Excavation and humidity induced extension veins of mudrock clasts in a ductile rock salt matrix, filled with secondary halite deposits (Northern Calcareous Alps, Austria, Bavaria)

The Alpine Haselgebirge Formation (Northern Calcareous Alps, Eastern Alps) in underground salt mines consists of a tectonic breccia of c. 50 % halite and 50 % mudrock. The mudrock clasts in the underground galleries undergo brittle deformation and develop <1 mm extension veins filled with white halite spaced in 1-10 mm range. The fillings often form well visible, flaky sheets, which give the rock its typical appearance in reflected light (Blattersalzgebirge). In-situ deformations of the galleries were analysed with extensometers in monitoring sections. Scanning electron microscopy, chemical analyses and swelling experiments were accomplished to elucidate the nature of mudrock and vein fillings. The mining induced extension veins strike parallel to the gallery and differ significantly from old, orogenic veins. After excavation, the stress in the rock salt matrix is accommodated by creeping of halite. However, the stress and the contact with water due to the high relative air humidity in the underground mines lead to initial cracks, subsequent swelling, and finally to complete disaggregation of the mudrock clasts. The saline solution can infiltrate cracks and fissures in the mudrocks. The cyclic crystallisation and dissolution of halite amplifies the disintegration of the mudrock clasts. The halite crusts behave similar to halite speleothems observed in salt karst environments.