DEFORMATION OF THE ALPINE HASELGEBIRGE FORMATION – STRUCTURES, 40Ar/39Ar POLYHALITE AGES AND THEIR INTERPRETATION

The Permian to Lower Triassic Haselgebirge of the Northern Calcareous Alps shows a number of unusual features different from other evaporite successions. The Mesozoic units of this fold-and-thrust belt were detached, thrusted and stacked along the evaporitic Haselgebirge Formation. Intact mudrocks allow reconstruction of early events, and we found a hitherto undetected Middle Triassic thermal event. Migrating mineralized hydrous fluids, released from mudrock, led to the replacement of halite by anhydrite retaining the shape of deformed halite. Coevally, polyhalite crystallized at ca. 230 Ma (⁴⁰Ar/³⁹Ar age).

The investigated rock salt deposits (Altaussee, Berchtesgaden-Dürrnberg) show a thermal overprint of likely Cretaceous age. Our fluid inclusions and vitrinite reflexion measurements yield a peak temperature of 180°C for Berchtesgaden and >240°C for Altaussee. Furthermore, fine-grained polyhalite mylonites with a ⁴⁰Ar/³⁹Ar age of ca. 113 Ma formed during the Early Alpine (Cretaceous) orogeny.

Rocksalt and mudrock form a two-component tectonite. By use of the temperature-independent subgrain-size piezometer for halite, the paleo-differential stress was calculated at ca. 2.5 MPa in Altaussee and ca. 4.5 MPa in Berchtesgaden. These paleo-stresses allow estimate temperatures at 150 ± 20°C and 110 ± 10°C, also implying very high strain rates (10^-9 to 10^-10 s^-1). During deformation, the halite deformed and recrystallized, and also crystallized in veins within mudrocks. We interpret high overpressure of the pore fluid to have significantly contributed to fracturing of the mudrock (Leitner et al., 2011, J. Struct. Geol. 33, 970–984).

The orientation of the foliation, the halite mineral lineation and other structures are consistent within each deposit and vary from deposit to deposit. In all cases, the axes of isoclinal folds are parallel to the lineation, what is typical for highly ductile rocks. White fibers within extensional veins have a preferred orientation and represent the last major structural stage of salt deformation within the deposits, whereby the mapped structures relate to the structures of the surroundings of the salt bodies. All Alpine salt bodies got their final shape and internal structure during Cenozoic deformation stages.