Structure and evolution of a rocksalt-mudrock-tectonite: The haselgebirge in the Northern Calcareous Alps

The Northern Calcareous Alps are part of the Eastern Alps in Austria and Germany. The Mesozoic units of this fold-and-thrust belt were detached, thrusted and stacked along the evaporitic Haselgebirge Formation. Exposed in salt mines, rocksalt and mudrock form a two component tectonite: The rock type “haselgebirge” consists of 10–70 wt % halite with silt- to gravel- or block-sized components within a halite matrix, and the “kerngebirge” with >70 wt % halite. All rock types studied are fault rocks. By use of a temperature-independent subgrain size piezometer, the paleo-differential stress of halite was calculated at ca. 2.5 MPa in Altaussee and ca. 4.5 MPa in Berchtesgaden. Including data from a grain-size piezometer, temperatures were estimated at ca. 150 ± 20 °C and 110 ± 10 °C. This implies very high strain rates, which are about 10−10–10−9 s−1. During the tectonic movement, the halite deformed, recrystallized, and crystallized as veins in mudrock fractures. We interpret high overpressure of the pore fluid to have significantly contributed to fracturing of the mudrock.