Oligocene— Miocene Duplexing Along The

Gangdese culmination model: Oligocene–Miocene duplexing along the India-Asia suture zone, Lazi region, southern Tibet

Item Type Article

Authors Laskowski, Andrew K.; Kapp, Paul; Cai, Fulong

Citation Andrew K. Laskowski, Paul Kapp, Fulong Cai; Gangdese culmination model: Oligocene–Miocene duplexing along the India- Asia suture zone, Lazi region, southern Tibet. GSA Bulletin ; 130 (7-8): 1355–1376. doi: https://doi.org/10.1130/B31834.1

DOI 10.1130/B31834.1

Publisher GEOLOGICAL SOC AMER, INC

Journal GEOLOGICAL SOCIETY OF AMERICA BULLETIN

Download date 09/10/2021 12:19:58

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Link to Item http://hdl.handle.net/10150/628513 Manuscript Text Click here to download Manuscript Lazi_Manuscript_Laskowski.docx

1 The Gangdese Culmination Model: Oligocene—

2 Miocene Duplexing along the India-Asia Suture Zone,

3 Lazi Region, Southern Tibet

4 Andrew K. Laskowski1*, Paul Kapp1, and Fulong Cai2

5 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

6 2Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau

7 Research, Beijing 100101, China

8 *Current address: Department of Earth Sciences, Montana State University, Bozeman,

9 Montana 59717, USA

10 ABSTRACT

11 The mechanisms for crustal thickening and tectonic exhumation along the

12 Yarlung (India-Asia) suture in southern Tibet are debated, as the magnitudes, relative

13 timing, and interaction between the two dominant structures—the Great Counter thrust

14 and Gangdese thrust—are largely unconstrained. In this study, we present new geologic

15 mapping results from the Yarlung suture zone in the Lazi region, located ~350 km west

16 of the city of Lhasa, along with new igneous (6 samples) and detrital (6 samples, 474

17 ages) U-Pb geochronology data to constrain the crystallization ages of Jurassic—

18 Paleocene Gangdese batholith rocks and provenance of Tethyan Himalayan and

19 Oligocene—Miocene Kailas Formation strata. We supplement these data with a

20 compilation of 124 previously published thermochronologic ages from Gangdese

21 batholith, Kailas Formation, and Liuqu Formation rocks, revealing a dominance of

22 Oligocene—Miocene (23-15 Ma) cooling contemporaneous with slip across the Great 23 Counter thrust system. These data are systematically younger than 98 additional compiled

24 thermochronologic ages from the northern Lhasa terrane, recording mainly Eocene

25 cooling. Structural and thermochronologic data were combined with regional geological

26 constraints—including INDEPTH seismic reflection data—to develop a new structural

27 model for the Oligocene—Miocene evolution of the Tethyan Himalaya, Yarlung suture

28 zone, and southern Lhasa terrane. We propose that a hinterland-dipping duplex beneath

29 the Gangdese mountains—of which the Gangdese thrust is a component—is

30 kinematically linked with a foreland-dipping passive roof duplex along the Yarlung

31 suture zone—the Great Counter thrust system. This interpretation, referred to as the

32 Gangdese Culmination model, explains why the Gangdese thrust system is only locally

33 exposed (at relatively deeper structural levels) and provides a structural explanation for

34 early Miocene crustal thickening along the Yarlung suture zone, exhumation of the North

35 Himalayan domes, relief generation along the modern Gangdese Mountains, Early

36 Miocene Yarlung river establishment, and creation of the modern internal drainage

37 boundary along the southern Tibetan Plateau.

38 INTRODUCTION

39 Documenting the structural style and timing of crustal thickening that produced

40 the ~5 km average surface elevation of the Tibetan Plateau is key to understanding the

41 response of continental crust to intercontinental collision and recognizing feedbacks

42 among climate, surface processes, and tectonics (e.g. Quade et al., 2003; Harrison et al.,

43 1992; Beaumont et al., 2001; Whipple, 2009). The significance and timing of Cenozoic

44 fault systems along the ~1300-km-long Yarlung (India-Asia) suture in southern Tibet

45 (Fig. 1), however, remain a subject of debate. Juxtaposition of deeply-exhumed magmatic 46 arc rocks of the southern Lhasa terrane against Indian passive margin strata, as well as

47 thermochronologic data and field mapping, led to the discovery of a north-dipping

48 mylonitic shear zone—the Gangdese thrust—that carried magmatic arc rocks southward

49 in its hanging wall (Yin et al., 1994; 1999). Primarily documented southeast of the city of

50 Lhasa, the Gangdese thrust was interpreted as a crustal-scale, Late Oligocene—Early

51 Miocene (27-23 Ma) structure based on 40Ar/39Ar thermochronology data, with a

52 minimum displacement of 46±9 km (Harrison et al., 1992; Yin et al., 1994; Copeland et

53 al., 1995). However, this structure is apparently not exposed along-strike to the west of

54 Lhasa, leading others to call into question its significance and along-strike continuity

55 (Aitchison et al., 2003). The dominant structures along the Yarlung suture west of Lhasa

56 are a system of south-dipping reverse faults called the Great Counter thrust (Heim and

57 Gansser, 1939; Yin et al., 1999; Murphy and Yin, 2003), which typically places Indian

58 passive margin rocks on suture zone mélange, mélange on forearc basin strata, and

59 forearc basin strata on the Oligocene-Miocene conglomerate, from south to north. A lack

60 of hanging wall cutoffs and no clear thermochronological date difference across

61 individual fault splays renders constraints on the timing and magnitude of Great Counter

62 thrust activity tenuous, but most studies agree that it was active by Late Oligocene—

63 Early Miocene time (Quidelleur et al., 1997; Harrison et al., 2000; Yin et al., 1999; Wang

64 et al., 2015), temporally overlapping or closely following the Gangdese thrust. Despite

65 the close spatial and temporal relationship between the Great Counter thrust system and

66 the Gangdese thrust (where the Gangdese thrust is exposed), the crosscutting or

67 branching relationships between them are not known. Whether the Gangdese thrust is an

68 orogen-scale structure that accommodated significant crustal shortening during Cenozoic 69 time, and the nature of its relationship to the more prominently exposed Great Counter

70 thrust system (Fig. 1), are open questions with major implications for Himalayan-Tibetan

71 tectonics.

72 To the north of the Yarlung suture zone is a belt of calc-alkaline plutonic rocks

73 that are dominantly Cretaceous to Paleogene in age (Schärer et al., 1984), referred to as

74 the Gangdese batholith, and related volcanic and volcaniclastic rocks that are dominantly

75 Paleocene to Eocene in age, referred to as the Linzizong Formation (Lee et al., 2009).

76 Collectively, these rocks compose the Gangdese magmatic arc, which developed along

77 the southern Lhasa terrane (Asian) margin during northward subduction of Neo-Tethyan

78 oceanic lithosphere and persisted during India-Asia collision (Kapp et al., 2007). The

79 Gangdese Mountains (Fig. 1) in southern Tibet (also called the Trans-Himalaya) are

80 composed mostly of Gangdese magmatic arc rocks. The Gangdese mountains define the

81 northern boundary of the Yarlung river watershed, in southern Tibet, and the southern

82 boundary of the internally drained portion of the Tibetan plateau.

83 Along the southern flank of the Gangdese Mountains (Fig. 1), an Oligocene-

84 Miocene, conglomerate-rich, continental unit referred to as the Kailas (Gangrinboche)

85 Formation (Gansser, 1964; Aitchison et al., 2002; DeCelles et al., 2011; 2016, Leary et

86 al., 2016b) is exposed in buttress unconformity atop Gangdese arc rocks. Nonmarine

87 strata of similar composition and structural position are continuous, with some variations

88 in sedimentary facies, for over 1300 km along the Yarlung suture zone (Leary et al.,

89 2016b). Some workers interpret the Kailas Formation as the product of contractional

90 deformation, associated with a lithospheric flexure during a late stage of India-Asia

91 collision (Aitchison et al., 2007), flexural foreland basin deposition related to the Great 92 Counter thrust system (Wang et al., 2015), or wedge-top sedimentation related to out-of-

93 sequence Great Counter thrust system activity (Yin et al., 1999). However, recent

94 investigations of the Kailas Formation sedimentology, fossil assemblages, and basin

95 architecture indicate that the Kailas Formation was deposited in an extensional basin

96 bounded by a north-dipping normal fault, perhaps related to Oligocene—Miocene

97 rollback of the subducted Great Indian slab (DeCelles et al., 2011; 2016; Wang et al.,

98 2013; Leary et al., 2016).

99 The Cretaceous—Paleogene Xigaze forearc basin (Einsele et al., 1994; Dürr,

100 1996, Wang et al., 2012; An et al., 2014; Orme and Laskowski, 2016) is exposed to the

101 south of the Kailas Formation across a splay of the Great Counter thrust system. Xigaze

102 forearc basin strata were deposited atop serpentinite mélange (Orme and Laskowski,

103 2016)—exposed along its southern margins in the Lazi region—suggesting that the 132-

104 122 Ma Yarlung suture zone ophiolites (Hébert et al., 2012; Chan et al., 2015) were in a

105 suprasubduction zone position at the onset of forearc basin deposition ca. 110 Ma (Huang

106 et al., 2015; Orme and Laskowski, 2016). Another conglomerate unit—the Liuqu

107 Formation—is locally exposed to the south of the Xigaze forearc basin, where it was

108 deposited mainly atop serpentinite-matrix mélange during Early Miocene time (Li et al.,

109 2015; Leary et al., 2016a) or late Paleocene time (Ding et al., 2017). The Yarlung suture

110 zone ophiolites and serpentinite-matrix mélanges structurally overlie a belt of subduction-

111 accretion, shale- and sandstone-matrix mélange with (meta-) sedimentary and (meta-)

112 basalt blocks (e.g. Cai et al., 2012). The sedimentary-matrix mélange separates rock units

113 that were located along the southern margin of the Lhasa terrane from the Cambrian-

114 Paleogene, (meta)sedimentary Tethyan Himalayan sequence, the majority of which was 115 deposited in a passive margin setting along the northern margin of India (Gaetani and

116 Garzanti, 1991; Liu and Einsele, 1994; Garzanti, 1999).

117 To the south of the Yarlung suture zone, approximately in the middle of the

118 Tethyan Himalayan physiographic zone (Fig. 1), a belt of gneiss domes is exposed, with

119 Tethyan Himalayan sequence rocks in the hanging wall of the roof faults and Greater

120 Himalaya sequence Neoproterozoic orthogneiss and paragneiss in the footwalls (Fig. 1).

121 The largest of these domes is the Mabja dome (Lee et al., 2004), located ~50 km south of

122 the Lazi region study area (Fig. 1). A variety of formation mechanisms have been

123 proposed for the North Himalayan domes, including 1) thrust duplexing related to north-

124 south contraction (Hauck et al., 1998; Makovsky et al., 1999), 2) diapiric rise of buoyant,

125 anatectic melts (e.g. LeFort, 1986), 3) Cordilleran-style metamorphic core complex

126 extension (Chen et al., 1990), and 4) paired vertical thinning and top-to-the-south

127 thrusting (Lee et al., 2000). Structures and metamorphic assemblages in the Mabja dome

128 record a pressure-temperature-deformation path suggesting initial north-south

129 contraction, thermal equilibration leading to peak metamorphism, high-strain ductile

130 vertical thinning (extension), and late development of a domal geometry possibly related

131 to upper-crustal normal faulting (Lee et al., 2004). Doming and upper crustal extension

132 took place during Middle Miocene time in the Mabja dome (Lee et al., 2004; Lee et al.,

133 2006; Lee and Whitehouse, 2007), whereas the timing of initial contractional deformation

134 is poorly constrained.

135 Thermochronology data from Gangdese batholith rocks exposed along the

136 Yarlung suture zone (Copeland et al., 1987; Harrison et al., 1992; Copeland et al., 1995;

137 Dai et al., 2013; Sanchez et al., 2013; Carrapa et al., 2014; Wang et al., 2015; Ge et al., 138 2016; Li et al., 2016; Laskowski et al., 2017;) reveal an orogen-scale Oligocene-Miocene

139 exhumation event, possibly associated with one or more tectonic, climatic, and erosional

140 factors (Carrapa et al., 2014; Carrapa et al., 2017). Low-temperature thermochronology

141 data from the Kailas Formation reveal a preponderance of Miocene (19-15 Ma) cooling

142 ages, interpreted to record efficient erosion associated with drainage reorganization and

143 establishment of the Yarlung River during Early Miocene time (Carrapa et al., 2014;

144 Lang and Huntington, 2014). Based on these data, the Gangdese magmatic arc appears to

145 have experienced semi-continuous exhumation throughout the period in which it has

146 variably been interpreted to be in the hanging wall of the Gangdese thrust (e.g. Yin et al.,

147 1994), in the footwall of the Great Counter thrust system, and/or in the hanging wall of a

148 north-dipping normal fault that created the accommodation space for burial by up to ~4

149 km of Oligocene-Miocene nonmarine strata (the Kailas Formation; DeCelles et al., 2011;

150 Wang et al., 2014; Leary et al., 2016b). Further complication arises from the broad range

151 of explanations for relief generation between the presently high-standing Gangdese arc

152 (Figs. 1-2) and low-lying Yarlung suture zone (Fig. 1), including flexure in a foreland

153 basin setting (Wang et al., 2015), crustal thickening driven by the Great Counter thrust

154 (Sanchez et al., 2013) or Gangdese thrust (e.g. Yin et al., 1994), and fluvial incision

155 influenced by the strengthening Asian monsoon coupled with renewed Indian

156 underthrusting (Carrapa et al., 2014). No structural model exists that reconciles the roles

157 of the Gangdese thrust and Great Counter thrust system, provides context for Oligocene-

158 Miocene sedimentary basin development along the suture zone, explains Gangdese

159 Mountains relief generation, and provides a mechanism for Yarlung River establishment

160 while maintaining compatibility with thermochronometric data. 161 In this study, we present a structural model for the Oligocene—Miocene evolution

162 of the southern Lhasa terrane, Yarlung suture zone, and Tethyan Himalayan based on

163 new regional-scale geologic mapping of a ~1,500 km2 area north of the town of Lazi,

164 Tibet (Figs. 1, 2a), encompassing three detailed (~1:50,000 scale) geologic mapping

165 locales (Figs. 2b-2d). In addition, we present five igneous U-Pb ages from Gangdese arc

166 and younger intrusive rocks and five detrital zircon U-Pb samples from sedimentary

167 rocks in the Lazi region. We interpret that the Great Counter thrust system is a passive

168 roof duplex associated with crustal-scale, hinterland-dipping duplexing—equivalent to

169 the Gangdese thrust. This model explains the occurrence of imbricated, foreland-dipping

170 thrust sheets in the absence of an emergent, hinterland-dipping detachment horizon. The

171 geometry of this system may also explain North Himalayan dome exhumation,

172 accomplished by top-north thrusting paired with ductile vertical thinning.

173 GEOLOGY OF THE LAZI REGION

174 Field Methods

175 We report data collected during fieldwork in 2012 and 2014 from the Yarlung

176 River valley ~10 km north of the city of Lazi, Tibetan Autonomous Region, China. The

177 city of Lazi, which is also referred to as Lhatse, Quxar, Quxia, or Chusar, is located ~350

178 km west-southwest of Lhasa city, and ~35 km north-northeast of the Mabja Dome (Lee et

179 al., 2004; 2006), crowned by Hlako Peak (~6500 m) (Fig. 1). Mapping was conducted at

180 approximately 1:100,000 scale across the study area (Fig. 2a), and 1:50,000 scale in three

181 areas (Figs. 2b-d) atop topographic maps generated from 3-arc-second Shuttle Radar

182 Topography Mission data with draped LandSat orthoimagery. Contacts were interpolated

183 between traverses using both Google Earth and LandSat orthoimagery. Cross sections 184 were drawn from our structural and mapping data, through the eastern and western

185 portions of the study area (Fig. 2a, Fig. 3).

186 Rock Units and Correlations

187 The southernmost rocks in the Yarlung suture zone are low-grade

188 metasedimentary rocks, dominated by slate and well-cemented quartz arenite in the Lazi

189 region (Fig. 2a), which we correlate to the Tethyan Himalayan sequence. In the western

190 third of the map area (Fig. 2a), shale-matrix mélange with blocks of sandstone, limestone,

191 chert and volcanic rocks is exposed to the north of the Tethyan Himalayan sequence. We

192 correlate these rocks to the Pomunong mélange (unit JKp), which was likely deposited

193 during Late Jurassic to Early Cretaceous time based on radiolarian fossils (Zhu et al.,

194 2005; Cai et al., 2012). Farther north, a unit of shale- and lithic-sandstone-matrix

195 mélange with blocks of chert, basalt, and gabbro is exposed between the Pomunong

196 mélange and serpentinite-matrix mélange. We correlate this unit to the Tangga mélange,

197 which was deposited during Late Triassic to Early Cretaceous time based on radiolarian

198 fossils (Ziabrev et al., 2003; Zhu et al., 2005), and is equivalent to the Bainang terrane of

199 Aitchison et al. (2000) and the radiolarian intra-ophiolitic thrust sheet of Tapponnier et al.

200 (1981). The Tangga and Pomunong mélanges likely formed in an accretionary wedge

201 setting during Neo-Tethyan oceanic subduction beneath the southern Lhasa terrane (Cai

202 et al., 2012).

203 In the southern portion of the study area (Fig. 2a), a sandstone- and chert-clast,

204 pebble-to-cobble conglomerate unit containing interbeds of sandstone and shale was

205 deposited in buttress unconformity atop chert- and matrix-dominated portions of the

206 Tangga mélange, the Pomunong mélange, and between the Tethyan Himalayan sequence 207 and serpentinite-matrix mélange, from west to east (Fig. 2a). We correlate this unit to the

208 Liuqu Formation (Yin et al., 1980), which was deposited in a contractional setting as part

209 of a fluvial and alluvial fan depositional system (Leary et al., 2016a). The preponderance

210 of geochronological and thermochronological data from the Liuqu Formation suggest that

211 it was deposited during a short interval between 20-19 Ma (Li et al., 2015; Leary et al.,

212 2016a). However, Ding et al. (2017) argue that the Liuqu Formation was deposited

213 during latest Paleocene time based on U-Pb geochronology of interbedded tuffs. More

214 work is needed to assess these competing hypotheses. In the Yarlung River valley, the

215 Liuqu Formation was deposited in angular unconformity atop chert blocks in the Tangga

216 mélange, and is dominated by red chert clasts (Fig. 4d). The maximum preserved

217 thickness of the Liuqu Formation is ~2 km, near the town of Liuxiang (Li et al., 2015),

218 whereas the maximum thickness of the Liuqu Formation in the Lazi region is ~200 m

219 (Leary et al., 2016a).

220 A belt of serpentinite- and gabbro-block mélange with a serpentinite-dominated

221 matrix is exposed north of the Liuqu Formation, and on both the south and north sides of

222 the Tangga mélange (Fig. 2a). We correlate these rocks to the laterally extensive and

223 variably tectonized south Tibetan ophiolites, which formed between 132 and 122 Ma

224 along-strike (Hébert et al., 2012; Chan et al., 2015). However, zircon U-Pb ages from a

225 gabbro block and a fine-grained, granitic intrusive rock in the serpentinite-matrix

226 mélange, collected within the study area near the town of Jiwa (Fig. 2a), indicate a

227 younger crystallization age of ~111 Ma (Orme and Laskowski, 2016). Xigaze forearc

228 basin strata, exposed to the north of Jiwa, were observed to be in depositional contact

229 with the serpentinite-matrix mélange (Fig. 2a). The onset of forearc basin deposition was 230 constrained to ~110 Ma based on the U-Pb age of a tuffaceous sandstone directly above

231 the basal unconformity, persisting until ~86 Ma based on detrital zircon maximum

232 depositional ages (Orme and Laskowski, 2016). North of the Xigaze forearc, a narrow

233 (1—2 km north-south width) but relatively thick (~1 km) east-west trending belt of

234 boulder-to-pebble conglomerate and sandstone of the Kailas (Gangrinboche) Formation

235 (e.g. Aitchison et al., 2002; DeCelles et al., 2011) is exposed in buttress unconformity on

236 Gangdese batholith rocks (Leary et al., 2016b). The thickest accumulations of the Kailas

237 Formation are up to 4 km thick, near the type locality at Mt. Kailas (Heim and Gansser,

238 1939; Gansser, 1964). Along the northern Yarlung River Valley, the Kailas Formation is

239 dominated by granite, volcanic, gneiss, limestone, and lithic sandstone cobbles. In the

240 northeast corner of the map area, a ~120 km2 leucogranitic pluton cross-cuts Xigaze

241 forearc, Kailas Formation, and Gangdese batholith rocks.

242 Fault Systems

243 The term “Great Counterthrust” was originally used to describe the south-dipping

244 reverse fault that placed Tethyan Himalayan sequence rocks on the Kailas Formation near

245 Mt. Kailas, ~650 km along strike to the west, where intervening sedimentary- and

246 serpentinite-matrix mélange and Xigaze forearc strata are absent (Heim and Gansser,

247 1939). Here we expand this nomenclature to include a system of moderately- to steeply-

248 south-dipping reverse faults that carry Tethyan Himalayan rocks in the structurally

249 highest position over Kailas Formation and Gangdese batholith rocks in the structurally

250 lowest position, with other splays juxtaposing the intermediary units (Fig. 2a). The

251 presence of multiple fault splays of the Great Counter thrust system might reflect a

252 shallower depth of exposure in the Lazi region than in the Mt. Kailas region, as Great 253 Counter thrust system splays have previously been interpreted to merge into a single fault

254 at depth (Laskowski et al., 2016).

255 Three distinct splays of the Great Counter thrust system were mapped in the Lazi

256 region (Fig. 2a). The southernmost splay juxtaposes the Tethyan Himalayan sequence

257 against serpentinite-matrix mélange, the Liuqu Formation, and Pomunong mélange from

258 east to west. This fault is poorly exposed but was inferred based on juxtaposition of rock

259 units, and dips to the south based on its relationship with topography. To the north, a

260 second splay juxtaposes hanging wall serpentinite-matrix mélange against Xigaze forearc

261 strata to the east, transitioning to a zone of anastomosing faults that juxtaposes the Liuqu

262 Formation, Pomunong mélange, serpentinite-matrix mélange, and Tangga mélange to the

263 west (Fig. 2a). Xigaze Forearc strata in the footwall of this fault zone are steeply dipping

264 and locally overturned (Fig. 2a), and the fault contact between serpentinite-matrix

265 mélange and Xigaze Forearc strata in the Yarlung River valley (Fig. 2a) is characterized

266 by a ~15 m thick zone of fault gouge and brecciated cataclasite. Similarly, a nearby fault

267 contact between serpentinite-matrix mélange and a chert block of the Tangga mélange is

268 exhibits brecciation and chlorite alteration across a ~20-m-thick fault zone. Some chert

269 blocks are entirely encased in serpentinite-matrix mélange and were likely emplaced as

270 tectonic slivers along the intra-ophiolitic splay fault (Fig. 2a). Preservation of the

271 depositional contact between Xigaze forearc strata and serpentinite-matrix mélange near

272 the town of Jiwa is likely the result of the anastomosing character of this fault zone (Fig.

273 2d). The faults dip 20-30º to the south in the eastern map area, 54º to the south near Jiwa

274 (Fig. 2d), 83º to the south ~4 km west of Jiwa, and between 35 and 75º to the south with 275 locally overturned exposures in the anastomosing zone along the Yarlung river valley

276 (Fig 2b).

277 Splays of the Great Counter thrust bound both the north and south sides of the

278 Xigaze forearc basin, which is ~15-km-wide in the Lazi region (Fig. 2a). Xigaze forearc

279 strata are folded across a broad syncline in-between these zones, producing a roughly-

280 fault-parallel average fold axial plane (Fig. 2a). At the smaller scale, the Xigaze forearc

281 strata are folded across open-to-tight, symmetrical folds with amplitudes on the hundred-

282 meter scale, which likely formed as parasitic folds during north-south contraction. In the

283 east, the ~50º-southeast-dipping, northern Great Counter thrust splay (Fig. 2a) juxtaposes

284 hanging wall Xigaze forearc basin strata against the Kailas Formation. Along the Yarlung

285 river valley to the west, the hanging wall of this splay consists of a fine-grained granitoid

286 that is in intrusive contact with Xigaze Forearc basin strata. This contact is sill-like in

287 geometry, and forearc strata structurally above the fine-grained intrusive rock display a

288 strong cleavage oriented ~100º, 60º southwest. The intrusive rock appears to be the

289 product of protracted hypabyssal intrusive activity, and was later subjected to shearing, as

290 foliation-parallel, fine-grained dikes of similar composition to the host rock were

291 observed.

292 The northernmost splay of the Great Counter thrust system (Fig. 2a) appears to

293 have experienced the largest magnitude of slip, as the Kailas Formation in the footwall

294 displays protomylonitic fabrics that were not observed along the southern splays. Along

295 the northern Yarlung River valley (Fig. 2c), the hanging wall consists of fine-grained,

296 felsic igneous rocks (Fig. 2a, 2c), characterized by cataclasis and brecciation within ~100

297 m of the fault. Structurally downsection, brittle fabrics give way to a 20-m-thick zone of 298 top-to-the-north, protomylonitic fabrics that are developed in the granitic clasts and

299 sandstone matrix of the Kailas Formation (Fig. 4a-b). We interpret that these rocks were

300 tectonically buried in the footwall of the Great Counter thrust to brittle-ductile conditions

301 (250—400 ºC), and were later accreted to the hanging wall to accommodate their

302 exhumation. Structurally below the mylonite zone, north-vergent recumbent folds were

303 observed within the Kailas Formation, similarly indicating top-north shearing (Fig. 2c,

304 4c). A weak fault-parallel foliation was observed structurally below the Kailas

305 Formation, in Gangdese batholith rocks ~1 km north of the mylonite zone (Fig. 2c).

306 The mylonitic fabrics observed along the Great Counter thrust in the Lazi region

307 allow us to estimate the depth of tectonic burial of Kailas Formation strata, and the

308 magnitude of slip along the northern splay of the Great Counter thrust system. Assuming

309 a geothermal gradient of 25 ºC/km, the mylonitic Kailas Formation rocks were likely

310 tectonically buried to depths of at least 10 km to achieve brittle-ductile conditions (250-

311 400 ºC) prior to exhumation in the fault zone hanging wall. Considering the measured

312 shear fabric orientation (45—50º to the south), our interpreted minimum burial depth (10

313 km), and assuming a steady geothermal gradient and the absence of convective heating

314 by igneous or hydrothermal activity, displacement across this splay of the Great Counter

315 thrust system totaled at least 13 km. Mylonitic fabrics developed in the proximal footwall

316 of the northernmost Great Counter thrust splay have also been reported near the town of

317 Langxian, located ~500 km along-strike to the east in southeastern Tibet (Wang et al.,

318 2015). The occurrence of mylonites at both localities and the >1000 km along-strike

319 continuity of the Great Counter thrust system suggests that it is a crustal-scale structure. 320 Within the detailed map area near the town of Jiwa (Figs. 2a, 2d), a ~35º north-

321 dipping reverse fault juxtaposes hanging wall sedimentary-matrix mélange against

322 footwall Liuqu Formation strata. Although this fault is not well exposed east of Jiwa, we

323 interpret that it extends along-strike, except where it is cut out by Great Counter thrust

324 splay with Tethyan Himalayan sequence and Liuqu Formation in the hanging wall (Fig.

325 2a). Within the footwall of this fault, bedding in the Liuqu Formation fans upsection from

326 locally overturned (steeply-north-dipping) to moderately south-dipping (Fig. 4e),

327 indicating syntectonic sedimentation. We interpret that this fault is an antithetic splay of

328 the Great Counter thrust system (Fig. 3);

329 Although fault dips and rock unit juxtapositions are highly variable along

330 different splays of the Great Counter thrust system, interpreted cross sections through the

331 Lazi region map area suggest that the regional structural geometry is dominated by a set

332 of three imbricate, south-dipping thrust sheets (Fig. 3). The steeply dipping faults are

333 interpreted to sole into moderately-southwest-dipping (~30º) master faults (Fig. 3),

334 especially in the anastomosting fault zone south of Xigaze forearc basin strata (Fig. 2a).

335 Foliation measurements from the Pomunong and Tangga mélange, bedding

336 measurements in the Liuqu Formation and the Kailas Formation near the northernmost

337 Great Counter thrust splay, and foliation in both Gangdese batholith and Kailas

338 Formation rocks surrounding this fault are consistent with this interpretation (Fig. 3). In

339 contrast, the Linzizong Formation volcanic rocks, exposed on the north side of the

340 Gangdese Mountains, display a regional northward dip (Fig. 1), suggesting that the

341 Gangdese batholith is exposed in the core of a broad anticline oriented parallel to the

342 Great Counter thrust system. There is no exposed north-dipping fault contact between 343 Gangdese batholith rocks and the Xigaze forearc basin, or Gangdese batholith rocks and

344 the Tethyan Himalayan sequence, that might correlate to the Gangdese thrust that has

345 been documented ~400 km along-strike to the east (Yin et al., 1994).

346 GEOCHRONOLOGY AND THERMOCHRONOLOGY

347 U-Pb Geochronology Methods

348 Five igneous and five detrital zircon U-Pb geochronology samples were collected

349 from map units across the study area to confirm map unit identities, constrain sediment

350 provenance, and determine the age of igneous intrusions. Zircons were extracted from ~2

351 kg samples by crushing followed by density and magnetic separation techniques. For

352 detrital samples, a large aliquot of grains (usually >1000) was mounted in epoxy together

353 with crystal standards. For igneous samples, 20-50 selected zircon grains were mounted

354 alongside crystal standards. Mounts were sanded to a depth of ~20 microns to expose

355 zircon cores, polished, imaged with back-scattered electron or cathodoluminescence

356 techniques for navigation purposes, and then cleaned prior to analysis. U-Pb

357 geochronology was conducted by laser-ablation inductively-coupled-plasma mass

358 spectrometry (LA-ICPMS) at the Arizona LaserChron center following the techniques

359 outlined in Gehrels et al. (2006; 2008) and Gehrels and Pecha (2014).

360 All samples were ablated using a Photon Machines Analyte G2 excimer laser

361 equipped with a HelEx ablation cell, with a spot size ranging from 10 to 30 microns

362 depending on the size of the target crystal zone. Typically, ablation pits are ~15 microns

363 in depth. One sample (61012AL3) was analyzed using an Element2 high-resolution

364 ICPMS, which sequences rapidly through U, Th, and Pb isotopes. Signal intensities were

365 measured with a secondary electron multiplier detector that operates in pulse counting 366 mode for signals less than 50,000 counts per second (cps), in both pulse-counting and

367 analog mode for signals between 50,000 and 5,000,000 cps, and in analog mode above

368 5,000,000 cps. The remaining samples (n = 9) were analyzed using a Nu high-resolution

369 ICPMS, which is equipped with a flight tube of sufficient width that U, Th, and Pb

370 isotopes are measured simultaneously. All Nu measurements were made in static mode

371 using Faraday detectors with 3x1011 ohm resistors for 238U, 232Th, 208Pb-206Pb, and

372 discrete dynode ion conters for 204Pb and 202Hg. Further details of the Element2 and Nu

373 analytical procedures can be accessed at the Arizona LaserChron Center web site

374 (Laserchron.org). Detrital zircon U-Pb analytical data are reported in Supplementary

375 Table S1 and igneous zircon U-Pb analytical data are reported in Supplementary Table

376 S2. Uncertainties in these tables are at the 1-sigma level and include only measurement

377 errors. Analyses that were >20% discordant or >5% reverse discordant were rejected.

378 Crystallization-age zircons were isolated from inherited ages in igneous U-Pb

379 samples using the Arizona LaserChron Center (www.laserchron.org) in-house program

380 AgePick, which provides tools for identifying inheritance, Pb loss, and/or overgrowth and

381 recrystallization of metamorphic zircon. Zircons interpreted as inherited in igneous

382 samples are displayed as probability distribution functions alongside weighted mean age

383 determinations incorporating analytical and systematic error (Fig. 5). Detrital zircon data

384 from this study and references from the literature are also displayed as probability

385 distribution functions (Fig. 6), all of which were created using the DZstats program

386 (Saylor and Sundell, 2016).

387 Igneous Zircon U-Pb Geochronology Results 388 Igneous U-Pb samples were collected from the Gangdese batholith (sample

389 61012AL1) in the northwestern portion of the map area (Fig. 2a, 2c), a fine-grained felsic

390 intrusive rock (61012AL7) in the hanging wall of the northernmost Great Counter thrust

391 splay directly below the sill-like intrusive contact with Xigaze forearc strata (Fig. 2a, 2c),

392 and from fine-grained dikes (6912AL1, 6912AL2) and a large, detached boulder of two-

393 mica leucogranite (61312AL1) in close proximity to a large (~100 km2) leucogranite

394 pluton (Fig. 2a). Sample 61012AL1, a hornblende-plagioclase-biotite-quartz granodiorite,

395 was located ~1.3 km north of outcrop exposure of the northernmost splay of the Great

396 Counter thrust, beneath the unconformable contact with the Kailas Formation (Fig. 2c).

397 The outcrop from which this sample was collected displays a weak foliation striking ~E-

398 W (87º) and dipping 60º to the south, similar to the orientation of shear fabrics along the

399 Great Counter thrust (Fig. 2c). The sample yielded a U-Pb age of 170.1 ± 3.8 Ma (Fig. 5).

400 This was the only sample of the five that lacked evidence for complexly zoned zircons

401 with xenocrystic cores. Sample 61012AL7 yielded a poorly constrained crystallization

402 age of 43.7 ± 0.8 Ma based on the weighted mean of the two youngest dates. Four other

403 zircons from this sample yielded dates between 48 and 52 Ma, whereas the remaining ten

404 analyses yielded Paleozoic and Proterozoic dates (Fig. 5). We interpret that all of the

405 dates not included in the weighted mean age determination are inherited based on the

406 presence of zircon cores identified using high-resolution back-scattered electron and

407 cathodoluminescence imagery; however, it is also possible that the true crystallization

408 age is younger than the poorly-defined youngest age population.

409 Samples 6912AL1 and 6912AL2 were collected along a ~north-south oriented

410 valley south of Zha Xilincun and the Phuntsoling Monastery, west of the leucogranite 411 pluton (Fig. 2a). Both samples were collected from fine-grained dikes that intruded sub-

412 parallel to E-W striking bedding of Xigaze forearc sandstone and shale, which dips

413 steeply to the north (strike ~350º, dip 70-85º). Sample 6912AL1 yielded an age of 9.9 ±

414 0.3 Ma (Fig. 5). Five 300-600 Ma dates from this sample are interpreted to reflect

415 inheritance (Fig. 5). Sample 6912AL2 yielded an age of 9.7 ± 0.6 Ma (Fig. 5). The 17

416 older dates from this sample include 300—600 Ma and older, Proterozoic age

417 populations that are also interpreted to reflect inheritance. Sample 61312AL1, a

418 phaneritic two-mica leucogranite, was collected due east of the dike samples on the east

419 side of the pluton (Fig. 2a) from a boulder in a large terminal moraine below a cirque

420 composed entirely of leucogranite. This sample yielded an age of 9.9 ± 0.3 Ma (Fig. 5).

421 Two 14—15 Ma dates and five others between 30 and 800 Ma are interpreted to reflect

422 inheritance. This pluton provides a minimum age of ~10 Ma for the Great Counter thrust

423 system, which it cross-cuts in the northeastern portion of the study area (Fig. 2a).

424 Detrital Zircon U-Pb Geochronology Results

425 Detrital zircon samples were collected from Tethyan Himalayan sequence rocks

426 (n = 3) and the Kailas Formation (n = 2) to confirm field identifications and refine

427 depositional ages. Two samples were collected from Triassic (62211PK3) and Jurassic

428 (62211PK5) Tethyan Himalayan sequence quartz arenites exposed 15—25 km southwest

429 of the city of Lazi outside of the detailed study area (Fig. 1) to examine previously

430 documented provenance differences between the Triassic interval of the Tethyan

431 Himalayan sequence and the rest of the Tethyan Himalayan sequence (e.g. Cai et al.,

432 2016). Sample 62211PK3 produced a detrital zircon age spectrum characterized by a

433 broad 180—1230 Ma age-probability peak with a few older Paleoproterozoic and 434 Archean ages (Fig. 6). In contrast, sample 62211PK5 produced an age spectrum

435 characterized by a broad distribution of ages between 490 and 1250 Ma, alongside a few

436 older Proterozoic and Archean ages, with a dominant age-probability peak at ~509 Ma

437 (Fig. 6). Sample 7712AL2 was collected from a Jurassic quartz arenite exposed on a

438 mountaintop ~1.5 km south of a Great Counter thrust splay (Fig. 2a, 2d), and is

439 characterized by a similar age spectrum to that of 62211PK5 with the addition of four

440 320—450 Ma ages (Fig. 2d).

441 Two samples were collected from sheared sandstone beds within the

442 conglomerate-dominated Kailas Formation, exposed in the footwall of the northernmost

443 Great Counter thrust splay (Fig. 2a, 2c). Both samples reveal a dominance of 40—100

444 Ma ages, largely distributed between ~50 Ma and 80—100 Ma age-probability peaks

445 (Fig. 6). Although these data suggest an Eocene maximum depositional age for the Kailas

446 Formation, another detrital zircon sample from the same locality presented in a separate

447 study provides a well-constrained Miocene maximum depositional age of 22.8 ± 0.3 Ma

448 (Leary et al., 2016b). In contrast, three other samples from the same study yielded older,

449 Eocene maximum depositional ages. Therefore, five of the six detrital zircon samples

450 from this locality (Fig. 2a, 2c) overestimate the depositional age of the Kailas Formation

451 by ~20 m.y., suggesting that zircon availability, extreme local sourcing, and/or complex

452 zonation (possibly with inherited cores and young rims that might have been missed

453 during analysis via LA-ICPMS) prevented reliable maximum depositional age

454 determination in this case.

455 Compilation of Thermochronological Data 456 Published medium- to low-temperature thermochronologic data were compiled

457 for the Gangdese batholith, Kailas Formation, and Liuqu Formation along a 1000-km-

458 long swath encompassing the Gangdese mountains and Yarlung suture zone, between Mt.

459 Kailas and the Lhasa region (Fig. 7). In addition, thermochronologic data from an Early

460 Cretaceous plutonic belt in the northern Lhasa terrane were compiled for comparison

461 with the southern Lhasa terrane data. The combined dataset includes 40Ar/39Ar biotite

462 (Copeland et al., 1987; Sanchez et al., 2013), zircon fission track (ZFT) (Wang et al.,

463 2015; Ge et al., 2016), zircon (U-Th)/He (Hetzel et al., 2011; Dai et al., 2013; Haider et

464 al., 2013; Li et al., 2016; Laskowski et al., 2017), apatite fission track (AFT) (Copeland et

465 al., 1995; Hetzel et al., 2011; Rohrmann et al., 2012; Haider et al., 2013; Carrapa et al.,

466 2014; Wang et al., 2015; Ge et al., 2016; Li et al., 2016), and apatite (U-Th)/He data

467 (Hetzel et al., 2011; Rohrmann et al., 2012; Haider et al., 2013; Dai et al., 2013; Ge et al.,

468 2016) for Gangdese batholith and northern Lhasa terrane plutonic rocks, zircon (U-

469 Th)/He and apatite fission track data (Carrapa et al., 2014) for the overlying Kailas

470 Formation, and apatite (U-Th)/He data for the Liuqu Formation (Li et al., 2015). Closure

471 temperatures for these systems can vary based on radiation damage, crystal chemistry,

472 grain size, zonation, and other factors, but the approximate closure temperatures are 350-

473 300 ºC for 40Ar/39Ar biotite (McDougall and Harrison, 1999), 250-230 ºC for ZFT (e.g.

474 Zuan and Wagner, 1985), 180-160 ºC for zircon (U-Th)/He (e.g. Guenthner et al., 2013),

475 120-60 ºC for AFT (e.g. Green et al., 1989), and 80-60 ºC for apatite (U-Th)/He (e.g.

476 Farley, 2000). Therefore, the data in this compilation record when samples were

477 exhumed through paleo-depths of ~17 km (40Ar/39Ar biotite) to ~2 km (apatite U-Th/He), 478 assuming a 20—30 ºC/km, steady-state geotherm and the absence of significant heat

479 convection by plutons and/or hydrothermal fluids.

480 The proximity of thermochronology samples to the Yarlung suture was calculated

481 by measuring the shortest straight-line distance from each sample to the nearest exposure

482 of ophiolitic rocks, and plotted against thermochronological age to reveal orogen-

483 perpendicular (generally north—south) trends (Fig. 7). Thermochronological data for the

484 Kailas Formation, Gangdese batholith, and northern Lhasa terrane reveal an increase in

485 cooling age with distance from the Yarlung suture, punctuated by an apparently sudden

486 transition across the Gangdese Mountains from mainly Oligocene-Miocene ages in the

487 southern Lhasa terrane to mainly Eocene and older ages in the northern Lhasa terrane

488 (Fig. 7) (Rohrmann et al., 2012). A plot of age vs. distance from the Yarlung suture

489 ophiolitic rocks reveals a dominance of 7—25 Ma ages within ~20 km, and a broader 6—

490 60 Ma range between 20 and 125 km (Fig. 7). The oldest ages in the southern Lhasa

491 terrane are from the zircon (U-Th)/He and ZFT systems, whereas the youngest ages are

492 mainly from the apatite (U-Th)/He and AFT systems (Fig. 7), likely reflecting the higher

493 and lower closure temperatures, respectively. In the Lhasa and Lopu Range regions,

494 biotite 40Ar/39Ar ages are between 17 and 27 Ma, overlapping in age with zircon (U-

495 Th)/He, AFT, and apatite (U-Th)/He data, likely indicating rapid cooling from ~300 ºC

496 (biotite 40Ar/39Ar) to ~80 ºC (apatite U-Th/He).

497 Probability distribution functions created from Yarlung suture zone data,

498 separated by thermochronological system and geologic unit (Fig. 8), reveal a dominance

499 of Late Oligocene—Middle Miocene cooling, except for apatite (U-Th)/He data from the

500 Gangdese batholith and Liuqu Formation that record Late Miocene cooling (Fig. 7, 8). 501 Age-probability peaks for Gangdese batholith and Kailas Formation samples display a

502 slight dependence on closure temperature (age decrease with decreasing closure

503 temperature). Aside from a few older ages from the Gangdese batholith in the Lhasa

504 region that may reflect igneous activity or conductive cooling after emplacement, the

505 majority of cooling through 40Ar/39Ar biotite to AFT closure temperature (approximately

506 350—60 ºC) took place between ~26 and 12 Ma (Fig. 7, 8). Therefore, we conclude that

507 if the Gangdese thrust is a major structure, it was likely active during this period, which

508 also overlaps in time with the permitted age range of the Great Counter thrust. Age

509 spectra from the Gangdese batholith are dominated by age-probability peaks at ~22 Ma

510 and ~17 Ma, accompanied by a ~25 Ma peak in the biotite 40Ar/39Ar data and a younger

511 ~9 Ma peak in apatite (U-Th)/He data that may reflect differences in closure temperature

512 (Fig. 8). Age-probability peaks for the Kailas Formation are between 20 and 12 Ma,

513 slightly younger than those of the Gangdese batholith, consistent with a southward

514 progression of exhumation. Thermochronologic data from the Liuqu Formation indicate

515 that it cooled through apatite (U-Th)/He closure between 10-4 Ma, likely recording

516 Yarlung river incision (e.g. Carrapa et al., 2014) and possibly influenced by Miocene

517 (~16 Ma) to recent orogen-parallel extension (e.g. Sundell et al., 2013) (Fig. 8).

518 DISCUSSION

519 Detrital Zircon Provenance Analysis

520 Detrital zircon samples from the Yarlung suture zone in the vicinity of the Lazi

521 region track sediment provenance prior to and during India-Asia collision. Samples

522 62211PK5 and 7712AL2 (Figs. 1, 2a), which we correlated with the Tethyan Himalayan

523 sequence, are dominated by Pan-African (500-600 Ma) detrital zircons that are typical of 524 both the Lhasa terrane, which rifted from Gondwana, and the Tethyan Himalayan

525 sequence (Fig. 6). However, the absence of younger detrital zircons that might have been

526 derived from the Late Triassic—Paleogene Gangdese magmatic arc is indicative of

527 derivation from Indian sources alone, as is typical of the majority of the Tethyan

528 Himalayan sequence (DeCelles et al., 2000; Gehrels et al., 2011). Similarities between

529 age spectra from these two samples and a composite reference curve for the Tethyan

530 Himalayan sequence (Fig. 6; Gehrels et al., 2011) support our field correlations. Sample

531 62211PK3, located northeast of sample 62211PK5 and southwest of sample 7712AL2

532 (Fig. 1), is characterized by Pan-African and older ages alongside younger, Paleozoic—

533 Early Jurassic (~180 Ma, n = 2) ages. The detrital zircon age spectrum for this sample is

534 similar to that of other Triassic—Early Jurassic Tethyan Himalayan rocks in southern

535 Tibet (Fig. 6) (Aikman et al., 2008; Cai et al., 2012; Li et al., 2015; Cai et al., 2016).

536 Since the Lhasa terrane had rifted from Gondwana prior to deposition of the Triassic—

537 Early Jurassic Tethyan Himalayan sequence strata (e.g. Li et al., 2016), and there is no

538 known Indian source of Permian—Early Jurassic detrital zircons, we interpret that

539 zircons in sample 7712AL2 were derived from crustal fragments along the northwestern

540 margin of Australia (modern day West Papua), transported westward onto the northern

541 margin of India, and incorporated into the Tethyan Himalayan sequence, in accordance

542 with previous interpretations of age-equivalent Tethyan Himalayan rocks in southern

543 Tibet (Cai et al., 2016).

544 Two detrital zircon samples from conglomeratic strata of the Kailas Formation

545 (61012AL2, 61012AL3) yielded markedly different age spectra than those of the Tethyan

546 Himalayan sequence (Fig. 6). Detrital zircons in these samples are mainly distributed 547 between two populations with age-probability peaks at ~50 Ma and ~90 Ma. A few

548 additional Middle Jurassic and Permian detrital zircons were present in sample

549 61012AL2 (Fig. 6). Comparison with a probability distribution function representative of

550 Gangdese arc activity generated from a compilation of igneous samples (Orme et al.,

551 2014) indicates that samples 61012AL2 and 61012AL3 were likely derived almost

552 exclusively from Gangdese magmatic arc rocks (Fig. 6). Similar detrital zircon age

553 spectra characterize the Kailas Formation at other locations along the Yarlung suture

554 zone (DeCelles et al., 2011; 2016; Leary et al., 2016b) and are indicative of local

555 derivation, as no Pan-African or older zircons from either the Lhasa terrane or India were

556 identified (Fig. 6). Interestingly, the granodiorite immediately below the Kailas

557 Formation close to the sample locality is Middle Jurassic in age (sample 61012AL1; Figs.

558 2a, 3), while only a small population of zircons with similar ages from sample 61012AL2

559 was identified. Therefore, the Middle Jurassic Gangdese batholith rocks were likely

560 exposed over a much smaller area compared to those of Late Cretaceous—Paleogene age

561 at the time of deposition. The youngest detrital zircons in both samples overlap in age

562 with the fine-grained granitic igneous rocks (sample 61012AL7), located in the hanging

563 wall of the northernmost Great Counter thrust splay, to the south (Fig. 2a). Therefore, it is

564 possible that these rocks were exposed to the south of the Kailas basin during deposition.

565 Observations that Guided Structural Model Development

566 Mapping, structural geology, geochronology, and thermochronology data

567 presented in this study, alongside constraints from previous studies, guided development

568 of our structural model for Oligocene—Miocene Yarlung suture zone evolution. At the

569 regional scale (Fig. 1), Gangdese batholith rocks are exposed in a belt north of the Xigaze 570 forearc that coincides with the highest elevation portion of the ~1600-km-long Gangdese

571 Mountains, with summit elevations that commonly exceed 5,500 m north of Lazi (Fig. 1).

572 In contrast, the Xigaze forearc basin and Yarlung suture zone mélanges to the south are

573 exposed in a relatively low-lying region, with peaks between 4,500 and 5,000 m

574 surrounding the ~3,900 m Yarlung River valley. Together, thermochronologic data (Fig.

575 7) and map relationships suggest that the Gangdese batholith was preferentially exhumed

576 in the southern Gangdese Mountains, at least partially prior to or during deposition of the

577 Kailas Formation. The Gangdese batholith appears to be exposed in the core of an east-

578 west trending antiform, as north-dipping Linzizong volcanic rocks (the Paleocene—

579 Eocene Gangdese magmatic arc volcanic carapace) and the variably (~35-60º) south-

580 dipping Kailas Formation are exposed along the northern and southern flanks of the

581 Gangdese Mountains, respectively (Fig. 1). The majority of thermochronologic ages

582 older than ~25 Ma in our dataset are from the samples farthest north in the Gangdese

583 Mountains, particularly in the region surrounding Lhasa, and in the northern Lhasa

584 terrane (Fig. 7).

585 Structural model development was guided by multichannel seismic reflection data

586 obtained during the International Deep Profiling of Tibet and the Himalaya (INDEPTH)

587 experiment (Brown et al., 1996; Nelson et al., 1996; Alsdorf et al., 1998; Hauck et al.,

588 1998), collected along a transect from the High Himalaya into the Lhasa Terrane (Fig. 9).

589 We integrated the orientation, depth, and previously published interpretations of major

590 reflectors by projecting a composite profile generated from migrated rift-parallel seismic

591 lines (Fig. 9; Alsdorf et al., 1998) ~250 km to the west into a roughly parallel, regional-

592 scale cross-section through the Lazi region (Fig. 9). A number of prominent reflectors are 593 visible in the INDEPTH data (Fig. 9), including three that were particularly relevant to

594 interpreting the subsurface geology of the Yarlung suture zone. The northernmost

595 reflector is the Yamdrok-Damxung Reflection band (YDR; Fig. 9) (Brown et al., 1996),

596 consisting of a band of reflectors with varying north and south dips between 12 km and

597 18 km depth that was interpreted either as a midcrustal partial melt layer (Nelson et al.,

598 1996) or as a pre-existing structural or lithologic boundary that was subsequently

599 deformed, likely after the shutdown of Gangdese arc magmatism (Alsdorf et al., 1998).

600 Three north-dipping reflectors between ~20-30 km depth are present beneath the southern

601 extent of the YDR (Fig. 9), displaying decreasing northward apparent dip with increasing

602 depth, the uppermost of which projects into the YDR. These were interpreted as a

603 hinterland-dipping duplex above a footwall ramp along a north-dipping fault that projects

604 to the surface within the Yarlung suture zone, possibly equivalent to the Gangdese thrust

605 (Alsdorf et al., 1998; Makovsky et al., 1999). To the south of the YDR, a near-horizontal

606 reflection band referred to as the Yarlung-Zangbo reflector (YZR) is visible at ~25 km

607 depth (Fig. 9), possibly representing a low-angle structural discontinuity below the

608 surface exposure of the Yarlung suture (Alsdorf et al., 1998), a hydrothermal or

609 magmatic boundary (Alsdorf et al., 1998), or an ophiolitic slab separating Indian- and

610 Asian- affinity rocks (Makovsky et al., 1999). Farther south, a series of south-dipping

611 reflections (referred to as the “backthrust system”) are visible in the INDEPTH data that

612 project beneath the North Himalayan domes (BTS; Fig. 9), including the Mabja dome

613 south of Lazi (Fig. 1). These reflectors were interpreted as the down-dip projection of the

614 Great Counter thrust system, interpreted to cut Greater Himalayan rocks that are currently

615 exposed in the North Himalayan domes (Fig. 1) (Hauck et al., 1998). 616 The Kailas and Liuqu Formations provide a rich record of sediment transport,

617 suture zone basin evolution, and thermal evolution during Oligocene—Miocene time.

618 The basal unconformity between the Kailas Formation and Gangdese arc plutonic rocks,

619 which are Middle Jurassic to Eocene in age in the Lazi region (Fig. 2a), implies that the

620 Gangdese batholith underwent significant exhumation prior to the onset of Kailas

621 Formation deposition, which occurred between 25—23 Ma in the Lazi region (Leary et

622 al., 2016b). Growth strata in the Kailas Formation (Wang et al., 2015) indicate

623 progressive southward steepening of the basal unconformity during deposition while

624 detrital zircon data, conglomerate clast counts, sandstone modal petrography and

625 paleocurrent indicators indicate initial Gangdese arc provenance from the north in the

626 lower Kailas Formation followed by provenance from the south in the upper Kailas

627 Formation (DeCelles et al., 2011; 2016; Leary et al., 2016b). Leary et al. (2016a)

628 interpret growth strata in the Liuqu Formation within the footwall of a Great Counter

629 thrust system splay (Fig. 4e; Leary et al., 2016a), indicating syntectonic sedimentation.

630 Thermochronologic data from Gangdese batholith rocks just to the east of the

631 Lazi region study area (Fig. 7) indicate that the Gangdese arc was being exhumed

632 immediately prior to, during, and shortly after Kailas Formation deposition, with

633 maximum age-probability between 20 and 15 Ma (Figs. 5,7). The most proximal

634 Gangdese batholith thermochronologic data, collected along a ~N-S transect ~5 km to the

635 northeast of the Lazi region geologic map (Figs. 2a, 8), include three apatite fission track

636 (AFT) ages between 25 and 23 Ma alongside one older age at ~28 Ma and two ages at ~9

637 Ma (Ge et al., 2016) and seven zircon (U-Th)/He ages between 23-17 Ma (Dai et al.,

638 2013; Ge et al., 2016). Also of note is the short duration between Kailas Formation 639 deposition (25-23 Ma) and subsequent exhumation, which began by 17 ± 1 Ma based on

640 zircon (U-Th)/He and AFT data from the Kailas Formation along-strike to the west

641 (Carrapa et al., 2014) (Fig. 8).

642 Any tectonic model of Yarlung suture zone evolution during Oligocene—

643 Miocene time must explain the geological, geophysical and thermochronological

644 constraints summarized above, and in particular provide a mechanism for Gangdese arc

645 exhumation both during and after deposition of the Kailas Formation, synchronous with

646 deposition of the upper Kailas Formation. If a north-dipping fault like the Gangdese

647 thrust were not active at the same time as the Great Counter thrust system, then we would

648 expect that that Gangdese batholith rocks would have experienced burial beneath the

649 Kailas Formation and additional tectonic burial in the Great Counter thrust footwall

650 during this period. Furthermore, between 25-23 Ma the Kailas basin was in a position of

651 low relative elevation and characterized by a warm and wet climate (DeCelles et al.,

652 2011; 2016), whereas today the Kailas Formation is exposed at high elevation (4800—

653 6700 m), draped along the southern margin of the Gangdese Mountains, with an arid- to

654 semi-arid climate and mean annual temperatures of approximately -5 ºC at 5000 m

655 elevation (Quade et al., 2011).

656 Despite the obscurity of north-dipping structures such as the Gangdese Thrust,

657 and the poorly-constrained magnitude of the Great Counter thrust due to a lack of

658 hanging-wall cutoffs, the Yarlung suture zone in southern Tibet appears to have

659 experienced large-magnitude shortening during Oligocene-Miocene time. Indeed, the

660 >1000-km along-strike continuity of the Great Counter thrust system indicates that it

661 accommodated significant shortening, while juxtaposition of Gangdese batholith rocks 662 against Xigaze forearc (Fig. 1) and Tethyan Himalayan strata (primarily east of Lhasa) is

663 consistent with the existence of a fault equivalent to the Gangdese thrust (Searle et al.,

664 1987, Yin et al., 1994). It is possible that the Gangdese thrust reactivated the original,

665 north-dipping megathrust—essentially the India-Asia suture projected down-dip—that

666 accommodated convergence between India and Asia. Modern convergent margins are

667 characterized by arc-trench distances (from the center of the magmatic arc to the trench)

668 on the order of ~300 km (Dickinson, 1995; Noda, 2016). In addition, a global

669 compilation of modern forearc basins reveals a constant forearc basin width-to-depth

670 ratio of 20:1 (Noda, 2016). In the Lazi region, the distance from the center of the

671 Gangdese batholith to the northernmost Tethyan Himalayan sequence rocks is only ~25

672 km, and the width of the Xigaze forearc basin is only ~15 km (Figs. 1, 2a). Estimation of

673 the original forearc basin width based on the maximum preserved thickness of Xigaze

674 forearc strata (~8 km; Einsele et al., 1994) predicts an original width of ~160 km,

675 implying ~90% shortening.

676 Structural Model

677 Passive roof duplexes, which are characterized by imbricated, foreland-dipping

678 thrust sheets in the absence of an emergent, hinterland-dipping fault, have been

679 recognized in the field and in seismic data since the 1980s (Banks and Wharburton,

680 1986). Physical experiments predict that passive roof duplexes are more common in

681 zones of efficient surface erosion (Mora et al., 2014, and references therein), consistent

682 with critical taper wedge mechanics of thin-skinned thrust belts (e.g. Davis et al., 1983).

683 Since their first recognition, it has been postulated that efficient sediment storage in the

684 adjacent monocline along the deformation front is critical to the formation of passive roof 685 duplexes (Mora et al., 2014). In the Lazi region, evidence that supports the existence of a

686 passive roof duplex includes: 1) the presence of imbricate, foreland-dipping (south-

687 dipping) thrust sheets, 2) the absence of an emergent, hinterland-dipping fault, and 3)

688 Early Miocene syntectonic sedimentation in a contractional setting in the upper Kailas

689 Formation and the Liuqu Formation 4) growth structures in the Kailas (Wang et al., 2015)

690 and Liuqu Formations (Fig. 4e; Leary et al., 2016a) that indicate increasing southward

691 dip of the basal unconformity throughout deposition, and 5) thermochronology data that

692 indicate significant exhumation of the Gangdese batholith and Yarlung suture zone

693 during Oligocene—Miocene time.

694 Our preferred model for southern Lhasa terrane, Yarlung suture zone, and

695 Tethyan Himalayan structural evolution during Oligocene-Miocene time (Fig. 10)

696 involves an initial, Oligocene—Miocene (~26-23 Ma) phase of extension along a top-to-

697 the-north normal fault, similar in orientation to the India-Asia suture projected down-dip,

698 to accommodate Kailas Formation deposition in an extensional basin. Later, this

699 structural discontinuity was reactivated as a top-to-the-north thrust system that roots into

700 a hinterland-dipping duplex beneath the Gangdese batholith. South-directed thrusting fed

701 slip into a system of top-to-the-north thrusts (the Great Counter thrust system), which

702 together comprise a foreland-dipping, passive-roof duplex. In this model, the hinterland-

703 dipping duplex, which is not exposed at the surface but is consistent with INDEPTH

704 seismic reflection data (Fig. 9), feeds slip into the Gangdese thrust of Yin et al. (1994),

705 which forms the basal detachment of the Great Counter thrust system passive roof

706 duplex. The lack of surface exposure of the Gangdese thrust in southern-central Tibet

707 likely reflects the shallower depth of exhumation relative to southeastern Tibet, where 708 Gangdese batholith rocks are juxtaposed against Tethyan Himalayan sequence rocks and

709 the Yarlung suture zone assemblages are absent.

710 The structural model (Fig. 10) initiates with deposition of the Kailas Formation

711 between 26-23 Ma in a fault-bounded, extensional basin associated with the inferred,

712 north-dipping Kailas normal fault (DeCelles et al., 2011; 2016; Leary et al., 2016b). We

713 favor this interpretation as it is explains the fanning southward-dipping growth strata in

714 both the lower and upper Kailas Formation (Wang et al., 2015), the narrow-but-deep,

715 “lacustrine sandwich” basin architecture that is typical of the Kailas Formation (DeCelles

716 et al., 2016), and organic geochemical data consistent with warm-water lacustrine

717 deposition that are interpreted to reflect deposition at lower elevation than at present

718 (DeCelles et al., 2011; 2016). The position of the Kailas basin adjacent to the Gangdese

719 magmatic arc, in the hinterland of the Himalayan thrust belt, implies that the Yarlung

720 suture was likely at high elevation during the Oligocene, following >20 Myr. of

721 collisional orogenesis (e.g. Najman et al., 2010; Hu et al., 2016). Therefore, the Kailas

722 basin might have transitioned to low elevation between 26-23 Ma as the result of

723 localized crustal extension (DeCelles et al., 2011). 25-23 Ma apatite fission track ages

724 from Gangdese arc rocks proximal to the Lazi region study area (Fig. 7; Ge et al., 2016)

725 might reflect erosional exhumation of Gangdese arc rocks during deposition of the Kailas

726 Formation.

727 During deposition of the upper Kailas Formation, a transition to southerly

728 provenance coincided with, or shortly preceded a return to contraction along the Yarlung

729 suture. Regional-scale exhumation of the Gangdese batholith and Kailas Formation

730 occurred between 23—15 Ma based on prominent 40Ar/39Ar biotite, zircon and apatite 731 fission track, and zircon and apatite (U-Th)/He age-probability peaks (Fig. 7, 8). We

732 interpret that 23—15 Ma exhumation was primarily driven by growth of the hinterland-

733 dipping duplex beneath the Gangdese Mountains, resulting in structurally higher fault-

734 bend folding to tilt the Kailas formation southward, drive erosional exhumation in the

735 southern Gangdese Mountains, and generate the regional northward dip of Linzizong

736 Formation, to the north (Fig. 10). Duplexing would be expected in the hinterland of the

737 Himalayan thrust belt, beneath the Yarlung suture zone, to regain crustal thickness and

738 build taper following Kailas basin extension. To the east of Lhasa, where the suture zone

739 is more deeply exhumed, it is possible that the hinterland-dipping detachment at the base

740 of this duplex—equivalent to the Gangdese thrust—is exposed at the surface. In the Lazi

741 region, the only north-dipping structure observed in the field was a reverse fault that

742 placed serpentinite-matrix mélange on the syncontractional, ~20 Ma Liuqu Formation (Li

743 et al., 2015; Leary et al., 2016a), which is likely an antithetic splay of the Great Counter

744 thrust system (Figs. 2a, 7).

745 North Himalayan dome exhumation in the center of the Tethyan Himalaya might

746 also be related to the propagation of the proposed hinterland-dipping duplex and

747 associated passive roof duplex (Fig. 1). We interpret that the Great Counter thrust system

748 roots into the detachment of the hinterland-dipping duplex. It is possible that this

749 structure branches with, or is the northerly extension of, the south-dipping “Back-Thrust

750 System” (BTS) INDEPTH reflector beneath the North Himalayan domes (Fig. 9). The

751 “Back-Thrust System” fault likely cuts the Greater Himalayan sequence based on the

752 relatively shallow, regional northward dip of the Tethyan Himalayan sequence north of

753 its basal contact along the northern margin of Mabja dome (Fig. 1, Fig. 10). Deep-rooted, 754 top-to-the-north thrusting, paired with vertical ductile thinning and upper-crustal

755 extension, is consistent with Middle Miocene upper-crustal exhumation ages from the

756 Mabja dome (Lee et al., 2004) and maintains bulk strain compatibility. The model

757 proposed here is similar to the kinematic model proposed by Lee et al. (2000) to explain

758 the exhumation of the Kangmar dome above the north-dipping Gyirong-Kagmar thrust,

759 with a modified vergence direction that is more compatible with the geophysical data

760 (Fig. 9; Hauck et al., 1998; Makovsky et al., 1999).

761 Our structural model provides an explanation for Miocene cooling ages in the

762 Kailas Formation and Gangdese batholith in southern Tibet (Fig. 7), which in a simple

763 structural model should have been experiencing tectonic burial rather than exhumation

764 during Great Counter thrust shortening. Cross section representations of our model are

765 consistent with the Lazi region geology (Fig. 10, Fig. 3), as well as the depth, orientation,

766 and previous structural interpretations of prominent seismic reflectors visible in

767 INDEPTH seismic reflection data (Fig. 9). Our model also provides mechanisms for

768 uplift of the Kailas Formation from relatively low to relatively high elevation, further

769 southward tilting of the upper Kailas Formation, and relief generation across the

770 Gangdese Mountains. As deformation progressed, three splays of the Great Counter

771 thrust system initiated, possibly propagating from north to south based on the

772 depositional ages of the upper Kailas Formation and the Liuqu Formation and trends in

773 the Gangdese batholith and Kailas Formation thermochronologic data (Fig. 7, 8).

774 Orogen-parallel extension initiated at ~16 Ma along the suture zone in southern

775 Tibet, providing a minimum age constraint for the Great Counter thrust system (Sanchez

776 et al., 2013; Sundell et al., 2014; Laskowski et al., 2017). Detrital zircon geochronology 777 of foreland basin deposits in India (Lang and Huntington, 2014), the termination of north-

778 south sediment transport associated with the Kailas and Liuqu Formations (Leary et al.,

779 2016a), as well as previous interpretations of Yarlung suture zone thermochronology

780 (Carrapa et al., 2014) suggest that the Yarlung-Siang-Brahmaputra river system also

781 initiated during early Miocene time. It is possible that the Yarlung River was established

782 through a combination of topographic inversion (i.e. uplift of the Himalayas to higher

783 elevation than the Yarlung suture zone)—thought to have taken place during Miocene

784 time based on paleoelevation records and structural interpretations (Murphy et al.,

785 2009)—and relief generation along the Gangdese Mountains related to duplexing. Middle

786 to late Miocene AFT and apatite (U-Th)/He ages from the Gangdese batholith, Kailas

787 Formation, and Liuqu Formation (Figs. 5, 9) likely reflect efficient erosional exhumation

788 along the Yarlung river valley following establishment of this continent-scale drainage

789 system (Carrapa et al., 2014). The ~10 Ma leucogranite pluton that cuts across the

790 Yarlung suture zone in the eastern portion of the study area (Fig. 2a) was possibly

791 generated by isothermal decompression driven by orogen-parallel extension. Its

792 exhumation might be related to diffuse normal faulting related orogen-parallel extension,

793 or efficient fluvial incision along the Yarlung river valley.

794 CONCLUSIONS

795 Structures and rocks units exposed in the Lazi region record a pronounced episode

796 of late Oligocene—early Miocene contractional deformation that drove exhumation of

797 the Yarlung suture zone and southern Lhasa terrane. Our mapping reveals the presence of

798 south-dipping imbricate splays of the Great Counter thrust system that juxtapose Tethyan

799 Himalayan Sequence strata, sedimentary- and serpentinite-matrix suture zone mélange, 800 Xigaze forearc basin strata, and the Kailas Formation, from south to north. Detrital zircon

801 provenance analysis of the Tethyan Himalayan sequence reveals a difference between

802 Triassic strata—derived at least partially from West Papua—and the rest of the Tethyan

803 Himalayan sequence—derived from more local, Indian sources. Dominance of

804 Cretaceous—Eocene detrital zircons in the Kailas Formation indicates local provenance

805 from the Gangdese arc, and the likely absence of Oligocene—Miocene igneous rocks

806 within the source area. Compilation of thermochronological data from a >1000-km-long

807 swath along the southern Lhasa terrane and Yarlung suture zone reveals a dominance of

808 late-Oligocene to early Miocene (~23-17 Ma) exhumation, concomitant with slip across

809 the Great Counter thrust system. The paradox of exhumation of the Kailas Formation and

810 Gangdese batholith at the same time or shortly after they were experiencing tectonic

811 burial to brittle-ductile conditions in the footwall of the Great Counter thrust system

812 supports the existence of a north-dipping fault that carried Gangdese batholith and Kailas

813 Formation rocks in its hanging wall. Using constraints from INDEPTH seismic reflection

814 data, we propose a new structural model for the Oligocene—Miocene Yarlung suture

815 zone in which the Great Counter thrust system is a passive-roof duplex that is

816 kinematically linked with the Gangdese thrust. The proposed existence of two

817 detachment horizons structurally above (the Great Counter thrust) and below (the

818 Gandese thrust) a duplex beneath the Gangdese Mountains explains the lack of Gangdese

819 thrust exposure west of the city of Lhasa in central-southern Tibet, and provides a

820 mechanism for suture zone exhumation concomitant with slip across the Great Counter

821 thrust system and deposition of the Liuqu Formation. We interpret that the Great Counter

822 Thrust cuts the Greater Himalayan sequence at depth, driving exhumation and upper- 823 crustal vertical thinning of these rocks as they were thrust northward. If this model is

824 correct, then the Oligocene—Miocene contractional deformation along the Yarlung

825 suture zone might have generated relief between the low-lying Yarlung suture zone and

826 the high-standing Gangdese Mountains, which largely define the southern boundary of

827 internal drainage on the Tibetan Plateau and the northern boundary of the incipient

828 Yarlung-Siang-Brahmaputra river watershed in southern Tibet.

829 ACKNOWLEDGEMENTS

830 We acknowledge Devon Orme and Kathryn Metcalf for field collaboration and

831 informative conversations, the Arizona Laserchron Center for analytical support, and Dr.

832 Ding Lin for field and laboratory collaboration. This research was supported by grants

833 from the U.S. National Science Foundation Continental Dynamics Program (EAR-

834 1008527), the U.S. National Science Foundation Instrumentation and Facilities program

835 (EAR-1338583) to the Arizona Laserchron Center, the China National Science

836 Foundation (41490610), and the Geological Society of America (student research grant).

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1137 conditions: Nuclear Tracks, v. 10, p. 303-307

1138 FIGURE CAPTIONS

1139 Figure 1 – Tectonic map of the Himalaya, Tethyan Himalayan physiographic zone, and

1140 southern Lhasa terrane in central-southern Tibet. Geology simplified from (insert ref for

1141 Tibet Plateau English Geo Map). The Lazi region study area is shown in the box, and the

1142 cross section line refers to the cross sections in Fig. 10. Inset map adapted from Guillot et

1143 al. (2008). STDS – South Tibetan Detachment system, GCT – Great Counter thrust, GT –

1144 Gangdese thrust.

1145 Figure 2 – A) Geologic map of the Lazi region study area, with geochronology sample

1146 locations and results, structural data for the Xigaze forearc (KEx) on an equal-area,

1147 lower-hemisphere stereonet, and geographical features referenced in the text. Detailed

1148 geologic mapping locales are indicated, and refer to B) the northern Yarlung River valley

1149 detailed mapping area, C) the southern Yarlung River valley detailed mapping area, and

1150 D) the Jiwa detailed mapping area.

1151 Figure 3 – Cross sections through the Lazi region map area along profiles A-A’ and B-B’

1152 (Fig. 2a) with no vertical exaggeration (1:1 vertical to horizontal scale). Map units and

1153 colors are keyed to Fig. 2. Structural data from nearby measurements was projected into

1154 the cross section plane.

1155 Figure 4 – Photographs from the Lazi region. A. Outcrop exposure of protomylonitic

1156 Kailas Formation conglomerate along the northernmost splay of the Great Counter thrust,

1157 displaying top-north sense of shear. B. Protomylonitic fabrics in both Kailas Formation

1158 matrix and cobbles along the exposure in photograph A. C. North-dipping recumbent fold 1159 and synthetic fault in the Kailas Formation beneath the Great Counter thrust system shear

1160 zone shown in A and B. D. Liuqu Formation pebble conglomerate deposited in angular

1161 unconformity atop a bedded, radiolarian chert chert block within the Tangga mélange.

1162 The Liuqu Formation at this locality coarsens upward to cobble and boulder

1163 conglomerate, and is dominated by chert clasts. E. Fanning beds in the Liuqu Formation

1164 that transition from overturned (to the north) to moderately-south-dipping (to the south)

1165 in the footwall of a north-dipping fault that we interpret as an antithetic splay of the Great

1166 Counter thrust system.

1167 Figure 5 – Igneous U-Pb results shown as both weighted mean age determinations with

1168 1-sigma error (left), plotted as a function of U/Th ratio, and probability distribution

1169 functions (PDFs) of U-Pb ages interpreted to reflect inheritance (right). Zircon dates

1170 excluded from the weighted mean age determination are indicated by the light weighted

1171 symbols. PDFs were generated using DZStats (Saylor and Sundell, 2016).

1172 Figure 6 – Detrital zircon U-Pb results presented in this study (top) alongside reference

1173 curves from literature (bottom). The color-coding indicates detrital-zircon based

1174 provenance correlation, discussed further in the text. Probability distribution functions

1175 were generated using DZStats (Saylor and Sundell, 2016). The solid gray bar indicates

1176 the range of ages most common in Kailas Formation samples, allowing for evaluation of

1177 our provenance interpretation. Peak ages, calculated using the AgePick program

1178 (available from www.laserchron.org), are labeled on the plots.

1179 Figure 7 – Compiled thermochronological data for Gangdese batholith and Kailas

1180 Formation rocks in the southern Lhasa terrane, and an Early Cretaceous pluton in the

1181 northern Lhasa terrane, with thermochronological system indicated by the symbol shape 1182 and age indicated by color. Zircon (U-Th)/He and apatite fission track ages for the Kailas

1183 Formation are indicated by the black dots and are labeled with thermochronologic age

1184 (zircon (U-Th)/He in bold). These data are plotted atop a digital elevation model from the

1185 Global Multi-Resolution Topography Synthesis (Ryan et al., 2009) annotated with

1186 geographical features discussed in the text (top). The Gangdese batholith and Lhasa

1187 terrane data are plotted as a function of distance from the Yarlung suture zone ophiolites

1188 and ophiolitic mélange (bottom). Compiled thermochronological data are available in

1189 Supplementary Table S3, which also contains references for data sources.

1190 Figure 8 – Summary of tectonic events discussed in the text, including magmatism,

1191 sedimentation, deformation (extensional and contractional), and metamorphism. In

1192 addition, probability distribution functions (PDFs) of compiled thermochronological data

1193 separated by system and rock unit (Supplementary Table S3) from the Yarlung suture

1194 zone across southern Tibet. Sample locations and ages are provided in Fig. 7. PDFs were

1195 generated using DZStats (Saylor and Sundell, 2016).

1196 Figure 9 – A. INDEPTH common-midpoint projected seismic reflection profile from

1197 Alsdorf et al. (1998), including labels for prominent seismic reflectors discussed in the

1198 text. These include the Moho, the MHT – Main Himalayan Thrust, the BTS – Backthrust

1199 Zone, the YZR – Yarlung Zangbo Reflector, the YDR – Yarlung Deep Reflector, and the

1200 GDR – Gangdese Deep Reflector. The simplified regional cross section presented in Fig.

1201 10 is shown atop these data to illustrate where our interpretations are supported by

1202 projected seismic reflection data. B. Generalized tectonic map of southern Tibet, showing

1203 the location of the regional-scale cross section C-C’ through the Lazi region map area 1204 (labeled with the box), and the location of INDEPTH seismic reflection profiles used to

1205 create the profile in A.

1206 Figure 10 – Restored (bottom) and present day cross-sections from the Himalayas,

1207 through the Lazi region study area, and into the southern Lhasa terrane (A-A’; Fig. 1)

1208 based on the geologic map in Fig. 1, INDEPTH seismic reflection data (Fig. 9), and our

1209 mapping results (Fig. 2a-d). No vertical exaggeration. Color-coding and unit names are

1210 keyed to Figs. 1 and 2a. Structural data are indicated by the ball and tick mark symbols,

1211 whereas the circled numbers indicate constraints from previous studies. Constraint #1 is

1212 the subhorizontal orientation of structural fabric referred to by previous workers as the

1213 Yarlung-Zangbo reflector (YZR; Fig. 9), constraint #2 refers to the south-dipping

1214 reflectors interpreted as the down-dip projection of the Great Counter thrust (BTS; Fig.

1215 9), and constraint #3 is the imbricate north-dipping reflectors beneath the YDR,

1216 previously interpreted as a hinterland-dipping duplex (Alsdorf et al., 1998). Constraint #4

1217 is the estimated thickness of the Greater Himalayan sequence beneath the Tethyan

1218 Himalaya based on exposures in the footwall of the Gurla Mandata metamorphic core

1219 complex ~600 km to the west of the study area (Murphy, 2007).

1220 1GSA Data Repository item 201Xxxx, containing U-Pb geochronologic data and

1221 compiled thermochronologic data, is available online at

1222 www.geosociety.org/pubs/ft20XX.htm, or on request from [email protected] or

1223 Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. Figure 1 87ºE Click here to download88ºE Figure Fig_1.pdf Indus 30ºN 35º GB ASIA C’ KPlv

Himalayasdetail GB 30º Yarlung Figure 10 Lhasa GB INDIA Kl 80º 95º

GB GANGDESE MTNS.GB GB Kl GB KPlv Kl

OMk Dogxu OMk ng River GB KEx Ngamring GCT L oph KEx 61012AL2,3 GCT mlg Yarl oph ung River Ml 7712AL2 mlg GCT Figure 2 Ml 62211PK3 Lazi

29ºN THS THS 62211PK5 No es rth H n Dom Hlako imalaya THS GHS L Peak L L GHS GHS THS Tingri L Mabja TETHYAN HIMALAYA THS

THS

L THS STDS C L L GHS L L Qomolangma L

28ºN HIMALAYA STDS (Everest) GHS L 50 km

Paleogene-Neogene Oligocene-Miocene Cretaceous L OMk oph leucogranite Kailas Formation Xigaze Ophiolite Jurassic-Paleogene Cretaceous Cambrian-Paleocene GB Kl THS Gandese Batholith Lhasa terrane Tethyan strata (undiv.) Cretac.-Paleogene Cretaceous Neoproterozoic Greater KPlv Kx GHS Linzizong Volcanics Xigaze Forearc Himalaya Sequence Miocene Sedimentary-matrix Ml mlg Liuqu Fm. mélange (undiv.) Cretaceous Xigaze JPg FigureQal Quaternary 2a alluvium Kx 87º45’ E 88ºClick E here to download Figure Fig_2a.pdf forearc strata, undiv. JPg Ng Neogene leucogranite JPg JPg L. Jurassic-E. Cretaceous A Miocene JKt Ml Tangga sed. mélange Liuqu Formation L. Jurassic-E. Cretaceous JKp OMk Oligocene-Miocene Pomunong sed. mélange JPg Kailas Formation N cht Radiolarian chert block Eocene (?) hypabyssal Ei oph felsic intrusive rock ophiolitic mélange A’ 29º 24’ N JPg Jurassic-Paleogene THS Tethyan Himalaya Sequence, Gangdese arc rocks undivided JPg Lithologic (thin) and fault (thick) contacts; Detail Map 1 solid lines, well located; dashed lines, JPg poorly located or inferred; dots, covered, red: active or recently active fault 61012AL1 B’ Thrust fault; triangle on hanging wall 170±4 37 OMk 61012AL2 OMk 30 Triangular arrow shows dip of fault, Ei OMk JPg diamond arrow shows trend and plunge 61012AL3 57 Phuntsoling of striae on fault surface Ei OMk Monastery OMk 49 45 44 55 55 Inclined, overturned and JPg vertical bedding 61012AL7 60 60 OMk 54 Kx 32 Foliation, arrow shows trend and plunge 44±1 Ma Kx OMk 40 of lineation Ng Ng Kx Town Kx Zha Xilincun Geochronology U-Pb zircon 4000 Multiple chronometers applied 50 Detrital zircon sample 55 70 35 40 l plane 65 axia 6912AL2 vg. 80 A 50 10±1 Ng 61312AL1 Kx Kx bedding 85 Kx Kx 48 10±1 n = 41 55 6912AL1 dikes 50 10±1 rubble

Kx 30 Yarlung River Kx Kx Kx 55 69 35 60 50 55 50 35 Kx Kx Kx 56 Detail Map 2 78 64 70 46 60 72 oph 42 77 Jiandacun 35 55 57 Detail Map 3 30 40 37 75 48 oph 52 20 cht 60 cht 37 cht 20 70 82 Ml JKt 65 oph oph oph 80 70 15 JKt 50 oph 80 75 77 49 68 Ml Jiwa 50 54 oph 60 50 oph 35 80 Ml 32 72 36 35 JKt Ml 65 Ml oph 80 74 THS JKp 35 JKp 83 54 65 Ml oph 55 Ml 51 7712AL2 THS JKp THS THS 20 10’ 50 Ml 29º 10’ N B 5 0 KILOMETERS 5 10 Lazicun THS A 45’ THS Figure 2b Click here to download Figure Fig_2b.pdf 87°44' E 87°40' E 87°42' E Gonggangcun 4000 B

Kx Kx Qal 50 55 Qal Kx 64

35 60 40 70 69 50 Kx 4000 35 Yarlung River 56 29°14' N Kx 55 42 78 30

4500 47 35 Jiandacun 48 oph 72 77 oph oph 60 37 37 oph cht 64 55 37 47 60 35 85 58 75 85 67 70 83 cht 40 Ml 60 Ml 70 JKt 75 Qal 15 80 JKt 49 cht 26 35 34 68 35 45 Ml cht 60 50 62 75 JKt Ml 44 50 29°12' N 32 80 36

35 JKt

JKp

35 JKp JKp

Youzhai JKp 55 N

Qal JKp 0.51 Km0 1 2 Figure 2c Click here to download Figure Fig_2c.pdf 87°47' E 87°49' E Qal JPg 4500 C JPg JPg

Qal

29°22' N

JPg 61012AL1 JPg 170±4 Ma

OMk 45 61012AL2 40 43 OMk recumbent fold 43 61012AL3 protomylonitic Kailas Fm. 4500 (fig. 5c) 49 cataclasite breccia

77 50 Qal cataclasite breccia Ei Ei 70 45

68 61012AL7 Kx 66 60 44±1 Ma 73 60 55

57 Qal

4000

Qal 29°20' N

1 0.5 0 KILOMETERS 1 2 N Figure 2d Click here to download Figure Fig_2d.pdf Kx 87°52' E 87°54' E 87°56' E 5000 D Kx 4500 Qal 29°14' N Qal 4500 5000

Kx Kx Kx

Qal 57

70 60 Qal oph 74 70 82 depositional 65 80 Kx 77 Kx oph 50 Qal oph 54 50 oph 65 oph 83 80 35 35 29°12' N Ml 32 70 65 25 72 oph 54 80 Ml 4500 Ml Ml Qal 65 Qal 4500 74 4500 72 Ml Ml 5000 THS 51 THS 7712AL2 THS

20 Qal Qal 25 50 THS 29°10' N THS N THS KM 0.51 0 1 2 5000 Figure 3 Click here to beddingdownload trace Figurebedding Fig_3.pdf A (SW) mylonite foliation A’ (NE) parasitic folding 5 THS Qal cht Ei Qal Ml Qal Qal Qal 4 JKp 3 oph JKt Kx JPg 2

Elev. (km) Elev. Kx OMk 1 0 oph no vertical exaggeration

B (SW) B’ (NE)

5 Ml oph KEx Qal Qal Qal OMk 4 THS JPg oph 3 Kx 2 JKp Ng Kx Elev. (km) Elev. 1 Ng oph 0 no vertical exaggeration Figure 4 Click here to download Figure Fig_4.pdf A B mylonitic fabric in Kailas Formation

foliation

sheared granite clast

south north south north

C Great Counter thrust system D

Liuqu Fm. folded Kailas Fm.

angular unconformity

Radiolarian chert block in Tangga mélange

south north

E hanging wall serpentinite-matrix mélange Liuqu Formation growth strata Great Counter thrust system

bedding

south north Figure15 5 Click here to download Figure Fig_5.pdf 61312AL1 61012AL7 14 xenocrystic zircon 13 n = 7 final age = 9.9 ± 0.3 2σ 12 MSWD = 0.8, systematic error = 0.8% n = 10, youngest population of 17 total 11 Age (Ma) 10

8 0.0 1.0 2.0 0 500 1000 1500 2000 U/Th Ma 60 58 61012AL7 61012AL7 56 xenocrystic zircon 54 n = 14 52 final age = 43.7 ± 0.8 2σ MSWD = 0.5, systematic error = 0.8% 50 n = 2, youngest population of 16 total 48 Age (Ma) 46 44 42 40 0.0 1.0 2.0 3.0 4.0 0 500 1000 1500 2000 Ma U/Th 15 6912AL1 6912AL1 extends off chart 13 xenocrystic zircon n = 5 11

9 Age (Ma) final age = 9.9 ± 0.3 2σ 7 MSWD = 1.0, systematic error = 0.8% n = 12, youngest population of 17 total

5 0.0 2.0 4.0 6.0 8.0 10 0 500 1000 1500 2000 U/Th Ma 15 6912AL2 6912AL2 13 xenocrystic zircon n = 17 11

9 Age (Ma) final age = 9.7 ± 0.6 2σ 7 MSWD = 0.1, systematic error = 0.8% n = 4, youngest population of 21 total 5 0.0 2.0 4.0 6.0 8.0 10 0 500 1000 1500 2000 U/Th Ma 190 61012AL1 185 180 175 170 165 Age (Ma) 160 final age = 170.1 ± 3.8 2σ MSWD = 0.1, systematic error = 0.7% 155 n = 16,16 total 150 0.0 1.0 2.0 U/Th Figure 6 Click here to download Figure Fig_6.pdf 49

87 n = 107 Kailas Formation: 61012AL3 50

90 177 n = 113 Kailas Formation: 61012AL2 n = 2 516 221 549 n = 6 180 Tethyan: 62211PK3 n = 70 515

546 Tethyan: 7712AL2 n = 87

509

543 Tethyan: 62211PK5 n = 89 554

0 50 100 150 200 250 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000

Asia Lhasa Terrane Igneous (Orme et al., 2014) Lhasa Terrane (Gehrels et al., 2011) n = 263 n = 733

Lazi Area Forearc (Orme and Laskowski, 2016) n = 2163

India Triassic Tethyan (Aikman et al., 2008; Cai et al., 2012; Li et al., 2015) n = 1413

Tethyan & Upper LHS (Gehrels et al., 2011) n = 3912

0 50 100 150 200 250 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 U-Pb Age (Ma) U-Pb Age (Ma) Figure 7 Click here to download Figure Fig_7.pdf 82ºE 88ºE Apatite-He AFT drainage divide Zircon-He ZFT 18 19 18 Biotite Ar-Ar 19 17 Mt. Kailas 20 17 Tibetan Plateau 7 17 14

30ºN 16 Ga ngd s Yarlung River ese Mountain 17 30ºN 17 16 21 14

Lazi study area

< 15 Mt. Everest

15-20 28ºN 20-23 23-27

Age (Ma) 27-38 > 38 400 km 88ºE error bars are 1σ 110

100

70 A-He AFT 60 Ar-Ar

Age (Ma) Z-He 50 ZFT 40

0 0 50 100 150 200 250 300 350 Distance North from Yarlung Suture Ophiolite (km) Figure 8Magmatism Sedimentation Deformation MetamorphismClick here to downloadThermochronology Figure Fig_8.pdf 0 Gangdese Arc Kailas Fm. Liuqu Fm.

10 E-W Extension Himalayan Himalayan Thrusting Liuqu Fm. 20 STDS GT GCT

Ma Kailas Fm. KF

30 North-HImalayan Leucogranites

40 Eo-Himalayan Tethyan Thrust Belt High-Himalayan Leucogranites

Gangdese Arc Extension Contraction

50 Ar/Ar Bio (n = 6) ZFT (n = 6) Z He (n = 15) AFT (n = 62) A He (n = 14) Z He (n = 5) AFT (n = 11) A He (n = 5)

Normalized Probability Figure 9SW C Regional Cross Section (Fig. 10) ClickC’ here to download Figure Fig_9.pdfNE A 15 YDR YDR YDR BTS YDR 30 MHT YZR MHT 45 Depth (km) 60 GDR km 25 50 Moho 75 INDEPTH multichannel common midpoint migrated seismic reflection profile

B km 50 100 s file ro p n o Lhasa Terrane ti 30ºN C’ c le f e r wide-angle c i segment

Yarlung Suture Zone

map area T P E D N I

Tethyan Himalaya

C 28ºN Greater Himalaya 87ºE 91ºE 20 10 km 0 10 20 10 km 0 10 Figure 10 C STDS 25-23 Ma 23-16 Ma THS LHS GHS LHS JKp Mabja Dome LHS 4 2 GHS JKt oph THS GHS JKp JKt THS OMl JKt KEx Kailas Fault oph reactivated THS Click here to download Figure Fig_10.pdf modern topographyandexposure KEx OMk JPg Mountains Gangdese OMk 1 sediment transport JPg KPlv JPg 3 JPg KPlv JPg 3 3 C’