The Middle-Jurassic Opalinus Clay is the foreseen host rock for radioactive waste disposal in central northern Switzerland. An extensive drilling campaign aiming to characterize the argillaceous formation resulted in a comprehensive drill core data set. The rheologically weak Opalinus Clay is only mildly deformed compared to the over- and underlying rock units but shows a variety of natural fractures. While these structures are hydraulically indistinguishable from macroscopically non-deformed Opalinus Clay today, their analysis allows for a better understanding of the deformation behaviour in the geological past. Here, we present an overview of the different fracture and fault types recorded in the Opalinus Clay and a detailed microstructural characterization of veins—natural dilational fractures healed by secondary calcite and celestite mineralizations. Macroscopic drill core analysis revealed five different natural fracture types that encompass tension gashes of various orientations with respect to bedding and small-scale faults with displacements typically not exceeding the drill core diameter. The occurrence of different fault types generally fits well with the local tectonic setting of the different drilling sites and with respect to the neighbouring regional fault zones. The microstructural investigations of the various vein types revealed their often polyphase character. Fibrous bedding-parallel veins of presumable early age were found to be overprinted by secondary slickenfibres. The polyphase nature of fibrous bedding parallel veins and slickenfibres is supported by differing elemental compositions, pointing towards repeated fracturing and mineralization events. Direct dating of vein calcites with U–Pb was unsuccessful. Nevertheless, age constraints can be inferred from structural orientations and fault slip kinematics. Accordingly, some of the veins already formed during sediment compaction in Mesozoic times, others possibly relate to Early Cenozoic foreland uplift. The youngest veins are most likely related to Late Cenozoic regional tectonic events, such as the Jura fold-and-thrust belt to the south and the Hegau-Lake Constance Graben to the northeast of the study area. During these latest tectonic events, previously formed veins acted as rheologically stiff discontinuities in the otherwise comparably weak Opalinus Clay along which deformation of the rock formation was re-localized.

The retreat of the Great Aletsch Glacier is accompanied by a series of slope failures in solid bedrock, which are heavily influenced by the presence of pre-existing deformation structures. Since the 1880's, the Great Aletsch Glacier has shortened by more than 3 km and decreased about 400 m in thickness. As a reaction to the loss of the stabilizing effect of the ice, one of the largest active deep-seated landslides in the European Alps with an affected surface area of about 1.5 km(2), called Moosfluh landslide, is evolving. In this study, a multimethod approach combining fieldwork, remote sensing techniques and microseismic monitoring is used to assess the effect of preexisting structures on the landslide deformation processes. The landslide evolution from 2008 to 2018 could be reconstructedwith high spatial resolution. Surface deformation analysis reveals the concentration of high deformation in narrow zones, allowing to directly link pre-existing tectonic and exhumation structures with landslide deformation processes. Toppling as the main gravity-driven process is enabled by reactivation of NE-SW striking, steeply SE dipping Alpine Handegg phase shear zones. Differences in the lateral detachment processes are attributed to shear zone bridges in the NE as well as fractures and shear zones similarly oriented to Alpine Oberaarb phase shear zones in the SW. At the landslide toe, a transition fromtoppling to slidingmechanism due to the formation of a continuous basal detachment surface can be observed, which is favored by the presence of exfoliation joints. The dramatic acceleration of theMoosfluh landslide in autumn 2016 is directly related to an increase in glacier height loss rate, which implies that glacier retreat is themain trigger of the landslide. A temporal stabilization of the landslide is recorded after 2017, most probably caused by the self-stabilizing properties of flexural toppling. However, microseismic data records a lateral propagation of the landslide, following the retreating Great Aletsch Glacier. (C) 2021 Elsevier B.V. All rights reserved.

Water pressures at the base of active glacial overdeepenings are known to fluctuate strongly on various time scales. Rapid peaks in basal water pressure can lead to fracturing of the glacier bed, a process that has been described at numerous sites around the world, mostly based on large hydrofracture systems. This article presents drill‐cores from the base of a >100‐m‐deep glacial overdeepening in the Lower Aare Valley in northern Switzerland that were investigated with high‐resolution imaging (including X‐ray computed tomography) as well as compositional and microstructural analyses. The drill‐cores recovered Jurassic limestones hosting palaeokarst voids infilled with blue clay. We identify this clay, based on its kaolinitic composition, as siderolithic Bolus Clay but in a rather atypical variety formed under reducing conditions. The surfaces of the palaeokarst walls show smoothly undulating as well as brecciated sections with form‐fit interlocking clasts, which are the result of an in situ brecciation process. We argue that these particular fractures are not related to (glacio‐)tectonics or frost action. Instead, we suggest an explanation by involving water‐pressure peaks that were transmitted to the void walls by the clayey karst filling, resulting in hydrofracturing. In addition to pervasive karstification and tectonic overprinting, this water pressure‐driven fracturing may have enhanced the deep incision of the overdeepening into the rheologically competent bedrock.

Dolomitic marble on the island of Naxos was deformed at variable temperatures ranging from 390 degrees C to >700 degrees C. Microstructural investigations indicate two end -member of deformation mechanisms: (1) Diffusion creep processes associated with small grain sizes and weak or no CPO (crystallographic preferred orientation), whereas (2) dislocation creep processes are related with larger grain sizes and strong CPO. The change between these mechanisms depends on grain size and temperature. Therefore, sample with dislocation and diffusion creep microstructures and CPO occur at intermediate temperatures in relative pure dolomite samples. The measured dolomite grain size ranges from 3 to 940 mu m. Grain sizes at T-max >450 degrees C show an Arrhenius type evolution reflecting the stabilized grain size in deformed and relative pure dolomite. The stabilized grain size is five times smaller than that of calcite at the same temperature and shows the same Arrhenius-type evolution. In addition, the effect of second phase particle influences the grain size evolution, comparable with calcite. Calcite/dolomite mixtures are also characterized by the same difference in grain size, but recrystallization mechanism including chemical recrystallization induced by deformation may contribute to apparent non-temperature equilibrated Mg-content in calcite. (C) 2016 Elsevier Ltd. All rights reserved.

Microstructures and textures of calcite mylonites from the Morcles nappe large-scale shear zone in southwestern Switzerland develop principally as a function of 1) extrinsic physical parameters including temperature, stress, strain, strain rate and 2) intrinsic parameters, such as mineral composition. We collected rock samples at a single location from this shear zone, on which laboratory ultrasonic velocities, texture and microstructures were investigated and quantified. The samples had different concentration of secondary mineral phases (<5 up to 40vol.%). Measured seismic P wave anisotropy ranges from 6.5% for polyphase mylonites (~40vol.%) to 18.4% in mylonites with <5vol.% secondary phases. Texture strength of calcite is the main factor governing the seismic P wave anisotropy. Measured S wave splitting is generally highest in the foliation plane, but its origin is more difficult to explain solely by calcite texture. Additional texture measurements were made on calcite mylonites with low concentration of secondary phases (≤10vol.%) along the metamorphic gradient of the shear zone (15km distance). A systematic increase in texture strength is observed moving from the frontal part of the shear zone (anchimetamorphism; 280°C) to the higher temperature, basal part (greenschist facies; 350–400°C). Calculated P wave velocities become increasingly anisotropic towards the high-strain part of the nappe, from an average of 5.8% in the frontal part to 13.2% in the root of the basal part. Secondary phases raise an additional complexity, and may act either to increase or decrease seismic anisotropy of shear zone mylonites. In light of our findings we reinterpret the origin of some seismically reflective layers in the Grône–Zweisimmen line in southwestern Switzerland (PNR20 Swiss National Research Program). We hypothesize that reflections originate in part from the lateral variation in textural and microstructural arrangement of calcite mylonites in shear zones.

We present a state-of-the-art review of the microstructural evolution in rocks under static and deformational conditions. First, the general concepts and processes are introduced using monomineralic aggregates. Then, they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase. The first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics. Based on comparisons between microfabrics of monomineralic and polymineralic rocks, we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures, modal compositions, spatial distribution of phases, crystallographic preferred orientations and associated processes. The article puts particular emphasis on the effect of coupled grain growth, mass transfer processes, and deformation mechanisms. We speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions. At the end of the article, we demonstrate the great potential of grain-size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations.

A detailed quantitative microstructural study coupled with cathodoluminescence and geochemical analyses on marbles from Naxos demonstrates that the analysis of microstructures is the most sensitive method to define the origin of marbles within, and between, different regions. Microstructure examination can only be used as an accurate provenance tool if a correction for the second-phase content is considered. If second phases are not considered, a large spread of different microstructures occurs within sample sites, making a separation between neighbouring outcrops difficult or impossible. Moreover, this study shows that the origin of a marble is defined more precisely if the microstructural observations are coupled with cathodoluminescence data.

Large-scale shear zones are often characterized by long lasting deformation with complex strain localization behavior. Based on a drill core across polymineralic carbonate mylonites of the Doldenhorn nappe (Helvetic Alps, Switzerland), the present study uses spatially resolved microstructural variations to discriminate unambiguously deformation microstructures from annealed ones, to infer the deformation mechanisms, and to reconstruct the strain localization history. These goals require quantitative microstructural and textural analyses of sample suites in sections across the shear zone. Particularly the occurrence of two distinct Zener trends, i.e., the relationship between grain size and volume fraction of matrix and second phase grains, allows for the identification of annealed from dynamic microstructures. The data presented here indicate that continued shearing after peak metamorphic conditions and during exhumation-induced cooling resulted in progressive strain localization at the base of the shear zone, while the inactive parts of the formerly high temperature shear zone were progressively overprinted by annealing.

Participants Ansorge Jörg (ETHZ) den Brok Bas (EAWAG-EMPA) Dèzes Pierre (SANW) Gonzalez Laura (University of Bern) Herwegh Marco (University of Bern) Hürzeler Jean-Pierre (University of Basel) Imper David (GeoPark) Mancktelow Neil (ETHZ) Mullis Josef (University of Basel) Nyffenegger Franziska (Fachhochschule Burgdorf, University of Bern) Pfiffner Adrian (University of Bern) Schreurs Guido (University of Bern) Schmalholz Stefan (ETHZ) Schmid Stefan (University of Basel) Wiederkehr Michael (University of Basel) Wilson Christopher (Melbourne University) Wilson Lilian (Melbourne University)

A new approach to quantify microstructures of coarse-grained marbles is presented. This technique is based on the intensity of light reflectance in dependence of the crystallographic orientation of calcite grains. The technique setup consists of a high-resolution camera, a strong light source and the polished sample in reflection position. Microstructural data obtained with this method are comparable with those obtained by light microscopy or by secondary electron microscopy. However, the new approach reflects the grain size distribution even more accurately than in the case of other techniques, because it allows an easy quantification of larger sample surfaces. Therefore, in contrast to established techniques, microstructural analysis and statistics can also be performed for coarse-grained samples with grains that exceed diameters of 1 cm. With this new technique, coarse-grained mylonitic microstructures from Naxos were quantified, which in turn allowed relating different microfabrics to different strain localization episodes within a large-scale shear zone complex. (c) 2008 Elsevier B.V. All rights reserved.

Microstructures of carbonate mylonites with 0-40 vol.% second-phase particles from different Helvetic nappes (Switzerland) have been analyzed. While the mean calcite grain size D increases with temperature (T), second phases pin calcite grain boundaries. Two types of second phases can be distinguished: small second phases included in calcite grains and larger second phases at calcite grain boundaries, both coarsening with volume fraction and T These trends are controlled by an interaction of different processes during deformation, including diffusion, dynamic recrystallization, grain growth and pinning. In terms of D T dependencies, the microstructures of the investigated nappes are similar. The relationship between D and T differs for high T and low T These observations can be explained by a change in the simultaneous activity of dominant mechanisms (e.g., diffusion and dislocation creep) as a function of T and second-phase content. In contrast. the relation log(d(p))-1/T (d(p) second-phase grain size) is linear with constant slope for all nappes indicating that growth processes of second phases are identical. Despite differences in fluid activity and strain rate between individual nappes, the microstructural relationships of D-cc, second phases and T remain similar. Therefore, grain coarsening maps of second phase and recrystallization controlled calcite aggregates are a representative tool to predict deformation conditions and mechanisms for other shear zones developed under similar physical conditions. (C) 2008 Elsevier B.V. All rights reserved.

Deformation in orogenic belts is frequently accommodated in calcite-rich lithologies, owing to their relatively low strength, particularly compared to quartz-rich rocks. Here, we investigate the coupling between calcite grain size, the presence and mineralogy of second phases, and crystallographic preferred orientation (CPO) in a transect through deformed limestones, perpendicular to the dominant foliation in the inverted limb of the Morcles nappe of the Swiss Helvetic Alps. Calcite grain size becomes progressively finer as the thrust contact is approached, and there is a concomitant increase in CPO intensity, with the strongest CPO's in the finest-grained, quartz-rich limestones, nearest the thrust contact. To understand the distribution of strain and the extent of strain localization, we compared the paleowattmeter and calcite flow laws from laboratory studies to previously published observations of microstructure at a range of locations, with varying peak metamorphic temperatures, along the Morcles nappe. The strain-rates predicted by extrapolation of these laboratory relationships agree well with the geologic constraints. We then applied the same approach to the samples from the present study. The results indicate that strain became progressively localized towards the thrust contact of the Morcles nappe, leading to an increase in strain rate of >1 order of magnitude in a zone <0.50 m thick. For localization to occur system and/or material softening is necessary. If dislocation activity is positively correlated with CPO, then softening cannot have occurred by a complete transition to diffusion creep in the finest grained samples. Rather, softening may have resulted from the formation of CPO, possibly coupled with effects related to the distribution of second phases and the overall geometry of the shear zone.

Carbonate mylonites with varying proportions of second-phase minerals were collected at positions of increasing metamorphic grade along the basal thrust of the Morcles nappe (Helvetic nappes, Switzerland). Variations of temperature, stress, and strain rate, changes in chemistry of solid and fluid phases, and differing degrees of strain localization and annealing were tracked by measuring the shapes, mean sizes, and size distributions of both matrix and second-phase grains, as well as crystal preferred orientation (CPO) of the matrix. Field structures suggest that strain rate was constant along the fault. The mean and distribution of the calcite grain sizes were affected most profoundly by temperature: Increased temperature, presumably accompanied by decreased stress, correlated with larger mean sizes and wider size distributions. At a given location, the matrix grains in mylonites with more second-phase particles are, on average, smaller, have narrower size distributions, and have more elongate shapes. For example, mylonites with 50 vol.% of second phases have matrix grain sizes half that of pure mylonites. Changes in calcite chemistry and the presence of synkinematic fluids seemed to influence microfabric only weakly. Temporal variations in conditions, such as exhumation-induced cooling, apparently provoke changes in temperature, stress, and strain rate along the nappe. These changes result in further strain localization during retrograde conditions and cause the grain size to be reduced by an additional 50%. The matrix CPO strengthens with increasing temperature or strain, but weakens and rotates with increasing second-phase content, These fabric changes suggest differing rates of grain growth, grain size reduction, and development of CPO owing to variations in the deformation conditions and, perhaps, mechanisms. To interpret natural mylonite structures or to extrapolate mechanical data to natural situations requires careful characterization of the microfabric, and, in particular, second-phase minerals. (c) 2007 Elsevier B.V, All rights reserved.

The Glarus thrust (Switzerland) offers a great field example of strain localization under retrograde conditions. Along the thrust, mylonitic microfabrics are characterized by a temperature/stress controlled balance of grain size reducing mechanisms and grain growth. Consequently, mean grain sizes decrease along the thrust with decreasing metamorphic conditions and towards the thrust contact. In an opposite manner, calcite twin densities increase towards the contact. CPOs are strongest between 0.5 and 15m away from the thrust, but become generally weaker in the last centimeters, where also cataclasites occur. The CPO weakening and grain size reduction towards the shear zone point to a change from predominant dislocation creep to granular flow finally ending in cataclastic deformation. The microfabric changes correlate with a decrease in δ13C and δ18O towards the thrust contact indicating the presence of fluids during cycles of brittle and plastic deformation. Based on the microfabric changes, variations in stable isotopes and cross-cut relationships, a subdivision into a low and high-temperature shear zone can be made. The microfabric modifications resulted from changes in deformation conditions due to ongoing thrusting and exhumation induced cooling, promoting further strain localization of an existing high-strain shear zone.

Polymineralic rocks undergo grain coarsening with increasing temperature in both static and deformational environments, as long as no mineral reactions occur. The grain coarsening in such rocks is complex because the different phases influence each other, and it is this interaction that controls the rate of grain coarsening of the entire aggregate. We present a mathematical approach to investigate coupled grain coarsening using a set of microstructural parameters, including grain size and volume fraction of both second phases and matrix mineral in combination with temperature information. Based on samples from polymineralic carbonate mylonites that were deformed at different temperatures, we demonstrate how the mathematical relation can be calibrated for this natural system. Using such data sets for other lithologies, grain coarsening maps can be generated, which allow the prediction of microstructural evolution in polymineralic rocks. Such predictions are crucial for all subdisciplines in the earth sciences that require fundamental knowledge about microstructural changes and rheology of an orogen at different depths, such as structural geology, geophysics, geodynamics, and metamorphic petrology.