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

Anfinson, Owen A.; Stockli, Daniel F.; Miller, Joseph C.; Möller, Andreas; Schlunegger, Fritz (2020). Tectonic exhumation of the Central Alps recorded by detrital zircon in the Molasse Basin, Switzerland. Solid Earth, 11(6), pp. 2197-2220. Copernicus Publications 10.5194/se-11-2197-2020

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Eocene to Miocene sedimentary strata of the Northern Alpine Molasse Basin in Switzerland are well stud- ied, 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 ac- tivity of the central Alpine Orogen from the late Eocene to mid Miocene. Between 35 and 22.5 ± 1 Ma, the detrital zir- con U–Pb age signatures are dominated by age groups of 300–370, 380–490, and 500–710Ma, with minor Protero- zoic 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 in- terpretations that initial basin deposition primarily recorded unroofing of the Austroalpine orogenic lid and lesser contri- butions from underlying Penninic units (including the Lep- ontine 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 exhuma- tion 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 detri- tal 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 crys- talline core of the Alpine Orogen. The lack of Alpine detri- tal 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 Periadri- atic intrusions located in the area surrounding the Periadriatic Line.

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