Tracking coarse-grained gravity flows by LASS-ICP-MS depth-profiling of detrital zircon (Aveto Formation, Adriatic foredeep, Italy)

Coarse-grained gravity flow deposits are quite common in the stratigraphic record, but their capability in transporting cobbles and boulders at great distance from the eroding sources is still poorly assessed. Here, we tackle this issue by the analysis of coarse-grained gravity flow deposits of the Aveto Formation, deposited in the Early Oligocene Adriatic foredeep and now exposed in the accretionary wedge of the Northern Apennines (Italy). We combine field observations and provenance constraints provided by Laser Ablation Split Stream (LASS)-ICP-MS depth-profiling of detrital zircon grains. We found that the polymodal grain-age distributions in the analyzed samples, dominated by Periadriatic, Variscan and Caledonian age populations, also include Late Cretaceous to Paleocene ages associated to thin epitaxial overgrowths on older zircon cores. These overgrowths display Th/U and REE patterns consistent with a metamorphic origin, and provide a diagnostic fingerprint that indicates the Bergell area in the Central Alps as the only viable source of coarse detritus, including cobbles and boulders of magmatic and metamorphic rocks, transported by gravity flows towards the nascent Adriatic foredeep. Our results provide not only pin-points for a reliable paleotectonic reconstruction of the Bergell-Aveto source-tosink system, one of the most remarkable features of the Oligocene Adria-Europe plate boundary, but also compelling evidence for ~300 km axial transport by gravity flows experienced by coarse detritus derived from the Oligocene Central Alps. The eruptions of the Bergell volcanic complex provided abundant pyroclastic material variably mixed with metamorphic detritus, that was quickly funneled into submarine canyons triggering southward-directed gravity flows. Active tectonics, pyroclastic material provided by volcanic eruptions, steep canyons close to the shoreline and the presence of fault-controlled submarine troughs, may effectively promote the long-distance transport of coarse material by gravity flows not only in the Oligocene, but also today.