Coupled micro-faulting and pressure solution creep overprinted on quartz schist deformed by intracrystalline plasticity during exhumation of the Sambagawa metamorphic rocks,southwest Japan

In the Sambagawa schist, southwest Japan, while ductile deformation pervasively occurred at D₁ phase during exhumation, low-angle normal faulting was locally intensive at D₂ phase under the conditions of frictional–viscous transition of quartz (c. 300 °C) during further exhumation into the upper crustal level. Accordingly, the formation of D₂ shear bands was overprinted on type I crossed girdle quartz c-axis fabrics and microstructures formed by intracrystalline plasticity at D₁ phase in some quartz schists. The quartz c-axis fabrics became weak and finally random with increasing shear, accompanied by the decreasing degree of undulation of recrystallized quartz grain boundaries, which resulted from the increasing portion of straight grain boundaries coinciding with the interfaces between newly precipitated quartz and mica. We interpreted these facts as caused by increasing activity of pressure solution: the quartz grains were dissolved mostly at platy quartz–mica interface, and precipitated with random orientation and pinned by mica, thus having led to the obliteration of existing quartz c-axis fabrics. In the sheared quartz schist, the strength became reduced by the enhanced pressure solution creep not only due to the reduction of diffusion path length caused by increasing number of shear bands, but also to enhanced dissolution at the interphase boundaries.     

Quartz ribbons in high grade granite gneiss: modifications of dynamically formed quartz c-axis preferred orientation by oriented grain growth

Quartz ribbons form a well-defined LS fabric in granitic gneisses sheared and metamorphosed in the upper-amphibolite facies. Boundaries of quartz grains are smooth and sub-perpendicular to ribbon walls, suggesting post-deformational growth, whereas c-axes display asymmetric girdle patterns consistent with the sense of shear indicated by mesoscopic fabrics. Subdivision of the microfabrics according to the proportion of ribbon length occupied by individual grains indicates that the c-axes of relatively short grains exhibit across-girdle pattern similar to that of the whole sample. Longer grains reproduce elements of the pattern of the short grains, suggesting that oriented grain growth occurred. Grain boundary mobility, which clearly ended in a post-deformational static regime, probably began during dynamic recrystallization. Owing to space limitations imposed by the feldspathic matrix, grain growth in the ribbons ceased before the initial dynamic fabric was erased. In summary, this study shows that the c-axes pattern of longer grains within quartz ribbons reproduces part of the dynamically formed pattern of the shorter grains, reflecting an oriented grain growth which concluded in a static episode, although possibly initiated in the dynamic regime.     

Quartz c-axis fabric development and large-scale post-nappe folding (Wandfluhhorn Fold,Penninic nappes)

Quartz c-axis fabrics have been investigated within a suite of quartz veins and monomineralic layers around a major post-nappe fold hinge (the Wandfluhhorn Fold) in the Bosco area (Swiss-Italian border) within the lower Penninic nappes.Two kinematic domains which are separated by the axial plane trace of the Wandfluhhorn Fold are recognized; on the lower limb the measured quartz c-axis fabric asymmetry indicates a sense of shear in which the overlying layers move to the southwest (i.e. top-to-SW) whereas on the upper limb the shear sense is reversed with the top moving to northeast. The shear direction (N60°E–N80°E), however, is constant in both areas and oblique to an older stretching lineation as well as to the D₃ fold hinge. Such a distribution of asymmetric quartz c-axis fabrics and the constant orientation of their interpreted shear direction, which is apparent only from the fabric data and not from field evidence, indicates fabric development pre- or early syn-Wandfluhhorn folding, with subsequent folding and modification of the existing textures and possibly rotation of the initial fold axis.An overall westward-directed shear has been suggested for the whole of the Lepontine Alps. However, this study demonstrates that this simple general pattern has been modified locally by later folding. It also demonstrates that the dominant lineation may be a finite stretching lineation due to more than one phase of deformation and is not necessarily related to any particular transport direction.     

Relationships between strain and quartz crystallographic fabrics in the Roche Maurice quartzites of Plougastel,western Brittany

An integrated microstructural and petrofabric study of the plastically deformed and partially recrystallized Roche Maurice quartzites of Plougastel, western Brittany, has revealed a clear correlation between the pattern of c-axis fabrics displayed by detrital quartz grains and the symmetry of the calculated strain ellipsoid. In specimens with flattening (k = 0) strains, c axes lie on a small circle girdle (opening angle 28–42°) centred about the principal finite shortening direction (Z). For specimens that exhibit approximate plane strain (k = 1), cross-girdle c-axis fabrics consisting of a small circle girdle centred about Z and connected through the intermediate principal extension direction (Y) were detected.Within individual specimens c-axis fabrics of syntectonically recrystallized new quartz grains within the matrix are similar to those of detrital quartz grains. c axes of new grains located within the relatively undeformed sections of the host detrital grains are commonly orientated at angles between 10 and 40° to the host c axis and are, in addition, statistically orientated at a higher angle to Z than their host c axes. These relationships are interpreted as indicating that both host grain control and the local strain (and/or stress) field may have influenced the process of recrystallization; the relative influence of these factors is, however, unknown.Microstructural and petrofabric studies indicate that the Roche Maurice quartzites have been subjected to essentially coaxial strain histories. The role of syntectonic recrystallization in facilitating continued plastic deformation in quartzites subjected to such strain histories is considered.     

Interaction of flexural shear,S–C fabrics,and oblique shear during folding of micaceous quartzite

This paper investigates the geometry, microstructure, and c-axis fabrics of an outcrop scale, micaceous quartzite fold produced under greenschist facies metamorphic conditions in the Moeda quartzite, Quadrilátero Ferrı́fero granite–greenstone terrain, southeastern Brazil. The fold limbs show development of opposed S–C fabrics and asymmetric quartz c-axis fabrics compatible with flexural slip along the fold surface. Towards the fold hinge, there is an increasing presence of oblique shear bands (here named S-bands) which gradually change to crenulations within the hinge zone. The oblique S-bands are interpreted to have formed through connection of several S-planes, increasing accommodation of antithetical shear along these S-planes and offset of the initial C-planes at intermediate stages of folding. This mechanism represents a kinematic inversion in the role played by the two sets of foliations in S–C structures. Our observations support flexural slip for early stages of folding. However, with progressive closure of the fold, the flexural slip mechanism involves increasing contributions from oblique shear on the S-bands, thus approximating an intermediate situation between flexural slip and passive folding (shear parallel to the axial plane).     

Quartz c-axes orientation determination using the rotating polarizer microscope

A fast and accurate optical method to calculate quartz c-axis orientations using the rotating polarizer stage and standard thin sections is presented. c-Axis orientations are calculated for each pixel and Achsenverteilungsanalyse (AVA) can easily be constructed to study problems that would normally require a prohibitive amount of tedious work. The computer controlled rotating polarizer stage replaces the polarizer and analyzer of the standard petrographic microscope and allows a thin section to remain fixed while the polarizing filters are rotated by stepper motors. Data are collected by stepping the polarizers through a 180° rotation, capturing a frame at each step and extracting information on the intensity of the pixel and position of the polarizers. The position data are used to calculate the trend or trend+180° of the quartz c-axes, while a simple mathematical relationship between intensity and plunge is derived which allows the plunges of c-axes to be calculated.     

Heterogeneous constrictional deformation in a ductile shear zone resulting from the transposition of a lineation-parallel fold

We use new (micro-)structural, petrofabric, strain and vorticity data to analyze the deformation path in a mesoscopic quartz mylonite zone. The mylonite zone resulted from the complete transposition of a stretching lineation-parallel isoclinal fold. Symmetric cleft-girdle quartz c-axis fabrics were recorded in the middle domain, which occupies the inner limbs of the precursor isoclinal fold, while asymmetric cleft- and crossed-girdle fabrics were observed in the upper and lower domains that represent the outer limbs. Constrictional strain, with increasing k values towards the middle domain, is inferred from petrofabric and 3D strain data. Oblique grain shape fabrics yield vorticity estimates of 0.72–0.90 in the zone. However, in the middle domain, pure shear dominated deformation is suggested by orthorhombic crystallographic fabrics. Strain rate is constant throughout the zone; a strain decrease towards the zone center implies that deformation ceased earlier in the middle domain. The data indicates that fold transposition and subsequent mylonitization started as pure-shear-dominated constrictional deformation and progressively changed to simple-shear-dominated, plane strain. During this flow path the asymmetric quartz c-axis fabrics likely developed by depopulation of cleft-girdle maxima rather than from the synthetic rotation of fabric maxima itself.     

The reliability of asymmetric c-axis fabrics of quartz to determine sense of vorticity

Asymmetric c-axis fabrics of quartz are commonly used to determine sense of vorticity in ductile shear zones. This method seems to work if the fabric pattern resembles a model fabric proposed by Lister and Hobbs (1980). Usually, however, c-axis fabrics are rather vague. The reliability of such vague fabrics was tested in a major shear zone with known sense of vorticity. Only 62% of the c-axis fabrics predict the correct sense. Great care should therefore be taken in applying this method to determine sense of vorticity.     

Textures of syntaxial quartz veins synthesized by hydrothermal experiments

Syntaxial quartz veins were synthesized by hydrothermal flow-through experiments using rock blocks (metachert, sandstone, and granite) containing slits. Based on analyses of vein textures using birefringence imaging microscopy, we identified two stages of crystal growth. During stage 1, quartz grain growth occurs without an increase in grain width. During stage 2, quartz grains develop facets and grow preferentially parallel to the c-axis orientation, and the aspect ratio of quartz grains shifts toward ～2.9. Competitive growth occurs significantly at stage 2, and the transition from stage 1–2 occurs at a critical distance from the vein wall, being approximately equal to the host-rock grain size. Crystal growth in the slits produce various textures controlled by the ratio of slit aperture (H) to host-rock grain size (d). In high H/d cracks, elongate–blocky texture develops by grain impingement during stage 2, whereas in low H/d cracks, crystals that bridge the crack form without competitive growth by grain impingement at stage 1. Heterogeneous structures of fracture porosity are produced during syntaxial vein formation, due to the anisotropy in the growth rate of quartz. Such “incompletely sealed” cracks may act as important fluid pathways and as weak planes in the upper crust.     

Piezoelectric effects in quartz-rich rocks

Piezoelectricity, a polarization of charge produced by an applied stress, occurs in many minerals. It is particularly strong in quartz. Aggregates of piezoelectric grains are themselves piezoelectric if the grains are suitably aligned. Such aggregates may be said to have a piezoelectric fabric. Thus quartz-rich rocks may possess a piezoelectric fabric and this paper discusses the various possible fabrics.To test whether a piezoelectric fabric might be detected in a quartz-rich rock, apparatus was built that hydraulically applied a sinusoidal stress to cubic specimens. The three resulting orthogonal polarizations of charge were measured via a charge amplifier. A specimen of pure quartz was used to verify the experimental method and to ensure that absolute piezoelectric moduli were being measured. Rocks with and without preferred orientation were tested. Of the latter types, those containing little or no free quartz (marble, basalt) did not exhibit measurable piezoelectric effects. However, all quartz-rich rocks (quartzites, granites, gneisses, mylonites) did show piezoelectric effects when stressed. These effects were in two categories

1. (1) effects due to piezoelectric fabrics, called true piezoelectric effects

2. (2) effects due to random distributions of the piezoelectric vectors, called statistical effects.

To distinguish between these two effects, three criteria were used. Firstly, the measured effects were compared with the expected statistical effect for a rock of that grain size and composition. Secondly, where possible, multiple specimens were cut from the one rock sample, all specimens with the same orientation. Specimens from a rock with a piezoelectric fabric should show similar results. Thirdly, the optically observed c-axis distribution and orientation was compared with the piezoelectrically predicted fabric and orientation.This paper shows that while most rocks gave results consistent with statistical effects from a non-polar or random distribution, some rocks exhibited a true piezoelectric effect due to fabric. This effect may be used, with some imprecision, to locate the a-axes and c-axes of quartz in the aggregate. The polarities of the a-axes are also obtained.     

Quartz and sheet-silicate preferred orientations of low symmetry, Pikikiruna Schist, New Zealand

The Pikikiruna Schist of Nelson, New Zealand, displays a fabric in which the patterns of quartz c-axes, the poles to planes of inequidimensional quartz grains, and the statistical maxima of poles to sheet-silicate cleavages are oblique to each other. The quartz c-axes patterns consist of type-1 and type-2 crossed-girdles. The triclinic fabric can be explained in terms of one complex rotational deformation of an essentially plane strain nature. Rotation of approximately 90° about the intermediate strain-axis was combined at a late stage with subsidiary rotations about the extension axis. The quartz c-axes patterns can be related to the kinematic framework rather than the finite strain-axes. On the other hand, the dimensional quartz preferred orientation may be closely related to the finite strain-axes, though the quantity of strain can not be measured because of recrystallisation.     

Shear zone origin of quartzite mylonite and mylonitic pegmatite in the Coyote Mountains metamorphic core complex,Arizona

Mylonites derived largely from granite, pegmatite and sedimentary quartzite occupy a 500 m thick, gently N-dipping zone along the northern flank of the Coyote Mountains, west of Tucson, in southeastern Arizona. The quartzite mylonites are exceptionally well developed and occur as discrete layers and lenses, 2–5 m thick, within yet thicker, boudinaged, sill-like lenses of mylonitic pegmatite. Mylonitization took place in the Tertiary within a normal-slip ductile shear zone. The shear zones formed in response to regional extension of continental crust. Extension is along a north-south line, and N-directed sense of shear is revealed by mica fish, oblique foliations in dynamically recrystallized quartz aggregates, and asymmetric quartz c-axis fabrics. The microstructures and c-axis fabrics, taken together, disclose that ductile and brittle deformation was achieved by intense, penetrative, non-coaxial laminar flow dominated by progressive simple shear.     

Qualitative analysis of three-dimensional strain using twins in calcite and dolomite

Turner's (1953) technique of locating the compression (C) and tension (T) axes from the known orientations of C-axis and twin pole ([022̄1] in dolomite and [011̄2] in calcite) for each grain yields the orientation of the unique stress system when a great majority of the grains in the rock shows only singlet twins. However, since it assumes the highest possible value for the coefficient of resolved shear stress s₀ (= 0.5), application of the technique to rocks in which a large number of grains show doublet and triplet twins results in great dispersion of the C- and T-axes. In such cases the unique stress system can be established by preparing separate C-axes fabric diagrams for the untwinned grains, grains with singlet twins, grains with doublet twins and grains with triplet twins; measurement of the orientations of the twin planes becomes unnecessary.     

Comparison of quartz microfabric with strain in recrystallized quartzite

The relationship between quartz c-axis microfabric and strain is examined in six specimens of recrystallized quartzite conglomerate in which strain was measured using pebble shapes. Four rocks subjected to plane strain display a direct relationship between the strength of preferred orientation and the strain intensity. The c-axis distributions in these rocks, as well as a rock subjected to moderate extensional strain, are crossed-girdles with maxima near the intermediate principal strain axis and connecting girdles at acute angles to the direction of maximum shortening. A rock subjected to moderate flattening strain has several maxima clustered near the direction of maximum shortening and a weak connecting girdle through the intermediate principal strain axis.These results are generally similar to those of other studies comparing strain and tectonite fabrics and also with experimental and computer simulation studies of fabrics. The degree of preferred orientation is related to total strain, and therefore microfabrics in quartzites may be cautiously interpreted as qualitative indicators of strain intensity. Uncertainties are greater, however, for correlations of fabric patterns with shapes of the strain ellipsoid. An observed increase in recrystallized grain sizes with increasing strain suggests that flow stress was lower in the more strained rocks.     

Construction of crossed girdles by superposing four subfabrics,each with a single maximum

Commonly, basal glide is the predominant deformation mechanism of quartz in tectonites. Therefore, local deformation is probably mostly progressive simple shear rotating the sheared domains as well as deforming them. If a tectonite body is constrained to be deformed irrotationally and approximately homogeneously throughout, it is necessarily traversed by closely spaced material surfaces that are approximately plane and orthogonal originally, and stay so through time. These surfaces act as internal boundaries and enforce cancellation of the rigid-body rotations of, in the general case, four distinct families of domains, with slip planes and directions mutually mirror-symmetric. The overall symmetry of the fabric is orthorhombic, with the mirror planes coinciding with the principal planes of strain. Certain grains with basal planes in favorable orientation for one of the four ideal simple shears could initiate the deformation, and because of the need for compatibility, entrain neighboring grains into a similar strain, making the surroundings of an initiating grain a shear zone. Compatibility also requires thec-axes of grains in a domain to be rotated progressively toward the direction of maximum shortening. If the original orientation of crystallographic axes was random, domains of one family thus acquire a fabric with a single maximum, and the four resulting fabrics with single maxima combine to form crossed-girdle patterns. Depending on the orientation of the average shear planes and slip directions in the four families, the crossed girdles can be of different types; most fabric types that have been observed in quartz tectonites can be obtained by superposition. Crossed-girdle fabrics with low symmetry result from non-coaxial strain histories.     

Strain path partitioning within thrust sheets: microstructural and petrofabric evidence from the Moine Thrust zone at Loch Eriboll,northwest Scotland

Quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the Loch Eriboll area of the Moine Thrust zone and are used to interpret the detailed processes involved in fabric evolution. The intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude. A correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry.Two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the Moine Thrust. Within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate, mutually interfering shear bands, globular low strain detrital quartz grains whose c axes are aligned sub-parallel to the principal finite shortening direction (Z) and quartz c axis fabrics which are symmetric (both in terms of skeletal outline and intensity distribution) with respect to mylonitic foliation and lineation. Non-coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation.Tectonic models offering possible explanations for the presence of kinematic (strain path) domains within thrust sheets are considered.     

Sense of shear and displacement estimates in the Abeibara-Rarhous late Pan-African shear zone,Adrar des Iforas,Mali

The late Pan-African Abeibara-Rarhous shear zone in the Adrar des Iforas (Mali) is described and studied with the aim of defining the direction, sense of movement and amount of displacement along the zone. It is a strike-slip shear zone, the dextral sense of which is demonstrated at the scale of the map by the rotation of the related mylonitic foliation and at the scale of the thin section with characteristic microstructures. Preferred orientation of quartz c-axes is tentatively used; three quartz-rich samples of 35% or more quartz indicate dextral strike-slip movement, but other samples do not show preferred orientation of quartz c-axes. Strain measurements have been performed on one half of the shear zone using established techniques and a new technique using the thickness of mylonitic layering. The results vary along the length of the shear zone when using the same method and for the same cross-section when using the three methods together. A mean value of 4 km is obtained for total displacement which is low when considering the apparent width of the shear zone. This result is discussed in view of the assumptions involved in the strain estimation. The tectonic history of the Abeibara-Rarhous shear zone and its significance in the Trans-Saharan Pan-African collisional belt are discussed.     

Unusual transition in quartzite dislocation creep regimes and crystal slip systems in the aureole of the Eureka Valley–Joshua Flat–Beer Creek pluton, California: a case for anhydrous conditions created by decarbonation reactions

Microstructures and quartz c-axis fabrics were analyzed in five quartzite samples collected across the eastern aureole of the Eureka Valley–Joshua Flat–Beer Creek composite pluton. Temperatures of deformation are estimated to be 740±50 °C based on a modified c-axis opening angle thermometer of Kruhl (J. Metamorph. Geol. 16 (1998) 142). In quartzite layers located closest (140 m) to the pluton-wall rock contact, flattened detrital grains are plastically deformed and partially recrystallized. The dominant recrystallization process is subgrain rotation (dislocation creep regime 2 of Hirth and Tullis (J. Struct. Geol. 14 (1992) 145)), although grain boundary migration (dislocation creep regime 3) is also evident. Complete recrystallization occurs in quartzite layers located at a distance of 240 m from the contact, and coincides with recrystallization taking place dominantly through grain boundary migration (regime 3). Within the quartzites, strain is calculated to be lowest in the layers closest to the pluton margin based on the aspect ratios of flattened detrital grains.The c-axis fabrics indicate that a slip operated within the quartzites closest to the pluton-wall rock contact and that with distance from the contact the operative slip systems gradually switch to prism [c] slip. The spatial inversion in microstructures and slip systems (apparent “high temperature” deformation and recrystallization further from the pluton-contact and apparent “low temperature” deformation and recrystallization closer to the pluton-contact) coincides with a change in minor phase mineral content of quartzite samples and also in composition of the surrounding rock units. Marble and calc-silicate assemblages dominate close to the pluton-wall rock contact, whereas mixed quartzite and pelite assemblages are dominant further from the contact.We suggest that a thick marble unit located between the pluton and the quartzite layers acted as a barrier to fluids emanating from the pluton. Decarbonation reactions in marble layers interbedded with the inner aureole quartzites and calc-silicate assemblages in the inner aureole quartzites may have produced high X_CO2 (water absent) fluids during deformation. The presence of high X_CO2 fluid is inferred from the prograde assemblage of quartz+calcite (and not wollastonite)+diopside±K-feldspar in the inner aureole quartzites. We suggest that it was these “dry” conditions that suppressed prism [c] slip and regime 3 recrystallization in the inner aureole and resulted in a slip and regime 2 recrystallization, which would normally be associated with lower deformation temperatures. In contrast, the prograde assemblage in the pelite-dominated outer part of the aureole is biotite+K-feldspar. These “wet” pelitic assemblages indicate fluids dominated by water in the outer part of the aureole and promoted prism [c] slip and regime 3 recrystallization. Because other variables could also have caused the spatial inversion of c-axis fabrics and recrystallization mechanisms, we briefly review those variables known to cause a transition in slip systems and dislocation creep regimes in quartz. Our conclusions are based on a small number of samples, and therefore, the unusual development of crystal fabrics and microstructures in the aureole to the EJB pluton suggests that further study is needed on the effect of fluid composition on crystal slip system activity and recrystallization mechanisms in naturally deformed rocks.     

Fabric development during shear deformation in the Main Central Thrust Zone, NW-Himalaya, India

Quartz microfabrics and associated microstructures have been studied on a crustal shear zone—the Main Central Thrust (MCT) of the Himalaya. Sampling has been done along six traverses across the MCT zone in the Kumaun and Garhwal sectors of the Indian Himalaya. The MCT is a moderately north-dipping shear zone formed as a result of the southward emplacement of a part of the deeply rooted crust (that now constitutes the Central Crystalline Zone of the Higher Himalaya) over the less metamorphosed sedimentary belt of the Lesser Himalaya. On the basis of quartz c- and a-axis fabric patterns, supported by the relevant microstructures within the MCT zone, two major kinematic domains have been distinguished. A noncoaxial deformation domain is indicated by the intensely deformed rocks in the vicinity of the MCT plane. This domain includes ductilely deformed and fine-grained mylonitic rocks which contain a strong stretching lineation and are composed of low-grade mineral assemblages (muscovite, chlorite and quartz). These rocks are characterized by highly asymmetric structures/microstructures and quartz c- and a-axis fabrics that indicate a top-to-the-south sense that is compatible with south-directed thrusting for the MCT zone. An apparently coaxial deformation domain, on the other hand, is indicated by the rocks occurring in a rather narrow belt fringing, and structurally above, the noncoaxial deformation domain. The rocks are highly feldspathic and coarse-grained gneisses and do not possess any common lineation trend and the effects of simple shear deformation are weak. The quartz c-axis fabrics are symmetrical with respect to foliation and lineation. Moreover, these rocks contain conjugate and mutually interfering shear bands, feldspar/quartz porphyroclasts with long axes parallel to the macrosopic foliation and the related structures/microstructures, suggesting deformation under an approximate coaxial strain path.On moving towards the MCT, the quartz c- and a-axis fabrics become progressively stronger. The c-axis fabric gradually changes from random to orthorhombic and then to monoclinic. In addition, the coaxial strain path gradually changes to the noncoaxial strain path. All this progressive evolution of quartz fabrics suggests more activation of the basal, rhomb and a slip systems at all structural levels across the MCT.