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1.
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   

2.
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   

3.
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   

4.
The main conclusion of this study is that non-coaxial strain acting parallel to a flat-lying D1 spaced cleavage was responsible for the formation of the D2 spaced crenulation (shear band) cleavage in Dalradian rocks of Neoproterozoic-Lower Ordovician age in the SW Highlands, Scotland. The cm-dm-scale D2 microlithons are asymmetric; have a geometrically distinctive nose and tail; and show a thickened central portion resulting from back-rotation of the constituent D1 microlithons. The current terminology used to describe crenulation cleavages is reviewed and updated. Aided by exceptional 3D exposures, it is shown how embryonic D2 flexural-slip folds developed into a spaced cleavage comprising fold-pair domains wrapped by anastomosing cleavage seams. The bulk strain was partitioned into low-strain domains separated by zones of high non-coaxial strain. This new model provides a template for determining the sense of shear in both low-strain situations and in ductile, higher strain zones where other indicators, such as shear folds, give ambiguous results. Analogous structures include tectonic lozenges in shear zones, and flexural-slip duplexes. Disputes over the sense and direction of shear during emplacement of the Tay Nappe, and the apparently intractable conflict between minor fold asymmetry and shear sense, appear to be resolved.  相似文献   

5.
Low-grade metamorphic rocks of Paleozoic–Mesozoic age to the north of Konya, consist of two different groups. The Silurian–Lower Permian Sizma Group is composed of reefal complex metacarbonates at the base, and flyschoid metaclastics at the top. Metaigneous rocks of various compositions occur as dykes, sills, and lava flows within this group. The ?Upper Permian–Mesozoic age Ardicli Group unconformably overlies the Sizma Group and is composed of, from bottom to top, coarse metaclastics, a metaclastic–metacarbonate alternation, a thick sequence of metacarbonate, and alternating units of metachert, metacarbonates and metaclastics. Although pre-Alpine overthrusts can be recognized in the Sizma Group, intense Alpine deformation has overprinted and obliterated earlier structures. Both the Sizma and Ardicli Groups were deformed, and metamorphosed during the Alpine orogeny. Within the study area evidence for four phases of deformation and folding is found. The first phase of deformation resulted in the major Ertugrul Syncline, overturned tight to isoclinal and minor folding, and penetrative axial planar cleavage developed during the Alpine crustal shortening at the peak of metamorphism. Depending on rock type, syntectonic crystallization, rotation, and flattening of grains and pressure solution were the main deformation mechanisms. During the F2-phase, continued crustal shortening produced coaxial Type-3 refolded folds, which can generally be observed in outcrop with associated crenulation cleavage (S2). Refolding of earlier folds by the noncoaxial F3-folding event generated Type-2 interference patterns and the major Meydan Synform which is the largest map-scale structure within the study area. Phase 3 structures also include crenulation cleavage (S3) and conjugate kink folds. Further shortening during phase 4 deformation also resulted in crenulation cleavage and conjugate kink folds. According to thin section observations, phases 2–4 crenulation cleavages are mainly the result of microfolding with pressure solution and mineral growth.  相似文献   

6.
The Hastings Block is a weakly cleaved and complexly folded and faulted terrain made up of Devonian, Carboniferous and Permian sedimentary and volcanic rocks. The map pattern of bedding suggests a major boundary exists that divides the Hastings Block into northern and southern parts. Bedding north of this boundary defines an upright box-like Parrabel Anticline that plunges gently northwest. Four cleavage/fold populations are recognised namely: E–W-striking, steeply dipping cleavage S1 that is axial surface to gently to moderately E- or W-plunging; F1 folds that were re-oriented during the formation of the Parrabel Anticline with less common N–S-trending, steeply dipping cleavage S2, axial surface to gently to moderately N-plunging F2 folds; poorly developed NW–SE-striking, steeply dipping cleavage S3 axial surface to mesoscopic, mainly NW-plunging F3 folds; and finally, a weakly developed NE–SW-striking, steeply dipping S4 cleavage formed axial surface to mainly NE-plunging F4. The Parrabel Anticline is considered to have formed during the D3 deformation. The more intense development of S2 and S3 on the western margin of the Northern Hastings Block reflects increasing strain related to major shortening of the sequences adjacent to the Tablelands Complex during the Hunter–Bowen Orogeny. The pattern of multiple deformation we have recorded is inconsistent with previous suggestions that the Hastings Block is part of an S-shaped orocline folded about near vertically plunging axes.  相似文献   

7.
The supracrustal enclave within the Peninsular Gneiss in the Honakere arm of the Chitradurga-Karighatta belt comprises tremolite-chlorite schists within which occur two bands of quartzite coalescing east of Jakkanahalli(12°39′N; 76°41′E), with an amphibolite band in the core. Very tight to isoclinal mesoscopic folds on compositional bands cut across in the hinge zones by an axial planar schistosity, and the nearly orthogonal relation between compositional bands and this schistosity at the termination of the tremolite-chlorite schist band near Javanahalli, points to the presence of a hinge of a large-scale, isoclinal early fold (F1). That the map pattern, with an NNE-plunging upright antiform and a complementary synform of macroscopic scale, traces folds 'er generation (F 2),is proved by the varying attitude of both compositional bands (S0) and axial pranar schistosity (S 1), which are effectively parallel in a major part of the area. A crenulation cleavage (S 2) has developed parallel to the axial planes of theF 2 folds at places. TheF 2 folds range usually from open to rarely isoclinal style, with theF 1 andF 2 axes nearly parallel. Evidence of type 3 fold interference is also provided by the map pattern of a quartzite band in the Borikoppalu area to the north, coupled with younging directions from current bedding andS 0 -S 1 inter-relation. Although statistically theF 1 andF 2 linear structures have the same orientation, detailed studies of outcrops and hand specimens indicate that the two may make as high an angle as 90°. Usually, in these instances, theF 1 lineations are unreliable around theF 2 axes, implying that theF 2 folding was by flexural slip. In zones with very tight to almost isoclinalF 2 folding, however, buckling attendant with flattening has caused a spread of theF 1 lineations almost in a plane. Initial divergence in orientation of theF 1 lineations due to extreme flattening duringF 1 folding has also resulted in a variation in the angle between theF 1 andF 2lineations in some instances. Upright later folding (F3) with nearly E-W strike of axial planes has led to warps on schistosity, plunge reversals of theF 1 andF 2 axes, and increase in the angle between theF 1 andF 2 lineations at some places. Large-scale mapping in the Borikoppalu sector, where the supposed Sargur rocks with ENE ‘trend’ abut against the N-‘trending’ rocks of the Dharwar Supergroup, shows a continuity of rock formations and structures across the hinge of a large-scaleF 2 fold. This observation renders the notion, that there is an angular unconformity here between the rocks of the Sargur Group and the Dharwar Supergroup, untenable.  相似文献   

8.
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   

9.
The Phyllite-Quartzite (PQ) Nappe constitutes an external, allochthonous complex of the Hellenides on the island of Crete and shows a polyphase structural history. A first phase of deformation (F 1) produced recumbent isoclinal folds, a penetrative schistosity, and boudinage under high-P/low-T metamorphic conditions. Mylonite formation at the top of the PQ Nappe, below the overriding Tripolitza Nappe, further boudinage, and schistosity (S 2) represent a late tectono-metamorphic episode. Post-metamorphic small folds (F 3), lineations, and a crenulation cleavage were formed synchronously with transport of the PQ Nappe. A last phase (F 4) developed small folds, a fracture/crenulation cleavage, and large-scale folds after nappe movement. It is suggested that high-P/low-T metamorphism in the PQ rocks originated during subduction. Nappe transport of the higher, unmetamorphosed units, which were thrust over the PQ Nappe, began under waning metamorphic conditions. Subsequent transport of the PQ Nappe itself also occurred after the completion of metamorphism and after the formation of the mylonite at its top.  相似文献   

10.
The lead-zinc bearing Proterozoic rocks of Zawar, Rajasthan, show classic development of small-scale structures resulting from superposed folding and ductile shearing. The most penetrative deformation structure noted in the rocks is a schistosity (S 1) axial planar to a phase of isoclinal folding (F 1). The lineations which parallel the hinges ofF 1 folds are deformed by a set of folds (F 2) having vertical or very steep axial planes. At many places a crenulation cleavage (S 2) has developed subparallel to the axial planes ofF 2 folds, particularly in the psammopelitic rocks. The plunge and trend ofF 2 folds vary widely over the area. Deformation ofF 2 folds into hook-shaped geometry and development of another set of axial planar crenulation cleavage are the main imprints of the third generation folds (F 3) in the region. In addition to these, there are at least two other sets of cleavage planes with corresponding folds in small scales. More common among these is a set of recumbent and reclined folds (F 4), developed on steeply dipping early-formed planes. Kink bands and associated sharp-hinged folds represent the other set (F 5). Two major refolded folds are recognizable in the map pattern of the Zawar mineralised belt. The larger of the two, the Main Zawar Fold (MZF), shows a broad hook-shaped geometry. The other large-scale structure is the Zawarmala fold, lying south-west of the MZF. Both the major structures show truncation of lithological units along their respective east ‘limbs’, and extreme variation in the width of formations. The MZF is primarily the result of superimposition ofF 3 onF 2.F 1 folds are relatively smaller in scale and are recognizable in the quartzite unit which responded to deformation mainly by buckle shortening. Large-scale pinching-and-swelling that appears in the outcrop pattern seems to be a pre-F2 feature. The structural evolutionary model worked out to explain the chronology of the deformational features and the large-scale out-crop pattern envisages extreme east-west shortening following formation ofF 1 structures, resulting in the formation of tight and isoclinal antiforms (F 2) with pinched-in synforms in between. These latter zones evolved into a number of ductile shear zones (DSZs). The east-west refolding of the large-scaleF 2 isoclinal antiforms seems to be the consequence of a continuous deformation and resultant migration of folds along the DSZs. The main shear zone which wraps the Zawar folds followed a curved path. Because of the penetrative nature of theF 2 movement, the early lineations which were at high angles to the later ones (as is evident in the west of Zawarmala), became subparallel to the trend ofF 2 folding over a large part of the area. Further, the virtually coaxial nature ofF 2 andF 3 folds and the refolding ofF 3 folds by a new set of N-S folds is an indication of continuous progressive deformation.  相似文献   

11.
Structural studies of Lower Permian sequences exposed on wave‐cut platforms within the Nambucca Block, indicate that one to two ductile and two to three brittle — ductile/brittle events are recorded in the lower grade (sub‐greenschist facies) rocks; evidence for four, possibly five, ductile and at least three brittle — ductile/brittle events occurs in the higher grade (greenschist facies) rocks. Veins formed prior to the second ductile event are present in some outcrops. Further, the studies reveal a change in fold style from west‐southwest‐trending, open, south‐southeast‐verging, inclined folds (F1 0) at Grassy Head in the south, to east‐northeast‐trending, recumbent, isoclinal folds (F1 0; F2 0) at Nambucca Heads to the north, suggesting that strain increases towards the Coffs Harbour Block. A solution cleavage formed during D1 in the lower grade rocks and cleavages defined by neocrystalline white mica developed during D1 and D2 in the higher grade rocks. South‐ to south‐southwest‐directed tectonic transport and north‐south shortening operated during these earlier events. Subsequently, north‐northeast‐trending, open, upright F3 2 folds and inclined, northwest‐verging, northeast‐trending F4 2 folds developed with poorly to moderately developed axial planar, crenulation cleavage (S3 and S4) formed by solution transfer processes. These folds formed heterogeneously in S2 throughout the higher grade areas. Later northeast‐southwest shortening resulted in the formation of en échelon vein arrays and kink bands in both the lower and higher grade rocks. Shortening changed to east‐northeast‐west‐southwest during later north‐northeast to northeast, dextral, strike‐slip faulting and then to approximately northwest‐southeast during the formation of east‐southeast to southeast‐trending, strike‐slip faults. Cessation of faulting occurred prior to the emplacement of Triassic (229 Ma) granitoids. On a regional scale, S1 trends east‐west and dips moderately to the north in areas unaffected by later events. S2 has a similar trend to S1 in less‐deformed areas, but is refolded about east‐west axes during D3. S3 is folded about east‐west axes in the highest grade, multiply deformed central part of the Nambucca Block. The deformation and regional metamorphism in the Nambucca Block is believed to be the result of indenter tectonics, whereby south‐directed movement of the Coffs Harbour Block during oroclinal bending, sequentially produced the east‐west‐trending structures. The effects of the Coffs Harbour Block were greatest during D1 and D2.  相似文献   

12.
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13.
F1 macroscopic folds in the Late Palaeozoic Coffs Harbour Beds in the SE portion of the New England Fold Belt are commonly transected by cleavage. These macroscopic folds are tight to isoclinal structures, with a consistent vergence to the NE. Axial surfaces are either steeply dipping to the SW or vertical, and are typically faulted. Anomalous bedding‐cleavage relations occur where the steeply dipping cleavage intersects overturned limbs of F1 macroscopic and some F1 mesoscopic folds. Elsewhere F1 mesoscopic folds have a well developed, axial‐surface cleavage and are rarely downward facing. Cleavage is commonly strike‐divergent from axial surfaces of F1 macroscopic folds, except adjacent to the Demon Fault System, where they are parallel. These anomalous cleavage‐folds relations possibly developed during the one deformation. D1 structures are refolded by kink‐like folds that are steeply plunging. The structural style of the D1 deformation indicates that it possibly resulted from accretionary processes at a consuming plate margin.  相似文献   

14.
The Paleoproterozoic Liaohe assemblage and associated Liaoji granitoids represent the youngest basement in the Eastern Block of the North China Craton. Various structural elements and metamorphic reaction relations indicate that the Liaohe assemblage has experienced three distinct deformational events (D1 to D3) and four episodes of metamorphism (M1 to M4). The earliest greenschist facies event (M1) is recognized in undeformed or weakly deformed domains wrapped by the S1 schistosity, suggesting that M1 occurred before D1. The D1 deformation produced small, mostly meter-scale, isoclinal and recumbent folds (F1), an associated penetrative axial planar schistosity (S1), a mineral stretching lineation (L1) and regional-scale ductile shear zones. Concurrent with D1 was M2 metamorphism, which occurred before D2 and produced low- to medium-pressure amphibolite facies assemblages. Regionally divergent motion senses reflected by the asymmetric F1 folds and other sense-of-shear indicators, together with the radial distribution of the L1 lineation surrounding the Liaoji granitoids, imply that D1 represents an extensional event. The D2 deformation produced open to tight F2 folds of varying scales, S2 axial crenulation cleavages and ENE-NE-striking thrust faults, involving broadly NW–SE compression. Following D2 was M3 metamorphism that led to the formation of sillimanite and cordierite in low-pressure type rocks and kyanite in medium-pressure rocks. The last deformational event (D3) formed NW-WNW-trending folds (F3), axial planar kink bands, spaced cleavages (S3), and strike–slip and thrust faults, which deflect the earlier D1 and D2 structures. D3 occurred at a shallow crustal level and was associated with, or followed by, a greenschist facies retrograde metamorphic event (M4).The Liaohe assemblage and associated Liaoji granitoids are considered to have formed in a Paleoproterozoic rift, the late spreading of which led to the occurrence of the early extensional deformation (D1) and the M1 and M2 metamorphism, and the final closing of which was associated with the D2 and D3 phases of deformation and M3 and M4 metamorphism.  相似文献   

15.
The southeastern Lachlan Fold Belt at Batemans Bay on the New South Wales south coast is an accretionary complex with a prolonged deformation history. Early features include synsedimentary folds, mélange, disaggregated bedding and faults. Fabrics within the clast-in-matrix mélange and mudstone match those found in cores from the lower slopes of modern accretionary prisms. At the toe of the accretionary prism, the contact between the craton-derived Adaminaby Group and ocean floor deposits of the Wagonga Group is conformable. As subduction continued, the early structures were overprinted by (D1) deformation that produced meridional north – south-trending, tight to isoclinal folds (F1) and associated axial-plane cleavage (S1). This west-dipping subduction occurred in the Late Ordovician/Early Silurian but probably began much earlier. A younger regional deformation (D2) resulted in north – south-trending, open to tight folds (F2), slightly oblique to F1, and an axial-surface cleavage (S2).  相似文献   

16.
Polyphase deformation chronologies established within the mid-crustal portions of orogenic belts have classically been attributed to regional-scale ‘events’ which generate distinct structural sequences that can be directly correlated across large tracts of the orogenic belt. However, concepts of progressive deformation in which minor structures may be continually generated, amplified and redeformed within a unifying kinematic framework suggest that regional correlation of minor structures is both misguided and misleading. Detailed structural analysis of lower amphibolite facies Dalradian metasediments in north-west Ireland does, however, demonstrate that a coherent and meaningful deformation chronology can be established within the framework of individual fold nappes. Protracted deformation has resulted in the generation of a series of overprinting, secondary structures (D4–D9), which are kinematically linked to the continued structural evolution and south-east directed translation of the crustal-scale (D3) Ballybofey (fold) Nappe. Secondary (D4) crenulation axes initiated at an oblique angle to the direction of nappe transport both rotate and amplify into larger scale folds, which are subparallel to transport and demonstrate successive stages of diachronous folding. Continued nappe-related deformation induces southwards verging contractional (D5) folds, which are particularly well developed and focused into reactivated ductile (D3) thrust zones generated during the initial stages of nappe translation. Subsequent to thickening-induced ductile extension and collapse of the nappe, a return to contractional tectonics is marked by major episodes of broad, open buckle folding developed orthogonal to both the overturned limb (D7) and upper limb (D8) of the nappe. Detailed structural analysis and investigation of secondary folds and overprinting fabrics provides a valuable insight into the protracted kinematic evolution of major fold nappes.  相似文献   

17.
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   

18.
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   

19.
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   

20.
S1 cleavage in the Hawick Rocks of the Galloway area is non-axial planar, cutting obliquely across the F1 folds in a predominantly clockwise sense. Individual S1 cleavage planes within cleavage-fans in F1 folds strike clockwise, locally anti-clockwise, of axial surfaces, and the mean plane to the S1 cleavage-fans dips predominantly more steeply than the axial surface. F1 folds investigated at scattered localities in Silurian and Ordovician rocks north of the Hawick Rocks are also transected by the S1 cleavage, indicating that non-axial planar S1 cleavage is widespread in the Southern Uplands. The S1 cleavage is a composite fabric. Objects deformed within sandstones and tuffs indicate oblate strain. F1 fold plunge varies from NE to SW and fold hinges locally are markedly curvilinear. Steeply plunging and locally downward-facing F1 folds are present along the southeast margin of the Hawick Rocks. The non-axial planar S1 cleavage relationships persist in the steeply plunging F1 folds. Synchronous development of the non-axial planar S1 cleavage and the variably plunging F1 folds is proposed.  相似文献   

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