Permo-Carboniferous Trough Research Articles

The Muschelkalk aquifer of the Molasse basin in SW-Germany: implications on the origin and development of highly saline lithium-rich brines in calcareous hydrothermal reservoirs

Highly saline lithium-rich hydrothermal fluids (measured chloride concentration up to 44 g kg−1, lithium concentration up to 162 mg kg−1) occur in the deep calcareous Muschelkalk aquifer beneath the northern Alpine foreland (Molasse) basin. We have combined geologic, hydraulic, hydrochemical, and stress field data of the Triassic Muschelkalk aquifer beneath younger sediments of Triassic–Jurassic successions and the Cenozoic Molasse basin of SW-Germany for a synthesis to constrain the origin and development of these brines. In contrast to the regional southeast plunge of Jurassic and Cenozoic strata, low-gradient groundwater flow in the Upper Muschelkalk aquifer is to the north, induced by regional recharge from west, south, and east. The investigated area is seismically active and north trending maximum horizontal stress likely fosters development of necessary fracture permeability for northward flow in the competent carbonates of the Upper Muschelkalk aquifer. The highest lithium concentrations and total dissolved solids (TDS) can be found in the southern parts of the Muschelkalk aquifer. Here, the Muschelkalk Group overlays directly a crystalline basement swell separating two ENE-trending Permocarboniferous troughs. We argue that the highly saline lithium-rich fluids originate from fluid–rock interaction of meteoric water with Variscan crystalline basement rocks and entered the Muschelkalk aquifer on top of the basement swell by permeable faults and fractures. The marginal calcareous sand-rich facies of the Muschelkalk enables the inflow of brines from crystalline basement faults and fractures into the aquifer. We thus argue for an external origin of these brines into the aquifer and further intra-reservoir development by dilution with meteoric water.

Evidence for synsedimentary differential tectonic movements in a low-subsidence setting: Early Jurassic in northwestern Switzerland

During the Early Jurassic (lasting ~ 27 Myr) only thin deposits (mostly ca. 30–50 m) of the Staffelegg Formation accumulated in wide parts of NW Switzerland while sea-level rise was in the range of ~ 60 m. Isopach and facies patterns provide clear evidence of differential subsidence while faults that formed in the basement during the late Palaeozoic became reactivated. Orientation of many relative thickness minima and maxima follows faults constituting either the Rhenish Lineament or the North Swiss Permo-Carboniferous Trough. Such pattern is seen on the isopach maps of the Schambelen, Beggingen, Weissenstein, Frick, Fasiswald, Mt. Terri, Breitenmatt, Rickenbach, Rietheim and Gross Wolf members of the Staffelegg Formation, independently upon if the individual lithostratigraphic units are condensed or display somewhat enhanced thickness. Onto a general trend of decreasing thickness to the S, often isopach anomalies of small areal extension are superimposed. They suggest that localized strike-slip movements affected a mosaic of basement blocks. Reactivation of faults in the basement during the Early Jurassic is also evidenced by temporally enhanced hydrothermal activity as documented by chronometric ages of veins and mineral alterations.

Helium diffusivity and fluxes from a sedimentary basin (Permo-Carboniferous trough, Northern Switzerland)

Abundances of radiogenic He and Ar isotopes, observed in ground waters of the Permian-Carboniferous terrigenous sediments (PCT) in Northern Switzerland, exceed those in the overlaying (Muschelkalk) and underlying (Crystalline basement) aquifers, eliminating external sources of these species. In this case a comparison of the observed abundances with those in situ generated sheds light on mobility of noble gases generated in the PCT.Detailed studies of the parent element and He isotope concentrations in the PCT rocks, mineral separates, pore and ground waters along with the data on γ-logging and porosity allow He isotope production and redistribution in the PCT rock-water system to be understood and quantified. The PCT shales generate most of 3He and 4He and both isotopes are almost completely released from the shales into pore space and migrate either into the aquifers with movable waters and further out of the PCT, or into gas-fluid inclusions of the rock forming minerals, quartz and plagioclase.Here we present a He concentration profile, based on: (i) helium measured concentrations in ground water samples from 5 aquifers available within the cross section studied; (ii) helium concentrations (in 15 PCT pore water “samples”), derived from the He partial pressure in gas-fluid inclusions, recovered by special extraction/saturation experiments.In order to estimates the rates of He migration through the PCT, we compare the produced and observed abundances within the frame of 1D diffusion model. Only two parameters, the diffusion coefficient for the whole sequence, DALL = 4.1 × 10−3 m2 year−1, and the rate of He removal from the crystalline aquifers, θCR ≈ 1.4 × 10−6 year−1, are required to obtained a good agreement between the observed and calculated He concentrations in PCT waters: the minimal square deviations are ≈(2 to 3) × 10−8 mol cm−3 H2O, comparable with the accuracy of measurements. The following parameters were derived from the modeling: the reduction factor DALL/D0 = 0.013; the He diffusion fluxes into the overlaying Muschelkalk and underlying Crystalline aquifers, FMU ≈ 1.4 × 10−7 mol m−2 year−1 and FCR = 0.24 × 10−7 mol m−2 year−1, respectively. From these values we calculated the mean helium residence times in PCT, <τPCT> ≈ 120 Ma, and in Muschelkalk aquifer, <τMU> ≈ 6 ka. The relationships between He diffusivities of the PCT and the underlying Crystalline basement are discussed. We further compare our results with other estimates of He mobility in low-permeable sediments.

Tectonics of the central Swiss Molasse Basin: Post‐Miocene transition to incipient thick‐skinned tectonics?

AbstractIn this paper we investigate the tectonics of the central Swiss Molasse Basin (SMB), situated between the central European Alps and the Jura fold‐and‐thrust belt. Based on (re)interpreted 2‐D reflection seismic data and geological 3‐D modeling, we present new insights into the structural configuration and evolution of the central SMB. During the thin‐skinned, main deformation phase of the Jura fold‐and‐thrust belt (~12–4 Ma), the central SMB was riding passively above a major detachment horizon within Triassic evaporites, which roots beneath the External Crystalline Massifs of the Central Alps. In the SMB, low‐amplitude detachment folds and detachment‐rooted WSW‐ENE striking thrust and reverse faults are related to this event. Younger, major NNE‐SSW and NNW‐SSE trending strike‐slip fault zones cross the basin over a length of several kilometers, affecting the Cenozoic and Mesozoic cover units and extending into the pre‐Mesozoic crystalline basement. In cases, the fault zones apparently reactivate inherited, Permo‐Carboniferous trough (PCT) bounding, Paleozoic normal faults. Seismic data further reveal mild inversion of WSW‐ENE striking PCTs. These structures clearly cut the basement‐cover contact, thus postdating the main deformation phase of the Jura fold‐and‐thrust belt. Present‐day earthquake focal mechanisms and epicenter distribution show strike‐slip faulting and basement‐seated seismicity, respectively, thus corroborating our observations from seismic interpretation. Our data suggest a change from late Miocene thin‐skinned to Pliocene to present‐day incipient thick‐skinned tectonics. This major change reflects the establishment of a midcrustal detachment and may be related to changes in slab dynamics of the subducting European lithosphere.

Production, redistribution and loss of helium and argon isotopes in a thick sedimentary aquitard-aquifer system (Molasse Basin, Switzerland)

In a rock–water system an apparent residence time for He can be derived from the bulk He concentration in the rock and the porewater and the He production rate provided that (1) the system is at steady state and (2) the He flux from external sources is negligible. This second condition is crucial and needs to be assessed by identifying the various He-sources in a given hydrologic environment. He and Ar isotope abundances were investigated in whole rock samples and mineral separates of the alternating shale–sandstone rock sequence of the Permo-Carboniferous Trough (PCT) of the Molasse Basin in northern Switzerland. 4 He concentrations and 40 Ar/ 36 Ar ratios in groundwater in the bounding aquifers are lower than those in PCT groundwaters, thus eliminating a possible external source of radiogenic noble gases. The elements producing radiogenic helium isotopes, U, Th and Li, are mainly concentrated in the shales, where they reside in biotite, muscovite, clay minerals and organic matter. Contents of these elements as well as chronological and chemical data allow us to calculate maximum “closed system” He isotope concentrations and the retention coefficients, e.g. 3 He measured / 3 He calculated ratios. As deduced from the low retention coefficients, almost all helium-3 that has been produced in these minerals since their deposition has been lost into the complementary reservoir, i.e. the porewater; the same conclusion is valid for 4 He. This is also indicated by the similarity between the calculated 4 He/ 3 He production ratio of the shales and that observed in PCT groundwater (1.3 × 10 7 ). Moreover, similar 4 He/ 3 He ratios are measured in quartz and plagioclase mineral grains from the sandstone layers, by far exceeding the production ratios derived for these minerals. Also, the measured concentrations of He isotopes (especially 3 He) are higher than the calculated production capacities of these minerals. These observations suggest that some of the He produced in the shale and released from these rocks migrates via the porewater into the sandstone layers and penetrates further into some rock-forming minerals, such as quartz and plagioclase. Measurements of the migration rate of He atoms through quartz crystals showed that equilibrium between internal (gas–fluid vesicles) and external (porewater) He-concentrations is reached on a short time scale of ~ 10 4 years at an in-situ temperature of ≈ 70 °C. The equilibrium allows the He concentrations in the porewater across the PCT sequence to be quantified using quartz crystals as detectors. For the PCT segment opened by the Weiach borehole these “quartz-derived” concentrations as well as those directly measured in PCT groundwaters are rather high implying a long residence time for the He-atoms in the PCT rock-porewater system, well exceeding 10 7 years. ► In shale/sandstone layers He-isotopes are mainly yielded in U, Th, Li rich shales. ► He releases from shales into porewater and migrates to He-poor sandstone layers. ► From water He migrates in sandstone minerals, e.g., quartz, acting as a sink for He. ► Observed 4 He/ 3 He ratio in sandstones is similar to the production ratio in shales. ► He residence time in P-C Trough, Molasse basin, N. Switzerland, exceeds 40 Ma.

Seismic source characterization of the Alpine foreland in the context of a probabilistic seismic hazard analysis by PEGASOS Expert Group 1 (EG1a)

Seismic source characterization is performed as part of the PEGASOS project for the assessment of the seismic hazard at the 4 sites of the Swiss Nuclear Power Plants. The analysis is performed according to the Level 4 procedures for expert elicitation defined in the guidelines of the US Nuclear Regulatory Committee whereby the quantification of uncertainties plays a crucial role. According to our analysis, which is one amongst four that were performed in the frame of PEGASOS, the most important epistemic uncertainty is related to the question as to weather basement-rooted faults at the margins of pre-existing Permo-Carboniferous troughs are prone for compressive or transpressive reactivation under the present-day stress field or not. The question after the present-day style of deformation in the Alpine foreland (thick-skinned versus thin-skinned) is closely related to this key question. Together with the consideration of uncertainties regarding the mapping of seismogenic zones and/or line sources alternative zonations are presented in form of a logic tree with 21 branches. Area sources play a predominant role in the working area located at the margin of a diffuse plate boundary. Earthquake recurrence relationships are discussed by taking into account a series of uncertainties. These concern the evaluation of b-values and the evaluation of a-values once the b-values were fixed. Both parameters in the Gutenberg-Richter law are based on non-perfect and incomplete catalogue data that were carefully analysed beforehand. Since PEGASOS demanded an analysis of annual probabilities down to one event in 107 years, the question after the value of the maximum possible earthquake magnitude M max and related error in M max estimates plays a crucial role. We estimate M max by using geological as well as statistical methods. M max = 6.9 cannot be excluded in most areas, in the Basel area M max = 7.3 is possible. Uncertainties in a, b and M max are again discussed in form of a logic tree, this time with 18 branches. Hence the final logic tree has 378 branches and represents the seismic source characterization input into PSHA that takes account of all uncertainties we are aware of.

Latest Pliocene to recent thick-skinned tectonics at the Upper Rhine Graben – Jura Mountains junction

The southernmost Upper Rhine Graben and adjacent Jura experienced basement-rooted shortening that occurred after the deposition of the Pliocene fluvial “Sundgau gravels”. Folds affecting the base of these gravels systematically trend NE to ENE. Combined evidence from reflection seismic lines and contour maps of the base-Tertiary and base-Pliocene levels indicates that these folds probably formed by thick-skinned reactivation of both NNE-SSW and WSW-ENE-striking faults. This thick-skinned shortening is NW-SE oriented, i.e. parallel to the maximum horizontal stresses inferred from seismotectonics. NNE-SSW-striking faults (paralleling the Upper Rhine Graben) have been reactivated in sinistral strike-slip mode. However, dextrally transpressive reactivation of the WSW-ENE-trending faults that belong to the Rhine-Bresse Transfer Zone is interpreted to predominate.Deflections of recent river courses around the crests of en-échelon-aligned surface anticlines suggest that the deformation is ongoing at present. Retro-deformation of the folds affecting the base of the Sundgau gravels indicates horizontal displacement rates of about 0.05 mm/a. This corresponds to a minimum strain rate in the order of 2·10−16 s−1, given the maximum time span of 2.9 Ma for this deformation, i.e. the biostratigraphically determined minimum age of the gravels. A change from thin-skinned tectonics, that prevailed during the main phase of Jura folding, to very probably still ongoing thick-skinned tectonics is inferred to have occurred in the Late Pliocene. We speculate that this change might be linked to the incipient inversion of Permo-Carboniferous troughs within the Alpine foreland in general. This inversion in dextrally transpressive or purely compressive mode along a WNW-ESE-trending basement fault, that is part of the Rhine-Bresse Transfer Zone, which in turn was prestructured during the formation of the Permo-Carboniferous troughs, could have triggered the 1356 Basel earthquake.

Evidence for an active fault below the northwestern Alpine foreland of Switzerland

SUMMARY This study is devoted to the analysis of a prominent concentration of earthquakes whose epicenters delineate an active 20‐30 km long N‐S trending tectonic feature near the town of Fribourg, in the Molasse Basin of western Switzerland. This feature coincides with the possible southward continuation of the NNE‐SSW trending Rhine Graben located approximately 80 km further north. In addition these epicenters are located in the vicinity of the Fribourg Syncline and the Alterswil Culmination, whose structural axes are oriented N‐S in this area, instead of being aligned with the predominant regional NE‐SW structural trend. Most of the earthquakes belong to one of three series of events that occurred over a time span of 2‐4 months in 1987, 1995 and 1999. They include four events with magnitudes between 3 and 4 and one with a magnitude of 4.3. Focal depths, constrained by modelling sPMP-PMP traveltime differences with synthetic seismograms, are around 2 km, which places these events in the sedimentary cover. Fault plane solutions correspond to almost pure strike-slip mechanisms with nearly N‐S and E‐W oriented nodal planes. High-precision relative locations of individual events within the different earthquake clusters as well as of the relative locations of the clusters to each other show that these earthquakes are associated with left lateral motion along a N‐S trending fault system. Deep reaching large scale flower structures in the Mesozoic and Tertiary overburden are observed on interpreted seismic profiles, close to the hypocenters. The unusual N‐S trend of the Fribourg Syncline can be attributed to movements along these faults during Oligocene and Miocene times. Also magnetic data support the assumption of a N‐S striking fault system in the Fribourg area, possibly related to a Permo-Carboniferous trough. Though the direct link between the fault traces in the overburden and the active fault system at depth could not be established in this study, their similar deformational style and their vicinity suggest that they are related. The total length of the inferred fault carries the potential of a magnitude 6 earthquake and thus constitutes a significant source of seismic hazard.

The low-temperature thermal history of northern Switzerland as revealed by fission track analysis and inverse thermal modelling

New zircon and apatite fission track (FT) data from four boreholes, which penetrate the Mesozoic and pre-Mesozoic sediments and crystalline basement of northern Switzerland, are presented. Inverse thermal modelling of the measured apatite FT parameters unravels the low-temperature (below ~120°C) thermal history of the crystalline basement of northern Switzerland. Zircon FT central and single-grain ages cluster around 250 Ma, thus maximum palaeotemperatures did not exceed ∼ 330°C after late-Variscan consolidation of the crystalline basement. Apatite FT central ages vary between 25 and 87 Ma. Confined mean track lengths range between 9.3 μm and 11.6 μm, suggesting substantial track annealing within all apatite samples. Modelled time-temperature paths offer a clear picture about the low-temperature thermal history of the crystalline basement of northern Switzerland: Cretaceous cooling is followed by an Eocene heating event and subsequent cooling to present-day temperatures. The Eocene heating episode is contemporaneous with the initial rifting stage of the nearby Upper Rhine Graben and the associated increasing volcanic activity. Crustal-scale faults of the Permo-Carboniferous Trough of northern Switzerland could have acted as major pathways for circulating hydrothermal fluids giving rise to the observed Middle to Late Eocene thermal event.

Post-Variscan evolution of the lithosphere in the area of the European Cenozoic Rift System

Abstract The European Cenozoic Rift System (ECRIS) transects Variscan basement, Permo-Carboniferous troughs and Late Permian to Mesozoic series, deposited in thermal sag basins, which are exposed on rift-related arches. We have analysed processes governing the transformation of the orogenically destabilized Variscan lithosphere into end-Cretaceous stabilized cratonic lithosphere, prior to its renewed Cenozoic rift-related destabilization. In the ECRIS area, crustal and lithospheric thicknesses at present are in the range of 24-35 km and 60-120 km, respectively. The Variscan orogen was characterized at the time of its end-Westphalian consolidation by 45-60 km deep crustal roots, marking major sutures. During the Stephanian-Early Permian wrench-induced collapse of the Variscan orogen, subducted lithospheric slabs were detached, causing upwelling of the asthenosphere, thermal thinning and/or partial delamina-tion of the lithospheric mantle, and regional uplift. By mid-Permian times, the crust was thinned to 28-35 km owing to its regional erosional unroofing, localized mechanical stretching and the interaction of mantle-derived melts with its basal parts. By mid-Permian times, when the temperature of the asthenosphere returned to ambient levels, thermal subsidence of the lithosphere commenced, controlling development of a system of Late Permian and Mesozoic intracratonic basins. These experienced repeated minor subsidence accelerations, related to the build-up of far-field stresses, which did not involve renewed lithospheric destabilization. Modelling of observed subsidence curves indicates that during the mid-Permian lithospheric thicknesses ranged in the ECRIS area between 40 and 80 km, but had increased by the end of the Cretaceous to 100-120 km. Cenozoic rifting and mantle-plume activity caused renewed lithospheric thinning.

Evolution of the lithosphere in the area of the Rhine Rift System

The Rhine Rift System (RRS) forms part of the European Cenozoic Rift System (ECRIS) and transects the Variscan Orogen, Permo-Carboniferous troughs and Late Permian to Mesozoic thermal sag basins. Crustal and lithospheric thicknesses range in the RRS area between 24–36 km and 50–120 km, respectively. We discuss processes controlling the transformation of the orogenically destabilised Variscan lithosphere into an end-Mesozoic stabilised cratonic lithosphere, as well as its renewed destabilisation during the Cenozoic development of ECRIS. By end-Westphalian times, the major sutures of the Variscan Orogen were associated with 45–60 km deep crustal roots. During the Stephanian-Early Permian, regional exhumation of the Variscides was controlled by their wrench deformation, detachment of subducted lithospheric slabs, asthenospheric upwelling and thermal thinning of the mantle-lithosphere. By late Early Permian times, when asthenospheric temperatures returned to ambient levels, lithospheric thicknesses ranged between 40 km and 80 km, whilst the thickness of the crust was reduced to 28–35 km in response to its regional erosional and local tectonic unroofing and the interaction of mantle-derived melts with its basal parts. Re-equilibration of the lithosphere-asthenosphere system governed the subsidence of Late Permian-Mesozoic thermal sag basins that covered much of the RRS area. By end-Cretaceous times, lithospheric thicknesses had increased to 100–120 km. Paleocene mantle plumes caused renewed thermal weakening of the lithosphere. Starting in the late Eocene, ECRIS evolved in the Pyrenean and Alpine foreland by passive rifting under a collision-related north-directed compressional stress field. Following end-Oligocene consolidation of the Pyrenees, west- and northwest-directed stresses originating in the Alps controlled further development of ECRIS. The RRS remained active until the Present, whilst the southern branch of ECRIS aborted in the early Miocene. Extensional strain across ECRIS amounts to some 7 km. Plume-related thermal thinning of the lithosphere underlies uplift of the Rhenish Massif and Massif Central. Lithospheric folding controlled uplift of the Vosges-Black Forest Arch.

Magnetic distortion of GDS transfer functions: An example from the Penninic Alps of Eastern Switzerland revealing a crustal conductor

We carried out sixty-four MT and GDS soundings in the eastern Swiss Alps to obtain information on the electrical conductivity distribution. One of the main findings is an anomalous directional behaviour of the real induction arrows over the entire period range (T = 1–300; s) on the Bündnerschiefer. The Bündnerschiefer occurs between the crystalline Aar and Gotthard massives to the North and the crystalline Penninic nappes (Adula, Tambo, Suretta) to the South within the investigation area. The sediments form an elongated eastwards plunging ramp with a possible conductive link to the Northern Swiss Permo-carboniferous trough (Molasse basin). We consider electric currents induced in various local and regional conductive structures and leaking into the Bündnerschiefer as possible causes for the observed effect upon the electromagnetic field. Applying Hypothetical Event Analysis (HEA) we find a spatial decoupling of the induction processes with depth and a conductivity anomaly presumably due to the northward indentation by the Adriatic plate.

Structural model for the Saxothuringian-Moldanubian suture in the Variscan basement of the Oberpfalz (Northeastern Bavaria, F.R.G.) interpreted from geophysical data

The Moldanubian-Saxothuringian boundary ∗ ∗ As defined by Kossmat. marks a prominent subduction zone in the internal parts of the European Variscides. In the Oberpfalz area deep reflection seismic profiles show a high density of various reflectors mainly in the upper crust indicating a complex polyphase deformation during the collision process. Seismic structures and related data from surface geology suggest that the Moldanubian-Saxothuringian collision consisted of two main phases. The first phase is marked by extensive NW-directed overthrusting of Moldanubian onto Saxothuringian crust along SE-dipping master décollements, some of which can be traced down to the Moho. During an advanced stage progressive overthrusting led to the development of wedge structures associated with SE-directed backthrusting and backfolding in the upper structural levels. Subsequently some of these wedges and their related décollements were folded to form the main NE-striking anticlines. The first phase of crustal imbrication correlates with a regional low-pressure-high-temperature metamorphism (at about 320 Ma) decreasing in grade northwestward. The second phase is characterized by a reorientation of the main direction of crustal shortening from NW-SE to SW-NE which is interpreted as the result of the N-directed indentation of the Vindelician terrane into the Moldanubian. This event is recorded by conjugate systems of overthrusting and subsequent strike-slip faulting in the Oberpfalz and the Bavarian Forest and the Black Forest-Vosges area. In the area of the southern German block between these basement outcrops the indentation front is marked by the arc-shaped axes of gravimetric and magnetic anomalies and Permo-Carboniferous troughs.

Salt-poor and salt-rich fluid inclusions in quartz from two boreholes in northern Switzerland

Microthermometric studies on fluid inclusions in quartz from two boreholes in northern Switzerland which cored the Böttstein granite at Böttstein and the crystalline basement with the overlying Permo-Carboniferous trough sediments at Weiach enable to elucidate the evolution and the P- T trapping conditions of the fluid compositions. Two fluid groups, one salt-poor and the other salt-rich, were identified within inclusions from boreholes. The salt-poor inclusions are older than the salt-rich inclusions, whose age is post-Stephanian. The salt-poor fluids consist mainly of NaCl and are trapped under relatively high temperatures (up to 350–400°C). The salt-rich inclusions reached temperatures of at least 100–130°C. Low temperatures observed during the first melting (− 70° to − 37°C) indicate that other chloride salts besides NaCl (i.e. CaCl 2, MgCl 2, etc.) must be present. These salt-rich fluids probably originated in the Permian and/or Triassic evaporites, and imply significant vertical fluid migration.