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Geosphere
Late Eocene crustal thickening followed by Early-Late Oligocene extension
along the India-Asia suture zone: Evidence for cyclicity in the Himalayan
orogen
Ran Zhang, Michael A. Murphy, Thomas J. Lapen, Veronica Sanchez and Matthew Heizler
Geosphere 2011;7;1249-1268
doi: 10.1130/GES00643.1
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Notes
© 2011 Geological Society of America
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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
1
Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA
New Mexico Geochronology Research Laboratory, New Mexico Bureau of Geology and Mineral Resources,
Socorro, New Mexico 87801, USA
2
ABSTRACT
INTRODUCTION
The timing of geologic events along the
India-Asia suture in southern Tibet remains
poorly understood because minimal denudation prevents widespread exposure of
structurally deep rocks. In this study, we
present geologic maps of two structurally
deep domes, cored by mylonitic orthogneisses, across the India-Asia suture zone
in southwestern Tibet. New U-Pb zircon
ages and rock textures indicate that core
orthogneisses are originally Gangdese arc
rocks that experienced Late Eocene prograde metamorphism, probably during
crustal thickening. Crosscutting leucogranite sills underwent northwest-southeast
extension related to slip along a brittleductile 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 rhyolitic dike. This rhyolite dike crosscuts a
widespread siliciclastic unit that was deposited across the detachment, which we correlate to the Kailas Formation. The Great
Counter thrust defines the surface trace of
the India-Asia suture zone; it cuts the Kailas
Formation, and is in turn cut by the Karakoram fault. A new 40Ar/39Ar muscovite age
of 10.17 ± 0.04 Ma for the Karakoram fault
footwall is consistent with published thermochronologic data that indicate Late Miocene transtension in southwestern Tibet.
Intercontinental collision between the Indian
subcontinent and central Asia resulted in widespread Cenozoic crustal thickening and surface
uplift that extends nearly 2000 km from northern India to Kazakhstan, a region encompassing over 7,000,000 km2. This continent-scale
event played an important role in the evolution
of Asian river systems (Brookfield, 1998) and
global climate change (Ruddiman and Kutzbach, 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; Royden, 1996; England and Molnar, 1997; Flesch et
al., 2001; Beaumont et al., 2004; Bendick and
Flesch, 2007). Because of the potential feedback between collision and other physical phenomena, 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 others 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 subduction 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 experience 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 IndusTsangpo suture and Yarlung-Tsangpo suture)
crustal shortening and related basin development was ongoing since 40 Ma in northeastern
Tibet in the Qilian shan and Fenghuo shanNangqian 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 crosscut folded TSS strata indicate that significant
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 deformation 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 Gangdese 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. Observations 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
*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 figures; 1 supplemental table file.
For permission to copy, contact [email protected]
© 2011 Geological Society of America
1249
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Zhang et al.
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. England and Searle (1986) and Kapp et al. (2007)
show that a retroarc thrust belt spanning southern 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 significantly
thickened immediately prior to the initial collision. 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 undertaken a field and geochronologic study of wellexposed 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 metamorphism, which we attribute to burial via crustal
thickening. This event is followed by the development 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 mountain range in Mandarin) in southwestern Tibet
(Fig. 1). Cretaceous–Tertiary granites that represent the Gangdese arc are exposed along the
eastern flank of the Ayi Shan whereas rocks of
the TSS crop out along the western flank. Our
geologic mapping (Fig. 1) shows that the geologic 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 southern 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 juxtaposes 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
1250
(present-day thickness). Immediately below
the detachment, the lower plate locally contains 10–20 m of chloritic, quartzofeldspathic
breccia. Exposures of the chloritic breccia
are common on the eastern side of the southern Ayi Shan and rare on the western side.
Beneath the chloritic breccias, where present, the lower plate metamorphic rocks can
be separated into three lithologically distinct
units. Immediately below the chloritic breccia is a sequence of mylonitic biotite schist
that reaches a thickness of 600 m. Primary
and secondary foliations (S, C, and C′ foliations) within the schist are defined by biotite
and recrystallized quartz, whereas the mineral stretching lineation is defined 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 normal 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 defined
by alternating hornblende + biotite rich layers and feldspar + quartz rich layers, whereas
the mineral stretching lineation is defined by
the aligned biotite clots as well as quartz and
feldspar grains. The structurally deepest rocks
observed in the northern dome are quartzofeldspathic 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 + muscovite + rutile + garnet, which corresponds to
amphibolite facies metamorphic conditions.
This mineral assemblage is present throughout the footwall from the highest to the lowest 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. Feldspar porphyroclasts range in size from 1 to 3 cm
in the long dimension. The orientation of the
mineral stretching lineation defined 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 corrugations. First- and second-order corrugations
are defined by the surface trace of the Ayi Shan
Geosphere, October 2011
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 symmetric (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
field. 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 parallel 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) defines the surface trace of the IYS. It
juxtaposes the TSS in its hanging wall against
Cretaceous–Tertiary granite and Tertiary siliciclastic 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, subparallel 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 fractured metabasalt, metagabbro, and peridotite,
which locally contain lenses of podiform chromite 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 structural position (Cheng and Xu, 1987; Liu, 1988;
Murphy et al., 2000; Murphy et al., 2009). The
Kailas Formation in the Ayi Shan lies unconformably 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).
Downloaded from geosphere.gsapubs.org on October 10, 2011
Crustal thickening and extension along the India-Asia suture zone
0
20
km
Zhaxigang
Nz and Ng - Neogene zada basin
and Gar basin sedimentary rocks
Tk - Early Miocene Kailas Formation
J-K
Northern Dome
32°30'N
Q
Q - undifferentiated Quaternary
surficial deposits
contour interval 400 m
K-Tgr
4700
10
80°E
79°30'E
'
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)
mgn + msch
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
consisting of right-lateral faults, normal faults,
and right-lateral ductile shear zones that strike
northwest. Fault-slip data show that the Karakoram fault accommodates right-lateral displacement with a minor normal dip-slip component
(Ratschbacher et al., 1994; Murphy et al., 2000;
Karakoram Fault System
J-K - undifferentiated Jurassic and
Cretaceous strata
N
K-Tgr - Cretaceous -Teritary plutonic
rocks (Gangdese arc rocks)
K-Tgr
5100
mgn +
msch
mgn + msch - mylonitic orthogneiss,
biotite schist and migmatite
K-Tgr
55
00
5100
Tss - Paleozoic-Mesozoic Tethyan
sedimentary sequence
43
00
00
43
Tss
an
Sh
79°30'E
Gar Airport
t
Sut
iaSu
Asisa
i-aAnddia
(In(I
S
KF
D
ST
00
39
Shiquanhe
dome axes
510
0
430
0
K-Tgr
Q
32°N
Figure 2B
K-Tgr
Southern Dome
84°
32°
Asia
Sut
u
re
Zo
ne
Q
Tss
0
550
4700
Ng
0
510
80°E
80°
79°30'0"E
M
80° FT
28°
4300
MBT
ia -
51
00
Ind
MCT
4700
76°
J-K
mgn + msch
uur
ereZ
Zoon
nee
80°
32°
active faults
(strike-slip and normal)
i
Ay
Nz
low-angle normal fault
KFS
5500
uus
tst
32°N
0
550
5900
rr
erTThh
ouunntet r
eatCCo
GGrer at
Q
GC
T
thrust fault
Q
Nz
76°
Figure 2A
K-Tgr
4700
84°
Figure 1. Geologic map of the Ayi Shan range showing the first-order geologic features exposed in the range and location
of detailed mapping. Abbreviations: GCT—Great Counter thrust; KFS—Karakoram fault system; MBT—Main Boundary 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
36
28
20
40
TSS
56
K-Tg
50
53
00
12 km
Geologic symbols
00
Qm
Qc
6
49
00
51
25
00
17
47
00
0
450
20
TSS
5300
msch
26
9
22
58
18
A
4900
48
5600
49
0
0
TSS
Qal
37
29
47
49 45
88
Tcg
cg
K-Tg
24
46
Qc
23
7
Qaf
39
55
35
18
5
22 42
4800
10
Anticline
TSS
Tcg
Qc
28
41
58
4
7
22
5100
10
TSS
Tcg
18 22
21
5500
0
570
35
5
25
19
16
magn
49
35
10
5800
14
42
3
54
AY-10
9
32
AY-11
msch
Tcg
Qaf
51
71
50
0
magn
mig 5
5600
K-Tg
5600
7
5
13
44
2
40
22
5
K-Tg
46
54
40
1
31
Qc
Qm
Qc
Qaf
Qal
31
6000
K
mig
58
00
32
0
44
0
39
35
Qaf
K-Tg
A′
45
0
0
6100
6200
Tertiary conglomerate
(correlated to the
Kailas Formation)
6100
10 0
Quaternary glacial
moraine deposits
6
Quaternary colluvial deposits
Quaternary alluvial fan deposits
5800
6300
6200
300
100
0
60
quartzofeldspathic migmatite
0
6260
6000
6 10 0
00
6 62
0
4800
AY-8 Rock sample location
6000
6000
4
0
600
0
Mylonitic
orthogneiss,
contains K-feldspar
0
6100 620
6
augens6201000
6000
6100 garnet
Mylonitic
biotite schist
600 0
00
59
Undifferentiated Cretaceous
strata
6 10 0
620
Cretaceous
granite
61 - Tertiary
62
0 6100
00
00
K-Tg contains large
K-feldspar phenocrysts
5900
58
62
00
5900
0
breccia
00 61
600Chloritic
00
00
55
0
magn
5500
4
28
Karakoram fault system
39
K
Qaf
4300
Undifferentiated Tethyan sedimentary
0
590
phyllite, slate,
6100sequence, gray and green
00
62
quartzite
0
630
6100
6100
msch
6000
TSS
6100
560
00
57
Kl
0
50
Quaternary
alluvium
0
Qaf
Qaf
00
Tcg6100
58
0
59 0 0
0
5 90
510
0
42
5
Qal
6000
Figure 2 (continued on following page). (A) Geologic map of the northern Ayi Shan.
small-scale anticline
TSS
14 41
29
21
AY-12
AY-8
Antiformal corrugation
(extension parallel fold)
7
0
00
10
55
5900
Qc
0
49
small-scale syncline
Syncline
40
mig
18
Qaf
magn
Qal AY-9
5
5000
10 23
Qm
Ayi Shan
detachment
0
570
mig
50
00
5100
Folds Arrow oriented parallel to the trace of the fold
axis indicates the plunge direction. Dashed where
460 0
approximately
located and dotted where concealed.
84
Great Counter
thrust
5300
30
48
15 14
0
transposed bedding mylonitic foliation bedding
Strike and dip of foliation and bedding
Detachment fault (low-angle normal fault)
box on hanging wall
53
13 35
Qc
70
00
5300
5500
4900
Normal fault bar and ball on hanging wall
40 30
Thrust fault barbs on hanging wall
59 50
Strike-slip fault opposing arrows indicate
sense of horizontal motion along the fault.
32 25
Lithologic and fault contacts dashed
where approximately located or inferred,
dotted where concealed. Arrows indicate
dip of fault and orientation of lineations.
72
11
37
0
540
0
N
46
00
4900
5000
49
00
Contour interval 100 m
0
00
550
0
49
00
48
00
56
45
60
00
Geologic map of the northern Ayi Shan
0
60
0
60
00
0
57
0
Geosphere, October 2011
0
00
00
64
0
56
47
6100
00
55
00
54
00
50
6100
61 0 0
00
5300
00
0
550
62 0 0
6000
61
60
00
53
0
5200
00
55
0
1252
43
6100
A
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Zhang et al.
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Crustal thickening and extension along the India-Asia suture zone
B
Karakoram fault system
Geologic map of the
southern Ayi Shan
Kg2
80°05′E
Kg1
Xing Gar Lake
bs
Kd
Qoal
Kg1
Contour interval 100 m
Kg2
A′
Qal
Qaf
48
00
N
37
26 32
21
52
0
2
4 km
Kg2
Qt
36
AY-14
15
magn
11
20
30
7
26
9
12
AY-7
Kg1
AY-5
32
8
32
34
41
47
67
Qt
0
Kg1
14 8 28 11
29
500
Kg2
5
AY-6
5
11
Namru
60
29
71
00
46
40
11
20
0
480
12
0
560
msch
Ayi Shan detachment
10
31
64
52
00
6066
AY-4
31
19
5
5
10
5400
45
0
AY-3
500
5200
0
18
10
msch
5000
Qal
Qoal
Alluvial and older alluvial deposits
cobble-pebble gravel, sand, and silt
Qt
River Terrace deposits cobble-pebble
gravel, sand, silt, and clay
Qc
Colluvial deposits boulder-cobblepebble-gravel, sand, and silt
Alluvial Fan Deposits cobble-pebble
gravel, sand, and silt
Qaf
Qc
42
49 55
magn
8
3
520
11
42
AY-1
AY-2
AY-15
25
50
35
Kg1
Tcg
41
32
um
32
um
Tcg
Tcg
Great Counter
5400
thrust (IYS)
Undifferentiated Tethyan sedimentary
TSS sequence, locally Mesozoic interbedded
00
52
Tcg
52
limestone, siltstone, and shale
38 42 75
35
39
TSS
Qal
um
Conglomerate unit boulder-cobblepebble gravel and sand
Kg1
granite ~15% modal biotite, locally
contains orthoclase phenocrysts
Kg2
granite ~5% modal biotite
TSS
5000
TSS
A
Kd
diorite
bs
Interlayered biotite schist and
quartzofeldspathic gneiss
msch
mylonitic biotite schist (lower plate of
Ayi Shan detachment)
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
1254
km
B
Qal
Qal
um
IYS
Kg
Southern Dome
mig
Σ
Kg1
Qal
magn
B′
6
km
Ayi Shan detachment
Gar Valley
basin
Karakoram fault system
Qc Qaf
K-Tg
A′
0
1
2
3
4
5
6
km
Geosphere, October 2011
2
3
4
5
6
um
Σ
TSS
Kg1
Tcg
Great Counter thrust
msch
magn
Kg1
chloritic breccia
Ayi Shan detachment
N46°E
chloritic breccia
Kg2
Kg1
Tcg
Qal
Kg1
gar valley
basin
2
3
4
5
-2
B
n
h
sc mag
m
chloritic breccia
Karakoram fault system
-1
?
K-Tg
Qal
o
at G
Ayi Shan detachment
-2
TSS
Tcg
Gre
st
hr u
er t
unt
N84°E
-1
0
1
2
3
4
5
6
A
IYS
N45°E
Zhang et al.
Figure 3. (A) Cross-section A–A′ across the northern Ayi Shan. (B) Cross-section B–B′ across the southern Ayi Shan. Displacement along the Ayi Shan detachment is predominately
out-of-plane (top-to-the-southeast). See Figure 2 for abbreviations and geologic symbols.
km
A
Northern Dome
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Downloaded from geosphere.gsapubs.org on October 10, 2011
Crustal thickening and extension along the India-Asia suture zone
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 surficial
deposits (Armijo et al., 1989; Chevelier et al.,
2005; Brown et al., 2002; Murphy and Burgess,
2006; Sanchez et al., 2010).
A
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 batholith, and (2) assess the age of metamorphism.
Seven orthogneiss samples were analyzed from
the southern dome (AY-1 –AY-7), three orthogneiss samples (AY-8–AY-10) were analyzed
from the northern dome, and two mylonitized
leucogranite sills (AY-11 and AY-12) were analyzed 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
Southern Dome
East
Ayi Shan detachment
Kgr - upper plate
Ayi Shan detachment
msch - lower plate
msch - lower plate
magn
B
Northern Dome
West
e
plat
wer
o
l
h
msc
Ayi Shan detachment
Kgr - upper plate
late
er p
w
o
h-l
msc
h
h
msc
s
dike
and
s
l
l
i
s
nite
ogra
c
u
e
h+l
msc
msc
sills
ranite
g
o
c
u
+ le
msch
ikes
and d
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.
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Zhang et al.
Southern Dome
C
ESE
D
Southeast
magn
msch
E
Southeast
Ayi Sh
a
K-Tg
n deta
chme
F
Southeast
nt
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
analyses. U-Pb isotopic data is available in the
Supplemental Table File1.
Methods
U-Pb geochronology of zircons was conducted by laser ablation–multicollector–
inductively coupled plasma–mass spectrometry
(LA-MC-ICP-MS) at the Arizona LaserChron
Center. Zircons were extracted from the rock
1
Supplemental Table File. PDF file of 14 supplemental tables. If you are viewing the PDF of this
paper or reading it offline, please visit http://dx.doi
.org/10.1130/GES00643.S1 or the full-text article
on www.gsapubs.org to view the Supplemental Table File.
1256
samples with a standard mineral separation process. Handpicked zircon grains were mounted
in epoxy and then polished to provide flat exposure of the interiors of the grains. The analyses
involved ablation of zircon with a New Wave/
Lambda Physik DUV193 excimer laser (operating at a wavelength of 193 nm) using a spot
diameter of 15–35 µm. The ablated material is carried with helium gas into the plasma
source of a GV Instruments IsoProbe, which is
equipped with a flight tube of sufficient width
that U, Th, and Pb isotopes are measured simultaneously. All measurements are made in static
mode, using Faraday detectors for 238U and
232
Th, 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 firing, and a 30 s
Geosphere, October 2011
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
204
Pb is unaffected by the presence of 204Hg
because backgrounds are measured on peaks
(thereby subtracting any background 204Hg and
204
Pb), 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 fifth 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 final
uncertainties are reported at the 2σ level.
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Crustal thickening and extension along the India-Asia suture zone
52
0.008
Sample AY - 1
Sample AY - 1
206
48
0.007
44
U
Pb/
238
44
40
40
0.006
36
206
Pb/
238
U Age (Ma)
48
32
0.005
36
28
32
Mea
Mean
ean
n = 45.53
45 53 ± 0.53
53 [1.5] 95%
95 conf.
con
onf.
3 of 34 rej.
rej MSWD = 3.0
0.004
0.00
0.04
28
0.16
Pb/ 235U
0.011
70
Sample AY - 2
66
Sample AY - 2
66
U
0.010
62
238
62
Pb/
238
U Age (Ma)
0.12
207
70
206
58
206
Pb/
0.08
58
0.009
54
54
Mean = 61.78 ± 0.92 [1.8]
1 of 22 rej. MSWD = 3.7
0.008
0.00
50
0.04
0.08
0.12
0.16
0.20
207
Pb/ 235U
0.016
95
100
U
238
206
65
206
Pb/
80
0.012
Pb/
75
Sample AY - 3
90
0.014
85
238
U age (Ma)
Sample AY - 3
70
0.010
60
0.008
55
Mean = 73.9 ± 1.4 [4.5]
1 of 19 rej. MSWD = 1.20
0.006
0.0
45
0.1
0.2
0.3
0.4
0.5
207
Pb/ 235U
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
define discordia lines that intersect concordia. The upper intersection is interpreted to reflect 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 reflect 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
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Zhang et al.
0.10
Sample AY - 4
44
42
0.08
1000
100
10
1
Sample AY - 4
0
206
Pb/
38
36
34
32
200
400
Pb/238U age (Ma)
0.06
238
U
40
206
206
Pb/
238
U Age (Ma)
Southern Dome
Samples AY-1 through AY-7 are orthogneiss 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, individual 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
206
Pb/238U ages is 45.53 ± 0.53 Ma (n = 31). We
ratios range from 1.1 to 8.9 (Table 3 in the Supplemental Table File [see footnote 1]). CL imaging of AY-3 zircon grains does not show typical 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 euhedral 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, respectively; U/Th ratios range from 1.2 to 458.4
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
U/Th
U-Pb Data
Mean = 38.0 ± 1.3 [3.2]
0 of 12 rej. MSWD = 7.8
0.04
Intercepts at
42 ± 31 & 504 ± 46 Ma
MSWD = 10.0
0.02
0.00
0.0
30
0.2
0.4
207
Pb/
0.6
0.8
235
U
0.12
206
Sample AY - 5
1000
100
10
1
0
U
238
39
35
0
0.08
206
300
0.04
0.02
33
500
200
400 600
Pb/238U Age (Ma)
0.06
Pb/
37
Mean = 37.43 ± 0.87 [0.6]
0 of 7 rej. MSWD = 1.5
Intercepts at
43 ± 47 & 558 ± 27 Ma
MSWD = 1.9
100
0.00
31
0.0
0.2
0.4
0.6
207
Pb/
U
238
Pb/
46
206
U age (Ma)
238
Pb/
206
48
44
Mean = 48.47 ± 0.58 [1.41]
1 of 31 rej. MSWD = 2.9
0.0078
48
0.0074
c
0.0070
44
0.0066
40
0.0058
0.00
0.02
0.04
207
38
Pb/
Figure 5 (continued).
1258
Sample AY - 6
52
0.0062
42
40
56
0.0082
50
Geosphere, October 2011
1.0
U
0.0086
Sample AY - 6
0.8
235
54
52
700
Sample AY - 5
U/Th
0.10
41
206
Pb/
238
U Age (Ma)
43
0.06
235
U
0.08
0.10
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Crustal thickening and extension along the India-Asia suture zone
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) define
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 reflect the age
of zircon overgrowth.
The zircon grains from AY-6 are dominantly 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
206
Pb/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 Supplemental Table File [see footnote 1], Fig. 5). The
0.10
36
1000
100
10
1
0
206
U
C
0.06
250
Pb/
0.04
206
30
Intercepts at
32.6 ± 5.0 & 477 ± 48 Ma
MSWD = 0.54
0.02
28
50
Mean = 31.55 ± 0.69 [0.2]
1 of 11 rej. MSWD = 3.0
0.00
0.0
26
0.2
0.4
207
0.0080
206
Sample AY - 8
46
U
238
Pb/
42
40
38
0.0068
42
0.0064
0.0060
38
0.0056
Mean = 38.74 ± 0.68 [0.8]
rej MSWD = 2.5
25
2 of 23 rej.
0 0048
0.0048
0.00
0.02
0.04
0.06
207
235
Sample AY - 10
0.016
100
0.014
90
238
U
100
Pb/
90
80
70
0.08
0.10
U
Sample AY - 10
80
0.012
206
238
U age (Ma)
Pb/
Pb/
Sample AY - 8
0.0052
32
206
U
34
36
110
0.8
46
0.0072
44
34
50
0.0076
206
Pb/
238
U Age (Ma)
50
0.6
235
Pb/
48
450
100 200 300 400 500
206
Pb/238U age (Ma)
238
32
Sample AY - 7
U/Th
0.08
Pb/
238
U Age (Ma)
Sample AY - 7
34
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
reflect 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 oscillatory 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
70
0.010
60
60
50
Mean = 87.4 ± 1.7 [20]
1 of 32 rej. MSWD = 4.3
0.008
50
0.006
40
0.0
0.1
0.2
207
Pb/
235
0.3
0.4
U
Figure 5 (continued).
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Zhang et al.
206
100
U/Th
Sample AY - 11
U
38
Pb/
34
32
0.1
700
0
206
Pb/
238
500
U age (Ma)
1000
500
0.08
300
Intercepts at
29 ± 42 & 876 ± 77 Ma
MSWD = 11.5
0.04
30
100
Mean = 34.8 ± 1.7 [2.8]
0 of 7 rej. MSWD = 2.3
28
0.00
26
0.0
0.4
0.8
207
Pb/
206
0.28
0.24
35
0.20
U
37
238
29
U
1400
1000
0.16
Pb/
31
1.6
Intercepts at
77 ± 87 & 1564 ± 200 Ma
MSWD = 17
0.12
1000
600
U/Th
Mean = 26.9 ± 1.1 [1.7]
2 of 12 rej. MSWD = 11.7
1.2
235
Sample AY - 12
Sample AY - 12
206
Pb/
238
U Age (Ma)
39
33
900
1
0.12
238
36
Sample AY - 11
10
206
Pb/
238
U Age (Ma)
0.16
40
0.08
100
10
1
27
0.04 200
0
206
500
1000
Pb/238U age (Ma)
25
0.00
0
23
1
2
3
4
207
Pb/ 235 U
Figure 5 (continued).
500
500
A
450
A
400
U/Th
300
350
200
U/Th
300
3
2
250
100
200
0
0
200
150
400
600
238
Pb/ U age (Ma)
1000
800
206
Relative probability
Relative probability
400
1
100
50
0
20
30
40
50
70
60
206
Interpretation
Pb/
80
90
100
110
120
130
238
U age (Ma)
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
140
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1500
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Crustal thickening and extension along the India-Asia suture zone
grains from both samples are dominantly euhedral 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 measured 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 reflect the age of the
igneous protolith.
Supplemental Table File [see footnote 1]). The
U-Pb isotope data of each sample spot plotted
on concordia diagrams (Fig. 5) define 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 reflect the age
of zircon overgrowth.
Northern Dome
AY-8 and AY-10 are orthogneiss samples
taken from the footwall of the Ayi Shan detachment in the northern Ayi Shan. The zircon
B
A AY-1-8
AY-2-17,-18
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 Sills
Samples AY-11 and AY-12 are samples of
mylonitized leucogranite from the footwall
of the Ayi Shan detachment in the northern
C
AY-4-12,-21
39.0±0.8 Ma
63.5±1.5 Ma
45.9±0.7 Ma
417.1±27.9 Ma
50 μm
100 μm
AY-6-16,-17
D
100 μm
65.8±1.5 Ma
E
AY-8-6,--7
F
AY-10-3,-10
48.1±1.1 Ma
86.6±1.9 Ma
39.2±0.4 Ma
77.0±2.5 Ma
90.1±4.7 Ma
87.2±2.7 Ma
46.1±0.8 Ma
100 μm
G 34.2±0.5 Ma
38.3±0.4 Ma
50 μm
100 μm
AY-13-14,-15
Figure 7. Cathodoluminescence (CL) images of some zircons analyzed in this study.
50 μm
649.1±35.9 Ma
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Zhang et al.
Ayi Shan. Metamorphic foliation cuts across
the contact between leucogranite and country
rock schist and gneiss. Individual laser spot
206
Pb/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 reflect the ages
of zircon overgrowth.
southern Ayi Shan gneiss dome. 40Ar/39Ar age
spectra from muscovite are flat for the majority 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 flat over the
majority of gas released and yield an age of
10.17 ± 0.04 Ma (Fig. 8). Because both age
spectra are flat we interpret both to be recording
rapid cooling of both rock samples.
40
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 provide 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, respectively. 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 (Honegger 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 orthogneisses is the Gangdese arc. This interpretation
is supported by the observation that both the
DISCUSSION
Protolith of Lower Plate Orthogneisses
Ar/39Ar THERMOCHRONOLOGY
40
Ar/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 spectrum analyses were conducted by step-heating
mineral separates within a double vacuum Mo
resistance furnace at the New Mexico Geochronology 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 Formation, and the other sample (AY-15) comes from
the south side of the Karakoram fault in its footwall (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
AY-14 muscovite
Int age = 10.17 ± 0.04 Ma
upper plate granite and the lower plate orthogneisses 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 ablation analyses define 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 define 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 interpret 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 Gangdese 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 developing topographic high that was being eroded in
the Eocene. We propose that this uplift reflects
AY-15 muscovite
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.
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Crustal thickening and extension along the India-Asia suture zone
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 Whitehouse, 2007; Larson et al., 2010). Our interpretation 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, northward across the India-Asia suture zone to the
Gangdese batholith.
Early to Late Oligocene Extension
Shear sense indicators show that the orthogneisses have undergone southeast-northwest–
directed stretching with dominantly top-tosoutheast 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 thinning 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 investigated field 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 eastern side of the Ayi Shan, this unfortunately led
to poorly constrained interpretations of the timing of geologic events and fault system geometry (compare figure 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 product of long-lived, continuous strike-slip deformation along the Karakoram fault. Valli et al.
(2007) show steeply dipping faults bounding
the Ayi Shan metamorphic core in the northern and southern domes and interpret that the
Karakoram fault and the Great Counter thrust
functioned together as a regional-scale positive
flower structure (transpressional deformation).
Our study extends across the entire range and
shows that the metamorphic rocks are not localized 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 displacement as suggested by Lacassin et al. (2004).
Strike-slip shear sense indicators along the
Great Counter thrust used to support the interpretation 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 field relationships
exposed across the entire range and geochronologic results presented here, we envision a more
complex geologic history than previously suggested (Lacassin et al., 2004; Valli et al., 2007,
2008). Figures 9 and 10 illustrate our interpretation of two cycles of shortening (vertical crustal
thickening) and extension (vertical crustal
thinning) in southwestern Tibet since the Late
Eocene. We envision that deformation represents 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 shortening of the Gangdese arc closely followed
behind or possibly overlaps in time with waning 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
Geosphere, October 2011
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, facilitated 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 Kailas 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 Kailas 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 provenance and paleoflow data, strata relationships,
and lithofacies patterns, that the Kailas Formation was not deposited in an overall contractional setting. Rather, they interpret the bulk of
the formation to have been deposited in a rift or
transtensional strike-slip basin. Their interpretation is consistent with our results, which indicate
that the Kailas Formation was deposited on vertically 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 (Murphy et al., 2009, 2010). Although the relationship 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 consistent 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 structural levels with respect to its position deeper in
the crust (Fig. 9C) (Ratschbacher et al., 1994;
Yin et al., 1999; Murphy and Yin, 2003).
1263
1264
Geosphere, October 2011
Decreased Gravitational
Potential Energy - φ
Increased Gravitational
Potential Energy - φ
Decreased Gravitational
Potential Energy - φ
Increased Gravitational
Potential Energy - φ
MCT
Moho
STD
India
LH duplexes and
imbricate thrusts
MFT
India
India
India
IYS
IYS
Burial
metamorphism
Moho
Moho
Karakoram fault - exposes metamorphosed
Gangdese arc
overthrusting of
Moho
metamorphosed Gangdese arc
Gangdese Arc
Thinning
Thickening
Thinning
Thickening/Burial
Metamorphism
Ayi Shan Events
Moho
Exhumation of metamorphosed
Gangdese arc
Ayi Shan
detachment
Gangdese arc
?
Gangdese arc
IYS - Great Counter thrust
suture zone
NHA
IYS
Tethyan Himalaya
thrust belt
Figure 9. Schematic diagram illustrating our interpretation of our results along the Indus-Yalu suture zone (IYS) (right-hand side of diagram) and how we view
our results in the broader context of the growth of the Himalayan orogenic wedge (left-hand side of diagram). Refer to text for details. Abbreviations: MCT—
Main Central thrust zone; STD—South Tibetan detachment; NHA—Northern Himalayan antiform; LH—Lesser Himalaya; MFT—Main Frontal thrust; φ—
taper angle of orogenic wedge. Black lines indicate active faults, dashed lines are future faults, and gray lines are inactive faults. Geologic symbols as in Figure 2.
(D) 15 Ma to present
(C) 18-15 Ma
(B) 32-26 (18?)Ma
(A) 40-31 Ma
Orogenic Wedge
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Zhang et al.
60
L
50
[Zhu et al. (2005)]
Intiation of collision
55
E
45
M
EOCENE
35
E
30
L
25
20
E
(U-Pb zircon ages, AY-11, 12)
32-26 Ma
Geosphere, October 2011
Ar/39Ar age
(AY-15)
40
5
E
[LPSZ (1), KF (2), GM (3)]
L
PLIOCENE
Extension (LPSZ, KF, GM)
16-14 Ma initiation
Ar/39Ar age
(AY-14)
40
L
10.17 ± 0.04 Ma
10
Shortening (GCT)
15
M
MIOCENE
18.10 ± 0.05 Ma
Extension (Ayi Shan detachment)
(U-Pb zircon ages, AY-4, 5, 7)
40-31 Ma
Shortening
(Burial metamorphism)
40
L
OLIGOCENE
0
QUATERNARY
Crustal thickening and extension along the India-Asia suture zone
Figure 10. Geologic events along the IYS in southwest Tibet based on our geologic mapping and geochronologic studies of rocks exposed in the Ayi Shan and previously published
results. Samples analyzed in this study are denoted by the AY prefix. Age constraints for the last phase of Transtension are from: (1) Thiede et al. (2006), (2) Phillips et al. (2004), and
Murphy and Copeland (2005). Abbreviations: GCT—Great Counter thrust; LPSZ—Leo Parghil shear zone; KF—Karakoram fault; GM—Gurla Mandhata detachment system.
65
E
PALEOCENE
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Zhang et al.
Stage D (15 Ma to Holocene)
The Great Counter thrust is cut by the Karakoram 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 suggests that the Karakoram fault in the vicinity of
the Ayi Shan was active during the Late Miocene. The Leo Parghil range is bounded by the
Leo Parghil shear zone, a top-to-WNW extensional shear zone. Zhang et al. (2000) showed
that its footwall on the SE side of the range contains ductile deformed garnet-bearing schists.
Leucogranite bodies in the footwall locally contain 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 complex (Fig. 2) (Murphy et al., 2000; Murphy and
Copeland, 2005). It is bounded by the top-tothe-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 indicate that extension initiated at ca. 15 Ma. The
timing of extension in this region is broadly the
same as that documented farther east in southcentral Tibet (Lee et al., 2011), and therefore we
interpret this to be a regional event, rather than a
local event associated with transtensional deformation 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 thickening) and energy dissipation (e.g., lateral spreading and hinterland extension) (Hodges et al.,
1996; Hodges, 2000). These processes compete to maintain a self-similar wedge geometry described by the taper angle (e.g., Dahlen,
1990). The first-order structural elements in
the central Himalaya have recently been integrated by Larson et al. (2010) into a conceptual
model describing orogenic wedge develop-
1266
ment characterized by two cycles of accumulation (increase in taper angle) and dissipation
(decrease in taper angle) of gravitational potential 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 forelanddirected lateral spreading resulting in a decrease
in the taper (decrease in gravitational potential
energy); (3) Middle Miocene hinterland thickening 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 deformation 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 thinning 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 interpreted to facilitate foreland-directed spreading 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 interpreted to represent a phase of renewed crustal
thickening and therefore surface uplift and an
increase in the taper angle. This event temporally correlates to the development of the North
Himalayan antiform, an ~700-km-long shortening 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. Initiation 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
Geosphere, October 2011
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 thinning in its hinterland are both processes that
assist in decreasing the taper angle of the orogenic 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
influence of the shape of the orogenic wedge in
controlling the kinematics of orogenic wedges.
CONCLUSIONS
Geologic mapping combined with geochronologic studies of rocks along the IYS in southwest 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 observations indicate that the protolith of orthogneisses exposed in the Ayi Shan is Gangdese
arc rocks and associated early Paleozoic country 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 orthogneisses 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 estimated 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
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Crustal thickening and extension along the India-Asia suture zone
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 hinterland events can be linked to deformation patterns recognized in the Himalayan foreland and
together support the idea that the evolution of
the orogen can be explained, at least qualitatively, 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 supported by a grant from the National Science Foundation (grant EAR 0438826 to Murphy).
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