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_Journal of Structural Geology 32 (2010) 792-802_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Sequential deformation from serpentinite mylonite to metasomatic rocks along the Sashu Fault, SW Japan Yusuke Soda*, Hideo Takagi Department of Earth Sciences, Faculty of Education and Integrated Arts and Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan Article info Article history: Received 17 October 2009; Received in revised form 28 April 2010; Accepted 12 May 2010; Available online 20 May 2010 Keywords: Serpentinite mylonite, Serpentinite breccia, Metasomatic rocks, LPO of antigorite Abstract The deformation of serpentinites along the Sashu Fault, Saganoseki Peninsula, SW Japan, involved both mylonitization and brecciation. The brecciation was accompanied by metasomatism, and the metasomatized rocks record additional ductile deformation. Serpentinite mylonite is composed mainly of antigorite with minor magnetite and Cr-spinel. Foliation and lineation in this rock type are defined by the shape preferred orientation (SPO) of antigorite and the alignment of fine-grained magnetite and carbonate minerals. The antigorite also shows a lattice preferred orientation (LPO), as measured with a universal stage, with c axes oriented near-perpendicular to the foliation and b axes oriented parallel to the lineation. Brecciation is concentrated in the reaction zone along the boundary between serpentinites and surrounding country rocks. Talc and chlorite schists developed in this zone as a result of associated metasomatism and deformation; consequently, they contain composite planar fabrics and drag folds. The breccias themselves are composed of serpentinite fragments cut by veins of calcite and talc. Analyses of fluid inclusions in calcite from the breccia indicate that brecciation occurred at 200-300°C at depths of 3.8-11.8 km. Metasomatic reactions, particularly the formation of talc, contributed to weakening of this serpentinite-bearing fault zone. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Serpentinite and its metasomatic products are important rocks to clarify the rheological properties along subduction zones (e.g., Peacock and Hyndman, 1999; Hermann et al., 2000) and along major faults such as the San Andreas Fault Zone (e.g., Andreani et al., 2005; Wibberley, 2007). It is possible that aseismic slip is facilitated by the occurrence of serpentinite along such shear zones, as indicated by the results of experimental studies (e.g., Moore et al., 1997; Reinen, 2000). The serpentine mineral group consists of antigorite, lizardite, and chrysotile. Antigorite is stable below 600-700°C, whereas lizardite and chrysotile are stable below 300°C (O’Hanley, 1996; Ulmer and Trommsdorff, 1999). It has been reported that antigorite is a major component of serpentinite mylonite, which is cohesive and characterized by a penetrative foliation and lineation (Norrell et al., 1989; Hermann et al., 2000; Li et al., 2004). Deformation experiments performed at pressures of 1-4 GPa and temperatures of 200-500°C reveal that viscous relaxation times of antigorite have a range equivalent to long-term deformations. * Corresponding author. Tel.: +81 3 5286 9864; fax: +81 3 3207 4950. E-mail address: soda@asagi.waseda.jp (Y. Soda). 0191-8141 $ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.05.003 After large earthquakes to slow earthquakes at subduction zones (Hilairet et al., 2007). Based on frictional deformation studies performed at low temperatures and pressures, it has been proposed that antigorite is conducive to seismic slip at shallow crustal levels (Dengo and Logan, 1981; Moore et al., 1997; Reinen, 2000); however, at low temperatures it is chrysotile rather than antigorite that is stable, meaning that chrysotile is a potential agent of aseismic slip (Moore et al., 1997; Reinen, 2000). Talc-bearing serpentinite is also thought to cause aseismic slip along the thrust boundaries of subducting plates, where the mantle wedge is serpentinized within the stability field of antigorite (Peacock and Hyndman, 1999; Hyndman and Peacock, 2003). Indeed, creep along the San Andreas Fault is attributed to the occurrence of talc-bearing serpentinite (Moore and Rymer, 2007; Wibberley, 2007). Several previous studies have undertaken petrological analyses of metasomatized serpentinites (e.g., Nishiyama, 1990; Schandl and Naldrett, 1992; Hansen et al., 2005; Spandler et al., 2008); however, the shear-deformation microstructures in such rocks have yet to be described in detail. Studies of natural examples of deformed serpentinite and associated metasomatic rocks are expected to provide new information on the rheology of subducting plate boundaries and major fault zones. In this paper, we describe a meter-scale transition from serpentinite mylonite, which contains antigorite with a lattice preferred orientation (LPO), to sheared metasomatic rocks (associated with brecciation) along the Sashu Fault, Japan, and go on to discuss the history of the shear deformation, P-T conditions, and the implications of our findings for the rheology of the fault zone. 2. Geological setting The serpentinites examined in this study are exposed along the Sashu Fault upon the Saganoseki Peninsula, Southwest Japan (Fig. 1a). The Sashu Fault marks the boundary between the Sambagawa metamorphic rocks to the north and the structurally overlying Upper Cretaceous Onogawa Group to the south (Teraoka et al., 1992; Yamakita et al., 1995). At the western end of the fault, the Nishikawauchi Formation, a Jurassic accretionary complex, occurs as a small lenticular mass (Saito et al., 1993). The fault strikes approximately NE-SW and dips 20-70° to the SE. The Sambagawa metamorphic rocks, which are distributed from Kanto to the Saganoseki Peninsula, underwent high-P T metamorphism during the Cretaceous (Banno and Sakai, 1989; Wallis and Banno, 1990). On the Saganoseki Peninsula, Sambagawa metamorphic rocks consist mainly of pelitic and basic schists, with subsidiary psammitic and quartzose schists. The mineral assemblage within the pelitic schist is indicative of the General geologic map of the Saganoseki Peninsula 130°E 132°E N 36° N 100km 134°E 136°E N 34° Fukuoka Osaka here Saganoseki Peninsula Sashu Fault Sashu N coast N 32° Nk Us Usuki 0 5km Cenozoic formations Onogawa Group Sambagawa metamorphic rocks Nk Nishikawauchi Formation Chichibu Group Us Usukigawa quartz diorite Ultramafic rocks Fault Inferred fault Outcrops of serpentinite Lithofacies map at the Sashu coast 0 32 31 32 Onogawa Group sandstone and mudstone Sambagawa metamorphic rocks basic schist pelitic schist quartz schist Serpentinite complex serpentinite gabbro 20 42 5 28 68 3 42 70 38 42 21 30 66 29 Block (B) 44 40 54 9 23 22 66 56 32 56 70 1 48 44 40 10 40 58 54 34 58 16 66 24 18 3 44 5 3 34 42 44 48 24 34 38 62 15 36 40 layered mafic rock pelitic phyllite 64 88 84 52 47 26 30 74 82 60 68 16 17 76 Stereographic projections B-2 B-1 A-9 A-8 A-7 A-6 A-5 A-4 A-3 A-2 A-1 798 Ключевые слова: cr spinel, deformation twin, brecciation, ?uid inclusion, reaction zone, homogenization temperature, range, saganoseki, hydrostatic condition, geophysical, fluid, fault zone, rock type, optic axis, serpentinite brecciation, serpentinite breccia, hermann, burkhard, ductile deformation, metamorphic, talc schist, spinel, schist, moore, society, melting temperature, source rock, metasomatism, talc, takagi, group, blocky, sashu fault, wa, reaction, journal structural, carbonate, mac rock, aggregate, university, vein, axis, cpl, calcite, ne-grained magnetite, fabric, foliation, serpentinite mylonites, fe-rich antigorite, structure, bodnar, sample, elsevier, serpentinite, kink band, fault, chlorite, magnetite, twin, parallel, peacock, reinen, size, universal stage, cut parallel, carbonate mineral, serpentinite mylonite, country rock, takagi journal, mantle wedge, pelitic, science, complex, metasomatic mineral, block, mineralogy, breccia, deformation structure, metasomatic rock, deformation, plane, serpentinites record, consists, grain, olivine, crustal rock, tectonophysics, antigorite, ?uid, ma?c, fault rock, dextral movement, depth, onogawa group, soda takagi, drag fold, hansen, geological society, cr-spinel, assemblage, observed, slip, journal structural geology, journal, ax, de?ned, mm, pelitic schist, calcite vein, study, teraoka, petrology, temperature, addition, spatial relation, stanford, mylonite, platy-euhedral grain, formation, uid pressure, lithostatic condition, previous study, yamakita, texture, occurred, zone, structural geology, serpentinite block, observed microstructures, nishiyama, serpentinite complex, metasomatic, mineral, minor magnetite, vogler, mylonitization, journal geophysical, coast, sashu coast, serpentinites, structural, subduction, norrell, geological, proportion, eds, metamorphic rock, scambelluri, ckm wallis, hyndman, rock, platy, uid inclusion, deformed, microstructures, sambagawa, sedimentary rock, platy antigorite, ni fe, nal stage, mineral assemblage, japan, sashu, sibson, geology, nishikawauchi formation, inclusion, pressure, lineation, aseismic slip, stage, optic, sibson streit, wibberley, matrix, type, onogawa, measured, condition, doi, ma?c rock, abstract, soda, fabric measured, nature, stability eld, deformed serpentinites, hanley, chlorite schist, mantle, fragment, geological survey, subduction zone, sambagawa metamorphic, serpentine mineral, orientation, serpentinite mylonitization, altered, shear