Frictional Behavior and Slip Localization in Simulated Faults of Halite at Sub-seismic to Seismic Slip Rates

Halite exhibits deformation behavior ranging from brittle to plastic at room temperature and at low pressures, and has been used to simulate deformation processes of the brittle-ductile transition zone. However, previous experiments on halite were performed at very low slip rates (10-9-10-3 m/s), requiring friction data at seismic slip rates for a more complete assessment of the applicability of the experimental results to natural earthquakes. We conducted friction experiments on halite at slip rate of 0.02-1.3 m/s and normal stresses of 0.8- 10.0 MPa using a high-velocity rotary shear apparatus. A thin layer (0.6-1.0 mm thick) of halite gouge was inserted between precut rock cylinders jacketed with Teflon sleeve. We found that mechanical behavior and deformation processes of halite gouge are remarkably different depending on slip rate and that frictional melting and dislocation creep can occur simultaneously at seismic slip rates. At sub-seismic slip rates of 0.02 to 0.05 m/sec, peak friction (μp = 0.76-0.85) was followed by steady-state friction (μss = 0.35-0.37). Gouge layer consists of a thin slip localization zone at the halite gouge-rock contact and a thick low slip-rate zone. The low slip-rate zone shows evidence for cataclastic flow with angular fragments set in a fine matrix. In contrast, the slip localization zone consists of very fine gouge with some remnants of fragments. At seismic slip rates of 0.1 to 1.3 m/sec, μp (0.64-0.99) was followed by μss (0.36-0.03). μss decreases with increasing slip rate. The shear zone consists of a thin slip localization zone at the halite gouge-rock contact and a thick low slip-rate zone. The low slip-rate zone consists of polycrystalline halite ribbons highly elongated obliquely to shear zone boundary and the oblique foliation is dragged into the thin slip localization zone. Each ribbon is also defined as a lattice preferred orientation domain by electron back-scattered diffraction (EBSD) analysis. Individual grains within the halite ribbons are also elongated with their long axis subparallel to the ribbons and their grain boundaries are either straight or slightly wavy. The size of the grains within the ribbons gradually decreases toward the slip localization zone. These microfabrics indicate that each halite ribbon results from plastic deformation of the original halite grain, with individual grains within the ribbon representing product of dynamic recrystallization. Their straight or slightly wavy grain boundaries suggest a static adjustment during cooling after deformation. In contrast, the thin slip localization zone consists of very fine euhedral grains (1-3 μm) at the margin of layer. We interpret that the very fine euhedral grains have grown from frictional melt and the plastic deformation in the low slip-rate zone was enhanced by heat conduction from the slip localization zone. Our experimental results suggest that pseudotachylyte and mylonite may develop simultaneously by 'slip-rate partitioning' at seismic slip rates, providing a new insight into the interpretation of their coexistence. Also, in view of preexisting halite friction data at lower slip rates, halite gouge exhibits a slight velocity weakening at low velocity regime (10-9-10-5 m/s), velocity strengthening at intermediate velocity regime (10-5-10-3 m/s), steady state at sub-seismic velocity regime (10-2-10-1 m/s) and remarkable velocity weakening at seismic velocity regime (> 10-1 m/s).