Experimental approaches to the study of deformation/metamorphism relationships

Abstract

Rock deformation and metamorphism can interact at the mechanistic level in the following ways: (a) facilitation of cataclasis through the release of high-pressure fluid during dehydration and decarbonation reactions; (b) facilitation of intracrystalline plasticity through the stresses induced during solid-state volume changes; (c) enhanced deformability through the transient existence of fine-grained reaction products; (d) modification of chemical potential gradients driving diffusion if a reaction can occur along the diffusion path; (f) changes in the resistance to intracrystalline plasticity through the effect of reaction-induced changes, pore fluid pressure and chemistry on point defect chemistry of the solid phases.Examples of experimental studies of each of these types of interaction are described. Special experimental problems arise through: (i) the effects of solid phase and pore space volume changes, and their effects on pore fluid pressure and measured strain; (ii) the effects of such microstructural changes on the determination of flow law parameters; and (iii) in many instances the need for very long duration deformation experiments if reaction kinetics are sluggish.There is an outstanding need for experimental studies of the effects of non-hydrostatic stress on the conditions for the onset of metamorphic reactions and phase transformations, as a basis for understanding some classes of deformation/metamorphism interaction. However, it is emphasized that the threefold classification of rock deformation mechanisms into cataclastic, crystal-plastic, and diffusive mass transfer processes, established from the study of deformation of monomineralic rocks, forms an essential framework for the understanding of deformation/metamorphism inter-relationships.

Episodic weakening and strengthening during synmetamorphic deformation in a deep-crustal shear zone in the Alps

“…Strain is initially produced by glide of dislocations, leading to production of dislocation tangles and strain hardening; recovery processes (dynamic recrystallization and dislocation climb) act to offset strain hardening by reducing internal strain energy. Processes associated with dislocation creep in quartz-and feldspardominated rocks have been documented both experimentally and in natural samples (Simpson 1983;Wintsch & Knipe 1983;Rutter & Brodie 1988;Hacker & Christie 1990;Tullis et al 1990;Hirth & Tullis 1992).…”