Modeling the influence of sinking anhydrite blocks on salt diapirs targeted for hazardous waste disposal

Due to the low permeability and high ductility of rock salt, many salt diap irs, such as those in Germany and the Netherlands, are targeted as long-ter m repositories for disposal of high-level radioactive and chemical wastes. Geophysical and subsurface data show that the Gorleben salt diapir, which i s one of the most extensively investigated diapirs in the world, and other salt diapirs of the Zechstein Formation in Germany contain large blocks (si milar to 80 m thick) of high-density anhydrite inclusions. These blocks, wh ich were carried upward by the rising salt, are considered to be detached s egments of intercalated layers that initially were deposited with the salt. Results of physical and numerical models, presented here, show that such d etached, high-density blocks, which were entrained and carried upward by th e diapir at an earlier stage, tend to sink in the late stages of diapiric e volution when the rate of diapiric rise slows down. During their descent, t hese high-density competent blocks deform by folding and create shear zones at the immediate contact with the less competent salt. The descending bloc ks initiate a secondary internal Bow within the salt diapirs they descend, and they may deform any repository built within such diapirs, which would o therwise be considered as tectonically inactive.