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Preprints
https://doi.org/10.5194/se-2020-44
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2020-44
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  29 Apr 2020

29 Apr 2020

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A revised version of this preprint was accepted for the journal SE and is expected to appear here in due course.

Tectonic Exhumation of the Central Alps Recorded by Detrital Zircon in the Molasse Basin, Switzerland

Owen A. Anfinson1,2, Daniel F. Stockli2, Joseph C. Miller3, Andreas Möller3, and Fritz Schlunegger4 Owen A. Anfinson et al.
  • 1Sonoma State University, Department of Geology, Rohnert Park, CA, 94928
  • 2University of Texas at Austin, Department of Geological Sciences, Jackson School of Geoscience, Austin, TX, 78712
  • 3University of Kansas, Department of Geology, Lawrence, KS
  • 4University of Bern, Institute of Geological Sciences, Bern, CH

Abstract. Eocene to Miocene sedimentary strata of the Northern Alpine Molasse Basin in Switzerland are well studied, yet they lack robust geochronologic and geochemical analysis of detrital zircon for provenance tracing purposes. Here, we present detrital zircon U-Pb ages coupled with rare earth and trace element geochemistry (petrochronology) to provide insights into the sedimentary provenance and to elucidate the tectonic activity of the central Alpine Orogen from the late Eocene to mid Miocene. Between 35–22.5 ± 1 Ma, the detrital zircon U-Pb age signatures were dominated by age groups of 300–370 Ma, 370–490 Ma, and 490–710 Ma, with minor Proterozoic age contributions. In contrast, from 21.5 ± 1 Ma to ~ 13.5 Ma (youngest preserved sediments), the detrital zircon U-Pb age signatures were dominated by a 252–300 Ma age group, with a secondary abundance of the 370–490 Ma age group, and only minor contributions of the 490–710 Ma age group. The Eo-Oligocene provenance signatures are consistent with interpretations that initial basin deposition primarily recorded exhumation and erosion of the Austroalpine orogenic cover and minor contributions from underlying Penninic units, containing reworked detritus from Variscan, Caledonian, and Cadomian orogenic cycles. The dominant 252–300 age group from the younger Miocene deposits is associated with the exhumation of Variscan-aged crystalline rocks of upper-Penninic basement units. Noticeable is the lack of Alpine-aged detrital zircon in all samples with the exception of one late Eocene sample, which reflects Alpine volcanism associated with incipient continent-continent collision. In addition, the rare earth and trace element data from the detrital zircon, coupled with zircon morphology and U/Th ratios, point to primarily igneous and rare metamorphic sources of zircon.

The observed change in detrital input from Austroalpine to Penninic provenance in the Molasse Basin at ~ 22 Ma appears to be correlated with the onset of synorogenic extension of the Central Alps. Synorogenic extension accommodated by slip along the Simplon fault zone promoted updoming and exhumation the Penninic crystalline core of the Alpine Orogen. The lack of Alpine detrital zircon U-Pb ages in all Oligo-Miocene strata also shows that the Molasse Basin drainage network was not accessing the prominent Alpine age intrusions and metamorphic complexes located in the southern portion of the Central Alps.

Owen A. Anfinson et al.

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Owen A. Anfinson et al.

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Short summary
Here we present new age data to provide insights into the source of sediment for the Molasse Sedimentary Basin in Switzerland. The paper aims to help shed light on the processes that built the Central Alpine Mountains between ~ 35 and ~ 15 million years ago. A primary conclusion drawn from the results is that at ~ 22 million years ago there was a significant change in the sediment sources within the basin. We feel this change in indicates major tectonic changes within the Central Alps.
Here we present new age data to provide insights into the source of sediment for the Molasse...
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