Oligo-Miocene Alpine Sediment Routing from Integrated Analysis of Seismic-Reflection Data and Detrital Zircon U-Pb Geochronology

Abstract:

We integrate detrital zircon geochronology and 3D seismic-reflection data to reconstruct Oligo-Miocene paleogeography and sediment routing from the Alpine hinterland to Austrian Molasse foreland basin. Three-dimensional seismic-reflection data image a network of deepwater tributaries and a long-lived (>8 Ma) foredeep-axial channel belt through which predominantly southerly and westerly turbidity currents are interpreted to have transported Alpine detritus >100 km. We analyzed 793 detrital zircon grains from ten sandstone samples collected from the seismically mapped network of channel fill. Grain age populations correspond with major Alpine orogenic cycles: the Cadomian (750-530 Ma), the Caledonian (500-400 Ma), and the Variscan orogenies (350-250 Ma). Additional age populations correspond with Eocene-Oligocene Periadriatic magmatism (40-30 Ma) and pre-Alpine, Precambrian sources >750 Ma. The abundances of these age populations vary between samples. Sediment that entered the foredeep-axial channel belt from the west (freshwater Molasse) and southwest (Inntal fault zone) is characterized by statistically indistinguishable, well-distributed detrital zircon ages. Sandstone from a shallow marine unit that was deposited proximal to the northern basin margin consists of >75% Variscan (350-300 Ma) zircon, which is believed to have originated from the Bohemian Massif to the north. Mixing calculations based on the Kolmogorov-Smirnoff statistic suggest that the Alpine fold-thrust belt was an important source of detritus to the deepwater Molasse basin. We document east-to-west provenance dilution within the axial channel belt via one or more southern tributaries. Our results have important implications for sediment dispersal patterns within continental-scale orogens, including the relative role of longitudinal versus transverse sediment delivery in peripheral foreland basins.