This section is dedicated to summarising key factors that influenced the present-day structural inventory. It focuses first on the encountered deformation style (i.e. influence of Paleozoic faults and mechanical stratigraphy) and then on the impact of the tectonic domains surrounding the siting regions on the structural inventory within the siting regions.
Influence of Paleozoic fault inventory on later deformation
The importance of geometric steps affecting the Base Mesozoic unconformity for subsequent deformation has been previously proposed for the Jura Fold-and-Thrust Belt (e.g. Laubscher 1961, Homberg et al. 2002, Schori et al. 2021, Schori 2021. The pre-Mesozoic tectonic evolution resulted in the formation of three distinct fault orientations (Hercynian or NW-SE-trending, Rhenish or NNW-SSW-trending, and Erzgebirgian or ENE-WSW-trending, Fig. 4‑55). These faults have acted as zones of weakness within the less deformed surrounding rock volume and were repeatedly reactivated throughout the tectonic evolution of Northern Switzerland (Section 4.3.5). Comparison of fault traces at various stratigraphic levels as performed by Schöpfer et al. (2024) indicates a strong control of these orientations on deformation in the sedimentary cover. In JO and NL, regional fault zones, with exception of the Siggenthal Anticline, are located above structures related to the Konstanz – Frick Trough (ENE-WSW-trending). This spatial coincidence calls for being consequential rather than coincidental. For NL, Butler et al. (submitted) investigated the influence of the basement topography and highlighted differences between the eastern and western segment of the Baden – Irchel – Herdern Lineament within the 3D seismic perimeter that could be associated with differences in the underlying Paleozoic fault geometry. Other examples of faults in the Mesozoic sedimentary cover with a typical Paleozoic orientation are the Neuhausen Fault, the Eglisau Fault and the Zweidlen Fault (NW-SE-trending). The Effingen Fault and the Trüllikon Fault and the Rheinau Fault are examples of faults trending NNE-SSW (Rhenish trend).
The influence of step morphology in the Base Mesozoic unconformity is further testified by a comparison between the Mandach Thrust and the Siglistorf Anticline. The Mandach Thrust is expressed throughout the Mesozoic succession as a rather narrow deformation zone. In contrast, deformation in the Mesozoic strata overlying the northern border of the Konstanz – Frick Trough in NL affects a broader zone. This marked difference is probably linked to the pronounced, single Paleozoic normal fault underlying the Mandach Thrust, whereas the Siglistorf Anticline is underlain by various smaller and more distributed Paleozoic faults (Nagra 2024a, 2024b).
The aforementioned examples from seismic reflection data interpretation provide supporting evidence for the proposed influence of the Upper Paleozoic trough and its fault pattern on the localisation of later deformation.
Mechanical stratigraphy and décollement horizons
In addition to the aforementioned stepped geometry of the Base Mesozoic unconformity, the variable mechanical properties of the Mesozoic strata also strongly influenced strain accommodation in Northern Switzerland. The overall stratigraphic column was introduced in Chapter 3 and Section 4.2. It is characterised by strong contrasts in the mechanical competence between the different units, with for instance the carbonates of the Malm Group and the Muschelkalk Group representing competent units and the clay-mineral-rich Opalinus Clay representing a less competent unit. The detailed geomechanical properties are reported in Sections 4.4 and 5.5 focusing on the Opalinus Clay. The mechanical competence contrast among different formations, also termed mechanical stratigraphy, is reflected in the fracture frequency distribution (Section 4.3.4), but also in the stress field characteristics (Section 4.4).
Two key observations in Northern Switzerland with respect to the understanding of the tectonic evolution are related to the mechanical stratigraphy. Firstly, the mechanical stratigraphy influences the fault architecture in the different units. In general, faults in Northern Switzerland appear highly segmented not only in map view, but also in vertical sections (Section 4.3.4). For instance, the surface expression of the Mandach Thrust varies along strike, being influenced by the mechanical stratigraphy and local changes in the stepped geometry of the Base Mesozoic unconformity (Malz et al. 2020). The mechanically less competent, clay-mineral-rich units are characterised by an even higher fault segmentation, with soft-linkage and fault relay zones (Roche et al. 2020, Zwaan et al. 2022). Fault architecture of these units is further detailed in Section 5.5.4.
Secondly, the typical stratigraphic column in Northern Switzerland contains multiple horizons that could act mechanically as décollement levels (e.g. Marro et al. 2023, for a recent compilation for the western Jura). Décollement level refers to the observation that the total strain accommodated above it differs from that below it. The evaporites in the Zeglingen Formation and in the Bänkerjoch Formation are considered as main décollement for the Jura Fold-and-Thrust Belt (e.g. Sommaruga et al. 2017). Additionally, the Opalinus Clay has also been proposed as a subsidiary décollement level (Matter et al. 1987, Marro et al. 2023). The detailed fault pattern observed in the Baden – Irchel – Herdern Lineament (Fig. 4‑67) requires an interval around the Opalinus Clay capable of decoupling the mechanical panel above from that below it (Laubscher 1985, Malz et al. 2016, Nagra 2024b). However, the competent panels above and below the incompetent Opalinus Clay, although independently folded in the Baden – Irchel – Herdern triangle zone in NL, are characterised by similar amounts of deformation. Therefore, at least in the siting regions, the Opalinus Clay did not allow the rock panel overlying it to be transported forelandward more than the rock panel underlying it. Also the overall degree of tectonic deformation observed within the Opalinus Clay in the boreholes within the siting regions shows that it was not acting as a secondary décollement north of the Baden – Irchel – Herdern Lineament (Section 5.5 and fracture frequency distribution in Fig. 4‑61, Fig. 4‑65, Fig. 4‑69).
As mentioned above, the evaporites in the Zeglingen Formation are considered as décollement level in the eastern Jura Fold-and-Thrust Belt. The halite intervals encountered in the recent deep boreholes vary substantially in vertical thickness (Fig. 4‑2). While the halite thickness reaches only 1.4 m in BOZ1, several decameter-thick halite deposits have been drilled in the central part of NL. In general, the halite is only slightly deformed with minor horizons of intense deformation (i.e. shear zones). Shortening estimates from the halite intervals are in broad agreement with the shortening estimates for the area north of the boreholes (distal with respect to the source of deformation, i.e. the Alps) based on balanced and restored cross-sections (Jordan et al. 2015). The halite interval in BOZ1 is more strongly deformed along with a generally more deformed Triassic evaporitic sequence than in NL (Madritsch et al. 2024; TBO reports in Tab. 2‑1). It is concluded that the Triassic evaporites encountered in the boreholes in the External Jura provide evidence for a décollement particularly in JO but also in NL. This observation based on the borehole records is well aligned with the proposed fault geometry from seismic reflection interpretation for the Siglistorf Anticline and the Mandach Thrust (Section 4.3.4).
Tectonic domains in Northern Switzerland
As introduced in Chapter 3 and detailed in the sections above, Northern Switzerland is at the interplay of the Konstanz – Frick Trough, the Jura Fold-and-Thrust Belt, the Hegau – Bodensee Graben and the Upper Rhine Graben. The following section is dedicated to illustrating the influence of these tectonic domains onto the structural inventory of the siting regions.
Paleogene to Early Neogene normal faulting in response to the uplift of the Black Forest related to the opening of the Upper Rhine Graben, the northward migration of the Alpine flexural forebulge and the lithospheric flexural bending in response to the Alpine orogeny affected Northern Switzerland (Section 4.3.5). This is recorded inter alia by (i) the general south dip of the Mesozoic strata; (ii) the net normal offset observed within Mesozoic strata along the eastern segment of the Baden – Irchel – Herdern Lineament; and (iii) the bending of the Mesozoic strata south of the Mandach Thrust, which is clearly related to a post-Mesozoic but pre-thrusting extensional reactivation of the master fault bounding the Konstanz – Frick Trough to the north. The occurrence of shear sense indicators indicative for normal faulting in the boreholes can be considered as additional evidence for this deformation stage (Fig. 4‑75).
The contractional deformation related to folding and thrusting associated with the Jura Fold-and-Thrust Belt is depicted foremost in surface topography. Its decrease towards the east is shown by the decreasing shortening estimates along 2D geological profiles across Northern Switzerland (Fig. 4‑75; Jordan et al. 2015). In addition, the relative abundance of reverse faulting shear sense indicators in the boreholes in JO and NL is also considered as evidence of the contraction-dominated deformation affecting the siting regions. The overall higher frequency of subseismic fracturing in the boreholes in JO (BOZ2, BOZ1) compared to the other boreholes is probably related to the stronger contractional overprint due to its more internal position with respect to the central Jura. The occurrence of the seismically mappable "Brugg Strukturzone", expressed in the Muschelkalk Group in the vicinity of BOZ1, could also influenced the subseismic fault density encountered along the entire borehole.
The Neuhausen Fault, marking the northeastern border of ZNO (Fig. 4‑75), is linked to the Hegau – Bodensee Graben. The oblique extension of the Hegau – Bodensee Graben probably resulted in the formation of high-angle faults in ZNO. It could further be the cause for the increased amount of normal faulting observed in the drill cores in ZNO compared to JO and NL (Fig. 4‑75).
Fig. 4‑75:Tectonic map of Northern Switzerland showing the influence of the neighbouring tectonic domains
Surface fault traces based on Nagra (2014b). Fault traces from seismic interpretation are based on 2D seismic reflection (Madritsch et al. 2013, Meier et al. 2014) and 3D seismic reflection interpretation (Birkhäuser et al. 2001, Nagra 2024a, 2024b, 2024c). They are shown on the highest stratigraphic horizon where the specific fault segment was picked (different colours in the map view). Only horizons from Top Opalinus Clay and higher stratigraphic levels are shown. Note that only a selection of the structures from 3D seismic interpretation is shown to highlight the structural trends of regional importance. See Fig. 4‑61, Fig. 4‑65 and Fig. 4‑69 for the complete fault pattern. The shortening estimates were calculated based on geometric 2D section restoration along 2D seismic reflection lines documented in Jordan et al. (2015). The pie charts show the relative abundance of shear sense indicators observed in the full cored sections. Micro faults with undefined shear sense are not displayed. The charts show the influence of the Jura Fold-and-Thrust belt with a predominance of reverse faulting or thrusting in JO and NL, the influence of the transtensional activity in the Hegau – Bodensee Graben with an increased portion of normal faulting in ZNO as well as the Paleogene extension related to the migration of the Alpine forebulge.