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1.
Four phases of deformation are recorded by minor structures in the New Harbour Group (NHG) of southern Holy Island. The regional schistosity in these rocks is a differentiated crenulation cleavage of D2 age. An earlier preferred orientation (S1) is commonly preserved as crenulations within the Q-domain microlithons of the S2 schistosity and is demonstrably non-parallel to bedding. F3 folds are widely developed in S2 and, to a lesser extent, in bedding. S3 crenulation cleavage is sporadically developed but can be intense locally. A major antiformal fold exists in the NHG near Rhoscolyn. This fold is of D3 age since it clearly deforms S2 schistosity and is consistent with the vergence of F3 minor structures. All planar structures are deformed by folds of D4 age. © 1997 John Wiley & Sons, Ltd.  相似文献   

2.
 本文以辽宁省鞍山附近元古宙千枚岩和片岩中的折劈S2为论述的基础。按照简单剪切原理计算出发育折劈的岩石中的γ(剪应变)值。通过γ等值线图及断面图、TM/TQF-α相关图和变形标志(石英)形态比的研究,初步认为,折劈岩石中矿物组成、结构、微构造和α角等的明显“分异”现象,主要由剪应变和伴随发生的物质迁移所造成。有限应变状态的特点是:剪应变高的带(M)和剪应变较低的带(QF)相间排列。相邻带剪应变差异控制着扩散物质迁移机制,对微构造(如微褶皱)的生成,有重要作用。折劈生成于T低于500℃,P 约为5kb 左右的绿片岩相变质环境,它标志着地壳处于区域性抬升状态,相继产生的共轭折劈和膝折带(属于 D,构造),则表明已抬升到足以引起岩石总体体积扩张的高度。  相似文献   

3.
《Geodinamica Acta》1999,12(1):25-42
The Early Eocene to Early Oligocene tectonic history of the Menderes Massif involves a major regional Barrovian-type metamorphism (M1, Main Menderes Metamorphism, MMM), present only in the Palaeozoic-Cenozoic metasediments (the so-called “cover” of the massif), which reached upper amphibolite faciès with local anatectic melting at structurally lower levels of the cover rocks and gradually decreased southwards to greenschist facies at structurally higher levels. It is not present in the augen gneisses (the so called “core” of the massif), which are interpreted as a peraluminous granite deformed within a Tertiary extensional shear zone, and lie structurally below the metasediments. A pronounced regional (S1) foliation and approximately N-S trending mineral lineation (L1) associated with first-order folding (F1) were produced during D1 deformation coeval with the MMM. The S1 foliation was later refolded during D2 by approximately WNW-ESE trending F2 folds associated with S2 crenulation cleavage. It is now commonly believed that the MMM is the product of latest Palaeogene collision across Neo-Tethys and the consequent internal imbrication of the Menderes Massif area within a broad zone along the base of the Lycian Nappes during the Early Eocene-Early Oligocene time interval. However, the meso- and micro-structures produced during D1 deformation, the asymmetry and change in the intensity and geometry of the F2 folds towards the Lycian thrust front all indicate an unambiguous non-coaxial deformation and a shear sense of upper levels moving north. This shear sense is incompatible with a long-standing assumption that the Lycian Nappes were transported southwards over the massif causing its metamorphism. It is suggested here that the MMM results from burial related to the initial collision across the Neo-Tethys and Tefenni nappe emplacement, whereas associated D1 deformation and later D2 deformation are probably related to the northward backthrusting of the Lycian nappes.  相似文献   

4.
Measurements of total, incremental and progressive strains associated with the development of small scale crenulation cleavage in some low-grade metamorphic rocks from Australia and Switzerland are applied to a discussion of the mechanical significance of the cleavage.Limits are placed on the amount of incremental and total slip or simple shear possible along the cleavage by the observation that the XY principal plane trace of bulk total crenulation strain coincides within 4° of the crenulation cleavage trace in all cases where this strain has been measured or estimated. The measurements are made on eight specimens using deformed porphyroblasts, crystal fibres in pressure-shadows around pyrite and flattened folds and include deformations with coaxial and non-coaxial histories.Further measurements derived from pressure-shadow fibres (eight specimens) show that the style and orientation of incremental deformation are essentially independent of the crenulation cleavage, except for a limit (43°) to the obliquity of the principal incremental extension axis during a given cleavage episode. The only special deformation related to the cleavage is the coaxial one. An indication of passive cleavage behaviour at high strain is shown by the progressive strain history of one specimen. Evidence for passive rotation of a transected axial plane is shown by another. A model is proposed to account for these observations, especially the conditions necessary for initiation and continued development of a new cleavage fabric.Some further applications of existing strain measurement techniques are described: of the Rf/Øf method to heterogeneously superposed tectonic strains and of an improved procedure of tα/α flattening analysis.  相似文献   

5.
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   

6.
Extensional structures in anisotropic rocks   总被引:2,自引:0,他引:2  
A distinct class of structures can form as a result of extension along a plane of anisotropy (foliation). The effect of the foliation is to decrease the ductility of the material in this orientation so that brittle fractures or shear-bands develop. Foliation boudinage is caused by brittle failure; extensional fractures cause symmetric boudinage, and shear fractures cause asymmetric boudinage.Extensional crenulation cleavage is defined by sets of small-scale ductile shear-bands along the limbs of very open microfolds in the foliation. The sense of movement on the shear-bands is such as to cause a component of extension along the older foliation. Conjugate cleavage sets indicate coaxial shortening normal to the foliation; the shortening axis bisects the obtuse angle between the sets. A single set indicates oblique or non-coaxial deformation.Extensional crenulation cleavage is microstructurally and genetically distinct from other types of cleavage. It does not occur as an axial plane structure in folds, and has no fixed relationship to the finite strain axes. It is common in mylonite zones, and may be favoured by crystal-plastic and cataclastic deformational mechanisms. These cause grain-size reduction, and hence softening, which favour the development of shear-bands.  相似文献   

7.
Porphyroblasts of garnet and plagioclase in the Otago schists have not rotated relative to geographic coordinates during non-coaxial deformation that post-dates their growth. Inclusion trails in most of the porphyroblasts are oriented near-vertical and near-horizontal, and the strike of near-vertical inclusion trails is consistent over 3000 km2. Microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain, and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures. This is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding.
Several crenulation cleavage/fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding. In the light of accumulating evidence that porphyroblasts do not generally rotate, the applicability of these models to deformed rocks is questionable.
Whether or not porphyroblasts rotate depends on how deformation is partitioned. Lack of rotation requires that progressive shearing strain (rotational deformation) be partitioned around rigid heterogeneities, such as porphyroblasts, which occupy zones of progressive shortening or no strain (non-rotational deformation). Therefore, processes operating at the porphyroblast/matrix boundary are important considerations. Five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast: (a) a thin layer of fluid at the porphyroblast boundary; (2) grain-boundary sliding; (3) a locked porphyroblast/matrix boundary; (4) dissolution at the porphyroblast/matrix boundary, and (5) an ellipsoidal porphyroblast/shadow unit.  相似文献   

8.
Two well-developed mesoscopic folds, D_2 and D_3, which postdate the middle amphibolite metamorphism, were recognized in the western hinterland zone of Pakistan. NW–SE trending D_2 folds developed during NE–SW horizontal bulk shortening followed by NE–SW trending D_3 folds, which developed during SE–NW shortening. Micro- to mesoscopically the NW–SE trending S2 crenulation cleavage, boudins and mineral stretching lineations are overprinted by D_3. The newly established NW–SE trending micro- to mesoscopic structures in Munda termed D_2, which postdated F_1/F_2, is synchronously developed with F3 structures in the western hinterland zone of Pakistan. We interpret that D_2 and D_3 folds are counterclockwise rotated in the tectonic event that has evolved the Hazara Kashmir Syntaxis after the main phase Indian plate and Kohistan Island Arc collision. Chlorite replacement by biotite in the main matrix crenulation cleavages indicates prograde metamorphism related with D_2. The inclusion of muscovite and biotite in garnet porphyroblasts and the presence of staurolite in these rocks indicate that the Barrovian metamorphic conditions predate D_2 and D_3. We interpret that garnet, staurolite and calcite porphyroblasts grew before D_2 because the well developed S2 crenulation cleavage wraps around these porphyroblasts.  相似文献   

9.
Shear zones are areas of intense deformation in localized zones which can be used as natural laboratories for studying deformation characteristics. Metre to-micro scale structures that develop in response to a progressive simple shear in a shear zone are characterized by a protracted history of deformation and are immensely useful in delineating the history of progressive deformation. To decipher these localized zones of deformation and to establish the continuous non-coaxial character of deformation, detail microstructural studies are very useful. Singhbhum shear zone (SSZ), a regional Precambrian tectonic dislocation zone in eastern India, depicting a top-to-south thrust movement of the hanging wall provides a scope for studying microstructural characteristics developed in response to a progressive shear at mid-crustal level. SSZ is characterized by intense stretching lineation, isoclinal folds, shear planes, superposed schistosity and deformed quartz veins. Quasi-plastic (QP) deformation mechanisms were predominantly active in the SSZ. The overprinting relationship between the earlier and later schistosity with a consistent sense of shear indicates that earlier schistosity is transposed to later schistosity through the intermediate stages of crenulation cleavage during a progressive non-coaxial deformation. The recrystallization of quartz in mylonitic quartzite suggests protracted history of deformation. The analysis of the character of quartz grains of both the porphyroclasts and recrystallized grains suggests that strain was partitioned between the most intensely deformed central part of the shear zone and the shear-related deformation zone outside the central part of the shear zone.  相似文献   

10.
Internal regions of orogenic belts may be characterized by an alignment of fold axes with mineral elongation lineations. This relationship is commonly interpreted as representing progressive tightening and rotation towards the shear direction of early buckle folds, the hinges of which were initiated orthogonal to this direction. Detailed structural analysis of lower amphibolite facies Dalradian metasediments of the Ballybofey (fold) Nappe, north-west Ireland, shows that an intense S3 schistosity is developed axial planar to mesoscopic and minor F3 folds. In areas of low D3 strain, F3 fold axes plunge gently towards the north-east, whereas in regions of greater strain plunges are towards the south-east subparallel to the constant mineral lineation. Minor folds which initiated at angles of 70–80° from the mineral lineation subsequently rotated towards the shear direction in a consistent clockwise sense. Progressive and variable non-coaxial deformation oblique to the original mean F3 orientation has resulted in a unimodal distribution pattern of fold axes. Analysis of the angular rotation of fold axes enables estimates of the bulk shear strain to be evaluated and models of progressive deformation to be assessed.  相似文献   

11.
The predominantly migmatitic Palaeoproterozoic Uusimaa belt preserves early lower-grade Svecofennian structures in the Orijärvi area in SW Finland. This study aims at explaining the deformational history responsible for its preservation and also at defining the age of the early Svecofennian deformation. Detailed structural analysis reveals that the preservation was enabled by polyphase strain partitioning, which initiated during the early Svecofennian D2 deformation, 1875 Ma ago, as revealed by ion microprobe U–Pb data on zircons from granodioritic and intermediate syn-D2 intrusive dykes. The D2 structures were low-strain upright folds at high crustal levels and sub-horizontal high-strain folds at deeper crustal levels. The sub-horizontal D2 structures were refolded into upright folds during the subsequent late Svecofennian D3 deformation, whereas the upright D2 structures behaved as almost rigid blocks that caused strain partitioning into high-strain zones along the block margins. This accounts for the low cumulative strain in specific parts of the Orijärvi area. Further strain partitioning during D4 caused reverse dip-slip movements along regional-scale shear zones. Crustal depth controlled the metamorphic grade during D2, when local migmatisation took place at deep crustal levels. Later metamorphic overprint during D3 deformation is evident from post-D2 growth of sillimanite and a second generation of andalusite.Similarities in the structural patterns between the Orijärvi area and the Tampere-Vammala area (100 km to the north) suggest that irrespective of the age of the later overprint, subsequent deformation was localised along the margins of the early formed upright domains, while the low-grade rocks within the domains were preserved.  相似文献   

12.
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   

13.
大别-苏鲁造山带不同岩片(块)经历了不同的褶皱变形.榴辉岩块(或透镜体)和硬玉石英岩片经历了高压-超高压背景下的两幕褶皱变形之后,在区域性第一幕变形期间主要发生透镜化为主,后期与围岩共同经历紧闭同斜第二幕褶皱.而其它岩片主要经历了现今野外可见的区域性三幕褶皱,其中区域性第一幕褶皱为片内残留褶皱,在斜长角闪岩透镜体中多见,宏观规律不明.区域性第二幕褶皱在露头尺度多见,轴面为折劈理,局部强烈置换成片理化带(复合片理或第二期片理),恢复第三幕褶皱改造作用后,揭示出各种岩片中的各级尺度的第二幕褶皱都为轴面北西倾南东倒、轴迹走向为NNE向的紧闭不对称褶皱,不对称性一致反映其指向与各种岩片向南东的逆冲运动有关.第三幕褶皱为以片理或折劈理为变形面的宽缓褶皱,轴迹走向NWW,枢纽向西倾伏.韧性剪切带为非透入性构造,分早晚两期,早期为韧性逆冲,新县穹隆以南,运动学标志指示向北逆冲,错切第二幕褶皱,结合新县穹隆北部向南的逆冲特征,反映这些韧性逆冲断层多数为第二幕大型褶皱翼部的次级逆冲断层;晚期为韧性滑脱带,其发育局限于几个岩性差异较大的接触带,带内伸展型折劈理发育,并对挤压构造样式有重要的改造作用.华北克拉通东部地块是华北克拉通的重要组成,其盖层古生界和三叠系在印支运动期间经历了一幕宽缓褶皱作用,其轴迹方向主体也为NWW向.这一褶皱构造明显在变形时间、变形样式和展布方向上都和大别-苏鲁造山带中的第三幕褶皱非常一致,说明它们具有动力学上的必然联系.同时,研究表明在华北克拉通东部地块中没有经历大别-苏鲁造山带中区域性第一、第二幕褶皱变形的记录,故本文认为印支期这两幕变形主要发生在华北板块东南缘的边界上,并没有波及到板内,而且从东向西高压-超高压岩石剥露具有穿时性.只有当华北板块和华南板块在第二幕变形之后构成了统一块体后,第三幕变形才波及华北板内.  相似文献   

14.
The relationship between deformation and dehydration has been investigated in Hercynian regionally metamorphosed rocks exposed on NW Sardinia. Two episodes of prograde mineral growth (M1 & M2) involving dehydration are recognized: growth of chlorite/phengite porphyroblasts at anchizone metamorphic conditions, contemporaneous with the first phase of deformation, D1, and growth of biotite from chlorite and phengite coincident with the second phase of deformation, D2. Deformation during both episodes of dehydration is characterized by penetrative axial planar foliations defined by well-developed phyllosilicate preferred orientations quantified by XRD textural goniometry, tight to isoclinal similar folds (interlimb angles <40°), and mineral-filled veins (hydrofractures) orientated parallel to axial planar foliations, that formed contemporaneously with the development of the penetrative foliations. No prograde mineral growth occurred during D2 at chlorite-zone conditions. D2 deformation in the absence of dehydration is characterized by non-penetrative crenulation cleavages, poorly developed phyllosilicate preferred orientations, relatively open (interlimb angles >40°), low-strain similar folds and minor brittle deformation. Systematic variations in macrofold interlimb angles, with respect to the timing of mineral growth, indicate that enhanced shortening (c. 80%) occurred during dehydration. Microfabrics show that the onset of dehydration is associated with the transition from a crenulation cleavage to a penetrative foliation. The presence of axial planar hydrofractures that formed coevally with dehydration and fabric development requires that supralithostatic fluid pressures and low differential stresses (<c. 20 MPa) accompanied dehydration. These features demonstrate a connection between the timing of dehydration and the style of deformation.  相似文献   

15.
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   

16.
The ENE-plunging macroscopic folds, traced by calc gneiss interbanded with marble and sillimanite schist within the Peninsular Gneiss around Suganapuram in the ‘Palghat gap’ in southern India, represent structures of the second generation (D2). They have folded the axial planes of a set of D1 isoclinal folds on stratification coaxially, so that the mesoscopic D1 folds range from reclined in the hinge zones, through inclined to upright in the limb zones of the D2 folds. Orthogonal relation between stratification and axial planar cleavage, and ‘M’ shaped folds on layering locate the hinge zones of the D1 folds, whereas folds on axial planar cleavage with ‘M’ shaped folds are the sites of the D2 fold hinges. Extreme variation in the shapes of the isoclinal D1 folds from class 1B through class 1C to nearly class 2 of Ramsay is a consequence of buckling followed by flattening on layers of widely varying viscosity contrast. The large ENE-trending structures in this supracrustal belt within the Peninsular Gneiss in the ‘Palghat gap’ could not have evolved by reorientation of NS-trending structures of the Dharwar tectonic province to the north by movement along the Moyar-Bhavani shear zone which marks the boundary between the two provinces. This is because the Moyar and Bhavani faults are steep dipping reverse faults with dominant dip-slip component. Deceased  相似文献   

17.
The Gran Paradiso nappe of the northwestern Alps mostly consists of augen gneisses derived from the Alpine deformation of Permian granitoids. The regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top-to-west sense of shear. The granitoid protolith is preserved in the kilometre-scale low-strain domain of the Piantonetto Valley and mainly consists of a porphyritic metagranite including joints, leucocratic dykes and biotite-rich schlieren. In this low-strain domain, the Alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite. The shear zones mostly dip towards S–SE in a shallow (shear zones 1) to steep inclination (shear zones 2). The shear zones show typical features that can be explained by reactivation of pre-existing joints and planar compositional heterogeneities. Palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening. The kinematics of individual shear zones depends on the orientation of the original heterogeneities (acting as nucleation planes) and by partitioning of strain components at the kilometre-scale with concentration of the flattening component to the Piantonetto low-strain domain. The strain geometry and the kinematics of individual shear zones within Piantonetto are not directly connected to the top-to-west sense of tectonic transport observed elsewhere in the Gran Paradiso nappe. However, the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the Gran Paradiso massif. We conclude that the shear zone network of the Piantonetto Valley is representative of the incipient stages of ductile deformation of a granite nappe. Even if its architecture is determined by the arrangement of pre-existing structural and compositional heterogeneities, aspects of the large-scale bulk strain can be derived from this local shear zone pattern.  相似文献   

18.
The structure of the Ciudad Rodrigo area (Iberian Massif, Central Iberian Zone) has been revisited in order to integrate new geological data with recent models of the evolution of the Iberian Massif. Detailed mapping of fold structures along with a compilation of field data have been used to constrain the geometry and relative timing of ductile deformation events in this section of the hinterland of the Variscan belt. The structural evolution shows, in the first place, the development of a regional train of overturned folds with associated axial planar foliation (D1). Towards the lower structural levels, the deflection of the fold limbs and a subhorizontal crenulation cleavage depict the upper structural boundary of a superimposed low angle shear zone (D2), which extends at least to the deepest parts of the basement exposed in the study area. The amplification and rotation of D1 folds about a horizontal axis also occurred within this shear zone. The flat-lying character of the D2 structures accounts for the attenuation of the previously thickened crust, which developed following gravity gradients during thermal re-equilibration. Subsequent deformation led to the formation of two orthogonal sets of upright folds (D3), representing a new shift between crustal thinning and crustal thickening in the region.  相似文献   

19.
Strongly deformed volcaniclastic metasediments and ophiolitic slices hosting the Sukari gold mineralization display evidence of a complex structural evolution involving three main ductile deformational events (D1–D3). D1 produced ENE-trending folds associated with NNW-propagating thrust slices and intrusion of the Sukari granite (689 ± 3 Ma). D2 formed a moderately to steeply dipping, NNW-trending S2 foliation curved to NE and developed arcuate structure constituting the Kurdeman shear zone (≤ 595 Ma) and East Sukari imbricate thrust belt. Major NE-trending F2 folds, NW-dipping high-angle thrusts, shallow and steeply plunging mineral lineation and shear indicators recorded both subhorizontal and subvertical transport direction during D2. D3 (560–540 Ma) formed NNE-trending S3 crenulation cleavage, tight F3 folds, Sukari Thrust and West Sukari imbricate thrust. The system of NW-trending sinistral Kurdeman shear zone (lateral ramps and tear faults) and imbricate thrusts (frontal ramps) forming the actuate structure developed during SE-directed thrusting, whereas the prevailing pattern of NNE-trending dextral Sukari shear zone and imbricate thrusts forming Sukari thrust duplex developed during NE-directed tectonic shearing. Sukari granite intruded in different pluses between 689 and 540 Ma and associated with at least four phases of quartz veins with different geometry and orientation. Structural analysis of the shear fabrics indicates that the geometry of the mineralized quartz veins and alteration patterns are controlled by the regional NNW- and NE-trending conjugate zones of transpression. Gold-bearing quartz veins are located within NNW-oriented sinistral shear zones in Kurdeman gold mine area, within steeply dipping NW- and SE dipping thrusts and NE- and NS-oriented dextral and sinistral shear zones around Sukari mine area, and along E-dipping backthrusts and NW-SE and N-S fractures in Sukari granite. The high grade of gold mineralization in Sukari is mainly controlled by SE-dipping back-thrusts branched from the major NW-dipping Sukari Thrust. The gold mineralization in Sukari gold mine and neighboring areas in the Central Eastern Desert of Egypt is mainly controlled by the conjugate shear zones of the Najd Fault System and related to E-W directed shortening associated with oblique convergence between East and West Gondwana.  相似文献   

20.
本文以辽宁省鞍山附近元古宙千枚岩和片岩中的折劈S2为论述的基础。按照简单剪切原理计算出发育折劈的岩石中的γ(剪应变)值。通过γ等值线图及断面图、TM/TQF-α相关图和变形标志(石英)形态比的研究,初步认为,折劈岩石中矿物组成、结构、微构造和α角等的明显“分异”现象,主要由剪应变和伴随发生的物质迁移所造成。有限应变状态的特点是:剪应变高的带(M)和剪应变较低的带(QF)相间排列。相邻带剪应变差异控制着扩散物质迁移机制,对微构造(如微褶皱)的生成,有重要作用。折劈生成于T低于500℃,P 约为5kb 左右的绿片岩相变质环境,它标志着地壳处于区域性抬升状态,相继产生的共轭折劈和膝折带(属于 D,构造),则表明已抬升到足以引起岩石总体体积扩张的高度。  相似文献   

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