C-axis Patterns Research Articles

Deformation conditions and kinematic vorticity within the footwall shear zone of the Wildhorse detachment system, Pioneer metamorphic core complex, Idaho

Detailed analysis of numerous sequential fabrics and microstructures preserved in the footwall shear zones of detachment fault systems can be used to clarify the spatial relationships and timing of deformation related to emplacement and exhumation of metamorphic core complexes. To quantify the kinematics during deformation, we conducted microstructural analyses along two ∼300 m transects across mylonitic sections of the footwall of the Eocene Wildhorse Detachment system within the Pioneer metamorphic core complex. These results enable comparison of deformation at different structural levels during exhumation and between monomineralic and polymineralic lithologies.In both transects, mylonitic rocks preserve normal-sense asymmetric structures such as recrystallized quartz oblique foliation, S/C–C′ structures, and asymmetric feldspar porphyroclasts. Quartz microstructures, CPO patterns, and kinematic vorticity analyses consistently indicate simple shear-dominated deformation, which is predicted for late stage, shallow-level deformation within a rolling hinge detachment system exhuming a metamorphic core complex. Variations in c-axis patterns and active slip systems are associated with temperature, influenced by structural level, and grain boundary processes related to lithology. To determine the degree of non-coaxiality and minimize limitations and uncertainties with kinematic vorticity methods, we used two methods to evaluate flow vorticity. Kinematic vorticity ranges from 0.55 to 0.95 for the oblique quartz foliation (δ/β) and C′-type shear bands (C′-c) methods. Quartz recrystallized grain size ranges from 10 to 37 μm, indicating maximum differential stresses of 80–110 MPa, which is consistent with the footwall shear zones of numerous metamorphic core complex detachment systems.

CPO and quantitative textural analyses within sheath folds

We investigate the intra- and inter-crystalline deformation processes involved in sheath fold development combining complementary fabric analysis techniques and 3D modelling by neutron tomography. The investigated sheath fold is a multi-layered sub-metre scale single-eye structure, developed in metapsammites from the Ben Hope Nappe, overlying the Moine Thrust Zone of NW Scotland. Crystallographic Preferred Orientations (CPOs) of quartz and biotite were acquired through a Neutron Diffractometer and an SEM-EBSD system to compare the full-fabric of the main phases and the active slip systems for an “in situ” structural control. Combined with orientation maps and grain size maps, results show that, despite the different structural positions of the investigated microdomains (upper vs lower fold limbs, inner vs outer sheath closures, distance from hinge of the sheath fold), quartz and biotite deformed uniformly, suggesting a constant differential stress and orientation of the kinematic vorticity axis. Previously recognized detachment horizons within the sampled sheath fold do not affect the fabric patterns recorded by quartz and biotite. This may be interpreted in two different ways: i) detachments formed during earlier active folding and prior to passive amplification of folds associated with more uniform flow to create the sheath fold geometries; ii) the quartz c-axis patterns are coeval with a late deformation phase (loading of the orogenic wedge) that pervasively obliterated the previous fabric and therefore did not preserve the active folding component. Several pieces of evidence reported here, such as top-to-SE normal-shear sense which is opposite to the regional kinematics, are more supportive of the second hypothesis. The analysis of mineral textures provides an improved dataset for the whole sheath fold and increases our understanding of recrystallization mechanisms active in shear zones.

Deformation mechanisms and rheology inversion of jointly deformed marble and quartzite in natural thermal gradient

Amphibolite facies marble surrounded by locally pure but mostly impure quartzite with ∼10–14% of dispersed white mica and crosscut by granite veins were jointly deformed within an extensional shear zone. Both marble and quartzite show three microstructural stages, M1m-M3m and M1q-M3qm, respectively, correlated with the onset of extension Stage 1 at ∼600 °C, and retrograde extension Stage 2 at ∼380 °C and Stage 3 at ∼300 °C. The coarse-grained microstructures M1m (1200 μm) in marble and M1q (300 μm) in pure quartzite showed the activity of basal ⟨a⟩ slip and prism ⟨a⟩ − rhomb ⟨a⟩ slip, respectively. The finer grained M2m (∼50 μm) microstructure in marble is associated with strain localization, subgrain rotation recrystallization and persistence of basal ⟨a⟩ slip. The M2q (175 μm) microstructure in pure quartzite shows a crossed girdle c-axis pattern, while the M2qm (120 μm) in impure quartzite shows locally c-axes parallel to the lineation, indicating the possible activity of prism [c] slip. Stage 3 in marble is characterised by minor recrystallization M3m that occurs mostly along microcracks formed subparallel to foliation. The impure quartzite M3qm microstructure shows further reduction in quartz grain size (80–90 μm) related to pining of well-dispersed white mica. The microstructure M1m-M1q and M2m-M2q formed by dislocation creep, while the M2qm and M3qm suggest the Rachinger grain boundary sliding of quartz grains along micas. Piezometric calculations suggest differential stresses of 1–5 MPa and ∼20 MPa for the M1m and M2m microstructures and 7 MPa and 11 MPa for the M1q and M2q microstructures, respectively. Comparison of these data with experimental flow laws confirmed that during Stage 1 marble is weaker than quartzite. However, during Stages 2 and 3, mica in impure quartzite became more dispersed but also locally aggregated; thus, the bulk strength of quartzite became governed by mixture of quartz and weak mica behaviour. This change in microstructure and the resulting deformation mechanism facilitated fluid transport across quartzite, which resulted in embrittlement of dominantly coarse-grained and impermeable competent marble. This process is accompanied by the formation of stretching faults developed parallel to weak layers of altered granite dykes in the vicinity of the marble-quartzite contact. The rheology inversion thus results from a mixture of deformation mechanisms in the weak polyphase quartzite and fluid-induced drop of effective pressure of strong and brittle marble at late stages of the extensional deformation.

Kinematic analyses of the asymmetric boudins in the Tutak area within the Zagros hinterland fold-and-thrust belt, Iran

Exhumation of the Tutak gneiss dome occurred in a transpressional regime between the Arabian plate and the Central Iranian microcontinent. The Tutak gneiss dome is located in the eastern part of the basement-involved Zagros hinterland fold-and-thrust belt. This dome consists of exhumed middle crust including the Tutak and Sourian metamorphic complexes, which are lied on its core and edges, respectively. Integrated structural, microstructural, and flow vorticity analyses in quartzo-feldspathic mylonites and well-developed asymmetrical boudins provide significant information regarding the kinematic characteristics coeval with the Tutak gneiss dome development. Asymmetrical boudins have been used as potential shear sense markers throughout the mantle of the Tutak gneiss dome. The deformed quarzitic layers within the slate belt host domino and shear-band boudins, demonstrating both sinistral and dextral shear senses. The mean vorticity number (Wm) was estimated from the quartzo-feldspathic mylonites and asymmetric boudins. Using the hyperbolic distribution (HD) and the RXZ strain ratio/quartz c-axis fabric (RXZ/β) methods. The estimated Wms from the HD method and quartz c-axis patterns are 0.69 to 0.82 and 0.6 to 0.92, respectively, suggesting a combination of 41% to 72% simple- and 28% to 59% pure-shear components. The close match confirms that asymmetric boudins can be a good strain estimator. The estimated deformation temperature (528.33 ± 50 °C) from the opening angles (OAs) of quartz c-axes suggests amphibolite facies conditions. Our results show that the Tutak gneiss dome developed under sub-simple shear conditions, which stretched and shortened the dome parallel and orthogonal to the transpressional boundaries, respectively.

Crystallographic preferred orientation (CPO) patterns in uniaxially compressed deuterated ice: quantitative analysis of historical data

AbstractStrain, temperature and strain rate are crucial factors governing the development of crystallographic preferred orientations (CPO) in ice. To better understand how CPO patterns change in response to these variables, we performed quantitative analyses on neutron diffraction data between 2010 and 2019, collected in situ during uniaxial compression experiments on deuterium ice. At strains >10% and temperatures <−10°C, thec-axis pattern switches from a single maximum (‘cluster’) to small circle (‘cone’), both oriented parallel to shortening. The diameter and mean width of the cone pattern decrease as strain and/or strain rate increases. Prismatic axis (aandm) patterns are characterised by great circles parallel to the pole figure margin and may be distinguishable from the patterns in ice deformed under simple shear. While strain has the main influence on the degree of preferred orientation (or CPO ‘strength’), both temperature and strain rate have minor influences, which limits the extent to which CPOs can be used to measure strain. As cluster patterns can be observed in thec-axes of ice deformed under both pure and simple shear settings, this may complicate interpretations of flow geometry in terrestrial ice unless the prismatic axis patterns are also considered.

Crystallographic preferred orientation of quartz deformed at granulite conditions: the Kalinjala Shear Zone, Port Neill, South Australia

Crystallographic preferred orientations of quartz aggregates in mylonitic orthogneisses from the highest strained central region of the Kalinjala Shear Zone, Port Neill South Australia, are products of a combination of constrictional and flattening strains. These rocks were deformed under granulite facies peak metamorphic conditions, reaching pressures of ∼1.0 GPa and temperatures of ∼800 ± 50 °C. Neutron diffraction textural and fabric analysis methods reveal a dominant single c-axis maximum parallel to the Y-strain axis. This is accompanied by a symmetric orthorhombic pattern, where the c-axis fabrics and corresponding pole figure densities of {a}, {m}, {r} and {z} planes are arranged around the XY foliation plane. These fabrics from coexisting adjacent layers suggest that the activation of prism-<a> slip is characteristic for these high-temperature deformation conditions. A small asymmetry in the [c], {a} and {m} pole figures suggests a minor component of dextral shearing. The c-axis pattern is weakened where the foliation is dominated by secondary phases, such as concentrations of biotite mica. The opening angles obtained from cross-girdle c-axis patterns produce temperature estimates that are ∼100 °C lower than the peak metamorphic temperature of ∼800 ± 50 °C, but closer to temperatures related to a component of flattening strain. These c-axis patterns are consistent with other structural characteristics and can be related to the late-stage flattening strains during the Kimban Orogeny. KEY POINTS Mylonitic orthogneisses are deformed under granulite facies conditions. Quartz fabrics were measured by bulk neutron diffraction and optical methods. Mylonite c-axis fabrics can be related to the high-temperature deformation and activation of the prism-< a > slip system. Crystal preferred orientations indicate a large component of flattening strain superimposed on the dextral shearing observed in the Kalinjala Shear Zone.

Mesozoic intracontinental ductile shearing along the Paleozoic Shangdan suture in the Qinling Orogen: Constraints from deformation fabrics and geochronology

AbstractThe Qinling Orogen, located between the North and South China Blocks, records subduction-collisional orogeny along the Paleozoic Shangdan and Triassic Mianlue sutures, and Mesozoic intracontinental orogenesis, which all played an important role in building the present tectonic framework and topography. The strike-slip Shagou ductile shear zone that overprints the Paleozoic Shangdan suture between the North and South Qinling Belts is crucial for understanding the Mesozoic intracontinental deformation in the Qinling Orogen. The microstructures, asymmetrical fabrics, and kinematic vorticities (0.81–0.95) suggest sinistral simple shear. The quartz c-axis patterns from felsic mylonites exhibit Y-maximum fabrics, indicating the activity of prism &amp;lt;a&amp;gt; slip, while those from amphibole-rich mylonites display both Y-maximum and Z-maximum fabrics showing the combined activity of prism &amp;lt;a&amp;gt; and basal &amp;lt;a&amp;gt; slip systems. In the mafic mylonites, the plagioclase fabrics were induced by combined (010)[100] and (001)[100] slip systems, while the amphibole fabrics were probably related to anisotropic growth or passive rotation of rigid clasts. Equilibrium P–T conditions of 4.28–6.12 kbar and 646–727 °C estimated from geothermobarometry suggest that the main deformation occurred under amphibolite facies conditions at middle–lower crustal depths. Zircon U-Pb ages constrain their protolith rocks to have crystallized at 816 ± 25 Ma and 726–718 Ma, while the intense mylonitization and sinistral shearing occurred at ca. 200–187 Ma. A U–Pb zircon age of 132 ± 3 Ma from a granitic dike cutting the foliation and an amphibole 40Ar/39Ar plateau age of 119.0 ± 0.9 Ma from mylonites together suggest that the Shagou shear zone evolved through decompression and exhumation stages in the time period of 132–119 Ma.

Geochronological Constraints for Boundary Shear Zones between Eastern Ghats Province and Bastar Craton: Implication for the Formation of Granulites and Their Exhumation History

Shear zones in the boundary between Eastern Ghats Province (EGP) and the cratons of Singhbhum in the north and Bastar in the west provide an excellent opportunity to study the tectonics of shear zone development and its timing in relation to the evolutionary history of the granulite suites. Detailed structural, microfabric and quartz C-axis patterns revealed a high temperature shear zone, at the western boundary between EGP and Bastar Craton (BC) around Paikmal. Petrological studies in this shear zone indicated decompression coeval with stretching in the sheared granulites. Geochronological constraints provided here indicate rapid exhumation of deep seated granulites in this boundary shear zone; the timing also is late in relation to the long-lived thermal (granulite formation) event in the EGP. Additionally, our geochronological data demonstrated the ~1600 Ma event in the Eastern Ghats Belt (EGB) involving sedimentation, magmatism, metamorphism and crustal anatexis, as a significant world event.

CPO and kinematic analysis of the Bitou S-tectonites (Central Cameroon shear zone): AMS and EBSD investigations

The development of foliation is not always associated with mineral stretching lineation in deformed rocks. Sometimes, S-tectonites only display foliation with no mineral stretching lineation, and it becomes a real challenge to perform kinematic analysis, i.e., to define the XZ section of the ellipsoid. In this study, we present in a portion of the Central Cameroon Shear Zone (CCSZ) (the Bitou biotite gneiss mylonite), the usefulness of anisotropy of magnetic susceptibility (AMS) studies to identify the three principal axes of the AMS ellipsoid (K1 ≤ K2 ≤ K3), equivalents respectively to the principal axes of the strain ellipsoid (X ≤ Y ≤ Z). The K1K3 plane of the AMS ellipsoid is equivalent to the XZ section of the strain ellipsoid. The fabrics developed in the studied mylonitised biotite gneiss strike ENE–WSW to E–W with steep dips for the mylonitic and magnetic foliations and moderate plunges for the magnetic lineation. The rock is paramagnetic. The AMS ellipsoids are mostly of oblate shape, while the quartz c-axis pattern is typical of non-coaxial flow. This implies that deformation partitioning took place during mylonitisation. Quartz crystallographic preferred orientation (CPO) measured using electron backscatter diffraction reveals the activation of prism <a> slip, implying that the mylonitisation occurs under moderate temperature conditions (450°C < T < 550°C). Microstructures observed in the K1K3 section of the AMS ellipsoid and CPO of the quartz c-axis indicate sinistral top-to-SW sense of shear. These results support the shear senses of movement that earlier studies in the CCSZ have emphasised and are assumed to be related to the early syn-D2 and D3 events of the Pan-African tectonic dated at ca. 613–585 Ma.

Dunhuang Tectonic Belt in northwestern China as a part of the Central Asian Orogenic Belt: Structural and U-Pb geochronological evidence

The Dunhuang Tectonic Belt (DTB) is located about 100 km south of the Beishan–Tianshan orogen in the Central Asian Orogenic Belt in NW China. It was previously considered as a part of the Tarim or North China craton.Detailed structural analyses reveal two episodes of deformation in the central DTB, D1 and D2. D1 is a north-side-up reverse shear, and D2 a dextral strike slip. Mineral assemblages, microstructures and quartz C-axis patterns indicate that D1 deformation took place under amphibolite facies conditions (500 to 600 °C) and D2 mostly under greenschist-facies conditions (300–450 °C). U–Pb zircon dating of eight granitoid/intermediate intrusions (mostly dikes, with well constrained cross-cutting relationships with the D1 and D2 structures) and an amphibolite gneiss indicates that D1 deformation took place before ca. 349 Ma and most likely at ca. 406 Ma, and D2 between ca. 249 Ma and ca. 241 Ma.The DTB has a structural, metamorphic and magmatic signature in the Paleozoic–Mesozoic that is typical of an orogenic belt. It shares a similar geological history with the Beishan–Tianshan orogen and is likely a part of the Central Asian Orogenic Belt. The DTB and the Beishan-Tianshan orogen might represent two separate Paleozoic mountain belts that developed more or less synchronously on the south and north sides, respectively, of the last vestige of the Paleo-Asian Ocean before its terminal closure in the Permian. The D1 reverse shearing in the DTB is interpreted to be related to a Silurian–Devonian terrane accretion/collision and the D2 dextral strike slip to post-accretional/collisional movement among terranes in Late Permian–Middle Triassic time.

Coalescing microstructure and fabric transitions with AMS data in deformed limestone: Implications on deformation kinematics

The microstructures of the heterogeneously deformed limestones of the Kolhan Group, Singhbhum Craton (SC), eastern India are investigated to study deformation mechanism, grain size and shape preferred orientation (SPO) development. The texture of the deformed rocks is analyzed by Electron Back-Scattered Diffraction (EBSD) in order to determine crystallographic preferred orientation (CPO) development in calcite grains. The c-axis pattern in protomylonite and mylonite define girdle patterns developed perpendicular to local shear planes showing monoclinic symmetry. The latter is interpreted in terms of a top-to-south shear, which is consistent with the observed microstructural and fabric data from the deformed rocks. The Anisotropy of Magnetic Susceptibility (AMS) studies are performed on 22 heterogeneously strained limestone specimens to document strain variation within the Kolhan rocks. The field fabric, SPO data measured from calcite and mica grains (κcal and κmi) and calcite CPO data are compared with observed variation in AMS fabric intensity (Pj) across the Kolhan basin. Integration of these data sets help to establish presence of large internal thrust planes within the Kolhan basin, which form a part of the regional foreland vergent thrust system developed along the northern margin of the SC.

Determining the Reference Frame for Kinematic Analysis in S-tectonites Using AMS

ABSTRACT In this paper a methodology based on anisotropy of magnetic susceptibility (AMS) measurements is presented to identify XZ section of the strain ellipsoid in order to perform kinematic analysis in S-tectonites, which are rocks that lack stretching lineation on the foliation plane. The similarity between orientations of three principal axes of the AMS ellipsoid (K1&amp;gt;K2&amp;gt;K3) and strain ellipsoid (X&amp;gt;Y&amp;gt;Z) is demonstrated by discussing the case of a mylonite (LS-tectonite) that contains both foliation and stretching lineation. Subsequently, the case of S-tectonite from the peninsular gneiss (south India) is discussed, where the K1K3 plane of AMS ellipsoid is equated with XZ section of strain ellipsoid. Quartz crystallo-graphic preferred orientations are investigated using SEM-EBSD studies in thin section prepared parallel to the K1K3 plane. Quartz c-axis patterns characteristic of rhomb &amp;lt;a&amp;gt; slip and indicating top-to-325° sense of shear are recorded. This supports the move-ment sense inferred by earlier researchers from northern parts of the sampled locality. Thus the usefulness of AMS in determining the reference frame for kinematic studies in S-tectonites is documented.

Quartz preferred orientation in naturally deformed mylonitic rocks (Montalto shear zone–Italy): a comparison of results by different techniques, their advantages and limitations

In the geologic record, the quartz c-axis patterns are widely adopted in the investigation of crystallographic preferred orientations (CPO) of naturally deformed rocks. To this aim, in the present work, four different methods for measuring quartz c-axis orientations in naturally sheared rocks were applied and compared: the classical universal stage technique, the computer-integrated polarization microscopy method (CIP), the time-of-flight (TOF) neutron diffraction analysis , and the electron backscatter diffraction (EBSD). Microstructural analysis and CPO patterns of quartz, together with the ones obtained for feldspars and micas in mylonitic granitoid rocks, have been then considered to solve structural and geological questions related to the Montalto crustal scale shear zone (Calabria, southern Italy). Results obtained by applying the different techniques are discussed, and the advantages as well as limitations of each method are highlighted. Importantly, our findings suggest that patterns obtained by means of different techniques are quite similar. In particular, for such mylonites, a subsimple shear (40% simple shear vs 60% pure shear) by shape analysis of porphyroclasts was inferred. A general tendency of an asymmetric c-maximum near to the Z direction (normal to foliation) suggesting dominant basal slip, consistent with fabric patterns related to dynamically recrystallization under greenschist facies, is recognized. Rhombohedral slip was likely active as documented by pole figures of positive and negative rhombs (TOF), which reveal also potential mechanical Dauphine twinning. Results showed that the most complete CPO characterization on deformed rocks is given by the TOF (from which also other quartz crystallographic axes can be obtained as well as various mineral phases may be investigated). However, this use is restricted by the fact that (a) there are very few TOF facilities around the world and (b) there is loss of any domainal reference, since TOF is a bulk type analysis. EBSD is a widely used technique, which allows an excellent microstructural control of the user covering a good amount of investigated grains. CIP and US are not expensive techniques with respect the other kind of investigations and even if they might be considered obsolete and/or time-consuming, they have the advantage to provide a fine and grain by grain “first round” inspection on the investigated rock fabric.

Recrystallization fabrics of sheared quartz veins with a strong pre-existing crystallographic preferred orientation from a seismogenic shear zone

Microstructural investigations were carried out on quartz veins in schist, protomylonite, and mylonite samples from an ancient seismogenic strike-slip shear zone (Sandhill Corner shear zone, Norumbega fault system, Maine, USA). We interpret complexities in the microstructural record to show that: (1) pre-existing crystallographic preferred orientations (CPO) in the host rock may persist in the new CPO patterns of the shear zone and (2) the inner and outer parts of the shear zone followed diverging paths of fabric development.The host rocks bounding the shear zone contain asymmetrically-folded quartz veins with a strong CPO. These veins are increasingly deformed and recrystallized with proximity to the shear zone core. Matrix-accommodated rotation and recrystallization may position an inherited c-axis maximum in an orientation coincident with rhomb <a> or basal <a> slip. This inherited CPO likely persists in the shear zone fabric as a higher concentration of poles in one hemisphere of the c-axis pole figure, leading to asymmetric crossed girdle or paired maxima c-axis patterns about the foliation plane.Three observed quartz grain types indicate a general trend of localization with decreasing temperature: (1) large (>100μm), low aspect ratio (<~5) and (2) high aspect ratio (~5–20) grains overprinted by (3) smaller (<~80μm), low aspect ratio (<~4) grains through subgrain rotation-dominated recrystallization. In the outer shear zone, subgrain rotation recrystallization led to a well-developed c-axis crossed girdle pattern. In the inner shear zone, the larger grains are completely overprinted by smaller grains, but the CPO patterns are relatively poorly developed and are associated with distinctively different misorientation angle histogram profiles (“flat” neighbor-pair profile with similar number fraction for angles from 10 to 90°). This may reflect the preferential activation of grain size sensitive deformation processes in the inner-most part of the shear zone where transient deformation associated with the seismic cycle may have localized.

Quantitative kinematic analysis of the asymmetric boudins of the Zagros accretionary prism, Iran

Asymmetric boudins that are widespread in the Seh-Ghalatoun area in the Neyriz, southwestern Iran are good indicators to estimate the kinematic vorticity number (Wk) and sense of shear. This area is part of the Sanandaj-Sirjan HP-LT metamorphic belt located within the Zagros orogenic belt. Boudins have been analyzed on the meso- and micro-scales. Domino and shearband boudins are developed in deformed quartzitic layers associated with quartzo-feldspathic gneiss, amphibolite, mica-schist and marble. Shearband boudins with backward rotation and domino boudins with forward rotation, mantled porphyroclasts, drag folds, and asymmetric folds which all display a dextral sense of shear. The mean kinematic vorticity number was estimated by using the asymmetric boudin hyperbolic distribution method and quartz caxis patterns. The estimated mean kinematic vorticity number is Wk = Wm = 0.85 ± 0.04, which indicates 34% pure shear and 66% simple shear components of deformation. The estimated mean deformation temperature based on the opening angle of the quartz c-axis fabrics is 530 ± 50 °C. These quantitative analyses suggest a sub-simple shear deformation developed at the epidote-amphibolite to amphibolite facies conditions at depth along the Sanandaj-Sirjan HP-LT metamorphic belt, which is associated with a transpressional flow regime with shortening perpendicular to the shear zone boundary.

Quartz c-axis fabric development associated with shear deformation along an extensional detachment shear zone: Chapedony Metamorphic Core Complex, Central-East Iranian Microcontinent

Lattice preferred orientations (LPOs) of quartz were used to establish differences in deformation geometry, finite strain and temperature across an extensional detachment shear zone within the Chapedony Metamorphic Core Complex in the Central-East Iranian Microcontinent along the northern flank of Gondwana. Quartz c-axis data show a continuous evolution across the core complex from asymmetric Type I crossed girdles at the southwest margin, to broken, asymmetric Type I crossed girdle and single girdle with a large concentration of axes plotted in the center of the stereoplot at the central parts of the core complex and small circle girdle pattern at the northeast margin. These variations in quartz c-axis patterns imply change in strain geometry during deformation from plane strain to general flattening and pure flattening. Integrating analyses of quartz c-axis opening angles, quartz c-axis patterns and recrystalization regimes of quartz and feldspar suggests deformation temperatures range between less than 400 °C and 650 °C, which yield greenschist to amphibolite facies conditions. Mean kinematic vorticity number (Wm) measured in the mylonite samples ranges between 0.67 and 0.71, which indicates that exhumation of the metamorphic rocks of the CMCC was facilitated by a significant component of pure shear strain within a general shear regime.

Ice deformed in compression and simple shear: control of temperature and initial fabric

AbstractLayered and polycrystalline ice was experimentally deformed in general shear involving axial compression (strain magnitude 0.5-17%) and simple shear (strain magnitude γ = 0.1-1.4). As the temperature is increased from -20°C to -2°C, there is at least a twofold enhancement in octahedral shear strain rate, which coincides with the onset of extensive dynamic recrystallization and a change in grain-size distribution at -15°C. Between -150C and -10°C the c-axis preferred orientation rapidly evolves with the initiation of two-maxima fabrics in shear zones. From -10°C to -2°C there is progressive evolution of a final c-axis pattern that is asymmetric with respect to the direction of shortening, with a strong maximum at ~5° to the pole of the shear zone, a sense of asymmetry in the direction of the shear, and a secondary maximum inclined at ~45° to the plane of shearing. An initial c-axis preferred orientation plays a critical role in the initial mechanical evolution. In contrast to established ideas, a strong alignment of basal planes parallel to the plane of easy glide inhibited deformation and there was an increased component of strain hardening until recrystallization processes become dominant.

The single-slip hypothesis revisited: Crystal-preferred orientations of sheared quartz aggregates with increasing strain in nature and numerical simulation

This study discusses the fabric development in naturally sheared quartz aggregates in comparison to results from texture modeling according to the polycrystalline plasticity theory with particular emphasis on the formation of a single c-axis maximum. The investigated natural shear zone samples were deformed at about 650 ± 50 °C with increasing strain up to γ ≈ 14 and show dynamic recrystallization microstructures of grain boundary migration recrystallization. Neutron diffraction texture analysis results in c-axis pole figures with a single maximum at the periphery of the pole figure. This maximum does not align with the shear plane normal towards higher strain, but rotates towards an inclined orientation in accordance with the sense of shear. Such a rotation is inconsistent with the single-slip hypothesis and suggests that the formation of this c-axis pattern is controlled by multi-slip on several slip systems. Based on the polycrystalline plasticity theory, this quartz fabric can develop if combined { 10 1 ¯ 1 } 〈 1 2 ¯ 10 〉 {r}<a>, { 1 ¯ 011 } 〈 1 2 ¯ 10 〉 {z}<a> and { 10 1 ¯ 1 } 〈 1 2 ¯ 10 〉 prism<a> slip dominates and must not be related to the commonly proposed ( 0001 ) 〈 1 2 ¯ 10 〉 basal<a> slip. The multi-slip texture development is in agreement with the shear sense interpretation from the asymmetry between well-defined quartz fabrics and the foliation. For dominant ( 0001 ) 〈 1 2 ¯ 10 〉 basal<a> slip in quartz and γ > 2, numerical simulations predict a single peripheral maximum perpendicular to the shear plane and two a-maxima with a ∼30°-inward position parallel to the shear plane. This simulation corresponds to naturally observed CPO patterns of quartz formed at different deformation conditions and it is in agreement with the single-slip hypothesis. Hence, our combined natural and numerical data suggest that the single-slip hypothesis is a possible explanation for a single c(0001)-maximum but not universally true.

Evaluating ice fabrics using fabric analyser techniques in Sørsdal Glacier, East Antarctica

AbstractIce cores (∼4 m long) obtained from areas of different surface velocities near the terminus of Sørsdal Glacier, East Antarctica, have been investigated using two versions of a fabric analyser (G50). In sections parallel to the flow plane, the microstructure is typically interlocking with elongate grains that parallel air-bubble elongation, X, reflecting their development in an earlier ductile regime. The c-axis fabric patterns vary with respect to X and vary from single–double maxima to asymmetric small-circle girdles oblique to the planar foliation, which can be attributed to a simple shear regime. The siteto-site variations in the c-axis patterns can be related to areas of different surface velocities, the asymmetry of fabrics correlating with localized strain variations and differences in the deformation path, but not to the current strain pattern recorded by the near-surface deformation conditions. Overprinting fractures have little effect on microstructure except for local dissolution and precipitation along stylolitic surfaces. Comparison of results from the two different fabric analysers reveals that with a higher pixel resolution the incorporation of additional monochromatic light-emitting diodes and repositioning of a retarder plate produce more reliable c-axis measurements.

Microstructure and elastic anisotropy of naturally deformed leucogneiss from a shear zone in Montalto (southern Calabria, Italy)

Abstract A strain gradient was mesoscopically recognized in sheared leucogneisses cropping out near Mount Montalto (Calabria, southern Italy) in the Aspromonte–Peloritani Unit on the basis of field observations. In order to investigate the relationship between textural and physical anisotropy, a microstructural and petrophysical study was carried out on selected mylonites exhibiting different stages of deformation. The main mineral assemblage is Qtz+Pl+Kfs+Wm, displaying S – C and shear-band textures; mica-fish and ribbon-like quartz are widespread. As strain increases K-feldspar, biotite and premylonitic low phengite white mica transformed to synmylonitic high phengite white mica and quartz, accompanied by an increasing albitization. Different quartz c -axis patterns are ascribable to non-coaxial progressive deformation; we suggest that deformation proceeded under greenschist- up to amphibolite-facies conditions owing to a local increase in shearing temperature. Laboratory seismic measurements were carried out on sample cubes (43 mm edged) cut according to the structural frame (foliation, lineation) of the rock. At 400 MPa and room temperature the averages of compressional ( V p ) and shear-wave velocities ( V s ) are very similar: 5.70–5.91 and 3.36–3.55 km s −1 , respectively. Seismic anisotropy and shear-wave splitting are related to the modal amounts of constituent minerals (in particular mica) and their crystallographic preferred orientation. Importantly, anisotropy is lowest in the most strained rock.