Fluid circulation in the depths of accretionary prisms: an example of the Shimanto Belt, Kyushu, Japan

Abstract

International audienceAccretionary prisms constitute ideal targets to study fluid circulation and fluid-rock interactions at depths beyond the reach of active margin deep drilling. The highest-grade rocks from the Shimanto Belt on Kyushu were buried under 3-5 kbars at ~ 300°C (Toriumi and Teruya, 1988). They contain abundant quartz veins, formed throughout burial and exhumation and variably affected by brittle and ductile deformation.Cathodoluminescence (CL) reveals the existence of two distinct types of quartz, characterized by a blue and brown color, respectively. CL-blue quartz fills macro-veins (width ≥ 10μm), while CL-brown quartz is present in micro-veins (width ~ 1 − 10μm) and ductilely recrystallized domains. On the basis of microstructures, the fluids associated with the CL-blue and CL-brown quartz are interpreted as “external” and “local”, respectively. Quartz growth rims of alternating CL colors as well as mutually cross-cutting veins show that the two fluids cyclically wetted the host rock.From fluid inclusions analysis, the fluid associated with CL-blue quartz has a salinity similar to seawater, while the fluid associated with CL-brown quartz is less saline. In addition, CL-blue quartz is richer in aluminum than the CL-brown one. In contrast to the salinity/aluminum signature, the δ18O isotopic signature of both quartz types is similar and buffered by host rock. The difference between the preservation of the salinity signature of the fluid and the loss of its δ18O signature is explained by quicker exchange kinetics and larger host rock buffering capacity for isotopic reequilibration.The “local” fluid, associated with CL-brown quartz, reflects the dilution of pore water by the pure water produced by prograde dehydration reactions of clay minerals. The “external” fluid associated with CL-blue quartz is interpreted as seawater or pore water from shallow (depth<1-2 km below seafloor) sediments. We propose that downward percolation of shallow water to depths ~ 10km is a transient process associated with mega-earthquakes

Evolution in H2O contents during deformation of polycrystalline quartz: An experimental study

“…It has been detected in quartz grains of gneiss (Gleason and DeSisto,538 2008) and in cherts (Ito and Nakashima, 2002) and is attributed by several authors to OH 539 groups bonded with Al +3 substituting for Si (Pankrath, 1991;Stalder and Konzett, 2012) 540 Kronenberg, 1994;Suzuki and Nakashima, 1999;Gleason and DeSisto, 2008). This 541 interpretation is supported in our case by the high content of aluminum in Hyuga quartz 542 (Raimbourg et al, 2015). 543 The band at 3363 cm -1 is observed only in deformed and recrystallized Brazil quartz, either in 544 the matrix (Figure 9a) or at recrystallized tails of porphyroclasts (e.g.…”

“…Hyuga quartz has a milky appearance due to the great abundance of many tiny fluid 206 inclusions . These inclusions range from a few to 10µm in diameter 207 (Raimbourg et al, 2015) and are heterogeneously distributed in the porphyroclasts which look 208 dark in thin section and grain mounts (Figure 1b-1). As shown by FTIR spectra, OH content 209 is variable as a consequence of the distribution of fluid inclusions (Figure 1b-1 (Figure 1b-3).…”

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

“…The thrust is well exposed along the coast of Nobeoka city in Miyazaki Prefecture across the deeper Morotsuka and Kitagawa Groups (hanging wall) in the north and the shallower Hyuga Group (footwall) in the south (e.g. Kondo et al, ; Okamoto, Kimura, Takizawa, & Yamaguchi, ; Okamoto, Kimura, Yamaguchi, Yamaguchi, & Kusaba, ; Raimbourg, Shibata, Yamaguchi, Yamaguchi, & Kimura, ; Raimbourg et al, ). It is inferred to have been active from 48 Ma to 40 Ma (Hara & Kimura, ).…”

“…The footwall rock is composed of a mélange of shale matrix with sandstone and basaltic blocks from the Eocene to early Oligocene Hyuga Group, brittlely deformed by cataclasis accompanied by pressure solution within the quartz aggregates in the sandstone blocks (Kondo et al, 2005). Cataclastic composite planar fabric and abundant small faults are distributed across the footwall strata and are occasionally filled by mineral veins of quartz and carbonate (Kawasaki et al, 2017;Kondo et al, 2005;Otsubo et al, 2016;Raimbourg et al, 2015;Yamaguchi et al, 2011).…”

K‐Ar Dating of Fossil Seismogenic Thrusts in the Shimanto Accretionary Complex, Southwest Japan

“…We determine the K‐Ar ages of authigenic illite within three cataclastic fault zones from the Shimanto belt of Japan (Figure ). These fault zones are situated at the top of regionally extensive zones of tectonic mélange—the Goshikinohama Fault at the top of the Yokonami mélange (Hashimoto et al, ; Hashimoto et al, ), the roof thrust of the Okitsu mélange (Ikesawa et al, ), and the Nobeoka Thrust at the top of the Hyuga mélange (Kondo et al, ; Raimbourg et al, ). We describe the tectonic setting of these three examples, and we compare the timing of fault slip to constraints on the timing of underthrusting and accretion.…”