Subalpine Molasse Research Articles

Estimating uncertainties in 3-D models of complex fold-and-thrust belts: A case study of the Eastern Alps triangle zone

Geological modeling commonly results in a single prescribed geometric representation of the subsurface with no consideration of uncertainties. Accounting for uncertainties is of particular importance in the triangle zone at the leading edge of deformation of the foreland fold-thrust belt of the European Alps, the Subalpine Molasse. Here, interpretations of the complex structures are limited to 2D and are based almost exclusively on subsurface data, lacking the constraint of surface data.Implicit modeling can be used to create automated 3D model realizations considering parameter uncertainty. In this sense, multiple possible models can be assessed within the uncertainty range assigned. As implicit modeling often yields artifacts or geologically unfeasible scenarios, the concept of geological topology can be used to constrain the modeling outputs, so that only models without artifacts are considered into the final model ensemble.Two experiments are designed to test out this workflow. The methodology is first tested using a very simple synthetic model, and later applied to a portion of the Subalpine Molasse, where different 2-D subsurface geometries have been proposed. For the first time, a 3D implicit model ensemble incorporating a full assessment of uncertainties is built in this area. Results show that it is feasible to use topological information for posterior model constraint, even in models of high structural complexity. To eliminate all non-meaningful models, however, it is necessary to use more than one topological constraint. A comparison of the prior and posterior model ensembles shows a correlation between model parameters and a shift in parameter probability density curves in the synthetic model experiment, and a decrease in entropy for both experiments. A topological constraint should thus be applied routinely when building a stochastic implicit model ensemble.

Constraining the 3-D Geometry of Fold-Thrust Belts Using Section Balancing vs. 3-D Interpolative Structural and Probabilistic Modeling

Quantitative uncertainty analysis, 2-D and 3-D modeling of the subsurface, as well as their visualization form the basis for decision making in exploration, nuclear waste storage and seismic hazard assessment. Methods such as cross-section balancing are well established and yield plausible kinematic scenarios. However, they are based on geological data with errors and subject to human biases. Additionally, kinematic models do not provide a quantitative measure of the uncertainty of structures at depth. New 3-D modeling approaches have emerged that use computational interpolation, which are less dependent on human biases. Probabilistic extensions enable the quantification of uncertainties for the modeled structures. However, these approaches do not provide information on the time evolution of structures. Here, we compare classical cross-section balancing (2-D, kinematic modeling) with 3-D computational modeling to pave the way towards a solution that can bridge between these approaches. We show the strengths and weaknesses of both approaches, highlighting areas where probabilistic modeling can possibly add quantitative structural uncertainty information to improve section balancing. On the other hand, we show where probabilistic modeling still falls short of being able to cover the observed geometric complexities. We ultimately discuss how a workflow that iteratively combines results of the approaches can improve structural and kinematic constraints. As an example, we use the fold-and-thrust belt of the northern Alpine Foreland, the so-called Subalpine Molasse, focusing on the Hausham Syncline (Bavaria) and adjacent areas. We take advantage of the fact that here the stratigraphy as well as the tectonic history are well constrained. We show that shortening within the syncline progressively increases from west to east, independent from structural uncertainties. Two equally viable models can explain this. First, strain in the west is accommodated underneath the syncline in a triangle zone that progressively tapers out, or second, the strain difference is accommodated in more internal units. This highlights the importance of introducing uncertainty modeling also in kinematic restorations, as it enables identifying key regions, where different hypotheses can be tested.

Improving Absolute Hypocenter Accuracy With 3D Pg and Sg Body‐Wave Inversion Procedures and Application to Earthquakes in the Central Alps Region

AbstractAccuracy of hypocenter location, in particular focal depth, is a precondition for high‐resolution seismotectonic analysis of natural and induced seismicity. For instance, linking seismicity with mapped fault segments requires hypocenter accuracy at the sub‐kilometer scale. In this study, we demonstrate that inaccurate velocity models and improper phase selection can bias absolute hypocenter locations and location uncertainties, resulting in errors larger than the targeted accuracy. To avoid such bias in densely instrumented seismic networks, we propose a coupled hypocenter‐velocity inversion procedure restricted to direct, first‐arriving, mainly upper‐crustal Pg and Sg phases. On the basis of synthetic tests and selected ground‐truth events we demonstrate that a sub‐kilometer hypocenter accuracy can be achieved by regional‐scale, three‐dimensional Pg and Sg velocity models combined with dynamic phase selection and a non‐linear location algorithm. The tomographic inversion uses about 60,000 Pg and 30,000 Sg quality‐checked phases of local earthquakes in the Central Alps region. The derived models image the VP and VS structure of the Central Alps upper crust at unprecedented resolution, including small‐scale anomalies such as those caused by Subalpine Molasse units below the Alpine front. The relocation procedure is applied to more than 18,000 earthquakes and the relocated hypocenters reveal previously unrecognized seismogenic structures, for instance in the Swiss Molasse basin south of Bern. The ML 4.6 Urnerboden earthquake of 2017 is used as an example to demonstrate how the derived 3D velocity structure and relocated hypocenters can be jointly interpreted to constrain the lithology hosting upper‐crustal seismicity in the Central Alps.

Absolute Age and Temperature Constraints on Deformation Along the Basal Décollement of the Jura Fold‐and‐Thrust Belt From Carbonate U‐Pb Dating and Clumped Isotopes

AbstractDuring its late‐stage evolution, the European Alpine orogen witnessed a northwest‐directed propagation of its deformation front along an evaporitic basal décollement into the foreland. This resulted in the decoupling of the northern Alpine Molasse Basin from its basement and the formation of the Jura fold‐and‐thrust belt. Here, we present the first absolute age and temperature constraints on deformation along this basal décollement using carbonate U‐Pb LA‐ICP‐MS dating and clumped isotope thermometry. We analyzed calcite veins associated with a thrust fault branching off from the basal décollement in the distal Molasse Basin and slickenfibers from thrusts and strike‐slip faults in the eastern Jura Mountains. Our U‐Pb data provide evidence for tectonic activity related to Alpine contraction between 14.3 and 4.5 Ma ago. According to the oldest deformation ages, the propagation of Alpine deformation into the distal foreland along the basal décollement occurred earlier than previously inferred by biostratigraphy, at Middle Miocene (Langhian) times at the latest. Younger deformation ages between 11.3 and 4.5 Ma correspond very well in time with shortening in the Subalpine Molasse and the Central Alps, proving simultaneous tectonic activity along both thrust fronts; i.e., the Jura Mountains and the Subalpine Molasse. Clumped isotopes reveal vein calcite precipitation at temperatures between 53 and 104 °C from fluids with oxygen isotope compositions between −6.2‰ and +9.5‰ reflecting distinct burial settings. Combined, our U‐Pb and clumped isotope data show that the burial conditions in the studied area remained constant between 14.3 and 4.5 Ma indicating that large‐scale foreland erosion did not initiate before 4.5 Ma.

Earthquakes in Switzerland and surrounding regions during 2017 and 2018

This report summarizes the seismicity in Switzerland and surrounding regions in the years 2017 and 2018. In 2017 and 2018, the Swiss Seismological Service detected and located 1227 and 955 earthquakes in the region under consideration, respectively. The strongest event in the analysed period was the ML 4.6 Urnerboden earthquake, which occurred in the border region of cantons Uri, Glarus and Schwyz on March 6, 2017. The event was the strongest earthquake within Switzerland since the ML 5.0 Vaz earthquake of 1991. Associated ground motions indicating intensity IV were reported in a radius up to about 50 km and locally approached intensity VI in the region close to the epicentre. Derived focal mechanisms and relative hypocentre relocations of the immediate aftershocks image a NNW–SSE striking sinistral strike-slip fault. Together with other past events in this region, the Urnerboden earthquake suggests the existence of a system of sub-parallel strike-slip faults, likely within in the uppermost crystalline basement of the eastern Aar Massif. A vigorous earthquake sequence occurred close to Château-d'Oex in the Préalpes-Romandes region in western Switzerland. With a magnitude of ML 4.3, the strongest earthquake of the sequence occurred on July 1, 2017. Focal mechanism and relative relocations of fore- and aftershocks image a NNE dipping normal fault in about 4 km depth. Two similarly oriented shallow normal-fault events occurred between subalpine Molasse and Préalpes units close to Châtel-St-Denis and St. Silvester in 2017/18. Together, these events indicate a domain of NE–SW oriented extensional to transtensional deformation along the Alpine Front between Lake Geneva in the west and the Fribourg Fault in the east. The structural complexity of the Fribourg Fault is revealed by an ML 2.9 earthquake near Tafers in 2018. The event images a NW–SE striking fault segment within the crystalline basement, which might be related to the Fribourg Fault Zone. Finally, the ML 2.8 Grenchen earthquake of 2017 provides a rare example of shallow thrust faulting along the Jura fold-and-thrust belt, indicating contraction in the northwestern Alpine foreland of Switzerland.

Long-wavelength late-Miocene thrusting in the north Alpine foreland: implications for late orogenic processes

Abstract. In this paper, we present new exhumation ages for the imbricated proximal molasse, i.e. Subalpine Molasse, of the northern Central Alps. Based on apatite (U-Th-Sm)/He thermochronometry, we constrain thrust-driven exhumation in the Subalpine Molasse between 12 and 4 Ma. This occurs synchronously to the main deformation in the adjacent Jura fold-and-thrust belt farther north and to the late stage of thrust-related exhumation of the basement massifs (i.e. external crystalline massifs) in the hinterland. Our results agree with other findings along the north Alpine foreland. While site-specific variations in the mechanical stratigraphy of the molasse deposits influence the pattern of thrusting at the local scale, we observe that late-Miocene thrusting is a long-wavelength feature occurring along the north Alpine foreland roughly between Lake Geneva and Salzburg. The extent of this thrusting signal as well as the timing suggests that late-Miocene thrusting in the north Alpine foreland coincides with the geometries and dynamics of the attached Central Alpine slab at depth. Interestingly, this implies that the slab geometry at depth does not coincide with the boundary between the Eastern and Central Alps as observed in the surface geology. Using this observation, we propose that thrusting in the Subalpine Molasse and consequently also the late stage of thrust-related exhumation of the external crystalline massifs, as well as the main deformation in the Jura fold-and-thrust belt are at least partly linked to changes in slab dynamics.

Thermal history of the westernmost Eastern Alps (Penninic Rhenodanubian Flysch nappes, Helvetic nappes, and Subalpine Molasse thrust sheets)

The frontal part of the westernmost Eastern Alps comprises from top to bottom of the Austroalpine and Penninic nappes, Ultrahelvetic slices, and two Helvetic thrust sheets, thrust upon the northern Alpine Molasse Basin. The thermal evolution of the Penninic Rhenodanubian Flysch nappes, the Helvetic nappes, and the allochthonous part of the Alpine Molasse Basin is constrained by vitrinite reflectance measurements and apatite fission track dating and implemented in a tectonic evolution scheme. Within the Helvetic nappes, vitrinite reflectance increases regionally from north to south and stratigraphically from the Campanian–Maastrichtian Wang Formation to the Toarcian Mols Member. Apatite fission track ages from Penninic and Subalpine Molasse units are consistently younger than the deposition age. They indicate therefore a post-depositional thermal overprint exceeding approximately 120 °C, the upper temperature limit of the apatite partial annealing zone. 1D thermal modelling suggests that the Penninic nappes attained deepest burial between the latest Cretaceous and Early Palaeocene with the Penninic basal thrust being located at approximately 8 km in the north compared to approximately 12 km in the south. Deepest burial of the upper Helvetic nappe occurred between the latest Eocene and Early Miocene. Its base was buried down to approximately 10.5 km in the north compared to 11.5 km in the south. Exhumation of the entire nappe stack started in the Early to Middle Miocene. For both Penninic and Helvetic models, a heatflow minimum during the Cenozoic deformation (max. 27–32 mW/m2), followed by an increase from the Middle Miocene onwards (up to 60 mW/m2), was assumed.

Geometry, amount, and sequence of thrusting in the Subalpine Molasse of western Austria and southern Germany, European Alps

In this paper we review the structure of the most external thrust belt of the Alps between the Rhein valley and Salzburg based on a new tectonic map and the (re)interpretation of seismic sections. Specifically we address the correlation between deformation in the Subalpine Molasse and the Alpine thrust belt in general and focus on the control of sedimentary facies on the structural style. A dramatic change in architecture from a ramp-flat structure to buckle folding is related to a change from coarse-grained fans to fine-grained deposits within the Subalpine Molasse. Additionally the interaction of escape tectonics with postcollisional shortening controls the decrease of late Early Miocene and younger shortening within the Subalpine Molasse from 50 km near the Rhine valley to almost zero near Salzburg. Transfer of shortening into the hinterland, which is the zone of lateral escape, ended foreland propagation of the Alpine thrusts and initiated a general break-back sequence of thrusting. Throughout this time the thrusts remained active. In such a scenario, tectonic units on top of the Subalpine Molasse are expected to undergo clockwise rotation around vertical axes. As thrusting in the Subalpine Molasse is closely related to contemporaneous transport and shortening within the tectonically higher Helvetic thrust sheets, amounts of Miocene differential shortening and related clockwise vertical axis rotation are minimum amounts. True clockwise vertical axis rotation is probably larger than the 12° deduced from the Subalpine Molasse thrust belt.

The Alpine nappe stack in western Austria: a crustal-scale cross section

Based on an N–S-oriented crustal-scale cross section running east of the Rhine Valley in Vorarlberg, western Austria, we address the Alpine nappe stack and discuss the boundary between Central and Eastern Alps. For our cross section, we used surface geology, drillings and reinterpreted seismic lines, together with published sections. The general architecture of the examined area can be described as a typical foreland fold-and-thrust belt, comprising the tectonic units of the Subalpine Molasse, (Ultra-)Helvetic, Penninic and Austroalpine nappes. These units overthrusted the autochthonous Molasse along the south-dipping listric Alpine basal thrust. The European Basement, together with its autochthonous cover, dips gently towards the south and is dissected by normal faults and trough structures. The seismic data clearly show an offset not only of the top of the European Basement, but also of the Mesozoic cover and the Lower Marine Molasse. This indicates an activity of the structures as normal faults after the sedimentation of the Lower Marine Molasse. The Subalpine Molasse is multiply stacked, forming a triangle zone at the boundary with the foreland Molasse. The shortening within the Subalpine Molasse amounts to approximately 45 km (~67 %), as deduced from our cross section with the Lower Marine Molasse as a reference. The hinterland-dipping duplex structure of the Helvetic nappes is deduced from surface and borehole data. There are at least two Helvetic nappes needed to fill the available space between the Molasse below and the Northpenninic above. This is in line with the westerly located NRP20-East transect (Schmid et al., Tectonics 15(5):1047–1048, 1996; Schmid et al., The TRANSMED Atlas: the Mediterranean Region from Crust to Mantle, 2004), where the two Helvetic nappes are separated by the Santis thrust. Yet in contrast to the Helvetic nappes in the NRP20-East transect, both of our Helvetic nappes comprise Cretaceous and Jurassic strata. This change is explained by an eastward down-stepping of the Santis thrust along a pre-existing, approximately N–S striking lateral ramp bounding an inverted Jurassic graben structure below the Rhine Valley. This causes the Santis thrust to detach the base Cretaceous west of the Rhine Valley and the base Jurassic units east of it. This graben-controlled change in detachment level leads to the formation of quite different nappe stacks on either side of the Rhine Valley and a “fault-controlled” appearance of the boundary between the Central and Eastern Alps.

Late Miocene to present deformation and erosion of the Central Alps — Evidence for steady state mountain building from thermokinematic data

We present new apatite fission track and apatite (U–Th)/He data from the Alpine orogenic front, the Austrian Subalpine Molasse. We show that the cooling signal reported from the Swiss part of the basin since 10Ma is also present farther east. Hence, it appears to be independent of the kinematic relation to thrusting in the external thrust belt present further west, the Jura Mountains. By reconstructing the Central Alpine pro-wedge geometry at 10Ma, we show that the taper of the Central Alps has not changed significantly and presumably remained close to the critical state since then. From cooling offsets at faults, the present day shortening rate of the pro-wedge ranges between 1.0 and 2.0mm/a and appears to have been constant since at least the Late Miocene. In conjunction with the observations of repeated out-of-sequence thrusting in the Subalpine Molasse, a stationary deformation front, constant shortening and erosion rates and the constant taper, we argue that the Central Alpine pro-wedge is at kinematic, as well as at mass flux steady-state since 10Ma. Our results suggest that distinct climate-driven erosion events – in contrast to more continuous erosion – are not distinguishable in the deformation record of the last 10Ma. Hence, kinematic and mass flux steady state in the Central Alpine pro-wedge along with stable localization of deformation restricted to the Subalpine Molasse may indicate a feedback between ongoing shortening and erosion at low rates during the Late Neogene to present.

True colors – experimental identification of hydrological processes at a hillslope prone to slide

Abstract. This study investigated runoff formation processes of a pre-alpine hillslope prone to slide. The experimental pasture plot (40 m × 60 m) is located in the northern front range of the Swiss Alps on a 30° steep hillslope (1180 m a.s.l., 1500 + mm annual precipitation). A gleysol (H-Go-Gr) overlies weathered marlstone and conglomerate of subalpine molasse. We conducted sprinkling experiments on a subplot (10 m × 10 m) with variable rainfall intensities. During both experiments fluorescein line-tracer injections into the topsoil, and sodium chloride (NaCl) injections into the sprinkling water were used to monitor flow velocities in the soil. The observed flow velocities for fluorescein in the soil were 1.2 and 1.4 × 10−3 m s−1. The NaCl breakthrough occurred almost simultaneously in all monitored discharge levels (0.05, 0.25 and 1.0 m depth), indicating a high-infiltration capacity and efficient drainage of the soil. These initial observations suggested "transmissivity feedback", a form of subsurface stormflow, as the dominant runoff process. However, the results of a brilliant blue dye tracer experiment completely changed our perceptions of the hillslope's hydrological processes. Excavation of the dye-stained soils highlighted the dominance of "organic layer interflow", a form of shallow subsurface stormflow. The dye stained the entire H horizon, vertical soil fractures, and macropores (mostly worm burrows) up to 0.5 m depth. Lateral drainage in the subsoil or at the soil–bedrock interface was not observed, and thus was limited to the organic topsoil. In the context of shallow landslides, the subsoil (Go/Gr) acted as an infiltration and exfiltration barrier, which produced significant lateral saturated drainage in the topsoil (H) and possibly a confined aquifer in the bedrock.

Linking the northern Alps with their foreland: The latest exhumation history resolved by low‐temperature thermochronology

The evolution of the Central Alpine deformation front (Subalpine Molasse) and its undeformed foreland is recently debated because of their role for deciphering the late orogenic evolution of the Alps. Its latest exhumation history is poorly understood due to the lack of late Miocene to Pliocene sediments. We constrain the late Miocene to Pliocene history of this transitional zone with apatite fission track and (U‐Th)/He data. We used laser ablation inductively coupled mass spectrometry for apatite fission track dating and compare this method with previously published and unpublished external detector method fission track data. Two investigated sections across tectonic slices show that the Subalpine Molasse was tectonically active after the onset of folding of the Jura Mountains. This is much younger than hitherto assumed. Thrusting occurred at 10, 8, 6–5 Ma and potentially thereafter. This is contemporaneous with reported exhumation of the External Crystalline Massifs in the central Alps. The Jura Mountains and the Subalpine Molasse used the same detachments as the External Crystalline Massifs and are therefore kinematically coupled. Estimates on the amount of shortening and thrust displacement corroborate this idea. We argue that the tectonic signal is related to active shortening during the late stage of orogenesis.

Rapid extensive erosion of the North Alpine foreland basin at 5-4 Ma

An extensive low-temperature thermochronology study of the Swiss part of the North Alpine Foreland Basin has been conducted with the aim of deciphering the late Neogene basin development. Apatite fission-track (AFT) ages from wells located in the distal and weakly deformed Plateau Molasse reveal rapid, km-scale erosion with an onset in early Pliocene times. The distribution of erosion implies that there was a strong gradient in late Miocene deposition rates along the strike of the basin, with an increase towards the northeast. Additionally, renewed tectonic activity and km-scale out-of-sequence thrusting during Plio-Pleistocene times is indicated by AFT data from wells within the thrusted, proximal Subalpine Molasse. Several different mechanisms driving late Neogene basin erosion and accelerated erosional discharge from the European Alps have been considered in the literature. Based on our AFT results, we reevaluate previously published hypotheses, and suggest that a change in climate and/or drainage reorganisation coincided and possibly interacted with preexisting tectonic and geodynamic forces in the Alpine region.

Transport and Variability of Fecal Bacteria in Carbonate Conglomerate Aquifers

Clastic sedimentary rocks are generally considered non-karstifiable and thus less vulnerable to pathogen contamination than karst aquifers. However, dissolution phenomena have been observed in clastic carbonate conglomerates of the Subalpine Molasse zone of the northern Alps and other regions of Europe, indicating karstification and high vulnerability, which is currently not considered for source protection zoning. Therefore, a research program was established at the Hochgrat site (Austria/Germany), as a demonstration that karst-like characteristics, flow behavior, and high vulnerability to microbial contamination are possible in this type of aquifer. The study included geomorphologic mapping, comparative multi-tracer tests with fluorescent dyes and bacteria-sized fluorescent microspheres, and analyses of fecal indicator bacteria (FIB) in spring waters during different seasons. Results demonstrate that (1) flow velocities in carbonate conglomerates are similar as in typical karst aquifers, often exceeding 100 m/h; (2) microbial contaminants are rapidly transported toward springs; and (3) the magnitude and seasonal pattern of FIB variability depends on the land use in the spring catchment and its altitude. Different groundwater protection strategies that currently applied are consequently required in regions formed by karstified carbonatic clastic rocks, taking into account their high degree of heterogeneity and vulnerability.

Detailed 3D seismic imaging of a fault zone beneath Lake Geneva, Switzerland

AbstractAn efficient high‐resolution (HR) three‐dimensional (3D) seismic reflection system for small‐scale targets in lacustrine settings was developed. In Lake Geneva, near the city of Lausanne, Switzerland, the offshore extension of a complex fault zone well mapped on land was chosen for testing our system. A preliminary two‐dimensional seismic survey indicated structures that include a thin (<40 m) layer of subhorizontal Quaternary sediments that unconformably overlie south‐east‐dipping Tertiary Molasse beds and a major fault zone (Paudèze Fault Zone) that separates Plateau and Subalpine Molasse (SM) units.A 3D survey was conducted over this test site using a newly developed three‐streamer system. It provided high‐quality data with a penetration to depths of 300 m below the water bottom of non‐aliased signal for dips up to 30° and with a maximum vertical resolution of 1.1 m. The data were subjected to a conventional 3D processing sequence that included post‐stack time migration. Tests with 3D pre‐stack depth migration showed that such techniques can be applied to HR seismic surveys.Delineation of several horizons and fault surfaces reveals the potential for small‐scale geologic and tectonic interpretation in three dimensions. Five major seismic facies and their detailed 3D geometries can be distinguished. Three fault surfaces and the top of a molasse surface were mapped in 3D. Analysis of the geometry of these surfaces and their relative orientation suggests that pre‐existing structures within the Plateau Molasse (PM) unit influenced later faulting between the Plateau and SM. In particular, a change in strike of the PM bed dip may indicate a fold formed by a regional stress regime, the orientation of which was different from the one responsible for the creation of the Paudèze Fault Zone. This structure might have later influenced the local stress regime and caused the curved shape of the Paudèze Fault in our surveyed area.

Slope instability mechanisms in dipping interbedded conglomerates and weathered marls—the 1999 Rufi landslide, Switzerland

In May of 1999, melting snow cover combined with heavy rainfalls in the northeastern part of Switzerland resulted in the occurrence of numerous shallow landslides. Many of these slides were located in the subalpine Molasse, a series of interbedded marls, conglomerates and sandstones. The subalpine Molasse is highly prone to such sliding activity given the dip of bedding, the surface topography and the weak nature of the marls that rapidly degrade when exposed to weathering. Historically, the subalpine Molasse has been the source of numerous slides of varying orders of magnitude and the surficial morphology is primarily dictated by previous rockslides, which periodically reactivate as secondary soil slips. Given the conditions contributing to the slope failure and the history of previous sliding, similar slides within the region are highly probable. This paper presents the findings from a detailed investigation of one such slide, the 1999 Rufi slide. Results from a detailed engineering geological investigation show that the initial rockslide failure plane predominantly developed and passed through the marl beds, as dictated by the penetration of the weathering front into the marls, and not along reactivated shears or the bedding plane contact between the marls and overlying conglomerates. Numerical modelling results based on a coupled hydromechanical distinct-element analysis were able to reproduce this mechanism by incorporating a progressive strength degradation procedure correlated to mapped weathering grades.

Alluvial fan sedimentation and structure of the southern Molasse Basin margin, Lake Thun area, Switzerland

The Chattian Lower Freshwater Molasse (USM) of the Thun area comprises a thick series of continental sediments which occur in three imbricate thrust sheets (Steffisburg, Zulg-Hombach, Blueme) of the Subalpine Molasse and over wide areas of the autochthonous Plateau Molasse. Facies analysis of outcrops, complemented by conglomerate clast counts, sandstone petrography, heavy mineral analysis and data from two deep exploration wells and seismics permit structural restoration of the thrust sheets, correlation of subalpine and autochthonous USM and consequently the reconstruction of the geometry and facies evolution of the alluvial fan systems. The most complete succession measuring more than 4500 m is preserved in the structurally highest Blueme thrust sheet

Processing, interpretation and modeling of seismic reflection data in the Molasse Basin of eastern Switzerland

Seismic reflection profiles crossing the Plateau Molasse, the Subalpine Molasse, and reaching into the Helvetic Nappes of eastern Switzerland have been reprocessed, interpreted and modeled. Final stacks and some migrated data are presented here. Foreward modeling with 2-D ray tracing is used to confirm the structural features interpreted from the seismic data. The basin is characterized by southward increase in thickness of the Tertiary strata which is associated with onlap structures. The Triassic-Middle Jurassic sediments thin southward and show a notable change in seismic character at the northern edge of the Alemannic land which might be due to Jurassic synsedimentary faulting. The Mesozoic strata are dipping at a shallow angle towards the south. An abrupt change to a steeper dip occurs at the southern end of the Plateau Molasse and is related to the northward pinch-out of the Lower Marine Molasse (UMM). Inversion of the thick UMM in the Neogene compression stage lead to the development of a classic triangle zone. Thrust faults within the Subalpine Molasse level out at depth according to the seismic data and are related to folding and thrusting on the northern flank of the Aar massif

Coalification in the northern Wildhorn nappe and adjacent units, western Switzerland. Implications for tectonic burial histories

Vitrinite reflectance of fourteen Eocene coal samples from the northern Wildhorn nappe in the Kander valley have been analyzed and compared to nine agewise similar coals from adjacent tectonic units. The degree of coalification decreases regularly from semi-anthracite in the Kander valley to lignite in the plateau molasse. The degree of coalification in the Tertiary coals appears to be solely dependent on their tectonic history and indicates and estimated 6 km of burial by higher (Pennine) nappes for the Kander valley. The rank at this locality (low-volatile bituminous to semi-anthracite) is significantly higher than in the Border chain at Beatenberg (high-volatile A bituminous). The latter locality therefore was never buried deeper than about 4 km. A further decrease in rank is found in the subalpine flysch and molasse (high-volatile B bituminous) which in turn is significantly more mature than lignites from the folded plateau molasse at Linden. Two samples of Dogger coal from the Pennine “Préalpes médianes” are medium- to low-volatile bituminous that is intermediate between the Kander valley and Beatenberg. Their rank, however, was achieved before their final emplacement north of the Wildhorn nappe. By comparing calculated TTI (Time-Temperature-Indices)-values for time-depth-burial curves with measured vitrinite reflectances, maximum burial temperatures were estimated for the various tectonic units. For the Wildhorn nappe, 160–180°C were obtained in the Kander valley and 100–130°C in the Border chain as a result of tectonic burial due to the thrusting by the Pennine Prealps. Sedimentary burial led to temperatures of 100–120°C in the subalpine molasse below the Border chain prior to its thrusting.

Dual Origin of Natural Gases in Subalpine Tertiary Basins: ABSTRACT

Many young sedimentary basins produce a great variety of natural gases ranging from deep wet gases to shallow dry gases. It is of considerable interest to find evidence for the origin of these gases, especially with regard to deep exploration. In the Tertiary subalpine Molasse basin of south Germany, dry gases occur in shallow Oligocene to Miocene reservoirs (^dgr13C1 ~ -70 to -60 parts per thousand), low C2+ gases in upper Eocene reservoirs (^dgr13C1 ~ -60 to -50 parts per thousand), and wet gases in other Eocene reservoirs (^dgr13C1 ~ -50 to 62 parts per thousand; the wet gases being partly associated with crude oils). Two alternatives for the origin of the gases should be considered: (1) the dry gases may be migrated wet gases which have been stripped of their C2+ components and have been enriched in carbon 12 isotopes; and (2) the dry gases are of biogenic origin. Carbon and hydrogen isotope analyses on gases in the Molasse basin have brought direct evidence for the dual origin of these gases. The shallow dry gases are of biogenic origin, as shown by the direct relation between the deuterium isotope ratios of the methane and their associated waters. The wet gases are of thermogenic origin. The scatter in the carbon isotopic composition and the C2+ concentration in this particular basin is due to downmixing of the bacterial gases to deeper strata owing to an underlying underpressured zone. C and H isotope analyses on gases from two other Tertiary subalpine basins (Austria and northern Italy) have shown the dual-origin concept to be generally applicable to these basins. In particular the D/H and 13C/12C patterns o the gases reflect mixing processes and thus give information on the general hydrodynamic situation of the basins. End_of_Article - Last_Page 522------------