Deformation processes in polycrystalline aggregates of gypsum

On the basis of both field and laboratory studies it is well established that polycrystalline gypsum is one of the weakest and most ductile rock materials found in the Earth's crust (e.g. Heard & Rubey, 1966; Murrell & Ismail, 1976; Baumann, 1985; Jordan, 1988; 1991; 1994). The deformation and densification behaviour of polycrystalline gypsum aggregates, and the underlying microphysical processes which control deformation, thus form a subject of considerable interest in a number of areas of structural geology, tectonophysics, geotechnical engineering and geomechanics. In the last 5 years, the mechanical properties of gypsum have also become a point of interest in building materials research. In the field of structural geology and tectonophysics, gypsum-dominated evaporite sequences have long been recognized to play an important role in controlling the strength of the Earth's upper crust, via their action as weak decollement horizons such as those characterizing the foreland thrust belts of the Pyrenees (Sole-Sugranes, 1978), the Alps (Beach, 1981; Laubscher, 1981; Murrell, 1981; Davis & Engelder, 1985; Mugnier & Vialon, 1986; Jordan, 1988; 1991; 1994; Ma1avieille & Ritz, 1989; Jordan et aI., 1990), the Greek Hellenides (Underhill, 1988) and the Atlas mountains of north Africa (Davis & Engelder, 1985). In addition, gypsum-dominated evaporites frequently develop large scale "halokinetic" structures such as pillows and diapirs (Wall et aI., 1961; Gould & De Mille, 1968; Dabbagh et aI., 1984; Doglioni, 1984; Laudon, 1984; Davis & Engelder, 1985; Van Berkel et aI., 1986; Simon & Suriano, 1986; Underhill, 1988; Kupfer, 1989) which are sometimes associated with, or directly trap, large oil and gas deposits (Balkwill, 1978). Like rocksalt, the characteristically low permeability of gypsum rock and their high hydrocarbon sealing/trapping potential, are of course determined by their low resistance to ductile flow and densification/compaction. For these reasons, interest exists in obtaining a quantitative, mechanism-based understanding of the deformation and densification behaviour of polycrystalline gypsum, suitable as input for numerical modelling studies of both crustal deformation and hydrocarbon migration phenomena.