The structure of the Hallstatt evaporite body (Northern Calcareous Alps, Austria): A compressive diapir superposed by strike-slip shear?

Abstract

Based on old detailed mining maps and own observations in the Hallstatt salt mine, we reinterpret the structure of the Hallstatt evaporite body of the Upper Permian to Lower Triassic Haselgebirge Fm. within the Northern Calcareous Alps (NCA). The Haselgebirge Fm. represents a rocksalt mylonite with abundant lenses of sulphates, mudstones and limestones. In comparison to published results of analogue modeling we interpret the present shape of the Hallstatt body as a WNW–ESE elongated compressive teardrop-like diapir. This is overprinted by NNE–SSW shortening and dominantly sinistral shearing along a W-trending shear zone. The internal structure shows steeply dipping rock units and foliation. Earlier dextral ductile shear fabrics of likely late Early Cretaceous age are preserved in sulphate rocks and are subsequently overprinted by mylonitic fabrics in rocksalt and cataclastic fabrics in other rocks.The low strength of halite results in recent subvertical shortening and a strain rate ε˙ of 8 × 10−10 [s−1] is deduced from deformed subhorizontal boreholes. This value is similar to such strain rates (10−10 to 10−9 s−1) estimated by the halite grain size distribution from other salt mines in the NCA and thus indicative of sub-recent formation of the halite microfabrics.