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Geochemistry and Geochronology of Gneiss Pebbles from Foreland Molasse Conglomerates: Geodynamic and Paleogeographic Implications for the Oligo‐Miocene Evolution of the Eastern Alps

Gneiss pebbles, deposited in foreland fans during the Miocene Epoch, were studied with geochemical and thermochronological methods to reconstruct the paleodrainage evolution of the Eastern Alps and the geodynamic behavior of the source areas. Clear discrimination between upper‐plate Austroalpine and lower‐plate Penninic gneiss pebbles was achieved by single‐pebble paleocooling paths. Austroalpine gneisses showed highly variable and regionally distinctive paleocooling paths, which record Variscan to Cretaceous Rb‐Sr and K‐Ar cooling ages. Penninic gneisses showed Paleogene to lower Miocene high‐temperature (Rb‐Sr, K‐Ar) and Miocene low‐temperature (zircon and apatite fission track) cooling ages. Cooling rates of Penninic gneisses indicate maximum exhumation rates at ∼17 Ma during the climax of lateral tectonic extrusion, recording dominance of tectonic unroofing. The paleodrainage evolution was characterized by a relatively stable catchment configuration in the western part of the Eastern Alps, where northward radial dewatering to fixed foreland entry points was documented throughout molasse sedimentation in Oligo‐Miocene times. The largest river system, represented by the Paleo‐Inn River, displayed a considerably larger catchment area than today and variable entry points into the foreland. The drainage pattern of the eastern part of the Eastern Alps was governed by the fault pattern that developed during lateral extrusion. Penninic basement rocks first became exposed in the eastern part of the Tauern window at ∼13 Ma, some 4 m.yr. after the climax of tectonic unroofing.