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.