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

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.



Upper Marine Molasse 268
Continental deposition of the LFM was followed by a shift to marine sedimentation of the 269 Upper Marine Molasse (UMM) in the Swiss foreland basin (Keller, 1989). This has been 270 interpreted as a change back to underfilled conditions. A return towards an underfilled basin 271 started already during LFM times at ~21 and was characterized by a continuous reduction in 272 sediment supply rates (Kuhlemann, 2000;Willett and Schlunegger, 2010). Marine conditions 273 began in the eastern Molasse basin and propagated westward (Strunck and Matter, 2002;274 Garefalakis and Schlunegger, 2019). These related effects appear to have been amplified by a 275 tectonically-controlled widening of the basin (Garefalakis and Schlunegger, 2019). This change 276 from overfilled non-marine to under-filled marine conditions is referred to as the Burdigalian 277 Transgression (Sinclair et al., 1991). However, the debate continues whether the cause of the 278 Burdigalian Transgression is due to: (i) an increase in sea level outpacing sedimentation (Jin et

Upper Freshwater Molasse 294
The Upper Freshwater Molasse (UFM) consists of non-marine conglomerates and 295 sandstones deposited in prograding alluvial fans and fluvial floodplains (Keller, 2000). The 296 thickest section of the preserved UFM (~1500 m) is situated to the west of Lucerne (Matter, 297 1964;Trümpy, 1980 (Willett and Schlunegger, 2011). 308 12 conglomerates in the Thun section (10SMB06) directly overly sandstones of the LMF II as a 340 tectonic interpretation of a seismic section has revealed (Schlunegger et al., 1993). Therefore, we 341 tentatively assigned an LMF II age to the 10SMB06 sample site. Finally, sampling and 342 stratigraphic age assignments for samples from the Bregenz Section were guided by the 343 geological map of Oberhauser (1994) and by additional chronologic (Kempf et al., 1999) and 344 stratigraphic work (Schaad et al., 1992). We considered an uncertainty of ±2 Ma to the age 345 assignment for the Bregenz Section samples. 346 347

Zircon U-Pb LA-ICPMS Methodology 348
The bulk of the detrital zircon samples were analyzed at the UTChron geochronology 349 facility in the Department of Geological Sciences at the University of Texas at Austin and a 350 smaller subset at the at the Isotope Geochemistry Lab (IGL) in the Department of Geology at the 351 University of Kansas, using identical instrumentation and very similar analytical procedures, but 352 different data reduction software (Table 1). All samples underwent conventional heavy mineral 353 separation, including crushing, grinding, water-tabling, magnetic, and heavy liquid separations, 354 but no sieving at any point. Separate zircon grains were mounted on double-sided tape (tape-355 mount) on a 1" acrylic or epoxy disc without polishing. For all samples 120-140 grains were 356 randomly selected for LA-ICPMS analysis to avoid biases and capturing all major age 357 components (>5%) (Vermeesch, 2004). All grains were depth-profiled using a Photon Machines 358 Analyte G2 ATLex 300si ArF 193 nm Excimer Laser combined with a ThermoElement 2 single 359 collector, magnetic sector -ICP-MS, following analytical protocols of Marsh and Stockli (2015). 360 30 seconds of background was measured followed by 10 pre-ablation "cleaning" shots, then 15 361 sec of washout to measure background, prior to 30 sec of sample analysis. Each grain was 362 ablated for 30 seconds using a 30 µm spot with a fluence of ~4 J/cm2, resulting in ~20 µm deep 363 ablation pits. For U-Pb geochronologic analyses of detrital zircon the masses 202 Hg, 204

Laser Ablation-Split Stream (LASS) Analyses of Detrital Zircon 391
In an attempt to glean additional provenance constraints from Molasse samples, we also 392 In an attempt to simplify data presentation and data reporting, the detrital zircon U-Pb 426 ages were lumped into genetically-related tectono-magmatic age groups that include the 427 Variscan, Caledonian, and Cadomian orogenic cycles. In addition to these three pre-Alpine 428 orogenic cycles we also considered the total number of Cenozoic (Alpine) ages, Mesozoic 429  Supplemental File 1 and all associated sample information can be found in Table 1.

Constrains on surface exhumation of external massifs 712
It has been suggested that rapid rock uplift of the external Aar Massif (Figure 1) Variscan material continuously increased through time (Figures 6 and 10). The material of this 728 region was derived from the eastern Swiss Alps, which includes the Austroalpine units, and 729 possibly the eastern portion of the Lepontine Dome (Kuhlemann and Kempf, 2002). This area 730 was not particularly affected by tectonic exhumation (Schmid et al., 1996). Therefore, we 731 interpret the continuous change in the age populations as record of a rather normal unroofing 732 sequence into the Alpine edifice.             Figure 3a) plotted as Kernel Density Estimations (KDE; Bandwidth set to 10), a Cumulative Distribution Plot (CDP), and as pie diagrams. Colored bars in the KDE and colored wedges in the pie diagrams show the relative abundance of age groups discussed in Section 5. Samples associated with each unit are referenced in Table 1. Plot only shows ages from 0-1000 Ma; however, a small number of older ages were analyzed and that data can be found in Supplemental File 1. N= (Number of samples, number of ages depicted/ total number of total ages).