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.
Tectonic Exhumation of the Central Alps Recorded by Detrital Zircon in the Molasse
Basin, Switzerland
