Tectonic exhumation of the Central Alps recorded by detrital zircon in the Molasse Basin, Switzerland

Abstract

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 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 and 22.5 ± 1 Ma, the detrital zircon U–Pb age signatures are dominated by age groups of 300–370, 380–490, and 500–710 Ma, with minor Proterozoic age contributions. In contrast, from 21 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 380–490 Ma age group and only minor contributions of the 500–710 Ma age group. The Eo-Oligocene provenance signatures are consistent with interpretations that initial basin deposition primarily recorded unroofing of the Austroalpine orogenic lid and lesser contributions from underlying Penninic units (including the Lepontine dome), containing reworked detritus from Variscan, Caledonian–Sardic, Cadomian, and Pan-African orogenic cycles. In contrast, the dominant 252–300 Ma age group from early Miocene foreland deposits is indicative of the exhumation of Variscan-aged crystalline rocks from the Lepontine dome 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 linked to incipient continent–continent collision. In addition, detrital zircon rare-earth and trace element data, coupled with zircon morphology and U∕Th ratios, point to primarily igneous and rare metamorphic sources. The observed switch from Austroalpine to Penninic detrital provenance in the Molasse Basin at ∼ 21 Ma appears to mark the onset of synorogenic extension of the Central Alps. Synorogenic extension accommodated by 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 corroborate the interpretations that between ∼ 25 and 15 Ma, the exposed bedrock in the Lepontine dome comprised greenschist-facies rocks only, where temperatures were too low for allowing zircon rims to grow, and that the Molasse Basin drainage network did not access the prominent Alpine-age Periadriatic intrusions located in the area surrounding the Periadriatic Line.

Miocene high elevation in the Central Alps

“…20 Ma onward, when basin-and orogen-related reorganizations are likely to have affected the topographic evolution on a regional scale since at least the early Miocene. These include exhumation in the Lepontine and Aar regions (Herwegh et al, 2017;Schlunegger and Willett, 1999), a hypothesized reversal in slab polarity (Kissling et al, 2006;Lippitsch, 2003) or slab delamination (Handy et al, 2021) beneath the Eastern Alps, a switch in the regional tilt of the Swiss Molasse Basin with an associated change in the basin-axial discharge direction at ∼ 17 Ma to a more complex pattern thereafter (Berger et al, 2005;Kuhlemann and Kempf, 2002;Kühni and Pfiffner, 2002;Garefalakis and Schlunegger, 2019), a change in sediment provenance in the SMB deposits (e.g., Anfinson et al, 2020;Von Eynatten et al, 1999), and the beginning of the reorganization of the drainage network into an orogenparallel E-W-oriented system (Bernard et al, 2021;Kühni and Pfiffner, 2002;Schlunegger et al, 2001). Four observations point to high spatial topographic heterogeneity in the mid-Miocene Central Alps: (1) the rapid and nearly vertical rise of the Aar Massif at ∼ 20 Ma is associated with a rearrangement of the drainage network, leading to a shift of the drainage divide towards the uplifted crystalline block (Bernard et al, 2021;Kühni and Pfiffner, 2001;Schlunegger et al, 2001), which is made up of basement rocks of the European plate comprising granites and granodiorites (Herwegh et al, 2020) with the lowest erodibility in the Central Alps (Kühni and Pfiffner, 2002).…”

Slab Load Controls Beneath the Alps on the Source-to-Sink Sedimentary Pathways in the Molasse Basin

“…This basin, which extends from Chambery to Linz over an along-strike distance of c. 850 km and which has a maximum cross-sectional width of c. 150 km (Figure 2), has evolved in response to the Alpine orogeny since the Late Cretaceous and records early underfilled (Flysch stage) and subsequently filled to overfilled conditions (Molasse stage) [18]. Numerous chronologic, stratigraphic, sedimentologic and provenance tracing analyses have resulted in a level of detail that is probably one of the highest for a foreland basin [12][13][14][15][16][17][19][20][21][22][23][24]. In this context, reconstructions of how the surface loads in the Alps evolved through space and time have been the preferred focus, if the aim was to use stratigraphic data from the Molasse basin to reconstruct Alpine orogenic events.…”

Two glaciers and one sedimentary sink: the competing role of the Aare and the Valais glaciers in filling an overdeepened trough inferred from provenance analysis

“…Provenance tracing revealed that the rocks of Iberian, African, and Piedmont origin constituted the major sediment sources for the Molasse sequences that were deposited in the Bern area (Spiegel et al, 2000(Spiegel et al, , 2001(Spiegel et al, , 2002von Eynatten, 2003). In contrast, material from the European External massifs was supplied to the Molasse Basin only after their exhumation around 14 Ma (Stutenbecker et al, 2019;Anfinson et al, 2020). Therefore, material derived from the External massifs is absent in the Molasse sequences in the Bern area.…”

“…Conversely, the absence of material of Penninic origin could indicate that the Valais Glacier did not advance to the Bern area and that material from Klippen sources was probably supplied by the Aare Glacier and mainly by the Saane Glacier. Finally, the bedrock that underlies the Quaternary succession at the Rehhag drill site consists of the Lower Freshwater Molasse (Schwenk et al, 2022), which is composed of the clastic sediment that was derived from the Klippen and the Penninic domains during the Late Oligocene and the Early Miocene (Schlunegger et al, 1993;Spiegel et al, 2002;von Eynatten, 2003;Anfinson et al, 2020). Therefore, it is likely that the distinction between a first-cycle Klippen and Penninic source signal and a Molasse signal is not straightforward if only the unmixing results are used.…”