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1.
In the Lesser Garhwal Himalaya, the North Almora Thrust separates the overlying medium-grade Dudatoli-Almora crystallines
of Precambrian age from the unmetamorphosed to partly metamorphosed rocks of the Garhwal Group of Late Precambrian age. The
crystalline nappe sheet consists of flaggy to schistose quartzites, granite gneisses and garnetiferous mica schist members
in an ascending order. In different localities. different members of the Dudatoli-Almora crystallines are exposed along the
thrust plane. Southwest of Adbadri fine-grained mylonitized schistose quartzites of Dudatoli-AImora crystallines are in contact
with the underlying metabasites of the Garhwal Group. The mylonitized schistose quartzites consist of alternating thick (1
to 2m) quartzite and thin (10 to 20cm) micaceous quartzite bands. The micaceous quartzites can be further differentiated into
alternating quartz-rich (0-5 to 2.0 cm thick) and mica-rich (0.2 to 1.0 cm thick) layers. In the quartzites the C-surfaces
are parallel to the S-surfaces defined by the alternating quartz-rich and mica-rich layers. Further, the S-surfaces exhibit
almost similar folds with multiple wavelengths where the axial planes are nearly parallel and enveloping surfaces are oblique
to the lithological layering. The evolution of these folds has been envisaged in three phases of deformation on the basis
of field evidence, fold geometry and microstructures.
During the first phase buckle folds (F
1) developed in thin micaceous quartzite layers. whereas thick quartzite bands underwent only layer parallel shortening. During
the second phase the stress orientation changed and the limbs ofF
1 folds were folded (F
2). During the third phase of deformation which coincided with thrusting, the rocks were sheared, mylonitized and developed
microstructures exhibiting dynamic recrystallization by the processes of subgrain rotation, and continual and discontinuai
grain boundary migration. This phase was also responsible for the development of C-surfaces parallel to the lithological layering.
Further, in the folded micaceous quartzite layers shearing resulted in the development of C-surfaces parallel to the axial
planes ofF
2 folds. 相似文献
2.
《Journal of Structural Geology》2001,23(6-7):845-856
A series of experiments is described in which layered specimens were shortened parallel to the layering. The specimens comprise two salt (NaCl) layers sandwiched between three layers of salt–mica synthetic schist. All specimens were prepared and deformed under the same conditions, except for the amount of shortening, which was varied. The resulting fold-shapes are variable, even where the amounts of shortening are the same. In one specimen, folds are believed to have developed essentially by buckling with very little concomitant bulk homogeneous shortening perpendicular to the axial-plane. The specimen lacks an axial-plane foliation. Other folds are believed to have experienced varying amounts of bulk homogeneous shortening before and during buckling, and all have axial-plane foliations that have developed by grain-scale transposition of the original bedding-parallel mica foliation. The difference in the behaviour of the various specimens is explained in terms of initial perturbations. These irregularities take the form of initial deflections in the almost planar bedding, variation in the degree of preferred orientation of the mica grains, and local compositional variation within individual salt–mica layers. 相似文献
3.
河南方城柏树岗金红石矿床含矿岩系主要由角闪(片)岩、变粒岩、斜长片岩、石英(片)岩及云母片岩组成、前者是主要含矿岩石。分为石英(片)岩、角闪(片)岩-云母斜长片岩、云母片岩-石英(片)岩三个岩性段,二段是主要含矿岩段。原岩为一套火山岩、正常沉积岩岩性组合。经历了两次变质作用,变质温压400～485℃、200～700MPa,变质时间416.1±0.5Ma,变质热液产生了少量细脉及脉岩。含矿岩系的形成变化是板块运动的结果。基性岩及基性岩质凝灰岩是钛的主要来源,变质作用是主要成矿作用。 相似文献
4.
Subir Kumar Ghosh 《Tectonophysics》1974,21(3):249-272
Two series of experiments were carried out with soft model-materials in order to assess the relative importance of initial homogeneous strain, external rotation and late-stage strain in reorienting early lineations during superposed buckle-folding. In the first series cylindrical buckling folds were produced in embedded planar sheets containing a “lineation”. In the second series noncylindrical folds were produced by compression of a set of cylindrical folds. The experiments indicate that the ratio of buckle shortening to layer-parallel strain is much smaller when the principal extension is parallel to the fold-axis than in the case when the principal extension is perpendicular to the fold-axis. In very competent rocks, the reorientation of old lineations is mainly by external rotation and by the associated concentric longitudinal strain. In moderately competent rocks, the orientation of early lineations always changes by initial homogeneous strain before buckling becomes significant. Because of the unlike amounts of initial strain in layers of different competences, orientations of unrolled lineations may not be parallel in disharmonically folded layers of unlike competences. Under certain conditions the early lineation may become virtually parallel to the later fold-axis. The experiments indicate that the effects of late-stage strain in buckle-folding are largely restricted to the incompetent layers of a multilayer. Hence, if orientation data of early lineations in both competent and incompetent rocks are lumped together, the pattern of orientation may become quite complex. Even for a single competent layer, the pattern of early lineations can locally become complex because of the complex nature of concentric longitudinal strain (and strain resulting from stretching of middle surface of the layer), development of conical folds, development of shear strain along hinge zones of deformed early folds and also because of the development of different orders of folds in both the first and the second deformations. 相似文献
5.
内蒙古查干通格地区中浅构造相韧性剪切变形地质体中构造形迹保存较好的有柳树沟岩组绢云石英片岩、石英岩,祖宗毛道基性岩墙和哈拉霍疙特组三段灰岩.变形岩石片理、矿物拉伸线理均非常发育,在不同构造部位和不同岩性中,矿物的变形特征存在明显差异,出现不同的构造样式.白云鄂博群哈拉霍疙特组三段灰岩中发育大量同斜倒转褶皱,且矿物拉伸线理与褶皱枢纽平行;绢云石英片岩中发育大量杆状构造、鞘褶皱.矿物拉伸线理、杆状构造均属a型线理,其延伸平行剪切运动方向.横向上中间变形强,向两侧逐渐变弱.宏观及微观的“S-C”组构、“云母鱼”等指示的运动方式为北东东向的斜下落剪切运动,变质程度为低绿片岩相. 相似文献
6.
J. P. Richert 《Australian Journal of Earth Sciences》2013,60(4):361-366
The geology of the Northern Flinders Range has been reinterpreted. Three clastic units, mapped previously (1, 2), were supposed to have been evidence of three late Proterozoic transgressions over the Archaean basement. Tectonic movements resulted in east‐west folds and major fractures zones. Recent structural and petrographic observations in the western part of the Mt Painter block lead to a reinterpretation of this region. Three tectonic phases may be observed in the Proterozoic rocks: the first phase is characterized by isoclinal folds with axial‐plane cleavage. Three thrust slices of quartzite, carbonate, and schist can be delineated. These thrust slices are separated by shear zones marked by mica schists which could be either basement or strongly deformed Adelaidean rocks. The second phase shows east‐west concentric upright folds with secondary cleavage in their hinges; this phase refolds the first‐phase structures and affects the underlying basement. The third phase created large strike‐slip faults which are superimposed on the first and second deformation. 相似文献
7.
This paper investigates the folding of single competent layers embedded in a less competent matrix, where the competence contrast is about 10: 1. Folds result from buckling during layer-parallel compression. A geometrical study of natural examples shows that individual folds tend to be grouped into fold complexes.The amplitude varies from a maximum at the centre of a complex to a minimum at each end. Each complex is often centred about a sedimentary lens or nodule which may have triggered the folding and localized the complex. The formation of folds of this kind has been simulated experimentally by deformation of models made from paraffin waxes of known rheological properties. Early in the deformation of a model, buckling starts at a localized site of disturbance, producing only one fold. With further deformation, new folds appear at either side of the initial one. The buckling then propagates along the layering, further folds appearing serially in time and distance. The end result is a complex with many individual folds and a regularly periodic shape.With a competence contrast of 10: 1, the rate of fold propagation is slow, and formation of a periodic complex requires an overall shortening of at least 15%. The shapes of folds formed experimentally are similar to those formed naturally. 相似文献
8.
The ENE-plunging macroscopic folds, traced by calc gneiss interbanded with marble and sillimanite schist within the Peninsular
Gneiss around Suganapuram in the ‘Palghat gap’ in southern India, represent structures of the second generation (D2). They have folded the axial planes of a set of D1 isoclinal folds on stratification coaxially, so that the mesoscopic D1 folds range from reclined in the hinge zones, through inclined to upright in the limb zones of the D2 folds. Orthogonal relation between stratification and axial planar cleavage, and ‘M’ shaped folds on layering locate the
hinge zones of the D1 folds, whereas folds on axial planar cleavage with ‘M’ shaped folds are the sites of the D2 fold hinges. Extreme variation in the shapes of the isoclinal D1 folds from class 1B through class 1C to nearly class 2 of Ramsay is a consequence of buckling followed by flattening on layers
of widely varying viscosity contrast.
The large ENE-trending structures in this supracrustal belt within the Peninsular Gneiss in the ‘Palghat gap’ could not have
evolved by reorientation of NS-trending structures of the Dharwar tectonic province to the north by movement along the Moyar-Bhavani
shear zone which marks the boundary between the two provinces. This is because the Moyar and Bhavani faults are steep dipping
reverse faults with dominant dip-slip component.
Deceased 相似文献
9.
10.
Dr. A. K. Jain 《International Journal of Earth Sciences》1975,64(1):230-248
In the Lesser Himalayan region of Garhwal, an elongate, NW-SE trending zone of mylonitic rocks is developed along the Singuni Thrust within the metasedimentary formation of the Deoban-Tejam Belt. Detailed petrography of various mylonitic rocks indicates that a quartz and felspar porphyry was emplaced along the Singuni Thrust. This was initially metamorphosed in the almandine-amphibolite facies before profound ruptural or cataclastic and crystalloblastic deformation evolved mylonitic rocks in the green schist facies. Southwesterly dipping foliation and an equally prominent mica lineation plunging in the same direction are developed in these mylonitic rocks. The quartzite is also intensely cataclastically deformed in the green schist facies and is highly schistose with a prominent mica lineation normal to the trace of Singuni Thrust, Uttarkashi Thrust and Main Central Thrust in the ‘a’ direction of tectonic transport. In quartzite and mylonitic rocks, a probable contemporaneous development of the metamorphic and structural elements has been postulated along the Singuni Thrust during large scale tectonic movements. Normally exposed Gamri Quartzite is more metamorphosed near its base along the Singuni Thrust and Uttarkashi Thrust while the intensity of deformation increases near the top of normally exposed quartzite along the Main Central Thrust and, thus, signifying the role of thrusting in cataclastically deforming the rocks and contributing to the phenomenon of widespread reversal of metamorphism in the Lesser Himalaya. 相似文献
11.
Dhruba Mukhopadhyay Mohan C. Baral Ranjan K. Niyogi 《Journal of Earth System Science》1997,106(4):259-276
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. 相似文献
12.
陕南秦巴山区是我国重要的金矿产区,尤其是安康西北部的羊坪湾-流芳金成矿带更是秦巴山区的典型代表,硝璜洞金矿就位于这一成矿带上.硝璜洞金矿产于志留纪大贵坪组第一岩性段的含炭质云英片岩夹薄层炭质石英岩内,显示岩性对金矿化的制约.矿体严格受韧性剪切带控制,主要赋存于与主剪切带呈“入”字形相交的次级韧性断裂带中.矿体呈似层状、透镜状、脉状产出.金矿物以自然金形式赋存,自然金粒度呈粗粒、巨粒.野外可识别出3期硅化蚀变,金矿化与第1期蚀变关系密切.矿床成因为韧性剪切带控制的变质热液型金矿床. 相似文献
13.
Marble, calc-silicate rock, quartzite and mica schist of Precambrian age in the ‘main Raialo syncline’ in the Udaipur district of central Rajasthan, India, have been affected by folding of four main generations (F1–F4), the first two of which are seen in the scale of map to microsection. The very tight to isoclinal F1 folds with long limbs and thickened hinges are generally reclined or inclined, and plunge gently castward or westward where least reoriented. The axial planes of the F1 folds have been involved in upright warps on east-west axes (F1′), nearly coaxial with the F1 folds, in some sectors. These folds have been overprinted by upright F2 folding of varying tightness with the axial planes striking north to northeast, resulting in interference patterns of different types in all scales. A penetrative axial plane foliation related to F1 folding and a crenulation cleavage parallel to the F2 axial pianes are seen in the micaceous rocks. Two sets of conjugate folds and kink bands of smail scale have been superimposed on the F1–F2 folds in thinly foliated rocks. The first of these sets (F3) has its conjugate axial planes dipping gently northeast and southwest, whereas the paired axial planes of the later set (F4) are vertical with north-northwest and east-west strikes. 相似文献
14.
根据Ramberg的纵弯褶皱粘性力学实验,在褶皱形态的分形分析基础上,利用分形理论和褶皱的流变学理论导出了褶皱的分数维(D)与岩层厚度(h)和粘度(μ)间的关系式,并探讨了褶皱复杂性对褶皱分数维的影响,从中获得有关复杂褶皱的流变学信息。影响分形褶皱复杂程度的因素很多,主要因素包括岩层的厚度和粘度。因此,对褶皱的分形测量和岩层厚度及粘度的分析,可以定量分析分形褶皱形成的流变机理。这一研究是褶皱的非线性流变学理论研究的一个尝试。 相似文献
15.
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。 相似文献
16.
Greenschist-facies sub-ophiolitic metamorphic rocks of Andaman Islands,Burma–Java subduction complex
《Journal of Asian Earth Sciences》2011,40(6):804-814
Sub-ophiolitic greenschist facies metamorphic rocks occur at the sole of ophiolite slices and as blocks in the mélange zone beneath the Andaman ophiolite. These are represented by metabasics as actinolite schist to actinolite–chlorite schist and metasediments as garnetiferous quartzo-feldspathic mica–chlorite schist and piemontite quartzite to piemontite bearing quartz–muscovite–chlorite schist to muscovite–quartz-chlorite schist. Actinolite occurs along the schistosity and also as porphyroblasts. Syn to post-tectonic garnet shows no compositional zoning and represent almandine–spessartine solid solution (Alm44–47, Sps23–27, Gros13–17, Pyr9–10). The metabasics are enriched in LILE and depleted in Zr and Y compared to N-MORB.The lithological features suggest that residual heat was the main heat source for greenschist-facies metamorphism. Top part of the subducting slab and overlying trench sediments were metamorphosed and dislocated by the close spaced thrusts in an accretionary prism setting. The field association indicates that metamorphism and the uplift of metamorphic rocks along with ophiolite slices were bracketed in between Cretaceous and Oligocene period. These processes were later than the Pre-Cretaceous emplacement of the ophiolites of Sumatra and Java. 相似文献
17.
First phase folds F1 developed in polydeformed Ajabgarh Group rocks of Proterozoic age are studied using various geometrical methods of analysis
for compatibility of homogeneous strain in both class 1–3 pairs by correlatingt′
ga/α plots with existing curves for competent layers and matchingt
ga/α plots with the flattening curves for the incompetent layers.
F1 folds were initiated by the process of buckling but underwent [(λ2/λ1) = 0.2 to 0.7] for competent layers andR- values of 1.1 to 5 for incompetent layers. The varying flattening is also revealed by the geometry of folds. The apparent
buckle shortening of folds which ranges between 49 and 67 per cent with a majority of the folds having shortening values between
50% and 55% (exclusive of layer parallel strain) and inverse thickness method strain up to 50%. Besides flattening, the fold
geometry was also modified by the pressure solution. This is borne by the presence of dark seams rich in phyllosilicates and
disseminated carbonaceous material offsetting limbs of buckled quartz veins in slates 相似文献
18.
David R. Gray John McL. Miller David A. Foster Robert T. Gregory 《Tectonophysics》2004,389(1-2):35-64
Controversy over the age of peak metamorphism and therefore the tectonic evolution of the Arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane-bearing eclogites from the Saih Hatat window beneath the allochthonous Samail ophiolite in NE Oman. These eclogites contain relicts of earlier fabrics, structures and metamorphic assemblages and provide a record of change from subduction to exhumation. The eclogites are part of a mafic layer that was disrupted into boudins up to 0.5 km in length within a lower plate shear zone (As Sifah shear zone). The megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ENE-trending lineation and sheathlike isoclines enveloped by the flat-lying schistosity. The boudin-bearing layer is isoclinally folded with calc-schist, mafic schist and quartz–mica schist, where the regional folds have axes parallel to the NE-trending stretching lineation (a-type folds). Textural evidence suggests multiple growth events for garnet and clinopyroxene, requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins. The surrounding calc-schist and quartz–mica schist are both intensely deformed with transposition foliation containing an NE-trending lineation in phengite and asymmetric shear indicators such as C′-type shear bands and asymmetric pressure shadows around garnets, that give top-to-the-NE sense of shear. Although consistent ENE-trending lineations in all the boudins suggest that they have largely acted as passive, nonrotating rigid bodies, the presence of NE-vergent asymmetric mesofolds, extensive dynamic recrystallisation, multiple generations of phengites and a range of 40Ar–39Ar apparent ages within the megaboudins suggest, however, that they have not acted entirely passively during the later deformation. Phengites isolated from the high-P/low-T fabrics show groupings in 40Ar–39Ar apparent ages interpreted as distinct metamorphic/cooling intervals at 140–135, 120–98 and 92–80 Ma. Microstructural relations suggest that age groupings younger than 100 Ma reflect phengite growth during exhumation with the top-to-the-NE shearing. The older ages (120–110 Ma) from fabrics that give top-to-the-S shear sense may reflect growth during the subduction phase. The combination of groupings of apparent argon ages older than the crystallisation age of the Samail Ophiolite, the suggestion of different geothermal gradients, and superposed metamorphism suggest that the eclogites and garnet blueschists formed as a result of underthrusting along a break that was not directly related to the metamorphic sole of the ophiolite. The glaucophane–eclogites are interpreted as having formed at different times under varying pressure–temperature conditions during underthrusting with variations in the rate of underthrusting, allowing thermal equilibration and/or rapid cooling at different crustal levels. 相似文献
19.
20.
Low grade submarine exhalative gold mineralization occurs at the London-Virginia deposit in central Virginia Piedmont. The deposit consists of a series of localized but conformable units — basal garnetiferous-chlorite schist, magnetite schist, quartz-muscovite schist, ferruginous quartzite, and chlorite-biotite schist — which represent a mixture of submarine epiclastic volcanic debris and exhalative chemical sediments. Finely disseminated gold occurs dispersed with minor amounts of pyrite, sphalerite, chalcopyrite, galena, and tennantite in the ferruginous quartzite and quartz-muscovite schist. The deposit is believed to have formed by processes analogous to those currently active in the Atlantis II Deep of the Red Sea. Silicarich, hypersaline brines discharged through fractures in the sea floor and ponded in a local basin. Epiosodic influx of clastic debris and extensive deposition of hydrothermal silica diluted the concentration of sulfides and gold to produce a low-grade, siliceous mineralized zone. Emanation from the exhalative vent was terminated when the basin was capped by a lava flow. Subsequent regional greenschist grade metamorphism has recrystallized the silica into a granular quartzite and produced minor remobilization of the gold and sulfides. 相似文献