Fault strength at the brittle-ductile transitional depth: Experimental study using deep fault materials along the Median Tectonic Line, Japan

Abstract:

To explore the fault strength at the middle crustal levels corresponding to the brittle-ductile transition zone, we conducted friction experiments of deep fault materials taken from the outcrops along the Median Tectonic Line (MTL) in SW Japan. The MTL is the longest exposed fault on the Japanese islands and has a long history of fault movements since the Early Cretaceous. Focusing on the influence of phyllosilicates in fault zones produced by hydrothermal activities, we prepared powdered samples of "mica-rich" and "chlorite-rich" cataclasites that contain total ~30 wt.% of mica and clay minerals. (The chlorite-rich cataclasite contains ~14 wt.% chlorite, and the mica-rich cataclasite contains ~14 wt.% white mica). Chlorite is a hydrous mineral commonly observed in fault zones, and is known to have low friction coefficients about 0.3 at high-pressure hydrothermal conditions (Okamoto et al., 2019, JGR). However, its mechanical effects in natural rock composition are almost unknown.

To simulate the deformation conditions at depth, large displacement friction experiments were conducted at temperature of 300°C, normal stress of 300 MPa under controlled pore fluid pressures, using the hydrothermal ring shear apparatus at Utrecht University. Sliding velocities were varied from 1 μm/s to 300 μm/s. The steady-state friction coefficients obtained in constant velocity tests were ~0.6–0.8. The effective stress law holds for cataclasite gouges in pore pressure and normal stress stepping tests. Velocity stepping tests yielded velocity-weakening or neutral behaviours, although existing laboratory data for pure chlorite gouges mostly show velocity-strengthening behaviours. Experimental products of the chlrite-rich sample exhibits microsctructural transition from "sheared" to "random" between slow (1 μm/s) and moderately fast (300 μm/s) runs. Natural chlrite-rich cataclasite along the MTL is also characterized by a random fabric with rounded crasts. These results indicate that fault strengths and sliding behaviours in the middle crusts are not drastically changed by the presence of phyllosilicates even if chlorite contents in fault zones are relatively high.