Fluid-enhanced low-temperature plasticity of calcite marble: Microstructures and mechanisms

Postmetamorphic carbonate fault rocks from the Damara orogen, Namibia, show the following features: (1) obvious contrasts between centimeter-scale grains in the clasts and nanometer- to micrometer-scale grains in the matrix, and between macroscopic cataclastic and microscopic mylonitic microstructures; (2) coexistence of tangled dislocations and organized dislocation walls; (3) occurrence of subgrains along margins of clasts and their transition into dynamically recrystallized grains in the matrix; and (4) new grains in grain sizes of a few nanometers to 3 μm in diameter. The coexistence of brittle and ductile microstructures is attributed to comprehensive intragranular twinning, and kinking, fracturing, and subsequent dislocation remobilization or reorganization and recrystallization. Fracturing is triggered by dislocation pileups due to dynamic loading, twinning, and kinking. It also generates free dislocations and tangled dislocations. Fractures provide fluid paths, increase fluid-rock interfaces, and enhance the possibility of fluid-rock interaction. Fracturing is subsequently accommodated by low-temperature plasticity that is attributed to hydrolytic weakening, i.e., fluid-enhanced recovery and dynamic recrystallization due to the infiltration of fluids into the deforming grains. During hydrolytic weakening, remobilized free dislocations and tangled dislocations climb toward incoherent boundary-like fractures. The dislocations are reorganized into dislocation walls that commonly constitute parts of subgrains developing into new grains. We conclude that: (1) fluids may increase the rate of dislocation glide and dislocation climb and may also enhance the recovery of strain-hardened rocks to accommodate fracturing processes in calcite marbles at low temperatures; and (2) calcite marble may have low-temperature plasticity and may undergo crystal plastic deformation due to hydrolytic weakening at shallow crustal levels.