Contrasts in physical properties between the hanging wall and footwall of an exhumed seismogenic megasplay fault in a subduction zone—An example from the Nobeoka Thrust Drilling Project

Abstract

[1] We examined the physical properties of an exhumed and fossilized subduction zone megasplay fault by analyzing geophysical logging data obtained by the Nobeoka Thrust Drilling Project, which provide a high-resolution transect of properties across the main fault zone. The footwall cataclasite exhibits higher averages of neutron porosity (7.6%) and lower values of electric resistivity (232 Xm) compared to the hanging wall phyllite (4.8%, 453 Xm). This clear contrast between the hanging wall and footwall may account for the difference in maximum burial and structural variation. Despite the contrast observed between the hanging wall and footwall in macroscopic scale, the resistivity and porosity data from both the hanging wall and footwall can be fit with a single curve using Archie's law, suggesting the similarities in microstructures and mineralogy in this low porosity range. Above the main fault core of the Nobeoka Thrust a brittle damage zone in the hanging wall contains pseudotachylyte as evidence of the seismogenic slip and does not follow Archie's law. Damage zones in the hanging wall are also observed in the modern splay fault at shallow depth in the Nankai Trough but with much thicker width, whereas the footwall damage zone is more extensive in the Nobeoka Thrust. Splay faults may exhibit strong deformation in the hanging wall in the early stage, and as fault rocks get buried deeper and as displacement and physical property contrast increase across the fault, the damage effect may eventually be enlarged in the footwall.

Changes in illite crystallinity within an ancient tectonic boundary thrust caused by thermal, mechanical, and hydrothermal effects: an example from the Nobeoka Thrust, southwest Japan

“…Such a displacement is comparable to that of the deep portion of the modern megasplay fault in the Nankai Trough (Kondo et al 2005;Hamahashi et al 2013). Several detailed structural investigations focused on good surface exposure have been performed (Kondo et al 2005;Okamoto et al 2006;Yamaguchi et al 2011;Kimura et al 2013).…”

“…(b) A simplified geologic map of the Shimanto Belt in northeast Kyushu, modified from Murata (1998). (c) Geologic map of the studied area and the locality of the drilling site (dot) for the hanging wall of the Nobeoka Thrust (modified from Kondo et al 2005;Kimura et al 2013;Hamahashi et al 2013). shales and sandstones (Okumura et al 1985;Kondo et al 2005), whereas the footwall is the Hyuga group, consisting of a cataclastic mélange of shale matrix with sandstone and basaltic blocks (Nishi 1988;Kimura et al 1991;Kondo et al 2005). Microfossil assemblages indicate the depositional age to be from middle Eocene to early Oligocene (Saito et al 1996).…”

Multiple damage zone structure of an exhumed seismogenic megasplay fault in a subduction zone - a study from the Nobeoka Thrust Drilling Project

“…In addition to physical property measurements on samples from the outcrop of the Nobeoka Thrust (Tsuji et al 2006), Hamahashi et al (2013) documented physical properties near the main fault core from geophysical logs. Clear structural and physical property contrasts across the thrust were characterized, which are partially due to different maximum burial depths of the hanging wall and footwall ).…”

Temporal stress variations along a seismogenic megasplay fault in the subduction zone: An example from the Nobeoka Thrust, southwestern Japan

“…The Nobeoka Thrust (Figure ), an on‐land example of an ancient megasplay fault, provides an excellent record of deformation at seismogenic depths (Kondo et al, ). The Nobeoka Thrust Drilling Project (NOBELL) sampled fault rock to study the mechanisms of megasplay faults and the geophysical and geochemical properties of the Nobeoka Thrust (Hamahashi et al, ).…”

“…(a) Geologic setting of the N obeoka T hrust and distribution of the S himanto B elt in southwest J apan. (b) Simplified geologic map of the S himanto B elt in northeast K yushu, modified from Murata () and Hamahashi et al (). (c) Geologic map of the study area, modified from Kondo et al () and Kimura et al ().…”