Experimental deformation of a single-layer anhydrite in halite matrix under bulk constriction. Part 2: Deformation mechanisms and the role of fluids

Composite natural samples, consisting of a single layer of anhydrite, embedded in matrix of rock salt, were experimentally deformed at temperature, T = 345 °C and strain rate, ė = 10−7 s−1. The geometry and kinematics of folds and boudins, which affect the stiff anhydrite layers, were described in Part 1 of the present study (Zulauf et al., 2009). The microfabrics, deformation mechanisms, and the role of fluids are treated in the present paper. Anhydrite was deformed under brittle–viscous conditions as is indicated by fracturing, twinning, and local recrystallization, the latter based on strain-induced grain-boundary migration. Viscous deformation of halite was accommodated by slip on {110}〈110〉 which led to formation and rotation of subgrains and a striking 001-maximum parallel to the long axis, X, of the strain ellipsoid. Differential stress obtained from subgrain size of halite is largely consistent with stresses recorded by the load cells of the machine (<5 MPa).Despite of the high deformation temperature and the low water content of the starting samples, NaCl brine and hydrocarbons, expelled from fluid inclusions of both halite and anhydrite, led to hydraulic fracturing and redistribution of matter. The fluids migrated along newly formed microfractures in anhydrite and along halite–anhydrite boundaries. Released hydrocarbons and NaCl brine were redeposited in open space of neck domains in the form of black organic coatings (condensate) and fine-grained halite, respectively. As these fluid-controlled phenomena are common in salt domes of northern Germany, and released fluids may led to contamination of bed rock and biosphere, the results of the present study should be important for workers who are dealing with radioactive waste deposits in rock salt.