Tectonic stylolites in anhydrite rock: Constraints on fluid-assisted deformation and isotopic ages of salt rocks (Gorleben salt dome, Germany)

Deformation microfabrics of anhydrite rock, collected from the Gorleben salt dome of northern Germany, revealed brittle-viscous deformation accommodated by fracturing, twinning, kinking, subgrain formation, strain-induced grain boundary migration, and incongruent dissolution-precipitation creep (IDPC), which led to stylolites aligned oblique to bedding. IDPC along the stylolite planes is documented by magnesite, which developed as a new phase replacing anhydrite. U-Pb dating of large primary anhydrite crystals yielded 251 ±18 Ma interpreted as formation age. U-Pb dating of stylolite-magnesite, on the other hand, yielded Oligocene and Miocene ages, which reflect the time of deformation and stylolite formation after the main phase of diapir emplacement.Experimental deformation of a competent (brittle-viscous) anhydrite layer, embedded in incompetent viscous rock salt matrix, at temperature, T = 345°C, strain rate, ė = 10-7 s-1, and a shortening strain, eZ = -30%, led to boudinage of the anhydrite layer by tensile fracture. Primary fluids, trapped in fluid inclusions of anhydrite, became mobile during boudinage but were not able to escape through the enclosing rock salt, which acted as a sealing matrix. This sealing behaviour explains the lack of stylolites in rock salt and is important for cavern industries and for the long-term safety of a repository for radioactive waste in salt structures. Moreover, the experimental deformation had a significant impact on the U-Pb isotopic system of both anhydrite and magnesite. Apart from one Miocene age of magnesite, with a large uncertainty, both yielded largely perturbed data, which cannot be used to calculate robust ages.