TECTONICS, VOL. 31, TC2014, doi:10.1029/2011TC003040, 2012
Miocene exhumation of the Pamir revealed by detrital
geothermochronology of Tajik rivers
C. E. Lukens,1 B. Carrapa,1,2 B. S. Singer,3 and G. Gehrels2
Received 4 October 2011; revised 6 February 2012; accepted 26 February 2012; published 18 April 2012.
[1] The Pamir mountains are the western continuation of the Tibetan-Himalayan system,
the largest and highest orogenic system on Earth. Detrital geothermochronology applied
to modern river sands from the western Pamir of Tajikistan records the history of sediment
source crystallization, cooling, and exhumation. This provides important information on
the timing of tectonic processes, relief formation, and erosion during orogenesis. U-Pb
geochronology of detrital zircons and 40Ar/39Ar thermochronology of white micas from
five rivers draining distinct tectonic terranes in the western Pamir document Paleozoic
through Cenozoic crystallization ages and a Miocene (13–21 Ma) cooling signal. Detrital
zircon U-Pb ages show Proterozoic through Cenozoic ages and affinity with Asian rocks
in Tibet. The detrital 40Ar/39Ar data set documents deep and regional exhumation of
the Pamir mountains >30 Myr after Indo-Asia collision, which is best explained with
widespread erosion of metamorphic domes. This exhumation signal coincides with
deposition of over 6 km of conglomerates in the adjacent foreland, documenting high
subsidence, sedimentation, and regional exhumation in the region. Our data are consistent
with a high relief landscape and orogen-wide exhumation at 13–21 Ma and correlate with
the timing of exhumation of the Pamir gneiss domes. This exhumation is younger in the
Pamir than that observed in neighboring Tibet and is consistent with higher magnitude
Cenozoic deformation and shortening in this part of the orogenic system.
Citation: Lukens, C. E., B. Carrapa, B. S. Singer, and G. Gehrels (2012), Miocene exhumation of the Pamir revealed by detrital
geothermochronology of Tajik rivers, Tectonics, 31, TC2014, doi:10.1029/2011TC003040.
1. Introduction
[2] The Pamir Mountains of Tajikistan and China are part
of the greater Himalayan-Tibetan orogenic system and comprise some of the highest mountains on Earth (Figure 1a).
The dramatic relief and altitude observed in the Himalayas
and Tibet have been largely associated with processes related
to pre- and syn-collisional shortening combined with Cenozoic erosion [Avouac and Tapponier, 1993; Hodges, 2000;
Yin and Harrison, 2000; DeCelles et al., 2002; Kapp et al.,
2007; Ritts et al., 2008]. In contrast, very little is known
about the timing and processes responsible for deformation,
exhumation and uplift of the Pamir. In particular, it is unclear
if the Pamir result directly from Indo-Asia collision or if they
are the result of significantly different, younger geodynamic
and erosional processes. Parallels have been drawn between
the Pamir and Tibet based on the fact that both comprise
several Paleozoic-Mesozoic accreted terranes [Harrison et al.,
1
Department of Geology and Geophysics, University of Wyoming,
Laramie, Wyoming, USA.
2
Department of Geosciences, University of Arizona, Tucson, Arizona,
USA.
3
Department of Geoscience, University of Wisconsin-Madison, Madison,
Wisconsin, USA.
Copyright 2012 by the American Geophysical Union.
0278-7407/12/2011TC003040
2000; Schwab et al., 2004; Kapp et al., 2007; Robinson et al.,
2007] and host Cenozoic magmatic rocks [e.g., Schwab et al.,
2004; Chu et al., 2006; Wang et al., 2008] and local extensional features [e.g., Hubbard et al., 1999; Kapp and Guynn,
2004; Amidon and Hynek, 2010]. However, the Pamir, which
currently lie well within the Asian continent, over 250 km
north of the western syntaxis (Figure 1a), may have a very
different tectonic history than those of Tibet and the Himalaya. Several authors have attempted to correlate tectonic
terranes in the Pamir with those in Tibet [Yin and Harrison,
2000; Schwab et al., 2004; Robinson et al., 2007; Robinson,
2009], based on the age and character of magmatic belts
and offset of individual local units across strike-slip faults.
Currently, a single correlation model does not exist and alongstrike correlation remains controversial.
[3] The Pamir differ from the Himalaya and Tibet in the
areal extent of metamorphic domes [Dmitriev, 1976;
Pospelov and Sigachev, 1987; Burtman and Molnar, 1993].
The Pamir domes are larger than those documented in Tibet,
they are Cenozoic in age, and have yielded predominantly
Miocene exhumation ages [Hubbard et al., 1999; Robinson
et al., 2007]. Previous workers have suggested that relief
(and related high rates of exhumation) developed in the
southern Pamir after 11 Ma (based on xenoliths and thermal
modeling [Ducea et al., 2003]) and, conversely, during
early mid Miocene time (based on the sedimentary record in
the Alai Valley [Hamburger et al., 1992; Coutand et al.,
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Figure 1. (a) Location map; white box is the approximate area in Figure 1b. (b) Map of the western
Pamir, showing major tectonic boundaries, surface geology, and locations of gneiss domes [Burtman
and Molnar, 1993; Schwab et al., 2004; Hacker, personal communication, 2011]. See section 2.1 for additional description of rock types within each tectonic terrane.
2002] (Figure 1a). A few thermochronologic studies have
suggested early mid Miocene exhumation on a local scale
[Hubbard et al., 1999; Amidon and Hynek, 2010], but
no regionally extensive sampling has previously been
executed to determine the exhumation history of the Pamir,
and thus the timing of orogenic-scale exhumation is
not constrained.
[4] Resolving both the crystallization and exhumation signal in the Pamir is important in order to resolve similarities
and differences in tectonic and exhumation history between
the Pamir and Tibet and to gain new insights on possible
fundamental differences in the geodynamics of the two
regions. To determine the timing and distribution of source
area crystallization and exhumation, we here use the detrital
geochronological and thermochronological signal recorded by
detrital minerals (zircon U-Pb and white mica 40Ar/39Ar)
derived from five major rivers draining the western Pamir
(Figures 1b and 2). Our new data set also contributes to
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Figure 2. Map of the western Pamir, showing the areal extent of watersheds above each sample point.
Tectonic boundaries are also shown (as in Figure 1) for reference.
further constraining along-strike affinity between terranes in
the Pamir and in Tibet.
2. Regional Setting
2.1. Geologic History of the Pamir
[5] The Pamir are generally divided into three distinct
tectonic terranes: the Northern, Central, and Southern Pamir
[e.g., Burtman and Molnar, 1993]. In the Northern Pamir,
the closure of a large ocean basin in late Carboniferous time
is documented by the presence of island arc igneous rocks
[Pospelov and Sigachev, 1987; Burtman and Molnar, 1993],
marine carbonates, and conglomerates [Pospelov, 1987;
Burtman and Molnar, 1993]. The suture associated with this
basin closure has been correlated to the Kunlun suture in
Tibet [Schwab et al., 2004], and represents the suturing of
the Northern Pamir onto the Asian continent [Burtman and
Molnar, 1993]. Volcanic rocks exposed in the northern
Pamir from the Kunlun arc are dated at 370–320 Ma
[Schwab et al., 2004].
[6] The rocks of the Central Pamir record the presence of a
small ocean basin in Mesozoic time. Deep marine sedimentary rocks, pillow basalts, and tuff of late Paleozoic to
Jurassic age were deposited [Pashkov and Shvol’man, 1979;
Burtman and Molnar, 1993], and the basin closed in late
Jurassic to Cretaceous time [Burtman and Molnar, 1993].
A thick Jurassic-Cretaceous sequence is today preserved in
the Tajik depression [Hamburger et al., 1992; Nikolaev, 2002]
and constitutes the erosional and depositional record of these
processes. Arc-type granitoids intruded into the Triassic
Rushan-Pshart marine basin sequence at 190–160 Ma may be
associated with subduction along the Rushan-Pshart suture
and closure of the basin [Schwab et al., 2004] (Figure 1b). The
Mesozoic basin evidenced in the Central Pamir is similar to
small basins documented in Afghanistan and in northwest
Tibet, but there is no evidence that these basins were
connected, and it is likely that several separate small basins
existed at this time [Burtman and Molnar, 1993].
[7] Farther south, the Indus-Tsangpo and Shyok sutures
(Figures 1b and 3) and associated ophiolite belts most likely
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Figure 3. Regional tectonic map, after Burtman and Molnar [1993].
document the collision of India with Asia in early Cenozoic
time [Burtman and Molnar, 1993]. Exposed bedrock geology in the Southern Pamir includes Precambrian and Paleozoic metamorphic rocks, Triassic, Cretaceous, and Paleogene
granites and diorites, and remnants of Carboniferous through
Paleogene sedimentary rock [Pashkov and Budanov, 1990;
Burtman and Molnar, 1993].
[8] Correlation between terranes across Tibet and the
Pamir is complicated by the ongoing controversy over displacement along the Karakorum fault and related structures,
which separate the two regions. The magnitude of slip
along the Karakorum strike-slip fault ranges from 65 km
[Murphy et al., 2000] to as much as 1000 km [Peltzer and
Tapponnier, 1988]. Recent studies have supported more
modest amounts of displacement. The correlation of antiformal domes in the Pamir and the Qiantang terrane in Tibet
by Schwab et al. [2004] suggest displacement of approximately 250 km, whereas the correlation of the Aghil
formation by Robinson [2009] revises that number to 149–
167 km of offset since Late Jurassic time.
[9] Under the correlation of Robinson [2009], the Southern Pamir correlate to the Qiangtang terrane in Tibet, and the
Northern Pamir correlate with the Sangpan-Garze terrane in
central Tibet (Figures 1b and 3). Between the two correlated
terranes lies the Central Pamir terrane, which has no Tibetan
equivalent [Burtman and Molnar, 1993; Robinson, 2009].
Alternatively, Schwab et al. [2004] correlate the Southern
Pamir with the Lhasa terrane, the Central Pamir with the
Qiangtang terrane, and the Northern Pamir with the SongpanGarze terrane.
[10] Some north-south striking extensional faults have
been mapped in the northeastern Pamir, but few extensional
structures have been documented in the western Pamir.
The Cenozoic history of the region is dominated by large
amounts of north-south shortening. Shortening has been
accommodated by northward movement of the entire Pamir
block by at least 300 km [Burtman and Molnar, 1993], by
strike-slip faulting along the eastern and western Pamir on
the edge of the Tarim basin and Tajik Depression [Burtman
and Molnar, 1993; Sobel and Dumitru, 1997; Sobel et al.,
2006] (Figure 3), and by internal shortening of at least
350 km [Burtman and Molnar, 1993; Schmidt et al., 2011].
High rates of convergence continue today, and geodetic
surveys have suggested that convergence across the northern
Pamir and Alai Valley is as much as 21 mm/yr [Bazhenov and
Burtman, 1990; Reigber et al., 2001]. Prior to about 29 Ma,
the Tarim Basin and Tajik Depression were connected by a
marine basin, of which the sedimentary record is preserved in
the Alai Valley (Figure 1a). This basin began to close around
the Oligocene-Miocene boundary [Leith, 1982; Hamburger
et al., 1992; Coutand et al., 2002; Nikolaev, 2002], when a
wedge of syn-tectonic Neogene conglomerates and coarse
sediment was deposited [Hendrix et al., 1994; Sobel and
Dumitru, 1997; Yin et al., 1999].
2.2. Metamorphic Domes
[11] Across the central and southern Pamir, a series of
Cenozoic metamorphic domes have been recognized, termed
the Pamir gneiss domes [e.g., Ratschbacher et al., 1997;
Hubbard et al., 1999; Schwab et al., 2004; Robinson et al.,
2007]. In the central Pamir, the domes are largely composed of amphibolite grade rocks, but also contain leucogranites [Schwab et al., 2004], and have yielded early
Miocene cooling ages from 40Ar/39Ar dating on biotite,
muscovite, and hornblende [Schwab et al., 2004]. In the
eastern Pamir, Robinson et al. [2007] correlated high-grade
schists and gneisses to rocks found in the Central Pamir
using field relations. Deformation in these rocks was dated
as late Oligocene to early Miocene using monazite inclusions in garnet, and exhumation to shallow crustal depths
was constrained to 7.5–9 Ma using 40Ar/39Ar dating of
biotite [Robinson et al., 2007]. In the southern Pamir (farther
south than the area sampled in this study), Hubbard et al.
[1999] described the “whiteschists” of the Goran series,
which are of upper amphibolite grade. 39Ar/40Ar dating of
biotite, phlogopite, and hornblende in these rocks shows
Miocene cooling (16.9 0.3 Ma for phlogopite, 18.3 0.2 Ma for biotite). Domes have been recognized in every
watershed sampled in this study, and in some cases contribute
large portions of exposed surface geology (Figure 1b).
[12] Exhumation of deep crustal rocks in the region during
late Miocene time has been documented by several authors
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based on the presence of deeply metamorphosed rocks and
xenoliths in ultra-potassic igneous rocks in the southern
Pamir [Hubbard et al., 1999; Ducea et al., 2003; Hacker
et al., 2005; Robinson et al., 2007]. Xenoliths that experienced temperatures of 1050–1200 C and pressures of 2.4–
2.7 GPa yield well-constrained eruption ages of 11 Ma,
based on 40Ar/39Ar dating of K-feldspar, groundmass,
and biotite [Ducea et al., 2003]. The presence of metamorphic domes in the Pamir has been used to constrain the
timing of high-pressure metamorphism and subsequent
exhumation of these rocks to the surface at 57–11 Ma
[Hubbard et al., 1999; Hacker et al., 2005]. Workers have
generally interpreted Miocene cooling ages to reflect tectonic exhumation due to local extension [Dmitriev, 1976;
Ratschbacher et al., 1997; Hubbard et al., 1999; Schwab
et al., 2004; Robinson et al., 2007], although the larger
geodynamic processes controlling dome formation and exhumation are poorly understood.
2.3. Tectonic Models for Exhumation in the Pamir
[13] The presence of metamorphic domes and xenoliths in
the Pamir [Ducea et al., 2003; Hacker et al., 2005] requires a
mechanism able to exhume rocks from depth in excess of
10km. Metamorphic domes, characterized by Miocene
thermochronological ages, are widespread within the Pamir
[Schwab et al., 2004; Hacker et al., 2005] but the processes
responsible for their formation and exhumation remain
largely unresolved.
[14] Several different models have been proposed for the
Cenozoic tectonic evolution of the Pamir, including exhumation via local extensional structures directly resulting
from Indo-Asia collision, shortening related to the collision
with the Tien Shan [Sobel and Dumitru, 1997], and following intracontinental subduction of thinned Asian crust
[Burtman and Molnar, 1993; Negredo et al., 2007]. In the
eastern Pamir, tectonic exhumation along extensional systems such as the Konghur Shan was responsible for significant exhumation of schists and gneisses in the late Miocene
[Robinson et al., 2004, 2007]. However, few extensional
systems have been mapped in the western and high (central)
Pamir that could accommodate similar exhumation.
[15] The exhumation of the Tien Shan, which lie north of
the Pamir and the Alai Valley (Figure 1), has been associated
with Indo-Asia collision. Exhumation from depths of 2–
5 km is recorded in the Tien Shan by apatite fission track
ages at 13–26 Ma, generally youngs southward [Sobel and
Dumitru, 1997; Sobel et al., 2006]. Several authors have
advocated the presence of an intracontinental subduction
zone along the Alai Valley, to the north of the Pamir, where
a south-dipping slab of Asian crust may currently be subducted to depths of more than 600 km [Burtman and
Molnar, 1993; Negredo et al., 2007]. Intracontinental subduction along this south-dipping subduction zone and subsequent collision between the thickened Pamir and Tien
Shan may have accommodated 300 km of northward
indentation of the Pamir salient along the south dipping
Main Pamir Thrust and Pamir Frontal thrusts (Figure 1b)
[Sobel and Dumitru, 1997; Hendrix et al., 1994; Coutand
et al., 2002]. Such indentation has been partly accommodated by strike-slip movements along western and eastern
structures (e.g., Karakoram and Kashgar-Yecheng transfer
system to the east) [Sobel et al., 2011].
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[16] Structural data from the Alai Valley, which represents
the last remain of a basin once connecting the Tajik
depression to the Tarim basin, indicate that deformation
started in the Oligocene [Coutand et al., 2002] and is still
active today [Arrowsmith and Strecker, 1999]. To the west,
where instead the Pamir and the Tien Shan are today in
contact, collision has been proposed to have occurred since
the Miocene-Pliocene [Pavlis et al., 1997].
[17] Intracontinental subduction is supported by seismic
tomography, which images a cold south-dipping slab under
the Pamir [Negredo et al., 2007]. Shallow- and intermediatedepth seismicity also follow a south-dipping band, and
suggest that a rigid body must be present at depth in which
earthquakes occur [Burtman and Molnar, 1993]. The high
rates of convergence and large amounts of shortening across
the Alai Valley [Reigber et al., 2001] are consistent with
intracontinental subduction.
[18] Subduction of thinned continental crust and/or the
removal of the upper crust can result in upper crustal shortening accommodated by the accretion or underplating
of sedimentary cover (and/or of the underlying basement
rocks) onto adjacent continental crust, and by internal crustal
thickening and thrusting [Molnar and Gray, 1979; Capitanio
et al., 2010]. Miocene exhumation in the Tien Shan [e.g.,
Sobel et al., 2006] supports high magnitude deformation
driven exhumation in the Cenozoic. Apatite fission track
ages from the eastern Pamir also support a Miocene age of
exhumation and suggest that deformation has been active
since 20 Ma [Sobel and Dumitru, 1997].
3. A Geothermochronologic Approach to Resolve
Exhumation and Tectonics in the Pamir
[19] In order to address the timing and distribution of
exhumation in the Pamir on an orogen-wide scale, a detrital
method is here applied. The distribution of detrital cooling
and crystallization ages has been used successfully in several
studies for provenance discrimination and to determine
timing of major tectonics events [e.g., Copeland and
Harrison, 1990; Najman et al., 1997; White et al., 2002;
Carrapa et al., 2003; Gehrels et al., 2003; DeCelles et al.,
2007; Ruhl and Hodges, 2005; Carrapa, 2010]. In particular, detrital geothermochronology of river sands has proven
to well represent the regional signature of the source areas
[e.g., Carrapa et al., 2004].
[20] Sand samples from five major modern rivers draining
the western side of the Pamir in Tajikistan (Figure 2) were
collected and analyzed using standard white mica 40Ar/39Ar
thermochronologic and zircon U-Pb geochronologic methods [Smith et al., 2003, 2006; Hodges, 2004; Gehrels et al.,
2006, 2008] (see Text S1 in the auxiliary material for
details).1 In general, one hundred grains [Vermeesch, 2004]
were targeted for analyses if available. However, due to poor
apatite and white mica mineral yields, less than one hundred
analyses per sample are here presented. Detrital zircon U-Pb
ages provide the timing of crystallization (or metamorphic
recrystallization) of different source rocks [e.g., Gehrels
et al., 2003] and thus help to discriminate between
1
Auxiliary material data sets are available at ftp://ftp.agu.org/apend/tc/
2011tc003040/. Other auxiliary material files are in the HTML. doi:10.1029/
2011tc003040.
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Figure 4. Summary of geothermochronologic data juxtaposed against U-Pb (zircon) ages from India and
Asia. Curves for 40Ar/39Ar and U-Pb in both the larger figure and inset are from this study. Curves for
Asian and Indian Plate affinity utilize data from the compilation of Henderson et al. [2010]. Inset includes
all data acquired in this study younger than 1 Ga; note logarithmic horizontal (time) scale.
different sources, whereas detrital white mica 40Ar/39Ar
thermochronology provides the timing of cooling (through
the 350–420 C isotherm; [McDougall and Harrison,
1999], assuming that source rocks are either igneous or
reached peak temperatures higher than the closure temperature during metamorphism) and exhumation of presumably
the same sources. The area sampled covers the western side
of the Pamir from north of the Kunlun suture to south of the
Rushan-Pshart suture (Figures 1b and 2) and include the
North (TJK08 and part of TJK04), Central (TJK07, and parts
of TJK04 and TJK06) and Southern Pamir (TJK05 and part
of TJK06) (Figures 1b and 2).
[21] Detrital U-Pb geochronologic analyses were performed using LA-MC-ICMPS in the LaserChron center at
the University of Arizona according to the methods of
Gehrels et al. [2008]. Between forty and ninety-eight grains
per sample yielded sufficiently precise and concordant ages
(Data Set S1 in the auxiliary material). U-Pb zircon ages
range from 32.6 3.4 Ma to 3485.4 30.5 Ma, with distinct peaks in age spectra at 36–42 Ma and 98–104 Ma.
As a whole, U-Pb ages indicate Asian crust affinity
[Henderson et al., 2010] (Figure 4), and reflect sources in
both plutonic (Mesozoic and Cenozoic), metasedimentary
(Paleozoic and Proterozoic), and possibly volcanic or
metavolcanic (Cenozoic) rocks (Figure 1b). The full U-Pb
data set can be found in the supplementary material (Data
Set S1).
[22] Between 54 and 85 white mica grains from each of
the five samples, for a total of 334 grains, were analyzed by
single-grain total fusion 40Ar/39Ar analysis at the Rare Gas
Geochronology Laboratory at the University of Wisconsin,
according to the method outlined by Smith et al. [2003,
2006]. Ages range from 13.57 0.55 Ma to 348 4 Ma,
with distinct age peaks at 15–21 Ma, 26–29 Ma, and
162–185 Ma (Figure 4 and Data Set S1). Age spectra
(peaks) for individual samples are reported as the maximum
age probability within a group of more than three analyses,
and are calculated using a weighted mean. Calculations were
performed using AgePick.
3.1. U-Pb and 40Ar/39Ar Age Distributions
of the Vanch River
[23] The Vanch River (TJK-08) (Figure 2) is the northernmost river sampled and drains the western slope of the
central Pamir, with source rocks including Cenozoic magmatic and volcanic rocks [Schwab et al., 2004], part of the
Yazgulom gneiss dome (B. Hacker, personal communication, 2011), and older sedimentary rocks from the Northern
and Central Pamir [Aghanabati, 1986].
[24] Ninety-eight grains from sample TJK-08 were analyzed for U-Pb geochronology. Ages range from range from
34.9 1.8 Ma to 3178.7 24.9 Ma. Most ages fall into one
large age peak (n = 68) at 40 Ma (Figure 4), and 5 grains
are older than 1 Ga. Sixty-nine grains were analyzed
for 40Ar/39Ar thermochronology. Ages range from 32.71 1.42 Ma to 348.18 3.94 Ma. Peaks are at 33 Ma (n = 5),
40 Ma (n = 10), and several peaks of n = 7–10 between
231 Ma and 280 Ma (Figure 5a).
3.2. U-Pb and 40Ar/39Ar Age Distributions
of the Murghab River
[25] The Murghab River (TJK-04) (Figure 2) drains an
area with >2 km of relief, deep in the high Pamir. Source
rocks include the Muskol Dome, Cenozoic magmatic (granitic) rocks [Schwab et al., 2004], and marine sedimentary
rocks of Carboniferous-Jurassic age from both the North and
Central Pamir.
[26] Forty grains from sample TJK-04 were analyzed for
zircon U-Pb geochronology. Ages range from 32.6 3.2 Ma to 3118.7 107.1 Ma. Major age peaks are present
at 37 Ma (n = 10) and 540 Ma (n = 6) (Figure 5b). Nine
grains are older than 1 Ga.
[27] Fifty-four grains from sample TJK-04 were analyzed for 40Ar/39Ar thermochronology. Ages range from
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Figure 5. (a–e) Summary of geochronologic and thermochronologic data for grains younger than 1 Ga
from each watershed sampled. Note the youngest age peak in Figures 5b–5e, which all fall between
13 and 21 Ma.
15.59 1.55 Ma to 21.14 1.54 Ma. One age peak dominates at 17 Ma (n = 52) (Figure 5b).
3.3. U-Pb and 40Ar/39Ar Age Distributions
of the Yazgulom River
[28] The Yazgulom River (TJK-07) drains an area of high
relief on the western slope of the central Pamir (Figure 2).
Source rocks include Cretaceous and Cenozoic magmatic
rocks, Paleozoic sedimentary rocks of the Central Pamir, and
the Yazgulom Dome [Aghanabati, 1986; Schwab et al., 2004].
[29] Forty-four grains from sample TJK-07 were analyzed
for U-Pb zircon geochronology; ages range from 39.8 0.9 Ma to 3485.4 30.5 Ma. The youngest age peak is at
65 Ma (n = 3), and the largest peak is at 614 Ma (n = 5).
Nine grains are older than 1 Ga (Figure 5c). Eighty-four
grains were analyzed for 40Ar/39Ar thermochronology. Ages
range from 17.82 0.75 Ma to 36.57 0.43 Ma. Peaks are
at 19 Ma (n = 12), and 27 Ma (n = 53) (Figure 5c).
3.4. U-Pb and 40Ar/39Ar Age Distributions
of the Bartang River
[30] The Bartang River (TJK-06) drains a very large area
(Figure 2), which covers much of the high terrain in the
central and southern Pamir as well as the western slope
of the mountain range. The drainage area includes the
Shakhdara, Muskol, and Sarez domes. Rocks outcropping
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within this watershed also include Mesozoic and Cenozoic
magmatic rocks, Paleozoic sedimentary rocks, and Paleozoic
basement rocks from parts of the Northern, Central, and
Southern Pamir [Aghanabati, 1986; Schwab et al., 2004;
Robinson et al., 2007].
[31] Eighty-five grains from sample TJK-06 were analyzed for zircon U-Pb geochronology. Ages range from
34.2 0.2 Ma to 2795.0 56.5 Ma. Small age peaks (n ≤ 8)
are present at 39 Ma, 104 Ma, 549 Ma, 559 Ma,
645 Ma, 802 Ma, and 839 Ma. The largest age peak
is at 972 Ma (n = 12), and twenty-four grains are older
than 1 Ga (Figure 5d).
[32] Sixty-nine grains were analyzed for 40Ar/39Ar thermochronology. Ages range from 13.57 0.55 Ma to 204 6.82 Ma, with five grains between 14 Ma and 23 Ma,
and 51 grains between 131 Ma and 199 Ma (Figure 5d).
Age peaks are numerous and contain less than five grains,
and thus are not reported individually here. The full data set
is available in the supplementary material (Data Set S1).
3.5. UPb and 40Ar/39Ar Age Distributions
of the Gunt River
[33] The Gunt River (TJK-05) drains an area of high relief
in southern Pamir (Figure 1b), and is sourced in Cretaceous
and Cenozoic magmatic rocks and Paleozoic sedimentary
rocks [Aghanabati, 1986; Robinson et al., 2007]. The
drained area includes the Shakhdara dome.
[34] Eighty-seven grains from sample TJK-05 were analyzed for U-Pb zircon geochronology. Ages range from
98.4 3.8 Ma to 2123.3 28.0 Ma. One large age peak
(n = 33) is present at 102 Ma (Figure 5e), and two grains are
older than 1 Ga.
[35] Forty grains were analyzed for 40Ar/39Ar thermochronology. Ages range from 14.19 1.25 Ma to 104.97 4.03 Ma, with distinct age peaks at 16 Ma (n = 40) and
27 Ma (n = 11). Only one grain is older than 39 Ma,
at 104.97 4.03 (Figure 5e).
4. Discussion
4.1. Pamir-Tibet Terrane Correlation
[36] Our new detrital zircon U-Pb ages generally resemble
those of Tibet, with a large percentage of Cenozoic (40–
55 Ma), Mesozoic (100–250 Ma) and ages that are generally
younger than 1 Ga [Murphy et al., 1997; Chu et al., 2006;
Leier et al., 2007a, 2007b, Wang et al., 2008; Wen et al.,
2008]. The Pamir data set contains an additional age spectra at 36–42 Ma that is likely sourced in the Cenozoic volcanic and plutonic rocks recognized by Schwab et al. [2004]
(Figure 1b). Comparison to Tibetan age distributions
(Figure 6) shows a strong similarity to ages from Qiangtang
terrane lavas (36–52 Ma) [Wang et al., 2008], and to
Gandese batholith ages from the Lhasa terrane (11–71 Ma,
with the largest age peak at 52 Ma) [Ji et al., 2009]. This
supports the terrane correlations of Robinson [2009], who
correlated the Qiangtang and Songpan-Garze terranes into
the Pamir, but does not exclude those of Schwab et al.
[2004], who included the Lhasa terrane in correlations with
the southern Pamir. A few grains older than 1 Ga are present,
which reflect an unknown older source. More geochronological work of this kind is needed in order to better resolve
terrane correlations between the Pamir and Tibet.
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4.2. Exhumation History of the Pamir
[37] When comparing zircon U-Pb and mica 40Ar/39Ar
ages it is possible to differentiate crystallization from cooling (exhumation) history. In the northernmost sample, from
the Vanch River, the youngest U-Pb zircon ages (34.9 1.8 Ma to 42.4 2.4 Ma) and 40Ar/39Ar ages (32.7 1.42 Ma to 42.0 1.2 Ma) are indistinguishable from each
other (Figure 5a). These ages are best explained to represent
crystallization of Cenozoic plutons rather than exhumation.
This interpretation is supported by the work of Schwab et al.
[2004], who mapped plutonic gneisses and volcanic rocks
of unspecified Cenozoic age within the same structural terrane. Our new data thus put a new constraint on the age of
these rocks. The remaining four samples (from the Murghab,
Gunt, Bartang, and Yazgulom Rivers), however, have
distinct lags of ≥15 Myr between the youngest zircon U-Pb
age and the youngest 40Ar/39Ar age (Figures 5b–5e). This
suggests that the 13–21 Ma youngest 40Ar/39Ar age peak
(Figure 5) represent exhumation ages rather than a plutonic
or magmatic signal. Given that all the rivers sampled drain
metamorphic domes, the young 40Ar/39Ar age component is
best explained to represent exhumation of these domes in the
Miocene. These metamorphic domes have been described
as amphibolite grade and leucocratic [Schwab et al., 2004],
and are a likely source of white micas.
[38] Overall, our new 40Ar/39Ar data indicate a remarkably
widespread early mid Miocene thermochronological signal
from four different major rivers. Cooling through the 350–
420 C isotherm (40Ar/39Ar closure temperature window)
[McDougall and Harrison, 1999] for a linear geothermal
gradient of ≥25 C corresponds to >11 km of crust removal
during the Miocene. This is supported by the presence of
>6 km-thick coarse alluvial deposits (e.g., Tavildara conglomerate, Figure 7) of Miocene age in the adjacent Tajik
Depression [Nikolaev, 2002] and Alai Valley [Coutand
et al., 2002]. This coarse detritus shed from the Pamir
represents a large volume of material removed by erosion
from proximal, high-relief hinterlands at this time. Subsidence analysis and isopach data of the Tajik Depression
[Leith, 1985; Nikolaev, 2002] support thermochronologic
data, documenting a sharp increase in sediment accommodation in the Tajik Depression, and thus tectonic loading and
thickening of the orogen, during Miocene time.
[39] The widespread 13–21 Ma 40Ar/39Ar signal indicates deeper exhumation in the Pamir with respect to Tibet,
where Cenozoic ages recorded by 40Ar/39Ar and fission
track thermochronology are limited [e.g., Guynn et al., 2006;
Rohrmann et al., 2011]. Very slow regional exhumation
characterizes Tibet since the Cretaceous [Kapp and Guynn,
2004], resulting in the widespread preservation of Mesozoic and Cenozoic sedimentary rocks and only localized
exposures of deeper crustal rocks [Kapp et al., 2000, 2007].
Remarkable differences also appear when the new data set is
compared to detrital 40Ar/39Ar ages from foreland basin
deposits in the Himalaya of India and Nepal [e.g., Szulc
et al., 2006; White et al., 2002]. Such deposits record a
significant Miocene detrital white mica 40Ar/39Ar age component (15–20 Ma) that has been associated with activation of the Main Central Thrust and South Tibetan
Detachment [Hodges, 2000, and references therein]. However, the ages recorded by Miocene through Quaternary
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Figure 6. Summary of U-Pb ages from this study and from the Lhasa and Qiangtang terranes in Tibet
Detrital samples are from Cretaceous deposits, and thus should not contain any grains younger than
65Ma. Data was complied from Leier et al. [2007a] (detrital samples), Wang et al. [2008] (Qiangtang
lavas), Chu et al. [2006] (Lhasa granites and Gangdese batholith), Ji et al. [2009] (Gangdese), and
Wen et al. [2008] (Gangdese).
foreland basin deposits in the frontal Himalaya are >1000 Ma
to <12 Ma [Szulc et al., 2006; White et al., 2002]. A similarly
wide spread of ages is observed in other collisional systems
such as in the Alps, where the exhumation record is a result
of erosion of different nappes that underwent multiple
tectono-thermal events [e.g., Carrapa et al., 2004].
[40] The detrital signal in the Pamir is derived from a large
area draining multiple terranes bounded by Paleozoic and
Mesozoic sutures and more recent (Cenozoic) structures,
and thus a much wider spectra of ages was expected in
the detrital record. The dominance of a Miocene signal
instead suggests provenance preferentially from the gneiss
domes, and is here interpreted to document the timing of
exhumation of these domes, which cover more than half the
overall sampled area. The dominance of sources in the
domes may also be influenced by focused erosion by rivers,
which may be preferentially eroding high elevation sources
in the core of the range. If the rivers have been stable
over long periods of time, they may have influenced the
exhumation of gneiss domes at the range core as suggested
by Pavlis et al. [1997]. However, this would require stability of several rivers over very long time scales, and is not
likely the sole mechanism to explain the magnitude of
exhumation observed.
[41] Geodynamic models suggest that extension could be
the direct result of collision, which may produce late-stage
normal faults and exhumation of deep-crustal rocks as a
result of the changing balance between downward-directed
shear traction and upward-directed buoyancy forces as collision progresses [Beaumont et al., 2009]. Depending on the
initial convergence velocity, ultra-high pressure rocks may
reach depths within 10 km of the surface within 11–7 Myr
after collision [Beaumont et al., 2009]. Our new data are
consistent with this model, including the exhumation of
xenolith-bearing ultra-high-pressure rocks observed in the
Southern Pamir [Ducea et al., 2003; Hacker et al., 2005].
However, if extensional structures are indeed responsible
for much of the Miocene dome exhumation in the Pamir,
then more detailed mapping of the region is needed, as few
extensional structures are currently recognized.
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et al., 2007]. In this scenario, the initiation of intracontinental subduction would have occurred some time prior to
21 Ma and thinned continental crust would underlie the
marine basin, which closed as subduction progressed until
collision occurred between the relatively thicker crust of the
Pamir and the Tien Shan.
[44] Both of these processes are expected to result in upper
crustal shortening and thickening of the Pamir in the early
Miocene, and thus may explain enhanced erosion. Intracontinental subduction of Asian crust beginning in the early
Miocene (25 Ma, when the Alai Valley began to close
[Coutand et al., 2002]) is consistent with Miocene exhumation of the Pamir gneiss domes and enhanced foreland
basin subsidence and deposition.
[45] Regardless of the preferred exhumation scenario, it
should be noted that these interpretations are based on a
relatively small sample size, and thus would greatly benefit
from additional study. Detrital studies of this kind typically
aim for >100 dated grains [Vermeesch, 2004], and previous
workers have suggested that sampling large rivers requires a
larger data set (100 grains) in order to effectively characterize the area sampled [Ruhl and Hodges, 2005]. Smaller
data sets have successfully been employed in characterizing
the timing of exhumation in other orogens [e.g., Copeland
and Harrison, 1990; Carrapa et al., 2004; Najman et al.,
2005], and although our samples are small, they are likely
to reflect the dominant age peak present in the source. The
data set presented here represents a first step in understanding the orogen-scale exhumation of the Pamir and future
work in this area is necessary in order to narrow the broad
range of possible interpretations presented here.
5. Conclusions
Figure 7. Photographs of the Miocene Tavildara conglomerate in the Tajik Depression. (a) Thickness of the conglomerate in this area varies up to over 5000 m. (b) A closer look
at this poorly sorted conglomerate. Subangular clasts up to
25 cm in diameter are predominantly limestone. Smaller,
well-rounded clasts of sedimentary and crystalline rocks
are also present, though less abundant. The conglomerate
is well-cemented, with a muddy, calcitic cement.
[42] Alternatively, the Miocene signal recorded by detrital
minerals of Tajik rivers may represent erosion of duplex
structures cored by gneiss domes, which were deeply incised
as a result of contractional deformation and increased relief
during collision with the Tien Shan. Crustal thickening and
subsequent exhumation may have resulted from a contractional regime across the Pamir and Tien Shan, resulting
directly from Indo-Asian collision. In the Alai valley, which
lies between the Pamir and Tien Shan, cessation of marine
deposition and initiation of intramontane deposition began at
25 Ma [Coutand et al., 2002]. The transition to terrestrial
deposition at this time is consistent with the closure of a
marine Tajik-Tarim basin [Burtman and Molnar, 1993] and
collision of the Pamir with the Tien Shan [Coutand et al.,
2002; Sobel et al., 2006].
[43] The closure of the marine Tajik-Tarim basin at this
time is also consistent with models that include intracontinental subduction [Burtman and Molnar, 1993; Negredo
[46] Our new U-Pb detrital zircon ages show an affinity to
age distributions from Tibet, and support the terrane correlation of Robinson [2009], in which the northern Pamir
correlate with the Sangpan-Garze terrane and the southern
Pamir correlate with the Qiangtang terrane. However, ages
that match those found in the Lhasa terrane [Ji et al., 2009]
are also present in samples from the Gunt and Bartang
Rivers, in agreement of the correlation of the southern Pamir
with the Lhasa terrane [Schwab et al., 2004].
[47] The fact that the bulk of detrital white mica 40Ar/39Ar
ages in the Pamir are early mid Miocene (13–21 Ma),
suggests younger and higher magnitude exhumation than
that observed in Tibet. Widespread exhumation recorded by
our new data set combined with the presence of proximal
and coarse-grained alluvial Miocene deposits in the Tajik
depression requires relatively high relief and rapid removal
of several kilometers of material from the Pamir at 13–
21 Ma. This timing correlates with closure of the Alai Valley
[Coutand et al., 2002] and collision with the Tien Shan
[Sobel and Dumitru, 1997; Sobel et al., 2006].
[48] Deep, extensive exhumation is consistent with a
higher magnitude of shortening in the Pamir with respect
to Tibet, which resulted in upper crustal deformation,
the exhumation of gneiss domes, and the rapid removal of
crustal material from the hinterland. We suggest that this
high magnitude, widespread exhumation is the result of
geodynamic processes that differ significantly from those
responsible for the development of the Tibetan-Himalayan
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system, and are related to a pre-Neogene geological history
that is unique to this region.
[49] Widespread exhumation may have resulted from
increased shortening and crustal thickening following IndoAsia collision, or following subduction of an intracontinental slab. Both of these models, or a combination of the
two, may explain the observed exhumation as well as the
presence of gneiss domes in the Pamir, and further study is
warranted to resolve the geodynamics of the area.
[50] Acknowledgments. This work was funded by a National
Geographic exploratory grant and by the University of Wyoming.
Geochronology at the Arizona LaserChron Center is supported by NSF
EAR 0732436. We thank Brian Jicha and John Trimble for help with
sample analysis and data reduction. This manuscript benefited from
thoughtful comments from Peter Molnar and Peter DeCelles.
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B. Carrapa and G. Gehrels, Department of Geosciences, University of
Arizona, 1040 E. 4th St., Tucson, AZ 85721, USA.
C. E. Lukens, Department of Geology and Geophysics, University of
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B. S. Singer, Department of Geoscience, University of WisconsinMadison, 1215 W. Dayton St., Madison, WI 53706, USA.
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