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One of the main hazards during tunnel boring machine (TBM) excavation in hard rock is related to the occurrence of unforeseen adverse conditions, leading to unfavourable rock mass behaviour during tunnel boring. In this respect, the continuous analyses of TBM excavation parameters may represent an interesting tool for a continuous monitoring of geotechnical conditions at the tunnel face and for early detection of changing ground conditions in the rock mass. This work provides a back‐analysis of TBM data recorded during the excavation of the Frejus Safety Tunnel, bored across the French‐Italian border in the Western Alps. It is a 13 km‐long tunnel excavated by a 9.4 m diameter, single‐shield TBM through strongly anisotropic calcareous schist, with cover up to 1800 m. Main problems faced during TBM excavation were related to the local occurrence of buckling‐related convergence and consequent asymmetrical loading on the precast segment rings. Correlating TBM performance parameters to the encountered geological and geomechanical conditions provides the possibility to identify which parameters are most affected by geomechanical properties of the rock mass at the tunnel face and by rock mass behaviour. Conversely, the analysis indicates which parameters can be adopted as monitoring tool of ground conditions during TBM advance. In addition, this work highlights how buckling phenomena can be distinguished, in terms of TBM operation and performance, from other adverse conditions as, e.g., blocky ground or squeezing. [ABSTRACT FROM AUTHOR]

Boreholes provided with biostratigraphic data indicate major changes in the Miocene sediment accommodation rates in the northern Campine Block, located in the southern North Sea Basin. Low sediment accommodation rates took place during the early to middle Miocene and abruptly increased into the late Miocene. Two processes likely explain these observations. First, the long‐time (±10 Myr) slow sedimentation of fine sands with a high—in part authigenic—glauconite content (of up to almost 50%) suggests that the southern North Sea area was sediment starved during the early to middle Miocene. Sediment starvation ended with the start of the late Miocene when the area became covered by west‐prograding clinoform sets of coarser sands with mainly reworked glauconite as part of the proto‐Rhine system. The latter sands filled the accommodation space that was left after early to middle Miocene sediment starvation. Second, accelerated subsidence during the late Miocene created additional accommodation space to be filled compared with the early to middle Miocene. A mechanism that explains well the regional accelerated subsidence is lithospheric folding or buckling. The timing and geometry of accelerated rates of subsidence in the southern North Sea area are indeed consistent with deformation that was thought to have been related to lithospheric folding in areas further south (in the direction of the Alps) under NW‐Alpine compression. [ABSTRACT FROM AUTHOR]

The Barrovian metamorphism of the Lepontine dome is manifested by isograds that cross‐cut tectonic nappe contacts, which is commonly interpreted as metamorphism that occurred after nappe emplacement. However, the pervasive mineral and stretching lineation in amphibolite facies, associated with top‐to‐foreland shearing, suggests that peak Barrovian conditions are coeval with nappe‐overthrusting. Here, we combine mapping and U‐Pb zircon dating to better constrain the relation between metamorphism and overthrusting. Metamorphic zircon rims show two age populations at 31–33 and 22–24 Ma. The younger population is locally observed in post‐foliation dikes (and associated metasomatism) likely sourced from deep‐migmatites exhuming along the Alpine backstop. The older population occurs regionally and is found in syn‐kinematic migmatites which occur along a crustal‐scale shear zone. Below this shear zone, magmatic and detrital zircon cores suggest that the Cima Lunga unit, previously interpreted as a tectonic mélange with Mesozoic fragments, was a pre‐Variscan metasedimentary sequence intruded by Permian granitic sills, now orthogneisses. This unit was strongly sheared along the top of the Simano nappe during overthrusting of a rock pile here‐termed Maggia‐Adula nappe. This large‐scale nappe emplacement imprinted the regional lineation and peak temperatures until 31–33 Ma. Péclet (1–10) and Brinkman (0.002–1.8) numbers, estimated for the overthrusting, suggest an advection‐dominated heat transfer caused by rock exhumation, with some diffusion (conduction) during nappe emplacement. Diffusion contributed to Barrovian isograds discordant to the thrust. Shear heating was important if stress times shearing rate >∼5·10−6 W·m−3 within the nappe. The thermal evolution after overthrusting was spatially heterogeneous until ca. 22 Ma. Plain Language Summary: The Lepontine area constitutes the core of the Central European Alps. It has a dome structure and it is internally formed by rock units which register pressure and temperature conditions typical of collisional orogens. The temperatures recorded by minerals are high and the origin of the heat that affected the Lepontine units is still unclear. In this study, we implemented different branches of geology to reveal the age of the Alpine events which juxtaposed the Lepontine units, their provenance and evolution. We performed extensive geological mapping to define the lithologies and structures of rocks. From 13 samples, we extracted 1158 zircon crystals that we analyzed and dated. Fieldwork permitted us to discover new rock units and better characterize the transition between the large‐scale units constituting the Lepontine dome. We propose a geodynamic scenario involving a major large‐scale Alpine unit. The emplacement of this unit generated the main heating event at 31–33 Ma, which is widespread and resulted in peak temperature conditions. The thermal evolution after this event was regionally complex and spatially heterogeneous. Locally in the south we document magmatic/fluid injections at 22–24 Ma, which were sourced from still‐hot regions in the roots of the orogen. Key Points: Geological mapping reveals widespread syn‐tectonic migmatitic rocks along a main crustal‐scale shear zone inside the Lepontine domeAlpine nappe emplacement is regionally recorded at 31–33 Ma, locally overprinted in the south by magmatic/fluid pulses at 22–24 MaMapping and detrital zircon crystals suggest a structurally coherent Cima Lunga unit of pre‐Variscan age, as part of the Simano nappe [ABSTRACT FROM AUTHOR]

The Ladinian Calcare Rosso of the Southern Alps provides a rare opportunity to examine the temporal relationships between tepees and palaeokarst. This unit comprises peritidal strata pervasively deformed into tepees, repeatedly capped by palaeokarst surfaces mantled by terra rossa. Palaeokarsts characterized by a regional distribution across the Southern Alps, occur at the base and at the top of the unit. Local palaeokarsts, confined to this part of the platform, occur within the Calcare Rosso and strongly affected depositional facies. Tepee deformation ranges from simple antiformal structures (peritidal tepees) to composite breccias floating in synsedimentary cements and internal sediments (senile tepees). Peritidal tepees commonly occur at the top of one peritidal cycle, in association with subaerial exposure at the cycle top, while senile tepees affect several pentidal cycles, and are always copied by a palaeokarst surface. Cements and internal sediments form up to 80% of the total rock volume of senile tepees. The paragenesis of senile tepees is extremely complex and records several, superimposed episodes of dissolution, cement precipitation (fibrous cements, laminated crusts, mega-rays) and deposition of internal sediments (marine sediment and terra rossa). Petrographical observations and stable isotope geochemistry indicate that cements associated with senile tepees precipitated in a coastal karstic environment under frequently changing conditions, ranging from marine to meteoric, and were altered soon after precipitation in the presence of either meteoric or mixed marine/meteoric waters. Stable isotope data for the cements and the host rock show the influence of meteoric water (average δ[SUP18]O = - 5.8&peermil;), while strontium isotopes (average [SUP87]Sr/[SUP86]Sr = 0.707891) indicate that cements were precipitated and altered in the presence of marine Triassic waters. Field relationships, sedimentological associations and... [ABSTRACT FROM AUTHOR]