Accepted Manuscript

Transpressional deformation, strain partitioning and fold superimposition in the southern Chinese Altai, Central Asian Orogenic Belt

Pengfei Li, Min Sun, Gideon Rosenbaum, Keda Cai, Ming Chen, Yulin He

PII: S0191-8141(16)30039-6 DOI: 10.1016/j.jsg.2016.04.006 Reference: SG 3333

To appear in: Journal of Structural Geology

Received Date: 9 December 2015 Revised Date: 13 March 2016 Accepted Date: 5 April 2016

Please cite this article as: Li, P., Sun, M., Rosenbaum, G., Cai, K., Chen, M., He, Y., Transpressional deformation, strain partitioning and fold superimposition in the southern Chinese Altai, Central Asian Orogenic Belt, Journal of Structural Geology (2016), doi: 10.1016/j.jsg.2016.04.006.

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Non-partitioning general shear strain

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High degree of strain partitioning

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fixed plate MANUSCRIPT Low degree of strain partitioning

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ACCEPTED P a g e | 1 ACCEPTED MANUSCRIPT

1

2

3 Transpressional deformation, strain partitioning and fold superimposition in the

4 southern Chinese Altai, Central Asian Orogenic Belt

5

6 Pengfei Li 1*, Min Sun 1, Gideon Rosenbaum 2, Keda Cai 3, Ming Chen 1, Yulin He 1

7 1 Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong,

8 China

9 2 School of Earth Sciences, The University of Queensland, Brisbane 4072, Queensland,

10 Australia

11 3 Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and MANUSCRIPT 12 Geography, Chinese Academy of Sciences, Urumqi 830011, China

13

14 *Corresponding author: Department of Earth Sciences, The University of Hong Kong,

15 Pokfulam Road, Hong Kong, China. Email: [email protected]; [email protected]

16

17 ACCEPTED P a g e | 2 ACCEPTED MANUSCRIPT

18 Abstract: Transpressional deformation has played an important role in the late Paleozoic

19 evolution of the western Central Asian Orogenic Belt (CAOB), and understanding the

20 structural evolution of such transpressional zones is crucial for tectonic reconstructions. Here

21 we focus on the transpressional Irtysh Shear Zone with an aim at understanding

22 amalgamation processes between the Chinese Altai and the West/East Junggar. We mapped

23 macroscopic fold structures in the southern Chinese Altai and analyzed their relationships

24 with the development of the adjacent Irtysh Shear Zone. Structural observations from these

25 macroscopic folds show evidence for four generations of folding and associated fabrics. The

26 earlier fabric (S 1), is locally recognized in low strain areas, and is commonly isoclinally

27 folded by F2 folds that have an axial plane orientation parallel to the dominant fabric (S2). S 2

28 is associated with a shallowly plunging stretching lineation (L ), and defines ~NW-SE MANUSCRIPT2 29 tight-close upright macroscopic folds (F 3) with the doubly plunging geometry. F 3 folds are

30 superimposed by ~NNW-SSE gentle F 4 folds. The F3 and F 4 folds are kinematically

31 compatible with sinistral transpressional deformation along the Irtysh Shear Zone and may

32 represent strain partitioning during deformation . The sub-parallelism of F 3 fold axis with the

33 Irtysh Shear Zone may have resulted from strain partitioning associated with simple shear

34 deformation along narrow mylonite zones and pure shear-dominant deformation (F 3) in fold

35 zones. The strainACCEPTED partitioning may have become less efficient in the later stage of

36 transpressional deformation, so that a fraction of transcurrent components was partitioned

37 into F 4 folds.

38 P a g e | 3 ACCEPTED MANUSCRIPT

39 Key words : Central Asian Orogenic Belt; Chinese Altai; Irtysh Shear Zone; Transpression;

40 Strain partitioning

41

42 1. Introduction

43 Transpressional deformation in orogenic belts commonly results from oblique plate

44 convergence (Harland, 1971; Carosi and Palmeri, 2002; Holdsworth et al., 2002; Sarkarinejad

45 et al., 2008; Zhang et al., 2010; Díaz-Azpiroz et al., 2014; Xu et al., 2015), and is

46 characterized by general shear strain that involves both simple and pure shear components

47 (Sanderson and Marchini, 1984; Fossen and Tikoff, 1993; Lin et al., 1998). Theoretically, the

48 general shear strain in transpressional zones can be either distributed homogeneously (Fig.

49 1a), or partitioned across the deformation zone due to the presence of mechanical anisotropies MANUSCRIPT 50 (Fig. 1b, c) (Fossen et al., 1994; Tikoff and Teyss ier, 1994; Jones and Tanner, 1995; Dewey et

51 al., 1998; Schulmann et al., 2003; Massey and Moecher, 2013). The simplest partitioning

52 model involves a series of discrete slip planes that accommodate simple shear deformation,

53 with pure shear deformation occurring in-between contractional domains (Fig. 1b; e.g.

54 Dewey et al., 1998). Alternatively, discrete slip planes may only accommodate a fraction of

55 the lateral displacement, with the remaining transcurrent component distributed in

56 contraction-dominantACCEPTED domains between discrete slip planes (Fig. 1c) (Tikoff and Teyssier,

57 1994; Teyssier et al., 1995; Miller, 1998). The degree of strike-slip partitioning (i.e. the

58 relative amount of lateral displacement partitioning into discrete slip planes) is dependent on

59 the orientation of the convergence vector relative to the boundary (Fossen et al., 1994; Tikoff P a g e | 4 ACCEPTED MANUSCRIPT

60 and Teyssier, 1994; Teyssier et al., 1995), and the rheological weakening along the shear zone

61 (Mount and Suppe, 1987; Zoback et al., 1987; Mount and Suppe, 1992).

62

63 The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens in the

64 world. It underwent a prolonged history of accretion from the late Mesoproterozoic to

65 Mesozoic (Zonenshain et al., 1990; Şengör et al., 1993; Zorin, 1999; Khain et al., 2002; Xiao

66 et al., 2003; Buslov et al., 2004a; Buslov et al., 2004b; Yakubchuk, 2004; Windley et al.,

67 2007; Wilhem et al., 2012; Han et al., 2015; Xiao et al., 2015; Zhang et al., 2015b), and was

68 characterized by multiple phases of deformation (e.g. Qu and Zhang, 1994; Allen et al., 2001;

69 Lin et al., 2009; Wang et al., 2010; Zhang and Cunningham, 2012). Recognizing the styles of

70 deformation is crucial for reconstructing the orogenic history and its associated episodes of MANUSCRIPT 71 terrane accretion. Within the CAOB, multiple genera tions of folds have been recognized, and

72 the origin of these structures was taken into account in large scale tectonic reconstructions

73 (Lehmann et al., 2010; Tian et al., 2013; Guy et al., 2014). For example, macroscopic Type 2

74 fold interference patterns in the Beishan region of the southern CAOB were inferred as

75 indicating two episodes of orthogonal convergence of the Tarim-North China Craton,

76 following the closure of the Paleo-Asian Ocean (Tian et al., 2013). Transpressional

77 deformation wasACCEPTED particularly common during the late Paleozoic amalgamation of the western

78 CAOB, and this deformation has apparently modified the earlier accretion-related

79 architecture and generated superimposed fold structures (Qu and Zhang, 1994; Allen et al.,

80 2001; Choulet et al., 2012; Li et al., 2015a). Understanding the structural evolution and strain P a g e | 5 ACCEPTED MANUSCRIPT

81 patterns across major transpressional zones is crucial for reconstructing the amalgamation

82 processes of the western CAOB in the late Paleozoic and the earlier accretion history.

83

84 This paper focuses on fold structures in the area of the Irtysh Shear Zone, southern Chinese

85 Altai (Fig. 2). This sinistral shear zone marks the front of an oblique collision between the

86 Chinese Altai and the intra-oceanic arc system of the West/East Junggar, extending >1000 km

87 from NE Kazakhstan, through NW China, to western Mongolia (Qu and Zhang, 1991; Qu

88 and Zhang, 1994; Laurent-Charvet et al., 2003; Liu et al., 2013; Li et al., 2015a). A series of

89 macroscopic (~10-50 km) doubly plugging folds that commonly show a dome shape in map

90 view, occur adjacent to the Irtysh Shear Zone (Figs. 2c and 3). It remains unclear whether

91 these doubly plunging folds resulted from superimposed folding or they were produced by a MANUSCRIPT 92 single phase of deformation. Available chronologica l data (e.g. Li et al., 2015b) suggest that

93 macroscopic folds in this area were developed in the Permian, which is the timing of

94 transpressional deformation along the Irtysh Shear Zone (Laurent-Charvet et al., 2003; Briggs

95 et al., 2007). It seems, therefore, that the doubly plunging folds are genetically linked to the

96 development of the Irtysh Shear Zone. As such, understanding the origin of these

97 macroscopic folds and their relationship with the regional shear zone could provide rigorous

98 constraints on theACCEPTED convergence angle and amalgamation processes between the Chinese Altai

99 and the West/East Junggar. To achieve this aim, we have conducted detailed structural

100 mapping around the macroscopic doubly plunging folds in the Qiongkuer Domain of the

101 southern Chinese Altai (Fig. 3). P a g e | 6 ACCEPTED MANUSCRIPT

102

103 2. Geological setting

104 The Chinese Altai is located in the junction between the peri-Siberian, West

105 Junggar-Kazakhstan-Tianshan, and Xing'an-Mongolian-East Junggar orogenic systems (Fig.

106 2a). It records Paleozoic accretionary processes along the southwestern margin of the

107 Siberian Craton, and the subsequent late Paleozoic convergence history with the intra-oceanic

108 arc systems of the West/East Junggar (Windley et al., 2002; Xiao et al., 2008).

109

110 The Chinese Altai mainly consists of Paleozoic meta-sedimentary/volcanic rocks, and is

111 divided into four ~NW-SE tectonic domains (Fig. 2c) (He et al., 1990; Windley et al., 2002;

112 Cai et al., 2011a). Based on the regional geological map (Li et al., 2008), these four tectonic MANUSCRIPT 113 domains can be traced along the strike into Mongoli a, Russia and Kazakhstan. The Northern

114 Altai Domain is the northernmost unit in the Chinese Altai, extending eastward into Mongolia.

115 It mainly includes Late Devonian to Early Carboniferous metasedimentary and metavolcanic

116 rocks with a metamorphic grade up to greenschist facies. The Central Altai Domain, which is

117 separated from the Northern Altai Domain by a normal fault, occupies the major part of the

118 Chinese Altai (Fig. 2c) as well as the Altai-Mongolian terrane in Russia and Mongolia. Rocks

119 in this domain areACCEPTED predominantly represented by Cambrian to Silurian marine-facies

120 turbidites and pyroclastic rocks of the Habahe Group and the Kulumuti Group, which were

121 interpreted as an accretionary complex developing along the adjacent Tuva-Mongolian

122 microcontinent (Fig. 2b) (Long et al., 2012). Metamorphism in this domain is characterized P a g e | 7 ACCEPTED MANUSCRIPT

123 by zonal metamorphic sequences centered by gneissic granitoids (Zhuang, 1994). Late

124 Ordovician to Devonian felsic volcanic rocks of the Dongxileke Formation and marine-facies

125 clastic rocks of the Baihaba Formation unconformably overlie the Habahe Group in the

126 westernmost Central Altai Domain (Fig. 2c) (Long et al., 2010). Farther south, the Qiongkuer

127 Domain is mainly occupied by Devonian volcanic and sedimentary rocks of the

128 Kangbutiebao and Altai formations, which were subjected to high temperature

129 metamorphism, locally up to granulite facies (Wang et al., 2009; Li et al., 2014; Tong et al.,

130 2014a; Yang et al., 2015b). The Qiongkuer Domain is possibly correlated with the Rudny

131 Altai in NE Kazakhstan and the Tseel terrane in Mongolia (Badarch et al., 2002). The

132 Southern Altai Domain is the southernmost tectonic unit of the Chinese Altai, and is

133 represented by schist, para-/ortho-gneiss, amphibolite, migmatite and metaschert of the Irtysh MANUSCRIPT 134 Complex (Qu and Zhang, 1991; Briggs et al., 2007; Li et al., 2015a). The origin of these

135 rocks is considered to be an accretionary complex (O'Hara et al., 1997; Xiao et al., 2009),

136 which can be traced into NE Kazakhstan, termed as the Irtysh-Zaisan Complex (Windley et

137 al., 2007) or the Kalba-Narym terrane (Buslov et al., 2004a; Safonova, 2013).

138

139 Rocks in the Qiongkuer and Southern Altai domains, as well as in the southern part of the

140 Central Altai Domain,ACCEPTED show evidence for late Paleozoic transpressional deformation along

141 the sinistral Irtysh Shear Zone (Qu and Zhang, 1991; Qu and Zhang, 1994; Laurent-Charvet

142 et al., 2002; Laurent-Charvet et al., 2003; Liu et al., 2013; Li et al., 2015a; Zhang et al.,

143 2015a; Li et al., 2016). This deformation phase was associated with a major uplift event in P a g e | 8 ACCEPTED MANUSCRIPT

144 the Permian as indicated by 40 Ar/ 39 Ar thermochronological data (Laurent-Charvet et al., 2003;

145 Briggs et al., 2007; Li et al., 2015b). The transpressional deformation has been attributed to

146 oblique convergence between the Chinese Altai and the West/East Junggar (Qu and Zhang,

147 1994; Li et al., 2015a), which was accompanied in the southern Chinese Altai by high

148 temperature metamorphism during the Permian (Wang et al., 2009; Li et al., 2014; Wang et

149 al., 2014b; Yang et al., 2015a; Yang et al., 2015b). In the Mesozoic and Cenozoic, the Irtysh

150 Shear Zone and/or other ~NW-SE faults in the southern Chinese Altai (Figs. 2 and 3) may

151 have been reactivated as the uplift of the Altai Mountain (Yuan et al., 2006; Glorie et al.,

152 2012; Delvaux et al., 2013).

153

154 A large number of granitoids occur in the Chinese Altai (Fig. 2c) and their ages are mostly MANUSCRIPT 155 Devonian and Permian (Zou et al., 1988; Liu, 1990; Yuan et al., 2007; Sun et al., 2009; Cai et

156 al., 2011b; Kröner et al., 2014; Tong et al., 2014b). The Devonian granitoids occur throughout

157 the Chinese Altai and were supposedly emplaced along a convergent continental margin

158 (Wang et al., 2006; Sun et al., 2008). In contrast, Permian granitoids are mainly distributed

159 along the southern Chinese Altai (Fig. 2c), and their origin is normally attributed to the

160 collision of the Chinese Altai with the West/East Junggar (Tong et al., 2014b). In addition, a

161 few Mesozoic graniticACCEPTED plutons were emplaced with a possible non-orogenic origin (Li et al.,

162 2013; Wang et al., 2014a).

163

164 3. Lithostratigraphy of the Qiongkuer Domain in the Fuyun area P a g e | 9 ACCEPTED MANUSCRIPT

165 The Qiongkuer Domain in the Fuyun area is occupied by two lithostratigraphic units: the

166 Kangbutiebao Formation and the Altai Formation (Figs. 2c and 3). A conglomerate layer (Fig.

167 4a) is recognized between the two formations, which together with the presence of an

168 underlying paleo-weathering layer, suggest an unconformable contact between the two

169 formations (BGMRX, 1978a). The spatial distribution of lithostratigraphic units is controlled

170 by macroscopic folds with the Kangbutiebao and Altai formations dominantly distributed in

171 the core areas of antiforms and synforms, respectively (Fig. 3).

172

173 Rocks of the Kangbutiebao Formation in the Fuyun area are metamorphosed up to granulite

174 facies (Li et al., 2014; Yang et al., 2015b), and are dominantly represented by migmatized

175 orthogneisses interlayered with minor amphibolite and metasedimentary rocks. The MANUSCRIPT 176 orthogneiss was likely derived from a felsic volcan ic protolith (BGMRX, 1978a), as indicated

177 by observed volcanic textures in the lower grade part of the Kangbutiebao Formation in the

178 Aletai region (Fig. 2c) (Chai et al., 2009). U-Pb zircon geochronology from orthogneiss and

179 metavolcanic rocks indicates an Early Devonian age for the Kangbutiebao Formation (Chai et

180 al., 2009; He et al., 2015).

181

182 The Altai FormationACCEPTED in the Fuyun area is lithologically distinct from the rest of the Altai

183 Formation in the Chinese Altai (Fig. 2c). It is characterized by the widespread occurrence of

184 amphibolite and amphibole schist, which rarely occur within the turbidite-dominant sequence

185 in other parts of the Altai Formation. Based on geochemical considerations, Xu et al. (2003) P a g e | 10 ACCEPTED MANUSCRIPT

186 suggested that the protoliths of these amphibolites originated in a back arc environment. The

187 internal stratigraphy of the Altai Formation is not well recognized due to the intense

188 deformation. Based on field observations and the interpretation of satellite images (Appendix

189 A), we here subdivide the Altai Formation in the Fuyun area into four lithostratigraphic units

190 (units 1-4; Fig. 3), which are approximately similar to the units in the 1:200,000 geological

191 map (BGMRX, 1978a). The lower part of the Altai Formation (unit 1) is characterized by

192 diagnostic white and black layers of quartzofeldspathic gneiss, amphibolite and banded chert

193 (particularly typical south of Kuerti, Figs. 3 and 4b, c). Rocks in unit 2 are relatively

194 homogeneous, and are dominated by amphibole schist and amphibolite with minor

195 porphyritic metavolcanic rocks and quartzite (Fig. 4d, e), in which deformed pillow basalt

196 was recognized (Fig. 4f). Further up, unit 3 is characterized by a thin layer of gray-white MANUSCRIPT 197 quartz schist. Unit 4, at the top of the sequence, mainly comprises gray-green chlorite

198 actinolite schist, meta-siltstone and porphyritic metavolcanic rocks.

199

200 4. Structural observations from the Qiongkuer Domain (Fuyun area)

201 A series of macroscopic doubly plunging fold structures with different sizes were recognized

202 in the Qiongkuer Domain of the southern Chinese Altai (Fuyun area, Fig. 3). Here we focus

203 on two sets of doublyACCEPTED plunging folds, termed the eastern and western doubly plunging folds,

204 respectively (Fig. 3). The eastern doubly plunging folds are characterized by a map-view

205 dome shape with >50 km long axis (Fig. 3). In order to constrain its geometry, we conducted

206 two structural transects in the hinge areas of this macroscopic fold structure (Figs. 3 and 5), P a g e | 11 ACCEPTED MANUSCRIPT

207 complemented by satellite image interpretation (Appendix A) (Fig. 5). In addition, detailed

208 structural observations were made throughout the western doubly plunging folds (Figs. 6 and

209 7).

210

211 4.1. Structures of the eastern doubly plunging folds

212 4.1.1. Mesoscopic structures (S 1 and S 2)

213 Two generations of outcrop-scale fabric were recognized around the eastern doubly plunging

214 folds. S 1 is locally recognized, and isoclinally folded with the axial plane parallel to S 2 (Fig.

215 8a). S 2 is a dominant fabric, and represents an axial planar fabric of F 2 folds (Fig. 8a, b). It is

216 predominantly associated with symmetric lens-shaped clasts and conjugate shear sets (Fig. 8c,

217 d). A shallowly plunging stretching lineation (L ), which is defined by preferred alignment of 2 MANUSCRIPT 218 amphiboles or stretched quartz and feldspar aggrega tes, is recognized within S 2 (Fig. 8e). On

219 a larger scale, S 2 shows variable orientations that define the ~NW-SE macroscopic folds of F3

220 (Fig. 5). Metamorphism associated with S 2 reaches the amphibolite facies in the core of the

221 macroscopic antiform as indicated by the mineral assemblage of amphibole, plagioclase and

222 garnet. The metamorphic grade progressively decreases to greenschist facies in the core of

223 macroscopic synform (i.e. unit 4 of the Altai Formation).

224 ACCEPTED

225 4.1.2. Macroscopic fold structures (F 3)

226 Macroscopic F 3 antiforms and synforms are characterized by tight to close inter-limb angles

227 (Fig. 5c, d). In the western transect (Figs. 3 and 5a), S 2 defines F 3 folds with β axis of 14-306 P a g e | 12 ACCEPTED MANUSCRIPT

228 (plunge angle-plunge) (Fig. 5e), which together with the map-view axial trace of F 3 (azimuth

229 126 °), indicates that the F3 axial plane must be vertical (Fig. 5g). Stretching lineations (L 2) in

230 this area gently plunge to ~NW (~11-304). Similarly, in the eastern transect, S 2 defines F 3

231 folds with β axis of 19-115 and a map-view axial trace of 114 °, which constrain the F3 axial

232 plane at 87-204 (Fig. 5h, j). The mean orientation of stretching lineations (L 2) across this

233 transect is 20-120 (Fig. 5i). Overall, L 2 stretching lineations are generally subparallel to the

234 axis of F 3 that shows similar axial plane orientations across the two transects. The opposite

235 plunging orientations of both F3 fold axes and stretching lineations (L2) in the eastern and

236 western transects (Fig. 5a, b), reveal the doubly plunging geometry of macroscopic F 3 folds.

237

238 At the outcrop scale, the dominant S fabric is folded by F and overprinted by S (Fig. 8f, g). 2 MANUSCRIPT3 3 239 The F 3 folds are commonly tight to close and rounded with a steeply dipping axial plane.

240 Folded L2 stretching lineation around F3 folds is also observed (Fig. 8h).

241

242 4.2. Structural observations from the western doubly plunging folds

243 The western doubly plunging folds are separated from the eastern doubly plunging folds by

244 ~E-W faults, which show sinistral strike-slip movements as manifested by the offset of

245 lithological layersACCEPTED (Fig. 3). The western doubly plunging folds are characterized by a

246 map-view dome shape with a ~15 km long axis and a ~4 km short axis (Figs. 3 and 6). Rocks

247 are gneissic granitoids in the core of the structure, surrounded by the Kangbutiebao and Altai

248 formations in the rim (Fig. 6). P a g e | 13 ACCEPTED MANUSCRIPT

249

250 4.2.1. Mesoscopic structures (S 1 and S 2)

251 Two generations of fabric (S 1 and S 2) were recognized around the western doubly plunging

252 folds (Fig. 6). S 1 is locally recognized in low strain areas (Fig. 9a), but is commonly

253 transposed and overprinted by S 2, which is the dominant fabric throughout the mapping area

254 (Fig. 9b, c). S 2 is parallel to the axial plane of F 2 folds, and is associated with a stretching

255 lineation (L 2, Fig. 9d). The development of S 2 in this area involved high temperature

256 metamorphism (amphibolite facies) and migmatization. In the area of the western doubly

257 plunging folds (Fig. 6), S 2 shows variable orientations that define macroscopic F 3 and F 4 fold

258 structures.

259 MANUSCRIPT 260 4.2.2. Macroscopic folds (F 3 and F 4)

261 Two generations of macroscopic folds were recognized with earlier ~NW-SE tight to close F 3

262 folds overprinted by ~NNW-SSE F 4 folds (Figs. 6 and 7). The map-scale superimposition of

263 F4 and F3 is manifested in the southwestern part of the western doubly plunging folds (bottom

264 left corner in Fig. 6). In order to understand the macroscopic geometry of F 3 and F 4, we

265 divided the map area into four domains (Fig. 6), with domains 1 and 4 dominated by the

266 geometry of F 3 andACCEPTED domains 2 and 3 dominated by the F4 geometry.

267

268 The macroscopic geometry of F 3 is best recognized in domains 1 and 4. Domain 1 shows a

269 series of map-scale ~NW-SE antiforms and synforms (Fig. 6). 58 measurements of S 2 define P a g e | 14 ACCEPTED MANUSCRIPT

270 a fold hinge at 31-311, which together with the map-view axial trace of F3 (azimuth 121 °),

271 constrain the dip and dip direction of the axial plane (74-031) (Fig. 6). In domain 4, only one

272 map-scale F 3 antiform was mapped, with a strike orientation of 104 °. The calculated F3 hinge

273 in this domain is 53-099, which together with the fold trace, constrain an axial plane of F 3 at

274 86-014 (Fig. 6). The opposite hinge orientations of F 3 in domains 1 and 4 show the doubly

275 plunging geometry of macroscopic F 3 folds, consistently with the opposite plunging

276 orientations of L 2 in domains 1 and 4 (see stereonets in Fig. 6). A series of ~NE-SW granitic

277 dykes occurs in domain 1, cutting macroscopic F 3 folds (Fig. 6). At the outcrop scale, F 3 in

278 domain 1 is commonly tight and rounded to angular, whereas it is generally open and

279 rounded in domain 4 (Fig. 9e, f). Spaced cleavage (S 3) was locally recognized in the core

280 areas of macroscopic F folds (Fig. 9g), with an orientation that is approximately parallel to 3 MANUSCRIPT 281 the F3 axial plane.

282

283 Macroscopic F 4 folds were mapped in domains 2 and 3. In domain 2, the calculated F4 fold

284 hinge (B 42 ) is 61-352 (Fig. 10a), which together with ~150° map-view axial trace of F 4,

285 defines an axial plane of 78-060 (Fig. 10a). F 4 in domain 3 yielded an axial plane of 81-240,

286 based on a calculated fold hinge (B 42 ) of 73-298 and the map-view axial trace of ~150° of F 4

287 (Fig. 10b). MacroscopicACCEPTED F 4 folds in both domains 2 and 3 show an asymmetric geometry with

288 sinistral vergence. Similarly, outcrop-scale F 4 folds are predominantly asymmetric, and 10

289 out of 12 observations show S-shaped folds, illustrating sinistral vergence (e.g. Fig. 9h).

290 These folds are commonly open and rounded to angular, and locally develop as F 4 P a g e | 15 ACCEPTED MANUSCRIPT

291 crenulations (Fig. 9h). Throughout the map area, the measured axial plane of outcrop-scale F 4

292 is consistently trending ~NNW-SEE (Fig. 10c). In the northern limb of the macroscopic F 3

293 fold, direct measurements of the F4 axial plane show a steep dip to NE (77-055), consistent

294 with the calculated F 4 axial plane in domain 2. Similarly, in the southern limb, the F4 dips to

295 the SW (77-235) (Fig. 10c), consistently with the calculated F 4 axial plane in domain 4. Field

296 measurements for the hinge of F 4 (B 42 ) show variable orientations.

297

298 5. Geochronology

299 In order to constrain the protolith age of the amphibolite-rich units of the Altai Formation in

300 the Fuyun area, and to provide a maximum age constraint for deformation, we collected two

301 samples for zircon U-Pb geochronology. Sample L14FY33 is a quartzite from unit 2 of the MANUSCRIPT 302 Altai Formation (Fig. 5b; GPS: 47°2'37"N, 89°21'57" E). The sample is characterized by a

303 weak S 2 fabric, and predominantly comprises quartz and some feldspar and amphiboles.

304 Sample L15FY22 is a gneissic granitoid with a pervasive S 2 fabric from the core of the

305 macroscopic doubly plunging fold (Fig. 6; GPS: 47°20'18"N, 88°50'56"E).

306

307 Zircon grains were separated using conventional crushing, heavy liquid and magnetic

308 techniques, and ACCEPTEDthen mounted in epoxy resin and polished to expose equatorial section.

309 Cathodoluminescence (CL) images were taken at the Department of Earth Sciences, the

310 University of Hong Kong (HKU). Zircon U-Pb isotopic analysis was conducted at the same

311 lab of HKU via a Nu Instruments MC-ICP-MS attached to a Resonetics Resolution M-50-HR P a g e | 16 ACCEPTED MANUSCRIPT

312 Excimer Laser Ablation System. The analytical procedure and instrument parameters follow

313 Xia et al. (2011) and Geng et al. (2014). We used the ICPMSDataCal software (Liu et al.,

314 2010) to process data, and the ISOPLOT program (Ludwig, 2003) for the weighted mean age

315 calculation and Concordia plots.

316

317 Zircons from both samples L14FY33 and L15FY22 are characterized by euhedral shape and

318 oscillatory zoning, indicating an igneous origin. U-Pb analytical results are presented in

319 Appendix B. The 17 analyses for gneissic granitoid sample (L15FY22) yielded a weighted

320 mean 206 Pb/ 238 U age of 419.6 ±2.1Ma (MSWD=0.84, Fig. 11a). As for quartzite sample

321 (L14FY33), the concordant 206 Pb/ 238 U ages (<10% discordance) are shown in

322 histogram/probability density plots. The 49 analyses for sample L14FY33 yielded a single MANUSCRIPT 323 age peak of ~392 Ma (Fig. 11b).

324

325 6. Discussion

326 6.1. Protolith age and the timing of deformation

327 Zircons from the quartzite (sample L14FY33) yielded a single age peak of ~392 Ma,

328 indicating that the protolith of this sample is either a meta-volcanic rock or a

329 meta-sedimentaryACCEPTED rock with a local source. This single zircon age population constrains the

330 maximum protolith age of the Altai Formation in the Fuyun area, which is consistent with

331 middle Devonian fossil assemblages within the limestone layers of the Altai Formation in the

332 Aletai area (Fig. 2c) (BGMRX, 1978b). P a g e | 17 ACCEPTED MANUSCRIPT

333

334 The timing of deformation is unfortunately poorly constrained. Structures around both the

335 eastern and western doubly plunging folds can be correlated with each other based on the

336 observations that the dominant fabric (S 2) in both areas are associated with shallowly

337 plunging stretching lineations and F 3 in both areas are characterized by doubly plunging

338 geometry and steeply dipping axial planes trending ~NW-SE. Our new zircon ages from the

339 gneissic granitoid and quartzite indicate that this deformation must have occurred after 392

40 39 340 Ma. Ar/ Ar step heating on syn-S2 amphibole from an amphibolite (Fig. 3) yielded a

341 plateau age of ~270 Ma (Li et al., 2015b). This age is consistent with other 40 Ar/ 39 Ar

342 amphibole ages from the Qiongkuer Domain and the Southern Altai Domain (Fig. 2) (Briggs

343 et al., 2007; Shen et al., 2013), but is younger than the age of metamorphic zircons (~300-280 MANUSCRIPT 344 Ma), which mark the time of peak high temperature metamorphism in the Permian (Li et al.,

345 2014; Wang et al., 2014b). Therefore, the ~270 Ma 40 Ar/ 39 Ar amphibole ages were interpreted

346 to indicate cooling of the southern Chinese Altai through the amphibole closure temperature

347 (Li et al., 2015b). This cooling has been attributed to a regional uplift event associated with

348 the development of ~NW-SE fold and fault system (Figs. 2c and 3) , which is interpreted to

349 represent transpressional deformation associated with the Irtysh Shear Zone at ~290-252 Ma

350 (Qu and Zhang, ACCEPTED1994; Li et al., 2015a). We therefore think that F3 folds in our study area

351 developed at ~270 Ma, which together with strike-slip and reverse movements of the Irtysh

352 Shear Zone (Qu and Zhang, 1994; Li et al., 2015a), were responsible for regional uplift. This

353 suggestion for the timing of deformation is also supported by the age of ~NE-SW granitic P a g e | 18 ACCEPTED MANUSCRIPT

354 dykes (~252 Ma, Zhang et al., 2012) that crosscut F3 fold structures (Figs. 3 and 6). The

355 timing of F4 is not well constrained, but most likely is in the Permian given that the ~252 Ma

356 dykes are not affected by this generation of folds (Fig. 6).

357

358 The Middle Permian 40 Ar/ 39 Ar amphibole age (~270 Ma), which was obtained by 40 Ar/ 39 Ar

359 step heating (Shen et al., 2013; Li et al., 2015b), contrasts with published 40 Ar/ 39 Ar

360 amphibole ages of ~249-244 Ma around the area of Fig. 5b (Laurent-Charvet et al., 2003).

361 The reason for this discrepancy, in our opinion, is associated with the fact that

362 Laurent-Charvet et al. (2003) have dated minerals using an in situ laser probe total fusion

363 technique. This analytic method could yield geologically younger ages, because the argon

364 system of the analyzed sample is commonly partially thermally disturbed leading to MANUSCRIPT 365 extraction and measurement of mixed gases from different reservoirs (see discussion in Li et

366 al., 2015b). Using a similar technique, Laurent-Charvet et al. (2003) have also obtained two

40 39 367 ~245 Ma Ar/ Ar biotite ages from the macroscopic F 3 folds in the area of Fig. 5b, but the

368 geological interpretation of these ages are equally enigmatic. On a larger scale, available

369 40 Ar/ 39 Ar biotite ages from the Qiongkuer Domain are variable and are predominantly

370 Triassic (Laurent-Charvet et al., 2003; Briggs et al., 2009; Li et al., 2015b). This variability

371 may either representACCEPTED a thermal disturbance associated with the emplacement of Triassic

372 granitoids, or along-strike variations of cooling and exhumation (Li et al., 2015b).

373

374 6.2. Structural interpretation and strain partitioning P a g e | 19 ACCEPTED MANUSCRIPT

375 Our structural observations indicate that the Qiongkuer Domain in the Fuyun area (Fig. 3)

376 was subjected to four phases of folding. The available age constraints indicate that

377 deformation occurred at ~392-252 Ma. During this period, the tectonic setting of the Chinese

378 Altai changed from subduction to collision, following the consumption of the Ob-Zaisan

379 Ocean (Laurent-Charvet et al., 2003; Xiao et al., 2004; Briggs et al., 2007; Xiao et al., 2009;

380 Li et al., 2015a; Xiao et al., 2015). The exact age of this collision is not well constrained. A

381 number of authors have suggested, based on the occurrence of collision-related magmatic

382 rocks, that the collision initiated in the Late Carboniferous or the latest Early Carboniferous

383 (Buslov et al., 2004a; Glorie et al., 2012; Safonova, 2013; Kuibida et al., 2016). However,

384 some ophiolitic rocks along the collisional zone may be as young as the early Permian, which

385 seems to support a Permian collision (e.g. Xiao et al., 2015). The strike-slip displacement MANUSCRIPT 386 along the Irtysh Shear Zone likely occurred after collision, based on the fact that the shear

387 zone bounds the Chinese Altai with the West/East Junggar. Therefore, the major phase of

388 shearing at ~290-252 Ma (Laurent-Charvet et al., 2003; Buslov et al., 2004a; Briggs et al.,

389 2007; Li et al., 2015a and reference therein), could provide a minimum timing constraint for

390 the collision, which is consistent with the fact that the youngest component of the Irtysh

391 accretionary complex along the southern Chinese Altai is Late Carboniferous (Alexeiev and

392 Gegtyarev, 2008;ACCEPTED Li et al., 2015a). In the following section, we discuss the structural

393 interpretation of four generations of structures (D1-D4) in the Qiongkuer Domain, as well as

394 their tectonic significance in the context of the subduction and collision history of the

395 southern Chinse Altai. P a g e | 20 ACCEPTED MANUSCRIPT

396

397 The tectonic origin of S 1 and S 2 is enigmatic. The penetrative fabric (S 2) was likely flat-lying

398 prior to F 3 folding, given the doubly plunging geometry of F 3 as discussed below. Such

399 flat-lying structures are also recognized in the Southern Altai Domain (Figs. 2c and 3).

400 Together with evidence for orogen-parallel stretching lineations, this penetrative fabric (S 2) is

401 interpreted to represent an episode of orogenic collapse during the convergence between the

402 Chinese Altai and the East Junggar in the late Paleozoic (Li et al., 2015a). Accordingly, the

403 earlier transposed fabric (S 1) in the Southern Altai Domain is interpreted to represent an

404 episode of contraction, which facilitated the subsequent crustal thinning as represented by the

405 flat-lying fabric and related stretching lineations (Li et al., 2015a). A similar stretching

406 lineation (L ) oriented parallel to the orogenic structural grains (~NW-SE) is also recognized 2 MANUSCRIPT 407 in the Qiongkuer Domain (Fuyun area) (Qu and Zhang, 1994; Laurent-Charvet et al., 2003;

408 Wang and Xia, 2005a; Li and Sun, 2014 and this study), which together with flat-lying S 2

409 foliations provides evidence for orogen-parallel extension. The available chronological data

410 constrain the timing of S1 and S 2 at ~392-270 Ma, but it remains unclear whether these

411 phases of deformation represent processes associated with the collision between the Chinese

412 Altai and the East/West Junggar (e.g. Li et al., 2015a), or whether they are related to the

413 earlier period ofACCEPTED subduction accretion. Laurent-Charvet et al. (2003) reported that the

414 penetrative S 2 fabric around the macroscopic F 3 folds in Fig. 5b is indicative of top-to-west

415 shearing. However, such observations were neither recognized in earlier work by Qu and

416 Zhang (1994) nor confirmed by our study. In contrast, we notice that S2 in the map area of P a g e | 21 ACCEPTED MANUSCRIPT

417 Fig. 5b is predominantly associated with symmetric lens-shaped clasts and conjugate shear

418 sets (Fig. 8c, d), which are indicative of symmetric deformation that contrasts with the

419 interpretation of top-to-the-W shearing.

420

421 The regional structural style is controlled by F3 folds, which show a doubly plunging

422 geometry (Fig. 3). We attribute the doubly plunging geometry to the fold growth in response

423 to buckling. According to Dubey and Cobbold (1977), folds nucleate with non-cylindrical

424 geometry and propagate laterally. When the lateral propagation of a nucleated fold is blocked,

425 the fold hinge would become highly curved (Holdsworth et al., 2002). Such an effect on

426 lateral fold propagation could result from variations in mechanical properties along

427 deformation zones (e.g. Ramsay and Huber, 1987). In the map area, the Altai Formation is MANUSCRIPT 428 rich in amphibolite, which is mechanically differen t than the turbidite-dominant sequence in

429 the adjacent areas. The contrast in competency between the two lithologies may have blocked

430 the lateral propagation of macroscopic F 3 folds, resulting in the doubly plunging geometry.

431 Alternatively, the doubly plunging geometry of F 3 may have partly been inherited from a

432 primary dome structure within the flat-lying S 2. Such domes are common in gneiss and

433 migmatite domains that were subjected to regional-scale sub-vertical flattening deformation

434 in response to crustalACCEPTED thinning (e.g. Whitney et al., 2004). An additional interpretation is that

435 the doubly plunging F3 folds are associated with a Type 1 F 3/F 4 fold interference pattern. We

436 think that this explanation is unlikely, because map-scale F 3 folds show close to tight

437 geometry (Figs. 5, 6 and 7), which tends to from a Type 2 interference pattern (Gruji ć, 1993). P a g e | 22 ACCEPTED MANUSCRIPT

438 Furthermore, the F4 folds appear to be kinematically linked to sinistral shearing subparallel to

439 F3 axial planes (see discussion below), which cannot generate a Type 1 fold interference

440 pattern.

441

442 On a larger scale, the doubly plunging F 3 folds in the Qiongkuer Domain (Fuyun area) are

443 part of a ~NW-SE fold and fault system associated with the development of the Irtysh Shear

444 Zone (Fig. 3) (Qu and Zhang, 1994). This shear zone, which is characterized by a series of

445 ~NW-SE fold zones bounded by sinistral mylonite zones with minor reverse components (Fig.

446 3), was interpreted to be a transpressional system in response to oblique convergence between

447 the Chinese Altai and the intra-oceanic arc system of the West/East Junggar (Qu and Zhang,

448 1991; Qu and Zhang, 1994; Li et al., 2015a). Within ~NW-SE fold zones of the Irtysh Shear MANUSCRIPT 449 Zone, fold axial planes are steeply dipping to the northeast (Li et al., 2015a), and are

450 consistent with the steeply dipping axial planes of macroscopic F 3 folds in the Qiongkuer

451 Domain, suggesting that ~NW-SE folds in both domains resulted from the same

452 transpressional event. On the other hand, chronological constraints on the formation of the

453 doubly plunging F 3 folds at ~270 Ma in the Qiongkuer Domain (Section 4.1) are

454 approximately similar to the timing of activity along the sinistral Irtysh Shear Zone

455 (~290-252 Ma) ACCEPTED(Laurent-Charvet et al., 2003; Buslov et al., 2004a; Briggs et al., 2007; Li et

456 al., 2015a and reference therein), thus further supporting the interpretation that ~NW-SE

457 macroscopic folds across the southern Chinese Altai are genetically linked with the Irtysh

458 Shear Zone. A series of ~NW-SE reverse faults were also recognized in the southern Chinese P a g e | 23 ACCEPTED MANUSCRIPT

459 Altai (Fig. 3) (Qu and Zhang, 1994; Wang and Xia, 2005b; Briggs et al., 2007; Briggs et al.,

460 2009). These faults are kinematically compatible with the transpressional deformation, but

461 the timing of fault activity is relatively poorly constrained. They were possibly active in the

462 Cenozoic given the topographic change across fault zones and the evidence for uplift of the

463 Altai Mountain in the Cenozoic (Yuan et al., 2006; Glorie et al., 2012).

464

465 F4 folds in the area of the western doubly plunging folds (Fig. 6) are superimposed on the F3

466 structures and are characterized by ~NNW-SSE axial planes. The angular relationships

467 between the F4 axial plane and the Irtysh Shear Zone, together with predominant sinistral

468 vergence of asymmetric F4 folds (Section 4.2.2), suggest that F4 folding corresponds to the

469 strain associated with the sinistral movement along the Irtysh Shear Zone. This interpretation MANUSCRIPT 470 is consistent with constraints on the timing of F4 folding (Permian and prior to the intrusion

471 of unfolded ~NE-SW granitic dykes), which overlap with the timing of activity along the

472 Irtysh Shear Zone (Laurent-Charvet et al., 2003; Buslov et al., 2004a; Briggs et al., 2007).

473

474 The spatial occurrence of folded zones bounded by sinistral mylonite zones across the

475 Qiongkuer and Southern Altai domains (Fuyun area) suggests that transpressional strain was

476 partitioned acrossACCEPTED the deformation zone, as commonly observed in oblique convergence

477 boundaries (Miller, 1998; Teyssier and Tikoff, 1998; Norris and Cooper, 2001). The

478 transcurrent component in a transpressional zone can either be accommodated in discrete slip

479 planes (Fig. 1b), or partitioned into both internal deformation of folded zones and slip on P a g e | 24 ACCEPTED MANUSCRIPT

480 discrete faults (Fig. 1c). The former is characterized by fold axial planes subparallel to

481 discrete faults, whereas the latter would result in oblique relationships between fold axial

482 planes and faults (Fig. 1b, c). In the Qiongkuer Domain, the axial plane of F 3 is sub-parallel

483 to the Irtysh Shear Zone. One possible interpretation is that F 3 folds were initially oblique

484 relative to the Irtysh Shear Zone, but were then rotated towards the orientation of the flow

485 apophyses. However, wrench-related folds commonly shows en echelon patterns, and

486 progressive fold rotation would lead to hinge-parallel extensional structures (e.g. Jamison,

487 1991; Teyssier and Tikoff, 1998; Titus et al., 2007), which are not observed in our study area

488 (apart of the L 2 stretching lineation which cannot be attributed to extension during F 3

489 folding).

490 MANUSCRIPT 491 An alternative interpretation is that sub-parallelism between the F 3 axial plane and the Irtysh

492 Shear Zone may have resulted from the partitioning of transpressional strain into simple shear

493 components along ~NW-SE mylonite zones and pure shear-dominant deformation in

494 ~NW-SE fold zones (e.g. F 3, Figs. 1b and 12a). F 3 folds are superimposed by F 4 folds that

495 show kinematic features of sinistral shearing, suggesting that a fraction of transcurrent

496 components may have been partitioned into ~NNW-SSE F 4 folds in the later stage of

497 transpressional deformation.ACCEPTED According to Tikoff and Teyssier (1994), one factor controlling

498 the efficiency of strain partitioning along discrete faults is the convergence angle, with the

499 transpressional strain being sufficiently partitioned into discrete faults when such an angle is

500 less than 20°. In the San Andreas fault system (central California), the plate convergent angle P a g e | 25 ACCEPTED MANUSCRIPT

501 is oriented ~5° relative to the fault zone, leading to a significant amount of simple shear

502 (>95%) that is accommodated by discrete strike-slip faults, and a pure shear-dominated

503 deformation in folded zones that are sub-parallel to the fault (Tikoff and Teyssier, 1994;

504 Teyssier et al., 1995; Teyssier and Tikoff, 1998; Argus and Gordon, 2001). A less efficient

505 strain partitioning in Sumatra is associated with convergence angle of ~50° that resulted in

506 only ~33% of the strike-slip components accommodated by discrete faults (Tikoff and

507 Teyssier, 1994; McCaffrey et al., 2000). Therefore, we postulate that the convergence angle

508 between the Chinese Altai and the East/West Junggar may have increased in a later stage of

509 transpressional deformation along the Irtysh Shear Zone (Fig. 12), thus leading to a decrease

510 in the partitioning of transcurrent components into the mylonite zones.

511 MANUSCRIPT 512 The degree of decoupling and weakening along the shear zones can also account for strain

513 partitioning. A weak fault zone can potentially generate independent contractional and

514 transcurrent zones that do not mechanically interact with each other (Mount and Suppe, 1987;

515 Zoback et al., 1987; Mount and Suppe, 1992). It is thus possible that during the early stage of

516 activity along the Irtysh Shear Zone, the shear zone was sufficiently weak and thus

517 accommodated transcurrent components along narrow mylonite zones and distributed

518 contractional deformationACCEPTED in ~NW-SE fold zones (e.g. F 3 in the Qiongkuer Domain). An

519 enhanced coupling in the later stage of transpressional deformation may have resulted in a

520 less efficient strain partitioning that led to a fraction of transcurrent components partitioned

521 into ~NNW-SSE F 4 folds. Indeed, structural observations show that the Irtysh Shear Zone P a g e | 26 ACCEPTED MANUSCRIPT

522 evolved from a high temperature stage (commonly associated with migmatization) to a more

523 brittle setting (Li et al., 2015a), thus leading to strain hardening and enhanced coupling.

524

525 Brittle strike-slip structures and their associated paleostress fields have been studied by

526 Glorie et al. (2012) along the Kazakhstan segment of the Irtysh Shear Zone. However, the

527 temporal relationships between the interpreted paleostress orientations and the superimposed

528 F3/F 4 folds studied by us, remains an open question. Glorie et al. (2012) have suggested that

529 an earlier ~E-W orientation of the maximum horizontal principal stress axis was subsequently

530 rotated into a ~ENE-WSW orientation. The latter could theoretically account for the

531 development of the ~NNW-SSE F4 folds, but the earlier ~NW-SE F 3 folds are not compatible

532 with the earlier ~E-W orientation of the maximum horizontal principal stress axis. The timing MANUSCRIPT 533 of brittle deformation is relatively poorly constra ined, and it is likely that the data

534 documented by Glorie et al. (2012) represent paleostress orientations that are younger than

535 the Permian (~270-252 Ma) ductile deformation associated with the superimposed folds.

536

537 7. Conclusions

538 Four generations of structures were recognized in the Qiongkuer Domain (Fuyun area) of the

539 Chinese Altai. S ACCEPTED1 is locally recognized in low strain areas and was isoclinally folded with

540 axial planes parallel to the dominant fabric (S 2). S 2 is the dominant fabric and is associated

541 with an orogen-parallel stretching lineation. On a larger scale, S 2 defines ~NW-SE

542 macroscopic upright folds with the doubly plunging geometry (F 3), which is overprinted by P a g e | 27 ACCEPTED MANUSCRIPT

543 ~NNW-SSE F 4 folds. Both F 3 and F 4 folds are kinematically compatible and overlap in time

544 with the adjacent Irtysh Shear Zone, and thus we interpret that these two phases of folds were

545 genetically associated with the transpressional deformation along this shear zone. The

546 sub-parallelism between F3 and the Irtysh Shear Zone may have resulted from a high degree

547 of strain partitioning, which led to pure shear-dominant deformation in fold zones (e.g. F 3)

548 and simple shear deformation along narrow mylonite zones. Strain partitioning may have

549 become less efficient in the later stages of transpressional deformation, thus forming F 4 folds

550 oblique to the Irtysh Shear Zone.

551

552 Acknowledgements: This study was financially supported by the Major Basic Research

553 Project of the Ministry of Science and Technology of China (Grant: 2014CB448000 and MANUSCRIPT 554 2014CB440801), Hong Kong Research Grant Council (HKU705311P, HKU704712P and

555 HKU17303415), National Science Foundation of China (41273048), HKU seed funding

556 (201111159137) and a HKU small grant (201309176226). We thank Rod Holcombe for the

557 discussion with structural interpretations, and Wenjiao Xiao, Damien Delvaux and Jianhua Li

558 for the helpful comments. Jean Wong and Hongyan Geng are acknowledged for their help

559 during the geochronological analysis. This work is a contribution of the Joint Laboratory of

560 Chemical GeodynamicsACCEPTED between HKU and CAS (Guangzhou Institute of Geochemistry),

561 IGCP 592 and PROCORE France/Hong Kong Joint Research Scheme. P a g e | 28 ACCEPTED MANUSCRIPT

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902

903

MANUSCRIPT

ACCEPTED P a g e | 45 ACCEPTED MANUSCRIPT

904 Figure caption

905 Fig. 1 . Conceptual models showing strain styles in monoclinic tranpressional zones

906 (Sanderson and Marchini, 1984; Tikoff and Teyssier, 1994; Dewey et al., 1998). (a)

907 Non-partitioning general shear strain; (b) High degree of strain partitioning with simple shear

908 components along discrete slip planes and pure shear deformation in contraction-dominant

909 domains, which results in the parallelism between fold axis and the deformation zone; (c)

910 Low degree of strain partitioning distributing simple shear components in both discrete slip

911 planes and contraction-dominant domain, which leads to obliquity of initial fold axis relative

912 to the deformation zone.

913

914 Fig. 2. Geological maps of the research area. (a) A simplified tectonic map showing major MANUSCRIPT 915 tectonic segments of the CAOB. In this paper, we follows Li et al. (2015a) to refer the

916 Xing'an-Mongolian-East Junggar orogenic system (XMEJOS) to represent arc systems in the

917 East Junggar and southern Mongolia as well as NE China, whereas the West

918 Junggar-Kazakhstan-Tianshan orogenic system (WJKTOS) and the peri-Siberian orogenic

919 system (PSOS) represent arc systems to the south and north of the Irtysh/Chara shear zones,

920 respectively. The topographic image is from Amante and Eakins (2009). (b) A simplified

921 tectonic map showingACCEPTED the major tectonic units around the Chinese Altai (based on Li et al.,

922 2008). ISZ: Irtysh Shear Zone; NEF: North-East Fault; CSZ: Chara Shear Zone. (c)

923 Geological map in the area of the Chinese Altai after Li et al. (2015b). The time range of

924 granitoids is based on the summary by Tong et al. (2014b) and Chen et al. (2010), whereas P a g e | 46 ACCEPTED MANUSCRIPT

925 major faults are based on Qu et al. (1994), Laurent-Charvet et al. (2003), Briggs et al. (2007),

926 Jiang et al. (2015) and Zhang et al. (2015a).

927

928 Fig. 3. Geological map in the southern Chinese Altai (Fuyun area) based on 1:20 000

929 geological maps. Note that the map highlights major structural elements and published

930 geochronological data (Laurent-Charvet et al., 2003; Briggs et al., 2007; Li et al., 2014; Li et

931 al., 2015b). The geometry of the Irtysh Shear Zone is after Qu and Zhang (1991) and Li et al.

932 (2015a).

933

934 Fig. 4. Photographs of representative rocks in the Qiongkuer Domain (Fuyun area). (a) A

935 pebble conglomerate layer marking the boundary of the Kangbutiebao and Altai formations; MANUSCRIPT 936 (b) White and black layers of quartzofeldspathic gneiss and amphibolite in unit 1 of the Altai

937 Formation; (c) Banded chert in unit 1 of the Altai Formation; (d) Amphibolite/amphibole

938 schist in unit 2 of the Altai Formation; (e) A quartzite layer in unit 2 of the Altai Formation,

939 which was folded with the axial plane parallel to S2; (f) Deformed pillow basalt within unit 2

940 of the Altai Formation. Note that the location information of all photos in this paper (Figs. 4,

941 8 and 9) is presented in Appendices A and C.

942 ACCEPTED

943 Fig. 5. (a-d) Structural maps and transects around two hinge areas of eastern doubly plunging

944 folds (See the location in Fig. 3). (e-j) Equal-area lower hemisphere stereonet plots for

945 dominant fabric (S 2) and associated stretching lineation (L 2). Note that the axial plane of P a g e | 47 ACCEPTED MANUSCRIPT

946 macroscopic F 3 is calculated based on map-view fold trace of F 3 and calculated fold hinge.

947

948 Fig. 6. Structural map in the area of western doubly plunging folds (see the location in Fig. 3).

949 Four domains were divided for geometric analysis and stereonet projection (equal area, lower

950 hemisphere).

951

952 Fig. 7. Two structural transects across doubly plunging F3 folds in Fig. 6. See the legend in

953 Fig. 6.

954

955 Fig. 8. Photographs of representative structures around the eastern doubly plunging F 3 folds.

956 (a) A local S fabric that was folded with the axial plane parallel to regional penetrative S 1 MANUSCRIPT2 957 (unit 1 of the Altai Formation). (b) A F 2 defined by folded veins with the occurrence of the

958 axial planar fabric (S 2, unit 1 of the Altai Formation). (c) Conjugate shear bands indicating

959 pure shear-dominant deformation during the development of S 2 (unit 1 of the Altai

960 Formation). (d) A symmetric lithic clast suggesting pure shear-dominant origin of S 2 (unit 1

961 of the Altai Formation). (e) Stretching lineations (L 2) defined by extended feldspar grains

962 (unit 1 of the Altai Formation). (f) Outcrop-scale F 3 folds in unit 3 of the Altai Formation. (g)

963 Outcrop-scale F ACCEPTED3 folds close to the contact of the Kangbutiebao Formation with the Altai

964 Formation. (h) Deflection of stretching lineations (L 2) by F 3 within unit 3 of the Altai

965 Formation.

966 P a g e | 48 ACCEPTED MANUSCRIPT

967 Fig. 9. Photographs of representative structures around the western doubly plunging F 3 folds.

968 (a) S 1 fabric overprinted and transposed by S 2. (b-c) Penetrative S 2 in amphibolite of the Altai

969 Formation and gneissic granitoids, respectively. (d) A stretching lineation (L 2) defined by

970 extended quartz/feldspar porphyroblasts in an orthogneiss of the Kangbutiebao Formation. (e)

971 Tight F 3 folds in domain 1 of Fig. 6. (f) Gentle F 3 folds in domain 4 of Fig. 6. (g) Spaced S 3

972 fabric locally occurs in the core area of macroscopic F 3. (h) S 4 crenulations overprinting S 3

973 (Altai Formation). Note sinistral vergence of asymmetric F4 folds.

974

975 Fig. 10. Structural maps of F 4 folds in the area of the western doubly plunging folds (see the

976 figure location of Fig. 10a, b in Fig. 6). (a) F 4 folds in domain 2 of Fig. 6. F 4 fold hinge (B 42 )

977 can be constrained to be 61°-352°, which together with ~150° map-view trace of F defines MANUSCRIPT4 978 an axial plane at 78°-060°. (b) F 4 in domain 3 of Fig. 6 yielded an axial plane of 81°-240° in

979 the basis of the fold hinge (B 42 ) of 73°-298° and the map view trace of ~150°. (c) Stereonet

980 projection of outcrop-scale S 4 (equal area, lower hemisphere) in the area of the western

981 doubly plunging folds.

982

983 Fig. 11 . (a) Concordia diagrams for zircon U-Pb analyses of sample L15FY22 (gneissic

984 granitoid); (b) AgeACCEPTED probability diagrams of zircons U-Pb analyses for quartzite (sample

985 L14FY33) in unit 2 of the Altai Formation.

986

987 Fig. 12. Schematic diagrams showing the development of ~NW-SE doubly plunging F 3 folds P a g e | 49 ACCEPTED MANUSCRIPT

988 and ~NNW-SSE F 4 folds, as well as the relationship with the evolution of the Irtysh Shear

989 Zone. Note that ~NW-SE reverse faults were possibly active during the development of the

990 Irtysh Shear Zone. (a) The sub-parallelism of F 3 fold axis with the Irtysh Shear Zone may

991 have resulted from strain partitioning associated with simple shear deformation along narrow

992 mylonite zones and pure shear-dominant deformation (F 3) in fold zones; (b) The obliquity of

993 F4 with the shear zone likely results from the less efficient strain partitioning in the later stage

994 of transpressional deformation, so that a fraction of transcurrent components were partitioned

995 into F 4 folds.

996 997

MANUSCRIPT

ACCEPTED ACCEPTED MANUSCRIPT

moving plate

fixed plate (a)

moving plate

fixed plate (b) MANUSCRIPT

moving plate

fixed plate (c)

Fig. 1 ACCEPTED ACCEPTED MANUSCRIPT

(a) (b) c n i n ya t a l o s Go t S t e r es a a ny W r id Siberian R A B l u lta C a d i Tuva-Mongolian r Craton n U Ir y N microcontinent c ty A E o s l F m h ta - i A p Z l C le a I ta x is S i S a Z -M Z n o n P S g S O o l L O S ia H n t a S P o k ? S e v e Z WJKTOS r d t JO ra n e o E W e M re est Jungg IS r n Fig. 2b u ar Z r e X ut a r s n ke e lon Fig. 2c So ISZ North China Microcontinent Fault Tarim Craton Island arc Inferred fault 1000 km Craton Sedimentary cover Eas t Junggar 250 km Continental marginal arc

87°E 89°E 91°E

(c)

N

′

5 4

° N

8 Central Altai 4 Domain M Kanasi o Qiongkuer n go Domain lia 0 50 100 km

M F a a e u rk lt a k Chonghuer u li Northern Altai H

N Habahe F a C ′ a b Domain

u a 5 h l h t e i 4 Southern n ° e

7 s

4 Altai Domain e Aletai A lt Buerjing Alahake a MANUSCRIPTi r Ir a ？ tys g h F g a un ult t J Wulungu s Lake Ir e ty Keketuohai W sh S he ar Z o ne Fig. 3 Fuyun Faults Normal faults

Qinhe Inferred faults Strike-slip faults Ea st J un Carboniferous strata gg in the Northern Altai Domain ar Devonian strata in the Northern Altai Domain

Altai Formation Baihaba Formation Sedimentary Triassic granitoids cover Amphibolite-dominant (Devonian)

Paleozoic strata of Kangbutiebao Dongxileke Formation Permian granitoids the East/West Junggar Formation (Devonian) (Ordovician-Devonian)

ACCEPTEDKulumuti Group Habahe Group Irtysh Complex Pre-Permian granitoids (Sillurian) (Cambrian-Sillurian)

Fig. 2 ACCEPTED MANUSCRIPT

88°54′E 89°18′E 89°42′E

Meso-Cenozoic Permian sedimentary cover granitic dyke Western doubly plunging folds Trace of dominant Fig. 6 Permian strata fabric Devonian- Carboniferous strata Macroscopic fold of the East Junggar ? Irtysh Complex Antiform

Central Altai Domain F u Altai Formation Synform y (unit 4)

u

N ′

n F 5

1 Altai Formation

° a Fault

7 E

a u (unit 3) 4 Qiongkuer Domain s l Fig. 5a t B t er n d Altai Formation Strike-slip fault ou ~245 Ma /mylonitic zone b (unit 2) ly p (map view) l un Kuerti gi Altai Formation ng f Thrust Irt Southern Altai Domain o (unit 1) ys ld h F F s ~270 Ma (map view) au uy lt un- Altai Formation Xib odu F (massive amphibolite) Strike-slip fault a ult MANUSCRIPT /mylonitic zone Altai Formation (profile view) T (undivided) u Thrust or inferred N er East Junggar S ho reverse fault S Ir h ng Kangbutiebao h ty ea sh (profile view) ea sh r Z a Formation N t

′ r Z e on 3 on e 0 e Kulumuti Group Metamorphic

° Fig. 5b zircon ages

7 4 0 5 10 km Habahe Group Monazite ages

Fuyun 40Ar/ 3 9Ar Irtysh Shear Zone Qiongkuer Domain Migmatites amphibole ages B ~265 Ma A A 40Ar/ 3 9Ar 1 km Permian granitoids biotite ages 0 0 ~287-269 Ma ~277 Ma -1 km -1 km Irty ~267-247 Ma ~278-264 Ma sh F 40Ar/ 3 9Ar au Gneissic granitoids ACCEPTED~284-267 Ma lt muscovite ages

Fig. 3 (a) ACCEPTED MANUSCRIPT(b)

Pebbles

(c) (d)

(e) (f)

e t i MANUSCRIPTPillow z t r basalt

a

u

Q

Fig. 4

ACCEPTED ″ (e) S2 in Fig. 5a 59 8 4 (a) ′ (b) ACCEPTED7 MANUSCRIPT B ° D β 4 7 46 4 N 41

34 ″ 31 41 6 3 ′ n=17 8 1 ° 68 (f ) L2 in Fig. 5a 7 64

4 74 56 75 N ″

6 42 19 78

39 3 ′ 40

6 56 69 ° 10 7

4 25

5 N 51

7 60 8 n=6 40 12 12 3 16 (g) F3 axial plane 74 76

″ in Fig. 5a 70 65 66 4

2 50 87 64 ′ β 7 60 75

1 38 9 ″ ° 0-

4 2 7 16 2 4 ′ 61 5 N

° 66 32 7

4 F3

N axial trace 55 (h) S2 in Fig. 5b 60 19

9 74

72 51 ″ ″ 2

2 β 1 ′ 1 ′ 6 4 1 ° ° n=85 7 7 46 4 4 73 5 N

N (I) L2 in Fig. 5b

72 16

76 62 5

A n=18 ′ 3 5 ° 61 1 ( j) F3 axial plane 7 ° 4 7 in Fig. 5b 4 11 N 72 65 N N MANUSCRIPT N C 84 8 68 5 9 7-2 L14FY33 04 β 0 1km 0 1km F3 axial trace E88°54′36″ E88°55′48″ E88°57′ N89°19′48″ N89°21′ N89°22′18″

A B (c) 1km

0

(d) D C 1km ACCEPTED 0

Gneissic Kangbutiebao Altai Formation Altai Formation granitoids Formation (unit 1) (unit 2)

Altai Formation Altai Formation Quaternary (unit 4) (unit 3) Irtysh Complex sediments

S2 L2 B32 S2 trace F3 axial trace Reverse Strike-slip Sample fault shear zone location

Fig. 5 ACCEPTED MANUSCRIPT

26 88 Domain 1 Domain 4 Domain 2 (Fig.10a) F3 axial 54 S2 F3 axial L2 S2 61 plane β plane β 74 B -0 86 31 -014 β 54 β

″ 90 4 Domain 1 5 ′

1 41 77 80 2 46 67 69 n=58 n=15 n=85 °

7 79

4 80 35 N 22 49 39 47 64 L2 41 66 31 74 47 51 14 D 76 78 76 40

58 57 71 71 n=20 85 77 56 55 76 80 74 88 34 82 47 31 69 79 69 69 68 66 ″ 74 82 2 46 66 4 ′ 21 80

0 46 2 ° 32 42 7 77 31 4

N 76 29 60 72 59 31 71 86 54 87 19 Domain 4 8 68 65 26 46 MANUSCRIPT51 86 68 74 88 ？ 73 63 78 41 L15FY22 53 35 84 59 86 45 75 49 45 68 89 77 A 81 81 84 89 54 64 37 74 78 74 89 88 34 64 81 69 75 33 73 85 26 88 83 78 69

″ 82 70 77 0 3 ′ 72 9 85

1 34

° N 7 Domain 3 84 50 4 76 N 79 76 (Fig. 10b) 72 60

0 2 km 82 81 C E88°48′36″ E88°51′ E88°53′24″ E88°55′48″

Gneissic Kangbutiebao Altai Formation Altai Formation Altai Formation Quaternary Granitic dyke granitoids Formation (unACCEPTEDit 1) (unit 2) (undivided) sediments

75 S2 First-order Second-order F4 axial Reverse Fault with strike Domain S2 L2 S3 B32 S4 B42 trace F3 axial trace F3 axial trace trace fault -slip component boundary

Fig. 6 ACCEPTED MANUSCRIPT

F3 F3 F3 A B 1km

0.5km 0.5km

F3

C D 1km 1km

0.5 km 0.5km MANUSCRIPT Fig. 7

ACCEPTED (a) ACCEPTED MANUSCRIPT(b)

S2

S1 S1 feldspar layers

S2

(c) (d)

S2

S2

1 mm

(e) (f) L2 MANUSCRIPT

S2 S3

Feldspar

(g) (h)

ACCEPTED B32

S2

S3 L2

Fig. 8 (a) (b) S2 ACCEPTED MANUSCRIPT

S2

S1 S1 S2

(c) (d)

S2

L2

(e) (f)

MANUSCRIPT

S2

S2 S3

S3

(g) (h) S2

S4 ACCEPTEDS3

S2

Fig.9 ACCEPTED MANUSCRIPT (a) F4 hinge

90 87 64 60 66 77 β 80

58 50 67 79

66 39 35 64 41 64 53 58 47 n=34

60 F4 axial surface F4

7 β 8 - 0 6 0

71 71 85 35 60

0 400m 76 F4 axial trace

(b) F4 hinge 81 87

34 81 78 74 84

83 85

89 β

75 78

82 89 88

70 62 70 77 F4 n=21 85 86 81 84 F4 axial surface 0 400m

Kangbutiebao β S2 B42 8 MANUSCRIPT1 - Formation 2 4 0 Altai Formation L2 S2 trace unit 1 S3 F4 axial trace Gneissic F4 granitoids S4 Fault axial trace

(c)

S4(northern limb of F3)

F3

n=7

S4(southern limb of F3) F3 N

0 2km ACCEPTEDn=5

Fig. 10 ACCEPTED MANUSCRIPT

(a) 0.071 Gneissic granitoid 440 (L15FY22) 419.6±2.1 Ma MSWD=0.84 0.069 n=17/20 U 8 3 2 / b P 6

0 0.067 2

410

0.065

400

0.063 0.47 0.49 0.51 0.53 0.55 0.57 0.59 0.61 207Pb/235U

(b) 20 392 Ma

Quartzite R

15 e

(L14FY33) l a t

n=49/70 i v r e p e b r m

10 o

u MANUSCRIPT b N a b i l i t y 5

0 300 320 340 360 380 400 420 440 460 480 Age (Ma)

Fig. 11

ACCEPTED ACCEPTED MANUSCRIPT

(a) N (b) F4 N 3 F F3 Cetral Altai Cetral Altai ? Domain ? Domain ? ? Q i Q o io ng n k gk ue u r D er D o o m Ir m ai ty a n sh in Ir S ty h sh e S a h r Z ea on r e Zo ne F3 F3

East Junggar East Junggar Fuyun

Irtysh Altai Formation Altai Formation Altai Formation Altai Formation Altai Formation Altai Formation Complex (unit 4) (unit 3) (unit 2) (unit 1) (undivided) (massive amphibolite)

Gneissic Kangbutiebao Antiform (F3) Synform (F3) Macroscopic Macroscopic Strike-slip Thrust Fault granitoids Formation fold (F3) fold (F4) shear zone

Fig. 12

MANUSCRIPT

ACCEPTED ACCEPTED MANUSCRIPT Research highlights

Four generations of structures are recognized in the southern Chinese Altai;

S1 and S2 may result from crustal thickening and thinning;

F3 and F4 are linked with partitioning of transpressional deformation;

Sub-parallelism of F3 with the shear zone results from high degree of partitioning;

Less efficient strain partitioning leads to the obliquity of F4 with the shear zone.

MANUSCRIPT

ACCEPTED