Vein-calcite–dominated fault rocks collected from several locations show evidence for intense intracrystalline plasticity and interface (twin and grain boundary) mobility, leading to dynamic recrystallization of calcite at temperatures (150–250 °C) significantly below those at which these features are commonly anticipated. These observations require a reappraisal of calcite deformation at low temperature, particularly the capability for dynamic recrystallization in the apparent absence of significant, thermally activated recovery processes. The cyclic introduction of coarse-grained calcite veins is observed to be essential for the initiation of intracrystalline deformation and associated dynamic recrystallization. The introduction of veins generates an essentially monomineralic rock of a grain size larger than the protolith. As a result, the mylonitization does not occur within a given protolith, but rather in the introduced secondary calcite. Through Hall-Petch–type grain- size–dependent dislocation interactions, stress is locally increased, and the resulting increase in dislocation densities promotes grain-boundary migration. The recognition that nominal high-temperature creep processes and associated microstructures can occur outside their expected temperature range has implications for fault rheology (strength) and fault permeability and porosity.

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