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_Journal of Structural Geology 32 (2010) 445e463_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Structure, kinematics and ages of transpression during strain-partitioning in the Chongshan shear zone, western Yunnan, China Bo Zhang a,*, Jinjiang Zhang a, Dalai Zhong b The Key Laboratory of Orogenic Belts and Crustal Evolution, Department of Geology, Peking University, Beijing 100871, China b Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Article info Article history: Received 15 June 2009 Received in revised form 4 February 2010 Accepted 15 February 2010 Available online 21 February 2010 Keywords: Kinematics Geochronology Transpression Chongshan shear zone Western Yunnan Abstract The Chongshan shear zone extends from the eastern Himalayan Syntaxis to the Lincang Granitic pluton in Yunnan Province, China. The structure and kinematics show that the shear zone comprises mainly of mylonitic gneissemigmatite and schist with a dextral-dominated strike-slip motion in an NeS trending northern segment, and a sinistral strike-slip shear in NWeSE trending middle and southern segments. Both were developed under a bulk, regional-scale sinistral transpression. SHRIMP and LA-ICPMS UePb and 40Ar/39Ar dating reveal two Tertiary magmatic events along the zone, followed by younger sinistral strike-slip shear. The Eocene magmatic event (c. 55e38 Ma), followed by metamorphism at c. 36 Ma, happened before the strike-slip motion. The strike-slip shear along the zone began c. 32 Ma, which generated shear heating from 32 to 22 Ma. The 40Ar/39Ar ages of syn-kinematic micas range from 19 to 14 Ma and indicate that the strike-slip shear continued to this time with coeval transpressional exhumation and uplift of the metamorphic rocks along the Biluoxueshane-Chongshan chain. The Chongshan zone is thus a Cenozoic shear zone, which was contemporaneous with motion on the left-lateral Ailao Shan-Red River shear zone and the right-lateral Gaoligong shear zone, and should be important in accommodating the northwards movement of India during collision. During Oligocene to Miocene times, the continental block that was extruded between the Ailao Shan-Red River and Gaoligong shear zones was dismembered into at least two major fragments by the Chongshan shear zone. 1. Introduction Following the IndiaeAsia collision at equatorial latitudes approximately since 54e49 Ma (Dewey et al., 1989; Zhu et al., 2005a), India has indented into the Asian continent c. 2000 km northwards, forming the Himalayan orogen, crustal thickening and the uplift of the Tibetan plateau (e.g. Searle et al., 2007). Related to this collision, widespread intra-continental deformation occurred in SE Asia along large-scale NWeSE trending strike-slip shear zones (Tapponnier et al., 1982, 1986; Peltzer and Tapponnier, 1988). Abundant geological and geochronological evidence implies that the Ailao Shan-Red River shear zone in southern China and Vietnam, the Wang Chao (or Mae Ping) and Three Pagadas strike-slip fault zones in northern Thailand, the Gaoligong shear zone and Sagaing fault in southern China and Burma all played a key role in the eastward movement of fault-bounded continental blocks during the northward indentation of the Indian continent. The Chongshan shear zone (Fig. 1), also referred as the Chongshan metamorphic zone because of the good outcrops of metamorphic and intrusive rocks, extends from the eastern Himalayan Syntaxis in the north, then along the Biluoxueshane-Chongshan mountain to the south, and finally converges with the Lincang Granite pluton in the south (Fig. 2; BGMRYP, 1987, 1990; Wang and Burch?el, 1997; Akciz et al., 2008). This zone marks the boundary between the Lanping-Simao terrane in the east and the Shan-Thai block (Subimasu) in the west (Fig. 2), a topographic high extending into Thailand (Morley, 2004; Searle et al., 2007). Metamorphic rocks exposed in this zone include mica schists, gneisses, marbles and quartzites with metamorphic grades ranging from greenschist to high-amphibolite facies, some of these rocks have been suggested to be Precambrian basement (BGMRYP, 1987; Heppe et al., 2007). Although the Chongshan shear zone is an important geological boundary, the structure, kinematics and geochronology along it have not been constrained very well other than a recently proposed early Cenozoic strike-slip shear event by Akciz et al. (2008). Other 446 B. Zhang et al. Journal of Structural Geology 32 (2010) 445e463 Fig. 1. Schematic tectonic map of SE Asia showing major fault and shear zones. Inset (A) shows the extrusion of Indochina in response to northward penetration by India (modi?ed after Peltzer and Tapponnier, 1988; Tapponnier et al., 1990; Leloup et al., 1995; Morley et al., 2001; Morley, 2007). (B) CSSZ, Chongshan shear zone; Xuelongshan shear zone (XLSSZ), Diancangshan shear zone (DCSSZ) and Ailao Shan shear zone (ALSSZ) belong to the Ailao Shan-Red River shear zone; EHS, Eastern Himalayan Syntaxis; WCFZ, Wang Chao fault zone; TPF, three Pagodas fault zone; SGF, Sagaing fault; MMB, Mogok metamorphic zone. recent geochronological analyses on the southern Chongshan mountain also revealed some Cenozoic events (Wang et al., 2006; Heppe et al., 2007). In order to get a better understanding of this poorly documented zone, we conducted a detailed study between latitudes 24 x14e28 x14 north and longitudes 99 x14e 100 x14 east (Fig. 1). We have carried out a structural and kinematic study along the Chongshan shear zone and across three sections in western Yunnan, China (Fig. 2). We also present sensitive high resolution ion microprobe (SHRIMP) and laser ablation-multi-collector-inductively coupled plasma mass spectrometer (LA-ICPMS) B. Zhang et al. Journal of Structural Geology 32 (2010) 445e463 447 Fig. 2. Simpli?ed geological map of the Chongshan shear zone and adjacent areas, showing stereograms of the foliation (large circle) and mineral stretching lineation (black dot) for observing sites along the Chongshan and Ailao Shan-Red River shear zones. All diagrams are equal-area Schmidt net, lower hemisphere. geochronological data to constrain the ages of magmatism, deformation and metamorphism in the zone. 40Ar/39Ar dating was also carried out to constrain the end of shearing and associated retrograde metamorphism along the zone. Combining these data with structure analysis, we then compare the tectonic evolution of the Chongshan shear zone and assess its relationship to other major strike-slip shear zones in the region. 448 B. Zhang et al. Journal of Structural Geology 32 (2010) 445e463 2. Geological setting The YunnaneMyanmar area comprises a triangular region of crust bounded on its western margin by the NeS striking Gaoligong shear zone and NNE-striking Sagaing fault, and on its eastern side by the NW-SE striking Ailao Shan-Red River shear zone (Fig. 1). The area is characterized by prominent, young strike-slip shear zones and faults (Zhong and Tapponnier, 1990; Leloup et al., 1995; Morley, 2007). From west to east these are: the Gaoligong; Chongshan and Ailao Shan-Red River shear zones. The Gaoligong shear zone separates the Burma and Shan-Thai blocks, and the Chongshan shear zone lies between the Shan-Thai block and Lanping-Simao terrane (Fig. 2). The Gaoligong shear zone extends southward from the eastern Himalayan Syntaxis to the eastern Tengchong area and then swings northeastward to join the Sagaing fault (Figs. 1 and 2). The zone is exposed completely along the Gaoligong mountain west of the Nujiang valley (Figs. 1 and 2; Wang and Burch?el, 1997; Zhong, 2000) where dextral ductile shear sense indicators are widely preserved. The zone is thought to form the western boundary of an extruded Indochina continental fragment (Ji et al., 2000; Wang et al., 2006; Akciz et al., 2008; Lin et al., 2009). Wang et al. (2006) stated that dextral strike-slip motions on the Gaoligong shear zone began at 32 Ma based on the 40Ar/39Ar dating of synkinematic minerals. However, Ji et al. (2000) dated syn-kinematic muscovites and hornblendes and found two peak deformation ages between 24e19 Ma and 14e11 Ma within the southern segment of the zone exhibiting dextral strike-slip. More recently, the main phase of dextral strike-slip shearing along the northern extension of the Gaoligong zone into the Jiali fault has been dated between 18 and 13 Ma (Lin et al., 2009). The Chongshan shear zone forms the over 250-km long boundary between the Lanping-Simao terrane and Shan-Thai block (Fig. 1B). This zone consists mainly of mylonitic gneisses with amphibolite enclaves, leucogranites, migmatites, pegmatites, marbles and garnet-bearing schists with well-defined foliation parallel to the trend of the zone. Petrological and thermobarometric studies revealed two metamorphic stages: an early high-amphibolite facies event (Zhang et al., 1993; Heppe et al., 2007) and a later greenschist facies overprint (Zhang et al., 1993; Wang et al., 2006; Akciz et al., 2008). The early peak metamorphic conditions are equivalent to c. 30 km in depth (Meng et al., 2008). Zhang et al. (1993) suggested that the shear zone protoliths formed originally as part of an early Precambrian arc overprinted by a low-P T regional metamorphism during the emplacement of the Lincang Granite, and that this pluton experienced a post-Triassic retrograde metamorphism (Heppe et al., 2007). During the Cenozoic the shear zone formed and exhumation (from a depth of approximately 20 km) occurred from the late Eocene to Miocene. 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