Effect of confining pressure on dilatation, recrystallization, and flow of rock salt at 150°C

Abstract

Abstract. Microstructural evidence for fluid-assisted dynamic recrystallization (FADRX) is widespread in naturally deformed rock salt. However, the principal experimental evidence for FADRX in salt has been obtained from stress relaxation experiments, and it is unclear whether the process occurs during steady state dislocation creep and what its effect might be. Here we report deformation experiments performed on natural rock salt at a constant strain rate of 3.5x10 -7s -•, 150øC, and confining pressures Pt. of 3-30 MPa. Samples deformed at Pt. = 3 MPa showed continuous work hardening and minor dilatation. Microstructurally, they exhibited intergranular cracking plus slip band and subgrain structures indicative of dislocation glide/creep. Electron backscatter diffraction analysis revealed a high frequency of boundaries with low-angle misorientations in the range 5 ø-10 ø. In contrast, samples deformed at Pt. > 6.5 MPa showed work hardening followed by steady state flow at strains > 6-7%. These samples compacted slightly, and crystal plastic deformation was accompanied by extensive FADRX, with a predominance of highangle boundaries (300-50 ø) over low-angle boundaries. We infer that FADRX is suppressed by dilatation at low pressures as a result of grain boundary disruption. At pressures high enough to prevent dilatation, however, FADRX acts as a "recovery" mechanism counteracting work hardening. The results offer a possible explanation for rheological variability seen in previous experiments conducted at pressures up to 30 MPa. A simple rate model for diffusion-and interface-controlled FADRX indicates that FADRX should become increasingly important toward natural halokinetic conditions, although the effect on flow stresses is likely to be small.

Solution-precipitation creep and fluid flow in halite: a case study of Zechstein (Z1) rocksalt from Neuhof salt mine (Germany)

“…This process might operate in other rock types, as suggested by the mobile hydrofracture model in Bons (2001). The detailed nature of the relation between the effective stress, differential stress and permeability was studied in a series of experiments by Urai et al (1986), Peach et al (2001) and Popp et al (2001). The authors noted that the permeability of rocksalt increases by five orders of magnitude if deformed at low temperature and mean effective stress due to microcracking and dilatancy.…”

Rheology of rock salt for salt tectonics modeling

“…Salt flow or halokinesis often occurs at temperatures ranging from 20 to 200°C. Salt deformation at those temperatures has been widely investigated in laboratories (Heard 1972(Heard , 1986Hansen 1979, Wawersik andHannum 1980;Wawersik andZeuch 1984, 1986;Hansen and Carter 1984;Wawersik 1985;Senseny 1988;DeVries 1988;Spiers et al 1990;Wawersik and Zimmerer 1994;Weidinger et al1997;Yang et al 1999;Hunsche and Hampel 1999;Peach and Spiers 2001;Hunsche et al 2003, Ter Heege et al 2005aSchoenherr et al 2007). The purpose of collecting the experimental data is to provide a basis for deformation modeling and to discuss the influence of different physical parameters on creep properties.…”

“…The creep controlled by dislocation mechanisms is widely investigated in laboratory experiments (Carter and Hansen 1983;Carter et al 1993;Rutter 1983;Urai et al 1986;Wawersik and Zeuch 1986;Heard and Ryerson 1986;Senseny et al 1992;van Keken et al 1993;Franssen 1994;Peach and Spiers 1996;Weidinger et al 1997;De Meer et al 2002;Hampel et al 1998;Brouard and Bérest 1998;Bérest et al 2005;Ter Heege et al 2005a, b). The creep equations mainly used in the salt mining industry are based on dislocation creep processes quantified in laboratory experiments (Ottosen 1986;Haupt and Schweiger 1989;Aubertin et al 1991;Cristescu 1993;Munson 1979Munson , 1997Jin and Cristescu 1998;Hampel et al 1998;Hampel and Schulze 2007;Hunsche and Hampel 1999;Peach et al 2001;Fossum and Fredrich 2002). The steady-state strain rate is related to the flow stress r using a power law creep (non-Newtonian) equation:…”

Current issues and new developments in deformation mechanisms, rheology and tectonics

“…6) involving deformation by a combination of dislocation creep on the weak slip system and grain boundary sliding (Hirth & Kohlstedt 1995). Dynamic recrystallization can result in other types of softening including, geometric softening (Ion et al 1982; and structural softening (Urai et al 1986;Peach et al 2001). Grainsize mm Fig.…”

“…Recrystallization during deformation has a strong effect on microstructure and texture development (Av6 Lallemant 1975;Karato 1988;Wenk et al 1997;Herwegh et al 1997) and may have an important influence on rheology of all types of materials (White 1977;White et al 1980;Urai et al 1986;Peach et al 2001). …”