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1.
In the history of superposed deformations of the iron formations at the western border of the Kolar Gold Field in S India, an important event was the successive growth of broadly coaxial plane noncylindrical folds in course of a progressive deformation concomitant with development of ductile mesoscopic shear zones. The noncylindrical folds were initiated as active folds by the creation of a buckling instability at successive stages on newly developed foliation surfaces. The nucleation of noncylindrical folds and the subsequent axial-plane folding of the tightened mature folds are explained by the mechanical inhomogeneity of the rocks and the heterogeneous character of strain. The correlation between increasing tightness and increasing noncylindricity of the folds indicates that the initial curvatures of hinge lines were accentuated by an extension parallel to the subhorizontal stretching lineation. From the patterns of deformed lineations over folds of varying tightnesses, it is concluded that the passive accentuation of hinge-line curvatures was mostly achieved when the folds had already become isoclinal or very tight. 相似文献
2.
Abstract Two varieties of charnockites are recognized in the Dharwar craton of southern India. The style and sequence of structures in one charnockite variety, and related intermediate to basic granulites, are similar to those in the supracrustal rocks of the Dharwar Supergroup and the adjacent Peninsular Gneiss. This style has isoclinal folds with long limbs and sharp hinges with an axial planar fabric in some instances. Additional evidence of flattening is provided by pinch-and-swell and boudinage structures, with basic granulites forming boudins in the more ductile charnockites/enderbites in the limbs of isoclinal folds. These folds are involved in near-coaxial upright folding resulting in the bending of the axial planes of the isoclinal folds and the associated boudins. All these structures are overprinted by non-coaxial upright folds with axial planes striking nearly N–S. The map pattern of charnockites suggests that this sequence of structures is present not only on a mesoscopic scale, but also on a macroscopic scale. Charnockites of this variety provide, in some instances, evidence of having been migmatized to give rise to hornblende–biotite gneiss and biotite gneiss, which form a part of the Peninsular Gneiss terrane.
The second variety comprises charnockite sensu stricto with an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non-coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process. 相似文献
The second variety comprises charnockite sensu stricto with an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non-coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process. 相似文献
3.
A method of kinematic analysis of structures, microstructuresand mineral preferred orientations, initially devised in the study of peridotites, has been applied to crustal rocks bearing evidence of large strains produced in metamorphic environments. Three tectonic lineaments (Angers-Lanvaux, Montagne Noire and Maydan) were selected. They illustrate a general situation arising in continental crusts when they are deformed by ductile transcurrent fault systems.The Angers-Lanvaux structure is bilaterally symmetric; its dominant feature is the horizontal stretching lineation which is parallel to the fold axes. The foliation and slaty cleavage in the most surficial formations wrap around the axis of the whole structure. The folds in the slates away from the axis also exhibit axes parallel to the general trend, but no stretching lineations. These folds are attributed to crustal shortening in a direction normal to the ductile fault. In the Montagne Noire recumbent folds are thrusted away from the axis of the structure over at least 25 km. The metamorphism is also centered on the structure and symmetrically reduced away from it. The core of the structure is occupied by a strongly lineated orthogneiss, cut by a late intrusive granite. The Maydan axial zone displays clear evidence of partial melting at various scales within the deformed gneisses: (1) in gashes perpendicular to the stretching lineation which in these anatectic formations tends to plunge at more than 45°; (2) in bands of deformed pegmatites; and (3) possibly in granites which on the one hand intrude the surrounding formations and on the other converge with increasing deformation on the fault zone. The quartz preferred orientations and microstructures in quartzite layers from Angers indicate that the plastic flow plane and direction lie, respectively, close to the foliation and lineation, the slight departure is ascribed to a flow with a rotational shear component.All this suggests a general model for the origin of such ductile zones. The horizontal relative displacement of crustal blocks along a ductile band is responsible for its overall steeply dipping foliation and horizontal lineations. Viscous heating progressively tends to concentrate the plastic flow along its axis. It is also responsible for the development of metamorphism and of anatexis at depth; the partially melted rocks tend to rise, building at shallower depth the arched structure in the axis of the ductile zone, with a continuing flowage parallel to this axis probably now in the solid state; they can also intrude the surrounding terrain as undeformed batholiths. The folds parallel to the stretching lineation in the axial zone are explained by the fact that, due to the escape of anatectic melts, the formations at depth flow in a narrowing channel. The upwelling of the axial structure induces a compression with folding in the surrounding sedimentary formations and gravity nappe sliding away from the axis. 相似文献
4.
Sheath folds or “eye” folds on decimetric to metric scales are well-developed in the metachert-marble-green rock interlayers of the Changchun Formation and in the marble lens of the Tienhsiang Formation, within the Tananao Group between Tienhsiang and Tailuko, along E-W cross-island highway of Taiwan. Closely associated with the sheath folds are the tight to isoclinal folds with rectilinear axes which are parallel to the hinge line of the “eyes”, and the directions of these folds range from N-S to N30°E with gentle plunges to the north or south.The sheath folds are believed to have been formed during the second phase of deformation in this region. The traces of the earlier folding can generally be found at the hinges or limbs of these sheath folds.The explanation presented here is that the sheath fold might be generated episodically during the F2 deformational phase throughout the entire history of progressive shearing as a result of episodic instability of the flow with successive refolding of metamorphic fabric, during Plio-Pleistocene deformation of Taiwan. 相似文献
5.
Carl E. Jacobson 《Journal of Structural Geology》1984,6(5):563-570
The Pelona Schist, which forms the lower plate of the Vincent thrust in the San Gabriel Mountains of southern California, has undergone a complex history of folding. The youngest folds in the schist (style 2 folds) range in shape from open to tight and fold both compositional layering and schistosity. These are superposed upon isoclinal folds with axial-plane schistosity (style 1 folds) that, in turn, overprint older isoclinal folds (also called style 1 folds). Samples from the hinges of style 2 folds contain two generations of muscovite. Muscovites of the older generation are parallel to the folded (style 1) schistosity. The newer muscovites recrystallized during and/or after style 2 folding. Microprobe analysis indicates that the two generations of muscovite are very similar in composition, although the new muscovites tend to have slightly higher paragonite and celadonite contents than the old muscovites. From the gross similarity of the two groups of muscovite, it is concluded that the style 1 and style 2 folds were produced during a single progressive deformation. The slightly higher paragonite and celadonite contents of the new muscovites are thought to indicate that both pressure and temperature were increasing during the deformation. This is consistent with the deformation being due to underthrusting of the Pelona Schist beneath the upper plate of the Vincent thrust. 相似文献
6.
The orientation of high temperature dikes has been studied withrespect to the plastic flow foliation and lineation in peridotitesfrom several massifs. Layering and dikes are composed of varioustypes of pyroxenites and gabbros; dunites, thought to representresidua along high temperature dikes, have been also studied.The layering is composed of dunites, websterites and/or orthopyroxenites,usually with the same minerals as in the host peridotite. Thesame rock types can be observed in the dikes emplaced earlyduring plastic deformation although the sequence tends to evolvetoward more ariegitic or gabbroic facies. Dikes emplaced during plastic flow display two dominant preferredorientations, one is at a high angle to the stretching lineation,the other is parallel to the shear planes deduced from the flowregime analysis in the peridotite (a unique shear plane obliqueto the foliation in the case of rotational flow, two shear planesconjugate with respect to the foliation in the case of irrotationalflow). Fracturing in all these orientations is ascribed to themagma pressure created by partial melting. Comparatively lowmagma pressure and high applied deviatoric stress result inshear fractures; high magma pressure and moderate applied stressresult in tension fractures (at high angles to the lineation).Dikes emplaced before or early during plastic flow are stronglydeformed and tectonically rotated toward the foliation and lineationdirections. For a large deformation they become parallel tothe foliation orientation. The ubiquitous mafic and ultramaficlayering present in most mantle peridotites may originate inthis way. 相似文献
7.
The northern fold belt away from the Singhbhum Shear Zone displays a set of folds on bedding. The folds are sub-horizontal
with E-W to ESE striking steep axial surfaces. In contrast, the folds in the Singhbhum Shear Zone developed on a mylonitic
foliation and have a reclined geometry with northerly trending axes. There is a transitional zone between the two, where the
bedding and the cleavage have become parallel by isoclinal folding and two sets of reclined folds have developed by deforming
the bedding—parallel cleavage. Southward from the northern fold belt the intensity of deformation increases, the folds become
tightened and overturned towards the south while the fold hinges are rotated from the sub-horizontal position to a down-dip
attitude. Recognition of the transitional zone and the identification of the overlapping character of deformation in the shear
zone and the northern belt enable the formulation of a bulk kinematic model for the area as a whole. 相似文献
8.
《Journal of Structural Geology》1988,10(6):555-565
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
9.
Rubén Díez Fernández Francisco J.Rubio Pascual Luis Miguel Martín Parra 《地学前缘(英文版)》2019,10(2):651-669
This contribution discusses about the rheological, kinematic and dynamic frameworks necessary to produce recumbent and upright folds from syn-orogenic granitic massifs that were formed during an early stage of magma genesis related to the onset of a migmatitic dome. Syn-kinematic granitoids occurring within the high-grade infrastructure of the Padron migmatitic dome(NW Iberia) are deformed into largescale recumbent folds(D_2) that are later affected by upright folds(D_3). Petrostructural analysis of a selected area of this dome reveals that after a period of crustal thickening(D_1), NNW-directed extensional flow gave way to recumbent folds and penetrative axial plane foliation(S_2). Superimposed subhorizontal compression resulted in upright folds(D_3). A closer view into the dynamics of the dome allows exploring the factors that may condition the nucleation of folds with contrasting geometries during progressive deformation of molten continental crust. The formation of folds affecting syn-kinematic granitoids suggests a cooling metamorphic path in migmatitic domes. Active and passive folding mechanisms require a crystallizing(cooling) magma to nucleate folds. A more competent metamorphic host inhibits fold nucleation from much less competent magmas. As it crystallizes, magma becomes more rigid(competent),and approaches viscosity values of its host. Passive folding is favored when no significant competence contrast exists between magma and host, so this folding mechanism is more likely shortly after magma genesis and emplacement. In such conditions, and under dominant subhorizontal flow accompanied by flattening(D_2),passive folding would produce isoclinal recumbent geometries. Further magma cooling introduces a shift into the rheological behavior of partially molten crust. Thereon, crystallizing magma bodies would represent significant competence contrasts relative to their host. At this point, buckling is a more likely folding mechanism, and more regular, buckle folds re-fold previous structures after significant cooling. The geometry of resulting folds is upright due to dominant subhorizontal compression(D_3) at this stage. 相似文献
10.
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
11.
甘肃李坝金矿变质岩构造与金成矿的关系 总被引：5，自引：1，他引：4
甘肃李坝金矿区舒家坝组变质岩,经历了３期不同性质构造变形：早期固态塑性流变,发生强烈面状构造置换和等斜褶皱;中期区域褶皱作用,形成一系列开阔褶皱;晚期伸展变形运动,引起断裂、岩浆活动。多期次的构造变形,造成含金建造叠加富集;成矿元素活化、迁移、沉淀。金矿床分布于岩体周围褶皱轴部的断层破碎带中,构造成矿作用显著。 相似文献
12.
Four types of soft-sediment folds of distinct geometry can be recognized in the upper part of the Talchir Formation (Lower Permian) of Jharia Basin, India. These folds, on systematic examination, indicate some events of progressive deformation. Experimental study reveals that if a layered stack of clay and overlying sand is allowed to flow slowly down a slope, differential velocity due to viscosity contrast leads to the deformation of the rheologic interface. The sharp planar contact gradually becomes wavy leading to the development of round-hinged folds involving sediments adjacent to it. With the advancement of the flow these folds gradually become overturned with the rotation of the axial plane in the direction of flow. Computer simulation suggests that progressive deformation of these folds by simple shearing may lead to the formation of tight isoclinal folds, which on dislocation along intrastratal normal faults may lead to the development of rootless isoclinal folds. The sheath folds observed in the studied section also indicate accentuation of the curved hinge due to simple shearing. The spatial distribution of these fold types in conjunction with the inferred direction of progressive deformation indicate basinward translation of the slump slice. If the same stack of sediments rapidly flows down the slope, the waveform generated at the interface quickly breaks in the form of roll-up recumbent fold due to Kelvin–Helmholtz instability. 相似文献
13.
Professor P. F. Williams 《International Journal of Earth Sciences》1983,72(2):589-604
A spectacular example of transposition, in the sense ofSander (1911), is described from a large (2–3 km2) area of continuous outcrop in northern Norway. Three generations of folds transpose an earlier layering into its persent orientation. It is shown that the volume problems associated with isoclinal folding are largely accommodated by disharmony of the folds. The large scale structure is considered and its relationship to regional structure is discussed. 相似文献
14.
Boudins with long axes (BA) oriented subnormal to bedding and to associated fold axes are observed in folded rocks in a thrust sheet exposed near the base of a regionally extensive allochthon in west-central Nevada, USA. Formation of the boudins is related to development of a regional fold-set coeval with major thrusting. The axes of boudins lie at a high angle to bedding, and in some instances, boudins define tight to isoclinal folds which are geometrically associated with the regional deformation. Quartz c-axis fabrics from oriented thin-sections of the boudins indicate extension parallel to the boudin axes (BA).
These relations and other mesoscopic structural data indicate a complex deformational history for boudin development. The history involves thin layers (to become boudins) deformed in folds disharmonic to major structures within the thrust sheet followed by flattening and associated extension parallel to fold axes. During flattening, arcuation occurred within the deforming mass resulting in rotation of fold axes and boudin axes (BA) toward the axis of finite extension (X). Extension parallel to BA recorded in the petrofabrics of boudins records incremental strain axes oriented at a high angle (50°) to the finite X and is probably related to an early plane-strain state associated with disharmonic folding. The finite extension (X) is down-dip in axial planes of major folds formed during thrusting and indicates a northwest to southeast transport for the thrusts. 相似文献
15.
In the metamorphic cores of many orogenic belts, large macroscopic folds in compositional layering also appear to fold one or more pervasive matrix foliations. The latter geometry suggests the folds formed relatively late in the tectonic history, after foliation development. However, microstructural analysis of four examples of such folds suggests this is not the case. The folds formed relatively early in the orogenic history and are the end product of multiple, near orthogonal, overprinting bulk shortening events. Once large macroscopic folds initiate, they may tighten further during successive periods of sub-parallel shortening, folding or reactivation of foliations that develop during intervening periods of near orthogonal shortening. Reactivation of the compositional layering defining the fold limbs causes foliation to be rotated into parallelism with the limbs.Multiple periods of porphyroblast growth accompanied the multiple phases of deformation that postdated the initial development of these folds. Some of these phases of deformation were attended by the development of large numbers of same asymmetry spiral-shaped inclusion trails in porphyroblasts on one limb of the fold and not the other, or larger numbers of opposite asymmetry spirals on the other limb, or similar numbers of the same asymmetry spirals on both limbs. Significantly, the largest disparity in numbers from limb to limb occurred for the first of these cases. For all four regional folds examined, the structural relationships that accompanied these large disparities were identical. In each case the shear sense operating on steeply dipping foliations was opposite to that required to originally develop the fold. Reactivation of the folded compositional layering was not possible for this shear sense. This favoured the development of sites of approximately coaxial shortening early during the deformation history, enhancing microfracture and promoting the growth of porphyroblasts on this limb in comparision to the other. These distributions of inclusion trail geometries from limb to limb cannot be explained by porphyroblast rotation, or folding of pre-existing rotated porphyroblasts within a shear zone, but can be explained by development of the inclusion trails synchronous with successive sub-vertical and sub-horizontal foliations. 相似文献
16.
In the high‐grade (granulite facies) metamorphic rocks at Broken Hill the foliation is deformed by two groups of folds. Group 1 folds have an axial‐plane schistosity and a sillimanite lineation parallel to their fold axes; the foliation has been transposed into the plane of the schistosity by these folds. Group 2 folds deform the schistosity and distort the sillimanite lineation so that it now lies in a plane. Both groups of folds are developed as large folds. The retrograde schist zones are zones in which new fold structures have formed. These structures deform Group 1 and Group 2 folds and are associated with the formation of a new schistosity and strain‐slip cleavage. The interface between ore and gneiss is folded about Group 1 axial planes but about axes different from those in the foliation in the gneiss. On the basis of this, the orebody could not have been parallel to the foliation prior to the first recognizable structural and metamorphic events at Broken Hill. The orebody has been deformed by Group 2 and later structures. 相似文献
17.
《Journal of Structural Geology》2003,25(10):1659-1673
Detachment folds represent a major structural element in a number of fold belts. They are common in the Jura Mountains, the Zagros fold belt, the Central Appalachian fold belt, the Wyoming fold-belt, the Brooks Range, the Parry Islands fold belt, and parts of the SubAndean belt. These structures form in stratigraphic packages with high competency contrasts among units. The competent upper units exhibit parallel fold geometries, whereas the weak lower unit displays disharmonic folding and significant penetrative deformation. Two distinct geometric types, disharmonic detachment folds, and lift-off folds have been recognized. However, these structures commonly represent different stages in the progressive evolution of detachment folds. The structures first form by symmetric or asymmetric folding, with the fold wavelength controlled by the thickness of the dominant units. Volumetric constraints require sinking of units in the synclines, and movement of the ductile unit from the synclines to the anticlines. Continuing deformation results in increasing fold amplitudes and tighter geometries resulting from both limb segment rotation and hinge migration. Initially, limb rotation occurs primarily by flexural slip folding, but in the late stages of deformation, the rotation may involve significant internal deformation of units between locked hinges. The folds eventually assume tight isoclinal geometries resembling lift-off folds. Variations in the geometry of detachment fold geometry, such as fold asymmetry, significant faulting, and fold associated with multiple detachments, are related to variations in the mechanical stratigraphy and pre-existing structure. 相似文献
18.
The majority of naturally occurring folds are noncylindrical if definitions are strictly applied. A new classification of noncylindrical folds using a triangular plot and based on measurements of interlimb angle and hinge angle is proposed. The end-members of the triangular plot are planes, cylindrical isoclines, and isoclinal domes. An infinite range of cylindrical and noncylindrical plane fold shapes may be represented. Noncylindrical nonplane folds may be represented on the plot using proportional circles to signify the degree of non-planarity. The triangular diagram is used to classify large-scale folds from north Norway and their origin is discussed. 相似文献
19.
20.
《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