Geologic repositories for radioactive waste are designed as multi-barrier disposal systems that perform a number of functions including the long-term isolation and containment of waste from the human environment, and the attenuation of radionuclides released to the subsurface. The rock laboratory at Mont Terri (canton Jura, Switzerland) in the Opalinus Clay plays an important role in the development of such repositories. The experimental results gained in the last 20 years are used to study the possible evolution of a repository and investigate processes closely related to the safety functions of a repository hosted in a clay rock. At the same time, these experiments have increased our general knowledge of the complex behaviour of argillaceous formations in response to coupled hydrological, mechanical, thermal, chemical, and biological processes. After presenting the geological setting in and around the Mont Terri rock laboratory and an overview of the mineralogy and key properties of the Opalinus Clay, we give a brief overview of the key experiments that are described in more detail in the following research papers to this Special Issue of the Swiss Journal of Geosciences. These experiments aim to characterise the Opalinus Clay and estimate safetyrelevant parameters, test procedures, and technologies for repository construction and waste emplacement. Other aspects covered are: bentonite buffer emplacement, highpH concrete-clay interaction experiments, anaerobic steel corrosion with hydrogen formation, depletion of hydrogen by microbial activity, and finally, release of radionuclides Geosci (2017) 110:3-22 DOI 10.1007 into the bentonite buffer and the Opalinus Clay barrier. In the case of a spent fuel/high-level waste repository, the time considered in performance assessment for repository evolution is generally 1 million years, starting with a transient phase over the first 10,000 years and followed by an equilibrium phase. Experiments dealing with initial conditions, construction, and waste emplacement do not require the extrapolation of their results over such long timescales. However, experiments like radionuclide transport in the clay barrier have to rely on understanding longterm mechanistic processes together with estimating safety-relevant parameters. The research at Mont Terri carried out in the last 20 years provides valuable information on repository evolution and strong arguments for a sound safety case for a repository in argillaceous formations.

In this paper we present newly acquired high‐quality wide‐angle seismic data of the GRANU95 project and first models of the crustal structure along two profiles (95‐A and 95‐B) beneath the Saxonian Granulites, a major ellipse‐shaped exposure of lower‐crustal material within the mid‐European Variscan belt in southeastern Germany (Saxony). The crust is subdivided into four layers. The crystalline basement with velocities higher than 6.0 km s−1 is generally reached at shallow depths, with three major sedimentary structures as prominent exceptions where velocities considerably lower than 6.0 km s−1 (as low as 5.1 km s −1 ) reach as deep as 4 km. The highest upper‐crustal velocities (up to 6.5 km s−1 ) are not seen below the exposed granulites themselves, but at shallow depths (4 km) SW of the exposure. These shallow high velocities correlate well in depth with highly reflective zones observed on three seismic‐reflection lines of DEKORP (85‐4N, 95‐01, 95‐02) at their intersection with the seismic‐refraction line 95‐B, where they appear as a set of NW–SE‐trending dome‐shaped reflections. On profile 95‐A this high‐velocity upper‐crustal layer (6.3 km s−1 ) dips from 5 to 9 km beneath the SE margin of the exposed granulites. These results suggest that the granulite dome and its western continuation are widely underlain by a NE‐trending antiformal structure (probably a sheet of metabasic rocks) where the exposed felsic granulites form just a local cap on top. Below the upper‐crustal high velocities, a layer with decreased velocity (6.2–6.25 km s−1 ) extends down to an average depth of 15 km along the Variscan strike (95‐B) and to 11–16 km depth (slightly dipping towards the SE) perpendicular to the terrane boundaries (95‐A). At mid‐crustal levels a weak reflection from a layer with a velocity of 6.4–6.6 km s−1 may indicate the classical Conrad discontinuity. At the depth range 22–24 km the velocity jumps to an average value of 7.0 km s−1, thus defining a prominent high‐velocity layer in the lower crust, which may be viewed as the well‐known laminated lower crust typical of Variscan structures, but with higher average velocity than usually detected. The crust–mantle boundary at about 30–31 km is typical for western Europe and confirms the extensional signature of the West European crust. Below the Moho, poorly constrained upper‐mantle velocities of about 7.9–8.0 km s−1 are derived. The high velocities observed in the lower‐crustal layer would not exclude the possibility of mantle‐derived intrusions, but the lack of any sign of an updoming Moho favours the interpretation of a more passively driven extension.

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Major bodies of high-pressure (HP) rocks in the Saxo-Thuringian Belt in East Germany (Saxonian Granulite Massif, Erzgebirge) are investigated using a variety of geophysical methods (seismic reflection and refraction survey, magnetotelluric studies, gravity modelling). The Saxonian Granulite Massif and the Erzgebirge are not a continuous feature, as can be seen from discontinuous reflections, offset of upper-crustal seismic refraction velocity layers, and crustal resistivity increasing towards the Erzgebirge. Their juxtaposition during the evolution of two Variscan-age thrust wedges may have controlled this geometry. The earlier thrust wedge emplaced the supracrustal Erzgebirge HP nappes from the southeast to the northwest onto the Saxo-Thuringian Basin, whereas the later one propagated southwards and uplifted the Saxo-Thuringian granulites from deeper levels. To the southwest, the granulites are observed at shallow depth as far as the Franconian Line; to the southeast they extend down to the Moho, or they continue at mid-crustal levels. The granulites beneath the Saxo-Thuringian Belt can only have originated in one of two subduction zones: either through 'subduction erosion' and subsequent underplating of parts of the Saxo-Thuringian Plate from the north, or by intracrustal plug flow of overheated material from the southeast.

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