Shape Fabrics Research Articles

Application of Geographical Information Systems to shape-fabric analysis

Abstract A Geographical Information System (GIS) has been applied to shape-fabric analysis and strain measurement. It works in either raster or vectorial format, allows determination of many geometrical features of grains and stores additional descriptive information linked to all these graphic elements. Some new applications for shape characterization and strain measurement have been developed, e.g. the shape parameter S , the Fry method installed as a variant that uses only contiguous grains (TFry method) and a new method (ASPAS method), based on the intersection lengths of the lines of a radial network with the grain boundaries. A compilation of finite-strain estimates on a tectonite sample has been used to compare the results obtained by the new methods. As the shape fabric does not register the full strain history, the strain ratios obtained from the shape-fabric analysis are lower in most cases than those obtained using pretectonic markers.

Physical weathering in arid landscapes due to diurnal variation in the direction of solar heating

Despite the prominent role of physical weathering in arid and semi-arid landscapes, there has been little study of the specifi c processes responsible for the rapid breakdown of subaerially exposed rocks. For example, many boulders and cobbles in deserts exhibit fi ne near-vertical cracks. Although workers have hypothesized that these and other cracks are initiated by diurnal heating and cooling, no convincing specifi c mechanism for their formation has been proposed. We have characterized these cracks at eight sites on surfaces of different ages in the Mojave, Sonoran, and Chihuahuan Deserts, and the high desert of central New Mexico. Our data reveal four basic types of cracks: longitudinal, surfaceparallel, fabric-related, and meridional. The orientations of the fi rst three types are associated with clast shape and rock fabric. The azimuths of meridional cracks, however, are preferentially aligned north-south, typically with a nonrandom multimodal distribution. We propose that these cracks are caused by tensile stresses that arise in the interior of clasts due to strong radial gradients in temperature that evolve and rotate in alignment with the sun’s rays. We suggest that the multimodal nature of crack orientations may be in part attributable to the seasonally varying, latitude-dependent solar elevation angle. Over millennial time scales, we suggest that this thermal cracking is an effi cient weathering process that, together with cumulic soil epipedon development, creates the key attributes of most desert pavements. In addition to individual clasts exposed on desert surfaces, this mechanism of cracking is potentially signifi cant in other climates and on other planets, as well as for rock outcrops and for man-made structures.

Folding by cataclastic flow: evolution of controlling factors during deformation

Folding at upper crustal levels occurs by bending of beds and flexural slip between beds. As a fold's interlimb angle decreases, changes in bed thickness and limb rotation are accommodated by various mechanisms, depending on deformation conditions. In the elastico-frictional (EF) regime, cataclastic flow may be the dominant mechanism for fold tightening. The Canyon Range (CR) syncline, located in the Sevier belt of central Utah, shows this type of deformation. The fold involves three thick quartzite units, with slight lithological variations between them. Fold tightening took place in the EF regime (<2 km overburden) by cataclastic flow, involving collective movement on a distributed network of fractures and deformation zones (DZs) from the micro- to the outcrop-scale. In detail, the degree of cataclastic deformation varies significantly across the fold due to minor variations in initial bedding thickness, grain size, matrix composition, etc. A cooperative relationship exists across different scales, and the fracture networks result in a fracture shape fabric that is relatively homogeneous at the outcrop-scale. The initial outcrop scale fracture/DZ network geometry is a product of the growth and linking of micro-scale cataclasite zones, which in turn is controlled by primary lithological variations. Once a fracture network forms, the material behavior of the fractured rock is unlike that of the original rock, with sliding of fracture-bound blocks accomplishing ‘block-controlled’ cataclastic flow. Thus, initial lithological variations at the micro-scale largely control the final deformation behavior at the largest scale. During progressive fold tightening, additional factors regulate cataclastic flow, such as fracture/DZ reactivation or healing, during folding. Although initial lithological variations in different units may produce unique network geometries, each unit's behavior may also depend upon the behavior of adjacent units. In the CR syncline, during the initial stages of cataclastic flow, the inherent nature of each quartzite unit results in unit-specific fracture network geometries. As deformation progresses, unit-specific networks begin to interact with those in surrounding units, resulting in feedback mechanisms regulating the later stages of network development. Thus, the nature of cataclastic flow changes dramatically from the initial to the final stages of folding.

Reference frame, angular momentum, and porphyroblast rotation

Rotation of small rigid objects in a deforming ductile matrix can produce two different types of microstructure: a shape fabric due to alignment of the principal axes of a population of elongate objects and the inclusion trail microstructure preserved in syntectonic porphyroblasts. We use numerical modeling to show that inclusion trails of elongate porphyroblasts are expected to be extremely complex. In contrast, snowball garnets are readily interpretable. But misuse of reference frame and kinematic misconceptions have obfuscated the discussion on the formation of porphyroblast inclusion trails in general and snowball garnet inclusion trails in particular. We clarify this point. Models for snowball garnet formation that are based on the notion of garnets being irrotational with respect to the earth can be rejected on a geometrical and kinematic basis. Further, the notion that rigid objects embedded in a deforming ductile matrix generally do not rotate is unsound—it violates the fundamental physical law of balance of angular momentum.

Tectonic evolution of the Brooks Range ophiolite, northern Alaska

Analysis of internal structures of the Brooks Range ophiolite at the three largest and well-exposed klippen reveals a NE–SW structural grain that may parallel the original axis of magmatism of a slow spreading marginal ocean basin. Sub-parallel directions of lattice fabrics in olivine of mantle peridotite and shape fabrics in pyroxene and plagioclase of layered gabbro indicate that asthenospheric and magmatic flow was closely coupled. These structures, including the petrologic moho, mostly dip steeply to the NW and SE, with slightly oblique flow lineations. Sedimentary and volcanic cover deposits also dip SE. The few exposures found of sheeted dike complexes generally strike parallel, but dip orthogonal to both the petrologic moho and cover deposits. These structural features are locally disturbed by syn- and post-magmatic normal faults emblematic of slow-spreading ridge processes. However, the consistent geometry of structures over a distance of 200 km demonstrates not only that the magmatic system was organized in a similar manner to an oceanic ridge, but that there was little to no rotation of individual klippe during tectonic emplacement. Ductile fabrics related to tectonic emplacement yield top-to-the NNW sense of shear indicators. The basal thrust and accompanying serpentinized shear zone is mostly flat-lying and truncates the steeply dipping ductile fabric of the ophiolite. This relationship and paleomagnetic data from the igneous sequence suggest that flow fabrics were most likely moderately inclined at the time the ophiolite formed. Similar relationships are found at diapiric centers along oceanic ridges and in other ophiolite bodies.

Microstructures and crystallographic preferred orientations of omphacite in Alpine eclogites: implications for the exhumation of (ultra-) high-pressure units

Microstructures and crystallographic preferred orientations (CPOs, textures) of omphacite within eclogites from distinct (ultra-) high-pressure units of the Alps were analysed in order to constrain the deformation conditions during and after high-pressure metamorphism. Especially the omphacite CPOs bear significant details for the reconstruction of the deformational evolution of several units. CPOs from the Austroalpine Koralm–Saualm Complex correspond to S-type fabrics, related to a deformation geometry within the flattening field. This geometry is documented by the shape fabrics, too. Textures described from the Dora Maira Massif show L>S- and L-type CPO fabrics, indicating a plane strain to constrictional deformation geometry. The CPOs from the Adula Nappe may all be described in terms of very well developed L-type fabrics formed within the constrictional field. Constrictional strain (axial elongation) is interpreted to have started at the peak of high-pressure metamorphic conditions and was continued along the retrograde path. Omphacite CPOs from the Eclogite Zone of the Tauern Window show a continuous transition from S- to L-type fabrics, corresponding to the transition from coarse-grained eclogites to fine-grained eclogite mylonites. This evolution is related to the final phases of the prograde evolution and coincides with a transition from a flattening to a constrictional deformation geometry. This is indicated by the shape fabrics, too. The constrictional geometry prevailed along the retrograde section of the PT path. The evolution of microstructures and CPOs is assumed to correlate with the deformation geometry, which is directly linked with the mechanisms of exhumation. S-type fabrics predominantely occur within eclogites exhumed by crustal extension, in particular the Koralm–Saualm Complex. L>S- and L-type fabrics predominantely occur within eclogites exhumed by extrusion within a low-angle corner (“subduction channel”), in particular the Dora Maira Massif, the Adula Nappe, and the Eclogite Zone. S-type CPOs from the Eclogite Zone were formed along the prograde path and are assumed to indicate compressional deformation related to the burial of this unit.

Viscous anisotropy of two-phase composites, and applications to rocks and structures

Anisotropy of viscosity in rocks has a significant effect on the development of geological structures, as demonstrated in several theoretical and analogue model studies. This paper develops the results of a self-consistent mechanics approach to modelling the bulk viscous properties of two-phase composites, to reveal the strength of the viscous anisotropy. Anisotropy factor, δ, is defined as the ratio of normal to shear viscosity, μ N/ μ S. The value of δ is determined for different types of composites of two viscous phases with varying viscosity ratio ( m=5, 10, 100), ranging from bilaminates to those with elliptical shape fabrics given by axial ratio, R. Bilaminates with equal layer thickness are the most highly anisotropic systems, where δ simply depends on m. These results are applied to rocks, modelled as two-phase composites of competent and incompetent lithologies, with the aim of quantifying the viscous anisotropy that is associated with natural rock deformation and the formation of geological structures. It is deduced that m=5 or 10 is an appropriate maximum viscosity contrast for many metasedimentary rocks (e.g. psammite/pelite composites), but that m=100 might be effectively reached in limestone/marl pairs, or in rocks containing surfaces of slip or zones of recrystallisation or grain-boundary sliding. These m values give rise to maximum δ values of 3, 5.5 and 25.5, respectively. An anisotropy factor of δ≫25 is effectively a composite of a rigid and a viscous phase, and may only be achieved in solid–melt mixtures. Anisotropic rocks, whether layered or containing a strong shape fabric, will develop folds more easily than isotropic rocks. Characteristic structures are similar folds, chevron folds, kink bands and shear bands. Geometric features of folds, especially fold angularity, may provide methods of quantifying the viscous anisotropy of rocks and composites.

An experimental and numerical study of the microstructural parameters contributing to the seismic anisotropy of rocks

The elastic properties of rocks are influenced by several microstructural variables, including the lattice preferred orientation and grain shape fabric of the mineral phases, variations in the spatial distribution of the mineral phases, the properties of the grain boundaries, and the presence of porosity/fractures. Consequently, in principle, directional variations in these variables can be inferred from seismic velocity anisotropy observations. Experimental seismic velocity measurements made on rocks of well‐characterized microstructure may be used to improve the quality of such interferences. However, since most rocks are microstructurally complex, in order to interpret the measurements fully, theoretical analyses which can accommodate all the relevant microstructural variables are required. Theoretical analyses of the requisite sophistication have only recently been developed. We have tested one of these (due to Ponte Castañeda and Willis) by using it to calculate the elastic properties of an upper mantle harzburgite and by then comparing the results with experimental velocity measurements to determine if the values of those microstructural variables which are difficult to quantify (grain boundary properties, fracture shape) are physically realistic. We find that they are and conclude that the Ponte Castañeda and Willis analysis provides a powerful method for a more detailed assessment of the causes of elastic property anistropy in rocks than has previously been possible.

Deformation partitioning during folding and transposition of quartz layers

Banded iron formation (BIF) from the Quadrilátero Ferrı́fero (southeastern Brazil) shows a compositional layering with alternating iron-rich and quartz-rich layers. This layering was intensively folded and transposed at a centimeter/millimeter scale through a component of bedding-parallel shear related to flexural slip at middle to high greenschist facies conditions (400–450 °C). The microstructure and c-axis fabrics of normal limbs, inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy (SEM) and digital image analysis. In the normal limbs, recrystallized quartz grains show undulose extinction, relatively dry grain boundaries, c-axes at high angle to foliation and a pervasive grain shape fabric (GSF) indicating operation of crystal-plastic processes. In the inverted limb, quartz grains show more serrated and porous (“wet”) grain boundaries; the GSF is similar to that of the normal limb, but c-axes are oriented at 90° to those of the normal limb. We interpreted these characteristics as reflecting operation of solution-precipitation deformation in inverted limbs, as a consequence of grains having been rotated to an orientation that was hard to basal 〈 a〉 glide, but easy to dissolution-precipitation creep. This deformation partitioning between crystal-plasticity and solution-transfer during folding/transposition of quartz may explain the common occurrence of layered quartz rocks, where individual layers show alternating c-axis fabrics with opposite asymmetries but a consistent GSF orientation. Such characteristics may reflect an earlier event of pervasive folding/transposition of a preexisting layering.

Experimental modeling of viscous inclusions in a circular high‐strain shear rig: Implications for the interpretation of shape fabrics and deformed enclaves

Deformation experiments with initially spherical and prolate viscous inclusions suspended in a viscous Newtonian matrix in a circular high strain annular shear rig provide insights on the shape development of inclusions in high strain shear zones during progressive deformation. Inclusions with a specific viscosity ratio with respect to the matrix material show distinct types of three‐dimensional shape development. For instance, at a high viscosity ratio between matrix and inclusion a pulsating ellipsoid develops, which both continuously rotates and changes its shape from a sphere to an ellipsoid and back to a sphere. The experiments show that the shape of an inclusion that has a viscosity different to the matrix material cannot be taken as a reliable indicator for the magnitude or type of finite strain. In naturally formed shear zones, in which strain rates are heterogeneous from boundary to center, viscosity contrast and power law flow will result in a large number of different shape developments trends for inclusions such as mafic and microgranitic enclaves, pebbles or mineral aggregates.

An Analysis of Differently Measured Moduli of Elasticity of Media Containing Shape Fabrics of Pores and Microcracks

An Analysis of Differently Measured Moduli of Elasticity of Media Containing Shape Fabrics of Pores and Microcracks

The anisotropy of itacolumite flexibility

Abstract Itacolumites are very special rocks due to their high flexibility. The investigated Brazilian itacolumites and associated non-flexible quartzites are of comparable composition but differ in their rock fabrics. The shape and size of quartz is mainly controlled by the mica fabric. Quartz textures and grain boundary migration features are indications for deformation at temperatures of about 500°C. The flexibility is mainly related to a penetrative network of open grain boundaries which enable a limited body rotation of individual quartz grains. Continuous layers of white mica display deformation features indicative of shear along the layer-parallel cleavage planes. As demonstrated by simple bending experiments, the flexibility is a highly anisotropic phenomenon which can be related to a directional dependence of grain shape fabrics and corresponding grain boundary pore spacing. According to quantitative estimates, the amount and anisotropy of bending can be explained by the rotation of separated quartz grains between layers of mica which act as flexural slip planes and are also responsible for the observed elastic rebound. Solution along grain boundaries, volumetric strain by thermal contraction of quartz and bulk extension are processes discussed for the origin of the extreme values of secondary grain boundary porosity.

Role of magma pressure, tectonic stress and crystallization progress in the emplacement of syntectonic granites. The A-type Estrela Granite Complex (Carajás Mineral Province, Brazil)

The Archaean, syntectonic, A-type Estrela Granite Complex (Carajás Mineral Province, Brazil) consists of three plutons emplaced in a greenstone sequence under low-pressure conditions (180< P<310 MPa). It is composed mainly of annite-, ferropargasite (±hedenbergite)- and ilmenite-bearing monzogranites. The contact aureole is affected by a subvertical penetrative schistosity conformable with the limits of the plutons. Meso- to microstructures and mineral reactions in the granites indicate that deformation occurred in a continuum from above-solidus to low- T subsolidus conditions. Two distinct planar structures are observed: (i) a concentrical primary foliation (S 0) corresponding to rhythmic, isomodal, phase layering associated with a faint grain shape fabric; it is horizontal in the centre and vertical towards the edges of the plutons; and (ii) a steep to subvertical foliation (S 1) associated with the deformation of S 0 and accompanied with emplacement of synplutonic dykes and veins of leucocratic granites and pegmatites. Emplacement, differentiation and consolidation of the Estrela Granite Complex are considered to result from a continuous evolution under decreasing temperatures in a single-stage strained crust (transpression), with two main periods. (1) The first period is controlled by body forces, and it corresponds to inflation with magma ponding. As long as the rheology is melt dominated, magma pressure is the critical parameter and almost no strain is recorded. With decreasing T, magmas crystallize and differentiate leading to a concentrical magmatic phase layering. The growing magmatic bodies are mechanically decoupled from the country rocks and their evolution depends on internal magma chamber processes. (2) For higher amount of crystallization (residual melt fraction F<0.5), the role of magma pressure becomes insignificant. Establishment of a continuous crystal framework leads to the coupling of plutons with their surroundings, and deformation in response to tectonic stress. Most of the strain is recorded during this period which starts from the rigid percolation threshold, and extends to subsolidus low-grade conditions. This leads to deformation of the partially crystallized volume and redistribution of fluid-enriched differentiated melts. The amount of crystallization through the rheological thresholds appears as the critical parameter determining the transition from magma-controlled processes (inflation and differentiation of the magma chamber, with development of a phase layering) to tectonic-controlled processes (deformation of the phase layering and redistribution of residual melts). This accounts for the fact that syntectonic plutons commonly display intermingled, boudinaged layers with distinct modal compositions and in some cases well-preserved rhythmic layering.

Crystallographic fabrics of omphacite, rutile and quartz in Vendée eclogites (Armorican Massif, France). Consequences for deformation mechanisms and regimes

This study aims at further understanding of the mechanisms how lattice-preferred orientations (LPO) develop during deformation in the main eclogite minerals. Microstructures and textures of deformed eclogites from the Les Essarts complex (Western France) were investigated using optical microscopy and electron backscatter diffraction (EBSD) in the scanning electron microscope. Microfabric analyses of eclogite-facies minerals are used to identify their deformation mechanisms, which define the rheology at high-pressure metamorphic conditions. Mechanisms of intracrystalline deformation by dislocation movement (dislocation creep) result usually in a non-linear flow law (typically power law), while diffusive processes (diffusion creep) correspond to linear flow laws. General microstructural observations may suggest intracrystalline deformation (dislocation creep) of omphacite. The omphacite LPO vary between S- and L-type and correlate with oblate or prolate grain shape fabrics, respectively. Until now, these LPO types have not been understood by plasticity models based on dislocation glide on the known slip systems in clinopyroxene. An alternative interpretation is given in terms of anisotropic growth and dissolution, with grain boundary diffusion as the rate controlling process. There are further indications suggesting diffusion creep with concomitant anisotropic growth and dissolution as a main deformation mechanism in omphacite. In omphacite around a hollow garnet, crystallographic and shape fabrics align with the c[001] axes parallel to the grain elongations defining the mineral lineation, which rotates locally with the inferred flow direction. In this part, the grain sizes of omphacite and rutile are larger than in the surrounding matrix. The geometry of both the shape and crystallographic fabrics is interpreted to represent the local stress regime (directions and ratios of the principal stresses). The LPO of rutile duplicate the LPO of omphacite and a similar distinction between S- and L-type was used. Rutile deformation mechanisms probably involve dislocation creep as well as diffusion creep. Quartz mainly occurs as an interstitial phase with weak LPO patterns interpreted as random. No representative obliquity of the LPO in omphacite nor rutile with respect to foliation and lineation was observed to be used as potential shear sense criteria. However, the rutile LPO was slightly rotated relative to the omphacite LPO consistently in most samples. The results suggest that diffusion processes are strongly involved in the deformation of eclogites. A linear flow law should be taken into account in tectonic models where eclogites are incorporated.

Modelling the bulk viscosity of two-phase mixtures in terms of clast shape

The rheological behaviour of many rock-types will depend on how mixtures of different minerals and rock clasts behave together. This paper develops two-dimensional modelling of the bulk viscosity of two-phase viscous mixtures with different shape fabrics in pure shearing, based on self consistent mechanics and behaviour of inclusion-matrix systems. An equation is presented for the bulk viscosity in terms of the phase viscosity contrast, phase fractions, and clast shape. Results are given for idealised two-dimensional mixtures of circles, squares or ellipses with a variety of phase viscosity contrasts, in pure shearing parallel/perpendicular to the shape fabric. The normalised bulk viscosities of all these mixtures fall between the theoretical upper and lower viscous bounds given by homogeneous strain-rate and homogeneous stress, respectively, but nearer to the upper bound with increasing shape fabric. Mixtures with a particular geometry are shown to have a constant ‘relative horizontal distance’ between these two bounds, as proposed by P.D. Bons.Using graphs for the normalised bulk viscosities of mixtures with variably elliptical aligned shape fabrics, the results for fabrics orthogonal to pure shearing can be extrapolated to: (a) diagonal fabrics in pure shearing, and (b) orthogonal or diagonal fabrics in simple shearing. These reveal the bulk viscous anisotropy of mixtures with shape fabric, in two dimensions. It is speculated that during a progressive finite deformation, two-phase mixtures might undergo changes in bulk viscosity over time, with steady stiffening during pure shear, and perhaps cyclic stiffening and softening during simple shear. The two-dimensional models provide an approximation to the three-dimensional properties of mixtures with shape fabrics, with potential applications to rocks, including conglomerates.

Rheological and microstructural evolution of Carrara marble with high shear strain: results from high temperature torsion experiments

This study investigated the rheological and microstructural evolution of Carrara marble deformed to large shear strain to understand how dynamic recrystallization and lattice-preferred orientation (LPO) are related to strain softening processes. Solid cylinders of Carrara marble were deformed in torsion up to a shear strain of γ=11 at constant twist rates, which correspond to a shear strain rate of 3×10−4s−1 at the outer surface, and at temperatures of 1000 and 1200K (727 and 927°C, respectively). For the initial grain size of 150μm, these conditions are within the dislocation creep regime.Substantial changes in both rheology and microstructure were observed as the marble deformed to high shear strain at 1000K (1200K). A peak stress was reached at about γ=1 (γ=0.5) followed by moderate strain weakening. An apparent steady-state flow stress was obtained at high shear strain of γ>5 (γ>2). The stress exponent n decreased slowly with strain from 10 (γ=1) to 6 (γ=9) at 1000K, but it remained approximately constant at 1200K (n around 10). At the maximum reached shear strain of γ=11 (γ=8.5), the marble had almost completely recrystallized to a fine grain size of about 10μm (20μm). A secondary foliation developed in the recrystallized matrix, which is at a large oblique angle to the shear zone boundary (SZB). LPO was measured by electron backscatter diffraction (EBSD). For both temperatures, the LPO evolved from an oblique deformation texture to a very sharp and symmetric single orientation component with r{101̄4} parallel to the shear plane and a〈1̄21̄0〉 parallel to the shear direction. It is concluded that strain weakening was associated with the development of a strong LPO during dynamic recrystallization to a finer grain size. Mechanical and microstructural steady-state is only reached at large shear strain. The steady-state lattice and grain shape fabrics can hardly be used as shear sense indicators in such recrystallized calcite mylonites.

Origin of a vertical lineation in conjugate transcurrent shear-zones at Broken Hill, Australia

Shear-zones at Little Broken Hill are interpreted as conjugate transcurrent structures. They developed before dolerite dyke emplacement in response to N–S horizontal shortening. They were reactivated as conjugate transcurrent structures after dyke emplacement in response to horizontal E–W shortening. Shear-zone deformation was accompanied by retrograde metamorphic conditions for at least the later part of the history. Displacement on the shear-zones was accommodated by coeval deformation within the shear-zone-bound blocks. This deformation involved shear on the pre shear-zone S 1 foliation and resulted in incomplete retrogression of earlier peak metamorphic mineral assemblages. It also resulted in a vertical extension and a component of dip slip on the shear-zones. A near-vertical stretching-lineation occurs parallel to the intersection of the shear-zones and is interpreted as a localised response to an overall vertical extension. Localisation of the lineation is explained in terms of grain-scale partitioning of deformation mechanisms. In the country rock, vertical extension was achieved by shear on S 1 with very little intragranular deformation. Consequently this deformation failed to produce a lineation. In the shear-zones, where all fabric elements were tending to vertical, the only deformation mechanism available for vertical extension was intragranular deformation, which resulted in a shape fabric, viz the stretching-lineation. Shear within the zones had both horizontal and vertical components parallel to the shear-zone foliation. Because the shear was achievable by slip on the foliation and did not involve intragranular mechanisms, it did not modify the lineation. Late shear-zone movement resulted in local crenulation of the shear-zone foliation about the early-formed stretching-lineation. The result was vertical plunging folds and a more pronounced “stretching” lineation. It is concluded that stretching-lineations in shear-zones should not be considered indicators of movement direction or of instantaneous or finite strain axes.

Implications of permeability and its anisotropy in a mica gouge for pore pressures in fault zones

Large permeability anisotropy due to anisotropic alignment of clay shape fabric may play an important role in focusing fluid flow along fault zones in clay rich rocks. The degree of permeability anisotropy depends on the differences in tortuosity of the fluid flow path and mean pore aperture, which are of a function of effective stress. We have conducted hydrostatic compaction and frictional sliding experiments on 1 mm thick artificial muscovite gouge layers with grain size <90 μm, at normal stresses on the layer ranging from 25 to 125 MPa. Permeability was measured both perpendicular and parallel to the gouge layer using water as pore fluid. Shear displacement up to 200 mm was achieved. During initial packing and subsequent hydrostatic compaction, muscovite basal planes were aligned parallel to the fault plane, leading to the largest possible contrast in tortuosity between flow path perpendicular and parallel to the fault zone. We found that fluid permeability parallel to the fault zone is about one order of magnitude higher than perpendicular to it and that the magnitude of permeability anisotropy does not change with increasing normal stress. During shear deformation, the permeability anisotropy decreased with increasing shear displacement, an effect interpreted as being due to progressive rotation of muscovite basal planes away from the fault plane. Previous hydrogeological modelling has shown that permeability anisotropy on the order of 3–5 orders of magnitude is needed to effectively focus fluid flow along fault zones. Thus, our results seem to indicate that fabric anisotropy alone is not enough to cause focused fluid flow along natural fault zones. During hydrostatic compaction of muscovite gouge, a linear relationship holds between ln permeability and normal stress and the slope is about 0.03 MPa −1, which is higher than for most porous sandstones. The value is reduced to about 0.006 MPa −1 after shear deformation. The values are lower by 1–2 orders of magnitude than the value used in Rice's fault model, suggesting that maintaining high pore pressures would be harder than he assumed.

Evolution of quartz microstructures and textures during polyphase deformation within the Tauern Window (Eastern Alps)

The interior of the Tauern Window exposes underplated Penninic continental lithosphere and the overlying obducted Penninic oceanic crust within a large antiformal dome in the internal zone of the Eastern Alps. These units have been affected by a polyphase deformation history. Generally, three deformation events are distinguished. D1 is related to underplating of, and top-to-the-N nappe stacking within, the Penninic continental units of the Tauern Window. Deformation stage D2 is interpreted to reflect the subsequent continent collision between the Penninic continental units and the European foreland, D3 is related to the formation of the dome structure within the Tauern Window. During thickening of continental lithosphere and nappe stacking (D1), and subsequent intracontinental shortening (D2), these tectonic units have been ductilely deformed close to a plane strain geometry. Conditions for the plastic deformation of the main rock-forming mineral phases (quartz, feldspar, dolomite, calcite) have prevailed during all three phases of crustal deformation. Generally, two types of quartz microstructures that are related to D1 are distinguished within the Tauern Window: (a) Equilibrated and annealed fabrics without crystallographic preferred orientations (CPO) have only been observed in the central part of the southeastern Tauern Window, corresponding with amphibolite-grade metamorphic conditions. (b) In the northeastern and central part of the Tauern Window microstructures are characterized by quartz grains that show equilibrated shape fabrics, but well preserved CPO with type-I cross girdle distributions, indicating a deformation geometry close to plane strain. During D2, two types of quartz microstructures are distinguished, too: (a) Quartz grains that show equilibrated shape fabrics, but well-preserved CPO. The c-axes distributions generally are characterized by type-I cross girdles, locally by type-II cross girdles, and in places, oblique single girdle distributions. (b) A second type of quartz microstructure is characterized by highly elongated grains and fabrics typical for dislocation creep and grain-boundary migration, and strong CPO. This type is restricted to the southern sections of the western and eastern Tauern Window. The c-axis distributions show type-I cross girdles in the western part of the Tauern Window and single girdles in the southeastern part. In the western part of the Tauern Window, a continuous transition from type (b) microstructures in the south to type (a) microstructures in the north is documented. The microstructural evolution also documents that the dome formation in the southeastern and western Tauern Window has already started during D2 and has continued subsequent to the equilibration during amphibolite to greenschist facies metamorphism. D3 is restricted to distinct zones of localized deformation. D3-related quartz fabrics are characterized by the formation of ribbon grains; the c-axes show small-circle distributions around the Z-axis of the finite-strain ellipsoid. During exhumation and doming (D3), deformation occurred under continuously decreasing temperatures.

Quartz crystallographic and morphologic fabrics during folding/transposition in mylonites

Quartz grain shape fabrics (GSF) and c-axis lattice preferred orientation (LPO) were investigated in folded quartz ribbons with variable fold geometry in low metamorphic-grade granitic mylonites from northwestern Argentina. The folds, which vary from open to isoclinal, show an asymmetric short limb–long limb geometry developed simultaneously with the quartz fabrics during progressive simple shear. GSF shows a consistent obliquity relative to the bulk shear plane throughout the folded ribbons. GSF inclination varies between 25° and 45° oblique to the shear plane, indicating that relatively small finite strain increments ( γ∼1.2) are attained before the GSF regeneration. In contrast, the c-axis preferred orientation varies throughout the folded ribbons. Unfolded ribbons and long limbs of folds show a typical c-axis maximum near the shear plane pole, while short limbs of tight and isoclinal folds show a c-axis maximum nearly orthogonal to that of long limbs. This pattern of domainal LPO is maintained in thickened ribbons derived from folding and transposition. This indicates that the GSF regeneration is comparatively much faster than a cycle of folding/transposition. Conversely, the LPO regeneration is much slower. We conclude that folding/transposition is a mechanism for development of domainal quartz LPO in kinematic frameworks with dominant simple shear.