NANO-SCALE DIFFUSION-ACCOMMODATED WEAKENING TRIGGERED BY FRICTIONAL SLIDING DURING EARTHQUAKES IN CARBONATE ROCKS?

The causes and grain-scale expression of dynamic weakening along faults at seismic slip rates (v ~ 1m/s) remain enigmatic. We present mechanical data and microstructural (SEM, TEM) observations from displacement-controlled friction experiments performed in a rotary shear apparatus on calcite gouges at seismic slip rates and normal stresses up to 18MPa. Friction initially increases to peak values (m=0.75) before decaying to lower steady-state values (m=0.2), with a typical slip distance d of ~20cm. Up to 80% of the peak friction is recovered during deceleration to arrest.

Microstructural observations on samples deformed up to the attainment of peak friction (μ=0.75) show the development of a localised, porous slip zone, <50 mm thick, with multiple striated slip surfaces coated by sub-rounded nano-grains. Although seismic slip rates (v > 0.7 m/s) are attained, no dynamic weakening is observed. Once the steady-state stage is established (μ=0.2), a 50 mm thick, low porosity cohesive slip zone develops composed of juxtaposed domains of nano-grains bounded by multiple, striated slip surfaces. The sub-micron domains comprise interlocking calcite grains with distributed sub-grains < 20nm in diameter, with straight, polygonal boundaries displaying 120° triple junctions. Textural evidence for dynamic recrystallization by dislocation creep is absent and slip zone internal architecture is not obliterated by the polygonal fabric development, i.e. no evidence of annealing.

We propose that cataclastic reduction down to nanoscale grainsizes and intense frictional heating due to shear localisation may have activated transient diffusion-dominated grain boundary sliding (GBS) mechanisms leading to dynamic weakening in the slip zone. Using constraints from the experimental conditions (strain rate, T) and microstructural observations (d), we calculated the predicted strengths using a range of constitutive flow laws for grainsize-sensitive and -insensitive mechanisms. At the very high strain rates achieved during the experiments, the flow law for Coble creep is the only one that predicts the measured strengths. The re-strengthening observed during the decelerating shearing phase is explained by falling temperatures “switching off” the nano-scale GBS leading to a return to frictional sliding and cataclasis.