Anisotropy along a N-S profile of mica rich lithologies in the western Tauern Window (Eastern Alps, Austria)

Anatomy and internal structure of the Alpine orogens are difficult to decipher as structural information is usually limited to surface and seismic data. Seismic results very much depend on the elastic wave velocity model of the rocks. Simple velocity models depend strongly on the rock composition. Seismic properties are directionally dependent. Anisotropy can be subdivided into intrinsic (CPO of minerals and alignment in rock/texture) and extrinsic (compositional layering or fractures) anisotropy. In the investigated rock samples, phyllosilicates are by far most decisive for the elastic anisotropy due to their platy shape. We present here the first results of fabric analysis in a N-S profile of phyllosilicate-rich samples (mainly Innsbruck quartzphyllite and Bündner schist) from the Brenner Base Tunnel Project in order to obtain a refined anisotropy and velocity model. Phyllosilicate-rich sections were selected from borehole and tunnel samples, from which 1.5 – 3.5 mm wide columns were drilled out from layers of different composition and structure. The CPO of phyllosilicates and graphite was measured using high energy X-ray diffraction at German Electron Synchrotron (DESY) and European Synchrotron Radiation Facility (ESRF). Pole figure data were directly extracted using single peak evaluation and compared to the optical microstructure and compositional distributions using µXRF measurements. Texture strength is variable along the section with peak values at the transition from the Innsbruck quartzphyllite to the upper Bündner schist. The texture strength correlates positively with the content and distribution of phyllosilicates and graphite. By measuring the smallest representative volume element, we estimate the upper bound of expected intrinsic velocity anisotropies. The effect of (micro)structure-based upscaling on these anisotropies will be discussed

Dislocation creep of dry quartz

International audienceSmall-scale shear zones within the Permian Truzzo meta-granite developed during the Alpine orogeny at amphibolite facies conditions. In these shear zones magmatic quartz deformed by dislocation creep and recrystallized dynamically by grain boundary migration with minor subgrain rotation recrystallization to a grain size of around 250–750 μm, consistent with flow at low differential stresses. Fourier transform infrared (FTIR) spectroscopy reveals very low water contents in the interior of recrystallized grains (in the form of discrete OH peaks, ~20 H/10 6 Si and very little broad band absorption, <100 H/10 6 Si). The spectral characteristics are comparable to those of dry Brazil quartz. In FTIR spectra, magmatic quartz grains show a broad absorption band related with high water concentrations only in those areas where fluid inclusions are present while other areas are dry. Drainage of fluid inclusions and synkinematic growth of hydrous minerals indicates that a hydrous fluid has been available during deformation. Loss of intragranular water during grain boundary migration recrystallization did not result in a microstructure indicative of hardening. These FTIR measurements provide the first evidence that quartz with extremely low intragranular water contents can deform in nature by dislocation creep at low differential stresses. Low intragranular water contents in naturally deformed quartz may not be necessarily indicative of a high strength, and the results are contrary to implications taken from deformation experiments where very high water contents are required to allow dislocation creep in quartz. It is suggested that dislocation creep of quartz in the Truzzo meta-granite is possible to occur at low differential stresses because sufficient amounts of intergranular water ensure a high recovery rate by grain boundary migration while the absence of significant amounts of intragranular water is not crucial at natural conditions

Deformation in the Greiner Shear Zone ̶ Pfitsch Valley, Southwestern Tauern Window

The Greiner Shear Zone is located within the Subpenninic core of the southwestern Tauern Window, Eastern Alps. It strikes SW-NE and separates the upright folded Zillertaler and Tuxer Zentralgneis Nappes from one another, whilst transecting their parautochthonous cover and allochthonous hanging-wall units. The Greiner Shear Zone is generally regarded as a transpressive shear zone, composed of multiple high strain zone splays, in which dextral, but dominantly sinistral shear sense indicators have been reported (Behrmann & Frisch 1990; Barnes et al. 2004). Deformation in the Greiner Shear Zone is pervasive and characterized by a sub-vertical foliation and west-southwest plunging lineation. However, the tectono-metamorphic history of the Greiner Shear Zone has not yet been fully clarified. To better constrain the structural architecture, kinematics, relative timing and spatial extent of the Greiner Shear Zone, geologic mapping within the Pfitsch Valley was carried out and structural data were collected. Optical and scanning electron microscopy analyses including EBSD were utilized to characterize the microstructure of deformed Furtschagl Schists incorporated within the shear zone. Deformed, inter-tectonic(ally grown) biotite porphyroblasts were characterized to ascertain the finite deformation history, as well as conduct a Schmid Factor analysis for (001)-slip of biotite grains, which is indicative of post-growth kinematics. Crystallographic dispersion axes of quartz grains were used to derive a vorticity axes distribution to better constrain the kinematics of the late Greiner Shear Zone. Furthermore, thermodynamic modelling using Theriak Domino was conducted to constrain the metamorphic evolution for shear zone samples stemming from the Furtschagl Schists, Venediger Nappe and Glockner Nappe. Geologic mapping resulted in a new geologic map of the study area, and three cross-sections constructed perpendicular to strike of the Greiner Shear Zone of the Pfitsch Valley section. Unoriented biotite grains in the Furtschagl Schist are interpreted to have grown a) over a pre-existing foliation and b) prior to the (late) Greiner Shear Zone activity, the latter resulting in a co-planar fabric with a rather minute overprint of the pre-existing deformation fabric. Schmid Factor analysis on those biotites indicates a sub-horizontal to N-plunging, N-S directed compression direction which resulted in sinistral shearing at the time deformation was ceasing. Results of the crystallographic dispersion axis analyses suggest shallow NE to E plunging axes on a shallow N- to steep NW-dipping flow plane, respectively. Based on the petrological investigations and thermodynamic modelling, a clockwise pT-path from blueschist facies to amphibolite facies conditions of approx. ~570°C and 6.8-7.5 kb could be derived for the Furtschagl Schists of the Venediger Nappe. Peak amphibolite facies conditions occur at the transition from early, syn-kinematic Greiner shearing to an inter-tectonic phase (Tauern Crystallization) as indicated by garnet and biotite growth. Therefore, post-Tauern Crystallization deformation of the Greiner Shear Zone within the Furtschagl Schists is the result of general shear dominated transpression at amphibolite facies metamorphic conditions, linked to sinistral strike-slip kinematics, which were active during N-S directed compression. The shear zone is further interpreted to exhibit a heterogenous monoclinic deformation symmetry, which is likely the result of an interconnected, anastomosing shear zone network

To sink, swim, twin, or nucleate: A critical appraisal of crystal aggregation processes

Crystal aggregates in igneous rocks have been variously ascribed to growth processes (e.g., twinning, heterogeneous nucleation, epitaxial growth, dendritic growth), or dynamical processes (e.g., synneusis, accumulation during settling). We tested these hypotheses by quantifying the relative orientation of adjacent crystals using electron backscatter diffraction. Both olivine aggregates from Kīlauea volcano (Hawaiʻi, USA) and chromite aggregates from the Bushveld Complex (South Africa) show diverse attachment geometries inconsistent with growth processes. Near-random attachments in chromite aggregates are consistent with accumulation by settling of individual crystals. Attachment geometries and prominent geochemical differences across grain boundaries in olivine aggregates are indicative of synneusis

Describing complex interactions of social-ecological systems for tipping point assessments: an analytical framework

Humans play an interconnecting role in social-ecological systems (SES), they are part of these systems and act as agents of their destruction and regulation. This study aims to provide an analytical framework, which combines the concept of SES with the concept of tipping dynamics. As a result, we propose an analytical framework describing relevant dynamics and feedbacks within SES based on two matrixes: the “tipping matrix” and the “cross-impact matrix.” We take the Southwestern Amazon as an example for tropical regions at large and apply the proposed analytical framework to identify key underlying sub-systems within the study region: the soil ecosystem, the household livelihood system, the regional social system, and the regional climate system, which are interconnected through a network of feedbacks. We consider these sub-systems as tipping elements (TE), which when put under stress, can cross a tipping point (TP), resulting in a qualitative and potentially irreversible change of the respective TE. By systematically assessing linkages and feedbacks within and between TEs, our proposed analytical framework can provide an entry point for empirically assessing tipping point dynamics such as “tipping cascades,” which means that the crossing of a TP in one TE may force the tipping of another TE. Policy implications: The proposed joint description of the structure and dynamics within and across SES in respect to characteristics of tipping point dynamics promotes a better understanding of human-nature interactions and critical linkages within regional SES that may be used for effectively informing and directing empirical tipping point assessments, monitoring or intervention purposes. Thereby, the framework can inform policy-making for enhancing the resilience of regional SES

Describing complex interactions of social-ecological systems for tipping point assessments: an analytical framework

Humans play an interconnecting role in social-ecological systems (SES), they are part of these systems and act as agents of their destruction and regulation. This study aims to provide an analytical framework, which combines the concept of SES with the concept of tipping dynamics. As a result, we propose an analytical framework describing relevant dynamics and feedbacks within SES based on two matrixes: the “tipping matrix” and the “cross-impact matrix.” We take the Southwestern Amazon as an example for tropical regions at large and apply the proposed analytical framework to identify key underlying sub-systems within the study region: the soil ecosystem, the household livelihood system, the regional social system, and the regional climate system, which are interconnected through a network of feedbacks. We consider these sub-systems as tipping elements (TE), which when put under stress, can cross a tipping point (TP), resulting in a qualitative and potentially irreversible change of the respective TE. By systematically assessing linkages and feedbacks within and between TEs, our proposed analytical framework can provide an entry point for empirically assessing tipping point dynamics such as “tipping cascades,” which means that the crossing of a TP in one TE may force the tipping of another TE. Policy implications: The proposed joint description of the structure and dynamics within and across SES in respect to characteristics of tipping point dynamics promotes a better understanding of human-nature interactions and critical linkages within regional SES that may be used for effectively informing and directing empirical tipping point assessments, monitoring or intervention purposes. Thereby, the framework can inform policy-making for enhancing the resilience of regional SES

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at root s(NN)=2.76TeV

Peer reviewe

The role of quartz during deformation of polyphase rocks

In this thesis, deformation mechanisms, rheology and the related microstructural evolution of quartz during deformation of polyphase rocks are investigated in natural, high temperature, small scale shear zones in granitoids. Selected shear zones are from the Gran Paradiso Nappe, Western Alps, Italy, and the Truzzo granite of the Tambo Nappe, Central Alps, Italy. The microstructural evolution inside strain gradients from non- or weakly deformed hostrocks to ultramylonitic shear zones is studied by optical microscopy, scanning electron microscopy and image analysis, crystallographic preferred orientations (CPO) by means of orientation imaging using computer integrated polarization microscopy (CIP) and electron backscatter diffraction (EBSD). Magmatic quartz grains recrystallize dynamically, and form polycrystalline aggregates that deform by dislocation creep inside a fine grained feldspar-mica matrix, which deforms by diffusion creep. With increasing strain, quartz aggregates form layers and finally disintegrate into a grain-scale polymineralic mixture with K-feldspar, plagioclase and mica. The disintegration of quartz aggregates marks the transition from a mylonite to an ultramylonite and occurs by intergranular dilatancy related to grain boundary sliding and by the precipitation of K-feldspar and biotite. Polycrystalline quartz aggregates deform as porphyroclasts embedded in a lower viscous matrix at low and constant differential stresses. This situation is approximated by Reuss-bound conditions. Quartz aggregates disintegrate because quartz fails to deform by dislocation creep compliant with the matrix. In the Gran Paradiso shear zones (lower amphibolite facies, ~500-550°C), a stable quartz grain size forms by a dynamic equilibrium. Subgrain rotation recrystallization and grain boundary migration recrystallization (synkinematic grain growth) contribute to a grain size decrease and increase respectively. In the ultramylonite the quartz grain size decreases below the quartz subgrain size and approaches the matrix grains size. The dynamic quartz grain size is decreased by a combination of pinning and dissolution processes. The constant quartz volume fraction requires quartz precipitation. Pinning eliminates the contribution of synkinematic grain growth, and dissolution permits a quartz grain size below the subgrain size. The ultramylonite deforms by dissolution-precipitation assisted diffusion creep. Quartz in the ultramylonite shows isotropic grain shapes and a random CPO. Polycrystalline quartz aggregates in the Gran Paradiso mylonites develop a strong CPO consistent with the activity of the basal- slip system. The CPO of each aggregate develops with respect to a local kinematic framework and not to the shear zone reference frame. The local reference frame is defined by the quartz aggregate and its position in the matrix. A strong CPO develops already at low strain, and peripheral [c]-axis maxima reach a stable position at about 70° with respect to the flow plane. Quartz aggregates show a local shear sense, which, at low strain is systematically opposite to the global shear sense. The inverse shear sense is interpreted to result from flow partitioning between the higher viscous quartz aggregate and the lower viscous matrix in bulk simple shear. The quartz fabric is in most cases related to the CPO such that the maximum of the surface orientation distribution function is synthetically rotated with respect to the sense of slip on the quartz basal plane. This situation implies a crystallographic control of the fabric development. Orthorhombic surface fabrics are suggested to form at high grain boundary mobility, monoclinic surface fabrics from at lower grain boundary mobility. In the Truzzo granite shear zones (amphibolite facies, ~550-650°C), dynamic recrystallization of quartz is dominated by grain boundary migration with an increasing contribution of subgrain rotation related microstructures during advanced stages of deformation. The ultramylonite part of the shear zones deforms by diffusion creep. Single quartz grains show a shape anisotropy and a very weak CPO, interpreted to result from a contribution of intracrystalline plasticity. Local biotite breakdown and subgrain rotation - bulging recrystallization occurs in thin, newly coalesced layers which represent the very latest structures in the ultramylonite. This relates to the inversion of the viscosity “contrast” between quartz and the feldspathic material at higher differential stresses and lower temperature. Theoretical and experimentally derived flow laws from literature are tested with the data obtained in this thesis. The quartz - feldspathic behavior can be simulated, resulting in geologically reasonable strain rates and viscosity ratios. In the Truzzo granite shear zones, Fourier transform infrared spectroscopy (FTIR) measurements inside single grains reveal that quartz remains dry during recrystallization at the main deformation event. FTIR spectra are flat and the water content is comparable to that of brazil quartz. Fluid inclusions are expelled during grain boundary migration recrystallization. Deformation took place at water present conditions, and quartz grain size piezometers suggest low differential stresses while dry quartz is considered extremely strong during experimental deformation. It is suggested that grain boundary processes contribute to the commonly observed weakening of polycrystalline quartz during fluid present conditions and that the low water concentrations might be sufficient for crystal plasticity at natural conditions