DETRITAL-ZIRCON U-PB GEOCHRONOLOGIC CONSTRAINTS ON PROVENANCE OF ADRIATIC TURBIDITES (ALPS-APENNINES SYSTEM)

Provenance of Oligo-Miocene strata from the Adriatic foredeep of the Alps-Apennines system is evaluated using detrital zircon U-Pb geochronology. Recent research suggests that the primary source area of these deposits is the Lepontine Dome of the Central Alps. Five samples collected from proximal deposits and eight samples from distal deposits accreted south of the Alps document significant changes in detrital zircon U-Pb age populations during the Oligocene-Miocene boundary. This shift in detrital zircon U-Pb age signature, when compared with data from modern sands, is likely correlative to a westward shift of the erosional foci within the Lepontine Dome, from the Ticino to the Toce subdome, due to the progressive indentation of Adria coeval with the progressive unroofing of Periadriatic magmatic rocks of Tertiary age along the Insubric Fault.

The lowermost Upper Oligocene proximal samples collected from the Como and Villa Olmo Conglomerates are dominated by Caledonian and Cadomian detrital zircon U-Pb age populations. The uppermost Oligocene and lower Miocene proximal samples collected from the Como Conglomerate are dominated instead by Periadriatic detrital zircon.

Distal samples collected from the Lower Oligocene Aveto Formation have a dominant Periadriatic age peak with lesser amounts of late Cretaceous, Variscan, Caledonian and Cadomian detrital zircon. The lowermost Upper Oligocene distal samples collected from the Macigno Formation contain populations of Periadriatic, Variscan, Caledonian, and Cadomian detrital zircon, with major shifts in relative abundance from the lower to upper strata. The most dramatic shift in provenance in the distal units is between two samples located relatively proximally to one another in the Modino unit: Upper Oligocene marls contains primarily Variscan and Caledonian zircon grains with no individuals yielding Periadriatic ages, whereas the Upper Oligocene – Lower Miocene sandstones of the same unit include dominant Periadriatic and Variscan age populations. The youngest distal sample, from the Lower Miocene Bobbio Formation, primarily contains Variscan detrital zircon ages.