineation X) and a tendency for the other crystal directions to be weakly constrained in their orientation about this dominant c-axis maximum. This results in ‘streaked’ pole figure patterns, with an axis of rotation parallel to the c-axis maximum. These atypical samples also have a distinctive optical microstructure, with advanced recrystallization and grain growth resulting in a strong shape fabric (SB) oblique to the dominant regional foliation (SA), whereas typical samples have a strong SA fabric outlined by very elongate, only partially recrystallized, ribbon grains. The recrystallized grains of the atypical samples are themselves deformed and show strong undulose extinction and a core-mantle recrystallization structure. The streaked texture is likely to be a direct consequence of lattice bending and kinking during heterogeneous slip on the favoured first-order prism (10 0) (a) system, the heterogeneity itself being due to problems in maintaining coherence across grain boundaries when insufficient independent easy-slip systems are available for homogeneous strain by dislocation glide. Such bending would be particularly prevalent in very elongate, thin ribbon grains, resulting in high internal strain energy and promoting recrystallization. Thus both the texture and the microstructure could be significantly modified by later strain increments affecting quartz grains with an already developed, nearly single-crystal texture.  相似文献   

8.
Phulad Shear Zone (PSZ) of Delhi Fold Belt in Rajasthan is a northeasterly striking ductile shear zone with a well developed mylonitic foliation (035/70E) and a downdip stretching lineation. The deformation in the PSZ has developed in a transpressional regime with thrusting sense of movement. The northeastern unit, i.e., the hanging wall contains a variety of rocks namely calc-silicates, pelites and amphibolites and the southwestern unit, i.e., the footwall unit contains only granitic rocks. Systematic investigation of the granites of the southwestern unit indicate a gradual change in the intensity of deformation from a distance of about 1 km west of the shear zone to the shear zone proper. The granite changes from weakly deformed granite to a mylonite/ultramylonite as we proceed towards the PSZ. The weakly deformed granite shows a crude foliation with the same attitude of mylonitic foliation of the PSZ. Microscopic study reveals the incipient development of C and S fabric with angle between C and S varying from 15 ° to 24 °. The small angle between the C and S fabric in the least deformed granite variety indicates that the deformation has strong pure shear component. At a distance of about 1 m away from the PSZ, there is abrupt change in the intensity of deformation. The granite becomes intensely foliated with a strong downdip lineation and the rock becomes a true mylonite. In mesoscopic scale, the granite shows stretched porphyroclasts in both XZ and YZ sections indicating a flattening type of deformation. The angle between the C and S fabric is further reduced and finally becomes nearly parallel. In most places, S fabric is gradually replaced by C fabric. Calculation of sectional kinematic vorticity number ( Wn) from the protomylonitic and mylonite/ultramylonite granites varies from 0.3 ± 0.03 to 0.55 ± 0.04 indicating a strong component of pure shear. The similarity of the geometry of structures in the PSZ and the granites demonstrates that the deformation of the two units is broadly synchronous and the deformation in both the units is transpressional.  相似文献   

9.
Caledonian eclogite facies shear zones developed from Grenvillian garnet granulite facies anorthosites and gabbros in the Bergen Arcs of western Norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation (CPO) in lower continental crust. Field relations on the island of Holsnøy show that the eclogites formed locally from granulite facies rocks by progressive development of: (1) eclogite adjacent to fractures; (2) eclogite in discrete shear zones (> 2 m thick); (3) eclogite breccia consisting of >80% well-foliated eclogite that wraps around rotated granulite blocks; and (4) anastomosing, subparallel, eclogite facies shear zones 30–100 m thick continuous over distances > 1 km within the granulite terrane. These shear zones deformed under eclogite facies conditions at an estimated temperature of 670 ± 50°C and a minimum pressure of 1460 MPa, which corresponds to depths of >55 km in the continental crust. Detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite/garnet-rich and kyanite/zoisite-rich layers. A consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite. The CPO of omphacite, determined from five-axis universal stage measurements, shows a strong b-axis maximum normal to foliation, and a c-axis girdle within the foliation plane with weak maxima parallel to the lineation direction. These patterns are consistent with deformation of omphacite by slip parallel to [001] and suggest glide along (010). The lineation and CPO data reveal a consistent sense of shear zone movement, although the displacement was small. Localized faulting of high-grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust. Field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth. Deformation of granulite and partial conversion to eclogite, such as is exposed on Holsnøy Island, may be an orogenic-scale process in the lowermost crust of collisional orogens.  相似文献   

10.
边千韬  林传勇 《地质科学》1996,31(2):170-175
在可可西里北缘发现的糜棱岩化带,经显微构造研究确定为韧性剪切带。此带发育流劈理及拉伸线理。糜棱岩化花岗岩和糜棱岩化石英脉中的石英发育亚晶粒构造、位错构造和动态重结晶,长石主要发育机械双晶。石英c轴组构属韧性剪切带中的典型形式,石英变形以位错蠕变机制和位错滑移机制共存为特征,石英的动态重结晶作用是由亚晶粒旋转机制形成。长石的变形主要是通过机械双晶实现的。此韧性剪切带形成时的温度约400℃,差异应力约30MPa,应变速率约1.9×10-13s-1.  相似文献   

11.
A fast and accurate optical method to calculate quartz c-axis orientations using the rotating polarizer stage and standard thin sections is presented. c-Axis orientations are calculated for each pixel and Achsenverteilungsanalyse (AVA) can easily be constructed to study problems that would normally require a prohibitive amount of tedious work. The computer controlled rotating polarizer stage replaces the polarizer and analyzer of the standard petrographic microscope and allows a thin section to remain fixed while the polarizing filters are rotated by stepper motors. Data are collected by stepping the polarizers through a 180° rotation, capturing a frame at each step and extracting information on the intensity of the pixel and position of the polarizers. The position data are used to calculate the trend or trend+180° of the quartz c-axes, while a simple mathematical relationship between intensity and plunge is derived which allows the plunges of c-axes to be calculated.  相似文献   

12.
The Canisp Shear Zone transects layered Lewisian gneisses near Lochinver, NW Scotland. It is a vertical ductile shear zone with a dextral shear sense, formed during Laxfordian amphibolite facies metamorphism, transposing the layering to new foliation and linear structures. Minerals in the layered gneisses show little or no shape fabric, while a strong shape fabric defines the foliation. For quartz, this shape fabric is accompanied by development of a preferred crystal orientation with fabric patterns reflecting the geometry of the shear deformation. The quartz fabric shows a pole-free area around the lineation with the c-axes concentrated in an asymmetric cross-girdle or a point maximum perpendicular to the shear plane, and a monoclinic symmetry consistent with the shear sense.  相似文献   

13.
Geometrical relations between quartz C-axis fabrics, textures, microstructures and macroscopic structural elements (foliation, lineation, folds…) in mylonitic shear zones suggest that the C-axis fabric mostly reflects the late-stage deformation history. Three examples of mylonitic thrust zones are presented: the Eastern Alps, where the direction of shearing inferred from the quartz fabric results from a late deformation oblique to the overall thrusting; the Caledonides nappes and the Himalayan Main Central Thrust zone, where, through a similar reasoning, the fabrics would also reflect late strain increments though the direction of shearing deduced from quartz fabric remains parallel to the overall thrusting direction. Hence, the sense of shear and the shear strain component deduced from the orientation of C-axis girdles relative to the finite strain ellipsoid axes are not simply related nor representative of the entire deformation history.  相似文献   

14.
Permian volcanic sediments at Bluff have been strained and thermally metamorphosed by Permian intrusives to metasediments of hornblende—hornfels facies. Quartz, which crystallised as a secondary mineral during metamorphism, has an unusual preferred orientation with c-axes either forming paired maxima in the plane containing the lineation (=maximum principal strain axis = direction of extension) and the perpendicular to schistosity (=minimum principal strain axis = shortening direction) or a broad maximum parallel to the lineation; the paired maxima are approximately 30° either side of the lineation. Some quartz grains are markedly elongate parallel to the lineation, and according to hypotheses of preferred orientation involving crystal plasticity, there should be some correlation between the shape of such grains and their c-axis orientations. Grain-size and shape analysis of Bluff quartz demonstrate that no such correlation exists; the analyses show that the preferred orientation results from oriented nucleation in the residual stress field immediately following the bulk straining of the rocks, with the distribution of c-axes as predicted by Kamb's hypothesis (1959). The time relationships of rock deformation, thermal metamorphism, and nucleation and growth of quartz are discussed.  相似文献   

15.
We performed deformation experiments on a foliated mylonite under high temperature and pressure conditions in this study. To investigate the effect of pre‐existing fabric on the rheology of rocks, our samples were drilled from natural mylonite with the cylinder axis parallel to the foliation (PAR) and perpendicular to the foliation (PER). We performed 25 tests on seven PAR samples and 21 tests on seven PER samples at temperatures ranging from 600 to 890 °C, confining pressures ranging from 800 to 1400 MPa, and steady‐state strain rates of 1 × 10−4, 1 × 10−5 and 2.5 × 10−6 s−1. In the temperatures of 600–700 °C, the deformation is accommodated by semi‐brittle flow, with the average stress exponent being 6–7 assuming power law flow; in the temperature range of 800–890 °C, deformation is mainly by plastic flow, with an average stress exponent of n = 3 and activation energies of Q = 354 ± 52 kJ/mol (PER and PAR samples). The experimental results show that the strengths of PER samples are higher than those of PAR samples. Deformation microstructures have been studied by optical and electron microscopy. The original foliation of PER samples is destroyed by deformation and replaced by a new foliation, but the deformation of PAR samples followed the original foliation. Electron backscatter diffraction (EBSD) measurements show a strong lattice preferred orientation (LPO) of the quartz c axis fabrics of the starting samples and deformed PER and PAR samples. However, the c axis fabric of quartz in experimentally deformed PER and PAR samples varied with temperature and strain rate is different from that seen in the starting mylonite sample. The initial quartz c axis fabric of the starting mylonite sample has been transformed into a new fabric during experimental deformation. Dehydration melting of biotite and hornblende occurred in both PER and PAR samples at temperatures of 800–890 °C. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

16.
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.  相似文献   

17.
《Tectonophysics》2003,361(3-4):171-186
Banded iron formation (BIF) from the Quadrilátero Ferrı́fero (southeastern Brazil) shows a compositional layering with alternating iron-rich and quartz-rich layers. This layering was intensively folded and transposed at a centimeter/millimeter scale through a component of bedding-parallel shear related to flexural slip at middle to high greenschist facies conditions (400–450 °C). The microstructure and c-axis fabrics of normal limbs, inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy (SEM) and digital image analysis. In the normal limbs, recrystallized quartz grains show undulose extinction, relatively dry grain boundaries, c-axes at high angle to foliation and a pervasive grain shape fabric (GSF) indicating operation of crystal-plastic processes. In the inverted limb, quartz grains show more serrated and porous (“wet”) grain boundaries; the GSF is similar to that of the normal limb, but c-axes are oriented at 90° to those of the normal limb. We interpreted these characteristics as reflecting operation of solution-precipitation deformation in inverted limbs, as a consequence of grains having been rotated to an orientation that was hard to basal 〈a〉 glide, but easy to dissolution-precipitation creep. This deformation partitioning between crystal-plasticity and solution-transfer during folding/transposition of quartz may explain the common occurrence of layered quartz rocks, where individual layers show alternating c-axis fabrics with opposite asymmetries but a consistent GSF orientation. Such characteristics may reflect an earlier event of pervasive folding/transposition of a preexisting layering.  相似文献   

18.
The regional development of distinct patterns of preferred orientation of quartz c-axes in the Saxony Granulites has been well documented in the literature. A suite of specimens representative of these fabrics has been examined both by optical universal stage, to determine quartz c-axis orientation, and by X-ray diffraction, to obtain orientation data from r, z, m and a. The data are combined to yield inverse pole figures of schistosity and lineation.The finite strain of the Saxony Granulites is thought to be essentially a flattening and there is no evidence that the deformation path is other than one of continuous flattening. Elongation in the plane of the schistosity is local and not extreme. Because of this apparently simple deformation picture, and because preliminary transmission electron microscopy reveals the presence of dislocation structures similar to those found in deformed metals, an attempt is made to interpret the quartz orientation in terms of dislocation slip mechanisms. There is some evidence that the activation of different mechanisms is perhaps primarily controlled by temperature. At least some of the patterns of preferred orientation of quartz were probably produced by deformation in the field of stability of α-quartz.  相似文献   

19.
Lattice Preferred Orientation (LPO) of quartz and muscovite are measured in a micaceous quartzite using SEM based Electron Backscatter Diffraction (EBSD) analysis. The measurements were done in a thin section prepared parallel to the K1K3 plane of the anisotropy of magnetic susceptibility (AMS) ellipsoid, which is equivalent to the XZ plane of the strain ellipsoid. Bulk data from the sample reveal that quartz c-axis develop an oblique single girdle LPO pattern indicating a dextral sense of shear. A similar pattern is produced by poles to the basal planes (001) of muscovite, which implies that muscovite controls the LPO of quartz in the rock. Petrofabric analysis of quartz is performed in domains viz. (I) not containing muscovite, (II) between two muscovite grains, and (III) rim of quartz grains around a muscovite grain. Quartz LPO pattern in Domain-III is noted to be similar to the bulk LPO recorded from the whole sample, thus confirming that muscovite was critical in textural evolution of the rock. The study thus establishes that a minor mineral phase in a rock can control the LPO of the major mineral phase, thus playing a significant role in development of bulk texture of the rock. The advantages of domainal petrofabric analysis in evaluation of slip systems in minerals in different parts of a rock, and vorticity quantification are discussed.  相似文献   

20.
三维参照变形及应变相研究评述   总被引：2，自引：0，他引：2  
三维参照变形和应变相是最近构造地质学领域中取得的重要进展,三维参照变形是理想化的三维变形分类,每一参照变形是共轴级分（拉伸、压扁或纯剪）和与其垂直的简单切组分同时作用的产物,三种可能的面理取向和三种可能的线理取向的不同组合构成六咱应变相,三维参照变形和应变相研究证明糜陵面理未必平行剪切带,可与剪切带斜交,甚至垂直,线理未必与剪切方向一致,可与剪切方向斜交,甚至垂直,出现横向面理时,剪切指向标志位于该面理内,出现横向线理时,剪切指向出现在与线理垂直的ａｃ面理内,三维变形分析不公可解决三维分析难以解释的横向面理和线理,而且可确定共轴组分的类型及其与单剪组分的结合方式。  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号