Phases of enhanced exhumation during the Cretaceous and Tertiary orogenies in the Eastern European Alps: new insights from thermochronological data and thermokinematic modeling

The Austroalpine nappes in the Eastern European Alps have preserved the record of two orogenic phases in the Cretaceous and Tertiary but their cooling and exhumation history remains poorly constrained. Here we use new low-temperature thermochronological data and thermokinematic modeling to unravel the exhumation history of the Austroalpine nappes in the Nock Mountains east of the Tauern Window (Wölfler et al., submitted). Our data show that the central Nock Mountains (Ötztal-Bundschuh and Drauzug-Gurktal nappes) cooled through the zircon fission track closure temperature (~240 °C) already in the Late Cretaceous. Apatite fission track ages cluster around 35-30 Ma, indicating that the rocks have been at depths of ≤5-6 km since the Eocene-Oligocene boundary. In contrast, the Radenthein and Millstatt Complexes, which are located south of the Hochstuhl Fault, cooled below 240 °C during the Eocene and show apatite fission track ages of ~15 Ma. Thermokinematic modeling of an age-elevation profile in the central Nock Mountains (near Innerkrems) revealed a phase of enhanced exhumation (~0.62 km/Ma) between ~100 and ~85 Ma, which we relate to syn- to late-orogenic Late Cretaceous extension. After a period of slow exhumation (~0.03 km/Ma), the exhumation rate increased to ~0.16 km/Ma at ~32 Ma. In contrast, thermokinematic modeling of an age-elevation profile near Millstatt shows that rocks of the Radenthein and Millstatt Complexes were rapidly exhumed (~0.78 km/Ma) from ~44 Ma to ~38 Ma during the initial Europe-Adria collision. After a phase of slow exhumation (~0.07 km/Ma) between ~38 and ~19 Ma, the exhumation rate increased to ~0.3 km/Ma with the onset of Miocene lateral extrusion in the Eastern Alps. Altogether, ~16 km of rock have been removed since ~100 Ma in the Innerkrems region, whereas ~11 km of rock have been removed in the last ~44 Ma in the Millstatt area. These findings are consistent with pressure-temperature estimates for the Ötztal-Bundschuh nappe and the Radenthein/Millstatt Complexes, respectively (Koroknai et al., 1999; Schuster, 2003; Krenn et al., 2003, 2011). The distinct differences in the cooling histories north and south of the Hochstuhl Fault further suggest that this fault, which has hitherto been considered as a dextral strike-slip fault during Miocene lateral extrusion (Polinski & Eisbacher, 1992; Linzer et al., 2002), also accommodated a considerable amount of thrust movement. The difference between the amount of exhumation north and south of the Hochstuhl Fault indicates ca. 5 km of vertical offset between ~44 and ~38 Ma.