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.
Within the Cambrian Jodoigne Formation in the easternmost part of the Anglo-Brabant Deformation Belt, sub-horizontal to gently plunging folds occur within the limbs of steeply plunging folds. The latter folds are cogenetic with cleavage and are attributed to the Brabantian deformation event. In contrast, although cleavage is also (1) virtually axial planar to the sub-horizontal to gently plunging higher-order folds, shows (2) a well-developed divergent fanning across these folds, (3) an opposing sense of cleavage refraction on opposite fold limbs, and (4) only very small cleavage transection angles, an analysis of the cleavage/bedding intersection lineation suggests that these higher-order folds have a pre-cleavage origin. On the basis of a comparison of structural and sedimentological features these higher-order folds are interpreted as slump folds. The seemingly ‘normal’ cleavage/fold relationship across the slump folds within the limbs of the large steeply plunging folds is due to the very small angle between cleavage and bedding.As such, a ‘normal’ cleavage/fold relationship is no guarantee for a syn-cleavage fold origin. It is not unlikely that also within undeformed, recumbent slump folds, a well-developed compaction fabric, formed parallel to the axial surface of the slump folds, may show fanning and contrasting senses of cleavage refraction on opposite fold limbs.  相似文献   

2.
A revised interpretation of a number of faults across the hinge and western limb of a large-scale anticlinal flexure in the Mount Isa district has been made in terms of the faults following earlier-formed be joints. Such joints often develop in weakly or moderately folded competent sediments, as a result of either tensile stresses that were active at a late stage during folding or the influence of residual stresses generated during tectonic uplift. The joints are oriented such that on a stereographic projection their poles plot parallel to the a axis of a fabric cross and at 90° to the fold axis (b). bc joints are thus approximately normal to bedding and contain the fold axis, and hence they fan around the axial plane of the fold containing them. Across the hinge and western limb of a steeply N-plunging large-scale F2 flexure in the Mount Isa district, a number of faults at high angles to bedding fan about the axial plane. Making use of the fold geometry and local bedding orientation it is possible to predict the orientation of ideal bc fractures at locations within the fold. These predictions fit well with the observed fault pattern. The movement on the faults, although apparently complex, appears consistent with continued shortening perpendicular to an axial-plane cleavage during the D2 deformation or as part of a later D2 deformation.  相似文献   

3.
The Canyon Range syncline, Central Utah, is composed of an alternating sequence of competent quartzite and incompetent argillite layers and is used here as a natural case study of multilayer folding processes. Geometric details of this fold are evaluated in terms of energy consumption in order to determine which kinematic components of folding are dominant at various stages of fold tightening. In addition, this paper attempts to evaluate what mechanism(s) (e.g. kink folding, fracture formation and sliding along surfaces) are involved in each kinematic component.In general, the patterns preserved in the Canyon Range syncline are comparable to multilayer folding models. In more detail, the following is concluded from this case study. (1) The competent and incompetent members deformed primarily by cataclastic flow and consumed approximately equal amounts of energy. (2) The roles of original competent and incompetent layers reversed during folding. (3) As the syncline tightened, less energy was consumed with increasing hinge fractions. (4) The least amount of energy was consumed with 40° limb dips (i.e., 100° interlimb angle). (5) With an open fold geometry (interlimb angle ≥140°), the hinge region consumed 70% of the fold's total energy. (6) Once the fold reached an interlimb angle of 60°, the limbs consume close to 70% of the total energy. (7) When the fold reached an interlimb angle of ≤60°, the incompetent layer(s) consumed 90% of the fold's energy.  相似文献   

4.
A non-coaxial deformation involving pre-folding initiation of cleavage perpendicular to bedding is proposed to explain non-axial planar cleavage associated with mesoscopic folds in part of the Appalachian foreland thrust-belt of southwest Virginia. Folds are gently plunging, asymmetric, upright to slightly inclined, sinusoidal forms with non-axial fanning cleavage. They show extreme local variations in type and degree of transection and the consistency of transection direction. These relations are further complicated by hinge migration.Cleavage-fan angles, bedding-cleavage angles and δ transection values appear influenced by fold tightness, and in part by fold flattening strain. Fold flattening increments are considered simultaneous with folding. Axial surface traces, and not cleavage traces, coincide with the principal extension direction in fold profiles. Geometric modelling of cleavage fanning and bedding-cleavage angle variations for various theoretical folding modes suggest that folding in limestone and sandstone layers was by tangential longitudinal strain. Significant shape modification and change in bedding-cleavage relations occurred after limb dips of 40 and 50° were attained in limestone and sandstone respectively. Mud-rock class 1C folds with convergent cleavage fans show features transitional between buckling and flexural flow. Initiation of ‘cleavage’ fabrics during layer-parallel shortening prior to significant folding may be important for cleavage evolution in some deformed rocks.  相似文献   

5.
In the Rhoscolyn area of Anglesey, the late Precambrian interbedded psammites and pelites of the Monian Supergroup are folded into a kilometre‐scale antiform, plunging about 25°NE and with an axial surface dipping about 40°NW. Numerous folds of up to a few tens of metres in wavelength are present on both limbs of this antiform. These smaller‐scale folds also plunge about 25°NE but clearly belong to two separate episodes of folding, and it has become a matter of longstanding controversy as to whether the larger antiform belongs to the first or second of these episodes. Close examination of the cleavage/bedding asymmetries from all the lithologies, however, shows that the large antiform is a second‐generation structure, and that on the gently dipping northwest limb, the sense of cleavage/bedding asymmetry of the earlier cleavage in the psammitic units has been almost uniformly and homogeneously reversed (so that it appears to be axial planar to the antiform), while in the pelitic units the sense of cleavage/bedding asymmetry of the earlier cleavage has been preserved. Many of the small‐scale complexities of the observed cleavage/bedding relationships may be explained by appealing to differences in the timing of the formation of buckling instabilities relative to this reorientation of the early cleavage in the psammites during the second deformation. A first‐order analysis of the finite strains from around the large‐scale antiform shows that the orientation of the first cleavage prior to the second deformation was steeply dipping to the southeast. The second deformation correlates with the southeast‐verging Caledonian deformation affecting the Monian and Ordovician units elsewhere in northwest Anglesey, while the northwest‐verging first deformation event, which is not present in the Ordovician rocks, must have occurred before they were deposited. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

6.
The so-called apparent rotation was defined as the angular deviation between a local paleomagnetic direction (after the standard bedding correction) and their corresponding paleomagnetic reference [J. Geophys. Res. 85 (1980) 3659]. In this paper, we make a theoretical exploration on this concept and we conclude that (depending on the number, sequence, orientation and magnitude of the deformation axes that have affected to the rock volume) the apparent rotation may be the addition of a vertical-axis rotation plus a spurious rotation. The later is an error whose origin is the inappropriate application of the bedding correction during the restoration (which does not fit the reverse sequence of deformations). Then, Apparent rot. (s.l.) (δ)=Spurious rot. (θ)+Vertical-axis rot. (β).Conical folds are complex geometries that cannot be restored by using the bedding correction. However, appearance of apparent and spurious rotations has not been studied even though the presence of this kind of folds is very common in fold and thrust belts. In this paper, we show a way to restore these structures and its associated paleomagnetic data by means of forward modelling on a stereographic projection. The modelling has to be based on a good characterization of the geometry (fold axis orientation) and understanding of the kinematics of the fold. General modelling has also allowed us to predict the apparent rotation in conical synclines. Its magnitude depends on the semiapical angle and on the degree of development of the fold; the sense of the rotation (clockwise or counter-clockwise) will depend on the sense of rotation the fold axis.The western External Sierras provide an excellent case study of apparent rotations due to the presence of a conical fold in the footwall (Ebro foreland basin) of the South Pyrenean sole thrust. In addition, a vertical-axis clockwise rotation up to 47° (32° in average) has been detected in the hagingwall. An apparent rotation up to 28° (20° in average) is observed in the footwall of the structure when a simple bedding correction is used. This deviation does not fit with the expected Ebro basin direction (reference) and is caused by the effect of the Riglos conical syncline, developed by the flexure of the foot wall ramp of the South Pyrenean sole thrust. The forward modelling carried out considering the geometry and kinematics (non significant rotations in the autochthonous foot wall) of this structure predicts very well the paleomagnetic observations in the field (geographic coordinates) with angular departures of only 5° (in average). The only application of the bedding correction would introduce errors (spurious rotations up to 21°, 12 in average) related to the conical geometry that would not allow the differentiation of these distinct structural units.  相似文献   

7.
Six experiments of single-layer folding with simple-shear boundary conditions were completed. Using materials of ethyl cellulose, the viscosity ratio of the stiff layer to matrix ranged from 20 to 100. The experiments were monitored by 10–14 photographs taken at equally spaced time intervals. Strain distributions in both the stiff layer and matrix were calculated from the displacements of over 300 ink dots distributed over the surface of each experiment. Both incremental strain (calculated from the relative displacements of the dots between successive photographs) and accumulating strain were determined on the two-dimensional profile of the materials as they folded.Symmetrical fold wavelengths occur and seem to be controlled by the wavelengths of initial perturbations in the stiff layer. If the Biot wavelength was not present initially, it will not occur in the final waveform. Consequently, in a group of natural folds, the mean value of wavelength/thickness ratios apparently reflects the initial perturbations. The mean value should not be confused with the Biot wavelength and should not be used to calculate viscosity ratios in naturally deformed rocks.Substantial layer thickening occurred only with viscosity ratios of 20. The amount of layer thickening also depends on initial perturbations of the stiff layer. If these perturbations are near the Biot wavelength, they are greatly amplified, the folds grow rapidly and layer thickening is small. If the perturbations are not near the Biot wavelength, amplification is small, the folds grow slowly and layer thickening is much greater.Principal elongations of the accumulated strain in the cores of some of the folds are not symmetrically distributed about axial planes and may cut across the axial plane at angles up to 20°. Strain shadows in the matrix, near the convex side of fold hinges, are also prominent. These triangular-shaped regions of low strain are not symmetrically disposed about fold axial planes, in contrast to strain shadows occurring in folds produced under pure-shear boundary conditions.The rotation of accumulating principal elongations in the stiff layer was calculated at fold inflections. Even though the folds themselves are generally symmetrical, these rotations at opposite fold inflections are not. One fold limb exhibits little rotation of principal elongations during folding while the other has rotations up to 70°. In contrast, folds formed in pure-shear boundary conditions have rotations of principal directions on opposite fold limbs equal in magnitude.  相似文献   

8.
Preferred orientations of phyllosilicate grains in the shaley rocks of the Labrador Trough were studied using a pole-figure goniometer. Transmitted X-rays permit determination of the basal planes of chlorite and muscovite. From their preferred orientation, strain is calculated according to March's theory. By choosing samples in successively more strongly deformed domains, a strain history can be recognized which started with compaction by loss of pore volume under an overburden and was later followed by an early homogeneous tectonic shortening parallel to bedding and at a right angle to the fold axis. Buckling occurred next, with strain in the limbs becoming different from that in the hinges; the limbs were lengthened parallel to bedding and nearly normal to the fold axis. Continuing appression, finally, led to pervasive shortening normal to the axial plane with concurrent stretching both updip along the axial plane and along the fold axis. This latter stretching may have occurred when variable-plunge basin and dome structures of the Labrador Trough were formed.  相似文献   

9.
In the western part of the North Singhbhum fold belt near Lotapahar and Sonua the remobilized basement block of Chakradharpur Gneiss is overlain by a metasedimentary assemblage consisting of quartz arenite, conglomerate, slate-phyllite, greywacke with volcanogenic material, volcaniclastic rocks and chert. The rock assemblage suggests an association of volcanism, turbidite deposition and debris flow in the basin. The grade of metamorphism is very low, the common metamorphic minerals being muscovite, chlorite, biotite and stilpnomelane. Three phases of deformation have affected the rocks. The principal D1 structure is a penetrative planar fabric, parallel to or at low angle to bedding. No D1 major fold is observed and the regional importance of this deformation is uncertain. The D2 deformation has given rise to a number of northerly plunging major folds on E-W axial planes. These have nearly reclined geometry and theL 2lineation is mostly downdip on theS 2surface, though some variation in pitch is observed. The morphology of D2 planar fabric varies from slaty cleavage/schistosity to crenulation cleavage and solution cleavage. D3 deformation is weak and has given rise to puckers and broad warps on schistosity and bedding. The D2 major folds south of Lotapahar are second order folds in the core of the Ongarbira syncline whose easterly closure is exposed east of the mapped area. Photogeological study suggests that the easterly and westerly closing folds together form a large synclinal sheath fold. There is a continuity of structures from north to south and no mylonite belt is present, though there is attenuation and disruption along the fold limbs. Therefore, the Singhbhum shear zone cannot be extended westwards in the present area. There is no evidence that in this area a discontinuity surface separates two orogenic belts of Archaean and Proterozoic age.  相似文献   

10.
The Doublespring duplex, located in the Lost River Range of Idaho, is a Sevier age fault-related fold complex in massive limestones of the Upper Mississippian Scott Peak Formation. Folds within the duplex closely resemble fault-bend fold geometrics, with open interlimb angles and low-angle bed cut-offs. Narrow, widely spaced, bedding-parallel shear zones with well-developed pressure solution cleavage alternate with massive, relatively undeformed layers on fold limbs. Shear zones are developed only on the limbs of anticlines, and have similar but unique morphologies in each of three different folds. Incremental strain histories reconstructed from antitaxial fibrous overgrowths and veins within the shear zones constrain the kinematics of folding. Shear zones experienced distributed bedding-parallel simple shear (flexural flow) towards pins near axial surfaces, while adjacent massive layers experienced rotation through an externally fixed extension direction. The absence of footwall synclines and morphological differences in shear zones from adjacent folds suggest that faulting preceded folding. Kinematic histories of folds that have experienced different translational histories are identical, and are not compatible with strain histories predicted from previous kinematic models of fault-bend folding. Shear zone development and fiber growth is instead interpreted to have occurred during low amplitude fixed-hinge buckling in response to initial resistance to translation of the thrust sheet. Fault-bend folding with mobile axial surfaces occurred with translation of the thrust sheets once the initial resistance to translation was overcome and resulted in no penetrative strain.  相似文献   

11.
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   

12.
拉卡兰褶皱带中,发育于Ballarat-Bandigo冲断带中的低级变质砂、泥岩的宏观构造以间离劈理和人字形褶皱为特征,而且劈理在褶皱中呈扇形发育。劈理和褶皱的几何关系分析显示:劈理和褶皱的形成为压溶作用、压扁作用、弯曲作用和被动旋转共同作用的结果,而褶皱砂、泥岩中变形构造则以与压溶作用和再沉淀过程有关的显微构造为其典型特征。Fry法进行的全岩应变测量显示,褶皱砂岩的内部应变相当低(X/Z=1.40—1.83),褶皱应变格局给出变形机制的信息包括:缩短过程中的压扁作用和压溶作用、褶皱过程中由弯滑导致的层平行剪应变、以及褶皱后期发育阶段内弧区强烈的压溶作用。宏观构造、显散构造以及应变特征多方面信息证明:低级变质的沉积岩在褶皱变形过程中,压溶作用为一重要的变形机制。应变分解显示在30％—50％的总地壳水平缩短量下,弯曲导致的缩短最为14％—36％,压扁导致的缩短量为3％—14％,压溶导致的缩短量为8％—26％,而且压溶作用主要发生在褶皱内弧区。  相似文献   

13.
The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590–1500 Ma). The earliest deformation elements preserved in the Hampden Synform are first‐generation tight to isoclinal folds and an associated axial‐planar slaty cleavage. Preservation of recumbent first‐generation folds in the hinge zones of second‐generation folds, and the approximately northeast‐southwest orientation of restored L1 0 intersection lineation suggest recumbent folding occurred during east‐west to northwest‐southeast shortening. First‐generation folds are refolded by north‐south‐oriented upright non‐cylindrical tight to isoclinal second‐generation folds. A differentiated axial‐planar cleavage to the second‐generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second‐generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (>45°). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late‐orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin‐skinned crustal shortening during the development of first‐generation structures to thick‐skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.  相似文献   

14.
Total strain patterns estimated across the Pulaski thrust sheet of the southwest Virginia Appalachians show an approximately homogeneous, plane strain deformation associated with folding and distortion above a subsurface décollement. Estimated strains are low (1.2 < < 2.0) with a subvertical extension. Chlorite fibers in pressure fringes on framboidal pyrite indicate that non-rotational deformation produced weak cleavage and pencil structure in mudrock. Variations in shape of pencils and fiber lengths in pressure fringes define highest strains in fold hinges and adjacent to contraction faults. Fabric transitions, delineated by distribution and intensity of cleavage, pencil structure and bedding fissility across the thrust sheet are strain dependent. Balanced cross-sections suggest 35% horizontal shortening due to regional folding and faulting within the Pulaski sheet. Strain integration techniques give 17–35% horizontal shortening associated with cleavage formation. Removal of this strain indicates that cleavage was superposed on open to tight, class-3 folds. Pre-existing thickness variations and anomalous low strains in tight folds require early folding accomodated by intergranular deformation (perhaps controlled grainboundary sliding). Suppression of cleavage formation and penetrative strain was possibly due to higher pore fluid pressure in the early stages of thrust sheet deformation. Observed variations in bedding-cleavage angle and low cleavage fans are compatible with this deformation sequence.  相似文献   

15.
A group of folds in alternating pelites and cross-laminated siltstones is described. An interpretation of the finite strain state, in the competent silt layers, is proposed on the basis of an analysis of the angle between cross-lamination and the principal surface of accumulation. Strain magnitudes are greatest in the fold hinge where domains of layer parallel shortening and layer parallel extension are separated by a neutral surface. Strain magnitudes in the fold limbs are small and are largely related to the development of the asymmetry of the folds. In the incompetent pelitic layers, strain in the fold limbs has a large, layer parallel shear component. Deformation in the pelites is accompanied by, and presumably partially achieved by, migration of quartz from areas where there is a tendency for volume to decrease, to areas where it is tending to increase. This process involves local increases in volume of more than 50%.A kinematic model is proposed for development of the folds. It involves early development of small symmetrical folds followed by their modification to asymmetrical, parasitic structures on the limbs of later folds. In the late stages of folding, continued shortening perpendicular to the axial surface orientation is achieved by development of a conjugate crenulation cleavage.  相似文献   

16.
Study of rotational inclusion fabrics in garnet porphyroblasts demonstrates that angles of rotation are dependent not only on the amounts of strain suffered by the host rocks but also on porphyroblast shape: near-spherical crystals suffer considerably more rotation than discoidal for a given amount of strain. Angles ranging from 160° to 0° have been measured but rotation took place during two distinct phases of deformation each associated with the formation of folds and attendant axial planar fabrics.The rotational inclusion fabrics permit a study to be made of the geometry and state of strain around two minor folds and thus suggest that the main mechanism of fold development was flexural flow. The differences in the amounts of strain, as recorded by rotational inclusion fabrics, around the two folds further suggest that there was unequal limb rotation during fold development and that the maximum compressive stress lay obliquely to the layering at the onset of folding.  相似文献   

17.
Three deformation phases are recognizable within the Lower Ordovician metasedimentary sequence of the Aberdaron area and they are similar to those described for Lower Palaeozoic sequences in other parts of North Wales. There is no certain evidence however for a major Aberdaron Syncline as described by some previous workers. The first deformation phase produced southcast verging mesoscopic folds with steep to moderate dipping axial surfaces and a sporadic axial plane cleavage. The second deformation was relatively weak and produced only a low-dipping crenulation cleavage at a few favoured localities. The third phase gave rise to numerous small buckle folds, kinks in some pelitic units where the first cleavage was well developed, an axial plane cleavage, and a suite of quartz veins. The orientation of the third phase minor structures is not uniform and the fold trend and strike of axial plane cleavage varies from east-northeast to south-southeast, although retains a constant angular relationship to the local strike of bedding. The distribution of the third cleavage is bimodal and the third deformation phase may have been brought about by conjugate shears during a late brittle fracture stage of NW–SE compression. The structural sequence affecting the Ordovician cannot be correlated with that in the Mona Complex and it seems likely that the Mona Complex was deformed before the Arenig.  相似文献   

18.
J. -B. Edel   《Tectonophysics》2003,363(3-4):225-241
Generally, the lack of bedding criteria in basement units hampers the interpretation of paleomagnetic results in terms of geotectonics. Nevertheless, this work demonstrates that successive remagnetizations recorded in Early Carboniferous metamorphic and plutonic units, without clear bedding criteria, can be used to constrain a polyphased tectonic evolution consisting of a regional clockwise rotation, followed by a folding phase, a tilting phase and a second regional clockwise rotation.Metamorphic, ultrabasic, tonalitic and granitic rocks from different parts of Limousin (western French Massif central; 45.5°N/1.25°E), which underwent metamorphism during Devonian–Early Carboniferous or were intruded in the Early–Middle Carboniferous, were sampled in order (a) to identify the magnetic overprinting phases and the related tectono-magmatic events and (b) to constrain the regional and plate tectonic evolution of Limousin. Paleomagnetic results from 32 new and 26 sites investigated previously show that at least 90% of the magnetization isolated in rocks older than 330 Ma are overprints. In agreement with results from adjacent areas of the Variscan belt, the major overprinting phases occurred: (a) in the last stages of the major exhumation phase [332–328 Ma; mean Virtual Geomagnetic Pole (VGP) “Cp”: 37°N/70.5°E], (b) during the post-collisional syn-orogenic extension (325–315 Ma; VGP “B”: 11°N/114°E), (c) in the Latest Carboniferous and Early Permian (VGP “A1”: 27°N/149°E) and (d) in the Late Permian (VGP “A”: 48°N/146°E). The Middle–Late Carboniferous overprints “Cp” and “B” are contemporaneous with emplacement of leucogranitic, crustal derived plutons, and probably result from the hydro-thermal activity related to the magmatism. The drift from “Cp” directions to “B” directions implies that after 330 Ma, Limousin underwent a clockwise rotation by 65°, together with the Central Europe Variscides. The “Bt” components, the VGPs of which deviate from the mean apparent polar wander path (APWP) of the belt, are interpreted as “B” overprints tilted during Late Variscan tectonics, that is, in the time range 325–315 Ma. The first and most important generation of “Bt” overprints was tilted during NW–SE folding associated with NE–SW shortening, updoming and emplacement of leucogranitic plutons. The second generation reveals southeastward tilting due to NE-striking normal faulting. The drift from “B” to “A1” directions implies that Limousin has participated to the second clockwise rotation by 40° of the whole belt in Westphalian times.  相似文献   

19.
Detailed structural analysis of part of the Variscan southcentral Pyrenees revealed the occurrence of several deformation generations, of which the most important one, called the mainphase folding and striking WNW-ESE, seems to be the oldest. Directional analysis of structural elements related to mainphase folding (sedimentary bedding, mainphase cleavage, small-scale foldaxes and intersection lineations) shows, however, that sedimentary bedding must have been non-planar before mainphase deformation took place. This observation suggests that premainphase folding occurred as well, and indeed the areal distribution of intersection lineations in the studied area demonstrates the existence of two early Variscan fold systems. They are characterized by very open NNW-SSE and WSW-ENE folds and have subvertical axial planes and subhorizontal foldaxes. In strong contrast to mainphase folds, penetrative axial plane foliations did not develop during deformation. Pre-mainphase folds in varying orientations have been reported from many other areas in the central Variscan Pyrenees, but a reinterpretation of existing maps and other data shows that also in these cases two pre-mainphase deformation generations must be present, rather than just one as suggested in most previous work. Again, the interference pattern of the two fold systems as well as field evidence indicates that axial planes are steep and strike approximately N-S and E-W, but locally strong reorientation due to Alpine deformation (mainly thrusting) has taken place. The significance of pre-mainphase folding in the Variscan Pyrenees is discussed in the light of an overall dynamic/ kinematic model involving alternating convergent and divergent right-lateral oblique-slip movements along the north-eastern boundary of the Iberian (micro-)plate. The occurrence of pre-mainphase folds is related to
  1. the transition from divergent to convergent obliqueslip movement (NNW-SSE folds), and
  2. initial oblique convergence of the Iberian and European plates (WSW-ENE folds) prior to mainphase collision.
  相似文献   

20.
This paper describes the experimental deformation of models made with sheets of paraffin wax, simulating a bedded cover resting on a basement wrench fault. During the experiments, “en échelon” folds appear in the cover. As a result of early fault motion, folds first appear at heterogeneities in the bedding and with axes at about 45° to the trace of the wrench fault. Further fault displacement causes a bulk rotation of fold axes towards parallelism with the basement wrench fault, and a resulting curvature of fold axes at larger fault displacement.Folding affects an area which tends to quickly stabilize in width, since folding weakens the sheared cover and subsequent deformation is concentrated in it. Axial surfaces of folds are initially upright, then tend to become inclined with an external vergence, forming a fan centered on the basement wrench fault. Deeper layer-deformation, close to the basement, involves fold reorientations that are greater than in the upper layers. Therefore, down a given vertical line, there is no continuity between surface and deep structures. The geometry and orientation of folds appearing at later stages of wrenching is controlled by the geometry and orientation of already extant folds.  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