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

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 × 10 0 to 7.61 × 10 3 m 3 .

Pseudotachylyte Veins in Accretionary Complexes: Melt or Mechanical Wear?

“…These two complexes investigated here are formed of stacks of units composed either of coherent turbidites or tectonic mélange units, bounded by thrust faults 35,36 . Some of these fault zones (two in Shimanto and one in Kodiak Accretionary Complex), contain Black Fault Rocks bearing characteristic features of pseudotachylytes 8,[12][13][14] . Black Fault Rocks are vitreous black thin veins, partially or totally devitrified 11 , which cut sharply across, with straight boundaries, the host-rock that often contains itself brittle microstructures.…”

“…Based on this straightforward idea, we developed a new high-resolution spatialized approach to Raman Spectroscopy, to unravel how the carbonaceous material crystallinity is distributed in space within rock samples. We applied this approach to three Black Fault Rocks described as pseudotachylyte in two exhumed accretionary complexes, the Shimanto Belt 8,12 in Japan and the Kodiak Accretionary Complex 13,14 in Alaska, in order to discriminate the respective effect of temperature increase and strain, and eventually to decipher the mechanical processes at the origin of the Black Fault Rocks.…”

“…Under SEM and TEM, the matrix seems composed of glassy amorphous material with vesicles and amygdules occurrences 10,13,37 . In the Nobeoka fault vein, EPMA element maps exposes flow textures 12 . Inside the matrix, scattered or aligned micro-to nanomicrolites and Fe-rich, Ti-rich sulfides are observed [12][13][14] .…”

“…Pseudotachylyte veins present typical features as injection structures, amorphous matrix or microlites 5 . More recently, Black Fault Rocks have been described in accretionary prisms [8][9][10][11][12][13][14] with macro-and micro-structures similar to the pseudotachylytes reported in crustal rocks 5 . These Black Fault Rocks are described as fault core filling material in zones of intense deformation of the host-rock at low to very low temperature conditions ranging from 160°C to 320°C.…”

Identification of hydrogeological evolution using hydrogeology-seismology analysis of groundwater head fluctuation related to the 1999 MW = 7.5 Chi-Chi earthquake

“…Several studies have discussed the abnormal groundwater head changes in response to this earthquake (Chia et al 2008;Wang et al 2016;Wang et al 2004b). However, large earthquakes may cause partial aquifer/aquitard rupture and an increase in the permeability of aquifer/aquitard (Elkhoury et al 2006;Hasegawa et al 2019;Katayama et al 2015;Ujiie and Kimura 2014), but few previous studies considered the earthquake-produced leaking/injecting effect on the post-seismic changes in groundwater head drawdown. Moreover, the post-seismic pore-elastic mechanical parameters and hydrogeological parameters associated with excess pore-water pressure release and effective stress relief should be in evolution, so many parameters may increase the uncertainties and error of simulating the drawdown in groundwater head.…”