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. Eocene to Miocene sedimentary strata of the Northern Alpine Molasse Basin inSwitzerland are well studied, yet they lack robust geochronologic andgeochemical analysis of detrital zircon for provenance tracing purposes.Here, we present detrital zircon U–Pb ages coupled with rare-earth and traceelement geochemistry to provide insights into the sedimentary provenance andto elucidate the tectonic activity of the central Alpine Orogen from thelate Eocene to mid Miocene. Between 35 and 22.5 ± 1 Ma, the detritalzircon U–Pb age signatures are dominated by age groups of 300–370,380–490, and 500–710 Ma, with minor Proterozoic age contributions. Incontrast, from 21 Ma to ∼ 13.5 Ma (youngest preservedsediments), the detrital zircon U–Pb age signatures were dominated by a252–300 Ma age group, with a secondary abundance of the 380–490 Ma agegroup and only minor contributions of the 500–710 Ma age group. TheEo-Oligocene provenance signatures are consistent with interpretations thatinitial basin deposition primarily recorded unroofing of the Austroalpineorogenic 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 isindicative of the exhumation of Variscan-aged crystalline rocks from theLepontine dome basement units. Noticeable is the lack of Alpine-ageddetrital zircon in all samples with the exception of one late Eocene sample,which reflects Alpine volcanism linked to incipient continent–continentcollision. In addition, detrital zircon rare-earth and trace element data,coupled with zircon morphology and U∕Th ratios, point to primarily igneousand rare metamorphic sources. The observed switch from Austroalpine to Penninic detrital provenance in theMolasse Basin at ∼ 21 Ma appears to mark the onset ofsynorogenic extension of the Central Alps. Synorogenic extensionaccommodated by the Simplon fault zone promoted updoming and exhumation thePenninic crystalline core of the Alpine Orogen. The lack of Alpine detritalzircon U–Pb ages in all Oligo-Miocene strata corroborate the interpretationsthat between ∼ 25 and 15 Ma, the exposed bedrock in theLepontine dome comprised greenschist-facies rocks only, where temperatureswere too low for allowing zircon rims to grow, and that the Molasse Basindrainage network did not access the prominent Alpine-age Periadriaticintrusions located in the area surrounding the Periadriatic Line.

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