Large amount of fluid migration around shallow seismogenic depth preserved in tectonic mélange: Yokonami mélange, the Cretaceous Shimanto Belt, Kochi, Southwest Japan

Abstract

Studying subduction zone fluid at shallow seismogenic depths is important to understand the nature of fault rocks at the updip limit of the seismogenic zone because fluid-rock interactions affect heat and mass transfer, and fault strength. In this study, we conducted detailed analyses of distribution of shear veins, and estimation of pressuretemperature conditions for shear vein formation for the Yokonami mélange, Shikoku, Southwest Japan, which is tectonic mélange zone in an on-land accretionary complex. We found a seismogenic fault at the upper boundary of the Yokonami mélange, indicating that the Yokonami mélange was active at seismogenic depth. The field-transect distribution of shear veins was examined. The frequency, the total and mean thicknesses of the shear veins were about 3.7 per meter, about 10 mm per meter, and about 3 mm per shear vein, respectively. Quartz within the shear veins shows elongate-blocky textures, suggesting precipitation from advective flow. The pressure and temperature conditions for shear vein formation were examined by fluid inclusion analysis, ranging 175-225°C and 143-215 MPa, respectively. The temperature is almost consistent with the paleotemperature determined from vitrinite reflectance, suggesting that the shear veins were formed at up to the maximum depth. The depth might be consistent with that where the seismogenic fault was formed. On the basis of the pressure and temperature conditions and the distribution of shear veins, we estimated the minimum volumetric ratio of fluid to host rocks, assuming that the shear veins had precipitated from advective flow. The estimated amount of fluid is about 106 m 3 per cubic meter of host rocks. The results suggest that a large amount of fluid migrates through mélange zones at shallow seismogenic depths. This fluid possibly originates from the dehydration of clay minerals from underthrusted sediments and an altered subducting slab.

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

“…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). …”

Geological evidence for shallow ductile-brittle transition zones along subduction interfaces: example from the Shimanto Belt, SW Japan

“…Within the mélange zone, the same shear zones may be stacked up by duplex structures to form underplated accretionary complexes (e.g., Hashimoto and Kimura, 1999). The thickness of each horse of ocean floor stratigraphy has been reported to be up to a few hundred meters in the mélange zone focused on in this study Hashimoto and Kimura, 1999;Ikesawa et al, 2005;Hashimoto et al, 2012). Although mélanges have been suggested to be sedimentary, tectonic, and diapiric, most mélange zones in the Shimanto Belt are inferred to be tectonic mélanges (e.g., Kimura and Mukai, 1991;Onishi and Kimura, 1995;Hashimoto and Kimura, 1999), with some exceptions (Osozawa et al, 2009).…”

“…Displacement along the shear vein is relatively small, that is, less than 1 m. Veins consist of quartz and calcite. Matsumura et al (2003) and Hashimoto et al (2012) presented the one-dimensional distribution of shear veins in the Mugi mélange and the Yokonami mélange, which are both in the Cretaceous Shimanto Belt, Shikoku Island. The number of shear veins in a 1-m interval perpendicular to the mélange foliations is approximately 2 to 3.…”

“…Studies have been conducted to connect geologic deformation features from on-land accretionary complexes with seismic and aseismic deformations or with the newly found failure styles (e.g., Ikesawa et al, 2003;Kimura et al, 2007;Meneghini et al, 2010;Fagereng and Toy, 2011;Saito et al, 2013). Pseudotachylyte and fluidized shear zones, which indicate dynamic weakening of the fault during displacement, were found in on-land accretionary complexes; this suggests that the geologic features of the seismogenic slips along subduction interfaces are visible in on-land accretionary complexes (e.g., Ikesawa et al, 2003;Kitamura et al, 2005;Ujiie et al, 2007;Meneghini et al, 2010;Hashimoto et al, 2012;Saito et al, 2013). Further detailed studies of onland accretionary complexes are needed to understand the nature of deformation features and how they relate to variable slip styles along subduction plate interfaces.…”

“…The spatiotemporal relation between plastic and brittle deformations, however, has been expressed as deformations in a heterogeneous mélange rheology (e.g., Fagereng and Sibson, 2010). Extension veins related to mélange fabrics and shear veins cutting the mélange fabrics have been well documented, with detailed pressure-temperature conditions from the Mugi mélange, Miyama formation, and Yokonami mélange in the Cretaceous Shimanto Belt (e.g., Hashimoto et al, 2002Hashimoto et al, , 2003Matsumura et al, 2003;Hashimoto et al, 2012) (see locations in Figure 1); these data can be a key in understanding the relation between ductile deformation for mélange and brittle failure (shear veins) in lithified shale matrices. In this study, we summarize the differences between observed deformation features and inferred deformation mechanisms for ductile mélange and shear vein formations in detail, with reference to published pressuretemperature conditions.…”