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1.
Three major episodes of folding are evident in the Eastern Ghats terrain. The first and second generation folds are the reclined type; coaxial refolding has produced hook-shaped folds, except in massif-type charnockites in which non-coaxial refolding has produced arrow head folds. The third generation folds are upright with a stretching lineation parallel to subhorizontal fold axes. The sequence of fold stylesreclinedF 1and coaxialF 2, clearly points to an early compressional regime and attendant progressive simple shear. Significant subhorizontal extension duringF 3folding is indicated by stretching lineation parallel to subhorizontal fold axes. In the massif-type charnockites low plunges ofF 2folds indicate a flattening type of deformation partitioning in the weakly foliated rocks (magmatic ?). The juxtaposition of EGMB against the Iron Ore Craton of Singhbhum by oblique collision is indicative of a transpressional regime.  相似文献   

2.
The structural geometry of a mylonite zone (the Woodroffe thrust) and the country rock in its immediate vicinity is described. Mylonitic schistosity formed axial planar to folds in country rock foliation and contains a mineral elongation lineation which is constant in orientation. However, the fold axes (and associated intersection lineation) spread in orientation within the mylonitic schistosity but with a strong maximum parallel to the mineral elongation lineation. It is demonstrated that the fold axes formed initially at approximately 90° to mineral elongation but rotated with increase in strain towards it. Where this phenomenon was homogeneous on a macroscopic scale, rotation of large blocks of country rock across zones of mylonitization accompanied reorientation of fold axes within the mylonite.The controversy of progressive simple versus progressive pure shear for mylonite zones is discussed in the light of recent fabric and other evidence. It is concluded that the inhomogeneous forms of both progressive pure shear and progressive simple shear played a part and that the former dominated initially but gradually gave way to the latter until brittle rupture with large simple-shear displacements on a zone lubricated by the formation of pseudotachylite, brought granulite over amphibolite facies rocks.  相似文献   

3.
Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km-long, NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike-slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated. These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones. The earlier fabric(S1), is locally recognized in low strain areas and SW-ward thrusts. S2 is associated with a shallowly plunging stretching lineation(L2), and defines ～NW-SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ～NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones. Dextral ENEstriking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NW-to NNW-striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.  相似文献   

4.
The patterns of deformed early lineations (L1) over later folds (F2) can be classified into several morphological types depending on the nature of variation of L1 F2 over the folds. The field relations indicate that the folds under consideration are neither shear folds nor parallel folds modified by flattening. The lineation patterns are therefore interpreted in terms of an empirical model of simultaneous buckling and flattening in which it is assumed that (i) the central surface of the folded layer remains a sine curve in transverse profile, (ii) the ratio of rates of buckle shortening to homogeneous strain is proportional to sin 2a, with a as the dip angle and (iii) the progressive deformation is coaxial with the Z-axis of bulk strain parallel to the planar segments of the early folds. The model gives an insight into the relative importance of different physical factors which control the development of dissimilar lineation patterns. Not all lineation patterns are explicable by this simplified model. Thus complex patterns with variable L1 F2 along the fold axis may develop by a progressive rotation of the geometrically defined fold hinge through successive material lines. The theoretical results have been applied to interpret the lineation patterns in Central Rajasthan, India. It is concluded that L1 was initially very close to the E-ESE trending subhorizontal Z-axis of bulk deformation during F2-folding and that the X-axis was subhorizontal or gently plunging with a N-NNE trend.  相似文献   

5.
Transpressional deformation has played an important role in the late Paleozoic evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here we focus on the transpressional Irtysh Shear Zone with an aim at understanding amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped macroscopic fold structures in the southern Chinese Altai and analyzed their relationships with the development of the adjacent Irtysh Shear Zone. Structural observations from these macroscopic folds show evidence for four generations of folding and associated fabrics. The earlier fabric (S1), is locally recognized in low strain areas, and is commonly isoclinally folded by F2 folds that have an axial plane orientation parallel to the dominant fabric (S2). S2 is associated with a shallowly plunging stretching lineation (L2), and defines ∼NW-SE tight-close upright macroscopic folds (F3) with the doubly plunging geometry. F3 folds are superimposed by ∼NNW-SSE gentle F4 folds. The F3 and F4 folds are kinematically compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may represent strain partitioning during deformation. The sub-parallelism of F3 fold axis with the Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation (F3) in fold zones. The strain partitioning may have become less efficient in the later stage of transpressional deformation, so that a fraction of transcurrent components was partitioned into F4 folds.  相似文献   

6.
Progressive ductile shearing in the Phulad Shear Zone of Rajasthan, India has produced a complex history of folding, with development of planar, non-planar and refolded sheath folds. There are three generations of reclined folds, F1, F2 and F3, with a striping lineation (L1) parallel to the hinge lines of F1. The planar sheath folds of F1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices. L1 swerves harmoniously with the curving of F1 hinge line. There is a strong down-dip mineral lineation parallel to the striping lineation in most places, but intersecting it at apices of first generation sheath folds. Both the striping and the mineral lineation are deformed in U-patterns over the hinges of reclined F2 and F3. Folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non-planar sheaths. The reclined folds of all the three generations were deformed by a group of subhorizontal folds. Each generation of fold initially grew with the hinge line at a very low angle with the Y-axis of bulk non-coaxial strain and was subsequently rotated towards the down-dip direction of maximum stretching. The patterns of deformed lineations indicate that the stretching along the X-direction was extremely large, much in excess of 6000 percent.  相似文献   

7.
Detailed field-structural mapping of Neoproterozoic basement rocks exposed in the Wadi Yiba area, southern Arabian Shield, Saudi Arabia illustrates an important episode of late Neoproterozoic transpression in the southern part of the Arabian-Nubian Shield (ANS). This area is dominated by five main basement lithologies: gneisses, metavolcanics, Ablah Group (meta-clastic and marble units) and syn- and post-tectonic granitoids. These rocks were affected by three phases of deformation (D1–D3). D1 formed tight to isoclinal and intrafolial folds (F1), penetrative foliation (S1), and mineral lineation (L1), which resulted from early E-W (to ENE-WSW) shortening. D2 deformation overprinted D1 structures and was dominated by transpression and top-to-the-W (?WSW) thrusting as shortening progressed. Stretching lineation trajectories, S-C foliations, asymmetric shear fabrics and related mylonitic foliation, and flat-ramp and duplex geometries further indicate the inferred transport direction. The N- to NNW-orientation of both “in-sequence piggy-back thrusts” and axial planes of minor and major F2 thrust-related overturned folds also indicates the same D2 compressional stress trajectories. The Wadi Yiba Shear Zone (WYSZ) formed during D2 deformation. It is one of several N-S trending brittle-ductile Late Neoproterozoic shear zones in the southern part of the ANS. Shear sense indicators reveal that shearing during D2 regional-scale transpression was dextral and is consistent with the mega-scale sigmoidal patterns recognized on Landsat images. The shearing led to the formation of the WYSZ and consequent F2 shear zone-related folds, as well as other unmappable shear zones in the deformed rocks. Emplacement of the syn-tectonic granitoids is likely to have occurred during D2 transpression and occupied space created during thrust propagation. D1 and D2 structures are locally overprinted by mesoscopic- to macroscopic-scale D3 structures (F3 folds, and L3 crenulation lineations and kink bands). F3 folds are frequently open and have steep to subvertical axial planes and axes that plunge ENE to ESE. This deformation may reflect progressive convergence between East and West Gondwana.  相似文献   

8.
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   

9.
Spatial variations in shear strain rate are expected in ductile shear zones. Where the variation is a change in shear strain perpendicular to the displacement direction, the effect is to rotate the shear slip planes. This is a mechanism for giving a rotation of fold axes into parallelism with the slip and extension direction in a rock. If such a variation in shear strain affects rocks with a strong planar anisotropy it is possible to produce a fabric with an apparent stretching lineation parallel to fold axes, but both significantly oblique to the slip direction. A possible example of this is seen in strongly deformed quartz-mica schists from Syros, Greece, where a stretching lineation is seen parallel to fold hinges over a range of fold axes orientations of at least 40°.  相似文献   

10.
The Dating rocks and Darjeeling gneisses, which constitute the Sikkim dome in eastern Himalaya, as well as the Gondwana and Buxa rocks of ‘Rangit Window’, disclose strikingly similar sequences of deformation and metamorphism. The structures in all the rocks belong to two generations. The structures of early generation are long-limbed, tight near-isoclinal folds which are often intrafolial and rootless. These intrafolial folds are associated with co-planar tight folds with variably oriented axes and sheath folds with arcuate hinges. Penetrative axial plane cleavage and mineral lineation are related structures; transposition of bedding is remarkable. This early phase of deformation (D 1) is accompanied by constructive metamorphism. The structures of later generation are open, asymmetrical or polyclinal; a crenulation cleavage or discrete fracture may occur. The structures of early generation are distorted by folds of later generation and recrystallized minerals are cataclastically deformed. Recrystallization is meagre or absent during the later phase of deformation (D 2). The present discussion is on structures of early generation and strain environment during theD 1 phase of deformation. The concentration of intrafolial folds in the vicinity of ductile shear zones and decollement or detachment surface (often described as ‘thrust’) may be considered in this context. The rocks of Darjeeling-Sikkim Himalaya display minor structures other than intrafolial folds and variably oriented co-planar folds. The state of finite strain in the rocks, as observed from features like flattened grains and pebbles, ptygmatic folds and boudinaged folds indicate combination of flattening and constrictional type strain. The significance of the intrafolial folds in the same rocks is discussed to probe the environment of strain during progressive deformation (D 1).  相似文献   

11.
Minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow. Using structures associated with WNW-directed Caledonian thrusting in N Scotland, we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction. These define a series of larger-scale fold culminations (reflecting ‘surging flow’) or depressions (reflecting ‘slackening flow’) that are bisected by transport-parallel culmination and depression surfaces. The fold patterns suggest a dominance of layer-normal differential shearing due to gradients in shear strain normal to transport. Culmination surfaces are marked by along-strike reversals in the polarity of structural facing and vergence of minor folds which, contrary to classic fold patterns, define reverse asymmetric relationships. Culmination surfaces separate folding in to clockwise (Z folds) and anticlockwise (S folds) domains relative to the transport lineation. The dip of fold axial planes systematically increases as their strike becomes sub-parallel to transport resulting in a 3D statistical fanning arrangement centred about the transport direction. Thus, mean S- and Z-fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved. Culminations display convergent fold patterns with fold hinges becoming sub-parallel to transport towards the culmination surface and underlying detachment, whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface. Thus, around culminations and depressions there are ordered, scale-independent relationships between transport direction, shear sense, fold facing, vergence and hinge/axial plane orientations. The techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow.  相似文献   

12.
Many folds of both deformation phases on Söröy exhibit a prominent lineation brought out by elongate micas or spindleform-quartz grains. This lineation displays a variety of patterns, depending on ils orientation relative to the fold axis. This dependence confirms the primary nature of the axial noncy lindrism.The lineation is a product of layer-parallel extension and represents the long axis of the sectional strain ellipse (X1), for the particular attitude of the layering. The formation of the lineation is confined to those layer orientations which cut the extension field of the strain ellipsoid. If the noncylindrical fold axes curve from normal into parallelism with the lineation, the patterns of lineation orientation developed will reveal the attitude of the major extension axis (X) and indicate the nature of the strain.The lineation developed in the earlier stages of fold growth and became a passive marker in later fold modification.The regional pattern of the lineation is similar to that for minor fold axes, and together they are held to identify the XY-plane of the bulk finite strain-field.  相似文献   

13.
Abstract Reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation. Its role has generally been insufficiently emphasized because evidence is best preserved where porphyroblasts which contain inclusion trails are present. Reactivation occurs when progressive shearing, operating in a synthetic anastomosing fashion parallel to the axial planes of folds, changes to a combination of coarse- and finescale zones of progressive shearing, some of which operate antithetically relative to the bulk shear on a fold limb. Reactivation of earlier foliations occurs in these latter zones. Reactivation decrenulates pre-existing or just-formed crenulations, generating shearing along the decrenulated or rotated pre-existing foliation planes. Partitioning of deformation within these foliation planes, such that phyllosilicates and/or graphite take up progressive shearing strain and other minerals accommodate progressive shortening strain, causes dissolution of these other minerals. This results in concentration of the phyllosilicates in a similar, but more penetrative manner to the formation of a differentiated crenulation cleavage, except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds. Reactivation can generate bedding-parallel schistosity in multideformed and metamorphosed terrains without associated folds. Heterogeneous reactivation of bedding generates rootless intrafolial folds with sigmoidal axial planes from formerly through-going structures. Reactivation causes rotation or ‘refraction’of axial-plane foliations (forming in the same deformation event causing reactivation) in those beds or zones in which an earlier foliation has been reactivated, and results in destruction of the originally axial-plane foliation at high strains. Reactivation also provides a simple explanation for the apparently ‘wrong sense’, but normally observed ‘rotation’of garnet porphyroblasts, whereby the external foliation has undergone rotation due to antithetic shear on the reactivated foliation. Alternatively, the rotation of the external foliation can be due to its reactivation in a subsequent deformation event. Porphyroblasts with inclusion trails commonly preserve evidence of reactivation of earlier foliations and therefore can be used to identify the presence of a deformation that has not been recognized by normal geometric methods, because of penetrative reactivation. Reactivation often reverses the asymmetry between pre-existing foliations and bedding on one limb of a later fold, leading to problems in the geometric analysis of an area when the location of early fold hinges is essential. The stretching lineation in a reactivated foliation can be radically reoriented, potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts. Geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events. Other aspects of reactivation, however, can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized; for example, D2-grown porphyroblasts may be dissolved against reactivated S1 and hence appear to have grown syn-D1.  相似文献   

14.
Abstract

An important generation of recumbent folds can be recognized in the Nevado-Filabride nappe complex in the Sierra de los Filabres in SE Spain.

Folding post-dates an initial phase of flattening and is prior to the main phase of non-coaxial deformation and thrusting in the upper part of the complex, involved in a large-scale movement zone with dominant sense of shear to the northwest.

Axial planes and axes of these folds are aligned sub-parallel to the plastic flow plane in quartz and the shear direction respectively. Relict quartz fabrics can be found however, which strongly suggest that some fold axes originated in this position and did not rotate towards parallelism with the shear direction. This apparently contradicts the generally accepted model of development of folds in mylonite zones.

The redistribution of linear structures of the older flattening fase over the folds, confirms that the original orientation of fold axes was close to the present and indicates that an important component of flexural slip was involved in fold formation.  相似文献   

15.
The Caprauna-Armetta Unit (CAU) is a Briançonnais cover nappe emplaced on the external margin of the Ligurian Briançonnais Zone. A structural analysis of the nappe indicates that there are four superposed deformations (D1-D4). D1 produced large recumbent isoclinal folds associated with a strong axial-plane cleavage and a SW-trending lineation. These folds can be related to a SW-directed overthrust shear. D2 produced open to moderately tight folds with subvertical axial planes, overturned towards the northeast. D3 and D4 are represented by large wavelength open folds affecting only the large-scale setting of the nappe.A finite strain map of the nappe has been compiled using data from an oolitic limestone layer. The measured strains appears to be essentially the product of the D1 phase. The measured ellipsoids are generally triaxial. The trend of the finite strain X axes is towards the southwest. Prolate ellipsoids with very high Rxz ratios occur on the inverted limbs and sometimes near the hinge zones of the anticlinal F1 folds. Oblate ellipsoids are prevalent on the normal limbs. This pattern of finite strain resulted from deformation in a ductile shear zone generated within the tectonic units trailed at the base of the huge Helminthoid Flysch Nappe during its motion towards the foreland.  相似文献   

16.
Analysis of shapes of folds, together with other structures such as axial plane foliation boudinage, mullions and cross joints, show that the F1 folds in the ‘main Raialo syncline’ were formed by buckling, and were subsequently modified by flattening normal to the axial planes and lengthening along the axis. The apparent buckle shortening of the F1 folds generally ranges between 70 and 80%. The folds were formed by simple shear (giving place to pure shear at certain stages) in an almost north-south direction on subhorizontal beds. Progressive deformation in the later stage of F1 folding resulted in gentle upright folding of F1 axial planes on F1′ axes slightly oblique to F1. The F2 folds, whose average shortening ranges from 20 to 30%, were also formed by buckling caused by horizontal compression in a nearly northwest-southeast direction. This folding was preceded and followed in some instances by homogeneous strain, as deduced fro mthe shapes of the F2 folds and the nature of variation of the F1 lineations. The F3 conjugate structures developed when the maximum compressive strain was vertical and the intermediate compressive strain northwest-southeast, almost normal to the subvertical F2 axial planes. The increase in the amplitude of the F2 folds in the last phase of F2 folding in certain zones resulted in an excess of vertical load, which dissipated with the formation of the F3 structures. In the last stage of movement (F4) the maximum compressive strain became horizontal along the strike of F2 axial planes, whereas the minimum compressive strain was normal to them. The F4 structures, therefore, point to a longitudinal shortening with reference to large scale F2 folding.  相似文献   

17.
In Cap Corse, progressive deformation during Late Cretaceous obduction of the ophiolitic Schistes Lustrés (sensu lato) as a pile of imbricate, lens-shaped units during blueschist facies metamorphism was non-coaxial. Two zones are recognized: a lower series emplaced towards the west is overlain by a series emplaced towards the south-southwest in Cap Corse. Equivalent structures (differing only in orientation) occur in both zones. The change in thrust direction was responsible for local refolding and reorientation of previously formed structures, parallel to the new stretching direction immediately below the thrust contact between the two zones, and within localized shear zones in the underlying series.Both zones are refolded about E-overturned F2 folds trending between 350 and 025°. Local minor E-directed thrusts occur associated with the F2 folds. This second deformation of Middle Eocene age is considered to be related to the backthrusting of an overlying klippe containing gneisses of South Alpine origin, and is followed by a third Late Eocene phase of upright 060°-trending F3 folds accompanied by greenschist facies metamorphism.  相似文献   

18.
Transected F1 fold structures in eastern Ireland are associated with subhorizontal stretching in the S1, cleavage whereas axial planar cleavage contains a vertical elongation direction. This suggests that the non-axial planar cleavage was influenced by a distributed strike-slip ductile shear. A major NE-SW trending F1 syncline is described in which the minor F1 folds show systematic variations in cleavage transection parameters. On the steep limb of the major syncline the cleavage transects the minor F1 folds in a consistently clockwise sense, whereas on the normal limb anticlockwise transected folds are seen. Axial planar cleavage occurs at the core of the major syncline. Fold profile analysis indicates that the buckling of the layers began before the initiation of the cleavage. Open, parallel folds at the major synclinal hinge zone are progressively ‘flattened’ on the steep limb towards a major D1 sinistral transcurrent fault. The angular transection, A, attains a maximum of 15° clockwise which diminishes to <5° at higher strains adjacent to the major fault. Incremental fibre growth in pressure shadows show a two-stage tectonic strain superposition of vertical pure shear followed by sinistral transcurrent simple shear during the development of the clockwise transecting cleavage. Anticlockwise transected folds were influenced by local dextral strike-slip on the southern margins of a rigid terrane. As a regional feature, the clockwise transection is explained by a sinistral transpressive deformation of end-Silurian age.  相似文献   

19.
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   

20.
It has long been recognised that within zones of intense non-coaxial deformation, fold hinges may rotate progressively towards the transport direction ultimately resulting in highly curvilinear sheath folds. However, there is a surprising lack of detailed and systematic field analysis of such “evolving” sheath folds. This case study therefore focuses on the sequential development of cm-scale curvilinear folds in the greenschist-facies El Llimac shear zone, Cap de Creus, Spain. This simple shear-dominated dextral shear zone displays superb three dimensional exposures of sheath folds defined by mylonitic quartz bands within phyllonite. Increasing amounts of fold hinge curvature (δ) are marked by hinge segments rotating into sub-parallelism with the mineral lineation (Lm), whilst the acute angle between the axial-planar hinge girdle and foliation (ω) also displays a sequential reduction. Although Lm bisects the noses of sheath folds, it is also clearly folded and wrapped-around the sheath hinges. Lm typically preserves a larger angle (θ) with the fold hinge on the lower limb (L) compared to the upper (U) limb (θL > θU), suggesting that Lm failed to achieve a steady orientation on the lower limb. Adjacent sheath fold hinges forming fold pairs may display the same sense of hinge arcing to define synthetic curvature, or alternatively opposing directions of antithetic curvature. Such patterns reflect original buckle fold geometries coupled with the direction of shearing. The ratio of long/short fold limbs decreases with increasing hinge curvilinearity, indicating sheath folds developed via stretching of the short limb, rather than migrating or rolling hinge models. This study unequivocally demonstrates that both hinges of fold pairs become curvilinear with sheaths closing in the transport direction recording greater hinge-line curvilinearity compared to adjacent return hinges. This may provide a useful guide to bulk shear sense.  相似文献   

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