Postseismic fluid discharge chemically recorded in altered pseudotachylyte discovered from an ancient megasplay fault: an example from the Nobeoka Thrust in the Shimanto accretionary complex, SW Japan

Ryota Hasegawa,Gaku Kimura,Tsuyoshi Ishikawa,Rina Fukuchi,Nobuhiro Ogawa,Yujin Kitamura,Yohei Hamada,Asuka Yamaguchi,Juichiro Ashi

doi:10.1186/s40645-019-0281-2

Abstract

Megasplay fault branching from plate boundaries of subduction zones is thought to be important sources of earthquakes and tsunamis. In this study, we performed structural and geochemical analyses on a fossilized megasplay fault (the Nobeoka Thrust of the Shimanto accretionary complex) to understand fluid-rock interaction and how the splay fault plays a role in fluid flow in the seismogenic zone. As a result of structural observations, we report that the principal slip zone (PSZ) of the Nobeoka Thrust is composed of foliated cataclasite originating from a sandstone-shale mélange and includes a thin (~ 1.5 mm thick) pseudotachylyte layer. Major and trace element composition analysis and EPMA element mapping revealed that the pseudotachylyte is enriched in Li and Cs within the PSZ, as well as in the slip zone of a minor fault in the footwall. Li and Cs enrichment in pseudotachylyte is interpreted as a result of fluid-rock interaction in the postseismic stage, because such an anomaly only results from a large fluid/rock ratio (R > 512–24 at 250–350 °C) under the influence of Li- and Cs-enriched fluids. The amount of fluid that reacted with the pseudotachylyte is estimated to be 1.78 × 100 to 7.61 × 103 m3.

Highlights

Megasplay fault branching from plate boundaries is thought to be one of the origins of subduction zone earthquakes
We investigated the record of fluid-rock interaction by applying geochemical analysis to the fault rocks of the Nobeoka Thrust in the Shimanto accretionary complex, Japan, which is presumed to be an inland analog of a megasplay fault in a subduction zone setting
We aim to clarify the fluid volume that likely reacted with fault rocks and the role that the megasplay fault plays in fluid flow in fault zones and subduction zones

Summary

Introduction

Megasplay fault branching from plate boundaries is thought to be one of the origins of subduction zone earthquakes. The megasplay fault of the Nankai Trough was the source of the 1944 Tonankai earthquake and the associated tsunami (Park et al 2002). Sibson (1973) proposed thermal pressurization as a factor in dynamic fault weakening during an earthquake. From theoretical and experimental investigations (e.g., Mase and Smith 1987; Rice 2006; Bizzarri and Cocco 2006), thermal pressurization is assumed to be one of the factors that causes a dynamic fault weakening at the time of an earthquake. Sibson (1975) proposed a seismic pumping model, i.e., fluid flows into cracks formed by faulting within a high pore pressure zone. Such fluid flow occurs every time faulting occurs

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