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1.
《Journal of Structural Geology》2001,23(6-7):1167-1178
S–C fabrics similar to those found in mylonites are observed in foliated cataclastic granitic rocks from the Nojima fault zone, southwest Japan. The foliated cataclastic rocks comprise cataclasite, fault breccia, gouge, and crushing-originated pseudotachylyte. The S–C fabrics observed in these cataclastic rocks involve S-surfaces defined by shape preferred orientation of biotite fragments or aggregates of quartz and feldspar fragments, and C-and C′-surfaces defined by microshears and shear bands, respectively, where fine-grained material is concentrated. Striations on the main fault plane are oriented parallel to the cataclasite lineations. A significant microstructural difference between the foliated cataclastic rocks and S–C mylonites is the absence of dynamically recrystallized grains in the foliated cataclasites. The striations, cataclastic lineations, and the S–C fabrics in the cataclastic rocks formed from the late Tertiary to the late Holocene indicate that the Nojima fault zone has moved as a dextral strike-slip fault, with a minor reverse component since it formed. S–C fabrics in cataclastic rocks provide important information on the tectonic history and are reliable kinematic indicators of the shear sense in brittle shear zones or faults.  相似文献   

2.
This quantitative microstructural study deals with textures of quartz domains within a mylonitized metapelite collected near a thrust surface corresponding to the tectonic contact between two metamorphic units, which crop out in the Aspromonte Massif, southern Calabria (Italy). The sample investigated lacks a mesoscopic stretching lineation. Therefore, quartz c-axis fabrics were investigated in two mutually orthogonal thin sections (a) parallel to the quartz rod lineation and perpendicular to the foliation (YZ plane) and (b) perpendicular to the quartz rods and perpendicular to the foliation (XZ plane); the data were generated using classical (manual measurements of quartz c-axis using U-stage) and modern methods (Computer Integrated Polarization microscopy). Both these sections show oblique foliations at ca. 40° from the main shear plane, implying that the actual X direction (stretching lineation that is absent on the mesoscopic scale) must lie between these two sections. Quartz c-axis data from the YZ section when rotated by 90° are similar with those from the XZ section. Hence, the data from the two sections are merged. These data when rotated by an angle of 50° from the direction of quartz rod lineation, gives an asymmetrical pattern indicating top-to-the-North sense of shear. This was confirmed by investigating quartz c-axis patterns in a section striking NS and perpendicular to the foliation. Based on the study it is thus concluded that this method can be used to do kinematic analysis in rocks that are devoid of stretching lineations. Apart from the above, the advantages and disadvantages of the classical and modern methods of quartz c-axis analysis are discussed.  相似文献   

3.
The Feiran–Solaf metamorphic belt consists of low-P high-T amphibolite facies, partly migmatized gneisses, schists, amphibolites and minor calc-silicate rocks of metasedimentary origin. There are also thick concordant synkinematic sheets of diorite, tonalite and granodiorite orthogneiss and foliated granite and pegmatite dykelets. The gneissosity (or schistosity) is referred to as S1, and is almost everywhere parallel to lithological layering, S0. This parallelism is not due to transposition. The gneissosity formed during an extensional tectonic event (termed D1), before folding of S0. S1 formed by coaxial pure shear flattening strain (Z normal to S0, i.e. vertical; with X and Y both extensional and lying in S1). This strain also produced chocolate tablet boudinage of some layers and S1-concordant sills and veins. S1 has a strong stretching lineation L1 with rodding characteristics. Within-plane plastic anisotropy (lower ductility along Y compared to along X) resulted in L1-parallel extensional ductile shears and melt filled cracks. Continued shortening of these veins, and back-rotation of foliations on the shears produced intrafolial F1 folds with hinges parallel to the stretching lineation. F1 fold asymmetry variations do not support previous models involving macroscopic F1 folds or syn-gneissosity compressional tectonics. The sedimentary protoliths of the Feiran–Solaf gneisses were probably deposited in a pre-800 Ma actively extending intracratonic rift characterizing an early stage of the break-up of Rodinia.  相似文献   

4.
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   

5.
In the Singhbhum Shear Zone of eastern India successive generations of folds grew in response to a progressive ductile shearing. During this deformation a mylonitic foliation was initiated and was repeatedly transposed. The majority of fold hinges were formed in an arcuate manner at low angles to the Y-axis in an E-W trending subhorizontal position and major segments of the fold hinges were then rotated towards the down-dip northerly plunging X-axis. The striping and intersection lineations were rotated in the same manner. The down-dip mylonitic lineation is a composite structure represented by rotated early lineations and newly superimposed stretching lineations. The consistent asymmetry of the folds, the angular relations between C and S surfaces and the evidence of two-dimensional boudinage indicate that the deformation was non-coaxial, but with a flattening type of strain with λ1 ⪢ λ2. The degree of non-coaxiality varied both in space and time. From the progressive development of mesoscopic structures it is concluded that the 2–3 km wide belt of ductile shear gave rise to successive anastomosing shear zones of mesoscopic scale. When a new set of shear lenses was superimposed on already sheared rocks, the preexisting foliation generally lay at a low angle to the lenses. No new folds developed where the acute angle was sympathetic to the sense of shear displacements. Where the acute angle was counter to the sense of shear, the pre-existing foliation, lying in the instantaneous shortening field, was deformed into a set of asymmetric folds.  相似文献   

6.
Lineations within mylonites exhumed in the hanging wall of New Zealand's active Alpine Fault zone have a complicated relationship to contemporary plate kinematics. The shear zone is triclinic and macroscopic object lineations are not usually parallel to the simple shear direction, despite high total simple shear strains (γ ≥ 150). This is mostly because the lineations are inherited from pre-mylonitic fabrics, and have not been rotated into parallelism with the mylonitic stretching direction (which pitches c. 44° in the fault plane). Furthermore, some lineations have been variably rotated depending on whether they are present in shear bands or microlithons, which accommodated bulk strains with different vorticities. Total strains required to obtain parallelism between the finite maximum principal stretching direction calculated from transpression models and these mylonitic lineations, are pure shear stretch, S1 ∼ 3.5; simple shear 11.7 < γ < 150. The observations and numerical models also show that linear features are not rotated much during simple shear because they initially lie within the shear plane, and that inherited fabric components may not be destroyed until very high simple shear strains have been attained.  相似文献   

7.
At Deobhog, migmatitic gneisses and granulites of the Eastern Ghats Belt are juxtaposed against a cratonic ensemble of banded augen gneiss, amphibolite and calcsilicate gneiss, intruded by late hornblende granite and dolerite. In the migmatitic gneiss unit, early isoclinal folds (syn‐D1M and D2M) are reoriented along N–S‐trending and E‐dipping shear planes (S3M), with (S1M–S3M) intersection lineations having steep to moderate plunges. The near‐peak P–T condition was syn‐D3M (≥900 °C, 9.5 kbar), as inferred from syn‐D3M Grt+Opx‐bearing leucosomes in mafic granulites, and from thermobarometry on Grt (corona)–Opx/Cpx–Pl–Qtz assemblages. The P–T values are consistent with the occurrence of Opx–Spr–Crd assemblages in spatially associated high‐Mg–Al pelites. A subsequent period of cooling followed by isothermal decompression (800–850 °C, c. 7 kbar) is documented by the formation of coronal garnet and its decomposition to Opx+Pl symplectites in mafic granulites. Hydrous fluid infiltration accompanying the retrograde changes is manifested in biotite replacing Opx in some lithologies. The cratonic banded gneiss–granite unit also documents two phases of isoclinal folding (D1B & D2B), with the L2B lineation girdle different from the lineation spread in the migmatitic gneiss unit. Calcsilicate gneiss (Hbl–Pl–Cpx–Scap–Cal) and amphibolite (Hbl–Pl±Grt±Cpx) within banded gneisses record syn‐D2B peak metamorphic conditions (c. 700 °C, 6.5 kbar), followed by cooling (to c. 500 °C) manifested in the stabilization of coronal clinozoisite–epidote. The D3B shear deformation post‐dates granite and dolerite intrusions and is characterized by top‐to‐the‐west movement along N–S‐trending, E‐dipping shear planes. Deformation mechanisms of quartz and feldspar in granites and banded gneisses and amphibole–plagioclase thermometry within shear bands in dolerites document an inverted syn‐D3B thermal gradient with temperature increasing from 350 to 550 °C in the west to ≥700 °C near the contact with the migmatitic gneiss unit. The thermal gradient is reflected in the stabilization of chlorite after hornblende in S3B shears to the west, and post‐D2B neosome segregation along D3B folds and shears to the east. The contrasting lithologies, early structures and peak metamorphic conditions in the two units indicate unconnected pre‐D3P–T –deformation histories. The shared D3 deformation in the two units, the syn‐D3 inverted thermal gradient preserved in the footwall cratonic rocks and the complementary cooling and hydration of the hanging wall granulites across the contact are attributed to westward thrusting of ‘hot’ Eastern Ghats granulites on ‘cool’ cratonic crust. It is suggested that the Eastern Ghats migmatitic gneiss unit is not a reworked part of the craton, but a para‐autochthonous/allochthonous unit emplaced on and amalgamated to the craton.  相似文献   

8.
Geometrical relations between quartz C-axis fabrics, textures, microstructures and macroscopic structural elements (foliation, lineation, folds…) in mylonitic shear zones suggest that the C-axis fabric mostly reflects the late-stage deformation history. Three examples of mylonitic thrust zones are presented: the Eastern Alps, where the direction of shearing inferred from the quartz fabric results from a late deformation oblique to the overall thrusting; the Caledonides nappes and the Himalayan Main Central Thrust zone, where, through a similar reasoning, the fabrics would also reflect late strain increments though the direction of shearing deduced from quartz fabric remains parallel to the overall thrusting direction. Hence, the sense of shear and the shear strain component deduced from the orientation of C-axis girdles relative to the finite strain ellipsoid axes are not simply related nor representative of the entire deformation history.  相似文献   

9.
The patterns of deformed early lineations (L1) over later folds (F2) can be classified into several morphological types depending on the nature of variation of L1 F2 over the folds. The field relations indicate that the folds under consideration are neither shear folds nor parallel folds modified by flattening. The lineation patterns are therefore interpreted in terms of an empirical model of simultaneous buckling and flattening in which it is assumed that (i) the central surface of the folded layer remains a sine curve in transverse profile, (ii) the ratio of rates of buckle shortening to homogeneous strain is proportional to sin 2a, with a as the dip angle and (iii) the progressive deformation is coaxial with the Z-axis of bulk strain parallel to the planar segments of the early folds. The model gives an insight into the relative importance of different physical factors which control the development of dissimilar lineation patterns. Not all lineation patterns are explicable by this simplified model. Thus complex patterns with variable L1 F2 along the fold axis may develop by a progressive rotation of the geometrically defined fold hinge through successive material lines. The theoretical results have been applied to interpret the lineation patterns in Central Rajasthan, India. It is concluded that L1 was initially very close to the E-ESE trending subhorizontal Z-axis of bulk deformation during F2-folding and that the X-axis was subhorizontal or gently plunging with a N-NNE trend.  相似文献   

10.
Progressive ductile shearing in the Phulad Shear Zone of Rajasthan, India has produced a complex history of folding, with development of planar, non-planar and refolded sheath folds. There are three generations of reclined folds, F1, F2 and F3, with a striping lineation (L1) parallel to the hinge lines of F1. The planar sheath folds of F1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices. L1 swerves harmoniously with the curving of F1 hinge line. There is a strong down-dip mineral lineation parallel to the striping lineation in most places, but intersecting it at apices of first generation sheath folds. Both the striping and the mineral lineation are deformed in U-patterns over the hinges of reclined F2 and F3. Folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non-planar sheaths. The reclined folds of all the three generations were deformed by a group of subhorizontal folds. Each generation of fold initially grew with the hinge line at a very low angle with the Y-axis of bulk non-coaxial strain and was subsequently rotated towards the down-dip direction of maximum stretching. The patterns of deformed lineations indicate that the stretching along the X-direction was extremely large, much in excess of 6000 percent.  相似文献   

11.
Deformation features on pebbles of the Alpine Molasse Basin are most clearly developed in carbonate components. Ductile distortion is small; most of the pebbles moved against each other to produce solution pits and slip-lineations on the pebble surfaces. The complete lineation field has a triaxial geometry. From a compressional axis of divergence with maximum solution, fields of diverging lineations extend to meet at a plane of convergence. Their ends bend away from an intermediate axis towards an (extensional) axis of convergence. The strain-symmetry is pure shear for orthogonal lineation-field axes, uniaxial compression and extension representing special cases. The angle α between the divergent and the convergent axes decreases from 90 to 0° with the transition from pure to simple shear. For Molasse pebbles α angles between 90 and 60° were usually observed. Regional compression developed perpendicular to the Alpine structures and parallel to bedding, with increasing deviations near the Alpine border. Zones are recorded of vertical and horizontal extension and of compression perpendicular to bedding and horizontal extension perpendicular to the Rhinegraben existing at the northern border of the Molasse Basin.  相似文献   

12.
Mubarak shear belt provides an opportunity to investigate quantitative finite strain (Rs), proportions of pure shear and simple shear components, sense of shear indicators, subhorizontal to steeply plunging mineral lineations, in a dextral transpressional zone. The structural style of the Mubarak shear belt is consistent with dextral transpression within the Central Eastern Desert where dextral and reverse shear have developed simultaneously with the regional foliation. The high strain zone of the Mubarak shear belt is characterized by steeply dipping foliation with sub-horizontal stretching lineation (simple shear) surrounded by thrust imbrications with slightly plunging stretching lineations. Strain estimates from the Mubarak shear belt are used to determine how pure and simple shear components of deformation are partitioned. The axial ratios in XZ sections range from 1.16 to 2.33 with the maximum stretch, S X , ranges from 1.06 to 1.48. The minimum stretch, S Z , ranges from 0.65 to 0.92 indicating a moderate variation in vertical shortening. Volcaniclastic metasediments and metagabbros were subjected to prograde low-grade regional metamorphism in the range of greenschist to lower amphibolite facies (450–650°C at 2–4 kbar). Medium pressure (6–8 kbar at 530°C) was estimated from the high strain zone within the dextral strike-slip shear zones. Retrograde metamorphism occurred at a temperature range of 250–280°C. There is a trend towards decreasing the ratio of 100Mg/(Mg + Fetot + Mn) away from the high strain zone of the Mubarak shear belt. Integrated strain and temperature estimates indicate that the simple shear (non-coaxial) components of deformation played a significant role in formation and exhumation of the Mubarak shear belt during the accumulation of finite strain and consequently during progressive transpression and thrusting.  相似文献   

13.
English summary Near the eastern base of the Taconic Range, in extreme southwestern Vermont, a complex of chlorite slate is exposed in the position of the floor of a thrust along which a mass of dolomite-limestone has been pushed from the east against, and over, the slate. In addition to the common structure, exhibiting westward-overturned open folds, with slip cleavage dipping eastward, about parallel with the axial pianos, the slate displays a number of subsidiary shears or thrust zones having the same orientation as the principal thrust. In these zones, a strong lineation as well as axes of small folds plunge E—SE, parallel with the direction of propagation of the thrust blocks. The origin of the lineation and lamination is believed to be identical with that of corresponding structures in rolled steel and glass.However, the formation of folds with axes parallel to the direction of thrust requires an additional shear stress acting perpendicularly to the direction of thrusting. The inhomogeneous composition, strength, and mobility of the flooring rocks are pointed out, and it is suggested that unequal rates of yielding of local rock masses below the thrust block generated these supplementary stresses, producing slight movements of small masses sideways. That this is a reasonable explantation is shown by experiments on salt dome structure byEscher andKuenen, in which also axes of folds and lineation parallel with the direction of maximum forward propagation were produced.  相似文献   

14.
Abstract

Large structures, lineations, foliations and sense of shear criteria are examined on the scale of the whole Ibero-Armorican Arc. Four sections (Galicia, Brittany-Vendée, Limousin and Eastern Massif Central) exemplify the major thrust sheets observed around the Arc. Stretching lineations are contemporaneous with the siluro-devonian metamorphism and are either transverse, oblique or parallel to the collision zone. A kinematic analysis shows that these lineations have resulted from a dominanüy transverse shear deformation which was followed by, or combined with, a longitudinal shear direction. On the scale of the entire Arc, this variation in the shear direction is interpreted as resulting from an early head on thrusting relative movement evolving to large scale movements parallel to the plate boundaries. Experiments with sand-silicone models support a model which generates the Arc by interaction between a transform sinistral direction, and a converning zone at a high angle to the transform direction.  相似文献   

15.
Abstract

The schists and gneisses of the Kanmantoo Group in the eastern Mt. Lofty Ranges show a well marked foliation and lineation. The foliation seen in the field is usually parallel to the bedding. The micas have a preferred orientation parallel to the lineation, resulting in girdles or partial girdles in the fabric diagrams. Quartz does not appear to have any preferred orientation. The lineations plunge to the S.S.E. or N.N.W., the mean plunge being about 20° to the S.S.E. This agrees with the plunge of the fold axes measured in the field and with the plunge of major structures deduced from field mapping. The area is thus one in which all the lineations are “b” lineations.  相似文献   

16.
Permian volcanic sediments at Bluff have been strained and thermally metamorphosed by Permian intrusives to metasediments of hornblende—hornfels facies. Quartz, which crystallised as a secondary mineral during metamorphism, has an unusual preferred orientation with c-axes either forming paired maxima in the plane containing the lineation (=maximum principal strain axis = direction of extension) and the perpendicular to schistosity (=minimum principal strain axis = shortening direction) or a broad maximum parallel to the lineation; the paired maxima are approximately 30° either side of the lineation. Some quartz grains are markedly elongate parallel to the lineation, and according to hypotheses of preferred orientation involving crystal plasticity, there should be some correlation between the shape of such grains and their c-axis orientations. Grain-size and shape analysis of Bluff quartz demonstrate that no such correlation exists; the analyses show that the preferred orientation results from oriented nucleation in the residual stress field immediately following the bulk straining of the rocks, with the distribution of c-axes as predicted by Kamb's hypothesis (1959). The time relationships of rock deformation, thermal metamorphism, and nucleation and growth of quartz are discussed.  相似文献   

17.
In some metamorphic terrains, lineations in folded surfaces are coaxial to the folds at their hinges, but show a systematic dispersion on the limbs. A simple theoretical model is presented, based on two assumptions: (1) the layering is folded according to two idealised models, “ideal compression folding” and “ideal shear folding”, which assume that the rock material is homogeneous and the layering passive; (2) the lineation is a manifestation of the total product of the pre-folding and folding strains. In an ideal compression fold, only apparent lineations can be dispersed away from the fold-axial trend; in an ideal shear fold, however, both real and apparent lineations are dispersed in a similar way, the degree of similarity depending on the X/Y ratio of the pre-fold strain. The lineation loci of the two models are sufficiently distinct for them to be used, together with other features of the fabric, to distinguish between folds produced by dominantly vertical movements, and those produced by dominantly horizontal movements.  相似文献   

18.
At Glenrock, near the southern end of the Peel Fault System, two fault zones are delineated by mélanges in which serpentinite is the main rock type.Protogranular and mylonitic textures are present in relicts of the parent peridotite and in blocks of massive pseudomorphic serpentinite that are surrounded by schistose serpentinite. In schistose serpentinite, the earliest foliation (S1) is defined, microscopically, by the parallel alignment of platy and fibrous serpentine minerals (lizardite and chrysotile) and by trains of magnetite and flattened serpentine pseudomorphs after olivine and pyroxene. It is considered that the schistosity formed perpendicular to the direction of maximum shortening, under conditions in which lizardite and chrysotile were ductile, but antigorite was not, by breakdown of pre-existing serpentine minerals and new growth of lizardite and chrysotile.Post-s1 foliations (S2andS3) superficially resemble crenulation cleavages in the field but, microscopically, show evidence of shear displacement and are referred to as microshear sets. They probably originated in the ductile-brittle transitional field of serpentine behaviour (Raleigh and Paterson, 1965).  相似文献   

19.
The E-W running Salem-Attur shear zone demarcates the tectonic boundary between Archaean Dharwar Craton in the north and Proterozoic Southern granulite terrane in the south. This study reveals that the shear zone is a low angle thrust. The thrust zone is around 10 m thick and it merges with the main shear zone along the strike. The thrust is developed on charnockite near Odyarpatti, which is retrograded into schists. Further, it is marked by gently dipping mylonitic foliation and subhorizontal lineation. The S-C fabric, mantled porphyroclasts and intragranular faults indicate northeasterly slip along the thrust. Recumbent shear folds SF1 are developed within the thrust zone. The thrust has been folded by late stage F2 fold which has brought variation in the orientation of the mylonitic foliation from subhorizontal to vertical attitude; the mylonitic lineations have been rotated to subvertical orientation also. Additionally, the F2 crenulations and shear cleavages and intersection lineations are superimposed on the mylonitic fabric. Thrusting along the Salem-Attur shear zone is probably the cause for upliftment of the charnockites to the upper crust. Post-upliftment stage has witnessed brittle deformation in the form of development of shear fractures in NNE-SSW and E-W directions. Pseudotachylites are emplaced along these fractures.  相似文献   

20.
The subduction and exhumation of accretionary prism metasedimentary rocks are accompanied by large‐strain ductile deformations which may be recorded in microstructures. Porphyroblast microstructures have been a key to unravel the kinematics in such deformed belts. Shape‐preferred orientation (SPO) of epidote and amphibole inclusions that define S‐shaped trails in prograde cores of plagioclase porphyroblasts were analysed from the high‐P/T Sambagawa metamorphic rocks. Inclusions are found to be elongate parallel to the [010] and [001] directions, respectively, and their long‐axis orientations define an internal foliation Si (best‐fit great circle) and lineation Li (maximum on the Si). S‐shaped inclusion trails in the orthogonal sections do not exhibit the same geometries, but rather are grouped into two types, where the foliation intersection axes (FIAs) are nearly perpendicular and parallel to Li, respectively. These two types of S‐shaped inclusion trails are seen in the sections inclined at low and high angles to the Li, respectively. However, the latter type commonly consists of composite trails, where the Si is first rotated about an FIA perpendicular to the Li (i.e. unique axis), and then about an FIA parallel to the Li. The S‐shaped inclusion trails are interpreted to have formed by the successive overgrowth of matrix minerals and rotation of the plagioclase porphyroblast cores about a unique axis in non‐coaxial deformation. The rotation of Si about an FIA nearly parallel to the Li is perhaps an apparent rotation, caused by the deflection of foliation around the growing prismatic plagioclase grain prior to inclusion into the porphyroblast. This study has for the first time documented the 3‐D geometry of S‐shaped inclusion trails in porphyroblasts from accretionary prism metasedimentary rocks and identified their origin, which helps to understand the flow kinematics in the deeper part of a subduction channel.  相似文献   

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