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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. [ABSTRACT FROM AUTHOR]

Foreland fold-and-thrust belts (FTBs) record long-lived tectono-sedimentary activity, from passive margin sedimentation, flexuring, and further evolution into wedge accretion ahead of an advancing orogen. Therefore, dating fault activity is fundamental for plate movement reconstruction, resource exploration, and earthquake hazard assessment. Here, we report U–Pb ages of syn-tectonic calcite mineralizations from four thrusts and three tear faults sampled at the regional scale across the Jura fold-and-thrust belt in the northwestern Alpine foreland (eastern France). Three regional tectonic phases are recognized in the middle Eocene–Pliocene interval: (1) pre-orogenic faulting at 48.4±1.5 and 44.7±2.6 Ma associated with the far-field effect of the Alpine or Pyrenean compression, (2) syn-orogenic thrusting at 11.4±1.1 , 10.6±0.5 , 9.7±1.4 , 9.6±0.3 , and 7.5±1.1 Ma associated with the formation of the Jura fold-and-thrust belt with possible in-sequence thrust propagation, and (3) syn-orogenic tear faulting at 10.5±0.4 , 9.1±6.5 , 5.7±4.7 , and at 4.8±1.7 Ma including the reactivation of a pre-orogenic fault at 3.9±2.9 Ma. Previously unknown faulting events at 48.4±1.5 and 44.7±2.6 Ma predate the reported late Eocene age for tectonic activity onset in the Alpine foreland by ∼10 Myr. In addition, we date the previously inferred reactivation of pre-orogenic strike-slip faults as tear faults during Jura imbrication. The U–Pb ages document a minimal time frame for the evolution of the Jura FTB wedge by possible in-sequence thrust imbrication above the low-friction basal decollement consisting of evaporites. [ABSTRACT FROM AUTHOR]

Foreland fold-and-thrust belts record long-lived tectonic-sedimentary activity, from passive margin sedimentation, flexuring, and further involvement into wedge accretion ahead of an advancing orogen. Therefore, dating fault activity is fundamental for plate movement reconstruction, resource exploration, or earthquake hazard assessment. Here, we report U-Pb ages of syntectonic calcite mineralizations from four thrusts and three tear faults sampled, at the regional scale, across the Jura fold-and-thrust belt in the northwestern Alpine foreland (eastern France). Four regional tectonic phases are recognized in the middle Eocene-middle Pleistocene interval: (1) pre-orogenic faulting at 44.7 ± 2.6 and 48.4 ± 1.5 Ma associated to the uplift of the Alpine forebulge, (2) syn-orogenic thrusting at 11.4 ± 1.1, 10.6 ± 0.5, 9.7 ± 1.4, 9.6 ± 0.3, and 7.5 ± 1.1 Ma associated to possible in-sequence thrust propagation, and (3) syn-orogenic tear faulting at 10.5 ± 0.4, 9.1 ± 6.5, 7.3 ± 1.9, 5.7 ± 4.7, 4.8 ± 1.7, and at 0.7 ± 4.2 Ma including the reactivation of a pre-orogenic fault as tear fault at 3.9 ± 2.9 Ma. Previously unknown faulting events at 44.7 ± 2.6 and 48.4 ± 1.5 Ma predate by ~ 10 Ma the accepted late Eocene age for tectonic activity onset in the Alpine foreland. In addition, we dated the previously inferred strike-slip faults re-activation as tear fault. The U-Pb ages demonstrate the long-lived tectonic history at the plate boundary between European and African plates and that the deformation observed in the foreland is directly linked to continental collision. [ABSTRACT FROM AUTHOR]

We combine structural and microstructural data with stable and clumped isotopes of syntectonic calcite veins or slickenfibers of five thrust and four strike-slip faults to characterize the paleofluid circulation in the Jura fold-and-thrust belt, eastern France. Syn-tectonic fluid circulation occurred along high permeability networks of breccias, foliated fault rocks, and discrete fault surfaces. At the regional scale, fluid circulation was dominated by cold meteoric fluids that infiltrated downward along both thrust and strike-slip faults, with various degrees of interaction with the carbonate host rocks. Fault-related mineralizations precipitated from fluids with δ ⁠18O values between −0.8‰ and − 7.8‰ (V-SMOW) at temperatures between 8 °C and 54 °C. The δ ⁠18O values of the paleofluids partly overlap those of modern meteoric waters of the region. The calcite mineralizations yielding the highest calculated δ ⁠18O paleofluid values are also the warmest ones and those with the heaviest carbon isotope signal, indicating strong host rock buffering and fluid heating at depth. A contribution of deep fluids from Permo-Triassic rocks cannot be excluded, but not likely. This could be related to the presence of Middle Jurassic Opalinus Clay Formation, which acted as a regional low-permeability barrier in the sedimentary succession. This can create two different fluid circulation systems characterized by warm and partly basement-derived fluids and by cold meteoric fluids, respectively, below and above the Middle Jurassic Opalinus Clay Formation. Results from this study can be used for modeling fluid circulation at the regional scale in carbonate-dominant foreland fold-and-thrust belts elsewhere in the world. [Display omitted] • Meteoric water circulated in thrust and strike-slip faults of the Jura fold-and-thrust belt. • Meteoric water channelized along high-permeability fault rocks. • Clays are regional low-permeability barrier for deep fluid upwelling. • Calcite precipitated in veins and slickenfibers at temperatures between 8 and 35 °C. • Tectonic deformation and fluid circulation occurred at depths between 0 and 0.8 km. [ABSTRACT FROM AUTHOR]