RESEARCH
Multisystem dating of modern river detritus from
Tajikistan and China: Implications for crustal evolution
and exhumation of the Pamir
Barbara Carrapa1,*, Fariq Shazanee Mustapha1, Michael Cosca2, George Gehrels1, Lindsay M. Schoenbohm3, Edward R. Sobel4,
Peter G. DeCelles1, Joellen Russell1, and Paul Goodman1
DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, ARIZONA 85721, USA
U.S. GEOLOGICAL SURVEY, DENVER FEDERAL CENTER, DENVER, COLORADO 80225, USA
3
DEPARTMENT OF CHEMICAL AND PHYSICAL SCIENCES, UNIVERSITY OF TORONTO–MISSISSAUGA, MISSISSAUGA, ONTARIO L5L 1C6, CANADA
4
INSTITUTE OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITY OF POTSDAM, POTSDAM-GOLM 14476, GERMANY
1
2
ABSTRACT
The Pamir is the western continuation of Tibet and the site of some of the highest mountains on Earth, yet comparatively little is known
about its crustal and tectonic evolution and erosional history. Both Tibet and the Pamir are characterized by similar terranes and sutures that
can be correlated along strike, although the details of such correlations remain controversial. The erosional history of the Pamir with respect
to Tibet is significantly different as well: Most of Tibet has been characterized by internal drainage and low erosion rates since the early
Cenozoic; in contrast, the Pamir is externally drained and topographically more rugged, and it has a strongly asymmetric drainage pattern.
Here, we report 700 new U-Pb and Lu-Hf isotope determinations and >300 40Ar/ 39Ar ages from detrital minerals derived from rivers in China
draining the northeastern Pamir and >1000 apatite fission-track (AFT) ages from 12 rivers in Tajikistan and China draining the northeastern,
central, and southern Pamir. U-Pb ages from rivers draining the northeastern Pamir are Mesozoic to Proterozoic and show affinity with the
Songpan-Ganzi terrane of northern Tibet, whereas rivers draining the central and southern Pamir are mainly Mesozoic and show some
affinity with the Qiangtang terrane of central Tibet. The εHf values are juvenile, between 15 and −5, for the northeastern Pamir and juvenile to
moderately evolved, between 10 and −40, for the central and southern Pamir. Detrital mica 40Ar/39Ar ages for the northeastern Pamir (eastern
drainages) are generally older than ages from the central and southern Pamir (western drainages), indicating younger or lower-magnitude
exhumation of the northeastern Pamir compared to the central and southern Pamir. AFT data show strong Miocene–Pliocene signals at the
orogen scale, indicating rapid erosion at the regional scale. Despite localized exhumation of the Mustagh-Ata and Kongur-Shan domes,
average erosion rates for the northeastern Pamir are up to one order of magnitude lower than erosion rates recorded by the central and
southern Pamir. Deeper exhumation of the central and southern Pamir is associated with tectonic exhumation of central Pamir domes.
Deeper exhumation coincides with western and asymmetric drainages and with higher precipitation today, suggesting an orographic effect
on exhumation. A younging-southward trend of cooling ages may reflect tectonic processes. Overall, cooling ages derived from the Pamir
are younger than ages recorded in Tibet, indicating younger and higher magnitudes of erosion in the Pamir.
LITHOSPHERE
GSA Data Repository Item 2014332
INTRODUCTION
The Pamir Mountains form the western
pro­
longation of the Tibetan-Himalayan collisional orogenic system (Fig. 1), which is the
locus of Earth’s highest mountains and largest
continental plateau. Although both Tibet and
the Pamir are characterized by similar rocks
and tectonostratigraphic architecture (Şengör,
1984; Dewey et al., 1988; Burtman and Molnar, 1993; Schwab et al., 2004; Robinson et al.,
2007, 2012; Robinson, 2009), current debate
exists about the exact correlation of terranes
and sutures along strike (Schwab et al., 2004;
Robinson et al., 2004, 2012; Robinson, 2009).
All models suggest northward displacement
*bcarrapa@email.arizona.edu
of the Pamir with respect to Tibet but differ in
the magnitude of offset along the Karakorum
fault as well in the width of individual geologic
terranes across the orogenic system and in the
degree of correlation (Fig. 2). In particular,
early work by Tapponnier et al. (1981) and the
more recent correlation of Schwab et al. (2004)
call for a large magnitude of slip (~250 km),
whereas others (Searle, 1996; Robinson et al.,
2004; Robinson, 2009) suggest a much smaller
offset (~150 km).
Tibet and the Pamir are strikingly different
in terms of morphology and exhumation history. Tibet is largely characterized by internal
drainage, high elevation, relatively low internal relief (Fielding et al., 1994), and limited
erosion since the early Cenozoic (Rowley and
Currie, 2006; DeCelles et al., 2007a, 2007b;
| Society of America | |For
LITHOSPHERE © 2014
Geological
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permission to copy, contact editing@geosociety.org
doi: 10.1130/L360.1
Rohrmann et al., 2012). Cenozoic exhumation is localized around Miocene rifts (Kapp
and Guynn, 2004) and the southeastern externally drained margin of the plateau (Clark et
al., 2005) and on the frontal (southern) flank
of the Himalaya (Thiede and Ehlers, 2013).
Because of the aridity, glaciers are for the most
part restricted to the crests of mountain ranges
north of the Himalaya (Owen, 2009), and
glacial erosion has not significantly affected
landscape morphology. The Pamir, in contrast,
is mostly externally drained, has high topographic relief (>2–3 km), and contains widely
exposed high-grade metamorphic domes that
have been exhumed since early Miocene time
(Fig. 1; Schwab et al., 2004; Schmidt et al.,
2011; Lukens et al., 2012). A striking feature
of the Pamir is the asymmetry in morphology
80°E
INDIA
100°E
20°N
30°N
D arvaz Faul
t
Triassic-Jurassic
Cretaceous
Paleozoic
Precambrian
Triassic-Jurassic
Cretaceous
Paleozoic/
undifferentiated
Murghab
1071-4
Strike Slip Fault
Town Locations
Sample Locations
1071-3
1071-2
gh
ab
1071-6
1071-7
Oytag
Tashkorgan
TJK-04
1071-1
1071-5
Muji
lt
ty
Kashgar (Kashi)
76°E
River sand samples
Western Pamir
TJK4 = Murghab River
TJK5 = Gunt River
TJK6 = Bartang River
TJK7 = Yazgulem River
TJK8 = Vanj River
Northeastern Pamir
1071- 3, 6, 7 = Tashkorgan River tributaries
1071- 1, 2 = Gez River
1071- 4 = Kalate River
Hindu Kush
Ak
s
F
Southern Pamir t
TJK-05
Thrust Fault
Normal fault
Khorog
TJK-06
TJK-07
Central Pamir
-Alai Range
Trans
Northern Pamir
Alai Valley
ur
M
u l
au
Neogene-Paleogene
an)
n Sh
Tie
TJK-08
(
A l a i Ra n g e
74°E
u
Fa
Cenozoic
Igneous Rocks
Tajik depression
90°E
Tibet
40°N
72°E
am
kor
a
r
Ka
Quaternary
Sed-Metased Rocks
Pamir
Tarim Basin
EURASIA
70°E
Al
100
lt
40°N
36°N
38°N
N
150 km
Tari
mB
asin
50
n -Tag
h Fau
0
78°E
Figure 1. (A) Inset map showing political borders. (B) Simplified geologic map of the Pamir, showing lithologic units and main sutures, compiled after Bershaw et al. (2012), Lukens et al.
(2012), Robinson et al. (2007), and Teraoka and Okumura (2007).
B
70°E
Tajik depression
A
68°E
CARRAPA ET AL.
| www.gsapubs.org | LITHOSPHERE
| RESEARCH
Multisystem dating of modern river detritus from the Pamir 70°E
40°N
80°E
A
90°E
Tajik
depression
B RPS
Pamir
Tarim Basin
TS
Qaidam
SS
AKMS
D
ram
Tibetan Plateau
BNS
Fa
u lt
India
in F
ront
200km
Hi m a
layas
70°E
al Th
rust
90°E
80°E
100°E
90°E
100°E
Tien Shan
Tajik
depression
Tarim Basin
B RPS TS
Pamir
Qaidam
SS
AKMS
D
ram
Tibetan Plateau
BNS
Fa
u lt
India
in F
ront
200km
Hi m a
layas
70°E
IYS
al Th
rust
80°E
70°E
30°N
Lhasa
Ma
90°E
80°E
C
Songpan-Ganzi
JS
Qiangtang
o
rak
Ka
30°N
0
40°N
Altyn Tagh Fault
MPT
A
C
40°N
IYS
80°E
70°E
30°N
Lhasa
Ma
B
Songpan-Ganzi
JS
Qiangtang
o
rak
Ka
30°N
0
40°N
Altyn Tagh Fault
MPT
A
C
40°N
100°E
Tien Shan
100°E
100°E
90°E
Tien Shan
Tajik
depression
Tarim Basin
Pamir
C
Qaidam
SS
AKMS
D
Qiangtang
o
rak
Ka
Fa
70°E
GEOLOGICAL SETTING OF THE PAMIR
30°N
Lhasa
Ma
India
Songpan-Ganzi
BNS
u lt
200km
JS
Tibetan Plateau
ram
30°N
0
40°N
Altyn Tagh Fault
MPT
A
B RPS TS
in F
ront
Hi m a
layas
IYS
al Th
rust
80°E
90°E
100°E
Figure 2. Simplified tectonic map of the Indo-Asian collision zone showing major active structures
and suture zones (after Burtman and Molnar, 1993; Yin and Harrison, 2000). (A) Terrane correlations
after Schwab et al. (2004); (B) terrane correlations after Robinson et al. (2004); (C) terrane correlations after Robinson (2009) and Robinson et al. (2012). Abbreviations: MPT—Main Pamir thrust;
IYS—Indus-Yarlung suture; BNS—Bangong-Nujiang suture; JS—Jinsha suture; AKMS—AyimaqinKunlun-Muztagh suture; RPS—Rushan-Pshart Suture; TS—Tanymas Suture. Terranes of western
Indo-Asian collision zone are: A—Northern Pamir; B—Central Pamir; C—Southern Pamir–Karakorum–Hindu Kush; D—Kohistan arc.
LITHOSPHERE | | www.gsapubs.org
and climate from west to east (Fig. 3). The
west side of the range receives up to 60 cm/yr
in rainfall, dominantly delivered by the midlatitude westerlies during the spring (Aizen et al.,
2001). Rivers are deeply incised, and the drainage divide is displaced far to the east within
the range. The western Pamir hosts some of
the largest glaciers outside of the polar regions
and the Himalaya and Karakoram (Fuchs et
al., 2013), including the Fedchenko glacier in
Tajikistan. Precipitation drops off dramatically
eastward across the Pamir to less than 10 cm/yr
by the midpoint of the range. Glaciers and permanent snow cover correspondingly decline
(Fuchs et al., 2013). Eastern drainages are
much less dissected and are dominated by the
structural basin in the hanging wall of the Kongur extensional system (Robinson et al., 2004).
A wealth of thermochronological, geochronological, and structural data exists from Tibet
(e.g., Copeland et al., 1995; Ratschbacher et al.,
1996; Murphy et al., 1997; Kapp et al., 2007;
Jolivet et al., 2001; Kirby et al., 2002; Rohrmann
et al., 2012), whereas only sparse data exist for
the Pamir (e.g., Amidon and Hynek, 2010; Robinson et al., 2012; Sobel et al., 2013; Stübner et
al., 2013a, 2013b; Thiede et al., 2013), leaving
the exhumation and tectonic history of this part
of the orogenic system largely unresolved. Here,
we apply geochronology, thermochronology,
and Hf isotope geochemistry to modern river
sand grains in order to determine the timing of
crustal evolution and the timing and pattern of
exhumation of the Pamir. The rivers sampled for
this study drain not only metamorphic domes
but also Paleozoic–Mesozoic and older rocks
(Fig. 1); therefore, the ages presented here represent the timing of regional tectonic and erosional processes. We also discuss the role that
climate-enhanced erosion may play in explaining the geomorphic and erosional differences
between the Pamir and Tibet.
The Pamir Mountains occupy a roughly
120,000 km2 region extending ~360 km north
from the Hindu Kush Mountains to the Alai
River valley and the Main Pamir thrust system
(Fig. 1), a group of south-dipping thrust faults
along which crust of the Tarim Basin and Tajik
depression has been underthrust beneath the
Pamir to a depth of at least 200 km (Schneider
et al., 2013). The Pamir is bounded by major
strike-slip faults on its eastern and western
flanks, and it is composed internally of three
or four large crustal blocks (or terranes) that
accreted onto Eurasia from Paleozoic to early
Cenozoic time (Burtman and Molnar, 1993).
The regional geology of the Pamir is dominated
0
0
105 cm/yr
70°E
1 cm/yr
Precipitation
B
69E
A
70°E
40°N
39°N
73°E
R.
Yazgulem
Dome
Sares Dome
73°E
76°E
TJK-04
Murghab R.
1071-1
_
^
75°E
-Ta
g
^^
_
_
Tashkorgan
1071-6
_
^
Tashko1071-3
rgam Rive
1071-7
r
ul
t
77°E
_1071-6
^
^1071-7
_
_ Tashkorgan R.
^
1071-3
Kongur ShanMustagh Ata Domes
76°E
21 m
8238 m
77°E
Elevation
Kashgar (Kashi)
75°E
^^
_
_1071-2
1071-4
_
^
74°E
Bartang R.
^
_
Moskol Dome
_
^
_
^
TJK-06
Gunt R.
_
^
Shakdara Dome
TJK-05
j
R.
an
em
TJK-08 V zgul
Ya
TJK-07
100km
71°E
72°E
_TJK-4
^
^
_
Murghab
_
^
_
TJK-6^
TJK-5
_ Southern Pamir
^
Khorog
TJK-8
TJK-7
100km
74°E
^1071-4 1071-5
_
Pamir
^1071-1
_
1071-2 Oytag
^
Muji _
Gez River
Central Pamir
t
hrus
mir t
a
P
Main
e
Northern
utur
lun s
re
Kun
sutu
s
a
m
y
n
Ta
Shan)
ien
T
(
A l a i Ra n ge
Alai Valley
72°E
0
100
200
D
0.40
0.60
0.80
1.00
300
400
0
1500
0.2
0.4
0.6
0.8
1
2000
distance (kilometers)
3000
4000
600
elevation (m)
500
drainage divide
5000
E
6000
0
0
700
10
20
30
40
50
60
Figure 3. (A) Digital elevation model (DEM) and shaded relief map of the Pamir on Shuttle
Radar Topography Mission (SRTM) base (http://www2​.jpl​.nasa.gov​/srtm/). Sample locations and upstream drainage areas are marked with stars and shaded regions, respectively. Major structures are indicated with black lines and standard structural symbols.
Location of swath profile (Fig. 3C) is indicated by black rectangle. (B) Topographic map
of the Pamir with drainages and precipitation, showing winter (December-January-February) average, long-term mean (1948–2007) rainfall data shown on shaded relief SRTM
base. Precipitation data are from the Precipitation Reconstruction Over Land data set
(Chen et al., 2002) with a spatial resolution of 0.5° × 0.5°. Location of the metamorphic
domes is indicated by black outlines with labels. Note that the location of the Sares
dome is from Schmidt et al. (2011) and differs from the location in Schwab et al. (2004).
Watersheds (gray) and sample sites (stars) are indicated. (C) Swath profile and precipitation data for box shown in A and B. Gray shaded region indicates maximum and minimum elevation, and black line indicates mean. Drainage divide, pushed far to the east,
is indicated. Relief is higher on the western side of the Pamir. Precipitation data show a
strong decrease from west to east. (D) Cumulative hypsometric curves for western (cool
colors) and eastern (warm colors) drainages. Note lower curves for less-incised eastern
drainages. (E) Hypsometric curves (same colors as D). Note plateau-like shape of eastern
drainages and sharper peaks of western drainages. Colors in D and E are from drainages
in A. SS—Shyok Suture.
relative area a/A
0.20
older
younger
(i.e. more active landscape)
C
0.00
0.00
0.20
0.40
0.60
0.80
1.00
0
1000
2000
3000
4000
5000
6000
elevation (meters)
relative altitude h/H
71°E
t
ar
Ru
su sha
tu nre Ps
h
38°N
37°N
36°N
40°N
Fa
39°N
m
ora
rak
a
K
38°N
n
37°N
Alt y
Kalate R.
lt
au
hF
Gez R.
precipitation (cm/a)
36°N
frequency
69°E
CARRAPA ET AL.
| www.gsapubs.org | LITHOSPHERE
| RESEARCH
Multisystem dating of modern river detritus from the Pamir by five domal structures (Fig. 3) developed in
medium- to high-grade metamorphic rocks
that yield pressure-temperature-time estimates
consistent with exhumation from midcrustal
depths, mostly during the Miocene (Schmidt et
al., 2011; Stübner et al., 2013b).
The Pamir terranes are referred to as the
Northern, Central, and Southern Pamir (Fig. 1)
and have been correlated in different ways to
terranes in Tibet (e.g., Schwab et al., 2004;
Robinson et al., 2007, 2012; Robinson, 2009).
Different correlation models (Fig. 2) have been
discussed in detail by Schwab et al. (2004) and
Robinson et al. (2004, 2012). The Schwab et
al. (2004) correlation model is based on geologic structures and geochemistry of magmatic
belts in the Pamir-Tibet orogen. This study correlates the Karakul granites, which are found
in parts of the Northern Pamir terrane, to the
Songpan-Ganzi terrane in Tibet. This correlation is supported by a continuous chain of
ca. 200 Ma plutons with similar geochemical
signatures that wraps around the Pamir from
the Karakul Basin into the Mazar region of
western Kunlun (Schwab et al., 2004). In the
Central Pamir terrane, the presence of midTriassic granitoids similar to those observed
in the Qiangtang block led the same authors to
propose a correlation between these terranes.
Schwab et al. (2004) also documented a longlasting Cretaceous magmatic history of intrusions in the Southern Pamir terrane. U-Pb geochronology of xenoliths in this region yielded
clusters of ages at 84–57 Ma, 170–146 Ma,
465–412 Ma, 890 Ma, and 1400 Ma, with the
youngest zircon age at 56.7 ± 5.4 Ma (Ducea et
al., 2003). These ages correlate with magmatic
activity of the Tirich Mir–Karakoram–Gangdese arc, which was active in the northernmost
part of the Southern Pamir and Hindu Kush–
Lhasa block and includes suites of granite and
granodiorites that yield crystallization ages of
ca. 1624 Ma, 902 Ma, 417 Ma, 355 Ma, and
196 Ma (Schwab et al., 2004). The proposed
correlation of the Northern Pamir terrane to
Songpan-Ganzi, the Central Pamir terrane to
Qiangtang, and the Southern Pamir terrane to
Lhasa by Schwab et al. (2004) implies a displacement of ~250 km along the Karakorum
fault (Fig. 2A).
The correlation model proposed by Robinson et al. (2004) (Fig. 2B), suggests a smaller
offset along the Karakorum fault of <200 km
(120–150 km—Searle, 1996; Searle et al., 1998;
60–70 km—Murphy et al., 2000). This model
correlates the Tanymas suture south of the
Northern Pamir to the Ayimaqin-Kunlun-Muztagh suture, suggesting a correlation between
the Northern Pamir terrane and the KunlunQaidam terrane in Tibet. Similarly, by correlat-
LITHOSPHERE | ing these suture zones, the Central Pamir terrane
is matched to the Songpan-Ganzi, the Southern
Pamir terrane is matched to the Qiangtang, and
the Kohistan arc is correlated to the Lhasa terrane (Searle, 1996; Searle et al., 1998; Murphy
et al., 2000). Yin and Harrison (2000) considered the Lhasa terrane and Kohistan arc to be a
continuous magmatic arc.
The correlation model of Robinson (2009)
(Fig. 2C) is a slight modification of the Robinson et al. (2004) model and is based on remote
mapping of the Late Triassic–Early Jurassic
Aghil Formation, which implies 149–167 km of
offset along the northern and southern portions
of the Karakorum fault. This study suggests a
correlation between the Bangong-Nujiang and
Shyok sutures, consistent with the interpreted
maximum offsets of the Miocene Baltoro granite (40–150 km). Further analysis of the Aghil
Formation shows that the antiforms in both
the Central Pamir and Qiangtang are not offset by the Karakorum fault. This model correlates the Northern Pamir with Songpan-Ganzi,
the Southern Pamir with Qiangtang, and the
Kohistan arc with Lhasa. This model also suggests that the Central Pamir is a crustal fragment
with no equivalent in Tibet (Burtman and Molnar, 1993; Robinson, 2009). Burtman (2010)
interpreted the Central Pamir to be a partially
rifted portion of the Southern Pamir–Qiangtang
terrane. More recent geochronological data
from the northeastern Pamir support correlation
of the Northern Pamir with the Songpan-Ganzi
terrane (Robinson et al., 2012). Despite differences between correlation models, all agree that
Pamir terranes have been offset northward relative to their Tibetan counterparts by slip along
the Karakoram strike-slip fault (Burtman and
Molnar, 1993; Lacassin et al., 2004; Schwab et
al., 2004; Robinson, 2009; Peltzer and Tapponnier, 1988).
DETRITAL APPROACH TO RESOLVING
TECTONICS AND EROSION
Geochronology and thermochronology
applied to detrital minerals provide information on the timing of crystallization, cooling,
and exhumation of the river catchment areas
and thus on regional tectonic and erosional
processes (e.g., Hodges et al., 2005; Carrapa,
2010). In particular, U-Pb geochronology combined with Lu-Hf geochemistry applied to zircons provide information on crystallization age
and crustal evolution (e.g., Patchett et al., 1982;
Griffin et al., 2000; Dickinson and Gehrels,
2009). The 40Ar/39Ar thermochronology method
applied to white mica and apatite fission-track
(AFT) thermochronology document the time of
mineral crystallization and cooling through the
| www.gsapubs.org
~350–80 °C temperature window (McDougall
and Harrison, 1999; Gleadow and Duddy, 1981;
Green et al., 1986), which corresponds to ~12–
2.5 km of crust removal (assuming a conservative 30 °C/km paleogeothermal gradient). Erosion and normal faulting (tectonic exhumation)
are responsible for cooling and exhumation of
Earth’s crust (England and Molnar, 1990). High
relief, precipitation, and active deformation can
enhance exhumation (Ring et al., 1999).
Detrital thermochronology is particularly
useful in areas where sampling in situ rocks
is challenged by high relief and the paucity of
roads, such as in the Pamir. In this study, we
let nature do the job of sampling the orogenic
system by utilizing rivers. We collected 12 samples of medium-grained sand from major rivers
draining catchments within the Northern, Central, and Southern Pamir terranes (Figs. 1 and
3); of these, nine samples yielded mica, and 11
yielded apatites. We present data from four river
samples from the Central and Southern Pamir
(western drainages), one from the interior of the
Pamir (TJK4) of Tajikistan, and seven river samples from the northeastern Pamir of China. The
Tajik Pamir samples were previously analyzed
for zircon U-Pb and 40Ar/ 39Ar geochronology
and thermochronology (Lukens et al., 2012).
Lu-Hf geochemistry and AFT thermochronology are here applied to all samples; U-Pb geochronology and 40Ar/39Ar thermochronology are
applied to the northeastern Pamir samples.
The Tajik river samples were collected
from the Vanj, Yazgulem, Bartang, Gunt, and
Murghab Rivers and drain a larger area than
the Chinese rivers, including mainly the Central and Southern Pamir and only part of the
Northern Pamir (Fig. 3). Drainage areas of
the Tajik rivers include Paleozoic sedimentary and basement rocks, Mesozoic and Cenozoic magmatic rocks, and large metamorphic
domes (Vlasov et al., 1991; Burtman and Molnar, 1993). The samples from the northeastern
Pamir were collected from the Kalate and Gez
Rivers and tributaries of the Tashkorgan River
that flow eastward, debouching into the Tarim
Basin (Fig. 3). The Chinese rivers drain the
northeastern Pamir, which is dominated by
Triassic and Jurassic sedimentary and metamorphic rocks (Fig. 1; Robinson et al., 2007).
Out of the seven samples we collected from
the northeastern Pamir, four (1071-1, 1071-2,
1071-4, 1071-6) come from the footwall of the
Kongur Shan normal fault, whereas the other
three (1071-3, 1071-5, 1071-7) come from
the hanging wall. Together, the sampled rivers
drain a land surface area, representing roughly
90% of the Pamir; the data provide the first
regional-scale assessment of exhumation ages
for almost the entire range.
CARRAPA ET AL.
A
P09D 1071-4 (n=65)
P09D 1071-5 (n=92)
P09D 1071-1 (n=90)
Age probability
P09D 1071-2 (n=98)
P09D 1071-7 (n=87)
P09D 1071-3 (n=85)
Southern Pamir source
P09D 1071-6 (n=88)
B
0
500
1000
1500
2000
2500
3000
Northeastern Pamir,
Eastern drainages
(this study; Bershaw et al., 2012)
Age probability
Zircon U-Pb and Hf geochronology was
conducted by laser ablation–multicollector–
inductively coupled plasma–mass spectrometry
(LA-MC-ICP-MS) at the Arizona LaserChron
Center (Gehrels et al., 2006, 2008; Cecil et al.,
2011; Gehrels and Pecha, 2014). For details on
analytical procedure, we refer the reader to the
GSA Data Repository1 and https://sites.google​
.com/a/laserchron.org/laserchron/. For zircon
U-Pb data from the Tajik rivers, see Lukens et
al. (2012).
In total, 700 zircons from all seven samples
from the northeastern Pamir were analyzed. Out
of the 700 grains, 605 were included in the data
reduction processes (Table DR1 [see footnote
1]). The excluded grains include those with high
206
Pb/238U error (>20%), high 204Pb (>100 cps
[counts per second]), and grains that produced
analyses that were >20% discordant or >5%
reverse discordant. Selected zircons were analyzed for Lu/Hf geochemistry, with an average
of 20 grains analyzed per sample.
Samples 1071-1, 1071-2, 1071-3, 1071-4,
and 1071-5 from the Gez and Tashkorgan Rivers
show mainly Permian–Triassic zircon U-Pb ages
(ca. 200–300 Ma; Fig. 4). A minor component at
400–500 Ma is present in most samples. Sample
1071-6, and to a lesser extent sample 1071-1,
contains a component at ca. 80 Ma. Sample
1071-4 contains a small Miocene (ca. 16 Ma)
component. Zircons from the Tajik rivers draining the Central and Southern Pamir terranes produced Proterozoic through Cenozoic U-Pb ages,
exhibiting affinity with Asian rocks. These data
were presented by Lukens et al. (2012) and will
be further discussed in this paper.
The Hf isotopic data from the zircon grains
exhibit widely variable εHf values (Table DR2
[see footnote 1]), ranging from +15 to −40
(Figs. 5A and 5B). A few highly juvenile values
(within 5 epsilon units of the depleted mantle
array) are present for grains between 300 and
900 Ma from the northeastern Pamir. Zircon
grains that crystallized between 600 and 200 Ma
have intermediate εHf values (near chondrite uniform reservoir [CHUR]).
Northeastern Pamir
Central, Southern Pamir,
Western drainages
(Lukens et al., 2012)
0
500
C
1000
TIBET
1500
2000
2500
3000
Northern Tibet/Songpan Ganzi (Weislogel et al., 2006; n=807)
Central Tibet/Qiangtang (Gehrels et al., 2011; n=2431)
Age probability
U-Pb and Lu-Hf Geochronology Methods
and Results
Souther Tibet/Lhasa (Gehrels et al., 2011; n=733)
Southern Pamir source
PAMIR
Northern Pamir (n=605)
Central Pamir (n=169)
U-Pb and Lu-Hf Data Interpretation
Southern Pamir (n=87)
We interpret the zircon U-Pb ages from the
northeastern Pamir to dominantly reflect Trias1
GSA Data Repository Item 2014332, Analytical details and data tables, is available at www​
.geosociety​.org​/pubs​/ft2014.htm, or on request from
editing@​geosociety​.org, Documents Secretary, GSA,
P.O. Box 9140, Boulder, CO 80301-9140, USA.
0
500
1000
1500
Age (Ma)
2000
2500
3000
Figure 4. Age probability plots for U-Pb geochronology. (A) Crystallization ages for northeastern
Pamir samples. (B) Comparison of crystallization ages between northeastern and western Pamir.
(C) Comparison of crystallization ages between Pamir and Tibet terranes. Shaded dark-green and
red box highlight the similarities between the corresponding signals from the different terranes
(Gehrels et al., 2011; Weislogel et al., 2006).
| www.gsapubs.org | LITHOSPHERE
| RESEARCH
Multisystem dating of modern river detritus from the Pamir A
20
Northeastern Pamir
Eastern drainages DM
15
10
5
CHUR
Epsilon Hf
0
-5
-10
-15
-20
1071-1
1071-2-P1
1071-3-P1
1071-4-P1
1071-5 P1
1071-6-P1
1071-7-P1
-25
-30
-35
-40
-45
B
0
200
400
600
800
1000
Age (Ma)
20
1200
1400
1600
Central, Southern Pamir
Western drainages DM
15
10
5
CHUR
0
Epsilon Hf
1800
-5
-10
-15
-20
-25
TJK-04
TJK-05
TJK-06
TJK-07
TJK-08
-30
-35
-40
-45
C
0
200
400
600
800
1000
1200 1400
Age (Ma)
20
1600
1800
2000
2200
2400
Pamir vs. Tibet
15
10
DM
5
CHUR
0
Epsilon Hf
-5
-10
-15
-20
-25
Central-S. Pamir
-30
NE Pamir
Qiangtang
Songpan Ganzi
Lhasa
-35
-40
-45
0
200
400
600
800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000
Age (Ma)
Figure 5. (A–B) εHf data of the analyzed zircons from the (A) northeastern Pamir and (B) western Pamir.
(C) Compilation of Pamir vs.Tibet Hf data (Songpan-Ganzi terrane data from Zhang et al. [2013]; Qiangtang terrane data from Zhai et al. [2013a, 2013b, 2013c]; Lhasa terrane data from Wu et al. [2010]). Refer
to Table DR2 (see text footnote 1) for more details and to Lukens et al. (2012) for zircon U-Pb ages from
the central and southern Pamir (western drainages). The gray arrow shows typical crustal evolution
with 176Lu/177Hf = 0.0115 (Vervoort and Patchett, 1996; Vervoort et al., 1999). Chondrite uniform reservoir
(CHUR)—Bouvier et al. (2008). Deplete mantle (DM)—Vervoort and Blichert-Toft (1999, 2009). ).
LITHOSPHERE | | www.gsapubs.org
sic magmatism recorded in the Kunlun arc of the
Karakul-Mazar Songpan-Ganzi terrane (Schwab
et al., 2004). Early Paleozoic ages (ca. 400–
500 Ma) may be associated with sources from
the northern and southern Kunlun magmatic
belts as suggested by Schwab et al. (2004). Samples 1071-5 and 1071-7, which were collected
close to the village of Oytag and the easternmost
section of the Tashkorgan River, record a different signal mainly characterized by early Paleozoic ages (ca. 400–600 Ma). Cretaceous ages
in sample 1071-6, and to a lesser extent sample
1071-1, could be derived from Cretaceous plutons (Jiang et al., 2014). In general, the zircon
U-Pb ages from rivers draining the northeastern
Pamir differ from the zircon U-Pb ages of rivers
draining the central and southern Pamir (Fig. 4);
western Pamir Rivers contain stronger 300 and
500 Ma components.
Juvenile εHf values for grains between 900
and 300 Ma record the generation of new continental crust during Neoproterozoic through
middle Paleozoic time. Intermediate εHf values
(near CHUR) record interaction with older
crust, perhaps of Mesoproterozoic age (Fig. 5).
Beginning at ca. 180 Ma, more variable εHf values, with some values near CHUR, record the
presence of younger (Neoproterozoic) crust as
well as more negative values that record the
presence of evolved crust. Overall ,the Hf data
are broadly similar to Hf results that have been
reported from Tibet (Fig. 5), supporting the idea
that crustal domains of the Pamir and Tibet have
common petrogenesis.
40
Ar/39Ar and AFT Thermochronology
Methods and Results
The 40Ar/39Ar analyses of white mica from
the northeastern Pamir samples were conducted at the U.S. Geological Survey Laboratory in Denver. Samples were irradiated for
10 MWH (Megawatt per hour) in the central
thimble position of the Denver USGS TRIGA
reactor with cadmium shielding. Sanidine from
the Fish Canyon Tuff was used as the neutron
influence monitor with a reference age of 28.20
± 0.8 Ma (Kuiper et al., 2008). For more details,
see Table DR3 (see footnote 1). For 40Ar/39Ar
analyses of Tajik rivers draining the central and
southern Pamir, we refer the reader to Lukens
et al. (2012). AFT analyses were conducted
at the University of Arizona following procedures described in Table DR4 (see footnote
1). In total, 330 white micas were analyzed by
40
Ar/39Ar thermochronology from the northeastern Pamir rivers, and 1154 grains from the 12
samples draining both the northeastern and central and southern Pamir were analyzed for AFT
thermochronology.
CARRAPA ET AL.
The 40Ar/39Ar single-grain ages range from
ca. 350 Ma to ca. 8 Ma (Fig. 6A). AFT age
density distributions and detrital populations
(calculated using DensityPlotter and Binomfit;
Brandon, 2002; Vermeesch, 2012) are mainly
late Cenozoic, but with a tail of early Cenozoic
ages (Fig. 6B); the youngest AFT population
is ca. 6 Ma (Fig. 7; Tables DR4 and DR5 [see
footnote 1]). Samples from the Vanj (TJK-8),
Yazgulem (TJK-7), Gunt (TJK-5), Bartang
(TJK-6), and Murghab (TJK-4) Rivers in Tajikistan, draining mostly the central and some of
southern Pamir, including the Yazgulen, Sares,
Muskol, and Shakdara metamorphic domes,
have prominent 40Ar/39Ar age components at
ca. 18 Ma and ca. 25 Ma (Lukens et al., 2012)
and late Miocene AFT ages (Fig. 6), with
youngest AFT populations between ca. 8 Ma
and ca. 13 Ma (Fig. 7). Samples from the Kalate
River (1071-4) and tributaries of the Gez (10711, 1071-2) and Tashkorgan (1071-3, 1071-7,
1071-6) Rivers, draining the northeastern Pamir,
including the Kongur Shan and Mustagh Ata
domes (Fig. 3), have prominent 40Ar/39Ar age
components at ca. 80 Ma, 100 Ma, and between
ca. 150 Ma and 200 Ma (Fig. 6) and early to
late Cenozoic AFT ages; the youngest AFT age
populations are between ca. 6 Ma and 22 Ma
(Fig. 7; Table DR5 [see footnote 1]).
Ar/39Ar and AFT Data Interpretation
40
Overall, the 40Ar/ 39Ar ages from rivers
drain­ing the northeastern Pamir are older than
40
Ar/ 39Ar ages from rivers draining the central
and southern Pamir, indicating older exhumation for the northeastern Pamir. Alternatively,
this pattern could indicate that exhumation in
the northeastern Pamir has been insufficient
to expose rocks recording Cenozoic 40Ar/ 39Ar
ages. Higher-magnitude exhumation of the
western Pamir is supported by Miocene synorogenic deposits >6 km thick preserved in the
Tajik depression (Nikolaev, 2002), indicating
rapid erosion of the Pamir at this time.
Detrital AFT ages of ca. 22–24 Ma are
consistent with initiation of exhumation of the
northeastern Pamir, possibly as the result of
southward-directed subduction or underthrusting of Asia (Amidon and Hynek, 2010; Sobel
et al., 2013). The presence of sparse late Miocene 40Ar/39Ar and AFT ages (between ca. 6 and
ca. 9 Ma) in the northeastern Pamir coincides
with the timing of tectonic exhumation of the
Kongur Shan detachment system as recorded by
regional in situ thermochronological and structural data (Robinson et al., 2004, 2007; Sobel
et al., 2011, 2013; Thiede et al., 2013; Cao et
al., 2013a) and supported by detrital zircon
fission-track ages from the Gez River (Cao et
al., 2013b). Older Cenozoic thermochronological ages in the northeastern Pamir have been
interpreted to be the result of an early phase of
subduction erosion along the Trans-Alai intracontinental suture (Sobel et al., 2013).
The new AFT ages from western Pamir range
from early to late Cenozoic. In particular, ages
from the Bartang River draining the southern
Pamir, including the Shakdara dome, show a significant late Cenozoic component (ca. 8–13 Ma;
Fig. 6) that is consistent with ages recorded in
basement rocks of the Shakdara dome (Stübner
et al., 2013b), suggesting that late-stage exhumation of the dome occurred during the late Miocene. The same signal is present in the Gunt and
Yazgulem River samples, indicating regional
exhumation of the Northern, Central, and Southern Pamir terranes during the Cenozoic.
Hypsometric Analysis
Hypsometry, hypsometric integral, and
cumulative hypsometry were calculated using
Shuttle Radar Topography Mission (SRTM)
90 m digital elevation data (bin size 100 m)
for four drainages in the central and southern Pamir representing the upstream drainage
areas for samples TJK-8, TJK-7, TJK-6, and
TJK-5, and three drainages in the northeastern
Pamir representing upstream drainage areas
for samples 1071-4, 1071-1, 1071-2, 1071-3,
1071-7, and 1071-6 (Fig. 3).
Hypsometric integrals for the western
drainages are, from north to south, 0.46 (corresponding to TJK-8), 0.51 (TJK-7), 0.49
(TJK-6), and 0.57 (TJK-5), with an average of
0.51 ± 0.4. Cumulative hypsometric curves are
S-shaped (Fig. 3D), and hypsometry displays
a prominent peak at around 4200 m (Fig. 3E).
The two smaller, northern drainages are more
asymmetric, with longer tails at low elevations.
Hypsometric integrals for the eastern drainages are lower. From north to south, values are
0.49 (corresponding to 1071-4), 0.42 (1071-1,
1071-2), and 0.45 (1071-7), with an average
of 0.45 ± 0.04. Curves are S-shaped but plot
slightly lower than the western cumulative hypsometries (Fig. 3D). The hypsometry displays
a broad plateau from ~3500 to 5000 m for the
southern two drainages. The plateau in the eastern drainage hypsometry could reflect glacial
erosion or the presence of the depositional basin
in the hanging wall of the Kongur extensional
system at an elevation of 3500–4500 m. Higher
hypsometric integrals in the western drainages
are consistent with evidence for more intense
fluvial erosion on that side of the range, which
includes higher precipitation, higher relief, and
the position of the divide well east of the midpoint of the range (Fig. 3).
DISCUSSION
Terrane Correlation and Crustal Evolution
The zircon U-Pb data from modern fluvial
sediments in rivers draining the Pamir document
an Asian crustal provenance. Compiled ages
yield three main zircon U-Pb age components
in the Pamir (Fig. 4). The youngest age group
contains Cenozoic and Cretaceous populations
younger than 100 Ma and constitutes more than
50% of the age spectra. The other two major age
components are the Permian–Triassic (ca. 200–
300 Ma) and Paleozoic (ca. 400–600 Ma), which
together represent less than 50% of the entire
age spectra. A striking regional difference is the
higher proportion of younger grains in central
and southern Pamir (western drainages) compared with northeastern Pamir rivers (Fig. 4).
The younger (<100 Ma) detrital zircon U-Pb
ages from rivers draining the central and southern Pamir are consistent with Cenozoic intrusions in the central Pamir domes (Schwab et al.,
2004). Therefore, we interpret the younger than
100 Ma detrital ages to mostly represent sources
from these domes and deep exhumation.
Schwab et al. (2004) correlated the central
Pamir to the Qiangtang block in Tibet. Detrital
U-Pb ages from our river samples support this
interpretation (Fig. 4). However, apart from the
small but significant presence of Paleozoic signals (ca. 400–600 Ma) in all the samples draining the central and southern Pamir (western
drainages), all samples are dominated by grains
younger than 100 Ma, suggesting an affinity
with Gangdese arc rocks of the Lhasa terrane in
Tibet (Ji et al., 2009). In contrast, zircon U-Pb
ages from the northeastern Pamir are mostly
older than 100 Ma and show affinity with the
Kunlun and Songpan-Ganzi arc rocks (Robinson et al., 2012). We note that sample 1071-6
(located in northeastern Pamir) contains a large
group of 80–100 Ma grains. We attribute this
to the fact that this sample comes from a river
draining part of the Southern Pamir terrane. Figure 4 illustrates the similarity of detrital patterns
between the Northern Pamir terrane (including
sample TJK-8 from Lukens et al., 2012) and
Songpan-Ganzi with two distinctive peaks at
ca. 200 Ma and ca. 550 Ma. Gehrels et al. (2011)
reported 212–537 Ma ages from the SongpanGanzi complex with peaks at ca. 264 Ma and
440 Ma, 720–850 Ma ages with age peaks at
770 and 793 Ma, 1730–2100 Ma ages with an
age peak at 1870 Ma, and 2360–2630 ages with
an age peak at 2515 Ma.
Based on the U-Pb zircon analyses of rivers
draining the Northern Pamir terrane, we make
the following interpretations: (1) The strong
200–300 Ma age component coincides with
| www.gsapubs.org | LITHOSPHERE
| RESEARCH
Multisystem dating of modern river detritus from the Pamir A
1071-4
(Kalate River; n=84)
B
Gez, Kalate rivers
(1071-1, 2, 4; n=300)
Eestern Pamir drainages
Northern Pamir
1071-1, n=78
Gez River
0
1071-2, n=83
10 20 30 40 50 60 70 80 90 100
Tashkorgan tributaries
(1071-3, 6, 7)
n=242
1071-6
(Tashkorgan tributary; n=85)
0
10 20 30 40 50 60 70 80 90 100
Vanj River
n=100
TJK8 (Vanj River; n=69)
TJK7 (Yazgulem River; n=84)
10 20 30 40 50 60 70 80 90 100
Yazgulem River
n=100
Western Pamir drainages
Central Pamir
0 10 20 30 40 50 60 70 80 90 100
TJK4 (Murghab River; n=54)
Murghab River
n=100
0 10 20 30 40 50 60 70 80 90 100
Bartang River
n=100
Southern Pamir
TJK6 (Bartang River; n=69)
TJK5 (Gunt River; n=58)
0
20
40
60
80
100 120 140 160 180 200 220 240 260 280 300 320 340 360 380
Ar/ Ar Age (Ma)
40
39
0
10 20 30 40 50 60 70 80 90 100
Gunt River,
n=102
0
10 20 30 40 50 60 70 80 90 100
AFT Age (Ma)
Figure 6. Kernel density distributions curves of detrital Ar/ Ar and apatite fission-track (AFT) ages (calculated using DensityPlotter; Vermeesch, 2012).
40
Ar/39Ar ages for the western Pamir drainages are from Lukens et al. (2012).
40
LITHOSPHERE | | www.gsapubs.org
39
CARRAPA ET AL.
Age (Ma)
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
TJK-8
TJK-7
TJK-4
TJK-6
TJK-5
40
Ar/39Ar ages
Vanj River
Yazgulem River
Muskol Dome
Murghab River
Sares and Muskol Domes
Shakdara Dome
Bartang River
Gunt River
S
youngest AFT population using DensityPlotter
Exhumation History
Cenozoic 40Ar/39Ar ages are widespread in
samples from central and southern Pamir (western drainages), whereas northeastern Pamir is
mainly characterized by Mesozoic ages (Fig. 6).
For example, the main 40Ar/39Ar age popula-
Figure 7. Distribution of younger
than 30 Ma 40Ar/39Ar and apatite
fission-track (AFT) ages and AFT
youngest populations (calculated using Binomfit; Brandon,
2002) from north to south; note
that most 40Ar/39Ar ages for the
northeastern Pamir are older
than 30 Ma. The majority of the
reported 40Ar/39Ar ages have
uncertainties of <2% (Table DR3
[see text footnote 1]). Errors for
the reported AFT ages are generally ~10% (Tables DR4 and DR5
[see text footnote 1]). We note
that the youngest AFT peaks generally include >50% of the whole
detrital spectrum for each sample
(Table DR5 [see text footnote 1]).
youngest 40Ar/ 39Ar age population using DensityPlotter
Ar/ 39Ar ages (<50 Ma)
youngest AFT population using Binomfit
40
the Triassic magmatism recorded in the Kunlun arc in the Karakul-Mazar Songpan-Ganzi
terrane (Schwab et al., 2004). This implies
that samples with these ages (1071-1, 10712, 1071-3, 1071-4, 1071-6) have affinity with
the Permian–Triassic Karakul-Mazar terrane.
(2) The early Paleozoic ages (ca. 400–600 Ma)
represent a source from the northern and southern Kunlun magmatic belts, as suggested by
Schwab et al. (2004).
Negative εHf values (as low as −42) in grains
between ca. 180 Ma and ca. 700 Ma are present
in samples derived from northeastern, central,
and southern Pamir, with higher proportions in
the central and southern Pamir samples, indicating a more-evolved crustal source. The εHf
values of samples from northeastern Pamir are
consistent with values recorded in the Songpan-Ganzi terrane, and εHf values from samples
derived from the central and southern Pamir
samples are consistent with εHf values recorded
in the Qiangtang and Lhasa terranes of Tibet
(Fig. 5C).
Eastern drainages
>50 Ma
Northern Pamir
M
Western drainages
1071-6
Kalate River
Central Pamir
1071-7
1071-3
Do
h-
ag
t
us
a
At
Southern Pamir
1071-1
1071-2
e
m
Tashkorgan River Gez River
Yazgulem Dome
Relative distance from the main Pamir thrust
1071-4
Footwall of Kongur Shan
detachment system
N
difference between youngest AFT and 40Ar/39Ar age
population (by DensityPlotter)
tion comprising 63% of the ages from the Gunt
River (Fig. 6; Table DR5 [see footnote 1]),
which is the largest drainage in the western
Pamir derived mostly from the Shakdara dome,
is 15.7 Ma. The main 40Ar/39Ar age population
consisting of 76% of the ages from the Tashkorgan River tributaries, derived mainly from
the Kongur-Shan and Mustagh Ata domes, is
78.6 Ma (Fig. 6; Table DR5 [see footnote 1]),
and the main 40Ar/39Ar age populations made of
61% and 42% of the ages from two tributaries
of the Gez River are 172.4 Ma and 98.7 Ma,
respectively (Table DR5 [see footnote 1]).
Although the bulk of 40Ar/39Ar ages are mainly
pre-Cenozoic in the northeastern Pamir (Fig. 6),
two samples (1071-6, 1071-1) exhibit 40Ar/39Ar
youngest population ages of 10.7 ± 0.2 Ma and
14.5 ± 0.2 Ma, consistent with in situ thermochronological ages (Thiede et al., 2013). These
samples were both derived from granitic intrusions. The southern sample (1071-6) is from
a river that drains a Miocene granite that has
yielded biotite 40Ar/39Ar ages as young as 11.45
± 0.3 Ma (Robinson et al., 2007), and could
explain the presence of late Miocene 40Ar/39Ar
ages. The northern sample (1071-1) taps into
an undated intrusion mapped as Triassic–Early
Jurassic (Robinson et al., 2007). Our data suggest the true age may be younger. Interestingly,
both samples are also located on the drainage
divide between the western and eastern Pamir.
Excluding these two anomalous samples,
the bulk of the 40Ar/39Ar age spectra are preCenozoic. Although some young (ca. 2–5 Ma)
40
Ar/39Ar ages have been recorded in the Kongur Shan gneiss dome (Robinson et al., 2004)
and reflect localized tectonic exhumation, our
data indicate that regional exhumation rates are
significantly higher in the central and southern
Pamir, which is the western and wetter side of
the range compared to the northeastern Pamir.
AFT ages for the central, southern and northeastern Pamir are mid- to late Cenozoic, indicating similar average rates of short-term exhumation (0.4 mm/yr; Table DR5 [see footnote 1]).
Late Cenozoic AFT ages (ca. 6 Ma) in samples
1071-2 and 1071-3 from the northeastern Pamir,
both of which tap into the footwall of the Kongur Shan extensional system, likely reflect
localized rapid exhumation associated with
movement along this young fault system. Modeling of thermochronologic data indicates exhumation of the Kongur Shan dome in the footwall
of the fault system at rates of 1.5–4 mm/yr since
the late Miocene (Robinson et al., 2010; Thiede
et al., 2013).
Assuming a conservative paleogeothermal
gradient of 30 °C/km, and closure temperatures
for white mica 40Ar/39Ar of ~350–425 °C (for
phengite—McDougall and Harrison, 1999; for
muscovite—Harrison et al., 2009), we obtain
average Cenozoic exhumation rates between 0.4
| www.gsapubs.org | LITHOSPHERE
| RESEARCH
Multisystem dating of modern river detritus from the Pamir and 0.2 mm/yr for the northeastern Pamir and
between 0.6 and 1.3 mm/yr for the central and
southern Pamir (Table DR5 [see footnote 1]).
For a lower paleogeothermal gradient, average
exhumation rates would be higher. The highest
exhumation rates are recorded by the Murghab
and Gunt Rivers (2.4 mm/yr and 2 mm/yr).
The high exhumation rates recorded by the
Murghab River, which drains a small area, may
reflect exhumation of the Muskol dome and or
localized exhumation associated with movement along the Karakorum fault (Amidon and
Hynek, 2010). Exhumation rates between 0.6
and 2 mm/yr recorded by the Gunt River detritus are interpreted here to reflect exhumation
of the Shakdara dome and are consistent with
rates recoded by in situ thermochronology
(Stübner et al., 2013b). Although the similarity between our detrital Cenozoic cooling ages
and the cooling ages of the metamorphic domes
supports this interpretation, the fact that the
sampled rivers also drain rocks other than the
domes (Fig. 1) allows for the possibility that the
grains with Cenozoic ages may be derived from
Mesozoic–Paleozoic and older rocks. If this is
the case, it would suggest that Cenozoic exhumation is orogen-wide rather than limited to the
domes, and it would be consistent with erosion
controlled by climate, rather than localized tectonic exhumation.
The different magnitude of exhumation of the
central and southern Pamir (western drainages)
versus northeastern Pamir is consistent with our
geomorphic data, which indicate greater erosion
on the western side of the range (Fig. 3). Western drainages exhibit higher hypsometric curves
and hypsometric integrals, and relief is higher
on the western side of the range (Fig. 3).
Figure 7 presents the younger than 50 Ma
40
Ar/39Ar ages and youngest 40Ar/39Ar and AFT
age populations. The most recent exhumation
signals from rivers draining the central and
southern Pamir domes occurred mainly between
ca. 19 Ma and ca. 8 Ma, and exhumation signals from the rivers draining the Kongur Shan
and Mustagh Ata domes are between ca. 15 and
ca. 6 Ma. The data also suggest a possible southward-younging trend of the youngest detrital
40
Ar/39Ar ages and youngest detrital AFT population ages for the central and southern Pamir
samples. Although this signal may be an artifact of sample size and or statistical treatment
of the data, the fact that both 40Ar/39Ar and AFT
youngest components show the same trend,
which is consistent with documented cooling
ages of metamorphic domes that also young to
the south (Schmidt et al., 2011), suggests that
our trend is geologically meaningful. We speculate that southward younging of exhumation of
the Pamir may be related to tectonic processes.
LITHOSPHERE | For example, northward subduction and underthrusting of India under Asia has contributed
to Cenozoic crustal thickening and east-west
extension in Tibet (DeCelles et al., 2002; Sundell et al., 2013) and most likely in the Pamir.
If this is correct, the present-day location of the
Indian slab under the Hindu Kush (Negredo et
al., 2007) requires southward slab rollback following underthrusting, as proposed for Tibet
during the Oligocene–Miocene (DeCelles et
al., 2011). Southward slab rollback of India
and asthenospheric upwelling, accompanied by
upper-plate extension, could explain the southward younging of thermochronological ages.
DISCUSSION AND CONCLUSIONS
Although both the Pamir and Tibet are composed of rocks with similar affinity, the exhumation history of the two regions appears to
be significantly different. Overall, zircon U-Pb
ages from modern river detritus derived from
the central and southern Pamir are most similar to zircon U-Pb ages recorded in the Qiangtang terrane of central Tibet; U-Pb ages from
modern rivers draining the northern Pamir are
most similar to zircon U-Pb ages recorded in
the Karakul-Mazar terrane (Fig. 1C), correlative
to the Triassic Songpan-Ganzi terrane of northern Tibet, and Kunlun magmatic belt. Detrital
zircon εHf values in the Pamir river sands are
consistent with εHf values reported for SongpanGanzi, Qiangtang, and Lhasa terranes in Tibet.
Thermochronological ages from the Pamir
record younger (mid-late Miocene), and on
average much faster, exhumation than that
recorded in Tibet. Lower magnitudes of exhumation in Tibet are supported by the surficial
geology, which is characterized by widespread
exposures of young volcanic and Mesozoic sedimentary rocks (Chung et al., 2005), indicating
a shallow level of erosion. Outside of late Miocene rifts and marginal areas (Clark et al., 2005),
low-temperature thermochronological ages are
almost exclusively pre-Oligocene (Hetzel et
al., 2011; Rohrmann et al., 2012; Duvall et al.,
2012). Together, these observations indicate that
most of the Tibetan Plateau has been internally
drained and did not experience significant exhumation during the Cenozoic. This is consistent
with increased aridification of large parts of the
Tibetan Plateau during the Eocene–Oligocene
(Dupont-Nivet et al., 2007) and with high elevations since at least the late Oligocene (Rowley
and Currie, 2006; DeCelles et al., 2007b; Quade
et al., 2011). In contrast, numerous early Miocene detrital thermochronological ages derived
from the Pamir interior presented in this study
indicate that most of the Pamir was undergoing
extensive exhumation during the Cenozoic at
| www.gsapubs.org
much higher rates than those observed in Tibet.
We hypothesize that greater precipitation on the
upwind, western Pamir led to greater long-term
exhumation, relief production, and migration
of the drainage divide progressively toward
the east. Alternatively, asymmetric exhumation may be related to asymmetric tectonics.
However, based on the correlation between the
locus of deep exhumation and higher precipitation (Fig. 3) in the Pamir, we favor climate as
the controlling factor on regional exhumation
by deep dissection of the western flank of the
orogen. This is supported by the highly asymmetric Pamir drainage divide positioned near
the eastern flank of the range and by the fact that
major rivers generally drain westward (Fig. 1).
Reanalysis data (Fig. 3) show that precipitation
comes from the west and is most intense during
the early spring. This precipitation is concentrated on the western side of the Pamir, which
forms an orographic barrier to eastward moisture transport. We suggest that relatively intense
precipitation on the windward side of the Pamir
has been a major factor since at least the early
Miocene in controlling bedrock exhumation and
eastward retreat of the orogenic drainage divide.
We also suggest that exhumation of the Pamir
domes was facilitated by erosion and should be
considered in tectonic models (Stübner et al.,
2013a). A trend of southward-younging cooling
ages between ca. 18 and ca. 6 Ma is observed
throughout the Pamir and can be explained by
southward migration of exhumation as a result
of southward rollback of the subducting Indian
slab under Asia. Overall, the magnitude of Ceno­
zoic regional exhumation in the Pamir is greater
than that observed in Tibet.
ACKNOWLEDGMENTS
We would like to acknowledge the support of National Geographic, the University of Wyoming, and the University of
Arizona for funds to B. Carrapa. E.R. Sobel and L.M. Schoenbohm thank the German Research Council (grant STR 373/201) for funding. We thank Jay Chapman, Peter Molnar, Steve
Roecker, Rebecca Bendick, and Claire Lukens for help in the
field and for scientific discussions, and Chen Jie for help with
Chinese river samples. U-Pb and Hf analyses were conducted
at the Arizona LaserChron Center, which is supported by
National Science Foundation grant EAR-1032156.
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