Mechanics of grain‐size reduction in fault zones

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Abstract
Recent observations of nanometer‐scale particles in the cores of exhumed fault zones raise questions about how such small particles are formed and how they survive, especially if significant shear heating is produced during an earthquake. Commercial crushing and grinding operations encounter a grind limit near 1 μm below which particles deform plastically rather than fracturing. A fragmentation model and low‐temperature plasticity mechanics indicate that it is not possible to produce under compressive loading and short timescale significantly smaller particles at any strain rate. However, shock loading and subcritical crack growth can produce nanometer‐scale fragments in compression. Under tensile loading the fragment size is determined by a competition between the nucleation of cracks and stress relaxation in their neighborhoods. Therefore higher tensile strain rates produce smaller fragments. The ultimate limit is determined by the availability of elastic strain energy, which does not place a significant constraint on the minimum grain size. Grain growth kinetics suggests that survivability of grains is very temperature sensitive. A 10 nm quartz fragment will double its size in 0.1 s at 1000°C, in 20 s at 800°C, in 14 h at 600°C, and in 10 years at 400°C. The observation of grains smaller than 10 nm places meaningful constraints on the dynamic fields and permeability of the fault zone during a large earthquake. Microstructural analysis of the grains and rock damage may be used to infer whether fragmentation occurred under macroscopic tension or compression.

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