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Geosphere

doi: 10.1130/GES00643.1 2011;7;1249-1268Geosphere

Ran Zhang, Michael A. Murphy, Thomas J. Lapen, Veronica Sanchez and Matthew Heizler orogenalong the India-Asia suture zone: Evidence for cyclicity in the Himalayan Late Eocene crustal thickening followed by Early-Late Oligocene extension

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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, USA2New Mexico Geochronology Research Laboratory, New Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico 87801, USA

1249

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

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

ABSTRACT

The timing of geologic events along the India-Asia suture in southern Tibet remains poorly understood because minimal denu-dation prevents widespread exposure of structurally deep rocks. In this study, we present geologic maps of two structurally deep domes, cored by mylonitic ortho-gneisses, across the India-Asia suture zone in southwestern Tibet. New U-Pb zircon ages and rock textures indicate that core ortho gneisses are originally Gangdese arc rocks that experienced Late Eocene pro-grade metamorphism, probably during crustal thickening. Crosscutting leucogran-ite sills underwent northwest-southeast extension related to slip along a brittle-ductile shear zone here designated the Ayi Shan detachment. The timing of shear along detachment is bracketed by zircon U-Pb ages of 26–32 Ma for these pre- to syn(?)- extensional leucogranites, and by a 40Ar/39Ar muscovite age of 18.10 ± 0.05 Ma for a rhy-olitic dike. This rhyolite dike crosscuts a widespread siliciclastic unit that was depos-ited across the detachment, which we cor-relate to the Kailas Formation. The Great Counter thrust defi nes the surface trace of the India-Asia suture zone; it cuts the Kailas Formation, and is in turn cut by the Kara-koram fault. A new 40Ar/39Ar muscovite age of 10.17 ± 0.04 Ma for the Karakoram fault footwall is consistent with published ther-mochronologic data that indicate Late Mio-cene transtension in southwestern Tibet.

INTRODUCTION

Intercontinental collision between the Indian subcontinent and central Asia resulted in wide-spread Cenozoic crustal thickening and surface uplift that extends nearly 2000 km from north-ern India to Kazakhstan, a region encompass-ing over 7,000,000 km2. This continent-scale event played an important role in the evolution of Asian river systems (Brookfi eld, 1998) and global climate change (Ruddiman and Kutz-bach, 1989; Molnar et al., 1993; Quade et al., 1995), and served as the testing ground for a wide variety of geodynamic models of orogenic processes (Houseman and England, 1996; Roy-den, 1996; England and Molnar, 1997; Flesch et al., 2001; Beaumont et al., 2004; Bendick and Flesch, 2007). Because of the potential feed-back between collision and other physical phe-nomena, it is critical to constrain the timing of deformational events.

The initial collision between India and Asia is estimated to have occurred during the Paleocene–Early Eocene (Besse et al., 1984; Gaetani and Garzanti; 1991; Rowley, 1996; Zhu et al., 2005), although some suggest it occurred earlier (latest Cretaceous–Paleocene) (Yin and Harrison, 2000; Ding et al., 2005) and even oth-ers have argued for a much later time (ca. 34 Ma) (Aitchison et al., 2007). The suture between India and Asia juxtaposes the Cretaceous– Tertiary Gangdese arc, which formed along the southern margin of Asia due to northward sub-duction of Tethyan oceanic lithosphere, against the Tethyan sedimentary sequence (TSS) which represents material deposited along the former

passive margin of the Indian subcontinent. Our present understanding of the history of the Gangdese arc indicates that it did not experi-ence collision-related deformation until at least 30 Ma (Yin et al., 1994; Harrison et al., 2000), some 25 Myr after the most widely accepted initiation estimates of intercontinental collision (Zhu et al., 2005). To the north of the Indus-Yalu suture zone (IYS) (also referred to as the Indus-Tsangpo suture and Yarlung-Tsangpo suture) crustal shortening and related basin develop-ment was ongoing since 40 Ma in northeastern Tibet in the Qilian shan and Fenghuo shan-Nangqian thrust belt (Zhang and Zheng, 1994; Yin and Harrison, 2000; Horton et al., 2004) and since 28 Ma in the Eastern Kunlun ranges (Yin et al., 2008). To the south of the suture, U-Pb ages of zircons from intrusive rocks that cross-cut folded TSS strata indicate that signifi cant crustal shortening within the Tethyan fold-thrust belt occurred prior to the mid-Eocene (Aikman et al., 2008). These observations imply that the Gangdese arc was capable of resisting deforma-tion or transmitting collisional related stresses while crustal shortening to its north and south was ongoing. This is at odds with Middle to Late Eocene ages of plutonic rocks of the Gang-dese arc (Honegger et al., 1982; Xu et al., 1985; Harrison et al., 2000; Wen et al., 2008), which suggest that the arc was hot and therefore weak during the early stages of the collision. Observa-tions that cast further doubt on the inference that the Gangdese arc escaped early collision-related deformation are the increased sedimentation rates and development of a collisional foredeep on the adjacent passive margin of the Indian



Zhang et al.

1250 Geosphere, October 2011

subcontinent at this time (Rowley, 1996; Ding et al., 2005; Zhu et al., 2005). These features imply the presence of a large crustal load to its north along the southern margin of Asia. Eng-land and Searle (1986) and Kapp et al. (2007) show that a retroarc thrust belt spanning south-ern Tibet developed north of the Gangdese arc between 105 and 53 Ma. Their studies indicate that the thrust belt likely roots into the Gangdese arc and therefore predict that it was signifi cantly thickened immediately prior to the initial colli-sion. This raises the possibility that the arc did not escape deformation, but rather transmitted the stress elsewhere due to its thickened state.

To better understand the geologic history of the Gangdese arc and the IYS, we have under-taken a fi eld and geochronologic study of well-exposed orthogneisses exhumed from deep structural levels along the IYS in southwestern Tibet. Our results indicate that the protolith of these orthogneisses are Gangdese arc rocks that experienced Late Eocene prograde metamor-phism, which we attribute to burial via crustal thickening. This event is followed by the devel-opment of a brittle-ductile extensional shear zone involving Gangdese arc rocks.

GEOLOGY OF THE AYI SHAN

The trace of the IYS trends subparallel to the crest of the Ayi Shan (shan means moun-tain range in Mandarin) in southwestern Tibet (Fig. 1). Cretaceous–Tertiary granites that rep-resent the Gangdese arc are exposed along the eastern fl ank of the Ayi Shan whereas rocks of the TSS crop out along the western fl ank. Our geologic mapping (Fig. 1) shows that the geo-logic framework of the Ayi Shan consists of three primary structural features (Figs. 2 and 3). They are, from oldest to youngest, the (1) Ayi Shan detachment, (2) the Great Counter thrust, and (3) the Karakoram fault system.

Ayi Shan Detachment

Two northwest-southeast elongated, doubly plunging structural domes exist in the south-ern and northern portions of the Ayi Shan and are mantled by a brittle-ductile shear zone, which we refer to as the Ayi Shan detachment (Figs. 1–3). The Ayi Shan detachment jux-taposes porphyritic granite in its upper plate (hanging wall) against metamorphic rocks in its lower plate (footwall). Although the shear zones in the southern dome and northern dome are possibly different structures, we refer to them collectively as the Ayi Shan detachment based on their structural similarities (Figs. 4A and 4B). Valleys crossing the Ayi Shan expose ~1500 m of lower plate (footwall) rocks

(present-day thickness). Immediately below the detachment, the lower plate locally con-tains 10–20 m of chloritic, quartzofeldspathic breccia. Exposures of the chloritic breccia are common on the eastern side of the south-ern Ayi Shan and rare on the western side. Beneath the chloritic breccias, where pres-ent, the lower plate metamorphic rocks can be separated into three lithologically distinct units. Immediately below the chloritic brec-cia is a sequence of mylonitic biotite schist that reaches a thickness of 600 m. Primary and secondary foliations (S, C, and C′ folia-tions) within the schist are defi ned by biotite and recrystallized quartz, whereas the min-eral stretching lineation is defi ned by smeared quartz grains and aligned clots of muscovite. Locally, discrete top-to-southeast shear zones exist within the top 100 m of the mylonitic schist. In the southern dome, high-angle nor-mal faults cutting the upper plate locally sole into the Ayi Shan detachment, implying they are kinematically linked with the detachment (Fig. 4C). Below the schist is an ~400-m-thick sequence of mylonitic orthogneiss (Fig. 4D). The foliation within the orthogneiss is defi ned by alternating hornblende + biotite rich lay-ers and feldspar + quartz rich layers, whereas the mineral stretching lineation is defi ned by the aligned biotite clots as well as quartz and feldspar grains. The structurally deepest rocks observed in the northern dome are quartzofeld-spathic migmatitic gneiss (Fig. 2), which is at least 500 m thick.

The characteristic mineral assemblage of the schist exposed in the footwall of the Ayi Shan detachment is biotite + plagioclase + musco-vite + rutile + garnet, which corresponds to amphibolite facies metamorphic conditions. This mineral assemblage is present through-out the footwall from the highest to the low-est exposed level. Garnets do not preserve an internal fabric and therefore no evidence for rotation during growth. The abundant ductilely deformed feldspar porphyroclasts suggest that deformation was occurring at temperatures of 400–500 °C (Tullis and Yund, 1991), which likely corresponds to mid-crustal depths. Feld-spar porphyroclasts range in size from 1 to 3 cm in the long dimension. The orientation of the mineral stretching lineation defi ned by biotite clots, quartz, and feldspar in the footwall rocks indicates the mean slip direction along the Ayi Shan detachment is S60°E.

The mylonitic foliation in the footwall rocks is folded at all scales. The most abundant folds are those with axes oriented subparallel to the stretching lineation, which we refer to as cor-rugations. First- and second-order corrugations are defi ned by the surface trace of the Ayi Shan

detachment and also by the foliation within its lower plate in northern and southern domes. The corrugations trend toward the southeast (parallel to the Ayi Shan) (Fig. 2) and are relatively sym-metric (Fig. 3). The age of these corrugations is not clear. If the corrugations are syn-kinematic with shearing along the Ayi Shan detachment, they likely formed in a constrictional strain fi eld. This is consistent with the local presence of L-tectonites in the southern Ayi Shan dome. Some amount of folding must have occurred after slip along the Ayi Shan detachment since it is folded (Fig. 3).

Variably deformed leucogranite bodies make up ~5%–10% of the footwall of the Ayi Shan detachment (Figs. 4E and 4F). Sills are more pervasive than dikes. Sills and dikes range between tens of centimeters up to 3 m thick. Generally, they display straight contacts with the country rock. Leucogranite dikes typically cut the mylonitic foliation and leucogranite sills at deep structural levels but are deformed at higher structural levels and transposed paral-lel to the mylonitic foliation. We interpret this relationship to indicate that emplacement of leucogranite bodies broadly occurred during shearing within the footwall of the Ayi Shan detachment.

Great Counter Thrust

The southwest-dipping Great Counter thrust (GCT) defi nes the surface trace of the IYS. It juxtaposes the TSS in its hanging wall against Cretaceous–Tertiary granite and Tertiary silici-clastic rocks in its footwall (Figs. 1 and 2). The TSS in the Ayi Shan region ranges in age from Ordovician to Cretaceous and consists of a wide variety of sedimentary and low-grade metasedimentary rocks (Cheng and Xu, 1987). The GCT strikes northwest-southeast, subparal-lel to the Ayi Shan. Shear sense indicators show that it accommodates top-to-northeast motion. Exposed in the immediate hanging wall of the GCT are fault-bounded lenses of highly frac-tured metabasalt, metagabbro, and peridotite, which locally contain lenses of podiform chro-mite that are elongated parallel to the trace of the GCT (Fig. 2B).

The footwall of the GCT typically consists of pebble, cobble, and boulder conglomerate. This rock sequence is correlated to the Kailas Formation based on its composition and struc-tural position (Cheng and Xu, 1987; Liu, 1988; Murphy et al., 2000; Murphy et al., 2009). The Kailas Formation in the Ayi Shan lies uncon-formably on Cretaceous–Tertiary granite and the lower plate of the Ayi Shan detachment, indicating that it postdates movement along the Ayi Shan detachment (Fig. 2A).



Crustal thickening and extension along the India-Asia suture zone

Geosphere, October 2011 1251

Karakoram Fault System

The eastern margin of the Ayi Shan coincides with the trace of the Karakoram fault system (Figs. 1–3). Deformation along the Karakoram fault occurs within a narrow zone (2–20 km wide)

consisting of right-lateral faults, normal faults, and right-lateral ductile shear zones that strike northwest. Fault-slip data show that the Kara-koram fault accommodates right-lateral displace-ment with a minor normal dip-slip component (Ratschbacher et al., 1994; Murphy et al., 2000;

Kapp et al., 2003). The Karakoram fault cuts, and therefore must be younger than, the GCT in the Ayi Shan. Thermochronologic studies along the Karakoram fault in the vicinity of the Ayi Shan show a rapid cooling event in its footwall (south side) at ca. 10 Ma, which is interpreted to have

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Great Counter Thrust (India-Asia Suture Zone

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.



Zhang et al.


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80°05′E

QalQoal

Alluvial and older alluvial depositscobble-pebble gravel, sand, and silt

River Terrace deposits cobble-pebble gravel, sand, silt, and clay

Alluvial Fan Deposits cobble-pebble gravel, sand, and silt

Conglomerate unit boulder-cobble-pebble gravel and sand

Undifferentiated Tethyan sedimentary sequence, locally Mesozoic interbedded limestone, siltstone, and shale

granite ~15% modal biotite, locally contains orthoclase phenocrysts

granite ~5% modal biotite

diorite

Interlayered biotite schist andquartzofeldspathic gneiss

mylonitic quartzofeldspathic augen gneiss

Serpentinized pyroxenite, gabbro, and basalt

Colluvial deposits boulder-cobble-pebble-gravel, sand, and silt

Qt

Qc

Qaf

Tcg

TSS

Kg1

Kg2

Kd

bs

magn

msch mylonitic biotite schist (lower plate of Ayi Shan detachment)

Geologic symbols - same as Figure 2A

0 2 4 km

NContour interval 100 m

Geologic map of the southern Ayi Shan

Karakoram fault system

um

B

30

2011

129

267

115

815

um

AY-7AY-6

AY-5

AY-1

AY-4

AY-2

AY-3

AY-14

AY-15



Zhang et al.


Σ

Σ

Kar

akor

am fa

ult s

yste

mA

yi S

han

deta

chm

ent

Kg1

Kg2

Kg1

Kg1

Qal

Kg1

Tcg

TS

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Kg1

Gre

at C

ount

er th

rust

Qal

Qal

Tcg

6 5 4 3 2

km

23456km

BB

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umm

sch

mag

n

chlo

ritic

bre

ccia

IYS

N46

°E

um

TS

SK

g

Gre

at G

ount

er th

rust

Tcg

Ayi

Sha

n de

tach

men

t

msc

hm

agn

mig

Kar

akor

am fa

ult s

yste

m

Qc

Qaf

Qal

Qal

AA

′

6 5 4 3 2 1 0 -1 -2

6 5 4 3 2 1 0 -1 -2

N45

°EN

84°E

K-T

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K-T

g

A

Ayi

Sha

n de

tach

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mag

n

km

Gar

Val

ley

bas

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IYS

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km

B

No

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ern

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me

Sou

ther

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om

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gar

valle

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r va

lley

bas

in b

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gar

valle

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chlo

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bre

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chlo

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Fig

ure

3. (A

) Cro

ss-s

ecti

on A

–A′ a

cros

s th

e no

rthe

rn A

yi S

han.

(B) C

ross

-sec

tion

B–B

′ acr

oss

the

sout

hern

Ayi

Sha

n. D

ispl

acem

ent a

long

the

Ayi

Sha

n de

tach

men

t is

pred

omin

atel

y ou

t-of

-pla

ne (

top-

to-t

he-s

outh

east

). S

ee F

igur

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for

abbr

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tion

s an

d ge

olog

ic s

ymbo

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resulted from slip along the Karakoram fault at that time (Arnaud, 1992). However, Valli et al. (2007) suggest it was continuously active since the Early Miocene, based on U-Pb zircon ages and petrofabric analyses of rocks exposed on its south side (footwall). The Karakoram fault is presently active as it offsets Quaternary surfi cial deposits (Armijo et al., 1989; Chevelier et al., 2005; Brown et al., 2002; Murphy and Burgess, 2006; Sanchez et al., 2010).

U-PB GEOCHRONOLOGY

Uranium-lead zircon geochronology of granitic and gneissic rocks collected from the southern and northern gneiss domes was undertaken to (1) test the possible genetic link between the rocks making up the core of the domes with Cretaceous–Tertiary granites exposed to the east within the Gangdese batho-lith, and (2) assess the age of metamorphism.

Seven orthogneiss samples were analyzed from the southern dome (AY-1 –AY-7), three ortho-gneiss samples (AY-8–AY-10) were analyzed from the northern dome, and two mylonitized leucogranite sills (AY-11 and AY-12) were ana-lyzed from the northern dome (Figs. 1, 5, and 6). Sample locations are shown on Figures 2A and 2B. We obtained cathodoluminescence images of most zircons after performing U-Pb analyses to evaluate zoning patterns and internal

B

Kgr - upper plate

msch + leucogranite sills and dikes

msch - lower plate

Ayi Shan detachment

West

Qtmsch + leucogranite sills and dikes

msch

msch

msch - lower plate

A East

Kgr - upper plate

msch - lower platemsch - lower plate

Ayi Shan detachment

magn

Ayi Shan detachment

Southern Dome

Northern Dome

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.



Zhang et al.


structure with respect to the location of the U-Pb analyses. U-Pb isotopic data is available in the Supplemental Table File1.

Methods

U-Pb geochronology of zircons was con-ducted by laser ablation–multicollector– inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) at the Arizona LaserChron Center. Zircons were extracted from the rock

Southeast

dike cut by extensional shear zone

msch - lower plate

E F

hammer

dikes

mylonitized sills

Southeast

msch

Ayi Shan detachment

K-Tg

Northern Dome Northern Dome

msch

ESEC D Southeast

magn

Southern Dome

Figure 4 (continued). (C) High-angle normal faults rooting into a low-angle normal fault. Cliff face is ~200 m. (D) Photo of duc-tilely 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.

samples with a standard mineral separation pro-cess. Handpicked zircon grains were mounted in epoxy and then polished to provide fl at expo-sure of the interiors of the grains. The analyses involved ablation of zircon with a New Wave/Lambda Physik DUV193 excimer laser (oper-ating at a wavelength of 193 nm) using a spot diameter of 15–35 µm. The ablated mate-rial is carried with helium gas into the plasma source of a GV Instruments IsoProbe, which is equipped with a fl ight tube of suffi cient width that U, Th, and Pb isotopes are measured simul-taneously. All measurements are made in static mode, using Faraday detectors for 238U and 232Th, an ion-counting channel for 204Pb, and faraday collectors for 208–206Pb. Each analysis consists of one 12-s integration on peaks with the laser off (for backgrounds), 12 one-second integrations with the laser fi ring, and a 30 s

delay to purge the previous sample and prepare for the next analysis. Common Pb corrections are accomplished by using the measured 204Pb and assuming an initial Pb composition from Stacey and Kramers (1975). Measurement of 204Pb is unaffected by the presence of 204Hg because backgrounds are measured on peaks (thereby subtracting any background 204Hg and 204Pb), and because very little Hg was present in the argon gas during this analytical session. In-run analysis of fragments of a large zircon crystal (generally every fi fth measurement) with a known age of 564 ± 4 Ma (2σ error) is used to correct for instrumental fractionation. The ages reported on each sample include both random and systematic errors associated with uncertainties in common Pb composition, age of the standard, and decay constant, and all fi nal uncertainties are reported at the 2σ level.

1Supplemental Table File. PDF fi le of 14 supple-mental tables. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00643.S1 or the full-text article on www.gsapubs.org to view the Supplemental Ta-ble File.





28

32

36

40

44

48

52

Meaean n = 4545.5353 ± 0.5353 [1.5] 9595% cononf.3 of 3434 rejrej. MSWD = 3.0

Mean = 45.53 ± 0.53 [1.5] 95% conf.3 of 34 rej. MSWD = 3.0

Sample AY - 1

Pb

/

U A

ge

(Ma)

206

238

58

62

66

70Sample AY - 2

50

54

Mean = 61.78 ± 0.92 [1.8] 1 of 22 rej. MSWD = 3.7

Pb

/

U A

ge

(Ma)

206

238

45

55

65

75

85

95

Mean = 73.9 ± 1.4 [4.5] 1 of 19 rej. MSWD = 1.20

Sample AY - 3

Pb

/

U a

ge

(Ma)

206

238

60

70

80

90

100

0.006

0.008

0.010

0.012

0.014

0.016

0.0 0.1 0.2 0.3 0.4 0.5

Sample AY - 3

Pb

/

U

206

238

Pb/ U 207 235

58

62

66

70

0.009

0.010

0.011

Sample AY - 2

54

0.0080.00 0.04 0.08 0.12 0.16 0.20

Pb

/

U

206

238

Pb/ U 207 235

28

32

36

40

44

48

0.004

0.005

0.006

0.007

0.008

0.00 0.04 0.08 0.12 0.16

Sample AY - 1

Pb

/

U

206

238

Pb/ U 207 235

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 associ-ated 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).



Zhang et al.


U-Pb Data

Southern DomeSamples AY-1 through AY-7 are ortho-

gneiss samples collected from the footwall of Ayi Shan detachment in the southern Ayi Shan range. Cathodoluminescence (CL) imaging of zircon grains from AY-1 show mottled and irregular CL patterns (Fig. 7). A few grains are rounded. Omitting three outlier spots, individ-ual 206Pb/238U ages range from 43.9 ± 0.9 Ma to 47.4 ± 0.9 Ma. U/Th ratios range from 0.4 to 7.5 (Table 1 in the Supplemental Table File [see footnote 1], Fig. 5). The weighted mean of all 206Pb/238U ages is 45.53 ± 0.53 Ma (n = 31). We

interpret this age as magmatic age due to the low U/Th ratios for all the zircon grains (Vavra et al., 1999).

Zircon grains from AY-2 are euhedral to subhedral. The individual spot 206Pb/238U ages range from 57.9 ± 0.6 Ma to 67.0 ± 0.8 Ma. U/Th ratios range from 0.7 to 4.2 (Table 2 in the Supplemental Table File [see footnote 1]). CL imaging of zircon grains shows oscillatory zonation consistent with igneous crystallization (Fig. 7). The weighted mean of all the ages is 61.78 ± 0.92 Ma (n = 21) (Fig. 5).

Most of the zircon grains extracted from AY-3 are subhedral. Individual spot 206Pb/238U ages range from 62.5 ± 10.1 Ma to 81.9 ± 7.7 Ma. U/Th

ratios range from 1.1 to 8.9 (Table 3 in the Sup-plemental Table File [see footnote 1]). CL imag-ing of AY-3 zircon grains does not show typi-cal oscillatory zonation and distinct rim-core domains as expected for igneous zircons. The weighted mean age is 73.9 ± 1.4 Ma (n = 18).

Zircon grains from AY-4 and AY-5 are euhe-dral and CL imaging of zircon grains shows igneous cores (low U/Th) and metamorphic rims (high U/Th) (Fig. 7C) (Vavra et al., 1999; Rubatto, 2002). The measured 206Pb/238U ages of individual spots range from 33.7 ± 0.8 Ma to 484.8 ± 4.7 Ma and 37.3 ± 0.4 Ma to 540.0 ± 9.0 Ma for samples AY-4 and AY-5, respec-tively; U/Th ratios range from 1.2 to 458.4

30

32

34

36

38

40

42

44

Mean = 38.0 ± 1.3 [3.2]0 of 12 rej. MSWD = 7.8

Sample AY - 4

Pb

/

U A

ge

(Ma)

206

238

31

33

35

37

39

41

43

Mean = 37.43 ± 0.87 [0.6]0 of 7 rej. MSWD = 1.5

Sample AY - 5

Pb

/

U A

ge

(Ma)

206

238

38

40

42

44

46

48

50

52

54

Mean = 48.47 ± 0.58 [1.41]1 of 31 rej. MSWD = 2.9

Sample AY - 6

Pb

/

U a

ge

(Ma)

206

238

0.00

0.02

0.04

0.06

0.08

0.10

0.0 0.2 0.4 0.6 0.8

Intercepts at 42 ± 31 & 504 ± 46 Ma

MSWD = 10.0

110

1001000

0 200 400

U/T

h

206Pb/238U age (Ma)

Sample AY - 4

Pb

/

U

206

238

Pb/ U 207 235

40

44

48

52

56

0.0058

0.0062

0.0066

0.0070

0.0074

0.0078

0.0082

0.0086

0.00 0.02 0.04 0.06 0.08 0.10

c

Sample AY - 6

Pb

/

U

206

238

Pb/ U 207 235

100

300

500

700

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.0 0.2 0.4 0.6 0.8 1.0


MSWD = 1.9

01

10100

1000

0 200 400 600

U/T

h

206Pb/238U Age (Ma)

Sample AY - 5

Pb

/

U

206

238

Pb/ U 207 235

Figure 5 (continued).





and 0.9 to 252.8, respectively (Tables 4 and 5 in the Supplemental Table File [see footnote 1]). The U-Pb isotope data of each sample plotted on concordia diagrams (Fig. 5) defi ne chords with upper and lower intercept ages. These are 504 ± 46 and 42 ± 31 Ma for sample AY-4 and 558 ± 27 and 43 ± 47 for sample AY-5, respectively. The weighted average age of laser spots that yielded U/Th ratios in excess of 20 is 38 ± 1.3 Ma (n = 12) for sample AY-4 and 37.43 ± 0.87 Ma for sample

AY-5, and we interpret these to refl ect the age of zircon overgrowth.

The zircon grains from AY-6 are domi-nantly euhedral to subhedral with a few that are rounded. CL imaging shows that most zircons display zoning patterns that are consistent with igneous derivation (Fig. 7D). The measured 206Pb/238U ages of individual spots range from 43.8 ± 1.5 Ma to 49.9 ± 0.8 Ma and U/Th ratios range from 0.7 to 2.7 (Table 6 in the Supple-mental Table File [see footnote 1], Fig. 5). The

weighted average age is 48.47 ± 0.58 (n = 30). Based on the U/Th ratio and zoning patterns in CL, we interpret the weighted average age to refl ect the age of the igneous protolith.

Zircons from AY-7 are mostly subhedral with a few of them being rounded. CL imaging shows that some of the zircon grains have oscil-latory zonations with distinct rim-core domains. Individual spot 206Pb/238U ages, range from 30.4 ± 2.4 Ma to 455.3 ± 16.6 Ma, and U/Th ratios range from 1.6 to 111.8 (Table 7 in the

50

250

450

0.00

0.02

0.04

0.06

0.08

0.10

0.0 0.2 0.4 0.6 0.8

Intercepts at 32.6 ± 5.0 & 477 ± 48 Ma

MSWD = 0.54

110

1001000

0 100 200 300 400 500

U/T

h

206Pb/238U age (Ma)

C

Sample AY - 7

34

38

42

46

50

0 0048

0.0052

0.0056

0.0060

0.0064

0.0068

0.0072

0.0076

0.0080

Sample AY - 8

0.00480.00 0.02 0.04 0.06 0.08 0.10

50

60

70

80

90

100

0.006

0.008

0.010

0.012

0.014

0.016

0.0 0.1 0.2 0.3 0.4

Sample AY - 10

26

28

30

32

34

36

Mean = 31.55 ± 0.69 [0.2]1 of 11 rej. MSWD = 3.0

Sample AY - 7

Pb

/

U

Ag

e (M

a)20

623

8

34

36

38

40

42

44

46

48

50

Mean = 38.74 ± 0.68 [0.8]2 of 23 rej MSWD = 2 5

Sample AY - 8

32

34 2 of 23 rej. = 2.5

Pb

/

U

Ag

e (M

a)20

623

8

40

50

60

70

80

90

100

110

Mean = 87.4 ± 1.7 [20]1 of 32 rej. MSWD = 4.3

Sample AY - 10

Pb

/

U a

ge

(Ma)

206

238

Pb

/

U

20

623

8 P

b/

U

206

238

Pb

/

U

20

623

8

Pb/ U 207 235

Pb/ U 207 235

Pb/ U 207 235




Zhang et al.


26

28

30

32

34

36

38

40

Mean = 34.8 ± 1.7 [2.8]0 of 7 rej. MSWD = 2.3

Sample AY - 11

Pb

/

U

Ag

e (M

a)20

623

8

23

25

27

29

31

33

35

37

39

Mean = 26.9 ± 1.1 [1.7]2 of 12 rej. MSWD = 11.7

Sample AY - 12

Pb

/

U

Ag

e (M

a)20

623

8

100

300

500

700

900

0.00

0.04

0.08

0.12

0.16

0.0 0.4 0.8 1.2 1.6


MSWD = 11.5

Sample AY - 11

0.1

1

10

100

0 500 1000

U/T

h

206Pb/238U age (Ma)

Pb

/

U

20

623

8

Pb/ U 207 235

200

600

1000

1400

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

0 1 2 3 4

Intercepts at 77 ± 87 & 1564 ± 200 Ma

MSWD = 17

1

10

100

1000

0 500 1000 1500

U/T

h

206Pb/238U age (Ma)

Sample AY - 12

Pb

/

U

20

623

8

Pb/ U 207 235


0

50

100

150

200

250

300

350

400

450

500

20 40 60 80 100 120 140

U/T

h

Relative probability

0

100

200

300

400

500

0 200 400 600 800 1000

U/T

h

Relative p

rob

ability

A

30 50 70 90 110 130

Pb/ U age (Ma)

A

23

1

2

3

Gangdese arc magmatism (orthogneisses)

Middle Eocene to Early Oligocene prograde metamorphism (orthogneisses) -> crustal shortening/thickening

Early to Late Oligocene anatectic melting (mylonitized leucogranite sills) -> crustal extension/thinning

Interpretation

1

238206

Pb/ U age (Ma)238206

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





Supplemental Table File [see footnote 1]). The U-Pb isotope data of each sample spot plotted on concordia diagrams (Fig. 5) defi ne chords with upper and lower intercept ages. These are 489 ± 28 Ma and 42 ± 18 Ma, respectively. The weighted average age of laser spots that yielded U/Th ratios in excess of 20 is 31.55 ± 0.69 Ma (n = 10) and we interpret these to refl ect the age of zircon overgrowth.

Northern DomeAY-8 and AY-10 are orthogneiss samples

taken from the footwall of the Ayi Shan detach-ment in the northern Ayi Shan. The zircon

grains from both samples are dominantly euhe-dral to subhedral with a few that are rounded. CL imaging of AY-8 shows that most zircons display zoning patterns that are consistent with igneous derivation (Fig. 7E). The mea-sured 206Pb/238U ages of individual laser spots range from 36.1 ± 1.9 Ma to 44.8 ± 3.0 Ma and U/Th ratios range from 0.5 to 3.2 (Table 8 in the Supplemental Table File [see footnote 1], Fig. 5). The weighted mean 206Pb/238U age is 38.74 ± 0.68 (n = 21). Based on the U/Th ratio and zoning patterns in CL, we interpret the weighted mean age to refl ect the age of the igneous protolith.

Individual laser spot 206Pb/238U ages from AY-10 zircons range from 54.0 ± 4.2 Ma to 95.0 ± 8.6 Ma (n = 31). CL images of AY-10 zircon grains do not show oscillatory zonation and distinct rim-core domains (Fig. 7F). U/Th ratios range from 0.7 to 2.6 (Table 10 in the Supplemental Table File [see footnote 1]). The weighted mean 206Pb/238U age is 87.4 ± 1.7 Ma (n = 31) (Fig. 5).

Mylonitic Leocogranite SillsSamples AY-11 and AY-12 are samples of

mylonitized leucogranite from the footwall of the Ayi Shan detachment in the northern

AY-8-6,--7

38.3±0.4 Ma

39.2±0.4 Ma

50 μm

AY-1-8

45.9±0.7 Ma45.9±0.7 Ma

50 μm

A AY-4-12,-21

417.1±27.9 Ma

39.0±0.8 Ma

100 μm

C

AY-6-16,-17

48.1±1.1 Ma

46.1±0.8 Ma

100 μm

D E AY-10-3,-10

86.6±1.9 Ma

87.2±2.7 Ma

90.1±4.7 Ma

77.0±2.5 Ma

100 μm

F

AY-13-14,-15

649.1±35.9 Ma

34.2±0.5 Ma

50 μm

G

AY-2-17,-18

65.8±1.5 Ma

63.5±1.5 Ma

100 μm

B

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



Zhang et al.


Ayi Shan. Metamorphic foliation cuts across the contact between leucogranite and country rock schist and gneiss. Individual laser spot 206Pb/238U ages for samples AY-11 and AY-12 range from 32.2 ± 3.0 Ma to 788.8 ± 14.8 Ma (n = 26) and 25.7 ± 1.4 Ma to 1782 ± 89 Ma (n = 19), respectively. U/Th ratios range from 0.7 to 62.3 and 3.8 to 386, respectively (Table 11 in the Supplemental Table File [see footnote 1]). The weighted average 206Pb/238U ages of laser spots that yielded U/Th ratios in excess of 20 are 34.8 ± 1.7 Ma (n = 7) for sample AY-11 and 26.9 ± 1.1 Ma for sample AY-12 (n = 10) (Fig. 5) and we interpret these to refl ect the ages of zircon overgrowth.

40Ar/39Ar THERMOCHRONOLOGY

40Ar/39Ar thermochronology was conducted on muscovite and biotite from rock samples from the southern Ayi Shan gneiss dome. Samples were irradiated for 7 h along with the standard Fish Canyon Tuff sanidine (FC-2) with an estimated age of 28.02 Ma (Renne et al., 1998) at the U.S. Geological Survey (USGS) TRIGA Reactor in Denver, Colorado. Age spec-trum analyses were conducted by step-heating mineral separates within a double vacuum Mo resistance furnace at the New Mexico Geo-chronology Research Laboratory, New Mexico Institute of Mining and Technology. Samples were targeted that could bracket the timing of geologic events that postdate movement along the Ayi Shan detachment. One sample (AY-14) comes from a dike that cuts the Kailas Forma-tion, and the other sample (AY-15) comes from the south side of the Karakoram fault in its foot-wall (Fig. 2B).

AY-14 is from a 25-cm-thick rhyolitic dike that cuts across sandstone and conglomerate beds in the Kailas Formation adjacent to the

southern Ayi Shan gneiss dome. 40Ar/39Ar age spectra from muscovite are fl at for the major-ity of gas released and yield an age of 18.10 ± 0.05 Ma (Fig. 8). AY-15 is from a granitoid in the hanging wall of the Ayi Shan detachment ~6 km west of the Karakoram fault. 40Ar/39Ar age spectra from muscovite are fl at over the majority of gas released and yield an age of 10.17 ± 0.04 Ma (Fig. 8). Because both age spectra are fl at we interpret both to be recording rapid cooling of both rock samples.

DISCUSSION

Protolith of Lower Plate Orthogneisses

A key element in the architecture of the IYS in southwestern Tibet is the lower plate of the Ayi Shan detachment. These rocks lie beneath the surface trace of the suture and therefore pro-vide information on the suture zone at depth. Lower plate samples AY-1, AY-2, and AY-3 from the southern dome yield U-Pb zircon ages with low U/Th values of 45.53 ± 0.53 Ma, 61.78 ± 0.92 Ma, and 76.1 ± 1.4 Ma, respec-tively. Similarly, lower plate samples AY-8 and AY-10 from the northern dome yield low U/Th ratios with 206Pb/238U ages of 38.74 ± 0.68 Ma and 87.4 ± 1.7 Ma, respectively. U-Pb zircon ages of upper plate granites collected from the southern Ayi Shan range from 47 to 50 Ma and have low U/Th ratios indicating that the zircons are recording crystallization ages (Wang et al., 2009). These ages, along with regional studies on the age of Gangdese arc magmatism (Hon-egger et al., 1982; Xu et al., 1985; Harrison et al., 2000; Wen et al., 2008) lie within the range of our geochronologic results suggesting that the protolith of the Ayi Shan lower plate ortho-gneisses is the Gangdese arc. This interpretation is supported by the observation that both the

upper plate granite and the lower plate ortho-gneisses contain distinctive large orthoclase phenocrysts and porphyroclasts, respectively.

Age of Prograde Metamorphism

Orthogneiss samples AY-4, AY-5, and AY-7 contain zircons that yield old ages in their cores and younger ages along their rims. Laser abla-tion analyses defi ne chords on U-Pb Concordia diagrams and suggest that each sample contains zircons that grew in two stages and/or suffered Pb loss. The older, mostly Paleozoic 206Pb/238U ages defi ne a chord and have low U/Th ratios. The younger rims (Fig. 7) are generally Late Eocene–Early Oligocene and have high U/Th ratios, which we interpret to be consistent with the growth of young zircon around an older core. The weighted average 206Pb/238U ages of the laser spots that yield U/Th ratios > 20, which we use to discriminate between mixed spot ages and those primarily from the rim, are 38 ± 1.3 Ma for AY-4, 37.43 ± 0.87 Ma for AY-5, and 31.55 ± 0.69 Ma for AY-7. We inter-pret the age of zircon overgrowths in samples AY-4, AY-5, and AY-7 to have occurred during prograde metamorphism likely associated with burial and crustal shortening (Fig. 6).

Because structurally deep exposures of Gang-dese arc and associated country rocks are rare, the regional extent of this metamorphic event is uncertain. However, sandstone petrology of Paleocene to Early Eocene clastic rocks in the Tethyan Himalaya of southern Tibet (86°42′E) indicate a transition from a continental block provenance, interpreted as Indian craton, to recycled orogen provenance, interpreted to be a Gangdese arc-trench system (Zhu et al., 2005). This implies that the Gangdese arc was a devel-oping topographic high that was being eroded in the Eocene. We propose that this uplift refl ects

AY-15 muscoviteInt age = 18.13 ± 0.07 Ma

AY-14 muscoviteInt age = 10.17 ± 0.04 Ma

cumulative % Ar released cumulative % Ar released0 20 40 60 80 100 0 20 40 60 80 100

10.29 ± 0.04 Ma

18.13 ± 0.07 Ma

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.





isostatic uplift due to crustal thickening and associated horizontal shortening.

The age of the Late Eocene–Early Oligocene metamorphic event that affected the Gangdese arc is coeval with Eohimalayan metamorphism (Hodges et al., 1988; Pêcher, 1989; Vannay and Hodges, 1996). Eocene–Oligocene crustal thickening within the Tethyan Himalaya (Godin et al., 1999; Godin et al., 2001; Lee et al., 2000; Aoya et al., 2005; Kellett and Godin, 2009; Aikman et al., 2008) is widely thought to have induced prograde metamorphism and anatectic melting within Greater Himalayan rocks (Godin et al., 2006; Aoya et al., 2005; Lee and White-house, 2007; Larson et al., 2010). Our inter-pretation of Middle Eocene to Early Oligocene crustal thickening of the Gangdese arc implies that the orogenic wedge at this time extended from the Tethyan Himalaya in the south, north-ward across the India-Asia suture zone to the Gangdese batholith.

Early to Late Oligocene Extension

Shear sense indicators show that the ortho-gneisses have undergone southeast-northwest–directed stretching with dominantly top-to-southeast displacement of the upper plate with respect to the lower plate. These data indicate the orthogneisses originated from underneath the upper plate granite to the southeast and were subsequently incorporated into the structurally shallow portions of the suture zone by way of shearing along the Ayi Shan detachment. This interpretation predicts that the Gangdese arc to the east of the Ayi Shan (near Mount Kailas) was vertically thinned. U-Pb ages of zircon from stretched mylonitic leucogranite sills that intrude the orthogneisses indicate vertical thin-ning occurred in the Early to Late Oligocene. This result is in agreement with previously reported geochronologic data from the Ayi Shan othogneisses and mylonitic leucogranite sills that southeast shear was ongoing during the Late Oligocene (Lacassin et al., 2004; Valli et al., 2007, 2008). These previous studies inves-tigated fi eld relationships and rocks exposed along the eastern margin of the Ayi Shan range along the Karakoram fault zone (Fig. 1). Because these studies were limited to the east-ern side of the Ayi Shan, this unfortunately led to poorly constrained interpretations of the tim-ing of geologic events and fault system geom-etry (compare fi gure 3 in Valli et al. [2007] to Figure 3 in this paper). Valli et al. (2007, 2008) interpret that the metamorphic rocks and the igneous rocks which intrude them are a prod-uct of long-lived, continuous strike-slip defor-mation along the Karakoram fault. Valli et al. (2007) show steeply dipping faults bounding

the Ayi Shan metamorphic core in the north-ern and southern domes and interpret that the Karakoram fault and the Great Counter thrust functioned together as a regional-scale positive fl ower structure (transpressional deformation). Our study extends across the entire range and shows that the metamorphic rocks are not local-ized along the Karakoram fault zone, but rather are restricted to the lower plate of the Ayi Shan detachment and are cut by the Karakoram fault zone. Fault slip data from the Great Counter thrust show that it facilitates NE-SW shortening along the length of the Ayi Shan (Murphy et al., 2009), rather than oblique strike-slip displace-ment as suggested by Lacassin et al. (2004). Strike-slip shear sense indicators along the Great Counter thrust used to support the inter-pretation by Lacassin et al. (2004) are located in the Mount Kailas area (southeast of the Ayi Shan) where the thrust trace coincides with that of the Karakoram fault. Our geologic mapping presented in this study along with fault-slip data presented in Murphy et al. (2009) indicate that oblique shear sense indicators along the thrust are not characteristic features, but instead, are local features present in the Mount Kailas area probably due to local strike-slip reactivation.

Deformation Cycles in Southwest Tibet

Taking into account the fi eld relationships exposed across the entire range and geochrono-logic results presented here, we envision a more complex geologic history than previously sug-gested (Lacassin et al., 2004; Valli et al., 2007, 2008). Figures 9 and 10 illustrate our interpreta-tion of two cycles of shortening (vertical crustal thickening) and extension (vertical crustal thinning) in southwestern Tibet since the Late Eocene. We envision that deformation repre-sents that occurring at middle to shallow crustal depths; this is represented by four stages (Fig. 9) described below.

Stage A (40–31 Ma)Burial metamorphism related to crustal short-

ening of the Gangdese arc closely followed behind or possibly overlaps in time with wan-ing arc magmatism (Fig. 9A)(Kapp et al., 2007). Although the timing in southwest Tibet is not known, we envision this metamorphic event coincided with development of the Tethyan fold-thrust belt as suggested by data in southern Tibet (Aikman et al., 2008). In this scenario, it is possible that the Tethyan fold-thrust belt roots into the Gangdese arc, implying that the Tethyan fold-thrust belt and Gangdese arc were part of the same thrust wedge during the Eocene. An implication of this interpretation is that the suture zone in the upper and middle crust was

translated southwards with respect to the suture zone in the lower crust and mantle lithosphere.

Stage B (32–26 Ma)Displacement along the Ayi Shan detachment

resulted in attenuation of the Gangdese arc, facil-itated exhumation of metamorphosed Gangdese arc rocks, and juxtaposed deep arc rocks against shallow arc rocks (Fig. 9B). The kinematics of extension is approximately parallel to the arcuate-shape Himalayan orogen. Since the Kai-las Formation is depositional on the lower plate of the Ayi Shan detachment, its age provides an upper bound on the timing of slip. The 40Ar/39Ar muscovite age of the rhyolitic dike (AY-14) that cuts the Kailas Formation indicates that the Kai-las Formation is older than ca. 18 Ma. Therefore slip on the detachment must predate 18 Ma. This age constraint is consistent with the U-Pb detrital zircon ages from the Kailas Formation, which are interpreted to record its deposition between 26 and 24 Ma (DeCelles et al., 2011). DeCelles et al. (2011) argue on the basis of prov-enance and paleofl ow data, strata relationships, and lithofacies patterns, that the Kailas Forma-tion was not deposited in an overall contrac-tional setting. Rather, they interpret the bulk of the formation to have been deposited in a rift or transtensional strike-slip basin. Their interpreta-tion is consistent with our results, which indicate that the Kailas Formation was deposited on ver-tically thinned Gangdese arc rocks. Moreover, the sandstone petrology of rocks at the base of the formation indicates that it was derived from deeply eroded Gangdese arc rocks (DeCelles et al., 2011). This is also consistent with our results that indicate exhumation of Gangdese rocks in the Late Oligocene.

Stage C (18–15 Ma)A second phase of shortening is evident by

the development of the north-directed GCT (Fig. 9C). The Kailas Formation is cut by the GCT in the Ayi Shan (Figs. 1 and 2), in the Gangdese shan near Mount Kailas (Yin et al., 1999), and farther east near Lopukangri (Mur-phy et al., 2009, 2010). Although the relation-ship between the rhyolitic dike (sample AY-14) and the Great Counter thrust is not known, we did not observe any igneous dikes that cut the Great Counter thrust and therefore the fault may postdate intrusion at ca. 18 Ma. This age is con-sistent with other studies that interpret the thrust to have been active during the Early to Middle Miocene (Ratschbacher et al., 1994; Yin et al., 1999). Slip along the GCT results in northward translation of the suture zone at shallow struc-tural levels with respect to its position deeper in the crust (Fig. 9C) (Ratschbacher et al., 1994; Yin et al., 1999; Murphy and Yin, 2003).



Zhang et al.


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Zhang et al.


Stage D (15 Ma to Holocene)The Great Counter thrust is cut by the Kara-

koram fault as well as by a large-magnitude extensional shear zone in the Leo Parghil range and Gurla Mandhata region (Fig. 9D; Thiede et al., 2006; Murphy et al., 2000, 2009, 2010; Murphy and Copeland, 2005). A K-feldspar from a leucogranite sample collected along the Karakoram fault system in the Zhaxigang area (Fig. 1) yields an age spectrum that indicates rocks on its south side (footwall) were rapidly cooled ca. 10 Ma (Arnaud, 1992). The 40Ar/39Ar muscovite age from sample AY-15, 10.17 ± 0.04 Ma, is consistent with this result and sug-gests that the Karakoram fault in the vicinity of the Ayi Shan was active during the Late Mio-cene. The Leo Parghil range is bounded by the Leo Parghil shear zone, a top-to-WNW exten-sional shear zone. Zhang et al. (2000) showed that its footwall on the SE side of the range con-tains ductile deformed garnet-bearing schists. Leucogranite bodies in the footwall locally con-tain an extensional shear fabric and yield a K-Ar age of 16–15 Ma. On the westside of the Leo Parghil range 40Ar/39Ar white mica ages from rocks in the footwall of the Leo Parghil shear zone of 16–14 Ma indicate a phase of rapid cooling possibly due to slip along the shear zone (Thiede et al., 2006). Southeast of the Ayi Shan is the Gurla Mandhata metamorphic core com-plex (Fig. 2) (Murphy et al., 2000; Murphy and Copeland, 2005). It is bounded by the top-to-the-west Gurla Mandhata detachment system, which is interpreted to be kinematically linked to the Karakoram fault. U-Pb zircon ages from extensionally sheared leucogranite bodies indi-cate that extension initiated at ca. 15 Ma. The timing of extension in this region is broadly the same as that documented farther east in south-central Tibet (Lee et al., 2011), and therefore we interpret this to be a regional event, rather than a local event associated with transtensional defor-mation along the Karakoram fault.

Implications for the Development of the Himalayan Orogenic Wedge

A current view on the growth of orogenic wedges describes their evolution as a function of the balance among processes responsible for energy accumulation (e.g., crustal thicken-ing) and energy dissipation (e.g., lateral spread-ing and hinterland extension) (Hodges et al., 1996; Hodges, 2000). These processes com-pete to maintain a self-similar wedge geom-etry described by the taper angle (e.g., Dahlen, 1990). The fi rst-order structural elements in the central Himalaya have recently been inte-grated by Larson et al. (2010) into a conceptual model describing orogenic wedge develop-

ment characterized by two cycles of accumu-lation (increase in taper angle) and dissipation (decrease in taper angle) of gravitational poten-tial energy (Fig. 9). Key stages of this evolution are as follows: (1) Eocene–Oligocene crustal thickening leading to an increase in the taper of the orogenic wedge (increase in gravitational potential energy); (2) Early Miocene foreland-directed lateral spreading resulting in a decrease in the taper (decrease in gravitational potential energy); (3) Middle Miocene hinterland thick-ening leading to a renewed buildup of the taper (increase in gravitational potential energy); and (4) Late Miocene to present lateral spreading instigating a decrease in the taper (decrease in gravitational potential energy). The deforma-tion cycles described above and illustrated in Figure 9 are consistent with the interpretation that the taper of the Himalayan orogenic wedge has increased and decreased twice since the Late Eocene. Within the context of a critical taper model, we interpret the geology of the Ayi Shan as recording deformation in the hinterland of the Himalayan orogenic wedge (Fig. 9). In this scenario, Late Eocene–Early Oligocene crustal thickening within the Gangdese arc rocks recorded in the Ayi Shan would have resulted in surface uplift, thus increasing the taper of the Himalayan orogenic wedge. Vertical thin-ning of upper and middle crustal rocks in this region followed soon after crustal thickening and therefore would have assisted in localized subsidence, thereby facilitating a decrease in the taper angle. This event temporally overlaps with movement along the Main Central thrust zone and South Tibet detachment, which are inter-preted to facilitate foreland-directed spread-ing of Greater Himalayan rocks (e.g., Hodges, 2000). Both hinterland vertical spreading and foreland-directed spreading are processes that assist in decreasing the taper of the orogenic wedge and therefore dissipate the gradient of the gravitational potential energy. Initiation of the Great Counter thrust, broadly bracketed to have occurred between 18 and 15 Ma, is inter-preted to represent a phase of renewed crustal thickening and therefore surface uplift and an increase in the taper angle. This event tempo-rally correlates to the development of the North Himalayan antiform, an ~700-km-long shorten-ing structure in the Himalayan hinterland (Lee et al., 2000, 2004; Godin et al., 2006; Larson et al., 2010). This implies that this renewed phase of hinterland thickening is a regional event. Ini-tiation of the large-magnitude extensional and transtensional shear zones (Leo Parghil shear zone, Gurla Mandhata detachment system, Karakoram fault system) at ca. 15 Ma indicates that the Himalayan hinterland transitioned to a phase of vertical thinning. In the Himalayan

foreland, age estimates of thrusting indicate that the locus of horizontal shortening and vertical thickening propagated toward the south (toward the foreland) (DeCelles et al., 2001). Foreland propagation of the thrust belt and vertical thin-ning in its hinterland are both processes that assist in decreasing the taper angle of the oro-genic wedge.

The kinematics of both phases of hinterland extension (32–26 Ma [18 Ma?] and 15 Ma to present) are approximately parallel to the arcuate-shaped Himalayan thrust belt. One hypothesis explains that arc-parallel extension in the Himalaya is a result of outward radial growth (spreading) since the Middle Miocene (e.g., Murphy et al., 2009). We propose that this is also a viable explanation for Oligocene arc-parallel extension in the Ayi Shan and highlights the infl uence of the shape of the orogenic wedge in controlling the kinematics of orogenic wedges.

CONCLUSIONS

Geologic mapping combined with geochro-nologic studies of rocks along the IYS in south-west Tibet reveal a geologic history we interpret to result from two cycles of shortening and extension. Our primary results are as follows.

(1) The U-Pb zircon ages and textural obser-vations indicate that the protolith of ortho-gneisses exposed in the Ayi Shan is Gangdese arc rocks and associated early Paleozoic coun-try rock.

(2) U-Pb ages and isotope systematics of zircon rims from the orthogneisses indicate that these rocks experienced a Late Eocene–Early Oligocene prograde metamorphic event which we attribute to burial via crustal thickening/shortening.

(3) Geologic mapping shows that the ortho-gneisses and overlying mylonitic schist are mantled by a top-to-the-southeast brittle-ductile detachment that we refer to as the Ayi Shan detachment. Shear along the detachment is coeval with intrusion of leucocratic dikes and sills into its lower plate. U-Pb zircon ages from these igneous bodies indicate that extension occurred during the Early to Late Oligocene.

(4) Initiation of the Great Counter thrust, broadly bracketed to have occurred between 18 and 15 Ma, is interpreted to represent a phase of renewed crustal thickening along the IYS. This regional shortening structure is cut by a system of strike-slip and extensional shear zones (Leo Parghil shear zone, Karakoram fault, Gurla Mandhata detachment system) that are esti-mated to have initiated between 16 and 15 Ma and are presently active.

(5) Our results show that the crust in the vicinity of the IYS experienced two cycles of





vertical thickening followed by vertical crustal thinning. Within the context of critical taper theory, vertical thickening and thinning in the Himalayan hinterland assist in increasing and decreasing the taper angle of the orogenic wedge, respectively. Moreover, these hinter-land events can be linked to deformation pat-terns recognized in the Himalayan foreland and together support the idea that the evolution of the orogen can be explained, at least qualita-tively, by critical taper models.

ACKNOWLEDGMENTS

We thank Peter DeCelles, Paul Kapp, and Alex Robinson for valuable discussions regarding the regional geologic implications of our results. We also thank Victor Valencia, Alex Pullen, and Scott Johnston for their assistance with U-Pb analyses. We also thank Matt Kohn and Kyle Larson for very helpful reviews of an earlier version of this paper. This study was sup-ported by a grant from the National Science Founda-tion (grant EAR 0438826 to Murphy).

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