Deformation of anhydrite rocks (Gorleben-Bank, z3OSM) in a high-strain domain of the Gorleben salt dome, Germany

Microfabrics and mineralogical-geochemical data of anhydrite and surrounding halite matrix of the Gorleben-Bank (z3OSM from drilling site 1.8 in the Gorleben exploration mine, Northern Germany), are used to constrain deformation mechanisms and history, while geochemical data are used to constrain the type of protolith. Dependent on layer thickness, anhydrite shows different macro- and microstructural characteristics of the brittle-ductile deformation regime. In thin layers, solution-precipitation creep plays an important role for accommodating finite strain. In thick layers, bulging, grain boundary migration and twinning are important deformation mechanisms in addition to solution-precipitation creep. In a younger deformation event, the anhydrite layers underwent boudinage by fracturing. Open boudin necks and fissures were filled with carnallite and halite. Microfabrics and geochemical data of matrix halite and of halite inside boudin necks suggest that rock salt has entered the necks by brine-assisted, viscous creep.