Late Eocene Crustal Thickening Followed by Early-Late Oligocene Extension Along the India-Asia Suture Zone: Evidence for Cyclicity in the Himalayan Orogen

Late Eocene crustal thickening followed by Early-Late Oligocene extension along the India-Asia suture zone: Evidence for cyclicity in the Himalayan orogen

Ran Zhang1,*, Michael A. Murphy1, Thomas J. Lapen1, Veronica Sanchez1, and Matthew Heizler2 1Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA 2New Mexico Geochronology Research Laboratory, New Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico 87801, USA

ABSTRACT INTRODUCTION passive margin of the Indian subcontinent. Our present understanding of the history of the The timing of geologic events along the Intercontinental collision between the Indian Gangdese arc indicates that it did not experi- India-Asia suture in southern Tibet remains subcontinent and central Asia resulted in wide- ence collision-related deformation until at least poorly understood because minimal denu- spread Cenozoic crustal thickening and surface 30 Ma (Yin et al., 1994; Harrison et al., 2000), dation prevents widespread exposure of uplift that extends nearly 2000 km from north- some 25 Myr after the most widely accepted structurally deep rocks. In this study, we ern India to Kazakhstan, a region encompass- initiation estimates of intercontinental collision present geologic maps of two structurally ing over 7,000,000 km2. This continent-scale (Zhu et al., 2005). To the north of the Indus-Yalu deep domes, cored by mylonitic ortho- event played an important role in the evolution suture zone (IYS) (also referred to as the Indus- gneisses, across the India-Asia suture zone of Asian river systems (Brookfi eld, 1998) and Tsangpo suture and Yarlung-Tsangpo suture) in southwestern Tibet. New U-Pb zircon global climate change (Ruddiman and Kutz- crustal shortening and related basin develop- ages and rock textures indicate that core bach, 1989; Molnar et al., 1993; Quade et al., ment was ongoing since 40 Ma in northeastern ortho gneisses are originally Gangdese arc 1995), and served as the testing ground for a Tibet in the Qilian shan and Fenghuo shan- rocks that experienced Late Eocene pro- wide variety of geodynamic models of orogenic Nangqian thrust belt (Zhang and Zheng, 1994; grade metamorphism, probably during processes (Houseman and England, 1996; Roy- Yin and Harrison, 2000; Horton et al., 2004) and crustal thickening. Crosscutting leucogran- den, 1996; England and Molnar, 1997; Flesch et since 28 Ma in the Eastern Kunlun ranges (Yin ite sills underwent northwest-southeast al., 2001; Beaumont et al., 2004; Bendick and et al., 2008). To the south of the suture, U-Pb extension related to slip along a brittle- Flesch, 2007). Because of the potential feed- ages of zircons from intrusive rocks that cross- ductile shear zone here designated the Ayi back between collision and other physical phe- cut folded TSS strata indicate that signifi cant Shan detachment. The timing of shear along nomena, it is critical to constrain the timing of crustal shortening within the Tethyan fold-thrust detachment is bracketed by zircon U-Pb deformational events. belt occurred prior to the mid-Eocene (Aikman ages of 26–32 Ma for these pre- to syn(?)- The initial collision between India and et al., 2008). These observations imply that the extensional leucogranites, and by a 40Ar/39Ar Asia is estimated to have occurred during the Gangdese arc was capable of resisting deforma- muscovite age of 18.10 ± 0.05 Ma for a rhy- Paleocene–Early Eocene (Besse et al., 1984; tion or transmitting collisional related stresses olitic dike. This rhyolite dike crosscuts a Gaetani and Garzanti; 1991; Rowley, 1996; Zhu while crustal shortening to its north and south widespread siliciclastic unit that was depos- et al., 2005), although some suggest it occurred was ongoing. This is at odds with Middle to ited across the detachment, which we cor- earlier (latest Cretaceous–Paleocene) (Yin and Late Eocene ages of plutonic rocks of the Gang- relate to the Kailas Formation. The Great Harrison, 2000; Ding et al., 2005) and even oth- dese arc (Honegger et al., 1982; Xu et al., 1985; Counter thrust defi nes the surface trace of ers have argued for a much later time (ca. 34 Ma) Harrison et al., 2000; Wen et al., 2008), which the India-Asia suture zone; it cuts the Kailas (Aitchison et al., 2007). The suture between suggest that the arc was hot and therefore weak Formation, and is in turn cut by the Kara- India and Asia juxtaposes the Cretaceous– during the early stages of the collision. Observa- koram fault. A new 40Ar/39Ar muscovite age Tertiary Gangdese arc, which formed along the tions that cast further doubt on the inference that of 10.17 ± 0.04 Ma for the Karakoram fault southern margin of Asia due to northward sub- the Gangdese arc escaped early collision-related footwall is consistent with published ther- duction of Tethyan oceanic lithosphere, against deformation are the increased sedimentation mochronologic data that indicate Late Mio- the Tethyan sedimentary sequence (TSS) which rates and development of a collisional foredeep cene transtension in southwestern Tibet. represents material deposited along the former on the adjacent passive margin of the Indian

*Corresponding author, present address: Shell Exploration and Production Company, 200 N. Dairy Ashford, Houston, Texas 77079, USA; [email protected].

Geosphere; October 2011; v. 7; no. 5; p. 1249–1268; doi: 10.1130/GES00643.1; 10 ﬁ gures; 1 supplemental table ﬁ le.

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Zhang et al.

subcontinent at this time (Rowley, 1996; Ding (present-day thickness). Immediately below detachment and also by the foliation within its et al., 2005; Zhu et al., 2005). These features the detachment, the lower plate locally con- lower plate in northern and southern domes. The imply the presence of a large crustal load to its tains 10–20 m of chloritic, quartzofeldspathic corrugations trend toward the southeast (parallel north along the southern margin of Asia. Eng- breccia. Exposures of the chloritic breccia to the Ayi Shan) (Fig. 2) and are relatively sym- land and Searle (1986) and Kapp et al. (2007) are common on the eastern side of the south- metric (Fig. 3). The age of these corrugations is show that a retroarc thrust belt spanning southern Ayi Shan and rare on the western side. not clear. If the corrugations are syn-kinematic ern Tibet developed north of the Gangdese arc Beneath the chloritic breccias, where pres- with shearing along the Ayi Shan detachment, between 105 and 53 Ma. Their studies indicate ent, the lower plate metamorphic rocks can they likely formed in a constrictional strain that the thrust belt likely roots into the Gangdese be separated into three lithologically distinct fi eld. This is consistent with the local presence arc and therefore predict that it was signifi cantly units. Immediately below the chloritic brec- of L-tectonites in the southern Ayi Shan dome. thickened immediately prior to the initial colli- cia is a sequence of mylonitic biotite schist Some amount of folding must have occurred sion. This raises the possibility that the arc did that reaches a thickness of 600 m. Primary after slip along the Ayi Shan detachment since not escape deformation, but rather transmitted and secondary foliations (S, C, and C′ folia- it is folded (Fig. 3). the stress elsewhere due to its thickened state. tions) within the schist are defi ned by biotite Variably deformed leucogranite bodies make To better understand the geologic history of and recrystallized quartz, whereas the min- up ~5%–10% of the footwall of the Ayi Shan the Gangdese arc and the IYS, we have under- eral stretching lineation is defi ned by smeared detachment (Figs. 4E and 4F). Sills are more taken a fi eld and geochronologic study of well- quartz grains and aligned clots of muscovite. pervasive than dikes. Sills and dikes range exposed orthogneisses exhumed from deep Locally, discrete top-to-southeast shear zones between tens of centimeters up to 3 m thick. structural levels along the IYS in southwestern exist within the top 100 m of the mylonitic Generally, they display straight contacts with Tibet. Our results indicate that the protolith of schist. In the southern dome, high-angle nor- the country rock. Leucogranite dikes typically these orthogneisses are Gangdese arc rocks that mal faults cutting the upper plate locally sole cut the mylonitic foliation and leucogranite experienced Late Eocene prograde metamor- into the Ayi Shan detachment, implying they sills at deep structural levels but are deformed phism, which we attribute to burial via crustal are kinematically linked with the detachment at higher structural levels and transposed paral- thickening. This event is followed by the devel- (Fig. 4C). Below the schist is an ~400-m-thick lel to the mylonitic foliation. We interpret this opment of a brittle-ductile extensional shear sequence of mylonitic orthogneiss (Fig. 4D). relationship to indicate that emplacement of zone involving Gangdese arc rocks. The foliation within the orthogneiss is defi ned leucogranite bodies broadly occurred during by alternating hornblende + biotite rich lay- shearing within the footwall of the Ayi Shan GEOLOGY OF THE AYI SHAN ers and feldspar + quartz rich layers, whereas detachment. the mineral stretching lineation is defi ned by The trace of the IYS trends subparallel to the aligned biotite clots as well as quartz and Great Counter Thrust the crest of the Ayi Shan (shan means moun- feldspar grains. The structurally deepest rocks tain range in Mandarin) in southwestern Tibet observed in the northern dome are quartzofeld- The southwest-dipping Great Counter thrust (Fig. 1). Cretaceous–Tertiary granites that rep- spathic migmatitic gneiss (Fig. 2), which is at (GCT) defi nes the surface trace of the IYS. It resent the Gangdese arc are exposed along the least 500 m thick. juxtaposes the TSS in its hanging wall against eastern fl ank of the Ayi Shan whereas rocks of The characteristic mineral assemblage of the Cretaceous–Tertiary granite and Tertiary silici- the TSS crop out along the western fl ank. Our schist exposed in the footwall of the Ayi Shan clastic rocks in its footwall (Figs. 1 and 2). geologic mapping (Fig. 1) shows that the geo- detachment is biotite + plagioclase + musco- The TSS in the Ayi Shan region ranges in age logic framework of the Ayi Shan consists of vite + rutile + garnet, which corresponds to from Ordovician to Cretaceous and consists of three primary structural features (Figs. 2 and 3). amphibolite facies metamorphic conditions. a wide variety of sedimentary and low-grade They are, from oldest to youngest, the (1) Ayi This mineral assemblage is present through- metasedimentary rocks (Cheng and Xu, 1987). Shan detachment, (2) the Great Counter thrust, out the footwall from the highest to the low- The GCT strikes northwest-southeast, subparal- and (3) the Karakoram fault system. est exposed level. Garnets do not preserve an lel to the Ayi Shan. Shear sense indicators show internal fabric and therefore no evidence for that it accommodates top-to-northeast motion. Ayi Shan Detachment rotation during growth. The abundant ductilely Exposed in the immediate hanging wall of the deformed feldspar porphyroclasts suggest that GCT are fault-bounded lenses of highly frac- Two northwest-southeast elongated, doubly deformation was occurring at temperatures of tured metabasalt, metagabbro, and peridotite, plunging structural domes exist in the south- 400–500 °C (Tullis and Yund, 1991), which which locally contain lenses of podiform chro- ern and northern portions of the Ayi Shan and likely corresponds to mid-crustal depths. Feld- mite that are elongated parallel to the trace of are mantled by a brittle-ductile shear zone, spar porphyroclasts range in size from 1 to 3 cm the GCT (Fig. 2B). which we refer to as the Ayi Shan detachment in the long dimension. The orientation of the The footwall of the GCT typically consists (Figs. 1–3). The Ayi Shan detachment jux- mineral stretching lineation defi ned by biotite of pebble, cobble, and boulder conglomerate. taposes porphyritic granite in its upper plate clots, quartz, and feldspar in the footwall rocks This rock sequence is correlated to the Kailas (hanging wall) against metamorphic rocks in indicates the mean slip direction along the Ayi Formation based on its composition and struc- its lower plate (footwall). Although the shear Shan detachment is S60°E. tural position (Cheng and Xu, 1987; Liu, 1988; zones in the southern dome and northern dome The mylonitic foliation in the footwall rocks Murphy et al., 2000; Murphy et al., 2009). The are possibly different structures, we refer to is folded at all scales. The most abundant folds Kailas Formation in the Ayi Shan lies uncon- them collectively as the Ayi Shan detachment are those with axes oriented subparallel to the formably on Cretaceous–Tertiary granite and based on their structural similarities (Figs. 4A stretching lineation, which we refer to as cor- the lower plate of the Ayi Shan detachment, and 4B). Valleys crossing the Ayi Shan expose rugations. First- and second-order corrugations indicating that it postdates movement along the ~1500 m of lower plate (footwall) rocks are defi ned by the surface trace of the Ayi Shan Ayi Shan detachment (Fig. 2A).

1250 Geosphere, October 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Crustal thickening and extension along the India-Asia suture zone

Karakoram Fault System consisting of right-lateral faults, normal faults, Kapp et al., 2003). The Karakoram fault cuts, and and right-lateral ductile shear zones that strike therefore must be younger than, the GCT in the The eastern margin of the Ayi Shan coincides northwest. Fault-slip data show that the Kara- Ayi Shan. Thermochronologic studies along the with the trace of the Karakoram fault system koram fault accommodates right-lateral displace- Karakoram fault in the vicinity of the Ayi Shan (Figs. 1–3). Deformation along the Karakoram ment with a minor normal dip-slip component show a rapid cooling event in its footwall (south fault occurs within a narrow zone (2–20 km wide) (Ratschbacher et al., 1994; Murphy et al., 2000; side) at ca. 10 Ma, which is interpreted to have

E ' 01020Q - undifferentiated Quaternary 0

80°E mgnmgn + mschmsch ° km surficial deposits 9

779°30'E79°30' contour interval 400 m Nz and Ng - Neogene zada basin and Gar basin sedimentary rocks KK-Tgr-Tgr 47004700 ZhaxigangZhaxigang J-K Tk - Early Miocene Kailas Formation Northern Dome N J-K - undifferentiated Jurassic and 32°30'N Cretaceous strata Q K-Tgr - Cretaceous -Teritary plutonic 51005100 K-TgrK-Tgr rocks (Gangdese arc rocks) mgnmgn + mgn + msch - mylonitic orthogneiss, mmschsch biotite schist and migmatite KK-Tgr-Tgr Tss - Paleozoic-Mesozoic Tethyan 55005 50 sedimentary sequence 0 430043 51005100 00 43004 3 0 0 Tss Figure 2A thrust fault K-TgrK-Tgr 47004700 Q 55005 5 59005 0 low-angle normal fault 9 0 0 G Nz 0 G r e r a e KFS active faults t a

t C (strike-slip and normal) Q oC A uunto y nu i J-KJ-K t n e t dome axes r e Gar Airport r 51005 T 550055 1 00 0 hhrT 0 hr SSha ur usstt h s t a ( I n nndi(I 430043 nd 00 dia i-a 32°N AAs- As KK-Tgr-Tgr iiasa i aS 76° Nz S uS mgnmgn + mmschsch 332°2°N tuu FigureFigure 2B2B turree re ZoZ 79°30'E Zo KFS onnee KK-Tgr-Tgr ne S T D GCT 80° 0 90 39003 Q 47004 Shiquanhe Southern Dome 7 32° 0 84° 0

In 32° MCT d 0 76° ia 0 - As 1 ia SSu 51005 ut ur e Zoon ne Q

43004 MF 3 TssTss MBT 80° T 0 0 47004700 28° 0 5500550 NNgg 0 5100510

79°30'0"E 84°

80°8

Figure 1. Geologic map of the Ayi Shan range showing the ﬁ rst-order geologic features exposed in the range and location of detailed mapping. Abbreviations: GCT—Great Counter thrust; KFS—Karakoram fault system; MBT—Main Bound- ary thrust; MCT—Main Central thrust; MFT—Main Frontal thrust; STD—South Tibetan detachment. Map is compiled from Cheng and Xu (1987), Murphy et al. (2000), Sanchez et al. (2010), and mapping presented in this paper.

Geosphere, October 2011 1251

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Zhang et al.

0 0 1 6

5 5 500 m

4 e

t 0

s 5

′ 5 000

0 6 0

y 8

g A 5 s

T 0

t 0

- 0

l 0 6 2 60

u 0

28 0 K-Tg K K-Tg 0

9 0

f 0

50 1

a 300 a 2

0 6 4 6

Q f 1

0 60

0 0

1 2 0

3 4 0 0

9 60

0 0

m 0

2 3 1 0 6 60

9 60 0

a 0 2 3

0 1 60 r 1

35 60 0

0 0

o 0 2

3 6

0 6 4400 0 k 0 0 20 1 6

60 2 a 6 0

6200

r 0 0 32

6 0

4500 a

0 0 0 0

0 3 0 0

4 1 6 0 Karakoram fault system K 6

1 6 6000

0 6

6 0 0

0 1 K 0

0 1 00 60 0 61

0 0 1 6 6 0 2

1 Rock sample location

6 0 6

7 l 6

50 K

0 0

0 0 0

0 8

1 0

6 5

6 0 0

5 0

0 9 7

1 5

6100 5

AY-8 0 6 0

0 Mylonitic orthogneiss, contains K-feldspar augens Chloritic breccia

Cretaceous - Tertiary granite contains large K-feldspar phenocrysts

Mylonitic garnet biotite schist

0 0 quartzofeldspathic migmatite Quaternary colluvial deposits

Quaternary glacial moraine deposits 0 Undifferentiated Tethyan sedimentary sequence, gray and green phyllite, slate, quartzite Quaternary alluvium

Undifferentiated Cretaceous strata Quaternary alluvial fan deposits Tertiary conglomerate (correlated to the Kailas Formation) 0

6 6000 60

600 600 6100

6000

Qaf 0

0 6100

0 0 6

1 0 0

500 60 0

0 9 4 Qaf 0

0 50

K 6

0 8

0 5500 5 Qc

31 6

Qal 0

0 mig Qm

Qaf 0

K-Tg 0 TSS Tcg msch

magn

0 0

6 8 0

42 50

9 5 0

00 5 0

9 53 5100

1 31 Qc

Qal

40 46

54 4800 T

0 - 5600 0

55 K-Tg K 5 n

16 25

g 5

a 35 magn m magn

5800 5700 g 14 c 22 42 5100 10

10 3

S 54 T Tcg Tcg

0 S 7 00

2 44 9 TSS T TSS 22 4

22 550

21 40 AY-10 6000 9 32

c 00 9 0 4

0 s 55

AY-11 5900 m msch msch c g g g 41 i Qc Q a 28 c mig m mig Tcg T m magn magn 58 g Qaf 5600 13 T 71 -

00 51 7 5 K-Tg K K-Tg

n 00 0 49 g 21 10

Qaf 550 a

00 41 AY-12 magn m TSS g 29

i 54 5300 14 mig m AY-9 AY-8 18 ). (A) Geologic map of the northern Ayi Shan. Ayi ). (A) Geologic map of the northern l

5 0 S Qc 42 a

35 500 S

Tcg 000 Q Qal Qal

7 18 5 TSS T TSS 22 23 55

39 4700 10 Qaf t

n Qm n

e a 7 h 5000

m 5100 46 Qc S h

00 i

c 4

5 corrugationAntiformal fold) parallel (extension 5000 y a

t S

small-scale anticline A g 5400 e TSS T TSS i

200 5600

5 Ayi Shan detachment d detachment 0 9

Qal 00 m mig mig 48 5700 29 37

45 4900 g 48 47

0 T

0 Anticline - 8

4 49 g 88 r K-Tg K 5600 cg c 10

e TcgT 0 A t 9

n 490 page continued on following u 26 5500 t o

s 5300 C u 18

r t Arrow oriented parallel to the trace of the fold of the fold the trace to parallel oriented Arrow h a t 84 small-scale syncline

e Syncline

r 0 0

0 0 6 5 4 msch Great Counter G Great Counter thrust thrust

22 4900 5 Folds Dashed where the plunge direction. axis indicates concealed. where and dotted located approximately 58

Figure 2 ( Figure 5300

35 20 0

5300 S

17 0

S 0

0 7

5300 0 4

TSS T 5500 4 25 Qc

4900 5100 N dashed 14

5300 15

30 mylonitic foliation bedding foliation mylonitic

50 11 g 32

40 (low-angle normal fault) (low-angle T

72 50 K-Tg K

opposing arrows indicate indicate opposing arrows 59 Qc bar and ball on hanging wall m barbs on hanging wall Geologic symbols Q Qm Qm

Geologic map of the northern Shan Ayi 5600 30 S Strike and dip of foliation and bedding and dip of foliation Strike 36 S 28 T TSS TSS 20 40 Contour interval 100 m 0 6 12 km Detachment fault Detachment box on hanging wall box Normal fault 37 Strike-slip fault Strike-slip sense of horizontal motion along the fault. sense of horizontal where approximately located or inferred, or inferred, located approximately where indicate Arrows concealed. where dotted dip of fault and orientation lineations. Lithologic and fault contacts transposed bedding transposed A fault Thrust

1252 Geosphere, October 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Crustal thickening and extension along the India-Asia suture zone

B Geologic map of the Karakoram fault system southern Ayi Shan 80°05′E Kg2 Kg1 bs Xing Gar Lake Kd Qoal A′ 37 Kg1 26 32 Qal Contour interval 100 m N Kg2 21 52 Qaf

4800 0 2 4 km Kg2 Qt 60 Namru AY-14 36

AY-5 Kg1 15 32 8 30 34 Qt magn 32 5000 7 Kg1 12 26 14 8 28 11 11 AY-7 9 29 Kg2 4600 20 AY-6 5 4800 29 5 71 41 67 40 11 47 11 20 12

5600 Ayi Shan detachment msch 10 31

5200 64 6066 AY-4

19 31 5 5 41 10 50 5400 35 Qal Alluvial and older alluvial deposits Kg1 42 magn 10 Qoal cobble-pebble gravel, sand, and silt 5000AY-3 Tcg 5200 Qt River Terrace deposits cobble-pebble AY-1 gravel, sand, silt, and clay 45 AY-15 AY-2 Colluvial deposits boulder-cobble- 8 msch Qc 25 5200 pebble-gravel, sand, and silt 3 11 5000 Alluvial Fan Deposits cobble-pebble 18 Qaf gravel, sand, and silt 42 Qc 32 Tcg Conglomerate unit boulder-cobble- 49 55 pebble gravel and sand um Tcg 32 um Undifferentiated Tethyan sedimentary Great5200 Counter TSS Tcg 5400 sequence, locally Mesozoic interbedded 52 thrust (IYS) limestone, siltstone, and shale 38 42 75 35 granite ~15% modal biotite, locally 39 Kg1 TSS contains orthoclase phenocrysts um Qal Kg2 granite ~5% modal biotite TSS 5000 Kd diorite

bs Interlayered biotite schist and TSS quartzofeldspathic gneiss

msch mylonitic biotite schist (lower plate of Ayi Shan detachment) A magn mylonitic quartzofeldspathic augen gneiss um Serpentinized pyroxenite, gabbro, and basalt Geologic symbols - same as Figure 2A

Figure 2 (continued). (B) Geologic map of the southern Ayi Shan.

Geosphere, October 2011 1253

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Zhang et al. 6 5 4 3 2 1 0 -1 -2 km ′ A K-Tg km 3 5 6 4 2 ′ B

y Qal E e l ° l n a i v s

r a N84 a b

Kg1 gar valley gar valley g basin basin Ayi Shan detachment Ayi Tcg Qal

magn Gar Valley Gar Valley basin Kg1 Qaf Qc Kg2 Karakoram fault system fault Karakoram Kg1 Karakoram fault system fault Karakoram chloritic breccia

Σ mig chloritic breccia across the southern Ayi Shan. Displacement along the Ayi Shan detachment is predominately Shan detachment is predominately Ayi Shan. Displacement along the Ayi the southern across ′ N46°E magn Kg1 Northern Dome Southern Dome Ayi Shan detachment Ayi Ayi Shan detachment Ayi msch

N45°E Kg chloritic breccia IYS Tcg

Great Counter thrust magn Kg1

um msch Qal

Qal K-Tg Great Gounter thrust Gounter Great Σ Qal TSS um ? across the northern Ayi Shan. (B) Cross-section B–B Shan. (B) Cross-section Ayi the northern across ′ IYS B 6 5 3 4 2 km Tcg TSS A B A 6 5 4 3 2 1 0 -1 -2 km out-of-plane (top-to-the-southeast). See Figure 2 for abbreviations and geologic symbols. abbreviations 2 for out-of-plane (top-to-the-southeast). See Figure Figure 3. (A) Cross-section A–A 3. (A) Cross-section Figure

1254 Geosphere, October 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Crustal thickening and extension along the India-Asia suture zone

resulted from slip along the Karakoram fault at U-PB GEOCHRONOLOGY Seven orthogneiss samples were analyzed from that time (Arnaud, 1992). However, Valli et al. the southern dome (AY-1 –AY-7), three ortho- (2007) suggest it was continuously active since Uranium-lead zircon geochronology of gneiss samples (AY-8–AY-10) were analyzed the Early Miocene, based on U-Pb zircon ages granitic and gneissic rocks collected from from the northern dome, and two mylonitized and petrofabric analyses of rocks exposed on the southern and northern gneiss domes was leucogranite sills (AY-11 and AY-12) were ana- its south side (footwall). The Karakoram fault is undertaken to (1) test the possible genetic lyzed from the northern dome (Figs. 1, 5, and presently active as it offsets Quaternary surﬁ cial link between the rocks making up the core of 6). Sample locations are shown on Figures 2A deposits (Armijo et al., 1989; Chevelier et al., the domes with Cretaceous–Tertiary granites and 2B. We obtained cathodoluminescence 2005; Brown et al., 2002; Murphy and Burgess, exposed to the east within the Gangdese batho- images of most zircons after performing U-Pb 2006; Sanchez et al., 2010). lith, and (2) assess the age of metamorphism. analyses to evaluate zoning patterns and internal

A East Southern Dome

Kgr - upper plate Ayi Shan detachment Ayi Shan detachment

msch - lower plate msch - lower plate

magn

Northern Dome B West

Ayi Shan detachment msch - lower plate

Kgr - upper plate msch sills and dikes msch - lower plate msch msch + leucogranite

msch + leucogranite sills and dikes Qt

Figure 4 (continued on following page). (A) Photo of the Ayi Shan detachment on the east side of the southern Ayi Shan. View to the south. (B) Photo of the Ayi Shan detachment on the west side of the northern Ayi Shan. View to the north. Cliff face is ~300–400 m.

Geosphere, October 2011 1255

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Zhang et al.

C Southern Dome Southeast ESE D

magn

msch

E K-Tg F Southeast Ayi Shan detachment Southeast

msch

msch - lower plate dike cut by extensional shear zone

dikes

mylonitized sills hammer Northern Dome Northern Dome

Figure 4 (continued). (C) High-angle normal faults rooting into a low-angle normal fault. Cliff face is ~200 m. (D) Photo of ductilely deformed orthogneiss containing top-to-the-southeast shear sense indicators (southern Ayi Shan). (E) Leucogranite bodies in the lower plate of the Ayi Shan detachment. Cliff face is ~300 m. (F) Photo of leucogranite dikes (northern Ayi Shan) cut by extensional ductile shear bands. Hammer is 40 cm. Abbreviations as in Figure 2.

structure with respect to the location of the U-Pb samples with a standard mineral separation pro- delay to purge the previous sample and prepare analyses. U-Pb isotopic data is available in the cess. Handpicked zircon grains were mounted for the next analysis. Common Pb corrections Supplemental Table File1. in epoxy and then polished to provide fl at expo- are accomplished by using the measured 204Pb sure of the interiors of the grains. The analyses and assuming an initial Pb composition from Methods involved ablation of zircon with a New Wave/ Stacey and Kramers (1975). Measurement of Lambda Physik DUV193 excimer laser (oper- 204Pb is unaffected by the presence of 204Hg U-Pb geochronology of zircons was con- ating at a wavelength of 193 nm) using a spot because backgrounds are measured on peaks ducted by laser ablation–multicollector– diameter of 15–35 µm. The ablated mate- (thereby subtracting any background 204Hg and inductively coupled plasma–mass spectrometry rial is carried with helium gas into the plasma 204Pb), and because very little Hg was present (LA-MC-ICP-MS) at the Arizona LaserChron source of a GV Instruments IsoProbe, which is in the argon gas during this analytical session. Center. Zircons were extracted from the rock equipped with a fl ight tube of suffi cient width In-run analysis of fragments of a large zircon that U, Th, and Pb isotopes are measured simul- crystal (generally every fi fth measurement) taneously. All measurements are made in static with a known age of 564 ± 4 Ma (2σ error) is mode, using Faraday detectors for 238U and used to correct for instrumental fractionation. 1Supplemental Table File. PDF fi le of 14 supple- 232Th, an ion-counting channel for 204Pb, and The ages reported on each sample include both mental tables. If you are viewing the PDF of this faraday collectors for 208–206Pb. Each analysis random and systematic errors associated with paper or reading it offl ine, please visit http://dx.doi .org/10.1130/GES00643.S1 or the full-text article consists of one 12-s integration on peaks with uncertainties in common Pb composition, age on www.gsapubs.org to view the Supplemental Ta- the laser off (for backgrounds), 12 one-second of the standard, and decay constant, and all fi nal ble File. integrations with the laser fi ring, and a 30 s uncertainties are reported at the 2σ level.

1256 Geosphere, October 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Crustal thickening and extension along the India-Asia suture zone

52 0.008 Sample AY - 1 Sample AY - 1 48 48 0.007 44

44 40 238 0.006 40 Pb/ U Pb/

238 36

206 Pb/ U Age (Ma) Age U Pb/ 36 0.005 32

206 28 32 MMeaneaean = 45.534545.5353 ± 0.530.5353 [1.5][1.5] 95%9595% conf.cononf. 0.004 3 ofof 3434 rej.rejrej. MSWDMSWD = 3.03.0 0.00 0.04 0.08 0.12 0.16 28 207Pb/ 235 U

70 0.011 70 Sample AY - 2 Sample AY - 2

66 66

0.010

62 8

238

58 U Pb/ 0.009 58

6 Pb/ U Age (Ma) Age U Pb/

206 54 54 Mean = 61.78 ± 0.92 [1.8] 0.008 1 of 22 rej. MSWD = 3.7 0.00 0.04 0.08 0.12 0.16 0.20 50 207Pb/ 235 U

95 0.016 100 Sample AY - 3 Sample AY - 3

0.014 90 85

38 0.012 75 2 70 Pb/ U Pb/ 0.010

238 06 60 65 2 Pb/ U age (Ma) age U Pb/ 0.008

206 55 Mean = 73.9 ± 1.4 [4.5] 1 of 19 rej. MSWD = 1.20 0.006 0.0 0.1 0.2 0.3 0.4 0.5 45 207Pb/ 235 U

Figure 5 (continued on following pages). Weighted average 206Pb/238U age and U-Pb concordia diagrams of zircons from samples AY-1– AY-8 and AY-10–AY-12. All uncertainties are at the 2σ level. Zircon U-Pb data from samples AY-4, AY-5, AY-7, AY-11, and AY-12 defi ne discordia lines that intersect concordia. The upper intersection is interpreted to refl ect the age of the protolith, and the lower intercept age is interpreted as the age of Pb loss or zircon overgrowth. The inset plots show the U/Th ratio versus age and indicate that the youngest ages correspond to elevated U/ Th ratios. The weighted mean 206Pb/238U ages of these samples refl ect only the data that plot near the lower intercept and have U/Th ratios >20. The uncertainties in the weighted averages include systematic errors associated with the age of standard zircons, fractionation corrections, and decay constant. The number in brackets refers to the weighted standard deviation of the data about the mean (Young, 1962). All plots were constructed with IsoPlot v. 3.5 (Ludwig, 2003).

Geosphere, October 2011 1257

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Zhang et al.

U-Pb Data interpret this age as magmatic age due to the ratios range from 1.1 to 8.9 (Table 3 in the Sup- low U/Th ratios for all the zircon grains (Vavra plemental Table File [see footnote 1]). CL imag- Southern Dome et al., 1999). ing of AY-3 zircon grains does not show typi- Samples AY-1 through AY-7 are ortho- Zircon grains from AY-2 are euhedral to cal oscillatory zonation and distinct rim-core gneiss samples collected from the footwall of subhedral. The individual spot 206Pb/238U ages domains as expected for igneous zircons. The Ayi Shan detachment in the southern Ayi Shan range from 57.9 ± 0.6 Ma to 67.0 ± 0.8 Ma. weighted mean age is 73.9 ± 1.4 Ma (n = 18). range. Cathodoluminescence (CL) imaging U/Th ratios range from 0.7 to 4.2 (Table 2 in Zircon grains from AY-4 and AY-5 are euhe- of zircon grains from AY-1 show mottled and the Supplemental Table File [see footnote 1]). dral and CL imaging of zircon grains shows irregular CL patterns (Fig. 7). A few grains are CL imaging of zircon grains shows oscillatory igneous cores (low U/Th) and metamorphic rounded. Omitting three outlier spots, individ- zonation consistent with igneous crystallization rims (high U/Th) (Fig. 7C) (Vavra et al., 1999; ual 206Pb/238U ages range from 43.9 ± 0.9 Ma to (Fig. 7). The weighted mean of all the ages is Rubatto, 2002). The measured 206Pb/238U ages 47.4 ± 0.9 Ma. U/Th ratios range from 0.4 to 61.78 ± 0.92 Ma (n = 21) (Fig. 5). of individual spots range from 33.7 ± 0.8 Ma 7.5 (Table 1 in the Supplemental Table File [see Most of the zircon grains extracted from AY-3 to 484.8 ± 4.7 Ma and 37.3 ± 0.4 Ma to 540.0 footnote 1], Fig. 5). The weighted mean of all are subhedral. Individual spot 206Pb/238U ages ± 9.0 Ma for samples AY-4 and AY-5, respec- 206Pb/238U ages is 45.53 ± 0.53 Ma (n = 31). We range from 62.5 ± 10.1 Ma to 81.9 ± 7.7 Ma. U/Th tively; U/Th ratios range from 1.2 to 458.4

0.10 44 Sample AY - 4 1000 100 Sample AY - 4 10 U/Th 42 0.08 1 0 200 400 206 238 40 Pb/ U age (Ma) 0.06 38

238

238 0.04 36 Pb/ U Pb/ Pb/ U Age (Ma) Age U Pb/

34 206 0.02 Intercepts at

206 42 ± 31 & 504 ± 46 Ma MSWD = 10.0 32 Mean = 38.0 ± 1.3 [3.2] 0.00 0 of 12 rej. MSWD = 7.8 0.0 0.2 0.4 0.6 0.8 30 207Pb/ 235 U

0.12 700 43 Sample AY - 5 1000 Sample AY - 5 100 0.10 10

41 U/Th 1 0 500 0.08 0 200 400 600 39 206Pb/238U Age (Ma)

238 0.06 37 300

238 Pb/ U Pb/ 0.04 35 6 Intercepts at

20 Pb/ U Age (Ma) Age U Pb/ 43 ± 47 & 558 ± 27 Ma 0.02 100 MSWD = 1.9 206 33 Mean = 37.43 ± 0.87 [0.6] 0 of 7 rej. MSWD = 1.5 0.00 31 0.0 0.2 0.4 0.6 0.8 1.0 207Pb/ 235 U

54 56 Sample AY - 6 0.0086 Sample AY - 6 52 0.0082 52

50 0.0078 48 48 0.0074

238 c 46 0.0070 44 Pb/ U Pb/ 44 0.0066

238

206 0.0062 40 42 Pb/ U age (Ma) age U Pb/ 0.0058 206 40 Mean = 48.47 ± 0.58 [1.41] 0.00 0.02 0.04 0.06 0.08 0.10 1 of 31 rej. MSWD = 2.9 38 207Pb/ 235 U Figure 5 (continued).

1258 Geosphere, October 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Crustal thickening and extension along the India-Asia suture zone

and 0.9 to 252.8, respectively (Tables 4 and 5 AY-5, and we interpret these to refl ect the age weighted average age is 48.47 ± 0.58 (n = 30). in the Supplemental Table File [see footnote of zircon overgrowth. Based on the U/Th ratio and zoning patterns in 1]). The U-Pb isotope data of each sample The zircon grains from AY-6 are domi- CL, we interpret the weighted average age to plotted on concordia diagrams (Fig. 5) defi ne nantly euhedral to subhedral with a few that are refl ect the age of the igneous protolith. chords with upper and lower intercept ages. rounded. CL imaging shows that most zircons Zircons from AY-7 are mostly subhedral These are 504 ± 46 and 42 ± 31 Ma for sample display zoning patterns that are consistent with with a few of them being rounded. CL imaging AY-4 and 558 ± 27 and 43 ± 47 for sample igneous derivation (Fig. 7D). The measured shows that some of the zircon grains have oscil- AY-5, respectively. The weighted average 206Pb/238U ages of individual spots range from latory zonations with distinct rim-core domains. age of laser spots that yielded U/Th ratios 43.8 ± 1.5 Ma to 49.9 ± 0.8 Ma and U/Th ratios Individual spot 206Pb/238U ages, range from in excess of 20 is 38 ± 1.3 Ma (n = 12) for range from 0.7 to 2.7 (Table 6 in the Supple- 30.4 ± 2.4 Ma to 455.3 ± 16.6 Ma, and U/Th sample AY-4 and 37.43 ± 0.87 Ma for sample mental Table File [see footnote 1], Fig. 5). The ratios range from 1.6 to 111.8 (Table 7 in the

36 0.10 1000 Sample AY - 7 Sample AY - 7 100

U/Th 10 34 0.08 C 1 450 0 100 200 300 400 500 206Pb/238U age (Ma) 0.06 32

238 0.04 250 30 Pb/ U Pb/

238

206 0.02 Intercepts at Pb/ U Age (Ma) Age U Pb/ 28 32.6 ± 5.0 & 477 ± 48 Ma 50 MSWD = 0.54

206 Mean = 31.55 ± 0.69 [0.2] 1 of 11 rej. MSWD = 3.0 0.00 26 0.0 0.2 0.4 0.6 0.8 207Pb/ 235 U

50 0.0080 50 Sample AY - 8 48 Sample AY - 8 0.0076

46 0.0072 46

44 0.0068 42

42 238 0.0064

238

40 U Pb/ 0.0060 38

206 Pb/ U Age (Ma) Age U Pb/ 38 0.0056

206 34 36 0.0052 Mean = 38.74 ±0.68 [0.8] 34 2 of 23 rejrej. MSWD = 2.5 0.0048 0.00 0.02 0.04 0.06 0.08 0.10 32 207Pb/ 235 U

110 Sample AY - 10 Sample AY - 10 0.016 100 100

0.014 90 90

238 80 80 0.012

238 Pb/ U Pb/ 70 Pb/ U age (Ma) age U Pb/ 70 206 0.010

206 60 60 0.008 50 50 Mean = 87.4 ± 1.7 [20] 1 of 32 rej. MSWD = 4.3 0.006 40 0.0 0.1 0.2 0.3 0.4 207Pb/ 235 U

Figure 5 (continued).

Geosphere, October 2011 1259

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Zhang et al.

0.16 40 Sample AY - 11 100 Sample AY - 11 900 10 38 U/Th 1 0.12 700 0.1 36 0 500 1000 206Pb/238U age (Ma)

34 238 500 0.08 Pb/ U Pb/ 238 32 300

206

Pb/ U Age (Ma) Age U Pb/ 30 0.04 Intercepts at 29 ± 42 & 876 ± 77 Ma

206 MSWD = 11.5 28 Mean = 34.8 ± 1.7 [2.8] 100 0 of 7 rej. MSWD = 2.3 0.00 26 0.00.40.81.21.6 207Pb/ 235 U

39 0.28 Sample AY - 12 Sample AY - 12 37 1400 0.24 Intercepts at 77 ± 87 & 1564 ± 200 Ma 35 0.20 MSWD = 17 1000 33 Mean = 26.9 ± 1.1 [1.7] 0.16 2 of 12 rej. MSWD = 11.7

238 31 0.12 1000

238

Pb/ U Pb/ 600 29 100 0.08 U/Th

206 10 Pb/ U Age (Ma) Age U Pb/ 27 1

206 0.04 200 0 500 1000 1500 206Pb/238U age (Ma) 25 0.00 23 01234 207Pb/ 235 U

Figure 5 (continued).

500 500 A 450 400 A 400 Relative probability 300 350 probability Relative U/Th 200 300 3 2 250 100 U/Th 200 0 0 200 400 600 800 1000 206 238 150 Pb/ U age (Ma) 1 100

0 2030 4050 60 70 8090 100 110 120130 140

206Pb/ 238 U age (Ma) Interpretation

1 Gangdese arc magmatism (orthogneisses)

2 Middle Eocene to Early Oligocene prograde metamorphism (orthogneisses) -> crustal shortening/thickening 3 Early to Late Oligocene anatectic melting (mylonitized leucogranite sills) -> crustal extension/thinning

Figure 6. Plot of 206Pb/238U zircon ages versus U/Th values and their relative probability.

1260 Geosphere, October 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Crustal thickening and extension along the India-Asia suture zone

Supplemental Table File [see footnote 1]). The grains from both samples are dominantly euhe- Individual laser spot 206Pb/238U ages from U-Pb isotope data of each sample spot plotted dral to subhedral with a few that are rounded. AY-10 zircons range from 54.0 ± 4.2 Ma to on concordia diagrams (Fig. 5) defi ne chords CL imaging of AY-8 shows that most zircons 95.0 ± 8.6 Ma (n = 31). CL images of AY-10 with upper and lower intercept ages. These are display zoning patterns that are consistent zircon grains do not show oscillatory zonation 489 ± 28 Ma and 42 ± 18 Ma, respectively. The with igneous derivation (Fig. 7E). The mea- and distinct rim-core domains (Fig. 7F). U/Th weighted average age of laser spots that yielded sured 206Pb/238U ages of individual laser spots ratios range from 0.7 to 2.6 (Table 10 in the U/Th ratios in excess of 20 is 31.55 ± 0.69 Ma range from 36.1 ± 1.9 Ma to 44.8 ± 3.0 Ma and Supplemental Table File [see footnote 1]). The (n = 10) and we interpret these to refl ect the age U/Th ratios range from 0.5 to 3.2 (Table 8 in weighted mean 206Pb/238U age is 87.4 ± 1.7 Ma of zircon overgrowth. the Supplemental Table File [see footnote 1], (n = 31) (Fig. 5). Fig. 5). The weighted mean 206Pb/238U age Northern Dome is 38.74 ± 0.68 (n = 21). Based on the U/Th Mylonitic Leocogranite Sills AY-8 and AY-10 are orthogneiss samples ratio and zoning patterns in CL, we interpret Samples AY-11 and AY-12 are samples of taken from the footwall of the Ayi Shan detach- the weighted mean age to refl ect the age of the mylonitized leucogranite from the footwall ment in the northern Ayi Shan. The zircon igneous protolith. of the Ayi Shan detachment in the northern

A AY-1-8 B AY-2-17,-18 C AY-4-12,-21

63.5±1.5 Ma 39.0±0.8 Ma 45.9±0.7 Ma

417.1±27.9 Ma

50 μm μ 100 μm 100 m 65.8±1.5 Ma

DEAY-6-16,-17 AY-8-6,--7 F AY-10-3,-10 48.1±1.1 Ma 86.6±1.9 Ma 77.0±2.5 Ma 39.2±0.4 Ma

90.1±4.7 Ma

46.1±0.8 Ma 87.2±2.7 Ma 100 μm 38.3±0.4 Ma 50 μm 100 μm

AY-13-14,-15 G 34.2±0.5 Ma

Figure 7. Cathodoluminescence (CL) images of some zircons analyzed in this study.

50 μm

649.1±35.9 Ma

Geosphere, October 2011 1261

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Zhang et al.

Ayi Shan. Metamorphic foliation cuts across southern Ayi Shan gneiss dome. 40Ar/39Ar age upper plate granite and the lower plate ortho- the contact between leucogranite and country spectra from muscovite are fl at for the major- gneisses contain distinctive large orthoclase rock schist and gneiss. Individual laser spot ity of gas released and yield an age of 18.10 ± phenocrysts and porphyroclasts, respectively. 206Pb/238U ages for samples AY-11 and AY-12 0.05 Ma (Fig. 8). AY-15 is from a granitoid in range from 32.2 ± 3.0 Ma to 788.8 ± 14.8 Ma the hanging wall of the Ayi Shan detachment Age of Prograde Metamorphism (n = 26) and 25.7 ± 1.4 Ma to 1782 ± 89 Ma (n ~6 km west of the Karakoram fault. 40Ar/39Ar = 19), respectively. U/Th ratios range from 0.7 age spectra from muscovite are fl at over the Orthogneiss samples AY-4, AY-5, and AY-7 to 62.3 and 3.8 to 386, respectively (Table 11 in majority of gas released and yield an age of contain zircons that yield old ages in their cores the Supplemental Table File [see footnote 1]). 10.17 ± 0.04 Ma (Fig. 8). Because both age and younger ages along their rims. Laser abla- The weighted average 206Pb/238U ages of laser spectra are fl at we interpret both to be recording tion analyses defi ne chords on U-Pb Concordia spots that yielded U/Th ratios in excess of 20 rapid cooling of both rock samples. diagrams and suggest that each sample contains are 34.8 ± 1.7 Ma (n = 7) for sample AY-11 zircons that grew in two stages and/or suffered and 26.9 ± 1.1 Ma for sample AY-12 (n = 10) DISCUSSION Pb loss. The older, mostly Paleozoic 206Pb/238U (Fig. 5) and we interpret these to refl ect the ages ages defi ne a chord and have low U/Th ratios. of zircon overgrowth. Protolith of Lower Plate Orthogneisses The younger rims (Fig. 7) are generally Late Eocene–Early Oligocene and have high U/Th 40Ar/39Ar THERMOCHRONOLOGY A key element in the architecture of the IYS ratios, which we interpret to be consistent with in southwestern Tibet is the lower plate of the the growth of young zircon around an older 40Ar/39Ar thermochronology was conducted Ayi Shan detachment. These rocks lie beneath core. The weighted average 206Pb/238U ages on muscovite and biotite from rock samples the surface trace of the suture and therefore pro- of the laser spots that yield U/Th ratios > 20, from the southern Ayi Shan gneiss dome. vide information on the suture zone at depth. which we use to discriminate between mixed Samples were irradiated for 7 h along with the Lower plate samples AY-1, AY-2, and AY-3 spot ages and those primarily from the rim, standard Fish Canyon Tuff sanidine (FC-2) with from the southern dome yield U-Pb zircon are 38 ± 1.3 Ma for AY-4, 37.43 ± 0.87 Ma for an estimated age of 28.02 Ma (Renne et al., ages with low U/Th values of 45.53 ± 0.53 Ma, AY-5, and 31.55 ± 0.69 Ma for AY-7. We inter- 1998) at the U.S. Geological Survey (USGS) 61.78 ± 0.92 Ma, and 76.1 ± 1.4 Ma, respec- pret the age of zircon overgrowths in samples TRIGA Reactor in Denver, Colorado. Age spec- tively. Similarly, lower plate samples AY-8 and AY-4, AY-5, and AY-7 to have occurred during trum analyses were conducted by step-heating AY-10 from the northern dome yield low U/Th prograde metamorphism likely associated with mineral separates within a double vacuum Mo ratios with 206Pb/238U ages of 38.74 ± 0.68 Ma burial and crustal shortening (Fig. 6). resistance furnace at the New Mexico Geo- and 87.4 ± 1.7 Ma, respectively. U-Pb zircon Because structurally deep exposures of Gang- chronology Research Laboratory, New Mexico ages of upper plate granites collected from the dese arc and associated country rocks are rare, Institute of Mining and Technology. Samples southern Ayi Shan range from 47 to 50 Ma and the regional extent of this metamorphic event were targeted that could bracket the timing of have low U/Th ratios indicating that the zircons is uncertain. However, sandstone petrology of geologic events that postdate movement along are recording crystallization ages (Wang et al., Paleocene to Early Eocene clastic rocks in the the Ayi Shan detachment. One sample (AY-14) 2009). These ages, along with regional studies Tethyan Himalaya of southern Tibet (86°42′E) comes from a dike that cuts the Kailas Forma- on the age of Gangdese arc magmatism (Hon- indicate a transition from a continental block tion, and the other sample (AY-15) comes from egger et al., 1982; Xu et al., 1985; Harrison et provenance, interpreted as Indian craton, to the south side of the Karakoram fault in its foot- al., 2000; Wen et al., 2008) lie within the range recycled orogen provenance, interpreted to be a wall (Fig. 2B). of our geochronologic results suggesting that Gangdese arc-trench system (Zhu et al., 2005). AY-14 is from a 25-cm-thick rhyolitic dike the protolith of the Ayi Shan lower plate ortho- This implies that the Gangdese arc was a devel- that cuts across sandstone and conglomerate gneisses is the Gangdese arc. This interpretation oping topographic high that was being eroded in beds in the Kailas Formation adjacent to the is supported by the observation that both the the Eocene. We propose that this uplift refl ects

AY-14 muscovite AY-15 muscovite Int age = 10.17 ± 0.04 Ma Int age = 18.13 ± 0.07 Ma 18.13 ± 0.07 Ma

10.29 ± 0.04 Ma

0 20 40 60 80 100 0 20 40 60 80 100 cumulative % Ar released cumulative % Ar released

Figure 8. Muscovite age spectra from samples AY-14 and AY-15. AY-14 are from a rhyolitic dike that intrudes the Kailas Formation. AY-15 is from granite in the upper plate of the Ayi Shan detachment and footwall (south side) of the Karakoram fault.

1262 Geosphere, October 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Crustal thickening and extension along the India-Asia suture zone

isostatic uplift due to crustal thickening and the Ayi Shan metamorphic core in the north- translated southwards with respect to the suture associated horizontal shortening. ern and southern domes and interpret that the zone in the lower crust and mantle lithosphere. The age of the Late Eocene–Early Oligocene Karakoram fault and the Great Counter thrust metamorphic event that affected the Gangdese functioned together as a regional-scale positive Stage B (32–26 Ma) arc is coeval with Eohimalayan metamorphism fl ower structure (transpressional deformation). Displacement along the Ayi Shan detachment (Hodges et al., 1988; Pêcher, 1989; Vannay Our study extends across the entire range and resulted in attenuation of the Gangdese arc, facil- and Hodges, 1996). Eocene–Oligocene crustal shows that the metamorphic rocks are not local- itated exhumation of metamorphosed Gangdese thickening within the Tethyan Himalaya (Godin ized along the Karakoram fault zone, but rather arc rocks, and juxtaposed deep arc rocks against et al., 1999; Godin et al., 2001; Lee et al., 2000; are restricted to the lower plate of the Ayi Shan shallow arc rocks (Fig. 9B). The kinematics Aoya et al., 2005; Kellett and Godin, 2009; detachment and are cut by the Karakoram fault of extension is approximately parallel to the Aikman et al., 2008) is widely thought to have zone. Fault slip data from the Great Counter arcuate-shape Himalayan orogen. Since the Kai- induced prograde metamorphism and anatectic thrust show that it facilitates NE-SW shortening las Formation is depositional on the lower plate melting within Greater Himalayan rocks (Godin along the length of the Ayi Shan (Murphy et al., of the Ayi Shan detachment, its age provides an et al., 2006; Aoya et al., 2005; Lee and White- 2009), rather than oblique strike-slip displace- upper bound on the timing of slip. The 40Ar/39Ar house, 2007; Larson et al., 2010). Our inter- ment as suggested by Lacassin et al. (2004). muscovite age of the rhyolitic dike (AY-14) that pretation of Middle Eocene to Early Oligocene Strike-slip shear sense indicators along the cuts the Kailas Formation indicates that the Kai- crustal thickening of the Gangdese arc implies Great Counter thrust used to support the inter- las Formation is older than ca. 18 Ma. Therefore that the orogenic wedge at this time extended pretation by Lacassin et al. (2004) are located slip on the detachment must predate 18 Ma. from the Tethyan Himalaya in the south, north- in the Mount Kailas area (southeast of the Ayi This age constraint is consistent with the U-Pb ward across the India-Asia suture zone to the Shan) where the thrust trace coincides with that detrital zircon ages from the Kailas Formation, Gangdese batholith. of the Karakoram fault. Our geologic mapping which are interpreted to record its deposition presented in this study along with fault-slip data between 26 and 24 Ma (DeCelles et al., 2011). Early to Late Oligocene Extension presented in Murphy et al. (2009) indicate that DeCelles et al. (2011) argue on the basis of prov- oblique shear sense indicators along the thrust enance and paleofl ow data, strata relationships, Shear sense indicators show that the ortho- are not characteristic features, but instead, are and lithofacies patterns, that the Kailas Forma- gneisses have undergone southeast-northwest– local features present in the Mount Kailas area tion was not deposited in an overall contrac- directed stretching with dominantly top-to- probably due to local strike-slip reactivation. tional setting. Rather, they interpret the bulk of southeast displacement of the upper plate with the formation to have been deposited in a rift or respect to the lower plate. These data indicate Deformation Cycles in Southwest Tibet transtensional strike-slip basin. Their interpreta- the orthogneisses originated from underneath tion is consistent with our results, which indicate the upper plate granite to the southeast and were Taking into account the fi eld relationships that the Kailas Formation was deposited on ver- subsequently incorporated into the structurally exposed across the entire range and geochrono- tically thinned Gangdese arc rocks. Moreover, shallow portions of the suture zone by way of logic results presented here, we envision a more the sandstone petrology of rocks at the base of shearing along the Ayi Shan detachment. This complex geologic history than previously sug- the formation indicates that it was derived from interpretation predicts that the Gangdese arc to gested (Lacassin et al., 2004; Valli et al., 2007, deeply eroded Gangdese arc rocks (DeCelles et the east of the Ayi Shan (near Mount Kailas) 2008). Figures 9 and 10 illustrate our interpreta- al., 2011). This is also consistent with our results was vertically thinned. U-Pb ages of zircon tion of two cycles of shortening (vertical crustal that indicate exhumation of Gangdese rocks in from stretched mylonitic leucogranite sills that thickening) and extension (vertical crustal the Late Oligocene. intrude the orthogneisses indicate vertical thin- thinning) in southwestern Tibet since the Late ning occurred in the Early to Late Oligocene. Eocene. We envision that deformation repre- Stage C (18–15 Ma) This result is in agreement with previously sents that occurring at middle to shallow crustal A second phase of shortening is evident by reported geochronologic data from the Ayi Shan depths; this is represented by four stages (Fig. 9) the development of the north-directed GCT othogneisses and mylonitic leucogranite sills described below. (Fig. 9C). The Kailas Formation is cut by the that southeast shear was ongoing during the GCT in the Ayi Shan (Figs. 1 and 2), in the Late Oligocene (Lacassin et al., 2004; Valli et Stage A (40–31 Ma) Gangdese shan near Mount Kailas (Yin et al., al., 2007, 2008). These previous studies inves- Burial metamorphism related to crustal short- 1999), and farther east near Lopukangri (Mur- tigated fi eld relationships and rocks exposed ening of the Gangdese arc closely followed phy et al., 2009, 2010). Although the relation- along the eastern margin of the Ayi Shan behind or possibly overlaps in time with wan- ship between the rhyolitic dike (sample AY-14) range along the Karakoram fault zone (Fig. 1). ing arc magmatism (Fig. 9A)(Kapp et al., 2007). and the Great Counter thrust is not known, we Because these studies were limited to the east- Although the timing in southwest Tibet is not did not observe any igneous dikes that cut the ern side of the Ayi Shan, this unfortunately led known, we envision this metamorphic event Great Counter thrust and therefore the fault may to poorly constrained interpretations of the tim- coincided with development of the Tethyan postdate intrusion at ca. 18 Ma. This age is con- ing of geologic events and fault system geom- fold-thrust belt as suggested by data in southern sistent with other studies that interpret the thrust etry (compare fi gure 3 in Valli et al. [2007] to Tibet (Aikman et al., 2008). In this scenario, it to have been active during the Early to Middle Figure 3 in this paper). Valli et al. (2007, 2008) is possible that the Tethyan fold-thrust belt roots Miocene (Ratschbacher et al., 1994; Yin et al., interpret that the metamorphic rocks and the into the Gangdese arc, implying that the Tethyan 1999). Slip along the GCT results in northward igneous rocks which intrude them are a prod- fold-thrust belt and Gangdese arc were part of translation of the suture zone at shallow struc- uct of long-lived, continuous strike-slip defor- the same thrust wedge during the Eocene. An tural levels with respect to its position deeper in mation along the Karakoram fault. Valli et al. implication of this interpretation is that the the crust (Fig. 9C) (Ratschbacher et al., 1994; (2007) show steeply dipping faults bounding suture zone in the upper and middle crust was Yin et al., 1999; Murphy and Yin, 2003).

Geosphere, October 2011 1263

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Zhang et al. — φ hrust;

Thickening

Thinning

Thickening/Burial Thickening/Burial Metamorphism

Thinning

Ayi Shan Events Ayi

Moho

Moho details. Abbreviations: MCT— Abbreviations: details. ide of diagram) and how we view faults. Geologic symbols as in Figure 2. faults. Geologic symbols as in Figure

Burial metamorphism

Exhumation of metamorphosed Gangdese arc

overthrusting of overthrusting metamorphosed Gangdese arc

Gangdese Arc

Gangdese arc

Ayi Shan Ayi detachment

Karakoram fault - exposes metamorphosed Karakoram Gangdese arc

Great Counter thrust Great Counter

IYS

IYS - IYS

suture zone

Tethyan Himalaya Himalaya Tethyan thrust belt

NHA

STD

MCT

Moho

LH duplexes and LH duplexes imbricate thrusts imbricate

India

MFT

Increased Gravitational Increased Gravitational Potential Energy - Energy Potential

Decreased Gravitational Decreased Gravitational Potential Energy - Energy Potential

Increased Gravitational Increased Gravitational Potential Energy - Energy Potential

Orogenic Wedge Orogenic

(A) 40-31 Ma

(B) 32-26 (18?)Ma

(D) 15 Ma to present taper angle of orogenic wedge. Black lines indicate active faults, dashed lines are future faults, and gray lines are inactive faults, and gray lines are future wedge. Black lines indicate active faults, dashed are angle of orogenic taper Figure 9. Schematic diagram illustrating our interpretation of our results along the Indus-Yalu suture zone (IYS) (right-hand s suture along the Indus-Yalu results of our interpretation 9. Schematic diagram illustrating our Figure to text for wedge (left-hand side of diagram). Refer of the Himalayan orogenic context of the growth in the broader results our t Himalaya; MFT—Main Frontal detachment; NHA—Northern Himalayan antiform; LH—Lesser Tibetan Main Central thrust zone; STD—South

1264 Geosphere, October 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Crustal thickening and extension along the India-Asia suture zone NARY QUATER- PLIOCENE (LPSZ, KF, GM) KF, (LPSZ,

)] 3 Ar age 39 (AY-15) (AY-15) Mandhata detachment system. Ar/ ), GM ( ), 40 2 (GCT) 10.17 ± 0.04 Ma Extension ), KF ( ), 1 16-14 Ma initiation [LPSZ ( Shortening xposed in the Ayi Shan and previously published Shan and previously Ayi xposed in the Ar age 39 (AY-14) (AY-14) Ar/ 40 18.10 ± 0.05 Ma 20 15 10 5 0 (Ayi Shan detachment) (Ayi 32-26 Ma 12) AY-11, ages, (U-Pb zircon Extension 40-31 Ma 7) 5, AY-4, ages, (U-Pb zircon Shortening (Burial metamorphism) x. Age constraints for the last phase of Transtension are from: (1) Thiede et al. (2006), (2) Phillips (2004), and (1) from: are Transtension the last phase of Age constraints for x. EOCENE OLIGOCENE MIOCENE Intiation of collision (2005)] [Zhu et al. 60 55 50 45 40 35 30 25 PALEOCENE EL E M LELE MLEL 65 results. Samples analyzed in this study are denoted by the AY preﬁ AY denoted by the Samples analyzed in this study are results. zone; KF—Karakoram fault; GM—Gurla thrust; LPSZ—Leo Parghil shear Counter GCT—Great Abbreviations: Murphy and Copeland (2005). Figure 10. Geologic events along the IYS in southwest Tibet based on our geologic mapping and geochronologic studies of rocks e studies of rocks geologic mapping and geochronologic based on our Tibet 10. Geologic events along the IYS in southwest Figure

Geosphere, October 2011 1265

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Zhang et al.

Stage D (15 Ma to Holocene) ment characterized by two cycles of accumu- foreland, age estimates of thrusting indicate that The Great Counter thrust is cut by the Kara- lation (increase in taper angle) and dissipation the locus of horizontal shortening and vertical koram fault as well as by a large-magnitude (decrease in taper angle) of gravitational poten- thickening propagated toward the south (toward extensional shear zone in the Leo Parghil range tial energy (Fig. 9). Key stages of this evolution the foreland) (DeCelles et al., 2001). Foreland and Gurla Mandhata region (Fig. 9D; Thiede are as follows: (1) Eocene–Oligocene crustal propagation of the thrust belt and vertical thin- et al., 2006; Murphy et al., 2000, 2009, 2010; thickening leading to an increase in the taper ning in its hinterland are both processes that Murphy and Copeland, 2005). A K-feldspar of the orogenic wedge (increase in gravitational assist in decreasing the taper angle of the oro- from a leucogranite sample collected along the potential energy); (2) Early Miocene foreland- genic wedge. Karakoram fault system in the Zhaxigang area directed lateral spreading resulting in a decrease The kinematics of both phases of hinterland (Fig. 1) yields an age spectrum that indicates in the taper (decrease in gravitational potential extension (32–26 Ma [18 Ma?] and 15 Ma rocks on its south side (footwall) were rapidly energy); (3) Middle Miocene hinterland thick- to present) are approximately parallel to the cooled ca. 10 Ma (Arnaud, 1992). The 40Ar/39Ar ening leading to a renewed buildup of the taper arcuate-shaped Himalayan thrust belt. One muscovite age from sample AY-15, 10.17 (increase in gravitational potential energy); and hypothesis explains that arc-parallel extension in ± 0.04 Ma, is consistent with this result and sug- (4) Late Miocene to present lateral spreading the Himalaya is a result of outward radial growth gests that the Karakoram fault in the vicinity of instigating a decrease in the taper (decrease in (spreading) since the Middle Miocene (e.g., the Ayi Shan was active during the Late Mio- gravitational potential energy). The deforma- Murphy et al., 2009). We propose that this is also cene. The Leo Parghil range is bounded by the tion cycles described above and illustrated in a viable explanation for Oligocene arc-parallel Leo Parghil shear zone, a top-to-WNW exten- Figure 9 are consistent with the interpretation extension in the Ayi Shan and highlights the sional shear zone. Zhang et al. (2000) showed that the taper of the Himalayan orogenic wedge inﬂ uence of the shape of the orogenic wedge in that its footwall on the SE side of the range con- has increased and decreased twice since the Late controlling the kinematics of orogenic wedges. tains ductile deformed garnet-bearing schists. Eocene. Within the context of a critical taper Leucogranite bodies in the footwall locally con- model, we interpret the geology of the Ayi Shan CONCLUSIONS tain an extensional shear fabric and yield a K-Ar as recording deformation in the hinterland of age of 16–15 Ma. On the westside of the Leo the Himalayan orogenic wedge (Fig. 9). In this Geologic mapping combined with geochro- Parghil range 40Ar/39Ar white mica ages from scenario, Late Eocene–Early Oligocene crustal nologic studies of rocks along the IYS in south- rocks in the footwall of the Leo Parghil shear thickening within the Gangdese arc rocks west Tibet reveal a geologic history we interpret zone of 16–14 Ma indicate a phase of rapid recorded in the Ayi Shan would have resulted to result from two cycles of shortening and cooling possibly due to slip along the shear zone in surface uplift, thus increasing the taper of extension. Our primary results are as follows. (Thiede et al., 2006). Southeast of the Ayi Shan the Himalayan orogenic wedge. Vertical thin- (1) The U-Pb zircon ages and textural obser- is the Gurla Mandhata metamorphic core com- ning of upper and middle crustal rocks in this vations indicate that the protolith of ortho- plex (Fig. 2) (Murphy et al., 2000; Murphy and region followed soon after crustal thickening gneisses exposed in the Ayi Shan is Gangdese Copeland, 2005). It is bounded by the top-to- and therefore would have assisted in localized arc rocks and associated early Paleozoic coun- the-west Gurla Mandhata detachment system, subsidence, thereby facilitating a decrease in the try rock. which is interpreted to be kinematically linked taper angle. This event temporally overlaps with (2) U-Pb ages and isotope systematics of to the Karakoram fault. U-Pb zircon ages from movement along the Main Central thrust zone zircon rims from the orthogneisses indicate extensionally sheared leucogranite bodies indi- and South Tibet detachment, which are inter- that these rocks experienced a Late Eocene– cate that extension initiated at ca. 15 Ma. The preted to facilitate foreland-directed spread- Early Oligocene prograde metamorphic timing of extension in this region is broadly the ing of Greater Himalayan rocks (e.g., Hodges, event which we attribute to burial via crustal same as that documented farther east in south- 2000). Both hinterland vertical spreading and thickening/shortening. central Tibet (Lee et al., 2011), and therefore we foreland-directed spreading are processes that (3) Geologic mapping shows that the ortho- interpret this to be a regional event, rather than a assist in decreasing the taper of the orogenic gneisses and overlying mylonitic schist are local event associated with transtensional defor- wedge and therefore dissipate the gradient of mantled by a top-to-the-southeast brittle-ductile mation along the Karakoram fault. the gravitational potential energy. Initiation of detachment that we refer to as the Ayi Shan the Great Counter thrust, broadly bracketed to detachment. Shear along the detachment is Implications for the Development of have occurred between 18 and 15 Ma, is inter- coeval with intrusion of leucocratic dikes and the Himalayan Orogenic Wedge preted to represent a phase of renewed crustal sills into its lower plate. U-Pb zircon ages from thickening and therefore surface uplift and an these igneous bodies indicate that extension A current view on the growth of orogenic increase in the taper angle. This event tempo- occurred during the Early to Late Oligocene. wedges describes their evolution as a function rally correlates to the development of the North (4) Initiation of the Great Counter thrust, of the balance among processes responsible Himalayan antiform, an ~700-km-long shorten- broadly bracketed to have occurred between 18 for energy accumulation (e.g., crustal thickening structure in the Himalayan hinterland (Lee and 15 Ma, is interpreted to represent a phase of ing) and energy dissipation (e.g., lateral spread- et al., 2000, 2004; Godin et al., 2006; Larson et renewed crustal thickening along the IYS. This ing and hinterland extension) (Hodges et al., al., 2010). This implies that this renewed phase regional shortening structure is cut by a system 1996; Hodges, 2000). These processes com- of hinterland thickening is a regional event. Ini- of strike-slip and extensional shear zones (Leo pete to maintain a self-similar wedge geom- tiation of the large-magnitude extensional and Parghil shear zone, Karakoram fault, Gurla etry described by the taper angle (e.g., Dahlen, transtensional shear zones (Leo Parghil shear Mandhata detachment system) that are esti- 1990). The ﬁ rst-order structural elements in zone, Gurla Mandhata detachment system, mated to have initiated between 16 and 15 Ma the central Himalaya have recently been inte- Karakoram fault system) at ca. 15 Ma indicates and are presently active. grated by Larson et al. (2010) into a conceptual that the Himalayan hinterland transitioned to (5) Our results show that the crust in the model describing orogenic wedge develop- a phase of vertical thinning. In the Himalayan vicinity of the IYS experienced two cycles of

1266 Geosphere, October 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Crustal thickening and extension along the India-Asia suture zone

Chevalier, M.-L., Ryerson, F.J., Tapponnier, P., Finkel, R.C., evolution of the Xining-Minhe and Dangchang basins, vertical thickening followed by vertical crustal Van Der Woerd, J., Haibing, L., and Qing, L., 2005, northeastern Tibetan Plateau: Magnetostratigraphic thinning. Within the context of critical taper Slip-rate measurements on the Karakorum fault may and biostratigraphic results: Journal of Geophysical theory, vertical thickening and thinning in the imply secular variations in fault motion: Science, Research, v. 109, doi:10.1029/2003JB002913. v. 307, p. 411–414, doi:10.1126/science.1105466. Houseman, G.A., and England, P., 1996, A lithospheric- Himalayan hinterland assist in increasing and Dahlen, F.A., 1990, Critical taper models of fold-and-thrust thickening model for the Indo-Asian collision, in Yin, decreasing the taper angle of the orogenic belts and accretionary wedges: Annual Review of Earth A., and Harrison, T.M., eds., The tectonic evolution of and Planetary Sciences, v. 18, p. 55–99, doi:10.1146/ Asia: New York, Cambridge University Press, p. 3–17. wedge, respectively. Moreover, these hinter- annurev.ea.18.050190.000415. Kapp, P., DeCelles, P.G., Gehrels, G.E., Heizler, M., and land events can be linked to deformation pat- DeCelles, P.G., Robinson, D.M., Quade, J., Ojha, T.P., Gar- Ding, L., Geologic records of Lhasa-Qiangtang and terns recognized in the Himalayan foreland and zione, C.N., Copeland, P., and Upreti, B.N., 2001, Indo-Asian collisions in the Nima area of central Stratigraphy, structure, and tectonic evolution of the Tibet: Geological Society of America Bulletin, v. 119, together support the idea that the evolution of Himalayan fold-thrust belt in western Nepal: Tecton- p. 917–932. the orogen can be explained, at least qualita- ics, v. 20, p. 487–509. Kapp, P., Murphy, M.A., Yin, A., Harrison, T.M., Lin, D., tively, by critical taper models. DeCelles, P.G., Kapp, P., Quade, J., and Gehrels, G.E., 2011, and Guo, J., 2003, Mesozoic and Cenozoic tectonic Oligocene-Miocene Kailas basin, southwestern Tibet: evolution of the Shiquanhe area of western Tibet: Tec- Record of postcollisional upper-plate extension in tonics, v. 21, doi:10.1029/2001TC001332. ACKNOWLEDGMENTS the Indus-Yarlung suture zone: Geological Society of Kellett, D.A., and Godin, L., 2009, Pre-Miocene deforma- America Bulletin, v. 123, p. 1337–1362, doi:10.1130/ tion of the Himalayan superstructure, Hidden val- We thank Peter DeCelles, Paul Kapp, and Alex B30258.1. ley, central Nepal: Journal of the Geological Society, Robinson for valuable discussions regarding the Ding, L., Kapp, P., and Wan, X., 2005, Paleocene-Eocene v. 166, p. 261–275, doi:10.1144/0016-76492008-097. record of ophiolite obduction and initial India-Asia col- Lacassin, R., Valli, F., Arnaud, N., Leloup, P.H., Paquette, regional geologic implications of our results. We also lision, south-central Tibet: Tectonics, v. 24, TC3001, J.L., Li, H., Tapponnier, P., Chevalier, M.-L., Guillot, thank Victor Valencia, Alex Pullen, and Scott Johnston doi:10.1029/2004TC001729. S., Maheo, G., and Xu, Z., 2004, Large-scale geom- for their assistance with U-Pb analyses. We also thank England, P., and Molnar, P., 1997, Active deformation of etry and offset of the Karakoram fault, Tibet: Earth Matt Kohn and Kyle Larson for very helpful reviews Asia: From kinematics to dynamics: Science, v. 278, and Planetary Science Letters, v. 219, p. 255–269, doi: of an earlier version of this paper. This study was sup- p. 647–650, doi:10.1126/science.278.5338.647. 10.1016/S0012-821X(04)00006-8. ported by a grant from the National Science Founda- England, P., and Searle, M., 1986, The Cretaceous-Tertiary Larson, K.P., Godin, L., Davis, W.J., and Davis, D.W., 2010, tion (grant EAR 0438826 to Murphy). deformation of the Lhasa block and its implications for Out-of-sequence deformation and expansion of the crustal thickening in Tibet: Tectonics, v. 5, p. 1–14, doi: Himalayan orogenic wedge: Insight from the Changgo 10.1029/TC005i001p00001. culmination, south central Tibet: Tectonics, v. 29, doi: REFERENCES CITED Flesch, L.M., Haines, A.J., and Holt, W.E., 2001, Dynam- 10.1029/2008TC002393. ics of the India–Eurasia collision zone: Journal of Lee, J., and Whitehouse, M.J., 2007, Onset of mid-crustal Aikman, A., Harrison, T.M., and Lin, D., 2008, Evidence Geophysical Research, v. 106, p. 16,435–16,460, doi: extensional fl ow in southern Tibet: Geology, v. 35, for early (>44 Ma) Himalayan crustal thickening, 10.1029/2001JB000208. p. 45–48, doi:10.1130/G22842A.1. Tethyan Himalaya, southeastern Tibet: Earth and Plan- Gaetani, M., and Garzanti, E., 1991, Multicyclic history of Lee, J., Hacker, B.R., Dinklage, W.S., Wang, Y., Gans, P., etary Science Letters, v. 274, p. 14–23, doi:10.1016/j the northern India continental margin (northwestern Calvert, A., Wan, J., Chen, W., Blythe, A.E., and .epsl.2008.06.038. Himalaya): The American Association of Petroleum McClelland, W., 2000, Evolution of the Kangmar Aitchison, J.C., Ali, J.R., and Davis, A.M., 2007, When Geologists Bulletin, v. 75, p. 1427–1446. Dome, southern Tibet: Structural, petrologic, and and where did India and Asia collide: Journal of Godin, L., Parrish, R.R., Brown, R.D., and Hodges, K.V., thermochronologic constraints: Tectonics, v. 19, Geophysical Research, v. 112, B05423, doi:10.1029/ 2001, Crustal thickening leading to exhumation of p. 872–895. 2006JB004706. the Himalayan metamorphic core of central Nepal; Lee, J., Hacker, B.R., and Wang, Y., 2004, Evolution of Aoya, M., Wallis, S.R., Terada, K., Lee, J., Kawakami, T., insight from U-Pb geochronology and 40Ar/39Ar North Himalayan gneiss domes: Structural and meta- Wang, Y., and Heizler, M., 2005, North-south extension thermochronology: Tectonics, v. 20, p. 729–747, morphic studies in Mabja dome, southern Tibet: Jour- in the Tibetan crust triggered by granite emplacement: doi:10.1029/2000TC001204. nal of Structural Geology, v. 26, p. 2297–2316, doi: Geology, v. 33, p. 853–856, doi:10.1130/G21806.1. Godin, L., Gleeson, T., Searle, M.P., Ullrich, T.D., and Par- 10.1016/j.jsg.2004.02.013. Armijo, R., Tapponnier, P., and Han, T., 1989, Late Cenozoic rish, R.R., 2006, Locking of southward extrusion in Lee, J., Hager, C., Wallis, S.R., Stockli, D.F., Whitehouse, right-lateral strike-slip faulting in southern Tibet: Jour- favour of rapid crustal-scale buckling of the Greater M.J., Aoya, M., and Wang, Y., 2011, Middle to Late nal of Geophysical Research, v. 94, p. 2787–2838, doi: Himalayan sequence, Nar valley, central Nepal, in Miocene extremely rapid exhumation and thermal re- 10.1029/JB094iB03p02787. Law, R.D., Searle, M.P., and Godin, L., eds., Channel equilibration in the Kung Co rift, Southern Tibet: Tec- Arnaud, N.O., 1992, Apports de la thermochronologie fl ow, ductile extrusion in continental collision zones: tonics, v. 30, doi:10.1029/2010TC002745. 40Ar/39Ar sur feldspath potassique à la connaisance de Geological Society [London] Special Publication 268, Liu, Z.Q.C., 1988, Geologic map of the Qinghai-Xizang Pla- la tectoniique Cenozoique d’Asia [Ph.D. dissertation]: p. 269–292. teau and its neighboring regions (1:1,500,000 scale): Clermont-Ferrand, France, Université Blaise Pascal, Godin, L., Brown, R.L., Hanmer, S., and Parrish, R.R., 1999, Beijing, Chengdu Institute of Geology and Mineral 160 p. Back folds in the core of the Himalayan orogen: An Resources, Geologic Publishing House. Beaumont, C., Jamieson, R., Nguyen, M., and Medvedev, S., alternative interpretation: Geology, v. 27, p. 151–154, Ludwig, K.R., 2003, User’s manual for Isoplot 3.00: A geo- 2004, Crustal channel fl ows: 1. Numerical models with doi:10.1130/0091-7613(1999)027<0151:BFITCO>2. chronological toolkit for Microsoft Excel. applications to the tectonics of the Himalayan-Tibetan 3.CO;2. Molnar, P., England, P., and Martinod, J., 1993, Mantle orogen: Journal of Geophysical Research, v. 109, Harrison, T.M., Yin, A., Grove, M., Lovera, O.M., Ryer- dynamics, the uplift of the Tibetan Plateau, and the p. 738–742, doi:10.1029/2003JB002809. son, F.J., and Zhou, X., 2000, The Zedong window: A Indian monsoon: Reviews of Geophysics, v. 31, Bendick, R., and Flesch, L., 2007, Reconciling lithospheric record of superposed Tertiary deformation: Journal of p. 357–396, doi:10.1029/93RG02030. deformation and lower crustal fl ow beneath cen- Geophysical Research, v. 105, p. 19,211–19,230, doi: Murphy, M.A., and Burgess, W.P., 2006, Geometry, kine- tral Tibet: Geology, v. 35, p. 895–898, doi:10.1130/ 10.1029/2000JB900078. matics, and landscape characteristics of an active G23714A.1. Hodges, KV., 2000, Tectonics of the Himalaya and south- transtension zone, Karakoram fault system, southwest Besse, J., Courtillot, V., Pozzi, J.P., Westphal, M., and Zhou, ern Tibet from two perspectives: Geological Society of Tibet: Journal of Structural Geology, v. 28, p. 268–283, Y.X., 1984, Paleomagnetic estimates of crustal short- America Bulletin, v. 112, p. 323–350. doi:10.1016/j.jsg.2005.10.009. ening in the Himalayan thrusts and Zangbo suture: Hodges, K.V., Hubbard, M.S., and Silverberg, D.S., 1988, Murphy, M.A., and Copeland, P., 2005, Transtensional Nature, v. 311, p. 621–626, doi:10.1038/311621a0. Metamorphic constraints on the thermal evolution of deformation in the central Himalaya and its role in Brookfi eld, M.E., 1998, The evolution of the great rivers the central Himalayan orogen: Philosophical Transac- accommodating growth of the Himalayan orogen: Tec- systems of southern Asia during the Cenozoic India- tions of the Royal Society of London, Series A, v. 326, tonics, v. 24, doi:10.1029/2004TC001659. Asia collision: Rivers draining southwards: Geo- p. 257–280. Murphy, M.A., and Yin, A., 2003, Sequence of thrust- morphology, v. 22, p. 285–312, doi:10.1016/S0169 Hodges, K.V., Parrish, R.R., and Searle, M.P., 1996, Tec- ing in the Tethyan fold-thrust belt and Indus-Yalu -555X(97)00082-2. tonic evolution of the central Annapurna range, Nep- suture zone, southwest Tibet: Geological Society of Brown, E.T., Bendick, R., Bourles, D.L., Gaur, V., Molnar, alese Himalayas: Tectonics, v. 15, p. 1264–1291, doi: America Bulletin, v. 115, p. 21–34, doi:10.1130/0016 P., Raisbeck, G.M., and Yiou, F., 2002, Slip rates of the 10.1029/96TC01791. -7606(2003)115<0021:SEASOT>2.0.CO;2. Karakoram fault, Ladakh, India, determined using cos- Honegger, K., Dietrich, V., Frank, W., Gansser, A., Thoni, Murphy, M.A., Yin, A., Kapp, P., Harrison, T.M., Lin, D., mic ray exposure dating of debris fl ows and moraines: M., and Trommsdoff, V., 1982, Magmatism and and Guo, J., 2000, Southward propagation of the Journal of Geophysical Research, v. 107, no. B9, doi: metamorphism in the Ladakh Himalayas (the Indus- Karakoram fault system into southwest Tibet: Timing 10.1029/2000JB000100. Tsangpo suture zone): Earth and Planetary Sci- and magnitude of slip: Geology, v. 28, p. 451–454, Cheng, J., and Xu, G., 1987, Geologic map of the Gerdake ence Letters, v. 60, p. 253–292, doi:10.1016/0012 doi:10.1130/0091-7613(2000)28<451:SPOTKF>2.0 region at a scale of 1:1,000,000 and geologic report: -821X(82)90007-3. .CO;2. Lhasa, China, Xizang Bureau of Geology and Min- Horton, B.D., Dupont-Nivet, G., Zhou, J., Waanders, G.L., Murphy, M.A., Saylor, J.E., and Ling, D., 2009, Landscape eral Resources. Butler, R.F., and Wang, J., 2004, Mesozoic-Cenozoic evolution of southwest Tibet based on integrated

Geosphere, October 2011 1267

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021 Zhang et al.

paleoelevation reconstructions and structural history: Sanchez, V., Murphy, M.A., Dupre, W.R., Ding, L., and Chemical Geology, v. 252, p. 191–201, doi:10.1016/j Earth and Planetary Science Letters, v. 282, p. 1–9, doi: Zhang, R., 2010, Structural evolution of the Neogene .chemgeo.2008.03.003. 10.1016/j.epsl.2009.01.006. Gar basin, western Tibet: Implications for releasing Xu, R.-H., Schärer, U., and Allègre, C.J., 1985, Magmatism Murphy, M.A., Sanchez, V., and Taylor, M.H., 2010, Syn- bend development and drainage patterns: Geological and metamorphism in the Lhasa block (Tibet): A geo- collisional extension along the India-Asia suture zone, Society of America Bulletin, v. 122, p. 926–945, doi: chronological study: The Journal of Geology, v. 93, south-central Tibet: Implications for crustal deforma- 10.1130/B26566.1. p. 41–57, doi:10.1086/628918. tion of Tibet: Earth and Planetary Science Letters, Stacey, J.C., and Kramers, J., 1975, Approximation of ter- Yin, A., and Harrison, T.M., 2000, Geologic evolution of v. 290, p. 233–243, doi:10.1016/j.epsl.2009.11.046. restrial lead isotope evolution by a two-stage model: the Himalayan-Tibetan orogen: Annual Review of Pêcher, A., 1989, The metamorphism in the central Hima- Earth and Planetary Science Letters, v. 26, p. 207–221, Earth and Planetary Sciences, v. 28, p. 211–280, doi: laya: Journal of Metamorphic Geology, v. 7, p. 31–41, doi:10.1016/0012-821X(75)90088-6. 10.1146/annurev.earth.28.1.211. doi:10.1111/j.1525-1314.1989.tb00573.x. Thiede, R., Arrowsmith, J., Bookhagen, B., McWilliams, Yin, A., Harrison, T.M., Ryerson, F.J., Chen, W.J., Kidd, Phillips, R.J., Parrish, R.R., and Searle, M.P., 2004, Age M., Sobel, E., and Strecker, M., 2006, Dome formation W.S.F., and Copeland, P., 1994, Tertiary structural constraints on ductile deformation and long-term slip and extension in the Tethyan Himalaya, Leo Pargil, evolution of the Gangdese thrust system in southeast- rates along the Karakoram fault zone, Ladakh: Earth northwest India: Geological Society of America Bul- ern Tibet: Journal of Geophysical Research, v. 99, and Planetary Science Letters, v. 226, p. 305–319, doi: letin, v. 118, p. 635–650, doi:10.1130/B25872.1. p. 18,175–18,201, doi:10.1029/94JB00504. 10.1016/j.epsl.2004.07.037. Tullis, J., and Yund, R.A., 1991, Diffusion creep in feldspar Yin, A., Harrison, T.M., Murphy, M., Grove, M., Nie, S., Quade, J., Cater, M.L., Ojha, T., Adam, J., and Harrison, T., aggregates: Experimental evidence: Journal of Struc- Ryerson, F.J., Wang, X., and Chen, Z., 1999, Tertiary 1995, Late Miocene environmental change in Nepal tural Geology, v. 13, p. 987–1000, doi:10.1016/0191 deformation history of southeastern and southwest- and the northern Indian subcontinent: Stable isotopic -8141(91)90051-J. ern Tibet during the Indo-Asian collision: Geological evidence from paleosols: Geological Society of Amer- Valli, F., Arnaud, N., Leloup, P.H., Sobel, E.R., Mahéo, Society of America Bulletin, v. 111, p. 1644–1664, ica Bulletin, v. 107, p. 1381–1397, doi:10.1130/0016 G., Lacassin, R., Guillot, S., Li, H., Tapponnier, P., doi:10.1130/0016-7606(1999)111<1644:TDHOSA>2 -7606(1995)107<1381:LMECIN>2.3.CO;2. and Xu, Z., 2007, Twenty million years of continu- .3.CO;2. Ratschbacher, L., Frisch, W., Liu, G., and Chen, C.C., 1994, ous deformation along the Karakorum fault, western Yin, A., Dang, Y.-Q., Wang, L.-C., Jiang, W.-M., Zhou, S.-P., Distributed deformation in southern and western Tibet Tibet: A thermochronological analysis: Tectonics, Chen, X.-H., Gehrels, G.E., and McRivette, M.W., during and after the India-Asian collision: Journal of v. 26, doi:10.1029/2005TC001913. 2008, Cenozoic tectonic evolution of Qaidam basin Geophysical Research, v. 99, p. 19,917–19,945, doi: Valli, F., Leloup, P.H., Paquette, J.-L., Arnaud, N., Li, and its surrounding regions (Part 1): The southern Qil- 10.1029/94JB00932. H., Tapponnier, P., Lacassin, R., Guillot, S., Liu, ian Shan-Nan Shan thrust belt and northern Qaidam Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., D., Deloule, E., Xu, Z., and Mahéo, G., 2008, New basin: Geological Society of America Bulletin, v. 120, Owens, T.L., and DePaolo, D.L., 1998, Intercalibra- U/Pb constraints on timing of shearing and long-term p. 813–846, doi:10.1130/B26180.1. tion of standards, absolute ages and uncertainties in slip-rate on the Karakorum fault: Tectonics, v. 27, doi: Young, H.D., 1962, Statistical treatment of experimental 40Ar/39Ar dating: Chemical Geology, v. 145, p. 117– 10.1029/2007TC002184. data: New York, McGraw-Hill, 172 p. 152, doi:10.1016/S0009-2541(97)00159-9. Vannay, J.-C., and Hodges, K.V., 1996, Tectonometamor- Zhang, J., Ding, L., Zhong, D., and Zhou, Y., 2000, Orogen- Rowley, D.B., 1996, Age of initiation of collision between phic evolution of the Himalayan metamorphic core parallel extension in the Himalaya: Is it the indicator India and Asia: A review of stratigraphic data: Earth between the Annapurna and Dhaulagiri, central Nepal: of collapse or the product in process of compressive and Planetary Science Letters, v. 145, p. 1–13, doi: Journal of Metamorphic Geology, v. 14, p. 635–656, uplift?: Chinese Science Bulletin, v. 45, p. 114–119, 10.1016/S0012-821X(96)00201-4. doi:10.1046/j.1525-1314.1996.00426.x. doi:10.1007/BF02884653. Royden, L.H., 1996, Coupling and decoupling of crust Vavra, G., Schmid, R., and Gebauer, D., 1999, Internal mor- Zhang, Y., and Zheng, J., 1994, Geologic overview in Kok- and mantle in convergent orogens: Implications phology, habit and U-Th-Pb microanalysis of amphib- shili, Qinghai and adjacent areas: Beijing, Seismologi- of strain partitioning in the crust: Journal of Geo- olite-to-granulite facies zircons: Geochronology of the cal Publishing House, 177 p. physical Research, v. 101, p. 17,679–17,705, doi: Ivrea Zone (Southern Alps): Contributions to Miner- Zhu, B., Kidd, W.S.F., Rowley, D.B., Currie, B.S., and 10.1029/96JB00951. alogy and Petrology, v. 134, p. 380–404, doi:10.1007/ Shaﬁ que, N., 2005, Age of initiation of the India-Asia Rubatto, D., 2002, Zircon trace element geochemistry: Par- s004100050492. collision in the east-central Himalaya: The Journal of titioning with garnet and the link between U-Pb ages Wang, S., Fang, X., Lai, Q., Zheng, D., and Wang, T., 2009, Geology, v. 113, p. 265–285, doi:10.1086/428805. and metamorphism: Chemical Geology, v. 184, p. 123– New radiometric dating constrains the time of ini- 138, doi:10.1016/S0009-2541(01)00355-2. tiation of the Karakorum fault zone: Tectonophysics, Ruddiman, W.F., and Kutzbach, J.E., 1989, Forcing of late v. 475, p. 503–513. Cenozoic northern hemisphere climate by plateau Wen, D., Liu, D., Chung, S., Chu, M., Ji, J., Zhang, Q., Song, uplift in southern Asia and the American west: Journal B., Lee, T., Yeh, M., and Lo, C., 2008, Zircon SHRIMP MANUSCRIPT RECEIVED 15 SEPTEMBER 2010 of Geophysical Research, v. 94, p. 18,409–18,427, doi: U-Pb ages of the Gangdese Batholith and implica- REVISED MANUSCRIPT RECEIVED 6 APRIL 2011 10.1029/JD094iD15p18409. tions for Neotethyan subduction in southern Tibet: MANUSCRIPT ACCEPTED 20 APRIL 2011

1268 Geosphere, October 2011

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1249/3341645/1249.pdf by guest on 30 September 2021