The general geological evolution of Northern Switzerland since the Paleozoic has been introduced in Chapter 3. In the following, more details regarding the tectonic and burial temperature evolution since the Early Jurassic are presented based on the observations reported in Section 4.3.4 combined with temperature and absolute age constraints obtained by clumped isotope thermometry coupled with U-Pb and U-Th dating of carbonate cements in veins and diagenetic structures such as pore fillings. The analysis of multiple generations of carbonate cements allows tracking of the evolution of burial temperature and pore fluids across geological timescales. The elemental and isotopic compositions of such carbonate cements record both the timing of precipitation as well as the temperature and composition of the mineral-forming fluid and thereby provide information on the ambient conditions at a specific point in time (e.g. Looser et al. 2021). Time of crystallisation is recorded by the incorporation of uranium into the carbonate lattice and the temperature of crystallisation by the clumped isotope composition. The carbonate clumped isotope palaeothermometer (Δ47) allows the direct reconstruction of the formation temperature and of the oxygen isotopic composition of the mineral-forming fluid (δ18Ofluid). Δ47/(U-Pb/U-Th) thermochronometry thus contrasts other burial temperature proxies such as apatite fission tracks and (U-Th)/He thermochronometers, vitrinite reflectance, or maturation-dependent biomarkers, which yield an integrated temperature signal over time.
In the following, the geodynamic evolution since the Early Jurassic is discussed with the focus on the younger evolution as it has a more direct impact on the present-day setting and its extrapolation to the future. Oxygen isotope data of pore fluids are included in the discussion as these provide additional information on the burial setting. Details on palaeo-fluid flow are discussed in Sections 4.5 to 4.7. The present-day temperature conditions in the siting regions are elaborated in Section 4.8.
Deposition, early diagenesis and onset of burial during the Early Jurassic
In the Early Jurassic, sedimentation occurred in an epicontinental sea (Chapter 3) with lithospheric stretching creating accommodation space. Deposition rates were very low throughout the Early Jurassic, resulting in a present-day thickness of the Lias Group of only 30 – 50 m in Northern Switzerland and shallow burial conditions until the beginning of the Middle Jurassic (Section 4.2.5; Reisdorf & Wetzel 2018, Müller et al. 2002). Void-filling calcite cements in ammonites, aragonite replacement cements in bivalves, and veins in the Beggingen Member (lower Lias Group) yield U-Pb ages overlapping with the stratigraphic age of the host rocks and thus document the early Jurassic geodynamic evolution (Fig. 4‑73). In the case of the early veins, this indicates that lithification of calcareous units in the Lias Group initiated shortly after deposition. The δ18Ofluid signatures of these cements document earliest diagenesis with the mineral-forming fluids sourcing from seawater (Fig. 4‑73). One sample from the Frick clay pit (Fig. 4‑60) indicates mixing with meteoric waters infiltrated during localised temporary emergence of the Beggingen Member during the Early Jurassic, in agreement with sedimentological evidence (Section 4.2.5, Reisdorf & Wetzel 2018 and references therein). Despite overlapping U-Pb ages, the temperatures of these early calcite phases show an inconsistent picture, with values ranging between 25 and 50 °C and, in the case of one outlier, even of 70 °C. While seawater-like Δ47 temperatures are in good agreement with marine to very shallow burial conditions, the extent of the observed high temperatures of other calcite phases as well as the coeval occurrence of very different temperatures raises questions about their plausibility. The observed high temperatures may result from near-surface related kinetic effects (e.g. degassing) and should thus be interpreted with caution.
Middle Jurassic to Early Cretaceous extension and thermal anomaly
Because of the absence of the Late Jurassic and Cretaceous sedimentary rocks due to non-deposition and/or erosion, the burial and thermal evolution during the Late Jurassic and Cretaceous has been debated. Several studies reported indications for heating during the Late Jurassic and Cretaceous, but the timing of occurrence and associated temperatures could not be precisely constrained (e.g. Schegg & Leu 1998, Mazurek et al. 2006, Timar-Geng et al. 2006, Omodeo-Salé et al. 2021, Villagómez Díaz et al. 2021). It thus remained unclear whether high temperatures resulted from deep burial under (later eroded) the latest Jurassic and Cretaceous sediments or from high basal heat flow. The presented Δ47/(U-Pb) data of calcite and saddle dolomite veins and pore-filling cements indicate that, between the Middle Jurassic and Early Cretaceous, burial temperatures increased to peak temperatures of 60 °C in the upper Malm Group, 80 – 85 °C in the middle Dogger Group, and up to 100 – 115 °C in the lower Lias Group at 150 – 135 Ma (Fig. 4‑73). Primary fluid inclusions in the same cements suggesting similar or even higher temperatures corroborate these Δ47 temperatures (i.e. pressure-corrected trapping temperatures of 80 °C for the upper Malm Group, 85 – 90 °C for the middle Dogger Group, and up to 110 – 115 °C for the lower Lias Group). These temperatures are in good agreement with temperatures of up to 104 ± 9 °C obtained from calcite veins in the Opalinus Clay reported by Akker et al. (2023), although U-Pb dating of those veins was unsuccessful.
The U-Pb ages of the carbonate cements recording these temperatures overlap within error or are only slightly younger than the depositional age of the youngest preserved Jurassic sediments (Wettingen Member, «Felsenkalke» and «Massenkalk») which have a Late Kimmeridgian stratigraphic age corresponding to 150 – 149 Ma. This suggests relatively shallow burial depths for the time of occurrence of peak burial temperatures and confirms high basal heat flow as the reason for the observed heating, as proposed by Mazurek et al. (2006), rather than deep burial. The δ18Ofluid signatures of the cements recording the peak burial temperatures reflect pristine to only slightly modified seawater and contradict extensive interaction between pore fluids and host rocks at high temperatures. Instead, the observed marine-like δ18Ofluid compositions document comparably short residence time of the fluids at the recorded high temperatures, in agreement with the inferred shallow burial depths. Low (non-radiogenic) 87Sr/86Sr signatures of these cements consistent with Jurassic to Early Cretaceous seawater show no indication for interaction of the pore fluids with the crystalline basement and/or Triassic formations and speak against hydrothermal fluid ascent into the Jurassic succession (Looser 2022). Therefore, the fluids sourced either externally from downward migrating Late Jurassic to Early Cretaceous seawater or internally from marine formation waters present in the host rocks and heated rapidly due to abruptly increased basal heat flow.
The temperature anomaly diminished during the early Middle Cretaceous, as recorded by 30 °C cooling in the lower Lias Group until 120 Ma, although burial depths are thought to have remained constant or even increased during that time (Crampton & Allen 1995, Schegg & Leu 1998, Mazurek et al. 2006). Although the general thermal evolution is comparable, the peak burial temperatures indicated by carbonate cements were hotter and reached earlier than suggested previously based on thermal modelling using apatite thermochronology, vitrinite reflectance and biomarker isomerisation data. For instance, Omodeo-Salé et al. (2021) estimated peak temperatures of ~ 95 °C for the lower Dogger Group that were reached at 110 Ma and Mazurek et al. (2006) estimated peak temperatures of ~ 60 °C for the top of the uneroded Malm Group, ~ 80 °C for the lower Dogger Group, and, interpolated from their estimates for the top of the crystalline basement, ~ 85 – 90 °C for the Lias Group at 120 – 105 Ma (Fig. 4‑73).
The time frame for the occurrence of high basal heat flow coincides with continental-scale crustal extension related to continued ocean opening and spreading at the Piemont – Liguria Ocean and Atlantic Ocean in the Late Jurassic – Early Cretaceous (e.g. Ziegler 1990, Handy et al. 2010). This tensional stress resulted in widespread extension tectonics (e.g. Trümpy 1980, Stampfli & Borel 2002) and accelerated subsidence of large parts of central Europe (Chapter 3, Section 4.2, e.g. Wildi et al. 1989). In Northern Switzerland, subsidence rates increased drastically at the beginning of the Middle Jurassic Era, as evidenced by orders of magnitude higher net sedimentation rates for the Dogger and Malm Groups compared to the Lias Group (Section 4.2; Müller et al. 2002). In this extensional regime, pre-existing Paleozoic fault systems in Northern Switzerland, such as the Konstanz – Frick Trough, are thought to have been episodically reactivated across the Jurassic (Allenbach 2002, Wetzel et al. 2003, Reisdorf & Wetzel 2018). The observed high basal heat flow could thus well have occurred related to thermal disturbances along such basement-rooted faults. Further evidence for thermal disturbances during the Jurassic and Early Cretaceous is provided by hydrothermal veins and mineral alterations in the pre-Mesozoic units in the Black Forest Massif, with radiometric ages covering the entire Jurassic and Early Cretaceous (Wernicke & Lippolt 1997, Pfaff et al. 2009, Staude et al. 2012, Brockamp et al. 2015 and references therein). As elaborated above, hydrothermal activity in Northern Switzerland would have been restricted to the pre-Mesozoic units and the lowermost Triassic sedimentary rocks without cross-formation fluid flow into the Jurassic succession whereby massive Triassic evaporites could have acted as effective seal (Mazurek et al. 2006, Looser 2022).
Eocene exhumation and evolution during the Oligocene
Cessation of the first interval of burial during the Late Mesozoic is recorded by a major erosional unconformity spanning ~ 100 Ma from the latest Jurassic to Eocene times (Herb 1988). Combined Δ47/(U-Pb) data show cooling of 25 °C at 55 to 50 Ma, which can be correlated with exhumation (Fig. 4‑73). Calcite cements precipitated in the Malm Group between Late Cretaceous and Early Eocene times show δ18Ofluid signatures overlapping with those of Jurassic porewaters. This indicates that the Mesozoic succession remained hydrologically undisturbed until the onset of exhumation at ~ 55 Ma. Starting at 50 Ma and continuing until 20 Ma, however, δ18Ofluid signatures gradually shifted to meteoric signatures with values as low as -7 ‰ resulting from infiltration of meteoric fluids and mixing with Jurassic marine porewaters. Possible transport processes are discussed in Section 4.7.
The earliest preserved deposits after erosion are continental karst infills in the uppermost Malm Group consisting of Siderolithic clays with a Middle Eocene to Early Oligocene depositional age (Mammal Paleogene zones MP13 – MP22, corresponding to 45 – 33 Ma (Rosselet 1991, Gradstein et al. 2020).
During the Oligocene, closure of the Piemont – Liguria and Valais Oceans was completed (e.g. Schmid et al. 1996). Subsequently, collision involved basal accretion of European upper crust into the Alpine wedge (Handy et al. 2010, Gleißner et al. 2021). Increased crustal loading of the lower plate resulted in down bending of the lithosphere and the consequent formation of a wedge-shaped foreland basin referred to as the Molasse Basin (Pfiffner 1986). Alternatively, it has been suggested that not flexural loading, but slab rollback is responsible for bending of the lower plate (Schlunegger & Kissling 2022). Whichever mode is preferred, the flexural foreland basin was deepest close to the orogenic front and became progressively shallower towards the north, resulting in a wedge geometry. Basin width (i.e. N-S extent) as well as possible formation of a distal forebulge is a function of the elastic thickness of the lower plate. The forebulge marked an area dominated by uplift and erosion. The lithospheric flexure resulted in a SE-directed tilting of the Mesozoic strata in Northern Switzerland (Fig. 4‑56, Fig. 4‑57 and Fig. 4‑58).
Fig. 4‑73:Evolution of burial temperature and oxygen isotope composition of pore fluids (δ18Ofluid) since the Early Jurassic
(a) Evolution of temperature since the Early Jurassic (after Mazurek et al. 2006). (b) Evolution of the oxygen isotope composition of mineral-forming fluids since the Early Jurassic. (c) Temperature evolution in the Malm Group since 25 Ma. (d) Evolution of the oxygen isotope composition of mineral-forming fluids in the Malm Group since 25 Ma. M.: Middle, U.: Upper, Paleo.: Paleocene, Oligo.: Oligocene, P.: Pliocene, Q.: Quaternary, Gr.: Group, Sid.: Siderolithic, VSMOW: Vienna standard mean ocean water, repl.: replacement.
Furthermore, flexing of the lower plate including pre-Mesozoic basement and Mesozoic cover resulted in extensional tectonics in Northern Switzerland during the Paleogene (e.g. Laubscher 1985). This normal faulting phase is recorded in Northern Switzerland, for instance, in the Mesozoic strata north of the Mandach Thrust which are at higher elevation than south of the Mandach (Nagra 2024a), in the present-day net normal offset preserved along the eastern continuation of the Baden – Irchel – Herdern Lineament (Fig. 4‑60; Meier et al. 2014) or in the normal faulting observed along the cores of the recent deep boreholes (Tab. 2‑1).
Preceded by Late Cretaceous volcanism, rifting in the southern Upper Rhine Graben started in Late Eocene times (42 – 31 Ma; Henrion et al. 2020, Ziegler 1992, Rotstein & Schaming 2011, Dèzes et al. 2004). The main rifting phase ended in the mid-Rupelian but localised extension related to a strike-slip-dominated deformation continued at least into the Pliocene (Giamboni et al. 2004, Rotstein & Schaming 2011). Rift-related surface uplift of the graben shoulders (i.e. Black Forest and Vosges Massifs) further increased southward tilting of the Mesozoic strata in Northern Switzerland (e.g. Müller et al. 2002).
Evolution during the Miocene
During the Miocene, the Alps grew by frontal and basal accretion. This has been attributed to either plate convergence (e.g. Bonnet et al. 2007, Schlunegger & Mosar 2011, Willett & Schlunegger 2010), or slab rollback and associated buoyancy-driven uplift (Schlunegger & Kissling 2015). NW-propagation of the Alpine deformation occurred along an intra-Triassic décollement level rooting underneath the External Crystlline Massifs (e.g. Buxtorf 1916, Laubscher 1961, Burkhard 1990, Ortner et al. 2024). Low strength Triassic evaporites acted as a décollement horizon facilitating the formation of a narrow taper as observed in the Jura Fold-and-Thrust Belt (e.g. Mosar 1999). Folding and thrusting in the Jura mountains 50 – 80 km north of the front of the Alpine nappes is inferred to have started in the Middle Miocene (Fig. 4‑74; Becker 2000, Looser et al. 2021, Smeraglia et al. 2021). Stratigraphic evidence from mammalian fauna place onset of Jura folding and thrusting at about 12 Ma (Becker 2000). Tectonic activity along the décollement (i.e. thin-skinned) is estimated from U-Pb calcite dating to 14.3 to 4.5 Ma in the Schafisheim borehole located south of the Internal Jura (Looser et al. 2021). Dating of regional fault zones in Northern Switzerland confirmed this age range and specifies that the main thin-skinned structures were established at 8 Ma (Fig. 4‑74; Madritsch et al. 2024). For instance, the Mandach Thrust, the Siggenthal Anticline and the Jura Main Thrust in JO and the Lägern Anticline in NL are associated with décollement-related (i.e. thin-skinned) deformation (Section 4.3.4).
Shortening north of the Subalpine Molasse generally decreases eastwards in the central North Alpine Foreland (e.g. Ortner et al. 2024 and references therein). This is also seen in the shortening estimates decreasing from JO to NL and in general from the shortening around 25 – 30 km in Central Jura and the maximum 5 km shortening estimated in JO (Fig. 4‑74; Burkhard 1990, Jordan et al. 2015). The eastward decrease of shortening and finally the change from allochthonous to autochthonous sedimentary rocks in the North Alpine Foreland can be associated with the strength of the décollement, where the thin-skinned salient of the Jura Mountains is associated with the distribution of evaporites.
As described by Madritsch et al. (2024), no clear trend of in-sequence or out-of-sequence thrusting can be deduced from the dense age data for the deformation sequence in the External Jura in Northern Switzerland (Fig. 4‑74). This is expected for a growing wedge, as it maintains its critical taper geometry (e.g. Willett 1999, Buiter 2012, Caër et al. 2018).
Fig. 4‑74:Simplified tectonic map of Northern Switzerland showing absolute ages obtained from veins sampled in outcrops and drill cores and shortening estimates
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. Absolute ages are from Madritsch et al. (2024) and Looser et al. (2021). Colour coding indicates the kinematic association of the sampled veins. Profile traces with shortening estimates are taken from Jordan et al. (2015) and refer to shortening estimates from kinematic 2D balancing.
The Hegau – Bodensee Graben also initiated during the Early Miocene with a period of transtension (Ibele 2015 and references therein). This was followed by a main phase of strike-slip tectonism in the Late Miocene to Pliocene (Schreiner 1992, Hofmann et al. 2000, Egli et al. 2017). U-Pb age dating of calcite showed that the Randen Fault, which is part of the Hegau – Bodensee Graben, was active at the same time as the main phase of the Jura folding and thrusting (Fig. 4‑74; Madritsch et al. 2024).
Continuing lithospheric flexure during the Late Miocene (e.g. Kuhlemann & Kempf 2002, Ford et al. 2006) led to a second interval of burial of the Mesozoic formations accompanying deposition in the Molasse Basin. Maximum burial temperatures during the Miocene decreased with increasing distance from the Alpine front, which is associated with the thinning of the Molasse sedimentary wedge towards the north (e.g. Pfiffner 1986, Kuhlemann & Kempf 2002). This second interval of burial is documented in calcite cements by a temperature increase starting at ~ 20 Ma and resulting in peak burial temperatures between 10 and 5 Ma, in broad agreement with the timing suggested by previous thermochronological data (Mazurek et al. 2006, Villagómez Díaz et al. 2021) and in agreement with the maximum age for the cessation of Molasse sedimentation of ~ 11.5 Ma in Northern Switzerland (Rahn & Selbekk 2007). Peak temperatures in the Malm Group ranged between 50 – 80 °C and are thus in broad agreement with previous estimates (Fig. 4‑73; Mazurek et al. 2006).
The southward rise in temperature and the development of the Jura Fold-and-Thrust Belt, which led to localised surface uplift along thrusts and consequently the creation of new fluid migration pathways, caused local variations in the temperature and oxygen isotopic composition of calcite cements. This is reflected by calcite veins formed coevally between 10 and 8 Ma in the Malm Group at different locations that show large variability in Δ47 temperatures and the evolution of δ18Ofluid. After relatively uniform compositions until 15 Ma, both Δ47 temperatures and δ18Ofluid data show diverging trends contemporaneously with the formation of the Jura Fold-and-Thrust Belt (Fig. 4‑73).
Evolution during the Pliocene
During Pliocene times, the Alpine deformation front continued propagating northwestwards, reaching into the Plateau Jura (Schlunegger & Mosar 2011). The major thin-skinned deformation phase of the Jura Fold-and-Thrust Belt (i.e. Miocene evolution) has been followed by a thin- and thick-skinned deformation at low strain rates until the present day (e.g. Philippe et al. 1996, Pfiffner et al. 1997, Mosar 1999, Giamboni et al. 2004, Ustaszewski & Schmid 2007, Madritsch et al. 2008, Schlunegger & Mosar 2011, Lacombe & Bellahsen 2016, Caër et al. 2023). Deformation involving the substratum below Middle Triassic décollement level is corroborated by interpretations from the recent 3D seismic reflection database (Section 4.3.4, Fig. 4‑66; Zambrano et al. submitted). For instance, the Weiach – Glattfelden – Eglisau Lineament roots in the Paleozoic strata but cuts through the décollement horizon and a footwall shortcut (i.e. thrusting cutting through the fault’s footwall) affecting the Paleozoic basement underneath the Mandach are evidence for the substratum involvement (Section 4.3.4). Caër et al. (2023) proposed lateral changes in deformation style along the Jura arc as the basement-involving deformation front follows the Alpine Front which has a different shape than the Jura arc. This results in deformation involving the basement in the eastern Jura Fold-and-Thrust Belt.
Miocene burial was followed by exhumation resulting in the present-day setting. The youngest preserved sedimentary rocks in Northern Switzerland are Upper Freshwater Molasse sedimentary rocks dated to 11.5 Ma and thus constrain the initiation of net erosion in the Molasse Basin (Rahn & Selbekk 2007). Based on existing data von Hagke et al. (2015) showed that exhumation may have commenced sometime between about 12 and 4 Ma and heat flux probably increased over time. Numerous studies propose onset of large-scale foreland exhumation to occur between 6 and 4.5 Ma (Kuhlemann & Kempf 2002, Cederbom et al. 2004, 2011, Schlunegger & Mosar 2011, von Hagke et al. 2014, Looser et al. 2021). Reported Δ47/(U-Pb) data from calcite cements of the Siggenthal Anticline and STA2 record cooling from 63 to 25 °C between 5.0 and 2.5 Ma (Fig. 4‑73) agreeing with the aforementioned estimates. This in turn is in agreement with rapid incision pulse triggered by capture of the former Aare-Danube River by the Rhone-Doubs system at 4.2 – 2.9 Ma with incision of 300 – 500 m in the lower Aare Valley (Chapter 3, Fig. 3‑11). The recorded 25 °C at 2.5 Ma suggests a remaining sedimentary overburden, in agreement with incision of another 100 – 200 m during the Pleistocene as recorded by gravel deposits in the lower Aare Valley (Chapter 3, Fig. 3‑11).
Quaternary tectonic activity
Tectonic deformation within the Quaternary has been postulated for Northern Switzerland. The present-day seismicity suggests that deformation in the North Alpine Foreland is ongoing (Kastrup et al. 2004, Lanza et al. 2022, Diehl et al. 2023). The inversion of focal mechanisms implies a strike-slip to normal faulting tectonic regime for the majority of the North Alpine Foreland. Indications for contraction to oblique contraction related seismicity are rare and restricted to the northwestern front of the Jura Mountains, where there is also geomorphic evidence for ongoing folding (Nivière & Winter 2000, Madritsch et al. 2010, Lanza et al. 2022). In addition, two samples provide U-Th ages within the Quaternary (Fig. 4‑73). The sample from the Villigen Formation (0.41 ± 0.01 Ma) was obtained from a slickenfibre indicating strike-slip faulting in TRU1. The other sample documents Quaternary fluid flow but is not related to tectonic activity. Further, discussion of the present-day tectonic activity (including vertical and horizontal velocities) is provided in Section 6.2 and in Nagra (2024l).