Karsten Kunze

Evolution of a calcite marble shear zone complex on Thassos Island, Greece: microstructural and textural fabrics and their kinematic significance

The deformation history of a monophase calcite marble shear zone complex on Thassos Island, Northern Greece, is reconstructed by detailed geometric studies of the textural and microstructural patterns relative to a fixed reference system (shear zone boundary, SZB). Strain localization within the massive marble complex is linked to decreasing P–T conditions during the exhumation process of the metamorphic core complex. Solvus thermometry indicates that temperatures of 300–350°C prevailed during part of the shear zone deformation history. The coarse-grained marble protolith outside the shear zone is characterized by symmetrically oriented twin sets due to early coaxial deformation. A component of heterogeneous non-coaxial deformation is first recorded within the adjacent protomylonite. Enhanced strain weakening by dynamic recrystallization promoted strong localization of plastic deformation in the ultramylonite of the calcite shear zone, where high strain was accommodated by non-coaxial flow. This study demonstrates that both a pure shear and a simple shear strain path can result in similar crystallographic preferred orientations (single c-axis maximum perpendicular to the SZB) by different dominant deformation mechanisms. Separated a-axis pole figures (+a- and −a-axis) show different density distributions with orthorhombic texture symmetry in the protolith marble and monoclinic symmetry in the ultramylonite marble consistently with the observed grain fabric symmetry.

Orientation analysis of localized shear deformation in quartz fibres at the brittle–ductile transition

Abstract The crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 270° to 370°C has been investigated. To explore the active mechanism during deformation of the quartz fibres, the complete crystallographic orientation of individual recrystallized grains and subgrains was determined by using a combination of CIP (computer-integrated polarization microscopy), EBSD (electron back-scatter diffraction), and additional specially developed image analysis techniques. It was found that the quartz fibres deformed along crystallographically controlled shear bands. From inverse pole figures of normals to the shear band boundaries it is evident that these bands occur along the positive and negative rhomb planes of quartz. It is suggested that they are initiated as fractures along these planes. Inside the shear bands, small grains broken off from the host grain show passive rotation of the lattice, consistent with the shear sense of the shear bands. Continued deformation and increasing temperature lead to different recrystallization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships.

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

Abstract 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.

TEXTURE OF SOLNHOFEN LIMESTONE DEFORMED TO HIGH STRAINS IN TORSION

Solnhofen limestone was deformed in torsion to shear strains (γ) ranging from 1 to 12, at a temperature of 750 °C, 300 MPa confining pressure and a maximum strain-rate of 10−3 s−1. These deformation conditions correspond to the intracrystalline power-law dislocation creep field close to the boundary to the grain-size-sensitive superplastic creep field. The grain-shape microstructure was observed using orientation contrast by backscattered electrons in the SEM. The grains remain sub-equant with an average grain size of around 4 μm, even to the highest strains. Lattice preferred orientation was determined using both X-ray texture goniometry and automated electron back-scatter diffraction. The c-axis preferred orientation develops from two main maxima with a weak sub-maximum, through two maxima to a single maximum perpendicular to the shear plane. The rate of increase of the intensity of the single maximum with increasing strain diminishes, and it appears that there is a tendency to a steady-state texture. The final single c-axis maximum is displaced slightly towards the shortening direction of the applied simple shear. The a-axes tend to a girdle perpendicular to the c-axis maximum. It is proposed that the partitioning of deformation between intra- and inter-crystalline mechanisms results in a pulsating strain state in the grains, contributing to the maintenance of sub-equant grains. It is argued that the lattices of constituent grains rotate continuously with no stable end orientation and that this can lead to a steady-state texture. The experimental preferred orientation compares well with that of natural calcite mylonites in the position of the c-axis maximum and the a-axis girdle.

Microstructures and lattice preferred orientations in experimentally deformed clinopyroxene aggregates

Microstructures and lattice preferred orientations (LPO) were analysed on experimentally deformed natural and hot-pressed clinopyroxene aggregates in order to understand the relationship between deformation processes and evolving microstructures. The LPO was measured using electron backscatter diffraction techniques in the scanning electron microscope (SEM). Microstructures were observed by polarized light microscopy and by orientation contrast in the SEM. Natural samples (Sleaford Bay pyroxenite) were deformed in axial compression stepping tests up to 16% shortening. These samples deformed mainly by twinning and dislocation glide with very little recrystallization. No clear LPO evolution apart from the initial LPO could be attributed to deformation. Synthetic clinopyroxenite samples were hot-pressed from powders of the same material with three different grain size ranges, and deformed in compression stepping experiments up to 28% shortening. In the samples with coarse (30 μm) and intermediate (20 μm) grain sizes, deformation was dominated by dislocation creep accommodated by subgrain rotation recrystallization. The recrystallized grains have sizes up to 8 μm and represent 15–25% of the sampled area. The sample with the finest initial grain size (5 μm) deformed dominantly by diffusion creep accompanied by grain boundary migration. All of the hot-pressed samples deformed in compression have a similar texture, consisting of a girdle of c[001] axes normal to compression and a point maximum of b[010] axes and a*(100) poles parallel to the compression direction. The LPO of recrystallized grains was separated on spatially resolved orientation maps and shows an S-type fabric with b[010] parallel to the compression axis and a random a*(100) distribution. The bulk fabric was largely present in the hot-pressed starting material and is interpreted as the result of compaction during cold-pressing. At moderate strains in axial compression, the initial compaction fabric has been weakly overprinted by the recrystallization fabric. One hot-pressed sample (coarse grain size) was deformed in torsion to a shear strain of γ=0.1. The microstructure indicates dominant dislocation creep and contains about 20% recrystallized grains. The texture evolved from the rotationally symmetric compaction LPO to an oblique fabric.

The influence of nano-scale second-phase particles on deformation of fine grained calcite mylonites

Grey and white carbonate mylonites were collected along thrust planes of the Helvetic Alps. They are characterised by very small grain sizes and non-random grain shape (SPO) and crystallographic preferred orientation (CPO). Presumably they deformed in the field of grain size sensitive flow by recrystallisation accommodated intracrystalline deformation in combination with granular flow. Both mylonites show a similar mean grain size, but in the grey mylonites the grain size range is larger, the grain shapes are more elongate and the dynamically recrystallised calcite grains are more often twinned. Grey mylonites have an oblique CPO, while the CPO in white mylonites is symmetric with respect to the shear plane. Combustion analysis and TEM investigations revealed that grey mylonites contain a higher amount of highly structured kerogens with particle sizes of a few tens of nanometers, which are finely dispersed at the grain boundaries. During deformation of the rock, nano-scale particles reduced the migration velocity of grain boundaries by Zener drag resulting in slower recrystallisation rates of the calcite aggregate. In the grey mylonites, more strain increments were accommodated by individual grains before they became refreshed by dynamic recrystallisation than in white mylonites, where grain boundary migration was less hindered and recrystallisation cycles were faster. Consequently, grey mylonites represent ‘deformation’ microfabrics while white mylonites are characterised by ‘recrystallisation’ microfabrics. Field geologists must utilise this different deformation behavior when applying the obliquity in CPO and SPO of the respective mylonites as reliable shear sense indicators.

Deformation behavior of silver submicrometer-pillars prepared by nanoimprinting

Scaling in material properties is of great importance in microsystems and microelectronics where the device dimensions continuously shrink. Recently, compression tests of micron-sized pillars produced using a focused-ion beam (FIB) have become standard in the investigation of scaling effects. The influence of the fabrication process on the mechanical properties of the samples has, however, not been conclusively resolved. In this study, 130 silver pillars were fabricated using a novel embossing technique that does not pose the issues associated with FIB milling, i.e. surface amorphization and gallium contamination. Displacement-controlled compression tests on pillars with diameters of 130–3000 nm reveal for submicrometer samples an inverse proportionality of flow strength to diameter, which is associated with a wide strength distribution and a deformation taking place in large discrete strain bursts. The largest pillars show instead near bulk-like behavior. Unlike studies on other fcc materials, mechanical twinning is also an important deformation mechanism in Ag due to its very low stacking fault energy. In addition, occasional preexisting twin boundaries constituted preferential planes for localized deformation if favorably sloped. Despite the absence of ion-induced artifacts, scaling laws and absolute values of strength are comparable to those obtained via FIB milling.

Texture analysis of a recrystallized quartzite using electron diffraction in the scanning electron microscope

Abstract The microstructure and crystallographic preferred orientation (texture) of a recrystallized quartzite from the Bergell Alps was quantitatively investigated using automated indexing of electron backscattered patterns (EBSP). The results are displayed in orientation maps of the microstructure and orientation as well as misorientation distribution functions. The orientation distribution function derived from EBSP compares excellently with texture data derived from independent neutron diffraction experiments. The misorientation distribution function of next neighbors reveals strong maxima for small angle grain boundaries (subgrain boundaries) and the Dauphine twin correlation. The Dauphine twins, which are generally not detectable optically, are not correlated with specific maxima in the orientation distribution function. Their origin and their role during recrystallization is discussed. During recrystallization no other preferred high angle misorientations developed. The frequent occurrence of small angle grain boundaries indicates progressive subgrain rotation as a recrystallization mechanism.

P-wave anisotropy in eclogites and relationship to the omphacite crystallographic fabric

Abstract Measurements of P-wave velocity at room temperature and confining pressures up to 500 MPa were carried out on three eclogite mylonites collected from a shear zone in the Monviso area (Western Alps). P-wave velocities at a pressure of 400 MPa range from 7.7 km/s to 7.9 km/s, yielding to a maximum anisotropy of 6%. From the CPO of omphacite we estimated a maximum contribution of omphacite to the P-wave anisotropy of only 1.3%. These results suggest that primarily the compositional layering and secondary the fabric of minor constituent minerals significantly contribute to the seismic anisotropy. Because of the anisotropy, the seismic reflectivity of subduction zones may vary with the direction of observation.

Ultrafine-grained quartz mylonites from high-grade shear zones: Evidence for strong dry middle to lower crust

Creep strength of the crust depends upon the rheology of the most common mineral, usually quartz. Recrystallized quartz grains in many high-grade shear zones from the middle to lower crust are typically large (millimeter sized), implying active grain boundary migration, but equivalents from old polymetamorphic and water-deficient basement sheared at similar crustal depths can be very small. For the latter, strong crystallographic preferred orientation (CPO) of quartz, with c axes aligned close to Y, progressive misorientation of crystals, subgrain and dislocation development, and core-mantle structures with recrystallized grains of 2‐8 m size, all point to dislocation glide dominantly on the prism a system with recrystallization by subgrain rotation. Recrystallized grain size piezometry of such quartz indicates high flow stress in fine-grained shear zones, while the synkinematic metamorphic mineral assemblage and the CPO are typical of amphibolite facies conditions. This is evidence that middle to lower crust is not inevitably weak due to its high temperature: water content also has an important influence.