Sources and physicochemical characteristics of fluids along a subduction‐zone megathrust: A geochemical approach using syn‐tectonic mineral veins in the Mugi mélange, Shimanto accretionary complex

Abstract

[1] The Mugi mélange in the Shimanto accretionary complex, southwest Japan, records faulting and fluid flow patterns at the updip limit of the seismogenic region of the Nankai subduction zone. To characterize the origin and behavior of syn-tectonic fluids, we investigated the carbon, oxygen, and strontium isotopic compositions, and rare earth element (REE) patterns of syn-tectonic calcite within veins along fault zones in the mélange, as well as the Sr isotopic compositions and REE patterns of surrounding host rocks. With the exception of intra-basalt veins formed prior to subduction, the d 13 C values of veins range from À10‰ to À19‰, suggesting a mixed carbon source (i.e., marine carbonate and organic matter). The vein-forming fluids have positive oxygen isotopic compositions (+2‰ to +9‰ (SMOW)) and high 87 Sr/ 86 Sr values (0.70794-0.70850), suggesting that the source was rock-buffered fluids affected both by terrigenous sediments and altered oceanic crust. The veins found in filling the fault zone associated with tectonic underplating have different REE patterns to those of the other veins, implying a difference in physicochemical processes affecting the fault zone near the subduction megathrust.

Alteration and dehydration of subducting oceanic crust within subduction zones: implications for décollement step-down and plate-boundary seismogenesis

“…The fact that oceanic basalts are exposed on the upper surface of the subducting plate in an area of duplex underplating means that S-C reactions are thought to be an important source of fluid for the plate-boundary fault system. Yamaguchi et al (2012) Sr values of several types of crack-fill veins associated with basalts in the Mugi mélange and used these data to suggest that the fluids that formed these veins were strongly influenced by basalt buffered fluids, a finding that is consistent with the bulk oceanic basalt dehydration processes identified during this study. The fluids resulting from basalt dehydration may also widely circulate within the mélange and can form pervasive sets of mineral veins within the seismogenic zone (Hashimoto et al 2012).…”

Quartz deposition and its influence on the deformation process of megathrusts in subduction zones

“…These veins are dominated by quartz, but abundant calcite veins are also observed, particularly near the basalt-derived fault zone (Ujiie et al 2007;Yamaguchi et al 2012). Heating and cooling experiments of fluid inclusions trapped in quartz veins suggest that these veins formed at temperatures of 125°C to 245°C and pressures of 92 to 145 MPa (Matsumura et al 2003).…”

“…Crack-fill veins are ubiquitous in the Mugi mélange (Figure 2a), and occur as two types: extensional cracks in the constricted parts of sandstone boudins (classified as 'vein I' in Matsumura et al (2003) or 'boudin neck veins' in Yamaguchi et al (2012)), and cracks along shear fabrics, such as Riedel and Y-surfaces ('vein II' in Matsumura et al (2003)). These veins are dominated by quartz, but abundant calcite veins are also observed, particularly near the basalt-derived fault zone (Ujiie et al 2007;Yamaguchi et al 2012).…”

“…Basalt-hosted veins were excluded from our analysis because it was not possible to discriminate between veins formed before subduction, associated with near-ridge fluid circulation, from those formed after subduction. When estimating the volume fraction, we also excluded veins in sedimentary horizons around the basal basalt, since Sr isotope analysis suggested that some of these veins have been influenced by dehydration of the basalt (Yamaguchi et al 2012).…”

Earthquake faulting in subduction zones: insights from fault rocks in accretionary prisms

“…Fault zones in exhumed accretionary complexes record fluid-rock interaction processes, many of which are interpreted in terms of a seismic cycle or coseismic fluid-rock interactions (Okamoto et al 2006;Meneghini and Moore 2007;Ujiie et al 2007a;Ishikawa et al 2008;Hamada et al 2011;Yamaguchi et al 2011aYamaguchi et al , 2012Kimura et al 2013).…”