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NARROW
Pebble population dating as an additional tool for provenance studies – examples from the Eastern Alps
Abstract Detrital fission-track (FT) dating can be successfully used in provenance studies of siliciclastic sediments to define the characteristic cooling ages of the source regions during erosion and sedimentation. In order to obtain more specific information about potential source regions we have developed the pebble population dating (PPD) method in which pebbles of specific lithotype are merged and dated. Dating of both zircon and apatite crystals from such pebble populations yields age distributions, which reflect the cooling ages of the given lithotype in the source area at the time of sedimentation. By this technique it is possible to define ‘FT litho-terrains’ in the source regions and thus outline palaeogeological maps. Two examples are presented from the Eastern Alps. (i) Comparison of FT ages from a sandstone sample and a gneiss PPD sample from an Oligocene conglomerate of the Molasse Basin shows that the youngest age cluster is present only in the sand fraction and derived from the Oligocene volcanic activity along the Periadriatic zone. The lack of the youngest ages in the gneiss pebble assemblage excludes the Oligocene exhumation of the crystalline basement from mid-crustal level. (ii) Pebble assemblages of red Bunter sandstone, gneiss and quartzite were collected from an Upper Miocene conglomerate of the Molasse Foreland Basin and merged as PPD samples. Apatite and zircon FT grain age distributions of these PPD samples, representing the largest ancient East Alpine catchment, allow generating a new combination of palaeogeological and palaeo-FT-age maps of the Eastern Alps for the Late Miocene.
An interdisciplinary study has been carried out on Naxos Island, located in the southern Aegean Sea (Greece), which shows Miocene geodynamic and environmental changes in a classic example of a collapsing orogen. Early to mid-Miocene siliciclastic deposits on Naxos have been shed from an uplifting mountainous realm in the south, which included a patchwork of at least four source terrains of different thermal histories. Petrography of pebbles suggests that the source units formed part of a passive continental margin succession (external Pelagonian unit), and an ophiolite succession mainly of deep-water cherts and limestones deposited on basalt substratum (Pindos unit). The continental margin source contributed rounded zircon crystals of Late Jurassic to Early Cretaceous age and broadly scattering Paleozoic zircon fission-track cooling ages. A distal pebble assemblage of Paleogene shallow-water carbonates passing into flysch-like, mixed calcarenitic and siliciclastic components with volcanic arc components is subordinately present. High-grade metamorphic components from the nearby metamorphic core complex are not present. The depositional evolution reflects increasing relief and, in some parts, a fluvial succession with rhythmic channel deposition, possibly due to runoff variability forced by orbital cyclicity. Upsection, the depositional trend indicates increasing seasonality and decreasing humidity in the source region. The Miocene sedimentary succession has been deposited on an ophiolite nappe. Juxtaposition of this ophiolite nappe occurred as an extensional allochthon during large-scale extension in the Aegean region at the margins of an exhuming metamorphic core complex.