Northern Alpine Foreland Basin Research Articles

Lithospheric- and crustal-scale controls on variations in foreland basin development in the Northern Alpine Foreland Basin

The Northern Alpine Foreland Basin (i.e., Molasse Basin) developed due to flexural subsidence from slab- and topographic loading during continental collision between the Adriatic and European plates. Previous studies highlight a diachronous transition from underfilled- to overfilled conditions in the Molasse Basin in response to orogen-parallel variations in flexural subsidence and sediment supply. In this contribution, we investigate the lithospheric- and crustal-scale orogenic process(es) that generated this diachronous transition. For this, we conducted a tectonostratigraphic analysis of the Molasse Basin. Firstly, we constructed a 3D geological model to derive the architecture of the European margin during the Eocene onset of flexure. Second, 2D/3D reflection seismic- and borehole data were used to characterise the spatiotemporal development of depositional environments and syn-flexural normal fault kinematics in the German Molasse. Our data show a stepwise rather than continuous eastward migration of the underfilled- to overfilled transition. Furthermore, syn-flexural fault kinematics document a Rupelian to early Burdigalian northward younging trend and higher cumulative Cenozoic offsets in the Eastern German Molasse (< 230 m) compared to the Western German Molasse (< 150 m). This implies a west-to-east increase in the curvature of bending of the European plate, induced by along-strike variations in the magnitude of applied loads and/or European lithospheric strength variations. These variations drove lateral variations in accommodation space and sediment supply. Subsequently, this led to the diachronous underfill-to-overfilled transition in the Molasse Basin. Taken together, we suggest that the diachronous transition was driven by spatiotemporal variations in the thickening of the orogenic wedge supported by slab detachment, promoted by subduction- and collision of the irregular European margin.

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.

The largest record of the minute beaver Euroxenomys minutus (Mammalia, Castoridae) from the early Late Miocene hominid locality Hammerschmiede (Bavaria, Southern Germany) and palaeoecological considerations

ABSTRACT In this study, we describe the dental remains of a very minute beaver from the early Late Miocene locality Hammerschmiede (MN 7/8) located at the Northern Alpine Foreland Basin (Southern Germany, Bavaria). The finds represent the so far most comprehensive (>1.000 dental specimens) collection of the trogontheriine castorid Euroxenomys minutus. Metrically and morphologically, the large data set demonstrates an extensive intraspecific variability of this beaver from Hammerschmiede that covers most finds of other European localities and confirms previous species assignments. The subspecies Euroxenomys minutus rhenanus from Dorn-Dürkheim (Germany), clearly isolates from this range of variation and can thus be validated. The performed age-frequency distributions for Euroxenomys minutus from the local stratigraphic levels HAM 5 (rivulet) and HAM 4 (river) correspond roughly and also resemble Euroxenomys from the swamp deposit Rudabánya (Hungary). Since previous mortality studies on the also occurring larger beaver Steneofiber depereti from Hammerschmiede demonstrated dominance of behavioural or internal factors, this cross-ecosystem similarity in mortality for the small beaver species is likely explained by external factors, such as predation. Supported by a taphonomic bone analysis, a multitude of predators are finally considered to prey on Euroxenomys minutus and indicate a predatory bone assemblage.

The origin of modern marlins (Teleostei: Istiophoridae): new fossil evidence from the Lower Miocene of Austria

ABSTRACT We report the oldest fossil record of Istiophoridae from the Northern Alpine Foreland Basin (Western Paratethys) in Pucking, Austria (Lower Miocene, lower Aquitanian, ∼22.4 Ma). The studied specimen consists of fragments of five articulated vertebrae and a partially preserved second dorsal fin. We found that the Pucking specimen is only surpassed in age by an isolated vertebra from the Chandler Bridge Formation (South Carolina, U.S.A.), with ∼24.7–23.5 Ma. Pucking and the Chandler Bridge Formation differ in age by ∼1.1–2.6 Ma. The fossil istiophorid from the Chandler Bridge Formation was considered controversial due to the large gap in the fossil record between the first appearance of Istiophoridae and the last appearance of any other extinct billfish clade. Here we report that Pucking (Lower Miocene) and South Carolina (Oligocene) share the occurrence of Istiophoridae, †Xiphiorhynchinae (Xiphiidae), and †Aglyptorhynchinae (†Palaeorhynchidae). Our finding demonstrates that the coexistence of different groups of billfishes is maintained over time and that the Chandler Bridge specimen is a reliable record. Our work closes a temporal gap of ∼4.4 Ma between the previously recognized oldest Istiophoridae (∼18 Ma) and the last appearance of †Xiphiorhynchinae and †Aglyptorhynchinae. This result supports a longer evolutionary history for Istiophoridae, establishes continuity in the billfish fossil record, and is significant for future time calibrated phylogenies.

Influence of rheologically weak layers on fault architecture: insights from analogue models in the context of the Northern Alpine Foreland Basin

We present a series of analogue models inspired by the geology of the Zürcher Weinland region in the Northern Alpine Foreland Basin of Switzerland to explore the influence of rheological weak, i.e. (partially) ductile layers on the 3D evolution of tectonic deformation. Our model series test the impact of varying weak layer thickness and rheology, as well as different kinematics of an underlying “basal fault”. Model analysis focuses on deformation in the weak layer overburden and, uniquely, within the weak layer itself. We find that for low to moderate basal fault displacements, the above-mentioned parameters strongly influence the degree of coupling between the basal fault and the weak layer overburden. Coupling between the basal fault and overburden decreases by reducing the strength of the weak layer, or by increasing the weak layer’s thickness. As a result, basal fault displacement is less readily transferred through the weak layer, leading to a different structural style in the overburden. By contrast, increasing the amount, or rate, of basal fault slip enhances coupling and leads to a more similar structural style between basal fault and overburden. Moreover, dip-slip displacement on the basal fault is more readily transferred to the overburden than strike-slip displacement of the same magnitude. Our model results compare fairly well to natural examples in the Northern Alpine Foreland Basin, explaining various structural features. These comparisons suggest that rheological weak layers such as the Jurassic Opalinus Clay have exerted a stronger control on fault zone architecture than is commonly inferred, potentially resulting in vertical fault segmentation and variations in structural style. Furthermore, the novel addition of internal marker intervals to the weak layer in our models reveals how complex viscous flow within these layers can accommodate basal fault slip. Our model results demonstrate the complex links between fault kinematics, mechanics and 3D geometries, and can be used for interpreting structures in the Alpine Foreland, as well as in other settings with similar weak layers and basal faults driving deformation in the system.

Apatites Record Sedimentary Provenance Change 4–5 Myrs Before Clay in the Oligocene/Miocene Alpine Molasse

Extracting information about past tectonic or climatic environmental changes from sedimentary records is a key objective of provenance research. Interpreting the imprint of such changes remains challenging as signals might be altered in the sediment-routing system. We investigate the sedimentary provenance of the Oligocene/Miocene Upper Austrian Northern Alpine Foreland Basin and its response to the tectonically driven exhumation of the Tauern Window metamorphic dome (28 ± 1 Ma) in the Eastern European Alps by using the unprecedented combination of Nd isotopic composition of bulk-rock clay-sized samples and partly previously published multi-proxy (Nd isotopic composition, trace-element geochemistry, U-Pb dating) sand-sized apatite single-grain analysis. The basin offers an excellent opportunity to investigate environmental signal propagation into the sedimentary record because comprehensive stratigraphic and seismic datasets can be combined with present research results. The bulk-rock clay-sized fraction εNd values of well-cutting samples from one well on the northern basin slope remained stable at ∼−9.7 from 27 to 19 Ma but increased after 19 Ma to ∼−9.1. In contrast, apatite single-grain distributions, which were extracted from 22 drill-core samples, changed significantly around 23.3 Ma from apatites dominantly from low-grade (&amp;lt;upper amphibolite-facies) metamorphic sources with Permo-Mesozoic and late Variscan U-Pb ages and εNd values of −4.4 to dominantly high-grade metamorphic apatites with late Variscan U-Pb ages and εNd values of −2.2. The change in apatite single-grain distributions at 23.3 Ma is interpreted to result from the exposure of a new Upper Austroalpine source nappe with less negative εNd values triggered by the ongoing Tauern Window exhumation. Combining these data with the clay-sized bulk-rock εNd values reveals that the provenance changed 4–5 Myrs later at 19 Ma in the clay-sized fraction. Reasons for the delayed provenance-change recording are rooted in the characteristics of the applied methods. Whereas single-grain distributions of orogen-wide sediment-routing systems can be dominated by geographically small areas with high erosion and mineral fertility rates, bulk-rock methods integrate over the entire drainage basin, thus diminishing extreme values. Hence, by combining these two methods, spatial information are uncovered, enabling a previously unattained understanding of the underlying environmental change.

The rock mass as the governing factor for successfully developing deep geothermal systems in Southern Germany

AbstractIn Southern Germany, geothermal wells target Upper Jurassic carbonates, formerly referred to as Malm, below the Northern Alpine Foreland Basin (NAFB). In the northern, shallower carbonates (3000 m depth), e.g., in the Munich area, hydrothermal systems prevail, and wells yield proper flow rates. Deeper carbonates (&lt; 4000 m) towards the south indicate a petrothermal system due to a decline in porosity and permeability. Inexplicably, fractures only contribute poorly to larger flow rates. This sets the goal for the project Dolomitkluft to analyse the rock mass for options countering low flow rates in two geothermal wells at Geretsried, Bavaria. Therefore, 20 m drill cores from 4600 to 4715 m total vertical depth (TVD) depict the initial structure of the rock mass: the rock parameters, the discontinuity network, and the in situ stresses. Alternating homogeneous, micritic, strong, to inhomogeneous, porous, weak limestones and massive, crystalline dolostones define the stratigraphy. Five joint sets from the cores can be distinguished into the bedding, two paleo anti‐ and synthetic normal faults and two recent strike‐slip faults. The latter, in combination with core disking, allows exclusively a strike‐slip (SS) stress regime and delimits the ratio to at least SH : Sv : Sh = 1.7 : 1.0 : 0.7. According to the results, including dilatation‐ and slip‐tendencies, drill trajectories of prosperous geothermal wells should head ESE/WNW, perpendicular to SS‐faults, and avoid the presence of normal faults.

Miocene high elevation in the Central Alps

Abstract. Reconstructing Oligocene–Miocene paleoelevation contributes to our understanding of the evolutionary history of the European Alps and sheds light on geodynamic and Earth surface processes involved in the development of Alpine topography. Despite being one of the most intensively explored mountain ranges worldwide, constraints on the elevation history of the European Alps remain scarce. Here we present stable and clumped isotope measurements to provide a new paleoelevation estimate for the mid-Miocene (∼14.5 Ma) European Central Alps. We apply stable isotope δ–δ paleoaltimetry to near-sea-level pedogenic carbonate oxygen isotope (δ18O) records from the Northern Alpine Foreland Basin (Swiss Molasse Basin) and high-Alpine phyllosilicate hydrogen isotope (δD) records from the Simplon Fault Zone (Swiss Alps). We further explore Miocene paleoclimate and paleoenvironmental conditions in the Swiss Molasse Basin through carbonate stable (δ18O, δ13C) and clumped (Δ47) isotope data from three foreland basin sections in different alluvial megafan settings (proximal, mid-fan, and distal). Combined pedogenic carbonate δ18O values and Δ47 temperatures (30±5 ∘C) yield a near-sea-level precipitation δ18Ow value of -5.8±1.2 ‰ and, in conjunction with the high-Alpine phyllosilicate δD value of -14.6±0.3 ‰, suggest that the region surrounding the Simplon Fault Zone attained surface elevations of &gt;4000 m no later than the mid-Miocene. Our near-sea-level δ18Ow estimate is supported by paleoclimate (iGCM ECHAM5-wiso) modeled δ18O values, which vary between −4.2 ‰ and −7.6 ‰ for the Northern Alpine Foreland Basin.

Detrital apatite geochemistry and thermochronology from the Oligocene/Miocene Alpine foreland record the early exhumation of the Tauern Window

AbstractThe early exhumation history of the Tauern Window in the European Eastern Alps and its surface expression is poorly dated and quantified, partly because thermochronological and provenance information are sparse from the Upper Austrian Northern Alpine Foreland Basin. For the first time, we combine a single‐grain double‐dating approach (Apatite Fission Track and U‐Pb dating) with trace‐element geochemistry analysis on the same apatites to reconstruct the provenance and exhumation history of the late Oligocene/early Miocene Eastern Alps. The results from 22 samples from the Chattian to Burdigalian sedimentary infill of the Upper Austrian Northern Alpine Foreland Basin were integrated with a 3D seismic‐reflection data set and published stratigraphic reports. Our highly discriminative data set indicates an increasing proportion of apatites (from 6% to 23%) with Sr/Y values &lt;0.1 up‐section and an increasing amount of apatites (from 24% to 38%) containing &gt;1,000 ppm light rare‐earth elements from Chattian to Burdigalian time. The number of U‐Pb ages with acceptable uncertainties increases from 40% to 59% up‐section, with mostly late Variscan/Permian ages, while an increasing number of grains (10%–27%) have Eocene or younger apatite fission track cooling ages. The changes in the apatite trace‐element geochemistry and U‐Pb data mirror increased sediment input from an ≥upper amphibolite‐facies metamorphic source of late Variscan/Permian age – probably the Ötztal‐Bundschuh nappe system – accompanied by increasing exhumation rates indicated by decreasing apatite fission track lag times. We attribute these changes to the surface response to upright folding and doming in the Penninic units of the future Tauern Window starting at 29–27 Ma. This early period of exhumation (0.3–0.6 mm/a) is triggered by early Adriatic indentation along the Giudicarie Fault System.

Quantifying Multiple Erosion Events in the Distal Sector of the Northern Alpine Foreland Basin (North-Eastern Switzerland), by Combining Basin Thermal Modelling with Vitrinite Reflectance and Apatite Fission Track Data

This work quantifies the amount of erosion associated with the Cretaceous and Miocene erosional unconformities recognised in the distal part of the Northern Alpine Foreland Basin (NAFB), north-eastern Switzerland. To achieve this goal, the basin thermal modelling approach is applied, calibrated by two different sets of data collected in previous studies: vitrinite reflectance (%Ro) and the temperature estimated from apatite fission tracks (AFT) data modelling. The novelty of this approach is the possibility to constrain the timing and magnitude of multiple erosion events by integrating thermal modelling with thermochronologic data. Combining these two methods allows the erosional events to be separated which would not be possible using only irreversible paleothermometers, such as vitrinite reflectance data. Two scenarios were tested, based on the data of two published thermochronology studies. For the Cretaceous unconformity, similar results are obtained for the two scenarios, both indicating that the deposition and the subsequent complete erosion of Lower Cretaceous deposits, in the order of 500–1300 m, depending on the area, are necessary, in order to attain the temperatures estimated by the thermal history modelling of AFT data. Thus, a depositional hiatus for this period is not likely. For the Miocene-Quaternary unconformity, the magnitude of erosion calculated for the two scenarios differs by 300–1400 m, depending on the AFT data considered. The two scenarios lead to a different evaluation of the subsidence and uplift rate of the study area, thus to a different interpretation of the tectono-stratigraphic evolution of this distal sector of the NAFB.

Volcanism and Volcanogenic Submarine Sedimentation in the Paleogene Foreland Basins of the Alps: Reassessing the Source-to-Sink Systems with an Actualist View

This work critically reviews the Eocene–Oligocene source-to-sink systems accumulating volcanogenic sequences in the basins around the Alps. Through the years, these volcanogenic sequences have been correlated to the plutonic bodies along the Periadriatic Fault System, the main tectonic lineament running from West to East within the axis of the belt. Starting from the large amounts of data present in literature, for the first time we present an integrated 4D model on the evolution of the sediment pathways that once connected the magmatic sources to the basins. The magmatic systems started to develop during the Eocene in the Alps, supplying detritus to the Adriatic Foredeep. The progradation of volcanogenic sequences in the Northern Alpine Foreland Basin is subsequent and probably was favoured by the migration of the magmatic systems to the North and to the West. At around 30 Ma, the Northern Apennine Foredeep also was fed by large volcanogenic inputs, but the palinspastic reconstruction of the Adriatic Foredeep, together with stratigraphic and petrographic data, allows us to safely exclude the Alps as volcanogenic sources. Beyond the regional case, this review underlines the importance of a solid stratigraphic approach in the reconstruction of the source-to-sink system evolution of any basin.

Revealing exhumation of the central Alps during the Early Oligocene by detrital zircon U\u2013Pb age and fission-track double dating in the Taveyannaz Formation

The late Eocene-to-early Oligocene Taveyannaz Formation is a turbidite series deposited in the Northern Alpine Foreland Basin (close to the Alpine orogenic front). Double dating of zircons with the fission-track and the U–Pb methods is applied on samples from the Taveyannaz Formation to reconstruct the exhumation history of the Central-Western Alps and to understand the syn-collisional magmatism along the Periadriatic lineament. Three samples from this unit show similar detrital zircon fission-track age populations that center at: 33–40 Ma (20%); 69–92 Ma (30–40%); and 138–239 Ma (40–50%). The youngest population contains both syn-volcanic and basement grains. Combined with zircon U–Pb data, it suggests that the basement rocks of Apulian-affinity nappes (Margna Sesia, Austroalpine) were the major sources of detritus, together with the Ivrea Zone and recycled Prealpine flysch, that contributed debris to the Northern Alpine Foreland Basin. Furthermore, the rocks of the Sesia–Lanzo Zone or of equivalent units exposed at that time presumably provided the youngest basement zircon fission-track ages to the basin. The Biella volcanic suite was the source of volcanogenic zircons. Oligocene sediment pathways from source to sink crossed further crystalline basement units and sedimentary covers before entering the basin from the southeast. The lag times of the youngest basement age populations (volcanic zircons excluded) are about 11 Myr. This constrains average moderate-to-high exhumation rate of 0.5–0.6 km/Myr in the pro-side of the orogenic wedge of the Central Alps during the late Eocene to early Oligocene.

Selective Recording of Tectonic Forcings in an Oligocene/Miocene Submarine Channel System: Insights From New Age Constraints and Sediment Volumes From the Austrian Northern Alpine Foreland Basin

Detailed characterization of variations in sediment architecture, flux, and transport processes in peri-orogenic basins offers insights into external climatic or tectonic forcings. We tested how four well-known tectonic/erosional events in the Oligocene/Miocene Alpine source area are recorded in the sediment-accumulation rates (SARs) of the deep marine sink in the Northern Alpine Foreland Basin (NAFB): exhumation of the Lepontine Dome (starting at 30 Ma) and the Tauern Window (23-21 Ma), erosion of the Augenstein Formation (~21 Ma), and the visco-elastic relaxation of the European Plate. The Upper Austrian NAFB offers a unique opportunity to investigate external forcings on sedimentary infill due to the large amount of data on the Alpine source and Foreland. Deep-marine sedimentation, forming the Puchkirchen Group and the basal Hall Formation, was controlled by a basin-axial submarine channel (3-5 km wide, >100 km length). Two basin-wide unconformities were recognized in seismic-reflection data: the Northern Slope Unconformity (NSU) and the Base Hall Unconformity (BHU). We combine biostratigraphic and chemostratigraphic analyses of 316 drill-cutting samples from three wells with a large 3D-seismic-reflection data set (3300 km2, >5 km depth) to determine age and duration of the unconformities and to calculate spatially averaged SARs for the submarine channel and its overbanks, separately. Deepening of the basin, recorded by the NSU, occurred between 28.1 and 26.9 Ma. The Puchkirchen Group (26.9–19.6 Ma) is characterized by constant SARs (within standard deviation) in the channel (432–623 (t/m2/Ma)) and on the overbanks (240–340 (t/m2/Ma)). The visco-elastic relaxation of the European Plate results in low SARs on the overbanks (186 (t/m2/Ma)), a decrease in sediment grain size in channel deposits and a decrease in sea level at the BHU (19.6–19.0 Ma). In the upper Hall Formation (19.0–18.1 Ma), clinoforms prograding from the south filled up the basin (1497 (t/m2/Ma)) within 1 Myrs. We conclude that only two of the tectonic signals are recorded in this part of the deep-marine sink, erosion of Augenstein Formation and visco-elastic relaxation of the European Plate; the exhumation of the Tauern Window and Lepontine Dome remain unrecorded.

Miocene basement exhumation in the Central Alps recorded by detrital garnet geochemistry in foreland basin deposits

Abstract. The Neogene evolution of the European Alps was characterized by the exhumation of crystalline basement, the so-called external crystalline massifs. Their exhumation presumably controlled the evolution of relief, distribution of drainage networks, and generation of sediment in the Central Alps. However, due to the absence of suitable proxies, the timing of their surficial exposure and thus the initiation of sediment supply from these areas are poorly constrained. The northern Alpine foreland basin preserves the Oligocene to Miocene sedimentary record of tectonic and climatic adjustments in the hinterland. This contribution analyses the provenance of 25 to 14 Myr old alluvial fan deposits by means of detrital garnet chemistry. Unusually grossular- and spessartine-rich garnet is found (1) to be a unique proxy for identifying detritus from the external crystalline massifs and (2) to occur abundantly in ca. 14 Myr old deposits of the foreland basin. In contrast to previous assumptions, we therefore propose that the external massifs were already exposed to the surface ca. 14 Myr ago.

Provenance and palaeogeographic evolution of Lower Miocene sediments in the eastern North Alpine Foreland Basin

Detrital heavy mineral analysis and sediment chemistry of Lower Miocene sediments of the Lower Austrian Northern Alpine Foreland Basin generally indicate provenance from metapelitic source rocks. The sediments of the Traisen Formation (Ottnangian) were primarily derived from Alpine sources such as the Austroalpine crystalline units of the Eastern Alps and reworked and eroded siliciclastics strata on top of the Northern Calcareous Alps and, in addition, low-grade metamorphic rocks of the Bohemian Massif. Ottnangian and Karpatian sediments in the subground of the northern Lower Austrian Molasse basin were mainly fed from the same southern sources, from low-grade metamorphic rocks of the Bohemian Massif and via resedimentation of Lower Miocene and Palaeogene of the Waschberg–Ždanice Unit and the Vienna Basin. A large-scale input of material from the Rhenodanubian Flysch Zone and of higher-grade metamorphic units and granites of the Bohemian Massif in the Lower Miocene Molasse basin can be ruled out. A compilation of well sections and heavy mineral data point to the existence of a partly emerged, N–S oriented ridge during the Egerian.

Architecture and Kinematics of the Constance‐Frick Trough (Northern Switzerland): Implications for the Formation of Post‐Variscan Basins in the Foreland of the Alps and Scenarios of Their Neogene Reactivation

AbstractPost‐Variscan basins are commonly inferred to represent important precursor structures of later extensional and contractional tectonic events in the foreland of the European Alps. Nevertheless, their structural characteristics are typically poorly constrained. Our investigation sheds new light on the architecture and kinematics of the Constance‐Frick Trough (CFT) in Northern Switzerland. According to our analysis of an extensive and revised 2‐D reflection seismic data set, the ENE‐WSW striking CFT is divided into an eastern segment and a western segment characterized by opposing half‐graben geometries. The transition between the two segments coincides with the prolongation of a preexisting WNW‐ESE trending strike‐slip fault zone exposed in the adjacent Black Forest Massif to the north. Field‐based analysis of outcrop‐scale faults in the latter area implies that the minimum horizontal stress during the formation of the CFT was roughly oriented NE‐SW, which is oblique to the trough axis. While previously published analogue models imply that some characteristics of the CFT could be explained by lateral linkage of graben segments under oblique extension, the pronounced half‐graben geometry of the CFT suggests that low‐angle normal faulting may also have contributed to trough formation. The observed structural complexities of the CFT and uncertainties regarding its deep structure must be taken into account for interpretations of similar Late Paleozoic basins inferred in the subsurface of the northern Alpine foreland basin, in particular, concerning scenarios of their Neogene reactivation during Alpine orogeny.

Evaluating igneous sources of the Taveyannaz formation in the Central Alps by detrital zircon U–Pb age dating and geochemistry

Late Palaeogene syn-tectonic volcanic products have been found in the Northern Alpine foreland basin and in the South Alpine hemipelagic basin. The source of abundant volcanic fragments is still in debate. We analyzed the geochronology and geochemistry of detrital zircons, and evaluated their temporal and genetic relationships with potential volcanic sources. The study shows that the detrital zircon U–Pb age patterns have two major age groups: a dominance (ca. 90%) of pre-Alpine zircons was found, as commonly observed in other Alpine flysch formations. These zircons apparently derived from erosion of the early Alpine nappe stack in South Alpine and Austroalpine units. Furthermore, a few Neo-Alpine zircons (ca. 10%) have ages ranging from Late Eocene to Early Oligocene (~ 41–29 Ma). Both source materials were mixed during long riverine transport to the basin margins before being re-deposited by gravity flows. These Palaeogene ages match with the activity of Peri-Adriatic magmatism, including the Biella volcanic suite as well as the Northern Adamello and Bergell intrusions. The values of REE and 176Hf/177Hf(t) ratios of the Alpine detrital zircons are in line with the magmatic signatures. We observe an in time and space variable supply of syn-sedimentary zircons. From late Middle Eocene to Late Eocene, basin influx into the South Alpine and Glarus (A) basins from the Northern Adamello source is documented. At about 34 Ma, a complete reorganisation is recorded by (1) input of Bergell sources into the later Glarus (B) basin, and (2) the coeval volcaniclastic supply of the Haute-Savoie basin from the Biella magmatic system. The Adamello source vanished in the foreland basin. The marked modification of the basin sources at ~ 34 Ma is interpreted to be initiated by a northwestern shift of the early Alpine drainage divide into the position of the modern Insubric Line.

The last erosional stage of the Molasse Basin and the Alps

We present a synoptic overview of the Miocene-present development of the northern Alpine foreland basin (Molasse Basin), with special attention to the pattern of surface erosion and sediment discharge in the Alps. Erosion of the Molasse Basin started at the same time that the rivers originating in the Central Alps were deflected toward the Bresse Graben, which formed part of the European Cenozoic rift system. This change in the drainage direction decreased the distance to the marine base level by approximately 1,000 km, which in turn decreased the average topographic elevation in the Molasse Basin by at least 200 m. Isostatic adjustment to erosional unloading required ca. 1,000 m of erosion to account for this inferred topographic lowering. A further inference is that the resulting increase in the sediment discharge at the Miocene–Pliocene boundary reflects the recycling of Molasse units. We consider that erosion of the Molasse Basin occurred in response to a shift in the drainage direction rather than because of a change in paleoclimate. Climate left an imprint on the Alpine landscape, but presumably not before the beginning of glaciation at the Pliocene–Pleistocene boundary. Similar to the northern Alpine foreland, we do not see a strong climatic fingerprint on the pattern or rates of exhumation of the External Massifs. In particular, the initiation and acceleration of imbrication and antiformal stacking of the foreland crust can be considered solely as a response to the convergence of Adria and Europe, irrespective of erosion rates. However, the recycling of the Molasse deposits since 5 Ma and the associated reduction of the loads in the foreland could have activated basement thrusts beneath the Molasse Basin in order to restore a critical wedge. In conclusion, we see the need for a more careful consideration of both tectonic and climatic forcing on the development of the Alps and the adjacent Molasse Basin.

Systematics and palaeoecology of Ruminantia (Artiodactyla, Mammalia) from the Miocene of Sandelzhausen (southern Germany, Northern Alpine Foreland Basin)

In more than 20 years of excavation at the fossil lagerstatte Sandelzhausen (Early/Middle Miocene boundary, Burdigalian/Langhian boundary, early middle MN5) a substantial amount of fossil remains of ruminants have been recovered. Currently, it is the largest recorded assemblage of ruminants from the Miocene Northern Alpine Foreland Basin. More than 1,000 teeth, almost 70 antler remains and one skull enable the identification of five ruminants, namely the tragulid Dorcatherium crassum, the palaeomerycid Germanomeryx n. g. fahlbuschi n. sp., and the cervids Lagomeryx parvulus, Lagomeryx pumilio, and Heteroprox eggeri n. sp. Lagomeryx parvulus and L. pumilio have the most extensive record yet known for these species, opening up a much more complete view of them 120 years after the discovery of the type materials. The newly established G. n. g. fahlbuschi n. sp. and H. eggeri n. sp. enlarge our knowledge on the taxonomic composition of Miocene European Ruminantia. Because of the exceptionally large number of specimens, nearly all tooth positions of all five species are documented, thereby completing hitherto partially known character sets. The investigation comprises extensive taxonomic descriptions of all species represented and an interpretation of the palaeoecology based on an analysis of the community structure. This clearly suggests a humid closed canopy forest interspersed with temporary and perennial waters and accompanying open areas. Moreover, a comparison with other, stratigraphically close Molasse Basin communities emphasizes the various peculiarities of the Sandelzhausen community (low species number, cervid-dominance, dominance of L. pumilio over L. parvulus, non-dominance of very small-sized ruminants, comparably high portion of palaeomerycids, all species being browsers, no Eotragus and no Amphimoschus). The investigation also clarifies the similarity with the communities from Undorf and Viehhausen (Germany, MN5). The deduced dynamics in community structure of the late Early and early Middle Miocene Northern Alpine Foreland Basin provides further support for the current hypothesis of a vast wetland environment under the strong influence of alternating dry and flood seasons.

Feeding preferences of Gomphotherium subtapiroideum (Proboscidea, Mammalia) from the Miocene of Sandelzhausen (Northern Alpine Foreland Basin, southern Germany) through life and geological time: evidence from dental microwear analysis

The objective of this study is to estimate changes in feeding preferences of the proboscidean species Gomphotherium subtapiroideum (Schlesinger 1917) by means of dental microwear analyses. The dietary changes are first evaluated through the ontogeny of this species, between juveniles and adults, and are then studied through geological time, from early Middle Miocene (MN5) to middle Late Miocene (MN8–9) localities of the German Molasse Basin. The microwear patterns of juvenile and adult individuals of G. subtapiroideum from Sandelzhausen (MN5) differ merely by the variable “length of scratches”, emphasizing longer jaw movements during mastication in adults. The microwear signatures of G. subtapiroideum do not vary significantly between the two geological time periods studied, but reflect mixed feeding preferences in both cases. These results imply that, despite an important environmental change at that time (drying and opening), the ecology of G. subtapiroideum and, especially, its feeding habits were not affected. Its dental microwear pattern is then compared with those of other species of Proboscidea from the Middle-Late Miocene of Germany, namely Deinotherium giganteum and Gomphotherium steinheimense.