Tectonic amalgamation of the Gaoligong shear zone and Lancangjiang shear zone, southeast of Eastern Himalayan Syntaxis

Accepted Manuscript Tectonic amalgamation of the Gaoligong shear zone and Lancangjiang shear zone, southeast of Eastern Himalayan Syntaxis Xuemeng Huang, Zhiqin Xu, Huaqi Li, Zhihui Cai PII: DOI: Reference:

S1367-9120(15)00102-9 http://dx.doi.org/10.1016/j.jseaes.2014.12.018 JAES 2275

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

2 September 2014 28 December 2014 31 December 2014

Please cite this article as: Huang, X., Xu, Z., Li, H., Cai, Z., Tectonic amalgamation of the Gaoligong shear zone and Lancangjiang shear zone, southeast of Eastern Himalayan Syntaxis, Journal of Asian Earth Sciences (2015), doi: http://dx.doi.org/10.1016/j.jseaes.2014.12.018

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Tectonic amalgamation of the Gaoligong shear zone and Lancangjiang shear zone, southeast of Eastern Himalayan Syntaxis

Xuemeng Huanga,b,c,★1, Zhiqin Xu a, Huaqi Li a, Zhihui Cai a

a

State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology,

Chinese Academy of Geological Sciences, Beijing 100037, China b

The Key Laboratory of Orogenic Belts and Crustal Evolution, Department of

Geology, Peking University, Beijing 100871, China c

Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, China

Earthquake Administration, Beijing 100085, China

1★

corresponding author:

Email address: [email protected] (Xue-meng Huang) -1-

Abstract The Gaoligong shear zone(GLGSZ) and Lancangjiang shear zone(LCJSZ) are two major large-scale shear zones southeast of Eastern Himalayan Syntaxis(EHS) and play important roles in the accommodation of the intracontinental deformation in southeast Asia. The two shear zones, which are mainly composed of high-grade metamorphic rocks and have long been regarded as the Precambrian basement of this area, are gradually merged northward and become tectonically amalgamated from Fugong to Gongshan area. Structural and kinematic analyses reveal that the amalgamation area has experienced strongly partitioned dextral transpression. N-S trending tight to isoclinal folds, sub-vertical foliation and sub-horizontal lineation are ubiquitously developed both within the shear zones and the Carboniferous flysch sediments of Baoshan block in between. Micro- and macro-kinematic indicators such as C-S structures, rotated porphyroclasts, asymmetric shear folds, sheath folds, domino and boudin structures suggest that the shear sense of the GLGSZ and LCJSZ is dextral in the amalgamation area. Comparative analyses of strata reveal that the Baoshan block is aligned with south Qiangtang block and the Lanping-Simao block was aligned with north Qiangtang block prior to the Cenozoic large scale tectonic extrusion and rotation. The amalgamation area, which has progressively switched positions from northeast of EHS to east and southeast of EHS, is just located at the neck of the large scale boudin of south Qiangtang-Baoshan block. A boudin model is proposed for the tectonic amalgamation of the GLGSZ and LCJSZ from Fugong to Gongshan area in response to the continued northward convergence between Indian plate and Eurasian plate since early Eocene, resistance of South China block, -2-

southeastward extrusion and clockwise rotation of Baoshan block around EHS.

Keywords: shear zone; Eastern Himalayan Syntaxis; tectonic amalgamation; boudin model

1. Introduction In response to the ongoing convergence between Indian and Eurasian plates, significant intracontinental deformation has been accommodated by crustal thickening on large-scale thrusts and lateral extrusion of continent-scale blocks along large-scale strike slip faults (Tapponnier et al., 1982, 1986; Peltzer and Tapponnier, 1988; Armijo et al., 1989; Dewey et al., 1989; Leloup et al., 1995; Lacassin et al., 1996; Yin et al., 2010, Fig.1). Several tectonic models, such as the indentation-extrusion tectonic model (Tapponnier et al., 1982,1986; Replumaz and Tapponnier, 2003), crustal thickening model (England and Houseman, 1989; Houseman and England, 1993) and lower crust flow model (Royden et al., 1997;Clark and Royden, 2000) have been advanced to elucidate the nature of the deformation of Tibet plateau. Additionally, Wang and Burchfiel(1997) considered that the extrusion was accompanied by strong internal deformation and rotation of smaller crustal fragments. GPS and geodetic data indicate that clockwise rotation of crustal blocks around the Eastern Himalayan Syntaxis is conspicuous (Chen et al., 2000; Wang et al., 2001; Zhang et al., 2004). The debates of these models center around whether the deformation is localized or distributed, and whether these boundary faults penetrate to the mantle (Tapponnier et -3-

al., 1982, 200; Leloup et al., 1995) or to the upper crust (Jolivet et al., 2001; Morley, 2007; Searle et al., 2010). The three rivers(Nujiang-Lancangjiang-Jinshajiang) areas, which have undergone significant crustal thickening, tectonic extrusion and re-orientation, are the best place to study the kinematics and dynamics of intracontinental deformation in SE Asia. Wide angle seismic tomography revealed that a comparable intra-crustal detachment exist in the three rivers area (Huang et al., 2009; Zhang and Wang, 2009). Based on magnetotelluric data, Bai et al.(2010) detected two major channels at a depth of 20-40km for the lower crust to flow, and underscores the role of the boundary faults shearing in the intracontinental deformation of SE Asia.

The southeast of Tibet is sliced by a network of large-scale shear zones, among which the Ailao Shan-Red River shear zone (ALS-RRSZ), the Gaoligong shear zone(GLGSZ) and Lancangjiang shear zone(LCJSZ) are the most prominent shear zones (Leloup et al., 1995; Roger et al., 1995; Wang and Burchfiel, 1997; Lee et al., 2003;Zhang et al., 2012b, Figs.1 and 2). These shear zones are the major boundaries of the extrusive blocks and keep accounts of the deformation history of this region, which is essential to unraveling the process and mechanism of the intracontinental deformation of Southeast Asia (Tapponnier et al., 1990a; Socquet and Pubellier, 2005; Searle, 2006). The Indochina block was extruded southeastward along the ALS-RRSZ to the east and GLGSZ to the west (Zhong et al., 1990; Tapponnier et al., 1990; Wang and Burchfiel, 1997; Yin et al., 1999; Zhang and Schärer, 1999; Wang et al., 2006; -4-

Leloup et al., 2001, 2007), and the displacement is over 500km (Leloup et al., 1995; Geissman et al., 2001, Fig. 1) . However, the Indochina block didn’t extruded as a single rigid block, it was dismembered into at least two major tectonic units, the Baoshan block to the west and the Lanping-Simao block to the east with the boundary of LCJSZ (Wang et al., 2006; Akciz et al., 2008; Xu et al., 2011). Geological investigation revealed that Tertiary shortening and shearing deformation are both prominent in southeast of Tibet (Leloup et al., 1995; Lacassin et al., 1996).

In recent years, a rapidly growing geochronological and structural studies have been systematically carried out on the GLGSZ (Ding, 1991; Zhong et al., 2000;Wang et al., 2006; Zhang et al., 2012b), LCJSZ (Wang et al., 2006; Akciz et al., 2008; Zhang et al., 2010, 2012a) and ALS-RRSZ (Tapponnier et al., 1990; Zhong et al., 1990; Harrison et al., 1992,1996; Leloup et al., 1995, 2001, 2007; Gilley et al., 2003; Searle, 2006; Anczkiewicz et al., 2007; Searle et al., 2010; Liu et al., 2012) to constrain the timings and kinematics of deformation of these shear zones. GLGSZ and LCJSZ are boundaries of the Baoshan block to the west and to the east respectively, and the two shear zones are gradually merged northward and become tectonically amalgamated from Fugong to Gongshan area. However, much less is known about the amalgamation mechanism of the two shear zones.

In this contribution, we present integrate field structural investigation, kinematic and fabric analysis of the two shear zones and rocks of Baoshan block in between. -5-

Combined with available data, we attempt to make clear the north continuation of Baoshan block prior to the tectonic extrusion and explore the possible mechanism of the tectonic amalgamation of GLGSZ and LCJSZ.

2.Geological setting The architectures of the amalgamation area in southeast of Tibet are composed of Langcangjiang shear zone, Baoshan block and Gaoligong shear zone (BGMRXZR, 1990; Wang and Burchfiel, 1997; Zhong et al., 2000), all of which have different rock types and deformation histories(Fig. 2).

2.1 Baoshan block The wedge-shaped Baoshan block, bounded between the GLGSZ to the west and LCJSZ to the east, is the northern part of Sibumasu block. This block is mainly composed of high-grade metamorphic Gaoligong Group, low to medium grade metamorphic Paleozoic strata, Mesozoic-Tertiary granites and Tertiary-Quaternary sediments (Song et al., 2010; Jin, 1994; Zhong et al., 2000). Cenozoic rocks are rare and characterized by folded upper Eocene-Oligocene conglomerate and sandstone that rest unconformably upon deformed older rocks(Akciz et al., 2008). Sedimentary facies, fossils in the Carboniferous and Permian strata combined with paleo-biogeographic evidences show that the Baoshan block may be a fragment of Gondwanaland (Wang, 1983; Jin, 2002;Metcalfe, 2002).Two stages of events have been recognized in the area, with the first one characterized by the unconformity -6-

between Eocene-Oligocene strata and lower folded Mesozoic rocks, and the second one characterized by the folded Eocene-Oligocene conglomerate and sandstone (Wang and Burchfiel, 1997; Akciz et al., 2008). The Baoshan block was amalgamated with the Yangtze Block along the Changning-Menglian suture zone due to the late Paleozoic subduction and collision (Zhong, 1998) . Based on the timing and kinematics of the shearing of the GLGSZ and LCJSZ, Wang et al.(2006) proposed that the Baoshan block escaped southeastward faster along the LCJSZ to the east and the GLGSZ to the west than the Northern Indochina block along the LCJSZ to the west and the ALS-RRSZ to the east.

2.2 Lancangjiang shear zone The LCJSZ, also called the Chongshan shear zone (Wang and Burchfiel, 1997) or Biluoxueshan-Chongshan shear zone (Zhang et al., 2012a), is the boundary between Baoshan block to the west and Lanping-Simao block to the east (Fig.2b). The shear zone mainly stretches along Biluoxueshan, Chongshan mountain, and Lincang batholiths pluton (BGMRYP, 1990; Wang and Burchfiel, 1997;Akciz et al., 2008; Zhang et al., 2010, Fig.2b). The north and south continuation of this shear zone remains unclear (Akciz et al., 2008).

The shear zone is mainly composed of mylonitic gneiss, migmatite, schist with metamorphic grades from greenschist to high-amphibolite facies, and some of the rocks have been considered to be Precambrian basement (BGMRYP, 1987;Heppe et -7-

al., 2007; Zhang et al., 2012a). This shear zone was a paired metamorphic belt: an eastern low-P/T belt which reaches upper amphibolite with local granulite facies(735° C at 5 kbar) and subsequently retrogressed at 450-500°C during post-Triassic time, and a western high-P/T belt which grades from west to east from blueschist through transitional blueschist/greenschist to epidote amphibolite facies at about 550–600°C (Zhang et al., 1993). The shear sense of the NW-SE trending southern segment(Chongshan shear zone) of the LCJSZ is sinistral (Wang et al., 2006;Akciz et al., 2008; Zhang et al., 2010; Zhang et al., 2012a), however, the shear sense of the S-N trending northern segment(Biluoxueshan shear zone) is dextral (Akciz et al., 2008; Zhang et al., 2012a). The timing of the shearing of this shear zone is 34-17Ma (Akciz et al., 2008). 40Ar/39Ar dating of syn-kinematic minerals revealed that the strike-slip shearing on the south segment of the LCJSZ at least began at ~32Ma (Wang et al., 2006). Based on available zircon U-Pb, monazite U-Th/Pb, and muscovite 40Ar/39Ar ages (Akciz et al., 2008; Zhang et al., 2010), Zhang et al. (2012a) reported that the transpressional deformation initiated at 57-46Ma, and strike slip shearing may begun at 34-32Ma. Socquet and Pubellier. (2005) postulated that the right-lateral shearing was later than the left-lateral shearing, and the later deformation phase may be related with a right-lateral drag by the GLGSZ. Zhang et al.(2010) proposed that this different shear sense of the two segments was caused by regional-scale strain partitioning of sinistral transpression.

2.3 Gaoligong shear zone -8-

The GLGSZ extends from the EHS, along the eastern side of Gaoligong range, and then to Longling area with an average elevation of 3500m. Southward, the shear zone bends along the Longling-Ruili fault with an average elevation of less than 2500m, and then extend further to join the Sagaing shear zone (Morley, 2007; Wang and Burchfiel, 1997;Zhang et al., 2012b,Figs.1and 2). The GLGSZ and Sagaing shear zone together act as the western boundary of extrusive Indochina block (Fig.2b). The GLGSZ is mainly composed of high-grade metamorphic rocks of Gaoligong Group, which has been regarded as Precambrian basement (BGMRYP, 1990;Zhong et al., 2000, Figs.2 and 3). The main rock units are metapelite, migmatitic gneisses and leucogranite (Zhong, 1998; Zhai et al., 1990;Wang et al., 2006; Song et al., 2010). Recent study reveals that Cenozoic metamorphism and deformation has reworked the preexisting Proterozoic rocks within the shear zone (Akciz et al., 2008; Eroğlu et al., 2013; Wang et al., 2006; Song et al., 2010; Zhang et al., 2012b). U-Pb ages on the granitic batholith reveal three stages of magmatism in the west of GLGSZ(Yang et al., 2006;Xu et al., 2008, 2012). The metamorphic history of the Gaoligong Group has been extensively studied (Ding, 1991; Ji et al., 2000; Zhong et al., 2000; Song et al., 2010). Three metamorphic stages have been identified from the GLGSZ (Ding, 1991; Zhong et al., 2000). Based on petrographic observations and thermobarometric estimates, Song et al. (2010) proposed two tectono-thermal events of the Gaoligong Group complex, that is an early regional metamorphism of medium to high pressure granulite facies along with magmatism due to crustal thickening prior to during ~53-22Ma, and a late metamorphism of greenschist facies restricted to narrow and -9-

strongly deformed shear zones. Structural overprinting relationship analyses discerned four episodes of deformation in the GLGSZ from Permian to late Tertiary (Zhang et al., 2012b).

A series of geochronological studies have been carried out along the shear zone. 39

Ar/40Ar ages of hornblende and plagioclase revealed that the shearing along the

GLGSZ occurred at 24-19Ma and 14-11Ma (Ji et al., 2000; Zhong et al., 2000). 39

Ar/40Ar ages of mica in mylonites revealed that the major phase of shearing is

18-23Ma (Lin et al., 2009), 16-10Ma (Zhang et al., 2012b), 27-29Ma (Wang et al., 2006). Multi-system geochronological data revealed three stages of cooling at 36~21Ma, 19~13Ma and 9~5Ma (Eroğlu et al., 2013). Based on the comparison of deformation time between Jiali shear zone and GLGSZ, Lee et al(2003) correlate the Jiali shear zone with GLGSZ through the Parlung fault.

3 Structure and kinematic analysis of the tectonic amalgamation area The GLGSZ and LCJSZ “sutured” directly from Fugong to Gongshan area, and only narrow belt of Paleozoic sediments of Baoshan block are intermittently exposed in between the two shear zones (Fig.3). Folds, sub-vertical foliations and sub-horizontal lineations are extensively developed in this area. In order to elucidate the mechanism of the amalgamation of these tectonic units, the structural, kinematics and fabrics of GLGSZ, LCJSZ and the Baoshan block in between are all studied.

- 10 -

3.1Deformation and kinematics of the GLGSZ The GLGSZ is mainly composed of mylonitic paragneisses, orthogneisses, augengneisses, leucocratic veins, migmatites, marbles and micaschists. The deformation history of the GLGSZ is complex and has experienced three stages of folding and a later ductile shearing from Permian to Tertiary (Zhang et al., 2012b). The GLGSZ is a long and narrow metamorphic belt with strongly developed mylonitic textures.

3.1.1 foliations and lineations L-S tectonic fabrics are pervasively developed along the GLGSZ, where both subvertical foliation and sub-horizontal stretching lineation are well developed. The foliation, formed by the preferred orientation of biotite, quartz, and feldspar ribbons, is nearly parallel to the Gaoligong mountain range and this shear zone. Stretching lineation, formed by elongation of quartz, feldspar, muscovite and biotite, long tails of porphyroclasts, and bouninaged felsic veins, is sub-horizontal in the foliation plane (Fig.4). Kinematic indicators of various types, such as rolled feldspar porphyroclasts, S/C structures, asymmetric shear folds, boudin and domino structures (Fig.4), are pervasive in the shear zone.

South of Lushui, prominent S-N lineation is marked by elongate quartz and feldspar ribbons (Fig.4A). S-N trending sub-horizontal stretching lineation is prominent on the sub-vertical S-N striking foliation plane at Gudeng town(site - 11 -

21-4)(Fig.4B). Fig.4C shows spectacular boudin structures of sheared and stretched leucocratic veins with dextral strike slip shear sense. Fig.4D shows σ-type rotated porphyroclast and flow fold of the felsic band in the paragneiss with dextral strike slip shear sense. South of Gudeng, Fig4E shows asymmetric shear folds with dextral shear sense. At Lumadeng county(site 21-3), down deep to the east bank of Lancangjiang river at this site, Fig.4F shows S-N trending boudin structures of domino-type fragmented feldspar porphyroclasts on the XZ plane of the strain ellipsoid with dextral strike slip shear sense, and Fig.4G shows S-N trending boudinage structures formed by strong lateral compression and elongation of the leucocratic band in the garnet+biotite+muscovite+sillimatite bearing paragneiss. The sigmoidal shape of the quartz porphyroclast indicates a right-lateral sense of shearing (Fig.4H).

The above different types of indicators are mutually consistent and show a right-lateral sense of shear. The coexistence of the fold, foliation and lineation suggest that the GLGSZ have experienced consistent dextral strike slip shearing and strong horizontal compression.

3.1.2 Structural cross sections at Gongshan The Dulongjiang river is a tributary of the Nujiang river and flows across the entire GLGSZ and part of the Tengchong block (Fig.5). A section along this tributary provides a fairly good opportunity to study the stratigraphy and structural style of the GLGSZ. This section mainly consists of three tectonic units, namely the granite of - 12 -

Tengchong block to the west, phyllite, schist and mylonitic gneisses of GLGSZ in the center, and Carboniferous marble of the Baoshan block to the east (Fig.6).

The GLGSZ is mainly composed of high grade mylonitic gneiss in the center and low grade schist at the two sides in Gongshan area (Fig.6). The high grade mylonitic gneiss mainly consists of biotite+feldspar+plagioclase bearing granitic augengneiss and mylonitic marble (Figs.7B and 7E), within which penetrative foliations and mineral stretching lineations are well developed. Marbles within the shear zone are strongly folded (Figs.6C and 7D). Concordant leucocranitic veins are folded with NNW trending axis and partly mylonitized (Fig.7E).The low grade schist belt mainly consists of foliated slate, phyllite and mica schist (Fig.7A). Additionally, the slate and marble were deformed by brittle faults with fault breccias and fault gouges (Fig.7C). Further to the west of the shear zone, granite and granodiorite of Tengchong block are slightly deformed and only develop some subvertical spaced cleavage at the marginal area (Figs.6A and 7F).

The intensity of deformation decreases from the center to both sides, especially to the western side (Figs.6A, 7A and 7C). The geometry and deformation intensity of the shear zone suggest that this shear zone may be a positive flower structure in cross section and the center part of the shear zone uplifts relatively faster than the two sides. The coexistence of folds, foliation and lineation within GLGSZ are characteristic of transpressional deformation under oblique convergent setting. - 13 -

3. 2 Deformation and kinematics of the LCJSZ The N-S trending segment of LCJSZ is located at the eastern side of the Nujiang River. This shear zone was regarded as a paired metamorphic zone, with lower grade schist belt to the east and high grade gneiss to the west (Zhang et al., 1993). The high-grade gneiss belt is mainly composed of garnet-biotite-sillimanite-bearing paragneiss, orthogneiss, augen gneiss with large feldspar porphyroblasts, migmatite, leucogranitic veins and granodiorite. The low-grade schist belt comprises severely folded phyllite with local mica schists (Zhang et al., 2012a).

Deeply dipping foliations and sub-horizontal lineations are well developed along this shear zone (Figs.8 and 9E-H). Penetrative foliations dip both to the west and east, with dip angles of 70-85°. Streching lineations are formed by the elongation of quartz, and feldspar porphyroclast with pitchs generally between 0 and 20°northward. Kinematic indicators, such as rolled feldspar and quartz porphyroclasts and folded leucocratic veins, show that the LCJSZ has consistent dextral strike slip shear sense

along the N-S trending segment (Figs.9A, 9D and 9F).

3.3 Deformation of the Paleozoic rocks of Baoshan block between the two shear zones The Carboniferous flysch sediments, sandwiched in between the two shear zones, are mainly consist of meta-quartz bearing sandstone, phyllite, slate and marble. The - 14 -

fact that Carboniferous strata of sandstone, limestone, mudstone and basaltic strata have undergone varying grades of metamorphism, suggests that regional high-grade metamorphism due to crustal thickening was not restricted to the shear zones (Song et al., 2010; Zhang et al., 2012a). These rocks have been extensively entangled in the later contraction and shearing deformation. Two kinds of folds are extensively developed in these rocks. One kind is tight to isoclinal upright folds with NNW-SSE striking axial planes (Fig.10E), and the other type is tight recumbent folds with nearly S-N striking axial planes (Figs.10D and10F). Sub-vertical foliations and sub-horizontal stretching lineations are also well developed in the Carboniferous meta-sedimentary rocks (Figs.10A and 10E). Various kinematic indicators, such as asymmetric feldspar porphyroclasts, C/S structures and sheared leucosome veins, show dextral strike slip shear sense (Figs.10B and10C). The combination of strike slip shearing and contractional deformation is consistent with a transpressional regime.

4 Discussion 4.1 Comparison of the cooling history of the GLGSZ and LCJSZ GLGSZ and LCJSZ, which are acted as two main boundary shear zones in the southeast of Tibet, play important roles in the accommodation of intra-continental deformation in response to the continuous convergence between India and Asia. The occurrence of Proterozoic, high grade metamorphic rocks at the surface along the GLGSZ and LCJSZ imply outstanding uplift. A large number of geochronological data, such as the ages of zircon, amphibolite, monazite, muscovite, biotite and apatite - 15 -

of different closure temperatures, have been published with regard to the timing of deformation and metamorphism of the two shear zones (Akciz et al., 2008; Lin et al., 2009; Song et al., 2010; Zhang et al., 2010; Zhang et al., 2012a; Eroğlu et al., 2013). Based on a synthesis of existing structural and geochronological data, we can tentatively depict the cooling path of the mylonites and deformed granites within the shear zones and elucidate the deformation history of the shear zones. The cooling and exhumation history of the two shear zones are nearly coincident (Fig.11). This similarity of the cooling history may imply that the two shear zones have nearly simultaneously shearing activity.

4.2 North continuation of Baoshan block Baoshan block and Lanping-Simao block are extruded blocks from Tibet, so that they must have some source relationship with some subcontinents within Tibet. Characteristics of the northern continuation of the Baoshan block and Lanping-Simao block have been strongly obscured by overprinting shortening and strike slip faulting around the Eastern Himlayan Syntaxis caused by the continued northward penetration of India into Asia.

The Qiangtang block was divided into North Qiangtang block(NQTB) and South Qiangtang block(SQTB) due to the existence of high pressure central Qiangtang metamorphic belt(CQMB) and Longmu Co-Shuanghu suture zone in center Qiangtang (Li et al., 2006; Zhai et al., 2011; Bao et al., 1999; Kapp et al., 2000, 2003). - 16 -

The NQTB exhibits warm-water faunas(Cathaysian affinity) and the SQTB exhibits cold-water biota and glacio-marine deposits(Gondwanan affinity) (Wang and Mu, 1983; Fan, 1985; Li, 1987; Li and Zheng, 1993; Chen and Xie, 1994). The boundary between Permo-Carboniferous sediments of Gondwana affinity and Cathaysian affinity extends from Longmu Co, Shuanghu to changing and Menglian (Li, 1987; Jin, 2002). Regionally, the only strata that may correlate with Lanping-Simao block are exposed in north Qiangtang block through the narrow belts of Mesozoic strata in the Three River merging area because they are commonly characterized by Jurassic terrestrial redbeds (Leloup et al., 1995; Akciz et al., 2008, Fig.1). The Carboniferous-Permian moraine strata of Gondwana affinity in Baoshan block can be correlated with the contemporaneous sediments within south Qiangtang block (Jin, 2002).

Palaeobiogeographically and palaeogeographically, on the basis of comprehensive comparison and correlation of Permian marine lithostratigraphy, biostratigraphy and faunal compositions among Tibetan blocks and adjacent blocks in western Yunnan, the south Qiangtang block should be aligned with the Baoshan block and the Lanping-Simao block should be aligned with north Qiangtang block(also called Qamdo block) (Jin, 2002; Metcalfe, 2006, 2013; Zhang et al., 2012c,Fig.12). Paleogeographic reconstruction also revealed that the south Qiangtang block was linked with Baoshan block and Sibumasu block (Metcalfe, 2006; Xu et al., 2012, - 17 -

2013). Geometrically, the south Qiangtang-Baoshan block is a large scale boudin and the areas around the EHS are just located at the neck of this Boudin.

4.3 Tectonic stress field of the study area GPS and geodetic observations revealed that the present day relative motion between India and Eurasian plate is about 40-54mm/yr with orientation of N20°E (DeMets et al., 1994; Paul et al., 2001; Socquet and Pubellier, 2005). Wang and Burchfiel.(2000) proposed that the assemblage of structures in southwestern Sichuan geometrically resembles structures in southern Yunnan that were positioned northeast of the EHS, and the present structures in southwest Sichuan can be serve as a guide to reconstruct the progressive tectonic development of the southwest Yunnan. Based on the earthquake focal mechanisms and fault slip data analysis in southwest Sichuan, Socquet and Pubellier (2005) inferred that the western Yunnan underwent dominant EW to ENE compression during Eocene to Miocene. In consideration of the long distance northward convergence of Indian plate, extrusion and clockwise rotation of materials around the EHS, southwestern Yunnan have been exposed to different stress patterns due to the different tectonic locations relative to the EHS. The stress pattern around EHS has been modeled from geological observations, earthquake data and geodetic data (England and Houseman, 1986; Holt et al., 1991; Huchon et al., 1994). The stress trajectory fans radially around the Eastern Himalayan Syntaxis, from SN in the Himalayan arc to EW in western Sichuan and South China, and finally to SN in Myanmar and Indochina. The main contributor of the stress pattern in Indochina is the - 18 -

northward migration of EHS (Holt et al., 1991; Huchon et al., 1994; Morley, 2007).

4.4 General deformation stages of the amalgamation area According to the tectonic reconstruction of the Tibet (Replumaz and Tapponnier, 2003; Replumaz et al., 2004, 2010; Royden et al., 2008; Xu et al., 2011;Pan et al., 2012), the amalgamation areas probably have progressively switched positions from the north of EHS to the east of EHS, and then escaped to the southeast of EHS due to the continuous convergence between Indian plate and Eurasian plate, tectonic rotation around the EHS and lateral extrusion of Baoshao block (Fig. 13). Given the obliquity of plate motion of India plate and different convergence angles relative to the shear zones, the study area has been located in a regional transpressional setting (Harland, 1971; Sanderson and Marchini, 1984;Dewey et al., 1998; Teyssier and Tikoff, 1998), and the study area probably has progressively experienced three stages of deformation from compression to pure shear dominated transpression, and to simple shear dominated transpression (Fig. 13). Transpressional deformation is prone to be partitioned into shear zone orthogonal compression and shear zone parallel strike slip shearing (Sanderson and Marchini, 1984;Dewey et al., 1998). In response to the shear zone orthogonal compression, folds are extensively developed in the Qiangtang block, Baoshan block and the two shear zones, and the transverse shortening is partly accommodated by horizontal extension and vertical uplift. In response to the shear zone parallel shearing, subvertical foliations and subhorizontal lineations are ubiquitous in the two shear zones. Due to the continuous indentation of the northeast - 19 -

corner of Indian plate, the strongest deformation occurred around the EHS and the south Qiangtang-Baoshan(SQT-BS) block narrowed significantly and bended clockwise around the EHS. Paleomagnetic analysis of the Cretaceous and Paleogene redbeds show that the Shan-Thai block has rotated clockwise since early Oligocene (Yang et al., 1995; Sato et al., 1999, 2001, 2007; Tong et al., 2013). Additionally, due to the southward extrusion of Baoshan block, enough space left behind was closed for the two boundary shear zones of GLGSZ and LCJSZ to juxtapose. The GLGSZ and LCJSZ

are

sutured

together

at

the

narrowest

area

of

the

south

Qiangtang-Baoshan(SQT-BS) block (Fig.14). As for the change in shear sense of the northern LCJSZ, the left/right-lateral shear of LCJSZ has been reported and discussed in previous studies (Soquet et al., 2005; Akciz et al., 2008; Zhang et al., 2010). One school of thoughts think that the right-lateral shear of the LCJSZ was a later event and was influenced by the drag of the motion of the GLGSZ (Soquet et al., 2005; Akciz et al., 2008). Another school of thoughts think that the left-lateral shear and right-lateral shear of LCJSZ were contemporaneous and were caused by progressive transpression (Zhang et al., 2010). The change of motion on shear zones is a common phenomenon and not an easy question in the rotated and extruded areas. Whether the geochronologcal data of mylonitic rocks in the amalgamation area between GLGSZ and LCJSZ represent the earlier left-lateral shear or later right-lateral shear needs more sophisticated studies, especially most of the early deformation relics were obliterated in the later deformation. I prefer to think that the left-lateral shearing of LCJSZ occurred before - 20 -

the tectonic amalgamation of LCJSZ and GLGSZ and the later right-lateral shear of LCJSZ may be occurred after the amalgamation of the two shear zones and caused by the drag of the motion of the GLGSZ.

5 Summary and Conclusions Structurally speaking, the GLGSZ, LCJSZ and strata of Baoshan block in between have experienced strong compressional and transpressional deformation, which are characterized by the strongly developed tight to isoclinal folds, subvertical foliations and subhorizontal lineations in the amalgamation area.

Paleogeologically, the south Qiangtang block and Baoshan block were linked together prior to the collision between India and Eurasian. South Qiangtang-Baoshan block have progressively transferred from north to east and southeast of EHS since early Eocene, during which the Baoshan block rotated clockwise around the EHS and extruded southward, and south Qiangtang-Baoshan block was bended and boundinaged as a result of progressive compressional to transpressional deformation.

GLGSZ and LCJSZ are amalgamated at the neck of the large scale boudin of south Qiangtang-Baoshan block. A boudin model is proposed for the tectonic amalgamation of GLGSZ and LCJSZ due to the continuous convergence between Indian plate and Eurasian plate, transpressional deformation, resistance of South China block, southeastward extrusion and clockwise rotation of Baoshan block. - 21 -

Acknowledgments This project was supported by the National Natural Science Foundation of China (No. 41302166). We would like to thank Duan Xiang-dong from Yunnan Bureau of Geological Survey for his careful logistic arrangement. We also thank anonymous reviewers for careful and critical reviews of the manuscript.

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Figure captions

Fig.1. Geologic sketch map of major tectonic features in Southeast Asia. modified after Peltzer and Tapponnier (1988) and Leloup et al.(1995). GLGSZ, Gaoligong shear zone; LCJSZ, Lancangjiang shear zone; ALSRRSZ, Ailaoshan-Red River shear zone; SGF, Sagaing fault; KF, Kunlun fault; XXF, Xianshuihe-Xiaojiang fault; WCF, Wangchao fault; TPF, Three Pagodas fault; DBPF, Dien Bien Phu fault; KJF, - 36 -

Karakoram-Jiali fault; HF, Haiyuan fault; QF, Qinling fault; PF, Poqu fault; LMSF, Longmenshan fault.

Fig.2.(a)Topographic map of Tibetan Plateau and surrounding areas. (b)Sketch map showing tectonic units，shear zones and brittle faults of southeastern East Himalayan Syntaxis. The white rectangle shows the location of the study area. The dominant tectonic units are labeled: BSB, Baoshan block; TCB, Tengchong block; LP-SMB, Lanping-Simao block; YZB, Yangtze block. NBSZ, Nabang shear zone; GLGSZ, Gaoligong shear zone; LCJSZ, Lancangjiang shear zone; ALS-RRSZ, Ailaoshan-Red river shear zone; (c)Tectonic section across the three shear zones(Socquet and Pubellier, 2005). 1.Magmatic rocks; 2. Precambrian metamorphic rocks;3. Shear zone and shear sense;4. Brittle fault and slip direction;5.Quaternary volcanic rocks; 6.Ophiolites; 7.Granites; 8.Quaternary-Neogene deposits; 9.Mesozoic-Paleozoic sediments; 10.Metamorphic rocks and schistosity; 11.Disconformity; 12.Shear sense; 13.Vertical shear sense.

Fig.3.Simplified geological map of the tectonic amalgamation area between Gaoligong shear zone(GLGSZ) and Lancangjiang shear zone(LCJSZ) at Fugong and Gongshan area. Geochronological data are from published papers(Lei et al., 2006;Wang et al., 2006;Akciz et al., 2008; Song et al., 2010; Zhang et al., 2012a, 2012b).

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Fig.4.Typical outcrop pictures of ductile structures of the GLGSZ. (A)N-S trending ‘pencil like’ subhorizontal mineral stretching lineations, southwest of Lushui. (site 25-10). (B)Sub-horizontal stretching lineation in the foliation of fine-grained paragneiss, south of Gudeng. (site 21-4). (C)Shearband boudins and asymmetric folds of felsic veins show dextral shearing, south of Gudeng. (Site 21-4, observed in the XZ plane). (D)Asymmetric shear folds of the leucogranitic band show dextral shearing, south of Gudeng. (site 21-4, observed in the XZ plane). (E)Asymmetric shear folds showing dextral shear sense, south of Gudeng. (site 21-6, observed in the XZ plane). (F)Domino structures show dextral shearing near Lumadeng. (site 22-3, observed in the XZ plane). (G)Bouninaged felsic veins in mylonitic garnet+muscovite+biotite+sillimatite geniss near Lumadeng. (site 22-3, observed in the XZ plane). (H)Stretched quartz grains and σ-type rotation of quartz porphyroclast shows dextral shearing south of Gudeng. (site 21-5, observed in the XZ plane).

Fig.5. Geological map of the Gongshan area, and Schmidt diagrams(lower hemisphere and equal area) for structural elements of mylonitic foliations and lineations at typical observation sites. Shaded contours represent poles to foliations of each observation sites. Black points represent stretching lineations.

Fig.6. a.Geological cross sections across Gaoligong shear zone(GLGSZ)at Gongshan in the eastern part of Dulongjiang pluton. b.Detailed geological map of the mylonitic gneiss. c.detailed geological map of the carboniferous sediments of Baoshan block. - 38 -

(see Fig.4 for location). 1Granite; 2Leucogranitic dyke; 3Granitic gneiss; 4Schist; 5Phyllite; 6Mylonitic marble; 7Carboniferous marble; 8Slate; 9Reverse fault.

Fig.7 Field observations along the Fugong geological cross section. (a)Crenulation folds in micaschist, site 23-1; (b)Biotite+feldspar+plagioclase granitic augengneiss, site 23-2; (c)Brittle fault in Carboniferous marble, site 23-3; (d)Disharmonious open fold in Carboniferous marble, site 23-4; (e)Leucogranitic dykes intruded into the mylonitic marble and some dykes are folded,(site 23-4,observed in the YZ plane); (f)Undeformed granodiorite of Cretaceous age, site 23-7.

Fig.8. Geological map of the Fugong area, and Schmidt diagrams(lower hemisphere and equal area) for structural elements of mylonitic foliations and lineations at typical observation sites. Shaded contours represent poles to foliations of each observation sites. Black points represent stretching lineations. MF, mylonitic foliations; SL, stretching lineations.

Fig.9. Field photographs of typical structures associated with ductile shearing along the

LCJSZ.

(A)S-N

trending

sub-horizontal

stretching

lineations

in

biotite+plagioclase+garnet mylonite, south of Fugong, site 25-1. (B)Tight folds in metabasalt, with axis trending NNW, south of Fugong, site 25-2. (C)Boudin structures and tight to isoclinal folds in Carboniferous marbles, south of Fugong, site 25-2. (D) - 39 -

Folded leucocratic veins show dextral shearing, south of Fugong. (observed in XZ plane, site 24-1). (E)Penetrative sub-horizontal stretching lineation, south of Fugong. site 24-1. (F)σ-type rotated quartz porphyroclasts show dextral shearing, south of Fugong. (observed in XZ plane, site 24-1). (G)Shear bands and folded leucocratic veins in biotite+garnet+sillimatite gneiss, north of Fugong. (observed in YZ plane, site

22-1).

(H)Sub-vertical

foliations

and

sub-horizontal

lineations

in

biotite+garnet+sillimatite gneiss, north of Fugong, site 22-1.

Fig.10 Representative field photographs showing deformation of the Carboniferous sediments. (A)Subhorizontal lineations in the Carboniferous limestone. north of Pihe, site 21-7, (B)Elongation of qurtz porphroblast inter-bedded in the Carboniferous slate showing dextral shear sense, south of Lishadi, site 22-4. (C) Folded quartz vein, south of Lishadi, site 22-4. (D)Recumbent fold structures with NNW trending axis in the Carboniferous marble. north of Pihe, site 21-7. (E)Sub-vertical foliations, sub-horizontal lineations and tight isoclinals fold in Carboniferous marble, south of Bingzhongluo, site 22-9. (F)Tight recumbent fold in Carboniferous marble, south of Gongshan, site 22-5.

Fig.11. Cooling path of deformed granites and mylonites from Gaoligong shear zone(GLGSZ) and Lancangjiang shear zone(LCJSZ). Age data include U-Pb zircon dates(Song et al., 2010; Zhang et al., 2010; Zhang et al., 2012a),U-Th/Pb monazite dates(Akciz et al., 2008), Rb/Sr muscovite dates(Eroğlu et al., 2013), 40Ar/39Ar dates - 40 -

from biotite and muscovite separates(Akciz et al., 2008; Lin et al., 2009; Zhang et al., 2010; Zhang et al., 2012a; Eroğlu et al., 2013), fission track dates for apatite(Wang et al., 2008; Eroğlu et al., 2013). Closure temperatures are taken from(Parrish, 2001).

Fig.12. General geologic framework of the Qinghai-Tibet Plateau, showing the main tectonic units and their boundaries(after (Zhang et al., 2012c). T, Tengchong block; B, Baoshan block; S, Simao block.

Fig.13 Tectonic reconstructions of the Himalayan-Tibet area especially southeast of Eastern Himalayan Syntaxis since early Eocene, the white rectangle represents the study area(amalgamation area). Modified after (Replumaz and Tapponnier, 2003; Royden et al., 2008; Pan et al., 2012; Xu et al., 2011, 2013) .

Fig.14 Boudin model of the tectonic amalgamation of GLGSZ and LCJSZ. TC, Tengchong block; SQT-BS, south Qiangtang-Baoshan block; LP-SM, Lanping-Simao block.

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Highlights: The Gaoligong shear zone(GLGSZ) and Lancangjiang shear zone(LCJSZ) are gradually merged northward and become tectonically amalgamated from Fugong to Gongshan area. Structural and kinematic analyses reveal that the amalgamation area has experienced strongly partitioned dextral transpression. The amalgamation area, which has progressively switched positions from northeast of EHS to east and southeast of EHS, is just located at the neck of the large scale boudin of south Qiangtang-Baoshan block.

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