Tectonic control on the palaeogeographical evolution of the Miocene seaway along the Western Alpine foreland basin

The Miocene of the Western Alpine foreland basin were deposited in a North-South seaway along the active alpine orogenic front. In the subalpine massifs and the southern Jura mountains, the revised Miocene stratigraphy documents a detailed chronology of thrust propagation at the western alpine front, where tectonic activity had a primary influence on seaway palaeogeographical evolution. Here we propose nine palaeogeographical maps during the Miocene, the first of which depicts the initial Miocene transgression at ∼21.0 Ma. Between ∼18.05 Ma and ∼12.0 Ma, a westward retreat of the Miocene Sea occurred in response to the activation of the basal thrust of the Belledonne massif, which in turn triggered successive fault zones from east to west. At ∼10.0 Ma, a major uplift phase intervened and induced a rapid southward retreat of the Miocene Sea. The reconstructed palaeogeographical maps outline the main controls on the foreland basin seaway evolution: (i) the timing of the main thrusts, (ii) the inherited palaeotopography and (iii) eustatic sea-level changes during the Miocene. These reconstructions are integrated at the basin scale, highlighting the southward to westward-directed seaway migration in response to the Belledonne thrust activity that deeply shaped the palaeogeographical evolution during the early to middle Miocene.

Miocene high elevation in the Central Alps

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 >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.

Paleoenvironmental change recorded in submarine fans: the Eocene-Oligocene climate transition in the Alpine foreland basin

The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifying the transition from Paleocene greenhouse to Oligocene icehouse conditions. Preservation of the sedimentary signal of such an environmental change is most likely in net-depositional environments, such as submarine fans, which are the terminal parts of sedimentary systems. Here, using sedimentological and stable isotope data from the Alpine foreland basin, we assess whether this major climatic transition influenced the stratigraphic evolution of submarine fans. Results indicate that submarine fan retreat in the Alpine foreland basin corresponds with positive δ13C excursions related to major global perturbations of the carbon cycle and cooling in the earliest Oligocene. Submarine fan retreat is suggested to be influenced by this cooling through enhanced aridity and reduced subaerial runoff from the Corsica-Sardinia hinterland. The influence of aridity was periodically overwhelmed by local environmental factors, such as hinterland uplift, which increased sediment supply to deep-water during arid periods. These results highlight that: 1) hinterland climate may play a greater role than sea-level in dictating sediment supply to deep-water and, 2) submarine fan evolution occurs through a complex interplay between climate, eustasy and tectonics, which makes robust interpretations of paleoenvironmental change from their stratigraphic record, without multi-proxy records, difficult.

Miocene high elevation and high relief in the Central Alps

Reconstructing Oligocene-Miocene paleoelevation contributes to our understanding of the evolutionary history of the European Alps and sheds light on geodynamic and Earth’s 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, however, remain scarce. Here we present stable and clumped isotope geochemistry measurements to provide a new paleoelevation estimate for the mid-Miocene (~14.5 Ma) European Central Alps. We apply stable isotope δ-δ paleoaltimetry on 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 ± 0.2 ‰ and in conjunction with the high-Alpine phyllosilicate δD record suggest that the region surrounding the SFZ attained surface elevations of > 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.

A new middle Miocene lineage based on taxonomic revision of the large and rare cricetid-rodent genus Lartetomys

Large-sized cricetid rodents (Cricetodotontini) immigrated to Central Europe at the beginning of the Middle Miocene Climate Transition at ca. 15 My and are thus of great value for biostratigraphic and palaeoecologic purposes. An important fossil material belonging to a relatively small species of Mixocricetodon from the North Alpine Foreland Basin is described. It is a rare component in faunas of equivalent age, and the new material confirms the synonymy with the genus Lartetomys. The lineage L. mirabilis-L dehmi is documented in its chronostratigraphic framework, and the origin of the genus is discussed but remains uncertain.

Dating (81Kr) deep thermal groundwaters in the Upper Austrian part of the Alpine Foreland Basin

<p>The Alpine Foreland Basin (AFB) extends from Geneva to Vienna. It is an highly populated area and also hosts important industrial sites. The competition between different areas of utilization, including groundwater, mineral water, geothermal energy, oil and gas as well as underground gas storage increased within the last decades and is still increasing. Therefore, the understanding of the subsurface, the hydrostratigraphic units and their interactions is essential to develop cross-boundary concepts for sustainable management, economic development and security of supply.</p><p>The aims of a three-years project, funded by the ÖAW (Austrian Academy of Science), were to delineate and characterise hydrostratigraphic units, to recognise water composition and to determine possible flow pathways within the Upper Austrian part of the AFB. As main data sources, hydrochemistry, stable isotopes, dissolved noble gases of groundwater and isotopic groundwater ages were used to proof and improve hydrogeological concepts. This presentation focuses on the <sup>81</sup>Kr investigations which were carried out within the project and were partly funded by the government.</p><p>Determining the age of deep groundwaters which are free of <sup>14</sup>C has been almost impossible for a long time. Improved analytical methods make it now possible to use <sup>81</sup>Kr for age characterisation. Therefore, nine water samples from deep wells representing different hydrostratigraphic units in the Upper Austrian AFB, were used for <sup>81</sup>Kr investigations. These samples include water from Upper Jurassic geothermal reservoirs.</p><p>Results imply a differentiated picture of groundwater residence times. <sup>81</sup>Kr model ages of Malmian water samples are uniform which is in line with hydrochemical analyses and stable isotopes of these samples. However, model ages are exceptional high (390000 – 550000 years) which would suggest low flow velocities. This seems to contradict all existing hydrogeological model concepts of a dynamic thermal water flow in Malmian carbonates. The water sample taken from the Eocene (Gallspach) exhibits a very old groundwater portion (> 900000 years), whereas samples from Oligocene strata show the youngest but strongly varying <sup>81</sup>Kr model ages (<25000 – 240000 years). The water sample of Bad Schallerbach is interpreted as a complex mixed system between young and middle-aged deep groundwaters with elevated mineralisation. A contribution from a deeper aquifer is postulated for the water sample in Andorf (240000 years). </p><p>In summary, results of the hydrochemical and stable isotope investigations together with the krypton analyses have shown that connections between the Upper Jurassic thermal water aquifer and younger groundwater systems (Eocene to Oligocene) are obvious. This confirms the hydrogeological model concepts, which assume a discharge of thermal waters east of the Upper Jurassic carbonate rocks into younger strata. Possible factors which may influence the model ages (e.g. diffusion processes, contact with formation waters containing hydrocarbons) were critically discussed on the gained database. However, the discrepancy between the derived <sup>81</sup>Kr model ages of the Malmian thermal waters and the current hydrogeological models could not be resolved yet. Further investigations should focus on recharge areas and therefore on aligning age data and hydraulic models.</p>

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.