Physical property anisotropy of foliated fault rocks: Study from the Nobeoka Thrust, Shimanto Belt, southwest Japan

Abstract

To investigate the physical property anisotropies of foliated fault rocks in subduction zones, the hanging wall phyllites and footwall cataclasites exhumed along the Nobeoka Thrust, a fossilized out‐of‐sequence‐thrust in the Shimanto Belt, Japan, was focused. Discrete physical property (electric resistivity, P‐ and S‐wave velocities, and porosity) measurements were conducted employing geologic coordinates (depth‐parallel direction, strike direction, and maximum dip direction of foliation), using the core samples obtained from the Nobeoka Thrust Drilling Project and compared the data to borehole geophysical logs. A higher sample P‐wave velocity (Vp), lower S‐wave velocity (Vs), higher Vp/Vs, and lower sample porosity and resistivity compared to the logs, are inferred to have been caused by the larger sampling scale of the logs and lower fluid saturation of the borehole. The phyllites and cataclasites exhibited substantial vertical and horizontal anisotropy of Vp (0.4–17.3 % and 2.7–13.8 %, respectively), Vs (0.5–56 % and 7.7–43 %, respectively), and resistivity (0.9–119 % and 2.0–65.9 %, respectively). The physical property anisotropies are primarily affected by the dip angles of foliation. The fault rocks that have gentler dip angles exhibit a higher Vp in the strike and maximum dip direction and a lower Vp in the depth‐parallel direction. In contrast, the fault rocks that have steeply dipping structures show a higher Vp in the strike and depth‐parallel directions with a lower velocity in the maximum dip direction. Resistivity anisotropy show a trend opposite to that of the Vp in relation to the dip angles. Our results show lower Vp anisotropy than those obtained in previous studies, which measured wave speeds perpendicular or parallel to foliation under confining pressure. This study highlights the significance of dip angles on vertical properties in geophysical surveys across foliated fault rocks.

P‐ and S‐Wave Velocities of Exhumed Metasediments From the Alaskan Subduction Zone: Implications for the In Situ Conditions Along the Megathrust

“…These physical properties have been investigated via scientific ocean drilling at shallow depths (Dugan et al, 2017;Moore et al, 2001) and studies of exhumed plate boundary fault rocks of ancient accretionary prisms (Byrne, 1986;Byrne & Fisher, 1987;Hamahashi et al, 2018;Rowe et al, 2009Rowe et al, , 2013Vannucchi & Maltman, 2000). Despite extensive laboratory experimental investigations of the mechanical, hydrological, and frictional behavior of both natural and synthetic fault and protolith material (e.g., Ikari et al, 2015;Kopf & Brown, 2003;Saffer & Marone, 2003), there are limited comparable data for appropriate subduction zone interface materials that define elastic properties, their anisotropy, or variations with stress state needed for rigorous and quantitative interpretation of in situ state and conditions from geophysical data in subduction systems (Hashimoto et al, 2010;Raimbourg et al, 2011).…”