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.

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.

Early Pleistocene complex cut-and-fill sequences in the Alps

<p>Deckenschotter (Cover Gravels in German) are Quaternary glacio-fluvial gravels, which unconformably overlie Tertiary Molasse or Mesozoic bedrock in the Northern Alpine Foreland. They comprise also the evidence of the Early Pleistocene glaciations. A significant phase of incision separated them into Höhere Deckenschotter (HDS: Higher Cover Gravels) and Tiefere Deckenschotter (TDS: Lower Cover Gravels) based on their topography. How the landscape evolved during Deckenschotter times is still not fully understood. The new cosmogenic nuclide chronology suggests that HDS deposited around 2 Ma and TDS around 1 Ma. In addition, 2 Ma old Deckenschotter are located at the same topographic elevation as the 1 Ma ones at Irchel (Canton of Zurich). This, indeed, points to cut-and-fill sequences and challenges the chronology based on the morphostratigraphy.</p><p>The aim of this study is to reconstruct the drainage patterns, base level changes, and thus the landscape evolution in the northern Alpine Foreland during the Early Pleistocene. Therefore, we focused on three Deckenschotter sites at Irchel and one in the area around Lake Constance. Sediments at these sites were analysed in detail to reveal their provenance, transport mechanism, depositional environment, and paleoflow regimes. Their chronology was established by isochron-burial dating. Our results indicate that the analysed sediments were transported from the Central and eastern Central Alps as well as from the Molasse to the foreland first by glaciers and then by rivers. They are deposited in a glacio-fluvial environment in the vicinity of a glacier. Based on the reconstructed chronology in this study and published cosmogenic nuclide ages, we propose that Deckenschotter are cut-and-fill sequences accumulated in three pulses between 2.5 Ma and 1 Ma. This cut-and-fill system implies that the regional base level was relatively constant during the Early Pleistocene. In addition, the depositional environment of Deckenschotter shows the presence of glaciers in the foreland. The 2.5 Ma old gravels, therefore, document the first advance of glaciers onto the Alpine Foreland. This seems to be synchronous with a first onset of glaciations on the northern hemisphere, which is assumed to occur at around 2.7 Ma.</p>

Delving deeper into cosmogenic 26Al-10Be isochron-burial dating of Swiss Deckenschotter deposits

<p>The oldest Quaternary deposits of the Swiss Northern Alpine Foreland are found on numerous hilltops, up to 300 m above the current valley bottoms. These Deckenschotter deposits consist mainly of glaciofluvial sediments intercalated with glacial sediments. Traditionally, the Deckenschotter are divided into two units: Höhere Deckenschotter (HDS – Higher Deckenschotter) and Tiefere Deckenschotter (TDS – Lower Deckenschotter). Elevation differences between the two suggest a phase of 100-150 m of incision (Graf, 2009).</p><p>Knowledge of their age of deposition is necessary for understanding the long-term landscape evolution as well as for assessing the long-term safety of the planned deep geological repository for nuclear waste in northern Switzerland (NTB 14-01, 2014). In this study, the method of isochron-burial dating was implemented to address the question of the age of the Deckenschotter. We aim to reconstruct the chronology of the alternating deposition and incision of the gravel units in the Northern Alpine Foreland. Our focus is placed on similar and complementary Deckenschotter sites located in the Northern Alpine Foreland in crucial locations in order to establish sound long-term landscape evolution scenarios. One of these is a former gravel pit, Feusi, situated in the southern slope of the hill chain called ‘Egg’ or ‘Schliniker Platten’, north of the village Oberweningen. The outcrop comprises several gravel units intercalated with glacigenic diamict layer in the upper part. Previous age estimates with the isochron-burial dating method indicate an age of 1.1 ± 0.2 Ma for the diamict layer (NAB 19-025, 2020). Knudsen et al. (2020) reported an age of 0.93 ± 0.13 Ma for the same layer based on a slightly different age calculation approach.</p><p>We sampled the lowermost accessible horizon, the Egg Schotter, of the Feusi outcrop at an altitude of ~580 m a.s.l. This horizon is located close to the base of the outcrop, just a few meters above the contact with the underlying Molasse and in a clear stratigraphic position, 20 m below the previously dated diamict. Study of the lowermost unit will allow us to temporally examine the earliest phases of Deckenschotter accumulation. Weathering horizons in the gravel layers overlying the Egg Schotter suggests periodic subaerial exposure. Therefore, the total time contained in the sediment package is difficult to estimate. Having two horizons dated at different depth in the same outcrop may provide insight into the timespan hidden between the deposition and weathering of different gravel layers. Indications of the timespan of HDS activity could be further gleaned by comparing to the age from the glacigenic sediment. In order to achieve this, eight clast samples of quartz-rich lithologies, of various shapes and sizes were collected in the Egg Schotter and processed for isochron-burial dating. The cosmogenic nuclides <sup>10</sup>Be and <sup>26</sup>Al were extracted and measured with the new MILEA accelerator at the accelerator mass spectrometry facility, ETH Zurich. The first results of this study will be presented.</p><p> </p><p>Graf, H.R. 2009: Quaternary Science Journal 58, 12–53</p><p>Nagra, NTB 14-01, 2014</p><p>Nagra, NAB 19-025, 2020</p><p>Knudsen, M.F. et al. 2020. Earth and Planetary Science Letters, 549, 116491</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.

Revealing exhumation of the central Alps during the Early Oligocene by detrital zircon U–Pb 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.

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

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.

New chronological constraints on the timing of Late Pleistocene glacier advances in northern Switzerland

Deposits of the Reuss Glacier in the central northern Alpine foreland of Switzerland are dated using luminescence methodology. Methodological considerations on partial bleaching and fading correction of different signals imply the robustness of the results. An age of ca. 25 ka for sediment directly overlying basal lodgement till corresponds well with existing age constraints for the last maximal position of glaciers of the northern Swiss Alpine Foreland. Luminescence ages imply an earlier advance of Reuss Glacier into the lowlands during Marine Isotope Stage 4. The presented data are compared to findings from other parts of the Alps regarding glacier dynamics and palaeoclimatological implications, such as the source of precipitation during the Late Pleistocene.