The Saint-Ursanne earthquakes of 2000 revisited: evidence for active shallow thrust-faulting in the Jura fold-and-thrust belt

The interpretation of seismotectonic processes within the uppermost few kilometers of the Earth’s crust has proven challenging due to the often significant uncertainties in hypocenter locations and focal mechanisms of shallow seismicity. Here, we revisit the shallow seismic sequence of Saint-Ursanne of March and April 2000 and apply advanced seismological analyses to reduce these uncertainties. The sequence, consisting of five earthquakes of which the largest one reached a local magnitude (ML) of 3.2, occurred in the vicinity of two critical sites, the Mont Terri rock laboratory and Haute-Sorne, which is currently evaluated as a possible site for the development of a deep geothermal project. Template matching analysis for the period 2000–2021, including data from mini arrays installed in the region since 2014, suggests that the source of the 2000 sequence has not been persistently active ever since. Forward modelling of synthetic waveforms points to a very shallow source, between 0 and 1 km depth, and the focal mechanism analysis indicates a low-angle, NNW-dipping, thrust mechanism. These results combined with geological data suggest that the sequence is likely related to a backthrust fault located within the sedimentary cover and shed new light on the hosting lithology and source kinematics of the Saint-Ursanne sequence. Together with two other more recent shallow thrust faulting earthquakes near Grenchen and Neuchâtel in the north-central portion of the Jura fold-and-thrust belt (FTB), these new findings provide new insights into the present-day seismotectonic processes of the Jura FTB of northern Switzerland and suggest that the Jura FTB is still undergoing seismically active contraction at rates likely < 0.5 mm/yr. The shallow focal depths provide indications that this low-rate contraction in the NE portion of the Jura FTB is at least partly accommodated within the sedimentary cover and possibly decoupled from the basement.

Subaqueous geomorphology and delta dynamics of Lake Brienz (Switzerland): implications for the sediment budget in the alpine realm

Non-invasive techniques such as seismic investigations and high-resolution multibeam sonars immensely improved our understanding of the geomorphology and sediment regimes in both the lacustrine and the marine domain. However, only few studies provide quantifications of basin wide-sediment budgets in lakes. Here, we use the combination of high-resolution bathymetric mapping and seismic reflection data to quantify the sediment budget in an alpine lake. The new bathymetric data of Lake Brienz reveal three distinct geomorphological areas: slopes with intercalated terraces, a flat basin plain, and delta areas with subaquatic channel systems. Quasi-4D seismic reflection data allow sediment budgeting of the lake with a total sediment input of 5.54 × 106 t sediment over 15 years of which three-quarter were deposited in the basin plain. Lake Brienz yields extraordinarily high sedimentation rates of 3.0 cm/yr in the basin plain, much more than in other Swiss lakes. This can be explained by (i) its role as first sedimentary sink in a high-alpine catchment, and by (ii) its morphology with subaquatic channel-complexes allowing an efficient sediment transfer from proximal to distal areas of the lake.

Seismic anisotropy of Opalinus Clay: tomographic investigations using the infrastructure of an underground rock laboratory (URL)

Seismic anisotropy and attenuation make claystone formations difficult to characterize. On the other hand, in many geotechnical environments, precise knowledge of structure and elastic properties of clay formations is needed. In crystalline and rock salt underground structures, high-resolution seismic tomography and reflection imaging have proven a useful tool for structural and mechanical characterization at the scale of underground infrastructure (several deca- to hundreds of meters). This study investigates the applicability of seismic tomography for the characterization of claystone formations from an underground rock laboratory under challenging on-site conditions including anisotropy, strong attenuation and restricted acquisition geometry. The seismic tomographic survey was part of a pilot experiment in the Opalinus Clay of the Mont Terri Rock Laboratory, using 3-component geophones and rock anchors, which are installed 2 m within the rock on two levels, thus suppressing effects caused by the excavation damage zone. As a source, a pneumatic impact source was used. The survey covers two different facies types (shaly and carbonate-rich sandy), for which the elliptical anisotropy is calculated for direct ray paths by fitting an ellipse to the separated data for each facies. The tomographic inversion was done with a code providing a good grid control and enabling to take the seismic anisotropy into account. A-priori anisotropy can be attributed to the grid points, taking various facies types or other heterogeneities into account. Tomographic results, compared to computations using an isotropic velocity model, show that results are significantly enhanced by considering the anisotropy and demonstrate the ability of the approach to characterize heterogeneities of geological structures between the galleries of the rock laboratory.

Alpine peak pressure and tectono-metamorphic history of the Monte Rosa nappe: evidence from the cirque du Véraz, upper Ayas valley, Italy

The Monte Rosa nappe consists of a wide range of lithologies that record conditions associated with peak Alpine metamorphism. While peak temperature conditions inferred from previous studies largely agree, variable peak pressures have been estimated for the Alpine high-pressure metamorphic event. Small volumes of whiteschist lithologies with the assemblage chloritoid + phengite + talc + quartz record peak pressures up to 0.6 GPa higher compared to associated metapelitic and metagranitic lithologies, which yield a peak pressure of ca. 1.6 GPa. The reason for this pressure difference is disputed, and proposed explanations include tectonic mixing of rocks from different burial depths (mélange) or local deviations of the pressure from the lithostatic value caused by heterogeneous stress conditions between rocks of contrasting mechanical properties. We present results of detailed field mapping, structural analysis and a new geological map for a part of the Monte Rosa nappe exposed at the cirque du Véraz field area (head of the Ayas valley, Italy). Results of the geological mapping and structural analysis shows the structural coherency within the western portions of the Monte Rosa nappe. This structural coherency falsifies the hypothesis of a tectonic mélange as reason for peak pressure variations. Structural analysis indicates two major Alpine deformation events, in agreement with earlier studies: (1) north-directed nappe emplacement, and (2) south-directed backfolding. We also analyze a newly discovered whiteschist body, which is located at the intrusive contact between Monte Rosa metagranite and surrounding metapelites. This location is different to previous whiteschist occurrences, which were entirely embedded within metagranite. Thermodynamic calculations using metamorphic assemblage diagrams resulted in 2.1 ± 0.2 GPa and 560 ± 20 °C for peak Alpine metamorphic conditions. These results agree with metamorphic conditions inferred for previously investigated nearby whiteschist outcrops embedded in metagranite. The new results, hence, confirm the peak pressure differences between whiteschists and the metagranite and metapelite. To better constrain the prograde pressure–temperature history of the whiteschist, we compare measured Mg zoning in chloritoid with Mg zoning predicted by fractional crystallization pseudo-section modelling for several hypothetical pressure–temperature paths. In order to reach a ca. 0.6 GPa higher peak pressure compared to the metapelite and metagranite, our results suggest that the whiteschist likely deviated from the prograde burial path recorded in metapelite and metagranite lithologies. However, the exact conditions at which the whiteschist pressure deviated are still contentious due to the strong temperature dependency of Mg partitioning in whiteschist assemblages. Our pseudo-section results suggest at least that there was no dramatic isothermal pressure increase recorded in the whiteschist.

New insights on the Early Cretaceous (Hauterivian–Barremian) Urgonian lithostratigraphic units in the Jura Mountains (France and Switzerland): the Gorges de l’Orbe and the Rocher des Hirondelles formations

The Hauterivian–Barremian series of the Jura Mountains were measured over more than 60 sections along a 200 km long transect between Aix-les-Bains (Savoie Department, France) and Bienne (Bern Canton, Switzerland), which prompted the need for a revision and improvement of the current lithostratigraphic scheme for this stratigraphic interval. A new formation, the Rocher des Hirondelles Formation, is proposed in replacement of the unsuitable Vallorbe Formation, while the Gorges de l'Orbe Formation is formally described. The Gorges de l'Orbe Formation, equivalent to the well-known “Urgonien jaune” facies, consists of two members, namely Montcherand Member and Bôle Member. The Rocher des Hirondelles Formation, equivalent to the “Urgonien blanc” facies, consists of three members, i.e. Fort de l'Écluse Member, Rivière Member and Vallorbe Member. The marly Rivière and Bôle members appear to present time-equivalent lithostratigraphic units, recording a major sedimentological event affecting contemporarily both formations. This study proposes a new sedimentary model opening a new point of view on the long-living controversies about the age of the Urgonian series from the Jura Mountains. The data point to strong diachronic ages of lithostratigraphic units with a late Hauterivian to early Barremian occurrence of the “Urgonian blanc” facies in the Meridional Jura area versus a latest Barremian age in the Central Jura area, reflecting a general progradation of the Urgonian shallow-water carbonate platform from the present-day Meridional Jura area toward external deeper-water shelf environments of the present-day Central Jura area and Molasse basin.

Seiches and the slide/seiche dynamics; subcritical and supercritical subaquous mass flows and their deposits. Examples from Swiss Lakes

Four historically documented large and potentially dangerous lacustrine waves in Swiss lakes show that these waves have been seiches (standing waves) triggered by sublacustrine slides; a statement which is in accordance with the experience of seismologists who see earthquakes triggering seiches in lakes. Nevertheless, large historical waves in Switzerland have recently been modeled as progressive shallow water waves (tsunamis), probably because the slide/seiche dynamics are not known, and experiments with subaquatic slides fail to generate seiches in test–flumes. It appears that these tests exhibit a small shear–energy/slide–energy ratio ε, if compared with the situation in lakes. These facts incite a shear–stress lemma that states that ε is the constituent factor for the slide/seiche coupling. The structure of the subaqueous mass flow deposit (MFD) in lakes Lucerne and Geneva suggests the occurrence of subcritical and of supercritical slide flows. The former would generate a contortite, a MFD with contorted bedding, the latter a debrite (mudclast conglomerate). Potential slide energy considerations are used for an estimation of the amplitudes of large seiches produced by subaquatic slides, a proceeding that yields partly similar and partly very different results, as compared with numerical tsunami simulations.

Eclogitic metamorphism in the Alpine far-west: petrological constraints on the Banchetta-Rognosa tectonic unit (Val Troncea, Western Alps)

The Banchetta-Rognosa tectonic unit (BRU), covering an area of 10 km2 in the upper Chisone valley, consists of two successions referred to a continental margin (Monte Banchetta succession) and a proximal oceanic domain (Punta Rognosa succession) respectively. In both successions, Mesozoic meta-sedimentary covers discordantly lie on their basement. This paper presents new data on the lithostratigraphy and the metamorphic evolution of the continental basement of the Monte Banchetta succession. It comprises two meta-sedimentary sequences with minor meta-intrusive bodies preserving their original lithostratigraphic configuration, despite the intense Alpine deformation and metamorphic re-equilibration. Phase equilibrium modeling points to a metamorphic eclogitic peak (D1 event) of 20–23 kbar and 440–500 °C, consistent among three different samples, analyzed from suitable lithologies. The exhumation P–T path is characterized by a first decompression of at least 10 kbar, leading to the development of the main regional foliation (i.e. tectono-metamorphic event D2). The subsequent exhumation stage (D3 event) is marked by a further decompression of almost 7–8 kbar associated with a significant temperature decrease (cooling down to 350–400 °C), implying a geothermal gradient compatible with a continental collision regime. These data infer for this unit higher peak P–T conditions than previously estimated with conventional thermobarometry. The comparison of our results with the peak P–T conditions registered by other neighboring tectonic units allows to interpret the BRU as one of the westernmost eclogite-facies unit in the Alps.