Towards the inversion of tectonic signals from deep-marine archives: Competing tectonic signal propagation from across the Alps into the marine sink
Final Report Abstract
The sedimentary fill of basins adjacent to mountains results from complex interactions between tectonics, climate, sediment transport, and transient storage. In turn, sediment archives are often used to reconstruct past tectonic activity and mountain building. However, our ability to quantitatively assess how tectonic processes impact erosion, sediment budgets, and the speed at which these signals propagate into marine records is limited. Our project quantified to determine how quickly tectonic disturbances in the hinterland become evident in sedimentary records, namely the sediment-routing systems of the European Alps the Upper Austrian foreland basin during the Oligocene/Miocene. Our previous work established deposition ages: Sediment transport through a submarine channel dominated from 26.9- 19.6 million years (Ma) (Puchkirchen formations). The Hall Formation (19.6–18.1 Ma) is characterized by a period of non-deposition (Base Hall Unconformity) around 19 Ma, followed fast basin fill northward propagating clinoforms. We analyzed the source of the sand-sized sediment using a single-grain apatite multi-proxy approach, studying 22 sandstone samples from the Puchkirchen and Hall formations. Apatite analysis, including trace-element geochemistry, fission track ages (AFT), U-Pb, and Sm-Nd isotopes, revealed a change in sediment source at 23.3 million years ago. This change involved increased contributions from a high-grade metamorphic source in the Upper Austroalpine region, indicated by higher eNd values. AFT lag times decreased after 24 Ma, suggesting an exhumation event starting at 29-27 Ma in the Eastern Alps. This was due to the initial exhumation of the Tauern Window, leading to gradual uplift (0.3-0.6 mm/year) of the Austroalpine units before significant acceleration at 23-21 Ma. In contrast, provenance analysis of claysized sediment, using Nd isotope analysis on 30 well-cutting samples, revealed stable source eNd values averaging around -9.7 (± 0.5) between 27 and 19 Ma. After 19 Ma, eNd values increased to approximately -9.2 (± 0.5). The total signal lag time, in response to the Tauern Window's exhumation from around 28 ± 1 Ma onward, was notably shorter (3.4 to 6 Ma) in sand-sized apatites compared to the clay-sized fraction (8.0 to 10.3 Ma). This difference stems from the methods used, where single-grain analysis can highlight localized source areas with high mineral fertility and erosion rates, while bulk-rock methods average such values. This divergence in recording provenance changes offers valuable insights into the extent of the perturbed source area. In summary, our study demonstrates how tectonic processes influence sediment flux and composition in a submarine foreland basin. The use of multiple methods to study the same parameter (flux or provenance) holds promise for future research, as it provides a more comprehensive understanding of environmental changes, given that different methods highlight distinct aspects of the same parameter.
Publications
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Oligocene/Miocene sediments from the Austrian Molasse did not reflect a large-scale surface response to the slab breakoff: results from detrital apatite fission track analysis. EGU Emile Argand Conference on Alpine Geological Studies (Sion)
Hülscher, J.; Sobel, E. & Bernhardt, A.
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Detrital apatite geochemistry and thermochronology from the Oligocene/Miocene Alpine foreland record the early exhumation of the Tauern Window. Basin Research, 33(6), 3021-3044.
Hülscher, Julian; Sobel, Edward R.; Verwater, Vincent; Groß, Philip; Chew, David & Bernhardt, Anne
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Selective recording of a tectonic forcing in different grain-size fractions in the Oligocene/Miocene Eastern Alpine Molasse Basin. Copernicus GmbH.
Hülscher, Julian; Sobel, Edward R.; Kallnik, Niklas; Hoffmann, J. Elis; Millar, Ian L. & Bernhardt, Anne
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Apatites Record Sedimentary Provenance Change 4–5 Myrs Before Clay in the Oligocene/Miocene Alpine Molasse. Frontiers in Earth Science, 10.
Hülscher, Julian; Sobel, Edward R.; Kallnik, Niklas; Hoffmann, J. Elis; Millar, Ian L.; Hartmann, Kai & Bernhardt, Anne
