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Kinematic analysis of sinistral cataclastic shear zones along the northern margin of the Mino Belt, central Japan
Journal of Asian Earth Sciences 24 (2005) 787–800 www.elsevier.com/locate/jaes
Kinematic analysis of sinistral cataclastic shear zones along the northern margin of the Mino Belt, central Japan Masakazu Niwaa,*, Kazuhiro Tsukadab, Shiro Tanakac a
Division of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan b The Nagoya University Museum, Nagoya 464-8601, Japan c Department of Earth and Planetary Sciences, Graduate School of Sciences, Nagoya University, Nagoya 464-8602, Japan Received 1 March 2003; accepted 20 May 2004
Abstract In this paper, cataclastic shear zones along the northern margin of the Mino Belt, central Japan are described, and the significance of the shearing in the tectonic evolution of SW Japan is examined. The Mino Belt in SW Japan is composed of accretionary complexes of Jurassic to Early Cretaceous age. Field investigation revealed that remarkable cataclastic shear zones trending east to northeast run along the northern margin of the Mino Belt. Closely spaced cleavage is developed in these shear zones. Lineation on the cleavage plunges at shallow to moderate angles. Deformation structures (e.g. composite planar fabric and asymmetric structure of clasts) in the sheared rocks clearly indicate a sinistral sense of shear. The shearing ceased by latest Cretaceous time, because the sheared rocks are overlain by unsheared Upper Cretaceous volcanic rocks. The sinistral shearing may be closely related to Cretaceous sinistral movement along the eastern margin of Asia. Sinistral shearing along the northern margin of the Mino Belt can be considered as a key for re-examining the tectonic development of SW Japan. q 2004 Elsevier Ltd. All rights reserved. Keywords: Sinistral shear zones; Foliated cataclasite; Mino belt; Central Japan
1. Introduction The Asian continental margin has developed by various processes such as accretion, collision and strike-slip movement. Accretion of trench-fill sediments and oceanic plate cover, collision of geologic belts and strike-slip tectonic movement also created the basic geotectonic framework of SW Japan. The accretionary complexes of SW Japan have been studied, and various tectonic models for their formation have been proposed (e.g. Suzuki and Hada, 1983; Taira et al., 1988; Wakita, 1988; Ditullio and Byrne, 1990; Nakae, 1990; Kimura and Mukai, 1991; Okamura, 1991; Kimura and Hori, 1993). However, the process of the collision and the role of the strike-slip tectonic movement in the evolution of SW Japan have not been fully explored. * Corresponding author. Tel.: C81-52-789-2526; fax: C81-52-7893033. E-mail address: [email protected] (M. Niwa). 1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2004.05.002
The Japanese Islands are divided into SW Japan and NE Japan by the Tanakura Tectonic Line (TTL; Fig. 1). SW Japan is subdivided into the Inner Zone and Outer Zone by the Median Tectonic Line (MTL; Fig. 1). The TTL and MTL are considered to be Cretaceous sinistral strike-slip tectonic zones (Koshiya, 1986; Takagi, 1986). Several studies (e.g. Klimetz, 1983; Xu et al., 1989, 1993) have indicated Mesozoic sinistral strike-slip displacement in East Asia, including along the TTL and MTL (Fig. 1a), with relation to the northward spreading of the oceanic plate in the Pacific area (Maruyama and Seno, 1986; Cox et al., 1989). The Hida Marginal Belt (HMB) of the Inner Zone of SW Japan is a tectonic zone dividing rocks of continental origin from a Jurassic accretionary complex. Several shear zones are developed in and around the HMB (Otoh and Yanai, 1996; Wakita et al., 2001; Otoh et al., 2003; Tsukada, 2003). Structural and paleomagnetical data, and the distribution of the geologic belts of the Inner Zone of SW Japan suggest that Mesozoic tectonic processes along the HMB provide a key for reconstructing the basic geotectonic framework of
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Fig. 1. (a) Distribution of major late Mesozoic sinistral faults in East Asia (after Xu et al., 1993). CSA, Central Sikhote-Alin Fault Zone; KR, Kurtuhin faults; MF, Mishun-Fushun faults; YQ, Yalujiang-Qingdao Fault Zone; TD, Taidong Fault; TL, Tancheng-Lujiang Fault Zone; KT, Kora-Taiwan Strait Fault; TB, Tienmushan-Baijishan Fault; CN, Changle-Nagao Fault Zone; SH, Shaowu-Heyuan Fault Zone; LH, Lishui-Haifeng Fault Zone; XE, Xingfong-Enping Fault Zone; MTL, Median Tectonic Line; TTL, Tanakura Tectonic Line. (b) Tectonic Map of SW Japan before the opening of the Sea of Japan. The tectonic divisions are from Wakita et al. (1992). The present position of the Japanese Islands is shown with a broken line. MTL, Median Tectonic Line; ISTL, ItoigawaShizuoka Tectonic Line; TTL, Tanakura Tectonic Line; HMB, Hida Marginal Belt; NTB, Nagato Tectonic Belt; UT, Ultra-Tamba Belt.
the Inner Zone of SW Japan (Wakita et al., 2001). The analysis of shear zones in and around the HMB is significant in order to understand the tectonic evolution of the Inner Zone of SW Japan. In this paper, shear fabrics along the northern margin of the Mino Belt, adjacent to the HMB, are examined, and the significance of this shearing in tectonic evolution of the Inner Zone of SW Japan is discussed.
2. Geological setting of the Inner Zone of SW Japan The Inner Zone of SW Japan is composed of the following geologic belts: Hida; Sangun; Akiyoshi; Maizuru; Ultra-Tamba; Hida Marginal; and Mino (Tamba-MinoAshio) Belts (Ichikawa, 1990; Wakita et al., 1992; Fig. 1b). The Hida Belt is composed mainly of Paleozoic metamorphic rocks having continental affinities (Maruyama and Seno, 1986). The Sangun Belt is composed of Paleozoic to Mesozoic metamorphic rocks. The Akiyoshi and UltraTamba Belts are composed of Permian accretionary complexes, and the Mino Belt is composed of a Jurassic to Early Cretaceous accretionary complex. The Maizuru Belt is interpreted as an Upper Paleozoic island arc system
overlain by Upper Paleozoic to Lower Mesozoic strata. The sinuous surface trajectories of the geotectonic boundaries and the occurrence of tectonic outliers and windows indicate that the most of the geologic belts occur as subhorizontal or gently-northward-dipping thin tectonic units and form a huge pile of nappes in the western part of the Inner Zone (e.g. Isozaki et al., 1990). The Sangun, Akiyoshi, Maizuru, and Ultra-Tamba Belts are widely distributed in the western part of SW Japan, whereas these geologic belts are absent, and the HMB is very narrow between the Hida and Mino Belts in the eastern part of SW Japan (Fig. 1b). The HMB includes fault-bounded blocks derived from basement rocks of the Sangun, Akiyoshi and Maizuru Belts (e.g. Chihara and Komatsu, 1982) and Paleozoic rocks of shelf facies (e.g. Wakita et al., 2001). The HMB is interpreted as a major transcurrent fault zone with a complex displacement history (Wakita et al., 2001). The Jurassic accretionary complex of the Mino Belt comprises coherent units and melange units (e.g. Wakita et al., 2001). The coherent units are composed of imbricate thrust sheets of accreted rocks. The melange units contain fragments derived from collapsed seamounts and peeled oceanic crusts in a Jurassic muddy matrix.
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Paleomagnetic data indicate that before the opening of the Sea of Japan in Middle Miocene time SW Japan was oriented in a northeast direction (Otofuji et al., 1985; Fig. 1b).
rocks (Figs. 2 and 3). In the northeasternmost part of the area rocks of the Mino Belt are metamorphosed by the intrusion of uppermost Cretaceous to Quaternary granitoids (Harayama, 1990).
3. Geological overview of the study area
4. Description of the shear zone
The study area is in the Hida Mountains, about 300 km to the west of Tokyo, central Japan (Fig. 1). Rocks of both the Mino and HMBs are exposed in this area (Fig. 2). The rocks of the Mino Belt are divided into a coherent clastic rock unit, a coherent chert-basalt unit and a melange unit. Sedimentary structures, such as graded bedding occur in alternating beds of sandstone and mudstone in the clastic unit. The rocks are folded into a synclinal fold with a half wave length from 10 to 15 km and the axis plunging gently westward, as shown on the geological map (Fig. 2). The melange unit crops out mostly on the northern limb of the fold. The melange unit in the study area includes various kinds of clasts (the definition of clast is after Wakita, 1988) set in a matrix of black mudstone and gray siltstone (Figs. 2 and 3). These clasts consist of sandstone, alternating beds of sandstone and mudstone, felsic tuff, siliceous mudstone, bedded chert, limestone, basaltic lava and volcaniclastic rocks. The bedding planes of clasts trend east-southeast to northeast, and dip moderately to steeply southward (Figs. 3a and b), but in the northeasternmost part of the study area the bedding planes trend northeast to north and dip moderately to steeply westward (Fig. 3c). Most of clasts are lenticular, showing a good alignment, subparallel to the bedding planes. Limestones in the study area, yield Carboniferous to Permian fusulinoideans (Isomi and Nozawa, 1957; Kojima, 1984). Bedded cherts yield Permian to early Middle Jurassic radiolarians (Adachi and Kojima, 1983; Kojima, 1984; Imazato and Otoh, 1993; Niwa et al., 2002a, 2003). Felsic tuff, siliceous mudstone and mudstone yield Early to Middle Jurassic radiolarians (Adachi and Kojima, 1983; Kojima, 1984; Imazato and Otoh, 1993; Harayama, 1990; Niwa et al., 2002a, 2003). According to this fossil data, accretion occurred in the Early to Middle Jurassic. In the northern part of the study area the melange unit is deformed into foliated cataclasite forming cataclastic shear zones (Fig. 3) with closely spaced cleavages. Many clasts in the shear zones are intensely elongated, and are rotated in the shear planes. To the north the rocks of the Mino Belt are in fault contact with Permian strata of the Junigatake Formation and Paleozoic shelf facies rocks of the HMB (Wakita et al., 2001; Niwa et al., 2002b; Figs. 2 and 3). The fault strikes east and dips to the south. Shear zones composed of foliated cataclasite to mylonite extend along the fault. All the preCretaceous rocks in the study area are covered unconformably by uppermost Cretaceous to Quaternary felsic volcanic
4.1. Classification of the sheared rocks Melange in the study area is mostly sheared, with a spaced cleavage in the muddy matrix. The cleavage is defined by partings along aligned fine-grained phyllosilicates, and corresponds to the P foliation (Rutter et al., 1986; Takagi, 1996). In this paper, rocks of the Mino Belt in the study area are classified into the following three types based on approximate mesoscopic spacing of the cleavage: Type 1—with a spacing less than 5 mm; Type 2—with a spacing ranging from 5 mm to 5 cm; Type 3—with a spacing of over 5 cm (Figs. 4–6). Type 1 rocks are composed of foliated cataclasite (protocataclasite to ultracataclasite; Takagi and Kobayashi, 1996), and are exposed in several belts running in an E–W to NE–SW direction (Fig. 5). In this paper, these belts are called ‘cataclastic shear zones’. Each cataclastic shear zone is several tens to hundreds meters in width and trends subparallel to the boundary between the Mino and HMBs. The rocks in the study area, especially in the cataclastic shear zones show different deformation structures (Figs. 6–9). The deformation structures in each rock Type are as follows: 4.1.1. Type 1 In the rocks of Type 1, mudstone of the melange matrix has a cleavage with a spacing of less than 5 mm (Fig. 6a). The cleavage planes are mainly sinuous or planar, but are sometimes anastomosing (Fig. 4). Fine-grained phyllosilicates in the mudstone show strong alignment. Pressure solution cleavages (or pressure solution seams; Borradaile et al., 1982; Groshong, 1988) are developed, showing a preferred orientation of aligned phyllosilicates (Fig. 8a). Sheared clasts are transected by pressure solution cleavage traces, filled with finer dark grains (Fig. 8c). In bedded chert and in alternating beds of sandstone and mudstone the internal stratal continuity is entirely broken, clasts and mineral grains are intensely fragmented and tectonically abraded (Fig. 6b). Clasts with a lenticular-shape are included in the sheared rocks, and are elongated subparallel to the cleavage (Figs. 7c, d and 8a, b). Pressure shadows (Hobbs et al., 1976; Groshong, 1988) commonly occur around lenticular clasts and mineral grains (Figs. 8a and b). Several clasts have tails composed of fine-grained material with the same composition as the host clast (Fig. 7d). Pervasive cracks filled with quartz or calcite are observed in the sheared clasts, mainly sub-perpendicular to the alignment of clasts (Figs. 7d and 8d).
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Fig. 2. Simplified geologic map of the study area. See Fig. 1 for location.
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Fig. 3. Geological maps and profiles of the northern marginal part of the Mino Belt. The distribution of cataclastic shear zones (Type 1) is shown. See Fig. 2 for location. 791
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Fig. 4. Classification of the sheared rocks in the study area. Terminology of the cleavage pattern is from Borradaile et al. (1982).
Foliated cataclasite in Type 1 displays a composite planar fabric (Figs. 7b, c, 8b and 9b). The composite planar fabric consists of P foliation, Y shear (slipping surface), and R1 Riedel shear (Rutter et al., 1986; Takagi, 1996). The P foliation is defined by an alignment of finegrained phyllosilicates. The P foliation and Y shear intersect at an angle less than 408. The R1 Riedel shears are spaced several to several tens of centimeters apart, and each of them appears as a thin layer of a finer grained aggregate, severely fragmented. They cut the P foliation and Y shears at an angle of 20–408 and show drag effects (Fig. 7c). Ductile shear zones several tens of meters in width are found at the boundary between the Mino and HMBs in the Fukuji area (Figs. 3 and 9). Foliation with a spacing of only a few hundreds of microns is developed in ductile shear zones. The foliation is subparallel to the orientation of dynamically recrystallized grains and pressure solution cleavages. Quartz and calcite grains are fine-grained and intensely elongated due to the dynamic recrystallization (Figs. 9c and d). Subgrain boundaries are observed in these grains. 4.1.2. Type 2 In the rocks of Type 2, mudstone of the melange matrix has anastomosing cleavage with a spacing ranging from 5 mm to 5 cm, and shows a scaly fabric (Agar et al., 1989; Vannucchi et al., 2003; Fig. 4). Fine-grained phyllosilicates in the mudstone are aligned parallel to the cleavage. Alternating beds of sandstone and mudstone, and felsic tuff layers are isolated by boudinage (Fig. 6c). In contrast in bedded chert most of the internal stratal continuity is preserved. Sandstone and felsic tuff clasts have been rotated and fragmented (Figs. 6c and d). Lenticular clasts of sandstone and felsic tuff are arranged along the cleavage, and have tails composed of fine-grained material of the same composition as the host clast. Clasts of chert, limestone and mafic volcanic rocks do not have associated tails. Cracking and mineralization in clasts of chert, limestone and mafic volcanic rocks are rarely observed. Pressure solution cleavages are developed along the margins of clasts which have a preferred alignment.
4.1.3. Type 3 Type 3 mudstones show mostly a random-fabric (Figs. 6e and f). But in places, the rocks are weakly foliated, with a rough cleavage widely spaced at over 5 cm (Fig. 4). Phyllosilicates in the mudstone are randomly oriented. Most clasts are weakly sheared and have an oval-shape. Cracking and disaggregation of clasts are rarely observed. Pressure solution cleavage may occur around some clasts of chert and mafic volcanic rocks, but these cleavages do not have any preferred orientation (Fig. 6f). 4.2. Stereographic analysis of the shear zones Cataclastic shear zones of Type 1 display a welldeveloped P foliation in mudstone forming a cleavage defined by partings along aligned fine-grained phyllosilicates. The cleavage in the cataclastic shear zones trends E to ESE and dips moderately to steeply to the S in the western part of the study area (Mt. Junigatake and Dayoshi areas; Fig. 5a). In the eastern part (Hirayu and Fukuji areas; Fig. 5b) the cleavage trends E to NE and dips moderately to steeply to the S. The cleavage in the northeasternmost part (Shin-Hodaka area; Fig. 5c) trends NNE and dips steeply NW. Lenticular clasts in the cataclastic shear zones show a preferred orientation of the cleavage. Two types of lineation are developed on the cleavage, i.e. cataclastic lineation and slickenline. Cataclastic lineations correspond to the direction of the long axes of lenticular clasts (Tanaka 1992; Fig. 7a). Cataclastic lineations and slickenlines generally indicate homogeneous orientations. In the Mt. Junigatake and Dayoshi areas, the lineations plunge SW or SE at shallow to moderate angles (Fig. 5a). In the Hirayu and Fukuji areas, the lineations plunge SW or E to NE at shallow to moderate angles (Fig. 5b). In some places of the Hirayu and Fukuji areas, the lineations plunge SE. In the Shin-Hodaka area, the lineations plunge SW to SSW at shallow to moderate angles (Fig. 5c). 4.3. Asymmetrical deformation structures Asymmetrical deformation structures in the cataclastic shear zones indicate a sinistral sense of shear. Composite planar fabrics such as the obliquity of the P surface with respect to the Y surface (P-Y fabric; Figs. 7b and 8b) and the inclination of the R1 Riedel shear (P-R1 fabric; Figs. 7c and 9b) are used as indicators of the sense of shear (Takagi, 1996). Clasts of bedded chert and alternating beds of sandstone and mudstone are sheared and deformed to form boudinage. Each boudin had been displaced along the R1 shear plane in a manner similar to that seen in extensional fault systems (Needham, 1987; Fig. 7c). Extension cracks in fragmented clasts and mineral grains are observed in the foliated cataclasites. Each particle of the fragmented clasts and mineral grains is rotated in a ‘domino style’ (Fig. 8b), and the rotation is analogous to the sheared
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Fig. 5. Equal-area lower hemisphere projections of cleavages (P foliations) and slip directions of hanging wall in the cataclastic shear zones (Type 1). The distribution of the cataclastic shear zones is also shown. See Fig. 2 for location.
stack of cards model (Etchecopar, 1977; Simpson and Schmid, 1983). Lenticular clasts and mineral grains are accompanied by asymmetrical pressure shadows (Fig. 8b). Clasts of
sandstone, felsic tuff and chert are associated with asymmetrical tails (Fig. 7d). These clasts and mineral grains form asymmetrical s-type or d-type objects (Simpson and Schmid, 1983; Passchier and Simpson, 1986).
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Fig. 6. Photographs showing the types of sheared rocks in the study area. (a) Field outcrop of Type 1 foliated cataclasite enclosing lenticular sandstone and chert clasts in a matrix of mudstone. (b) Photomicrograph of the Type 1 foliated cataclasite (PPL). Fragmented and abraded grains are indicated by the white arrows. (c) Field outcrop of boudinaged interbedded sandstone (ss) in mudstone (ms) of Type 2. The mudstone shows a scaly fabric. (d) Photomicrograph of the Type 2 rocks including felsic tuff (ft) and sandstone (ss) lenses in a mudstone matrix (PPL). The lenses are rotated and fragmented. (e) Field outcrop of Type 3 rocks showing melange with a random-fabric. The melange includes clasts of felsic tuff, sandstone, chert, limestone, and mafic volcanic rocks in matrices of black mudstone and gray tuffaceous siltstone. (f) Photomicrograph of the Type 3 rocks (PPL). The rounded basalt clast (bs) is accompanied by pressure solution cleavages indicated by the white arrows.
5. Discussion 5.1. Cataclastic shear zones by secondary sinistral shearing The Mino Belt in the study area consists of following two lithological assemblages: (1) terrigenous clastic rocks such as sandstone and mudstone; (2) bedded chert, limestone and mafic volcanic rocks having affinities with collapsed seamounts and peeled oceanic crust. Pairing of the two types of lithology and the occurrence of melange
suggest that the Mino Belt represents an accretionary complex (e.g. Isozaki et al., 1990; Kojima and Kametaka, 2000). Melange is a common component of uplifted ancient accretionary complexes (Raymond, 1984; Cowan, 1985; Lash, 1987). Various geological processes have influenced melange formation, i.e. tectonic (Hsu¨, 1968), olistostromal (Abbate et al., 1970) and diapiric (Barber et al., 1986). The melange in the Mino Belt may have been formed at the accretion stage (e.g. Wakita, 1988; Nakae, 1990).
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Fig. 7. Photographs showing field outcrops of cataclastic shear zones (Type 1). (a) Cataclastic lineation plunging southwest on the P foliation of the foliated cataclasite (white arrow). (b) Strongly sheared bedded chert. The P-Y fabric indicates a sinistral sense of shear (white arrows). (c) R1-Y fabric in boudinage of alternating beds of sandstone and mudstone. The fabric indicates top-to-the-east movement (white large arrows). The sandstone lens is displaced on the R1 shear plane (white small arrow) and forms asymmetrical boudinage. (d) Foliated cataclasite including lenticular chert clasts in mudstone. The lenticular chert clast (outlined with white broken line) is associated with asymmetrical tails that indicate top-to-the-east movement (white large arrows). Cracks in the chert clast (white small arrows) are sub-perpendicular to the orientation of the aligned clasts.
Rocks along the northern margin of the Mino Belt in the study area are deformed into foliated cataclasite with sinistral shearing. Several deformation structures (e.g. scaly fabric, pressure solution cleavage, cracks filled with quartz or calcite, lenticular clasts with tails or pressure shadows and boudinage) recognized in this area have been reported from other areas of ancient and modern accretionary complexes (e.g. Fisher and Byrne, 1987; Needham, 1987; Nakae, 1990; Kimura and Mukai, 1991; Labaume et al., 1997), however, cataclastic shearing of Type 1 in the study area probably followed shortly after melange formation at the accretion stage, for the following reasons: 1. Foliated rocks of melange units in accretionary complexes generally show a scaly fabric having
disjunctive and anastomosing cleavage (Vannucchi et al., 2003). Mesoscopic spacing and the patterns of cleavages developed in rock Types 2 and 3 are nearly the same as those in common accretionary complexes. On the other hand, the Type 1 cleavage is different from the scaly cleavage common in accretionary complexes. The foliated cataclasite of Type 1 has cleavage with a spacing of less than 5 mm, and the closely spaced cleavage is mainly sinuous or planar. In addition, Type 1 rocks include ductile shear zones composed of deformed rocks formed by dynamic recrystallization (Fig. 9). 2. The south-dipping fault along the boundary of the Mino and HMBs is clearly not an accretion-related structure, because accretionary complexes are formed by the tectonic transfer of strata from the descending
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Fig. 8. Photomicrographs of foliated cataclasite (Type 1) in XZ thin sections (PPL). (a) Sandstone clasts associated with pressure shadows (ps) in foliated mudstone. Dark pressure solution cleavages are arranged subparallel to the foliation (white arrows). (b) Foliated cataclasite showing a sinistral P-Y fabric. Imbrication of cracked quartz grains (black arrows) and sandstone clasts (ss) with s-type pressure shadows show a sinistral sense of shear (white arrows). (c) Crushed sandstone clast associated with pressure solution cleavages (black arrows). (d) Sandstone clasts (ss) with cracks. The cracks filled with quartz (black arrows) are subperpendicular to the orientation of the aligned clasts.
oceanic plate into the overlying continental plate, consequently accreted strata underlie the continental plate. But in the study area the accretionary complex of the Mino Belt overlies HMB rocks of shelf facies along the south-dipping fault. Type 1 shear zones extend along the south-dipping fault between the Mino and HMBs. Cleavage in these shear zones trends E–W and dips S, parallel to the boundary fault. Therefore shear zones of Type 1 are most likely related to postaccretion faulting along the boundary between the Mino and HMBs.
5.2. Timing of sinistral shearing Sinistral cataclastic shear zones transect the melange unit of the Mino Belt in the study area. Since the melange mudstone matrix in this area includes middle to late Middle Jurassic radiolarians (Adachi and Kojima, 1983; 1984; Kojima, Harayama, 1990; Imazato and Otoh, 1993; Niwa et al., 2003), melange formation occurred after the Middle Jurassic. In the northern Fukuji area, a remarkable sinistral shear zone composed of foliated cataclasite is developed in
the Lower Cretaceous Tochio Formation of the Tetori Group (Tsukada, 2003; Fig. 2). This suggests that the sinistral shearing occurred during the Cretaceous, after the formation of the accretionary complex in the Jurassic. The sinistral cataclastic shear zones are overlain unconformably by unsheared felsic volcanic rocks of the Oamamiyama Group and Kasagatake Rhyolites (Figs. 2, 3 and 10a). Felsic tuff in the lower part of the Oamamiyama Group yields palynomorphs of Maastrichtian age (Kasahara, 1979). Lava of the Kasagatake Rhyolites has given radiometric ages of 68–55 Ma (Harayama, 1990). Moreover, the sheared rocks in the northeastern part of the Mino Belt are intruded by unsheared granitic rocks with radiometric ages of 64–54 Ma (Harayama, 1990; Fig. 10b). Hence, the sinistral shearing in and around the HMB probably occurred after Early Cretaceous time and had finished by the latest Cretaceous. 5.3. Significance of the cataclastic shearing in relation to the tectonic evolution of SW Japan In this study, it has been established that Cretaceous sinistral cataclastic shear zones are developed along the northern margin of the Mino Belt (Figs. 2 and 3).
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Fig. 9. (a) Photograph showing the field outcrop of the ductilely deformed rocks of Type 1. (b) Photograph showing a polished surface of (a). The R1-Y fabric indicates sinistral sense of shear (white arrows). (c) Photomicrograph of (b), showing pressure solution cleavages (psc) and dynamically recrystallized quartz (qtz) and calcite (cal) in a XZ thin section (XPL). (d) Close-up view of dynamically recrystallized quartz (XPL).
Sasaki et al. (2001) has also described several cataclastic shear zones along the northern margin of the Mino Belt which form a sinistral strike-slip imbricate fan. Major fault zones of Cretaceous age showing sinistral shearing are widely distributed along the continental margin of East Asia (Xu et al., 1989, 1993; Fig. 1a). In Japan, Cretaceous sinistral fault systems have played an important role in the tectonic history of Japan (e.g. Takagi, 1986; Taira and Tashiro, 1987; Otsuki and Ehiro, 1992). During Late Jurassic to Early Cretaceous time, the fast, relative northward, movement of the Pacific oceanic plate (Izanagi Plate) caused the development of sinistral fault systems along the Asian continental margin (Maruyama and Seno, 1986; Cox et al., 1989). According to the tectonic reconstruction proposed by Klimetz (1983); S¸engo¨r and Natal’in (1996); Wan and Zhu (1997), the eastern margin of Asia drifted northwards along these sinistral fault systems in Cretaceous time, driven by the northward movement of the western Pacific oceanic plate. Mizutani et al. (1990) suggested that the Jurassic to Early Cretaceous accretionary complexes, including
the Mino Belt, which are distributed along the eastern continental margin of Asia, drifted northward during or after accretion in the Cretaceous. Paleomagnetic data also suggests that the accretionary complex of the Mino Belt moved from a more southerly location to its present northerly position by sinistral movements in Cretaceous time (Hattori, 1982; Hirooka, 1990). Mizuno and Otsuka (1997) suggested that map-scale asymmetric half-basin and half-dome synclinal folds of bedded chert in the Mino Belt, demonstrate that sinistral simple shear occurred during Cretaceous time. Kimura (1999) reported that in a coherent unit of the accretionary complex within the Mino Belt, sinistral strike-slip faults overprint subduction-related thrust faults. The sinistral shear zones reported in this paper may provide further evidence for the northward shift of the accretionary complex of Mino Belt after or during the accretion. Kinematic data along the northern margin of the Mino Belt described in this paper support the model of the Cretaceous sinistral movement along the HMB. Tsukada (2003) has suggested that sinistral movement along the HMB might have resulted from a sinistral collision between
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References
Fig. 10. Photographs showing the boundary between the cataclasites in the Mino Belt and Upper Cretaceous rocks. (a) Sheared basalt clast (bs) in the cataclastic shear zone is overlain unconformably (A-A 0 ) by undeformed felsic tuff of the Maastrichtian Oamamiyama Group. (alt) Alternating beds of red felsic tuff and gray felsic tuff breccia; (ft) green felsic tuff; (cgl) basal conglomerate. (b) Foliated cataclasite is intruded by unsheared granitic rocks (black arrows). The intrusion age of the dikes has been dated by the Rb–Sr method at around 54 Ma.
the Mino and HMBs in the Cretaceous. Further structural analyses of deformation structures in other areas of the Paleozoic–Mesozoic geologic belts in Japan are required to determine the detailed tectonic history of SW Japan. Acknowledgements We wish to thank Prof. M. Adachi, Associate Prof. M. Takeuchi, and Associate Prof. H. Yoshida of Nagoya University for helpful discussion and advice. We are indebted to Prof. S. Kojima of Gifu University and Associate Prof. S. Otoh of Toyama University for valuable discussion. Special thanks go to Prof. H. Takagi of Waseda University and Dr A.J. Barber of University of London for critical reading of the manuscript. The work was supported by Grant-in-Aid of Fundamental Scientific Research (nos. 10003433 and 11740278) from the Ministry of Education, Science and Culture, Japan.
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M. Niwa et al. / Journal of Asian Earth Sciences 24 (2005) 787–800 Isomi, H., Nozawa, T., 1957. Geology of the Funatsu district. Quadrangle Ser., scale 1:50,000. Geological Survey of Japan (in Japanese with English Abstract). Isozaki, Y., Maruyama, S., Fukuoka, F., 1990. Accreted oceanic materials in Japan. Tectonophysics 181, 179–205. Kasahara, Y., 1979. Geology of the Oamamiyama Group—Latest Cretaceous acid volcanism on the Hida Marginal Belt, Central Japan. Memoir of Geological Society of Japan 17, 177–186 (in Japanese with English Abstract). Kimura, K., 1999. The slip direction of thrust faults—a case study from a chert-clastic sequence in the Mino-Tamba Belt, central Japan. Journal of Geological Society of Japan 105, 208–226 (in Japanese with English Abstract). Kimura, K., Hori, R., 1993. Offscraping accretion of Jurassic chert-clastic complexes in the Mino-Tamba Belt, central Japan. Journal of Structural Geology 15, 145–161. Kimura, G., Mukai, A., 1991. Underplated units in an accretionary complex: me´lange of the Shimanto Belt of eastern Shikoku, southwest Japan. Tectonics 10, 31–50. Klimetz, M.P., 1983. Speculations on the Mesozoic plate tectonic evolution of eastern China. Tectonics 2, 139–166. Kojima, S., 1984. Paleozoic–Mesozoic strata in the Takayama area, Gifu Prefecture, central Japan: their stratigraphy and structure. Journal of Geological Society of Japan 90, 175–190 (in Japanese with English Abstract). Kojima, S., Kametaka, M., 2000. Jurassic accretionary complexes in East Asia. Memoir of Geological Society of Japan 55, 61–72 (in Japanese with English Abstract). Koshiya, S., 1986. Tanakura Shear Zone: the deformation process of fault rocks and its kinematics. Journal of Geological Society of Japan 92, 15– 29 (in Japanese with English Abstract). Labaume, P., Maltman, A.J., Bolton, A., Tessier, D., Ogawa, Y., Takizawa, S., 1997. Scaly fabrics in sheared clays from the de´collement zone of the Barbados accretionary prism. Proceedings of the Ocean Drilling Program, Scientific Results 156, 59–77. Lash, G.G., 1987. Diverse me´langes of an ancient subduction complex. Geology 15, 652–655. Maruyama, M., Seno, T., 1986. Orogeny and relative plate motions: example of the Japanese Islands. In: Zonenshain, L.P. (Ed), Tectonics of the Eurasian Fold Belts, Tectonophysics 127, 305–324. Mizuno, M., Otsuka, T., 1997. Large-scale folds in the Kamiaso area, Mino Terrane, central Japan. Journal of Faculty of Science, Shinshu University 32, 27–36 (in Japanese with English Abstract). Mizutani, S., Shao, J.A., Zhang, Q.L., 1990. The Nadanhada terrane in relation to Mesozoic tectonics on continental margins of East Asia. Acta Geologica Sinica 3, 15–29. Nakae, S., 1990. Melanges in the Mesozoic sedimentary complex of the northern part of the Tamba Belt, Southwest Japan. Journal of Geological Society of Japan 96, 454–469. Needham, D.T., 1987. Asymmetric extensional structures and their implications for the generation of me´langes. Geological Magazine 124, 311–318. Niwa, M., Tsukada, K., Kojima, S., 2002a. Early Jurassic radiolarians from pelitic rocks in the Mino Belt, Nyukawa Village, Gifu Prefecture, central Japan. Journal of Geological Society of Japan 108, 16–23 (in Japanese with English Abstract) Niwa, M., Tsukada, K., Kojima, S., 2002b. Permian clastic formation in the Yokoo area, Nyukawa Village, Gifu Prefecture, central Japan. Journal of Geological Society of Japan 108, 75–87 (in Japanese with English Abstract). Niwa, M., Kashiwagi, K., Tsukada, K., 2003. Jurassic, Triassic and Permian radiolarians from the Hirayu Complex of the Mino Belt in the Nyukawa-Hirayu area, Gifu Prefecture, central Japan. The Journal of Earth and Planetary Sciences, Nagoya University 50, 13–42.
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Kinematic analysis of sinistral cataclastic shear zones along the northern margin of the Mino Belt, central Japan Masakazu Niwaa,*, Kazuhiro Tsukadab, Shiro Tanakac a
Division of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan b The Nagoya University Museum, Nagoya 464-8601, Japan c Department of Earth and Planetary Sciences, Graduate School of Sciences, Nagoya University, Nagoya 464-8602, Japan Received 1 March 2003; accepted 20 May 2004
Abstract In this paper, cataclastic shear zones along the northern margin of the Mino Belt, central Japan are described, and the significance of the shearing in the tectonic evolution of SW Japan is examined. The Mino Belt in SW Japan is composed of accretionary complexes of Jurassic to Early Cretaceous age. Field investigation revealed that remarkable cataclastic shear zones trending east to northeast run along the northern margin of the Mino Belt. Closely spaced cleavage is developed in these shear zones. Lineation on the cleavage plunges at shallow to moderate angles. Deformation structures (e.g. composite planar fabric and asymmetric structure of clasts) in the sheared rocks clearly indicate a sinistral sense of shear. The shearing ceased by latest Cretaceous time, because the sheared rocks are overlain by unsheared Upper Cretaceous volcanic rocks. The sinistral shearing may be closely related to Cretaceous sinistral movement along the eastern margin of Asia. Sinistral shearing along the northern margin of the Mino Belt can be considered as a key for re-examining the tectonic development of SW Japan. q 2004 Elsevier Ltd. All rights reserved. Keywords: Sinistral shear zones; Foliated cataclasite; Mino belt; Central Japan
1. Introduction The Asian continental margin has developed by various processes such as accretion, collision and strike-slip movement. Accretion of trench-fill sediments and oceanic plate cover, collision of geologic belts and strike-slip tectonic movement also created the basic geotectonic framework of SW Japan. The accretionary complexes of SW Japan have been studied, and various tectonic models for their formation have been proposed (e.g. Suzuki and Hada, 1983; Taira et al., 1988; Wakita, 1988; Ditullio and Byrne, 1990; Nakae, 1990; Kimura and Mukai, 1991; Okamura, 1991; Kimura and Hori, 1993). However, the process of the collision and the role of the strike-slip tectonic movement in the evolution of SW Japan have not been fully explored. * Corresponding author. Tel.: C81-52-789-2526; fax: C81-52-7893033. E-mail address: [email protected] (M. Niwa). 1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2004.05.002
The Japanese Islands are divided into SW Japan and NE Japan by the Tanakura Tectonic Line (TTL; Fig. 1). SW Japan is subdivided into the Inner Zone and Outer Zone by the Median Tectonic Line (MTL; Fig. 1). The TTL and MTL are considered to be Cretaceous sinistral strike-slip tectonic zones (Koshiya, 1986; Takagi, 1986). Several studies (e.g. Klimetz, 1983; Xu et al., 1989, 1993) have indicated Mesozoic sinistral strike-slip displacement in East Asia, including along the TTL and MTL (Fig. 1a), with relation to the northward spreading of the oceanic plate in the Pacific area (Maruyama and Seno, 1986; Cox et al., 1989). The Hida Marginal Belt (HMB) of the Inner Zone of SW Japan is a tectonic zone dividing rocks of continental origin from a Jurassic accretionary complex. Several shear zones are developed in and around the HMB (Otoh and Yanai, 1996; Wakita et al., 2001; Otoh et al., 2003; Tsukada, 2003). Structural and paleomagnetical data, and the distribution of the geologic belts of the Inner Zone of SW Japan suggest that Mesozoic tectonic processes along the HMB provide a key for reconstructing the basic geotectonic framework of
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Fig. 1. (a) Distribution of major late Mesozoic sinistral faults in East Asia (after Xu et al., 1993). CSA, Central Sikhote-Alin Fault Zone; KR, Kurtuhin faults; MF, Mishun-Fushun faults; YQ, Yalujiang-Qingdao Fault Zone; TD, Taidong Fault; TL, Tancheng-Lujiang Fault Zone; KT, Kora-Taiwan Strait Fault; TB, Tienmushan-Baijishan Fault; CN, Changle-Nagao Fault Zone; SH, Shaowu-Heyuan Fault Zone; LH, Lishui-Haifeng Fault Zone; XE, Xingfong-Enping Fault Zone; MTL, Median Tectonic Line; TTL, Tanakura Tectonic Line. (b) Tectonic Map of SW Japan before the opening of the Sea of Japan. The tectonic divisions are from Wakita et al. (1992). The present position of the Japanese Islands is shown with a broken line. MTL, Median Tectonic Line; ISTL, ItoigawaShizuoka Tectonic Line; TTL, Tanakura Tectonic Line; HMB, Hida Marginal Belt; NTB, Nagato Tectonic Belt; UT, Ultra-Tamba Belt.
the Inner Zone of SW Japan (Wakita et al., 2001). The analysis of shear zones in and around the HMB is significant in order to understand the tectonic evolution of the Inner Zone of SW Japan. In this paper, shear fabrics along the northern margin of the Mino Belt, adjacent to the HMB, are examined, and the significance of this shearing in tectonic evolution of the Inner Zone of SW Japan is discussed.
2. Geological setting of the Inner Zone of SW Japan The Inner Zone of SW Japan is composed of the following geologic belts: Hida; Sangun; Akiyoshi; Maizuru; Ultra-Tamba; Hida Marginal; and Mino (Tamba-MinoAshio) Belts (Ichikawa, 1990; Wakita et al., 1992; Fig. 1b). The Hida Belt is composed mainly of Paleozoic metamorphic rocks having continental affinities (Maruyama and Seno, 1986). The Sangun Belt is composed of Paleozoic to Mesozoic metamorphic rocks. The Akiyoshi and UltraTamba Belts are composed of Permian accretionary complexes, and the Mino Belt is composed of a Jurassic to Early Cretaceous accretionary complex. The Maizuru Belt is interpreted as an Upper Paleozoic island arc system
overlain by Upper Paleozoic to Lower Mesozoic strata. The sinuous surface trajectories of the geotectonic boundaries and the occurrence of tectonic outliers and windows indicate that the most of the geologic belts occur as subhorizontal or gently-northward-dipping thin tectonic units and form a huge pile of nappes in the western part of the Inner Zone (e.g. Isozaki et al., 1990). The Sangun, Akiyoshi, Maizuru, and Ultra-Tamba Belts are widely distributed in the western part of SW Japan, whereas these geologic belts are absent, and the HMB is very narrow between the Hida and Mino Belts in the eastern part of SW Japan (Fig. 1b). The HMB includes fault-bounded blocks derived from basement rocks of the Sangun, Akiyoshi and Maizuru Belts (e.g. Chihara and Komatsu, 1982) and Paleozoic rocks of shelf facies (e.g. Wakita et al., 2001). The HMB is interpreted as a major transcurrent fault zone with a complex displacement history (Wakita et al., 2001). The Jurassic accretionary complex of the Mino Belt comprises coherent units and melange units (e.g. Wakita et al., 2001). The coherent units are composed of imbricate thrust sheets of accreted rocks. The melange units contain fragments derived from collapsed seamounts and peeled oceanic crusts in a Jurassic muddy matrix.
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Paleomagnetic data indicate that before the opening of the Sea of Japan in Middle Miocene time SW Japan was oriented in a northeast direction (Otofuji et al., 1985; Fig. 1b).
rocks (Figs. 2 and 3). In the northeasternmost part of the area rocks of the Mino Belt are metamorphosed by the intrusion of uppermost Cretaceous to Quaternary granitoids (Harayama, 1990).
3. Geological overview of the study area
4. Description of the shear zone
The study area is in the Hida Mountains, about 300 km to the west of Tokyo, central Japan (Fig. 1). Rocks of both the Mino and HMBs are exposed in this area (Fig. 2). The rocks of the Mino Belt are divided into a coherent clastic rock unit, a coherent chert-basalt unit and a melange unit. Sedimentary structures, such as graded bedding occur in alternating beds of sandstone and mudstone in the clastic unit. The rocks are folded into a synclinal fold with a half wave length from 10 to 15 km and the axis plunging gently westward, as shown on the geological map (Fig. 2). The melange unit crops out mostly on the northern limb of the fold. The melange unit in the study area includes various kinds of clasts (the definition of clast is after Wakita, 1988) set in a matrix of black mudstone and gray siltstone (Figs. 2 and 3). These clasts consist of sandstone, alternating beds of sandstone and mudstone, felsic tuff, siliceous mudstone, bedded chert, limestone, basaltic lava and volcaniclastic rocks. The bedding planes of clasts trend east-southeast to northeast, and dip moderately to steeply southward (Figs. 3a and b), but in the northeasternmost part of the study area the bedding planes trend northeast to north and dip moderately to steeply westward (Fig. 3c). Most of clasts are lenticular, showing a good alignment, subparallel to the bedding planes. Limestones in the study area, yield Carboniferous to Permian fusulinoideans (Isomi and Nozawa, 1957; Kojima, 1984). Bedded cherts yield Permian to early Middle Jurassic radiolarians (Adachi and Kojima, 1983; Kojima, 1984; Imazato and Otoh, 1993; Niwa et al., 2002a, 2003). Felsic tuff, siliceous mudstone and mudstone yield Early to Middle Jurassic radiolarians (Adachi and Kojima, 1983; Kojima, 1984; Imazato and Otoh, 1993; Harayama, 1990; Niwa et al., 2002a, 2003). According to this fossil data, accretion occurred in the Early to Middle Jurassic. In the northern part of the study area the melange unit is deformed into foliated cataclasite forming cataclastic shear zones (Fig. 3) with closely spaced cleavages. Many clasts in the shear zones are intensely elongated, and are rotated in the shear planes. To the north the rocks of the Mino Belt are in fault contact with Permian strata of the Junigatake Formation and Paleozoic shelf facies rocks of the HMB (Wakita et al., 2001; Niwa et al., 2002b; Figs. 2 and 3). The fault strikes east and dips to the south. Shear zones composed of foliated cataclasite to mylonite extend along the fault. All the preCretaceous rocks in the study area are covered unconformably by uppermost Cretaceous to Quaternary felsic volcanic
4.1. Classification of the sheared rocks Melange in the study area is mostly sheared, with a spaced cleavage in the muddy matrix. The cleavage is defined by partings along aligned fine-grained phyllosilicates, and corresponds to the P foliation (Rutter et al., 1986; Takagi, 1996). In this paper, rocks of the Mino Belt in the study area are classified into the following three types based on approximate mesoscopic spacing of the cleavage: Type 1—with a spacing less than 5 mm; Type 2—with a spacing ranging from 5 mm to 5 cm; Type 3—with a spacing of over 5 cm (Figs. 4–6). Type 1 rocks are composed of foliated cataclasite (protocataclasite to ultracataclasite; Takagi and Kobayashi, 1996), and are exposed in several belts running in an E–W to NE–SW direction (Fig. 5). In this paper, these belts are called ‘cataclastic shear zones’. Each cataclastic shear zone is several tens to hundreds meters in width and trends subparallel to the boundary between the Mino and HMBs. The rocks in the study area, especially in the cataclastic shear zones show different deformation structures (Figs. 6–9). The deformation structures in each rock Type are as follows: 4.1.1. Type 1 In the rocks of Type 1, mudstone of the melange matrix has a cleavage with a spacing of less than 5 mm (Fig. 6a). The cleavage planes are mainly sinuous or planar, but are sometimes anastomosing (Fig. 4). Fine-grained phyllosilicates in the mudstone show strong alignment. Pressure solution cleavages (or pressure solution seams; Borradaile et al., 1982; Groshong, 1988) are developed, showing a preferred orientation of aligned phyllosilicates (Fig. 8a). Sheared clasts are transected by pressure solution cleavage traces, filled with finer dark grains (Fig. 8c). In bedded chert and in alternating beds of sandstone and mudstone the internal stratal continuity is entirely broken, clasts and mineral grains are intensely fragmented and tectonically abraded (Fig. 6b). Clasts with a lenticular-shape are included in the sheared rocks, and are elongated subparallel to the cleavage (Figs. 7c, d and 8a, b). Pressure shadows (Hobbs et al., 1976; Groshong, 1988) commonly occur around lenticular clasts and mineral grains (Figs. 8a and b). Several clasts have tails composed of fine-grained material with the same composition as the host clast (Fig. 7d). Pervasive cracks filled with quartz or calcite are observed in the sheared clasts, mainly sub-perpendicular to the alignment of clasts (Figs. 7d and 8d).
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Fig. 2. Simplified geologic map of the study area. See Fig. 1 for location.
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Fig. 3. Geological maps and profiles of the northern marginal part of the Mino Belt. The distribution of cataclastic shear zones (Type 1) is shown. See Fig. 2 for location. 791
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Fig. 4. Classification of the sheared rocks in the study area. Terminology of the cleavage pattern is from Borradaile et al. (1982).
Foliated cataclasite in Type 1 displays a composite planar fabric (Figs. 7b, c, 8b and 9b). The composite planar fabric consists of P foliation, Y shear (slipping surface), and R1 Riedel shear (Rutter et al., 1986; Takagi, 1996). The P foliation is defined by an alignment of finegrained phyllosilicates. The P foliation and Y shear intersect at an angle less than 408. The R1 Riedel shears are spaced several to several tens of centimeters apart, and each of them appears as a thin layer of a finer grained aggregate, severely fragmented. They cut the P foliation and Y shears at an angle of 20–408 and show drag effects (Fig. 7c). Ductile shear zones several tens of meters in width are found at the boundary between the Mino and HMBs in the Fukuji area (Figs. 3 and 9). Foliation with a spacing of only a few hundreds of microns is developed in ductile shear zones. The foliation is subparallel to the orientation of dynamically recrystallized grains and pressure solution cleavages. Quartz and calcite grains are fine-grained and intensely elongated due to the dynamic recrystallization (Figs. 9c and d). Subgrain boundaries are observed in these grains. 4.1.2. Type 2 In the rocks of Type 2, mudstone of the melange matrix has anastomosing cleavage with a spacing ranging from 5 mm to 5 cm, and shows a scaly fabric (Agar et al., 1989; Vannucchi et al., 2003; Fig. 4). Fine-grained phyllosilicates in the mudstone are aligned parallel to the cleavage. Alternating beds of sandstone and mudstone, and felsic tuff layers are isolated by boudinage (Fig. 6c). In contrast in bedded chert most of the internal stratal continuity is preserved. Sandstone and felsic tuff clasts have been rotated and fragmented (Figs. 6c and d). Lenticular clasts of sandstone and felsic tuff are arranged along the cleavage, and have tails composed of fine-grained material of the same composition as the host clast. Clasts of chert, limestone and mafic volcanic rocks do not have associated tails. Cracking and mineralization in clasts of chert, limestone and mafic volcanic rocks are rarely observed. Pressure solution cleavages are developed along the margins of clasts which have a preferred alignment.
4.1.3. Type 3 Type 3 mudstones show mostly a random-fabric (Figs. 6e and f). But in places, the rocks are weakly foliated, with a rough cleavage widely spaced at over 5 cm (Fig. 4). Phyllosilicates in the mudstone are randomly oriented. Most clasts are weakly sheared and have an oval-shape. Cracking and disaggregation of clasts are rarely observed. Pressure solution cleavage may occur around some clasts of chert and mafic volcanic rocks, but these cleavages do not have any preferred orientation (Fig. 6f). 4.2. Stereographic analysis of the shear zones Cataclastic shear zones of Type 1 display a welldeveloped P foliation in mudstone forming a cleavage defined by partings along aligned fine-grained phyllosilicates. The cleavage in the cataclastic shear zones trends E to ESE and dips moderately to steeply to the S in the western part of the study area (Mt. Junigatake and Dayoshi areas; Fig. 5a). In the eastern part (Hirayu and Fukuji areas; Fig. 5b) the cleavage trends E to NE and dips moderately to steeply to the S. The cleavage in the northeasternmost part (Shin-Hodaka area; Fig. 5c) trends NNE and dips steeply NW. Lenticular clasts in the cataclastic shear zones show a preferred orientation of the cleavage. Two types of lineation are developed on the cleavage, i.e. cataclastic lineation and slickenline. Cataclastic lineations correspond to the direction of the long axes of lenticular clasts (Tanaka 1992; Fig. 7a). Cataclastic lineations and slickenlines generally indicate homogeneous orientations. In the Mt. Junigatake and Dayoshi areas, the lineations plunge SW or SE at shallow to moderate angles (Fig. 5a). In the Hirayu and Fukuji areas, the lineations plunge SW or E to NE at shallow to moderate angles (Fig. 5b). In some places of the Hirayu and Fukuji areas, the lineations plunge SE. In the Shin-Hodaka area, the lineations plunge SW to SSW at shallow to moderate angles (Fig. 5c). 4.3. Asymmetrical deformation structures Asymmetrical deformation structures in the cataclastic shear zones indicate a sinistral sense of shear. Composite planar fabrics such as the obliquity of the P surface with respect to the Y surface (P-Y fabric; Figs. 7b and 8b) and the inclination of the R1 Riedel shear (P-R1 fabric; Figs. 7c and 9b) are used as indicators of the sense of shear (Takagi, 1996). Clasts of bedded chert and alternating beds of sandstone and mudstone are sheared and deformed to form boudinage. Each boudin had been displaced along the R1 shear plane in a manner similar to that seen in extensional fault systems (Needham, 1987; Fig. 7c). Extension cracks in fragmented clasts and mineral grains are observed in the foliated cataclasites. Each particle of the fragmented clasts and mineral grains is rotated in a ‘domino style’ (Fig. 8b), and the rotation is analogous to the sheared
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Fig. 5. Equal-area lower hemisphere projections of cleavages (P foliations) and slip directions of hanging wall in the cataclastic shear zones (Type 1). The distribution of the cataclastic shear zones is also shown. See Fig. 2 for location.
stack of cards model (Etchecopar, 1977; Simpson and Schmid, 1983). Lenticular clasts and mineral grains are accompanied by asymmetrical pressure shadows (Fig. 8b). Clasts of
sandstone, felsic tuff and chert are associated with asymmetrical tails (Fig. 7d). These clasts and mineral grains form asymmetrical s-type or d-type objects (Simpson and Schmid, 1983; Passchier and Simpson, 1986).
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Fig. 6. Photographs showing the types of sheared rocks in the study area. (a) Field outcrop of Type 1 foliated cataclasite enclosing lenticular sandstone and chert clasts in a matrix of mudstone. (b) Photomicrograph of the Type 1 foliated cataclasite (PPL). Fragmented and abraded grains are indicated by the white arrows. (c) Field outcrop of boudinaged interbedded sandstone (ss) in mudstone (ms) of Type 2. The mudstone shows a scaly fabric. (d) Photomicrograph of the Type 2 rocks including felsic tuff (ft) and sandstone (ss) lenses in a mudstone matrix (PPL). The lenses are rotated and fragmented. (e) Field outcrop of Type 3 rocks showing melange with a random-fabric. The melange includes clasts of felsic tuff, sandstone, chert, limestone, and mafic volcanic rocks in matrices of black mudstone and gray tuffaceous siltstone. (f) Photomicrograph of the Type 3 rocks (PPL). The rounded basalt clast (bs) is accompanied by pressure solution cleavages indicated by the white arrows.
5. Discussion 5.1. Cataclastic shear zones by secondary sinistral shearing The Mino Belt in the study area consists of following two lithological assemblages: (1) terrigenous clastic rocks such as sandstone and mudstone; (2) bedded chert, limestone and mafic volcanic rocks having affinities with collapsed seamounts and peeled oceanic crust. Pairing of the two types of lithology and the occurrence of melange
suggest that the Mino Belt represents an accretionary complex (e.g. Isozaki et al., 1990; Kojima and Kametaka, 2000). Melange is a common component of uplifted ancient accretionary complexes (Raymond, 1984; Cowan, 1985; Lash, 1987). Various geological processes have influenced melange formation, i.e. tectonic (Hsu¨, 1968), olistostromal (Abbate et al., 1970) and diapiric (Barber et al., 1986). The melange in the Mino Belt may have been formed at the accretion stage (e.g. Wakita, 1988; Nakae, 1990).
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Fig. 7. Photographs showing field outcrops of cataclastic shear zones (Type 1). (a) Cataclastic lineation plunging southwest on the P foliation of the foliated cataclasite (white arrow). (b) Strongly sheared bedded chert. The P-Y fabric indicates a sinistral sense of shear (white arrows). (c) R1-Y fabric in boudinage of alternating beds of sandstone and mudstone. The fabric indicates top-to-the-east movement (white large arrows). The sandstone lens is displaced on the R1 shear plane (white small arrow) and forms asymmetrical boudinage. (d) Foliated cataclasite including lenticular chert clasts in mudstone. The lenticular chert clast (outlined with white broken line) is associated with asymmetrical tails that indicate top-to-the-east movement (white large arrows). Cracks in the chert clast (white small arrows) are sub-perpendicular to the orientation of the aligned clasts.
Rocks along the northern margin of the Mino Belt in the study area are deformed into foliated cataclasite with sinistral shearing. Several deformation structures (e.g. scaly fabric, pressure solution cleavage, cracks filled with quartz or calcite, lenticular clasts with tails or pressure shadows and boudinage) recognized in this area have been reported from other areas of ancient and modern accretionary complexes (e.g. Fisher and Byrne, 1987; Needham, 1987; Nakae, 1990; Kimura and Mukai, 1991; Labaume et al., 1997), however, cataclastic shearing of Type 1 in the study area probably followed shortly after melange formation at the accretion stage, for the following reasons: 1. Foliated rocks of melange units in accretionary complexes generally show a scaly fabric having
disjunctive and anastomosing cleavage (Vannucchi et al., 2003). Mesoscopic spacing and the patterns of cleavages developed in rock Types 2 and 3 are nearly the same as those in common accretionary complexes. On the other hand, the Type 1 cleavage is different from the scaly cleavage common in accretionary complexes. The foliated cataclasite of Type 1 has cleavage with a spacing of less than 5 mm, and the closely spaced cleavage is mainly sinuous or planar. In addition, Type 1 rocks include ductile shear zones composed of deformed rocks formed by dynamic recrystallization (Fig. 9). 2. The south-dipping fault along the boundary of the Mino and HMBs is clearly not an accretion-related structure, because accretionary complexes are formed by the tectonic transfer of strata from the descending
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Fig. 8. Photomicrographs of foliated cataclasite (Type 1) in XZ thin sections (PPL). (a) Sandstone clasts associated with pressure shadows (ps) in foliated mudstone. Dark pressure solution cleavages are arranged subparallel to the foliation (white arrows). (b) Foliated cataclasite showing a sinistral P-Y fabric. Imbrication of cracked quartz grains (black arrows) and sandstone clasts (ss) with s-type pressure shadows show a sinistral sense of shear (white arrows). (c) Crushed sandstone clast associated with pressure solution cleavages (black arrows). (d) Sandstone clasts (ss) with cracks. The cracks filled with quartz (black arrows) are subperpendicular to the orientation of the aligned clasts.
oceanic plate into the overlying continental plate, consequently accreted strata underlie the continental plate. But in the study area the accretionary complex of the Mino Belt overlies HMB rocks of shelf facies along the south-dipping fault. Type 1 shear zones extend along the south-dipping fault between the Mino and HMBs. Cleavage in these shear zones trends E–W and dips S, parallel to the boundary fault. Therefore shear zones of Type 1 are most likely related to postaccretion faulting along the boundary between the Mino and HMBs.
5.2. Timing of sinistral shearing Sinistral cataclastic shear zones transect the melange unit of the Mino Belt in the study area. Since the melange mudstone matrix in this area includes middle to late Middle Jurassic radiolarians (Adachi and Kojima, 1983; 1984; Kojima, Harayama, 1990; Imazato and Otoh, 1993; Niwa et al., 2003), melange formation occurred after the Middle Jurassic. In the northern Fukuji area, a remarkable sinistral shear zone composed of foliated cataclasite is developed in
the Lower Cretaceous Tochio Formation of the Tetori Group (Tsukada, 2003; Fig. 2). This suggests that the sinistral shearing occurred during the Cretaceous, after the formation of the accretionary complex in the Jurassic. The sinistral cataclastic shear zones are overlain unconformably by unsheared felsic volcanic rocks of the Oamamiyama Group and Kasagatake Rhyolites (Figs. 2, 3 and 10a). Felsic tuff in the lower part of the Oamamiyama Group yields palynomorphs of Maastrichtian age (Kasahara, 1979). Lava of the Kasagatake Rhyolites has given radiometric ages of 68–55 Ma (Harayama, 1990). Moreover, the sheared rocks in the northeastern part of the Mino Belt are intruded by unsheared granitic rocks with radiometric ages of 64–54 Ma (Harayama, 1990; Fig. 10b). Hence, the sinistral shearing in and around the HMB probably occurred after Early Cretaceous time and had finished by the latest Cretaceous. 5.3. Significance of the cataclastic shearing in relation to the tectonic evolution of SW Japan In this study, it has been established that Cretaceous sinistral cataclastic shear zones are developed along the northern margin of the Mino Belt (Figs. 2 and 3).
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Fig. 9. (a) Photograph showing the field outcrop of the ductilely deformed rocks of Type 1. (b) Photograph showing a polished surface of (a). The R1-Y fabric indicates sinistral sense of shear (white arrows). (c) Photomicrograph of (b), showing pressure solution cleavages (psc) and dynamically recrystallized quartz (qtz) and calcite (cal) in a XZ thin section (XPL). (d) Close-up view of dynamically recrystallized quartz (XPL).
Sasaki et al. (2001) has also described several cataclastic shear zones along the northern margin of the Mino Belt which form a sinistral strike-slip imbricate fan. Major fault zones of Cretaceous age showing sinistral shearing are widely distributed along the continental margin of East Asia (Xu et al., 1989, 1993; Fig. 1a). In Japan, Cretaceous sinistral fault systems have played an important role in the tectonic history of Japan (e.g. Takagi, 1986; Taira and Tashiro, 1987; Otsuki and Ehiro, 1992). During Late Jurassic to Early Cretaceous time, the fast, relative northward, movement of the Pacific oceanic plate (Izanagi Plate) caused the development of sinistral fault systems along the Asian continental margin (Maruyama and Seno, 1986; Cox et al., 1989). According to the tectonic reconstruction proposed by Klimetz (1983); S¸engo¨r and Natal’in (1996); Wan and Zhu (1997), the eastern margin of Asia drifted northwards along these sinistral fault systems in Cretaceous time, driven by the northward movement of the western Pacific oceanic plate. Mizutani et al. (1990) suggested that the Jurassic to Early Cretaceous accretionary complexes, including
the Mino Belt, which are distributed along the eastern continental margin of Asia, drifted northward during or after accretion in the Cretaceous. Paleomagnetic data also suggests that the accretionary complex of the Mino Belt moved from a more southerly location to its present northerly position by sinistral movements in Cretaceous time (Hattori, 1982; Hirooka, 1990). Mizuno and Otsuka (1997) suggested that map-scale asymmetric half-basin and half-dome synclinal folds of bedded chert in the Mino Belt, demonstrate that sinistral simple shear occurred during Cretaceous time. Kimura (1999) reported that in a coherent unit of the accretionary complex within the Mino Belt, sinistral strike-slip faults overprint subduction-related thrust faults. The sinistral shear zones reported in this paper may provide further evidence for the northward shift of the accretionary complex of Mino Belt after or during the accretion. Kinematic data along the northern margin of the Mino Belt described in this paper support the model of the Cretaceous sinistral movement along the HMB. Tsukada (2003) has suggested that sinistral movement along the HMB might have resulted from a sinistral collision between
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References
Fig. 10. Photographs showing the boundary between the cataclasites in the Mino Belt and Upper Cretaceous rocks. (a) Sheared basalt clast (bs) in the cataclastic shear zone is overlain unconformably (A-A 0 ) by undeformed felsic tuff of the Maastrichtian Oamamiyama Group. (alt) Alternating beds of red felsic tuff and gray felsic tuff breccia; (ft) green felsic tuff; (cgl) basal conglomerate. (b) Foliated cataclasite is intruded by unsheared granitic rocks (black arrows). The intrusion age of the dikes has been dated by the Rb–Sr method at around 54 Ma.
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