The Tauern Window (TW) in the European Eastern Alps is one of Earth’s largest tectonic windows. It comprises nappes that were formed by the southward subduction of the European plate beneath the Adriatic plate. These nappes were stacked during the Late Eocene and, subsequently refolded during the Miocene due to the northward push of the eastern Southalpine Indenter. This process exhumed the western TW by up to 25 km, and coevally caused lateral escape and extensional tectonics. However, the Miocene deformation history of the western TW is still under ongoing debate. This study focuses on the Miocene deformation history of the western TW using 2-D, 3-D, and 4-D approaches.We first restore a N-S oriented cross-section along the Brenner Base Tunnel using published zircon fission-track and P-T data. Restoration reveals two deformation phases: upright folding of the top of the nappe stack started to cease around 17 Ma, followed by thrusting of the entire nappe stack along the Sub-Tauern ramp. Contemporaneously, the hanging-wall nappes experienced 44–50% thinning due to W–E extension.Our static 3-D reconstruction of the present-day structure of the western TW integrates published maps, cross-sections, and structural field data. The model discloses lateral structural changes, e.g., the transition of upright folds in the east into overturned folds in the west with varying plunge of the fold axes. We hypothesize that detachment of the lower crust of the eastern Southalpine Indenter caused different styles of deformation in front of it during indentation.To prove our hypothesis, we restore the western TW in 4-D using the same method as for our 2-D reconstruction. We displace the nappe stack of the western TW downwards along the Sub-Tauern ramp (ca. 10 km over 15 Ma), followed by unfolding under high-temperature conditions, which allows viscous deformation. Finally, we will integrate strain information to restore the component of lateral escape.

Laser ablation U-Pb single zircon geochronology was applied to four peraluminous granite and granodiorite samples from the Bassi & egrave;s pluton in the Central Pyrenees (France) yielding a wide range of concordant ages from early Carboniferous (Tournaisian, 351 Ma) to early Permian (Artinskian, 285 Ma). Emplacement of the Bassi & egrave;s pluton occurred incrementally during the main Variscan deformation phase, with increased activities every 7-15 Myr and a peak at around 321 Ma. Evolution of the Bassi & egrave;s pluton is more complex than the previously published single age of 312 Ma implied, underscoring the importance of synorogenic Variscan magmatism within the Axial Zone. Supported by geochronologic data from other plutons and gneiss domes, the notion that magmatism in the Axial Zone was confined to the late- to post-orogenic phase (315-295 Ma) is refuted. A possible thermal source for the long-lasting widespread plutonism in the Variscan crust is the TUZO mantle plume.

The Tauern Window (TW) in the European Alps is one of the largest tectonic windows in the world. Its formation started in the Cretaceous with subduction of the Penninic realm beneath the northern margin of Adria leading to the collision between Europe (Subpenninic) and the Adria margin (Austroalpine). The resulting Penninic and Subpenninic nappe stack was exhumed by ca. 20 km by the approach of the Dolomites Indenter (Eastern Southern Alps) in the Miocene. This last deformation stage resulted in synkinematic N-S shortening of the western TW (ca. 70 km), W-E extension and lateral extrusion towards the east. However, how the Subpenninic core (Venediger Duplex; VD) and the Penninic and Austroalpine nappes (PN and AN, respectively) in the hanging-wall were tectonically stacked, upright folded and emplaced is poorly understood. This study investigates the deformation accommodated by each major tectonic basement unit of the western TW, and contributes to a better understanding of orogenic processes in general.We kinematically restore the cross-section of [1] along the Brenner Base Tunnel (W of the TRANSALP seismic profile) using the software MOVEtm (Petroleum Experts), focusing firstly on the VD. We choose area balancing as minimum criteria, because we do not know how much material was transported out of the plane of cross-section by extension and lateral extrusion. We integrate zircon fission-track data (ZFT) as a temporal constraint and test different geothermal gradients. Petrological data are used to define the maximum depth the VD reached at the time of indentation and as marker for the transition from brittle to viscous conditions of the felsic rocks of the VD (lowest temperature for folding). Finally, we reconstruct the hanging-wall nappes above the restored VD, thereby precisely constraining the position of the AN at that time. The surface samples taken from the AN must have reached thermal conditions between the annealing zones of apatite fission-tracks and ZFT (115°C and 180°C, respectively) as only the former system was reset in the Miocene.We first displace the entire VD down along the Sub-Tauern Ramp below the 300°C isotherm (brittle to viscous transition of felsic rocks). For this, the geothermal gradient of 50°C/km fits well to the petrological data. ZFT ages reveal upright folding of the VD terminated at ca. 17 +/- 2 Ma. Subsequent unfolding of the gneiss cores, while conserving surface area, reveals the model to be extended ca. 70 km to the south (i.e. thus equaling indenter shortening), which means that no material left the plane of cross-section by W-E extension or lateral extrusion. However, the situation for the hanging-wall nappes is different: The total thickness of the northern limbs of the AN and the PN together is twice as much after restoration compared to today. We postulate that the extension on the Brenner Normal Fault mainly caused this tectonic thinning, which is approximately 10 km. References[1] Reiter, F., Freudenthaler, C., Hausmann, H., Ortner, H., Lenhardt, W., & Brandner, R. (2018). Tectonics, 37(12), 4625-4654.

The interpretation of seismic data in orogens is usually difficult to decipher as structural information is limited to surface and borehole data. Seismic interpretations very much depend on the elastic wave velocity model, which in the simplest case is a function of rock composition. Seismic velocities can also be anisotropic, i.e. depend on the wave propagation direction inside the rock. Seismic anisotropy can be subdivided into intrinsic (crystallographic preferred orientation (CPO)) and extrinsic (shape preferred orientation, compositional layering or fractures) anisotropy. Microstructures in thin section scale have an impact not only on millimeter-scale but also on larger anisotropies in the field such as meter- to kilometer-scale folds. Here we explore the effect of microstructure (mainly folding and crenulation) on the homogenization of seismic anisotropy from samples of millimeter to thin section scale. The investigated samples are phyllosilicate- and graphite-rich samples (Innsbruck quartzphyllite and Bündner schist) from the N-S running Brenner Base Tunnel Project (NW-Tauern Window). Phyllosilicate-rich sections with layers of different composition and structure were selected from drill core samples of the exploration tunnel. The CPO of phyllosilicates and graphite from 1.5 – 3.5 mm thick cylinders was measured using high energy X-ray diffraction at DESY (Hamburg, Germany) and the ESRF (Grenoble, France). Pole figure data was directly extracted using single peak fitting. The CPO of quartz was determined by using EBSD. Seismic velocities for each sample were computed using µXRF-based modal composition and single crystal stiffness tensors. We measured the smallest representative volume element which we consider to be undisturbed by microstructural effects. Therefore, we estimate an upper bound of expected intrinsic velocity anisotropies. Thin section-scale anisotropies were modeled from the upper bound anisotropy and the observed microstructure, i.e., small-scale folding. Computed velocities were compared to Vp-anisotropy measurements on the drill cores. The velocity anisotropy is primarily governed by the content and distribution of phyllosilicates and graphite. Given the crystal symmetry and the low single crystal elastic anisotropy, phases such as feldspar, quartz or calcite can be considered as irrelevant with respect to seismic anisotropies. The simulation of a crenulation cleavage has a stronger impact than centimeter-size folding: The crenulation cleavage reduces the anisotropy for example from 14 % to 12 %. Centimeter-size folding with observed interlimb angles of 140° in contrast is negligible. The effect of microstructures like centimeter-scale folds and crenulation has only a limited impact on anisotropies of foliated rocks during homogenization from millimeter to thin section-scale. We assume that during homogenization to a larger scale, the effect of folding with small interlimb angles or different fold axes within the homogenized volume will have a stronger influence on seismic anisotropy.

The Penninic and Subpenninic nappe stack of the Tauern Window (TW) in the European Alps was formed by collision between Europe (Subpenninic) and the Adria margin (Austroalpine), and finally exhumed by the northward push of the Southalpine Dolomites indenter in the Miocene. In this study, we kinematically restore a cross-section along the trace of the Brenner Base Tunnel, concentrating mainly on the Subpenninic nappes (Venediger duplex; VD). We integrate zircon fission-track data (ZFT) as a temporal constraint for the termination of viscous deformation and test different geothermal gradients (GG). P-T-t data are used to define (a) the depth of the brittle-viscous transition (ca. 300 degrees C) and (b) pre-indenter depth. We displace the VD down along the Sub-Tauern ramp below the 300 degrees C isotherm. At that time, a GG of ca. 50 degrees C/km prevailed. ZFT data reveal that viscous conditions allowing folding of the VD started to cease slightly earlier than 17 +/- 0.6 Ma. Unfolding of the VD, while conserving surface area, yield that the model is extended by ca. 70 km (thus equaling indenter shortening), which means that in the westernmost TW, the VD was not significantly affected by W-E extension. Reconstruction of the hanging-wall nappes (Austroalpine and Penninic nappes) above the restored VD reveals that the total pre-indenter thickness of their northern limbs was 25%-48% greater than today. We interpret this as tectonic thinning, which was mainly caused by the Brenner normal fault.

Microstructures that preserve quantitative information about deformation conditions during viscous flow of crystalline materials are rare, the most common example being the dynamically recrystallized grain or subgrain size to infer differential stress. There are many instances in which identification of recrystallized grains or subgrains is challenging. Thus, another microstructural attribute that relates to differential stress would be useful. We use electron backscatter diffraction (EBSD) data from experimentally deformed Black Hills quartzite to show that the perimeter-area fractal dimension of quartz aggregates (the slope of the log-log relationship between perimeter and diameter), which we term the grain boundary dimension (GBD), strongly correlates with differential stress (sigma) in rocks deformed by dislocation creep. Unlike traditional methods for estimating differential stress, the GBD method does not require identification of recrystallized grains or subgrains. Analysis of 9 samples yields both power-law and logarithmic calibrations (sigma in MPa) and , which show excellent agreement with published grain-size piezometers. Analysis of kernel average misorientation maps, guided by theoretical considerations, suggests that GBD develops through heterogeneous grain boundary migration related to spatial variation of driving force from dislocations and dislocation walls in adjacent grains. Our calibrations cover dislocation-creep conditions in which local grain boundary migration and subgrain rotation are the main dynamic recrystallization processes. Further work is needed to refine the calibration, test extrapolation to natural conditions, assess its applicability to general-shear deformation, and extend the method to other minerals such as calcite, rock salt, olivine and ice.

The shortening of sediments in accretionary prisms is accomplished by localized faulting as well as non-localized deformation. While faulting is often easily recognized from seismic sections, accessing the amount and extent of non-localized deformation is rather challenging. In order to address this challenge, we explore samples from the active accretionary prism offshore Gisbourne, NZ at the Hikurangi margin which contains accreted sediments of Pliocene to recent age. Drilling at Site U1318F of IODP Expedition 375 recovered non- to semi-lithified sediments from a major accretionary fault, the Papaku Fault, including its hanging wall and footwall. The crystallographic preferred orientation (CPO) of the clay minerals is a measure for their alignment and was determined in 66 sediment samples from the drill core (250-500 mbsf) using high energy X-rays. The results show that the CPO strength of the clay mineral basal planes (00l) is in general weak and no depth-related trend can be observed. In the hanging wall of the Papaku Fault, (00l) pole figures have non-rotationally symmetric, unimodal density distributions displaying incomplete girdles. In the footwall, most (00l) pole figures exhibit unimodal, rotationally symmetric to weak girdle density distributions, with most maxima pointing parallel or subparallel to the drill core axis. Fault zone samples also exhibit rotationally symmetric, unimodal (00l) distributions, with maxima perpendicular to the fault plane. We assume that pre-shortening and pre-faulting, sediments had a weak initial CPO related to sedimentation and compaction with a rotationally symmetric, unimodal (00l) distribution. The girdle shape of the distribution in the hangingwall and to a minor extent in the footwall is introduced by non-localized deformation which results in grain-scale folding. Accordingly, diffuse shortening was larger in the present-day hanging wall than in the present-day footwall. Furthermore, we interpret the CPO in the Papaku fault itself to be a result of sediment shearing, overprinting any pre-existing CPO. The position of the Papaku fault is compatible with fault initiation where diffuse shortening was unable to propagate sufficiently towards the foreland. While our results also confirm existing tectonic models from this part of the Hikurangi margin, more importantly they demonstrate implications for strain distribution in fault and thrust systems as well as the usefulness of clay mineral CPO for unravelling deformation and tectonic processes in accretionary prism sediments.

The impact of fluid-rock interaction during deformation is difficult to characterize at the regional scale because of the lack of continuous outcrops to provide textural evidence and meaningful spatial comparisons. Herein we introduce the use of bench top μXRF data as a cost-effective means to quantify chemical and deformation gradients within representative specimens from across the strain gradient associated with the Pocologan Kennebecasis shear zone (PKSZ) in the Canadian Appalachians. Within the Pocologan Harbour granitoid belt, the main foliation is defined by three distinct mineral assemblages characterized by: (A) zoisite, no muscovite; (B) zoisite and muscovite; (C) muscovite and potassium feldspar. We first test whether these mineral assemblages are indicative of primary lithological variation or hydrothermal alteration using major and trace element geochemistry. Then, we test if these mineral assemblages impact the fabric anisotropy using quantitative image analysis of thin section phase maps derived using a benchtop μXRF. We found that whole rock compositions of specimens with an assemblage characterized by potassium feldspar and muscovite show significant element mobility compared to the least altered specimens and that the thin section phase map anisotropy best defines the deformation gradient of the PKSZ regardless of the metasomatic assemblage. This deformation gradient is mimicked by the shape anisotropy of quartz, plagioclase, biotite and muscovite aggregates. Within the most altered specimens, sericite alteration of plagioclase increased the mineral aggregate anisotropy through strain localization in shear bands while the crystallization of potassium feldspar at the expense of plagioclase of muscovite, plagioclase and quartz decreased the mineral aggregate anisotropy which indicate an overall strengthening of the shear zone. Moreover, the combination of grain-size reduction mechanisms and syn-deformation metasomatism in the high strain portion of the shear zone triggered a change of deformation mechanism from plastic deformation to diffusion creep.

The alignment of clay minerals in sediments is of high importance for their mechanical and physical properties. The development of this alignment starts with the deposition of clay, its strength is measured by the crystallographic preferred orientation. So far, the early stages of sedimentation have been restricted to post-mortem observations. Here we present particle settling experiments in four dimensions (time and orientation, as a function of overburden and composition) observed in situ using synchrotron diffraction, in which kaolinite and kaolinite-illite mixtures were sedimented in water columns. The alignment strength in freshly settled sediments increases with overburden, but is higher in deionized water than in seawater. Alignment strength increases within the first few millimetres of overburden and stagnates afterwards. With illite added, the resulting alignment strength is substantially decreased. Our results demonstrate that electrostatic interactions between particles are overcome by gravitational forces already within the upper millimetres of sediment. Clay minerals align in a preferred orientation already during the first stages of sediment settling, prior to deeper sediment compaction, according to particle settling experiments using 4D synchrotron diffraction and crystal-preferred orientation analysis

The Tauern Window in the European Alps has a high tectonic complexity. It is a key area to understand a number of important orogenic processes, including nappe stacking, exhumation, indentation as well as escape tectonics. The polyphase Alpine deformation history of the Tauern Window began with subduction and accretion of the Penninic realm beneath the northern margin of Adria (Austroalpine) in the Cretaceous. Ongoing convergence led to collision between Europe (Subpenninic) and the Adria margin and to the formation of the Penninic and Subpenninic nappe stack in the southward dipping orogenic wedge from Eocene to early Oligocene. The W-E trending Periadriatic Fault System (PFS) located within the Adriatic units south of the Tauern Window was active as dextral strike-slip fault at this time, as indicated by the deformation of the Eocene and Oligocene Periadriatic intrusions [1]. Indentation of the Dolomites Indenter (Eastern Southalpine) bent the primarily PFS and finally caused this fault system to be sinistrally offset by the NNE-SSW striking Giudicarie fault system in the Miocene. This last deformation stage (D5 after [2]) caused strong N-S shortening (~65 km) of the western Tauern Window in front of the Dolomites Indenter, accompanied by lateral extrusion towards the east of at least ~100 km involving major strike-slip faults (e.g., Inntal Fault, PFS, SEMP). W-E extension further led to the formation of the Katschberg and Brenner Normal Fault (on the eastern and western borders of the Tauern Window, respectively). The latter, perhaps in combination with slab break-off and mantle upwelling, led to rapid exhumation of the Tauern Window.Balancing a cross-section is an excellent tool to analyze the kinematic evolution of mountain belts. Therefore, we collected a structural dataset along a N-S trending cross-section through the western Tauern Window based on the Brenner Base Tunnel profile [3]. Before balancing, however, basic assumptions have to be typically considered: (1) Whether plane-strain deformation is applicable, which means that no material should move lateral into or out of the cross-section plane (2) Conservation of the area (or volume), and (3) line-length should be preserved. Hence, such a balancing is not simply possible in the western Tauern Window because of the last deformation stage (D5 after [2]), when contemporaneous N-S shortening, W-E extension, and vertical uplift led to penetrative deformation and non-plane strain conditions, respectively. We focus on the restoration of the last deformation stage; first with plane-strain and second with non-plane, oblate strain. The results reveal the effect of the W-E extension on the nappe geometry in the footwall of the Brenner Normal Fault – a topic that is controversially discussed. This is the basis for further backward restoration that needs to incorporate all the tectonic movements out of the cross-section plane, and will be carried out as balancing in 3-D at a later stage of the project. References[1] Pomella, H. et al. (2011). International Journal of Earth Sciences, 100(8), 1827-1850.[2] Schmid, S. M. et al. (2013). Swiss Journal of Geosciences, 106(1), 1-32.[3] Brandner, R. et al. (2008). Geo Alp, 5, 165-174.

<p>The microstructural evolution of clay-rich sedimentary rocks starts with the settlement and alignment of particles. With the accordance between shape preferred orientation and crystallographic preferred orientation (CPO) in the case of disk-like clay particles, the parallel alignment can be quantified measuring the clay CPO.</p> <p>In order to quantify the influence of the sedimentation conditions on the CPO of primary layering we performed sedimentation experiments combined with in-situ synchrotron diffraction measurements. The experimental procedure involved a sediment suspension drip inserted at the top of a 30 cm water column at regular intervals. The fluid in the column was either deionized water or seawater, and the sediment suspension contained either kaolinite with of disk-like particle shapes or a mixture of kaolinite and polycrystalline illite, the latter with more compact particle shapes. Time-resolved CPO development in the experiments was measured at ESRF, beamline ID22.</p> <p>The formation of a CPO is readily observed at the water-sediment interface with an initially higher CPO strength in deionized water experiments than in seawater. The resulting sediment shows a pronounced layering in both fluids when using pure kaolinite. In the layered sediments in deionized water the CPO strength varies strongly between different layers. At some measurement positions, a high initial CPO strength drops fast in the first 150 minutes, interpreted to result from dewatering-related reorganisation of the microstructure. A stable CPO strength can be observed after ~200 min. In the seawater experiments the CPO strength does not vary in different layers and increases slowly but constantly with time and overburden indicating a successive rotation of particles. CPO in kaolinite experiments is higher than in kaolinite + illite experiments as compact particles locally inhibit the alignment.</p> <p>The evolution of clay particle orientation and therefore microstructure can be quantified in space and time. It is suggested that the initial microstructure is crucial for the progressive development of the rock during diagenesis and hence e.g. resulting physical properties. </p>

Electron backscatter diffraction (EBSD) was employed to probe the structural order of a zoned radiation -damaged zircon (ZrSiO4) on the sub-micron scale. The amorphous fraction of the growth-zones is in the range of-45-80%, due to variations in the amount of incorporated uranium and thorium (-0.22- 0.43 wt% UO2 and-0.02-0.08 wt% ThO2) and the resulting alpha-decay events over time. The obtained Kikuchi patterns' band contrast (bc) and band slope (bs) are indicative of the degree of atomic-scale or-der. The excellent correlation of both parameters with the evolution of the elastic modulus validates the methods reliability.(c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

At the Hikurangi convergent margin the Pacific plate is subducted westward beneath the Australian plate. This margin has been the location of major earthquakes as well as slow slip events related to the ongoing subduction. Drill site U1518 which was drilled during IODP Expedition 375, 73 km offshore Gisborne (New Zealand), targeted the Papaku fault, a splay fault of the major decollement in sediments of the frontal accretionary prism. We selected samples from the mostly hemipelagic, weakly consolidated mudstones in the fault zone, as well as from hangingwall and footwall. In order to investigate localized and distributed deformation in the fault zone, we analysed composition, microstructure and crystallographic preferred orientation (CPO). For that we applied µXRF measurements and optical microscopy, as well as synchrotron texture analysis at DESY in Hamburg. The samples from hanging- and footwall sediments show a relatively homogeneous microstructure with local compositional layering. While CPO strength in the hangingwall is slightly increasing with depth for all analysed clay mineral phases, the CPO in the footwall samples is in general lower and does not show a clear trend with depth. This might be interpreted as different deformation histories in hangingwall and footwall which is in accordance with previous studies. Fault zone samples show a variety of microstructures, such as mingling of different sedimentary components, locally overprinted by microfaults. CPO strength in the faulted sediments is also variable, with zones showing strong alignment of phyllosilicates and zones showing weak alignment of phyllosilicates. Variations in CPO and variable distribution of sedimentary components indicate a heterogeneous deformation within the fault zone which might be due to local compositional variations.

Physical properties of rocks are mainly controlled by the modal composition, crystallographic preferred orientation (CPO) and microstructure of a rock. One of the most relevant physical properties related to the interpretation of seismic data are the elastic properties of a mineral aggregate. Changes of elastic properties - and hence changes in our interpretation of the tectonic architecture of certain regions - can be related to mineral reactions and deformation. In order to explore the impact of mineral reaction and deformation on elastic anisotropy, we study oceanic serpentinites formed at low-grade metamorphic conditions by hydration of peridotites. Samples are obtained from the Atlantis Massif, which is an Oceanic Core Complex located at 30°N, Mid-Atlantic Ridge. During IODP Expedition 357, oceanic serpentinites were recovered from drill cores along the southern wall of the Massif. Fully serpentinized samples displaying variable microstructures were analyzed regarding the influence of microstructure and CPO on the overall elastic anisotropy. Microstructure analysis was based on optical microscopy and large area micro X-ray fluorescence mapping. For CPO analysis synchrotron high energy X-ray diffraction in combination with the Rietveld method was applied and the derived CPO was used to compute seismic properties. Serpentinites with a typical mesh microstructure are interpreted to represent undeformed samples and show a close to uniform CPO. The increase in fabric anisotropy of vein-like magnetite aggregates is interpreted as an increase in deformation. Samples show a single c-axis-maximum and enhanced CPO. Calculated seismic anisotropies show up to >5% anisotropy for compressional waves (Vp) and shear wave splitting up to 0.15 km/s in the deformed samples. Hence, such an anisotropy can be used to differentiate deformed from undeformed zones in seismic data sets using the elastic anisotropy data.

Subduction and exhumation are key processes in the formation of orogenic systems across the world, for example, in the European Alps. For geophysical investigations of these orogens, it is essential to understand the petrophysical properties of the rocks involved. These are the result of a complex interaction of mineral composition and rock fabric including mineral textures (i.e., crystallographic preferred orientations). In this study we present texture-derived elastic anisotropy data for a representative set of different lithologies involved in the Alpine orogeny. Rock samples were collected in the Lago di Cignana area in Valtournenche, in the Italian northwestern Alps. At this locality a wide range of units of continental and oceanic origin with varying paleogeographic affiliations and tectono-metamorphic histories are accessible. Their mineral textures were determined by time-of-flight neutron diffraction. From these data the elastic properties of the samples were calculated. The data set includes representative lithologies from a subduction-exhumation setting. In subducted lithologies originating from the oceanic crust, the P-wave anisotropies (AVPs [%]) range from 1.4 % to 3.7 % with average P-wave velocities of 7.20–8.24 km/s and VP / VS ratios of 1.70–1.75. In the metasediments of the former accretionary prism the AVPs range from 3.7 % to 7.1 %, average P-wave velocities are 6.66–7.23 km/s and VP / VS ratios are 1.61–1.76. Continental crust which is incorporated in the collisional orogen shows AVP ranging from 1.4 % to 2.1 % with average P-wave velocities of 6.52–6.62 km/s and VP / VS ratios of 1.56–1.60. Our results suggest that mafic and felsic rocks in subduction zones at depth may be discriminated by a combination of seismic signatures: lower anisotropy and higher VP / VS ratio for mafic rocks, and higher anisotropy and lower VP / VS ratio for felsic rocks and metasediments.

Abstract. The physical properties of claystones, shales, and slates are highly dependent on the alignment of phyllosilicate minerals. With increasing overburdening, the shape and the crystallographic preferred orientation of these minerals are affected by uniaxial shortening as well as tectonic processes including recrystallization under elevated pressure and temperature conditions. The microstructural anisotropy expressed mainly by the alignment of phyllosilicates significantly predetermines the orientation of fractures, hence the shear strength and stability of clay-rich sediments and rocks. A quantitative analysis of phyllosilicate alignment is therefore essential to evaluate the properties and the mechanical behavior of these rocks. This can be carried out by analyzing the crystallographic preferred orientation (texture). Although texture analysis is a common tool in geosciences, it becomes more difficult in fine-grained rocks owing to for example particle size, heterogeneity, the polyphase composition, and difficulties in sample preparation. Methods such as electron backscatter diffraction, neutron diffraction, or laboratory X-ray diffraction are restricted with respect to preparation artifacts, sampling size and statistics, water content, etc. To overcome these issues, we successfully apply high-energy X-ray diffraction as available at synchrotron research facilities, e.g., at the German Electron Synchrotron Facility (DESY) in Hamburg, Germany, or the European Synchrotron Research Facility (ESRF) in Grenoble, France. In combination with Rietveld refinement we analyze the bulk texture of phyllosilicate-rich rocks. Here we present the results of texture analysis from a wide range of these rocks: Pleistocene poorly consolidated mud (rocks), affected only by sedimentation and burial; more highly consolidated but tectonically largely unaffected Jurassic claystone from the Opalinus Formation of the Swabian Alb; Carboniferous shales from the Harz mountains representing low-grade metamorphic and deformed rocks. Our methodical approach to quantifying the microstructural anisotropy using texture analysis in fine-grained rocks allows for the quantification of physical properties resulting from the alignment of phyllosilicates. Furthermore, it enables the prediction of direction-dependent mechanical strength, which is crucial for the establishment of long-term repositories for radioactive waste in shales and claystones.

The crust within collisional orogens is very heterogeneous both in composition and grade of deformation, leading to highly variable physical properties at small scales. This causes difficulties for seismic investigations of tectonic structures at depth since the diverse and partially strong upper crustal anisotropy might overprint the signal of deeper anisotropic structures in the mantle. In this study, we characterize the range of elastic anisotropies of deformed crustal rocks in the Alps. Furthermore, we model average elastic anisotropies of these rocks and their changes with increasing depth due to the closure of microcracks. For that, pre-Alpine upper crustal rocks of the Adula Nappe in the central Alps, which were intensely deformed during the Alpine orogeny, were sampled. The two major rock types found are orthogneisses and paragneisses; however, small lenses of metabasites and marbles also occur. Crystallographic preferred orientations (CPOs) and volume fractions of minerals in the samples were measured using time-of-flight neutron diffraction. Combined with single crystal elastic anisotropies these were used to model seismic properties of the rocks. The sample set shows a wide range of different seismic velocity patterns even within the same lithology, due to the microstructural heterogeneity of the deformed crustal rocks. To approximate an average for these crustal units, we picked common CPO types of rock forming minerals within gneiss samples representing the most common lithology. These data were used to determine an average elastic anisotropy of a typical crustal rock within the Alps. Average mineral volume percentages within the gneiss samples were used for the calculation. In addition, ultrasonic anisotropy measurements of the samples at increasing confining pressures were performed. These measurements as well as the microcrack patterns determined in thin sections were used to model the closure of microcracks in the average sample at increasing depth. Microcracks are closed at approximately 740 MPa yielding average elastic anisotropies of 4 % for the average gneiss. This value is an approximation, which can be used for seismic models at a lithospheric scale. At a crustal or smaller scale, however, local variations in lithology and deformation as displayed by the range of elastic anisotropies within the sample set need to be considered. In addition, larger-scale structural anisotropies such as layering, intrusions and brittle faults have to be included in any crustal-scale seismic model.

Abstract. Permeability of crystalline rocks depends on parameters such as density and interconnectivity of fractures and pores. While in pristine crystalline rocks porosity is usually considered to be low, low-grade solution phenomena such as the formation of episyenites occur occasionally and may cause a local dramatic increase in porosity and permeability. These solution phenomena can be effective in otherwise unaltered rocks and may result in the preferential removal of certain mineral phases, especially of quartz so that porosities correspond to the spatial distribution of the previously existing mineral phase if no subsequent mineralization occurs (e.g., Pennacchioni et al., 2016). Using light-optical and scanning electron microscopy, X-ray tomography, micro-XRD, as well as digital image analysis, the differences in connectivity and hence permeability between, for example, quartz-depleted granite, gneiss, and schist can be characterized and quantified. We demonstrate that such porosities do not necessarily result in high permeabilities in an undeformed granodiorite from the Central Gneiss unit of the Tauern Window (Lago di Neves area, Italy), since former quartz aggregates are not interconnected due to their relatively late crystallization age and the preservation of the magmatic fabric; however, in the case of moderate mylonitic deformation, quartz as rheologically weak phase forms interconnected aggregates and layers. Its dissolution results in an extremely increased permeability. Therefore, not only the content and grain size but also the distribution, shape and alignment of minerals are crucial for rock permeability and need to be carefully investigated when searching for a final repository of highly radioactive waste in crystalline rocks. Especially since local shear zones may form in otherwise undeformed intrusive bodies, a detailed structural analysis beyond the exclusion of the presence of fractures is required to mitigate the risk of a long-lasting nuclear waste disposal.

Crystallographic preferred orientation (CPO) and the associated seismic anisotropy of serpentinites are important factors for the understanding of tectonic settings involving hydrated Earth´s mantle, for example, at slow‐spreading mid‐ocean ridges. CPO of lizardite and magnetite in low‐grade metamorphic serpentinites from the Atlantis Massif oceanic core complex (Mid‐Atlantic Ridge, 30°N) were determined using synchrotron high energy X‐ray diffraction in combination with Rietveld texture analysis. Serpentinite mesh structures show weak CPO while deformed samples show a single (0001) maximum perpendicular to the foliation. Seismic anisotropies calculated from CPO show up to >11% anisotropy for compressional waves (Vp) and shear wave splitting up to 0.38 km/s in the deformed samples. This indicates that deformation in shear zones controls elastic anisotropy and highlights its importance in defining the seismic signature of hydrated upper mantle.

Quartz c-axis pole figures are hugely popular for the estimation of various deformation conditions, such as strain state, slip system interpretation or deformation temperature. Most of these relations are purely empirical. Here we present quantitative results of the relation between microstructure and quartz c-axis pole figure data to add to the insights between deformation processes and texture development. We analyze EBSD data of experimentally sheared quartzite (kinematic vorticity number Wk = 0.9, experiments of Heilbronner & Tullis, 2006), a mylonitic quartzite from Eriboll (Wk = 0.5, Lloyd’s pers. collection) and a deformed quartz vein from the Tonale line (Wk = 0.4, Stipp & Kunze, 2008). All samples are composed of deformed old grains and recrystallized (by bulging and/or subgrain rotation) and deformed grains in variable proportions.C-axis pole figures can be decomposed into several components (girdles and point maxima) which occupy distinct positions. These components can be related to two simple microstructural parameters, aspect ratio and long axis direction of grains. While the grain shape evolution in each of the samples differ in detail, they have several features in common:1) c-axes of equiaxed grains occupy a position close to the inferred instantaneous shortening direction,2) c-axes of grains with higher aspect ratios contribute to single girdle distributions,3) the girdle position depends on the grain long axis direction,4) grains with long axes parallel to the foliation (inferred XY plane of finite strain) provide highest c-axis concentrations in the center of the pole figure,5) grains contributing to an oblique grain shaped foliation (“freshly” recrystallized, deformed grains) show elongated, peripheral maxima grading into single girdles inclined with the sense of shear and6) grain shapes which relate to antithetic flow (in the low Wk samples), relations 3-5 hold, with the exception that the resulting peripheral maximum or girdle is also inclined against the sense of shear.We interpret the individual c-axis pole figure components to reflect contributions from different processes which relate to oriented nucleation or growth (in the case of bulging recrystallization), as well as to a grains’ strain history. This strain history depends on the ratio of how fast a grain is straining (by glide) to how fast it is recrystallizing. The final c-axis pole figure of a polycrystalline aggregate simply reflects the weighted mixture of these components based on the synchronous contribution of each process.The individual contribution of each process depends on several parameters (e.g., stress as a driving force for local grain boundary migration, grain boundary mobility, or rate of deformation among others). Since many of these parameters are also temperature-dependent, we suggest, for instance, that the variability of the c-axis opening angle with temperature is merely the result of the temperature different dependencies of the contributing processes. Hence, it is unsurprising that the so-called c-axis opening angle cannot be universally applied as a thermometer and is a good example of unrelated cause and correlation and may be expected to give arbitrary results.References:Heilbronner, R., Tullis J., 2006 https://doi.org/10.1029/2005JB004194, 2006.Stipp, M. and Kunze, K., 2008 https://doi.org/10.1016/j.tecto.2007.11.041, 2008.