The long-lasting exhumation history of the Ötztal-Stubai Complex (Eastern European Alps): New constraints from zircon (U-Th)/He age-elevation profiles and thermo-kinematic modeling

Hölzer, K; Wolff, R; Hetzel, R; Dunkl, I

Zusammenfassung

The Eastern European Alps formed during two orogenic cycles, which took place in the Cretaceous and Cenozoic, respectively. In the Ötztal-Stubai Complex – a thrust sheet of Variscan basement and Permo-Mesozoic cover rocks – the record of the first (Eoalpine) orogeny is well preserved, because during the second (Alpine) orogeny the complex remained largely undeformed. Here, new zircon (U-Th)/He (ZHe) ages are presented and thermo-kinematic modeling is applied to decipher the cooling and exhumation history of the central part of the Ötztal-Stubai Complex since the Late Cretaceous. The ZHe ages from two elevation profiles increase over a vertical distance of 1500 m from 56±3 to 69±3 Ma (Stubaital) and from 50±2 to 71±4 Ma (Kaunertal), respectively. These ZHe ages and few published zircon and apatite fission track ages were used for inverse thermo-kinematic modeling. The modeling results show that the age data are well reproduced with a three-phase exhumation history. A first phase with relatively fast exhumation (~250 m/Myr) during the Late Cretaceous ended at ~70 Ma and is interpreted to reflect the erosion of the Eoalpine mountain belt. As Late Cretaceous normal faults occur at the margins of the Ötztal-Stubai Complex, normal faulting may have also contributed to the exhumation of the study area. Subsequently, a long period with slow exhumation (<10 m/Myr) prevailed until ~16 Ma. This long-lasting phase of slow exhumation suggests a rather low topography with little relief in the Ötztal-Stubai Complex until the mid-Miocene, even though the Alpine orogeny had already begun in the Eocene with the subduction of the European continental margin. Accelerated exhumation since the mid-Miocene (~230 m/Myr) is interpreted to reflect the erosion of the mountain belt, due to the development of high topography in front of the Adriatic indenter and repeated glaciations during the Quaternary.