Hashimoto and Yamano Earth, Planets and Space 2014, 66:141 http://www.earth-planets-space.com/66/1/141 LETTER Open Access Geological evidence for shallow ductile-brittle transition zones along subduction interfaces: example from the Shimanto Belt, SW Japan Yoshitaka Hashimoto* and Natsuko Yamano Abstract Tectonic mélange zones within ancient accretionary complexes include various styles of strain accommodation along subduction interfaces from shallow to deep. The ductile-brittle transition at shallower portions of the subduction plate boundary was identified in three tectonic mélange zones (Mugi mélange, Yokonami mélange, and Miyama formation) in the Cretaceous Shimanto Belt, an on-land accretionary complex in southwest Japan. The transition is defined by a change in deformation features from extension veins only in sandstone blocks with ductile matrix deformation (possibly by diffusion-precipitation creep) to shear veins (brittle failure) from shallow to deep. Although mélange fabrics represent distributed simple to sub-simple shear deformation, localized shear veins are commonly accompanied by slickenlines and a mirror surface. Pressure-temperature (P-T) conditions for extension veins in sandstone blocks and for shear veins are distinct on the basis of fluid inclusion analysis. For extension veins, P-T conditions are approximately 125 to 220°C and 80 to 210 MPa. For shear veins, P-T conditions are approximately 185 to 270°C and 110 to 300 MPa. The P-T conditions for shear veins are, on average, higher than those for extension veins. The temperature conditions overlap in the range of approximately 175 to 210°C, which suggests that the change from more ductile to brittle processes occurs over a range of depths. The width of the shallow ductile-brittle transition zone can be explained by a heterogeneous lithification state for sandstone and mudstone or high fluid pressure caused by clay dehydration, which is controlled by the temperature conditions. Keywords: Shallow ductile-brittle transition zone; Tectonic mélange; Shear vein; Pressure-temperature condition; Accretionary complex; Shimanto Belt Findings mechanical behavior of wedge materials and plate bound- Introduction aries (e.g., Wang and Hu, 2006). The physical properties of sediments evolve during un- The seismogenic zone has been described as the derthrusting along subduction thrust interfaces because temperature-controlled zone of plate boundary coupling, of mechanical (e.g., compaction) and chemical (e.g., de- where interplate earthquakes nucleate (e.g., Hyndman hydration and cementation) processes (e.g., Moore and and Wang, 1993). Recently, additional types of failure Saffer, 2001). Each of these components of the lithifica- have been found along active subduction zones, such as tion process can contribute to changes in deformation slow slip events (e.g., Hirose et al., 1999; Kato et al., 2012; features and deformation mechanisms (Knipe, 1989). The Ito et al., 2013), episodic tremor and slip (e.g., Rogers and evolution of physical properties of sediments associated Dragert, 2003; Obara et al., 2004, Ishida et al., 2013), very with changes in deformation features and deformation low frequency earthquakes (e.g., Ito and Obara, 2006; mechanisms are reflected in both wedge geometry and the Sugioka et al., 2012), and dynamic overshoot to the trench kinematics of earthquakes brought on by changes in the axis (e.g., Ide et al., 2011). Studies have been conducted to connect geologic deformation features from on-land ac- cretionary complexes with seismic and aseismic deforma- * Correspondence: [email protected] tions or with the newly found failure styles (e.g., Ikesawa Department of Applied Science, Faculty of Science, Kochi University, et al., 2003; Kimura et al., 2007; Meneghini et al., 2010; Akebonocyo 2-5-1, Kochi 780-0915, Japan © 2014 Hashimoto and Yamano; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Hashimoto and Yamano Earth, Planets and Space 2014, 66:141 Page 2 of 9 http://www.earth-planets-space.com/66/1/141 Fagereng and Toy, 2011; Saito et al., 2013). Pseudotachy- understanding the relation between ductile deformation lyte and fluidized shear zones, which indicate dynamic for mélange and brittle failure (shear veins) in lithified weakening of the fault during displacement, were found in shale matrices. In this study, we summarize the differences on-land accretionary complexes; this suggests that the between observed deformation features and inferred de- geologic features of the seismogenic slips along subduc- formation mechanisms for ductile mélange and shear vein tion interfaces are visible in on-land accretionary com- formations in detail, with reference to published pressure- plexes (e.g., Ikesawa et al., 2003; Kitamura et al., 2005; temperature conditions. We propose that the shallow Ujiie et al., 2007; Meneghini et al., 2010; Hashimoto et al., ductile-brittle transition zone is reflected by the change 2012; Saito et al., 2013). Further detailed studies of on- from extension veins within sandstone lenses contained in land accretionary complexes are needed to understand the ductile shale matrices to shear veins in brittle shale nature of deformation features and how they relate to matrices. Finally, we discuss the interpretation of the variable slip styles along subduction plate interfaces. ductile-brittle transition zone in shallower portions of The evolution of deformation features and deform- the subduction plate interface. ation mechanisms along subduction zones has been re- vealed from on-land geology in exhumed accretionary Tectonic mélanges complexes, not only in the Shimanto Belt (e.g., summa- Mélanges are characteristic components of on-land ac- rized in Kimura et al., 2007) but also in other accretion- cretionary complexes. A mélange is defined as a map- ary complexes (e.g., Fisher and Byrne, 1987; Fagereng scale geologic body with coherent blocks in a sheared and Toy, 2011; Rowe et al., 2013). By reviewing many matrix texture. Blocks are composed mainly of sand- previous studies, Fagereng and Toy (2011) pointed out stone or oceanic material such as basalt or cherts, and that ductile flow by diffusion-precipitation creep and the blocks are surrounded by a shale matrix (Figure 2A). cataclastic deformation coexist as the major deformation The thickness of the mélange zone in the Shimanto mechanisms in seismogenic crust. The spatiotemporal complex is typically less than 1 km based on field obser- relation between plastic and brittle deformations, however, vations for the mappable distribution of mélange zones has been expressed as deformations in a heterogeneous (e.g., Taira et al., 1988). Rowe et al. (2013) determined mélange rheology (e.g., Fagereng and Sibson, 2010). Exten- the thickness of a shear zone along a subduction plate sion veins related to mélange fabrics and shear veins cut- interface and found it to be up to a few hundred meters ting the mélange fabrics have been well documented, with in thickness. Within the mélange zone, the same shear detailed pressure-temperature conditions from the Mugi zones may be stacked up by duplex structures to form mélange, Miyama formation, and Yokonami mélange in underplated accretionary complexes (e.g., Hashimoto and the Cretaceous Shimanto Belt (e.g., Hashimoto et al., 2002, Kimura, 1999). The thickness of each horse of ocean floor 2003; Matsumura et al., 2003; Hashimoto et al., 2012) stratigraphy has been reported to be up to a few hundred (see locations in Figure 1); these data can be a key in meters in the mélange zone focused on in this study Figure 1 Distribution of the Shimanto Belt in central and southwest Japan. Locations of the Yokonami mélange, Mugi mélange, and Miyama formation are also represented. Hashimoto and Yamano Earth, Planets and Space 2014, 66:141 Page 3 of 9 http://www.earth-planets-space.com/66/1/141 Figure 2 Textures of tectonic mélange showing sandstone blocks surrounded by shale matrix (A). Extension veins, which developed only in sandstone blocks, were observed. Scaly cleavages are well developed in the shale matrix. Shear veins cutting the shale matrix (B). Scanned image of a thin section of a shear vein under cross-polarized light (C). The scale is the same as Figure 2D. Sketch of Figure 2C (D). White, quartz; gray, clay layers; and black, calcite. Slickenlines and slickensteps on the shear vein surface (E). (Hashimoto and Kimura, 1999; Ikesawa et al., 2005; texture and dissolved radiolaria fossils with the black Hashimoto et al., 2012). Although mélanges have been seams (e.g., Onishi and Kimura, 1995; Hashimoto and suggested to be sedimentary, tectonic, and diapiric, most Kimura, 1999), which indicates that dissolution and pre- mélangezonesintheShimantoBeltareinferredtobetec- cipitation creep contributed to mélange formation. This tonic mélanges (e.g., Kimura and Mukai, 1991; Onishi and feature is the plastic strain at the outcrop scale. Extension Kimura, 1995; Hashimoto and Kimura, 1999), with some veins developed only in the mélange blocks are observed exceptions (Osozawa et al., 2009). The characteristics of pervasively at both the outcrop and the microscopic scales tectonic mélanges are as follows: (1) the blocks have con- (Figure 2A) (Hashimoto et al., 2003). The extension veins sistent asymmetric shapes and the surrounding