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1.
Marble, calc-silicate rock, quartzite and mica schist of Precambrian age in the ‘main Raialo syncline’ in the Udaipur district of central Rajasthan, India, have been affected by folding of four main generations (F1–F4), the first two of which are seen in the scale of map to microsection. The very tight to isoclinal F1 folds with long limbs and thickened hinges are generally reclined or inclined, and plunge gently castward or westward where least reoriented. The axial planes of the F1 folds have been involved in upright warps on east-west axes (F1′), nearly coaxial with the F1 folds, in some sectors. These folds have been overprinted by upright F2 folding of varying tightness with the axial planes striking north to northeast, resulting in interference patterns of different types in all scales. A penetrative axial plane foliation related to F1 folding and a crenulation cleavage parallel to the F2 axial pianes are seen in the micaceous rocks. Two sets of conjugate folds and kink bands of smail scale have been superimposed on the F1–F2 folds in thinly foliated rocks. The first of these sets (F3) has its conjugate axial planes dipping gently northeast and southwest, whereas the paired axial planes of the later set (F4) are vertical with north-northwest and east-west strikes.  相似文献   

2.
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   

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 Peninsular Gneiss around Gorur in the Dharwar craton, reported to be one of the oldest gneisses, shows nealy E-W striking gneissosity parallel to the axial planes of a set of isoclinal folds (DhF1). These have been over printed by near-coaxial open folding (DhF12) and non-coaxial upright folding on almost N-S trend (DhF2). This structural sequence is remarkably similar to that in the Holenarasipur schist belt bordering the gneisses as well as in the surpracrustal enclaves within the gneisses, suggesting that the Peninsular Gneiss has evolved by migmatization synkinematically with DhF1 deformation. The Gorur gneisses are high silica, low alumina trondhjemites enriched in REE (up to 100 times chondrite), with less fractionated REE patterns (CeN/YbN < 7) and consistently negative Eu anomalies (Eu/Eu* = 0.5 to 0.7). A whole rock Rb-Sr isochron of eight trondhjemitic gneisses sampled from two adjacent quarries yields an age of 3204 ± 30 Ma with Sr i of 0.7011 ± 6 (2σ). These are marginally different from the results of Beckinsale and coworkers (3315 ± 54 Ma, Sr i = 0.7006 ± 3) based on a much wider sampling. Our results indicate that the precursors of Gorur gneisses had a short crustal residence history of less than a 100 Ma.  相似文献   

5.
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   

6.
In the Kolar Schist Belt well-preserved small-scale diastrophic structures suggest four phases of folding (F1 — F4). The near coaxial F1 andF 2folds are both isoclinal with long-drawn out limbs and sharp hinges. The axial planes of bothF 1andF 2folds are subvertical with N-S strikes; these control the linear outcrop pattern of the Schist belt. The later folds (F 3and F4) are important in small-to-intermediate scales only and are accommodation structures formed during the relaxation period of the early folding episodes. Mesoscopic shear zones, post-F2 but pre-F3 in age, are present in all the rock types in this area. The F1 and F2 folds and the mesoscopic shear zones were formed during a continuous E-W subhorizontal compression. Available geochemical and isotopic data show that the Kolar Schist Belt with ensimatic setting is bounded by two granitic terrains of contrasting evolutionary histories. This, together with E-W subhorizontal compression over a protracted period of time, strengthens the recent suggestions that the Kolar Schist Belt represents a suture. This belt then marks the site of a continent-continent collision event of late Archaean-early Proterozoic age.  相似文献   

7.
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   

8.
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   

9.
The earliest decipherable record of the Dharwar tectonic province is left in the 3.3 Ga old gneissic pebbles in some conglomerates of the Dharwar Group, in addition to the 3.3–3.4 Ga old gneisses in some areas. A sialic crust as the basement for Dharwar sedimentation is also indicated by the presence of quartz schists and quartzites throughout the Dharwar succession. Clean quartzites and orthoquartzite-carbonate association in the lower part of the Dharwar sequence point to relatively stable platform and shelf conditions. This is succeeded by sedimentation in a rapidly subsiding trough as indicated by the turbidite-volcanic rock association. Although conglomerates in some places point to an erosional surface at the contact between the gneisses and the Dharwar supracrustal rocks, extensive remobilization of the basement during the deformation of the cover rocks has largely blurred this interface. This has also resulted in accordant style and sequence of structures in the basement and cover rocks in a major part of the Dharwar tectonic province. Isoclinal folds with attendant axial planar schistosity, coaxial open folds, followed in turn by non-coaxial upright folds on axial planes striking nearly N-S, are decipherable both in the “basement” gneisses and the schistose cover rocks. The imprint of this sequence of superposed deformation is registered in some of the charnockitic terranes also, particularly in the Biligirirangan Hills, Shivasamudram and Arakalgud areas. The Closepet Granite, with alignment of feldspar megacrysts parallel to the axial planes of the latest folds in the adjacent schistose rocks, together with discrete veins of Closepet Granite affinity emplaced parallel to the axial planes of late folds in the Peninsular Gneiss enclaves, suggest that this granite is late-tectonic with reference to the last deformation in the Dharwar tectonic province. Enclaves of tonalite and migmatized amphibolite a few metres across, with a fabric athwart to and overprinted by the earliest structures traceable in the supracrustal rocks as well as in a major part of the Peninsular Gneiss, point to at least one deformation, an episode of migmatization and one metamorphic event preceding the first folding in the Dharwar sequence. This record of pre-Dharwar deformation and metamorphism is corroborated also by the pebbles of gneisses and schists in the conglomerates of the Dharwar Group. Volcanic rocks within the Dharwar succession as well as some of the components of the Peninsular Gneiss give ages of about 3.0 Ga. A still younger age of about 2.6 Ga is recorded in some volcanic rocks of the Dharwar sequence, a part of the Peninsular Gneiss, Closepet Granite and some charnockites. These, together with the 3.3 Ga old gneisses and 3.4 Ga old ages of zircons in some charnockites, furnish evidence for three major thermal events during the 700 million year history of the Archaean Dharwar tectonic province.  相似文献   

10.
The Betsimisaraka Suture (B.S.) of Madagascar is an important structural zone defining the collision between Eastern and Western Gondwana. It is represented by highly deformed high-grade metamorphic rocks with mineral assemblages typical of ophiolitic material, including chromitite and nickel bearing rocks consistent with a suture zone setting. Analysis of satellite imagery coupled with field investigations has helped to elucidate the structure and evolution of the B.S. Digital image processing of Landsat ETM+ data was integrated with Synthetic Aperture Radar (SAR) to reduce the effects of the dense vegetation cover. Enhanced false color composite (7-4-2 and 4-2-3), single Landsat bands and band ratio composites including 2/7-1/7-2/5, 5/3-5/1-7/3, 5/1-7/1-4/1 and 5/7-5/1-5/4 × 3/4 (in RGB) improve the lithologic contrast, reduce the effects of topography and enhance the structural lineaments. Ductile deformation deduced from structural features mapped on Landsat enhanced images indicates three generations of folding (F1, F2, and F3) coupled with shearing: (1) F1 folds with NE striking axial surfaces; (2) F2 related with N–S striking axial surfaces, (3) and F3 associated with ENE–WSW axial surfaces, indicating NNW and SSE contractional strain similar to the deformation in the southern B.S. Mapping these structures enables three types of shearing to be delineated: (1) NW–SE dextral shearing as seen in the Befandriana region; (2) NW–SE sinistral shearing defined by sigmoidal bodies in Mandritsara and Ankijanilava-Marotandrano regions, (3) and NE–SW striking dextral shears recorded in the Lake Alaotra region. Several faults, joints and fractures represent brittle deformation events. Lineaments analyzed within the B.S. are divisible into two groups of brittle structures: (1) N–S trending lineaments correlated with the Gondwanan collision events and (2) much younger NE and NW trending lineaments that are mainly found in the Antananarivo block. The latter may represent an active tectonic event in the central plateau that bounds the western part of the B.S.  相似文献   

11.
Multiple deformation in all the Precambrian metamorphic-migmatitic rocks has been reported from Rajasthan during the last three decades. But, whereas the Aravalli Group and the Banded Gneissic Complex show similarity in the style and sequence of structures in all their details, the rocks of the Delhi Group trace a partly independent trend. Isoclinal folds of the first generation (AF1) in the rocks of the Aravalli Group had gentle westerly plunge prior to later deformations. These folds show reclined, inclined, and upright attitude as a result of coaxial upright folding (AFla). Superposition of upright folds (AF2) of varying tightness, with axial plane striking N to NNE, has resulted in interference patterns of diverse types in the scale of maps, and deformation of earlier planar and linear structures in the scale of hand specimens. The structures of the third generation (AF3) are either open recumbent folds or reclined conjugate folds with axial planes dipping gently towards NE or SW. Structures of the last phase are upright conjugate folds (AF4) with axial planes striking NNE-SSW and E-W. The Banded Gneissic Complex (BGC) underlies the Aravalli Group with a conglomerate horizon at the contact, especially in southern Rajasthan. But, for a major part of central and southern Rajasthan, migmatites representing BGC show a structural style and sequence identical with those in the Aravalli Group. Migmatization, broadly synkinematic with the AF1 folding, suggests extensive remobilization of the basement. Very rare relict fabric athwart to and overprinted by structures of AF, generation provide tangible evidence for a basement. Although the structures of later phases in the rocks of the Delhi Group (DF3 and DF4) match with the late-phase structures in the Aravalli Group (AF3 and AF4), there is a contrast in the structural history of the early stages in the rocks of the two groups. The folds of the first generation in the Delhi Group (DF1) were recumbent to reclined with gentle plunge towards N to NNE or S to SSW. These were followed by coaxial upright folds of varying tightness (DF2). Absence of westerly trending AF1 folds in the Delhi Group, and extreme variation in plunge of the AF2 folds in contrast with the fairly constant plunge of the DF2 folds, provide evidence for an angular unconformity between the Aravalli and the Delhi Groups. Depending on the importance of flattening attendant with and following buckling during AF2 deformation, the lineations of AF1 generation show different patterns. Where the AF1 lineations are distributed in circular cones around AF2 axes because of flexural-slip folding in layered rocks with high viscosity contrast, loci of early lineations indicate that the initial orientation of the AF1 axes were subhorizontal, trending towards N280°. The orientation of the axial planes of the earlier folds has controlled the development of the later folds. In sectors where the AF, axial planes had N-S strike and gentle dips, or E-W strike with gentle to steep dips, nearly E-W horizontal compression during AF2 deformation resulted in well-developed AF2 folds. By contrast, where the AF, axial planes were striking nearly N-S with steep dips, E-W horizontal compression resulted in tightening (flattening) of the already isoclinal AF1 folds, and probably boudinage structures in some instances, without the development of any AF2 folds. A similar situation obtains when DF4 deformation is superposed on earlier structures. Where the dominant S-planes were subhorizontal, N-S compression during DF4 deformation resulted in either chevron folds with E-W striking axial plane or conjugate folds with axial plane striking NE and NW. In zones with S-planes striking E-W and dipping steeply, the N-S compression resulted in flattening of the earlier folds without development of DF4 folds.  相似文献   

12.
The banded iron-formation in the southeastern Bababudan Hills display a macroscopic synformal bend gently plunging towards WNW. The bedding planes in smaller individual sectors show a cylindrical or conical pattern of folding. The dominant set of minor folds has WNW-ESE trending axial planes and the axes plunge towards WNW at gentle to moderate angles, though there is considerable variation in orientation of both axes and axial planes. A later set of sporadically observed folds has N-S trending axial planes. The macroscopic synformal bend within the study area forms the southeastern corner of a horseshoe shaped regional synformal fold closure which encompasses the entire Bababudan range. The minor folds are buckle folds modified to a varying extent by flattening. In some examples the quartzose layers appear to be more competent than the ferruginous layers; in others the reverse is true. The folds are frequently noncylindrical and the axes show curvature with branching and en echelon patterns. Such patterns are interpreted to be the result of complex linking of progressively growing folds whose initiation is controlled by the presence of original perturbations in the layers. Domes and basins have at places developed as a result of shortening along two perpendicular directions in a constrictional type of strain. Development of folds at different stages of progressive deformation has given rise to nonparallelism of fold axes and axial planes. The axes and axial planes of smaller folds developed on the limbs of a larger fold are often oriented oblique to those of the latter. Progressive deformation has caused rotation and bending of axial planes of earlier formed folds by those developed at later stages of the same deformational episode. Coaxial recumbent to nearly reclined fold locally encountered on the N-S limb of the macroscopic fold may belong to an earlier episode of deformation or to the early stage of the main deformation episode. The E-W to ESE-WNW strike of axial plane of the regional fold system in the Bababudan belt contrasts with the N-S to NNW-SSE strike of axial planes of the main fold system in the Chitradurga and other schist belts of Karnataka.  相似文献   

13.
Kilometer-size fold interference patterns in the Beishan Orogenic Collage (BOC) in the southernmost Altaids formed by fold superimposition in fossiliferous Permian sedimentary rocks. First-phase (F1), upright and almost north-trending folds, were refolded by E- to ENE-trending F2 folds, whose axial planes and axes are vertical or subvertical. From east to west there is a regional change in style of interference patterns from lobate–cuspate-, to crescent- to mushroom-shape. This variation is accompanied by a westward decrease in the F2 interlimb angle and related to a higher percentage of coarse-grained clastic rocks, suggesting a dependence of the F2 deformation on lithology. Axial planar slaty cleavages are well developed in F1 and poorly developed in F2 folds. The superposed folds mainly underwent flexural-slip and flexural flow folding to give rise to the lobate–cuspate pattern, and to the crescent pattern caused by flattening and flexural flow folding where the sediments were unconsolidated and enriched in fluids. The two folding events are interpreted to result from a major change in plate configuration that caused the inversion of an inter-arc basin during the final amalgamation of the BOC in the latest Permian to early to mid-Triassic. The two folding events bracketed between a maximum detrital zircon age of <273 Ma, and the youngest age of an intruded dyke at 219.0 ± 1.2 Ma suggest rapid plate reconfiguration related to final amalgamation of the Altaids orogen.  相似文献   

14.
The Chengde-Pingquan region is located in the central part of the Yanshan Orogenic Belt (YOB). At Daheishan and Pingquan in the central YOB, thrusts and folds of variable trends are displayed in 2 km-scale fold interference patterns. Detailed field mapping was conducted to decipher the geometry of these two superimposed structures. Map-view geometry and stereonet plots for outcrop-scale folds indicate that the superimposed structures form arrowhead interference pattern where NW-SE-trending F1 folds are refolded by later ENE-WSW F2 folding. After remove the effects of later faulting, restored map-views of the superimposed structures show that when the F1 folds have inclined axial surfaces but with no an overturned limb, an arrowhead interference pattern (here called modified type-2 pattern) can form. Our field data and reinterpretation of the findings of previous studies suggest that five major shortening phases have occurred in the Chengde-Pingquan region. The first two phases, which formed the superimposed folds, occurred earlier than the Late Triassic (D1) and during the Late Triassic to Early Jurassic (D2). These two phases were followed by three deformation phases that are mainly characterized by thrusting and strike-slip faulting, which strongly modified the large-scale fold interference patterns.  相似文献   

15.
An assessment of the southern Betsimisaraka Suture (B.S.) of southeastern Madagascar using remote sensing and field investigation reveals a complex deformation history. Image processing of Landsat ETM+data and JERS-I Synthetic Aperture Radar (SAR) imagery was integrated with field observations of structural geology and field petrography. The southern B.S. divides the Precambrian basement rocks of Madagascar in two parts. The western part includes Proterozoic rocks whereas the eastern part is an Archean block, named the Masora block. The southern part of the B.S. includes high-grade metamorphic rocks, recording strong deformation and has mineral deposits including chromite, nickel, and emerald, characteristic of oceanic material that is compatible with a suture zone.Large-scale structural features indicate ductile deformation including three generations of folding (F1, F2, and F3) associated with dextral shearing. The first folding event (F1) shows a succession of folds with NE striking axial planes. The second folding event (F2) mainly has north–south striking axial planes and the last event (F3) is represented by mega folds that have ENE–WSW axial plane directions and have NNW and SSE contractional strain patterns. Closure of the Mozambique Ocean between two components of Gondwana sandwiched rocks of the B.S. and formed upright folds and shortening zones which produced N–S trending lineaments. Later dextral movements followed the contraction and formed NW–SE trending lineaments and N–S trending normal faults associated with dextral strike slip faults and fractures.  相似文献   

16.
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   

17.
Detailed structural analysis of part of the Variscan southcentral Pyrenees revealed the occurrence of several deformation generations, of which the most important one, called the mainphase folding and striking WNW-ESE, seems to be the oldest. Directional analysis of structural elements related to mainphase folding (sedimentary bedding, mainphase cleavage, small-scale foldaxes and intersection lineations) shows, however, that sedimentary bedding must have been non-planar before mainphase deformation took place. This observation suggests that premainphase folding occurred as well, and indeed the areal distribution of intersection lineations in the studied area demonstrates the existence of two early Variscan fold systems. They are characterized by very open NNW-SSE and WSW-ENE folds and have subvertical axial planes and subhorizontal foldaxes. In strong contrast to mainphase folds, penetrative axial plane foliations did not develop during deformation. Pre-mainphase folds in varying orientations have been reported from many other areas in the central Variscan Pyrenees, but a reinterpretation of existing maps and other data shows that also in these cases two pre-mainphase deformation generations must be present, rather than just one as suggested in most previous work. Again, the interference pattern of the two fold systems as well as field evidence indicates that axial planes are steep and strike approximately N-S and E-W, but locally strong reorientation due to Alpine deformation (mainly thrusting) has taken place. The significance of pre-mainphase folding in the Variscan Pyrenees is discussed in the light of an overall dynamic/ kinematic model involving alternating convergent and divergent right-lateral oblique-slip movements along the north-eastern boundary of the Iberian (micro-)plate. The occurrence of pre-mainphase folds is related to
  1. the transition from divergent to convergent obliqueslip movement (NNW-SSE folds), and
  2. initial oblique convergence of the Iberian and European plates (WSW-ENE folds) prior to mainphase collision.
  相似文献   

18.
狼山地区叠布斯格岩群变形研究及其构造意义   总被引:1,自引:0,他引:1  
牛鹏飞  曲军峰  张进  张北航  赵衡 《地质学报》2019,93(8):1867-1884
阿拉善东北缘狼山地区的叠布斯格岩群(杂岩)作为阿拉善地块前寒武纪基底之一,主要出露有条痕状黑云斜长片麻岩、斜长角闪片麻岩及透镜状斜长角闪岩,夹透辉石大理岩和磁铁石英岩等。本次研究通过构造解析与填图,系统分析了狼山地区叠布斯格岩群构造变形样式、变形期次与时限。研究显示,古元古代变质杂岩叠布斯格岩群至少记录了四期变形,第一期变形主要表现为片麻岩早期面理的枢纽近E- W向褶皱变形(D1),轴面倾向NNW,应与华北克拉通统一化过程有关;第二期为近N- S向褶皱变形(D2),褶皱枢纽向NNE倾伏,古生代时期阿拉善地块与华北板块增生拼合,在阿拉善地块东缘产生近东西向挤压,在狼山地区形成枢纽近N- S向的褶皱;第三期变形为NE向巴彦乌拉山- 狼山断裂带的左行韧性走滑剪切作用(D3),中-晚三叠世扬子板块与华北板块碰撞造成的阿拉善地块相对华北板块沿巴彦乌拉山- 狼山断裂发生左行剪切运动,使早期构造发生逆时针旋转,是狼山地区一期重要的变形事件;第四期为NE- SW向紧闭褶皱(D4),褶皱轴面多倾向NW,晚侏罗世来自古太平洋的俯冲和鄂霍茨克洋的闭合产生的NW- SE向挤压,使叠布斯格岩群片麻岩及后期糜棱岩化花岗岩再次发生枢纽NE- SW向褶皱变形。  相似文献   

19.
Abstract Two varieties of charnockites are recognized in the Dharwar craton of southern India. The style and sequence of structures in one charnockite variety, and related intermediate to basic granulites, are similar to those in the supracrustal rocks of the Dharwar Supergroup and the adjacent Peninsular Gneiss. This style has isoclinal folds with long limbs and sharp hinges with an axial planar fabric in some instances. Additional evidence of flattening is provided by pinch-and-swell and boudinage structures, with basic granulites forming boudins in the more ductile charnockites/enderbites in the limbs of isoclinal folds. These folds are involved in near-coaxial upright folding resulting in the bending of the axial planes of the isoclinal folds and the associated boudins. All these structures are overprinted by non-coaxial upright folds with axial planes striking nearly N–S. The map pattern of charnockites suggests that this sequence of structures is present not only on a mesoscopic scale, but also on a macroscopic scale. Charnockites of this variety provide, in some instances, evidence of having been migmatized to give rise to hornblende–biotite gneiss and biotite gneiss, which form a part of the Peninsular Gneiss terrane.
The second variety comprises charnockite sensu stricto with an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non-coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process.  相似文献   

20.
Large-scale structures, textures and mineral assemblages in the Precambrian rocks of the Banded Gneissic Complex and the overlying Delhi Group in north-central Aravalli Mountain reveal a complex deformational-crystallization history. In the basement Gneissic Complex at least three deformational events, D0, D1 and D2, and two separate episodes of metamorphism, M1 and M2, are recognized. The supracrustal Delhi Rocks display only two phases of deformation, D1 and D2, associated with a single protracted period of metamorphism, M2.The first phase of deformation (D1) of the Delhi orogeny (1650-900 m.y.) produced large isoclinal folds that are overturned towards the southeast and have gentle plunges in NE and SW directions. The second phase of deformation (D2) gave rise to tight open folds on the limbs and axial-plane surfaces of the D1 folds. These folds generally plunge towards the N and NNW at 30°–80°. In the Basement Complex one more deformation (D0) of the Pre-Delhi orogeny (> 2000 m.y.) is recorded by the presence of reclined and recumbent folds with W to WNW trending fold axes. The D0 folds were superimposed by D1 and D2 folds during the Delhi orogeny.The three deformational events have been correlated with the crystallization periods of minerals in the rocks and a setting in time is established for this part of the Aravalli range.  相似文献   

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