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JOURNAL OF GEOPHYSICAL RESEARCH: SOLID EARTH, VOL. 118, 2408–2416, doi:10.1002/jgrb.50185, 2013
Kinematics of the Pamir and Hindu Kush regions
from GPS geodesy
Anatoli Ischuk,1 Rebecca Bendick,2 Anatoly Rybin,3 Peter Molnar,4 Shah Faisal Khan,5
Sergey Kuzikov,3 Solmaz Mohadjer,2 Umed Saydullaev,1 Zhyra Ilyasova,1
Gennady Schelochkov,3 and Alexander V. Zubovich6
Received 29 August 2012; revised 14 March 2013; accepted 17 March 2013; published 8 May 2013.
[1] GPS velocities measured in the Pamir and surrounding regions show a total of
~30 mm/yr of northward relative motion between stable Pakistan and Eurasia. The
convergence budget is partitioned into 10–15 mm/yr of localized shortening across the
Trans-Alai Thrust, which bounds the Pamir on the north, consistent with southward
subduction of intact lithosphere. Another 10–15 mm/yr of shortening is distributed across
the Chitral Himalaya and Hindu Kush, suggesting that Hindu Kush seismicity might be
related to northward subduction of Indian lithosphere. Modest shortening at <5 mm/yr
occurs north of the Trans-Alai Thrust, across the South Tien Shan and between the
Ferghana Valley and Eurasia. Negligible north-south shortening occurs within the high
Pamir, but as much as 5 mm/yr, and perhaps 10 mm/yr, of east-west extension occurs
within this region. This extension is matched by a comparable amount of east-west
shortening in the Tajik Depression. The localization of shortening to the margins of the
Pamir combined with observations of distributed internal extension implies that the
east-west vertically averaged, horizontal compressive normal stress is smaller than the
north-south compressive stress.
Citation: Ischuk, A., et al. (2013), Kinematics of the Pamir and Hindu Kush regions from GPS geodesy, J. Geophys. Res.
Solid Earth, 118, 2408–2416, doi:10.1002/jgrb.50185.
1.
Introduction
[2] The Pamir-Hindu Kush region of Central Asia (Figure 1),
like the Tibetan Plateau to its east, has accommodated the
collision of the Indian subcontinent with Asia by crustal
shortening and thickening. Moreover, the same belts of
ophiolites and related magmatic zones that reflect suturing
of crustal fragments identified within Tibet also extend
through the Pamir [e.g., Burtman and Molnar, 1993;
Schwab et al., 2004]. These observations combined with
Additional supporting information may be found in the online version of
this article.
1
Institute of Geology, Earthquake Engineering and Seismology, The
Academy of Sciences of the Republic of Tajikistan, Dushanbe, Tajikistan.
2
Department of Geosciences, The University of Montana, Missoula,
Montana, USA.
3
Research Station of the Russian Academy of Sciences, Bishkek,
Kyrgyzstan.
4
Cooperative Institute for Research in Environmental Sciences (CIRES),
University of Colorado, Boulder, Colorado, USA.
5
National Centre of Excellence in Geology (NCEG), University of
Peshawar, Peshawar, Pakistan.
6
Central Asian Institute for Applied Geosciences (CAIAG), Bishkek,
Kyrgyzstan.
Corresponding author: R. Bendick, Department of Geosciences, The
University of Montana, 32 Campus Dr., Missoula, MT 59812, USA.
([email protected])
©2013. American Geophysical Union. All Rights Reserved.
2169-9313/13/10.1002/jgrb.50185
comparable mean elevations of ~5000 m for the Tibetan
Plateau and ~4000 m for the Pamir have been used to argue
for similar evolution and dynamics of the two areas [e.g.,
Burtman and Molnar, 1993; Ducea et al., 2003; Hacker,
2005; Mechie et al., 2011; Yin and Harrison, 2000].
[3] Differences between the two regions, however, are as
important as their similarities. Northward underthrusting is
well documented beneath the Himalaya and southern Tibet,
but intermediate depth seismicity is sparse and cannot be
related to subduction of Indian lithosphere. Intermediate
depth seismicity is abundant beneath the Pamir however,
and this seismicity, which occurs north and east of the adjacent Hindu Kush region, has been interpreted as occurring
either within lithosphere attached to the India plate and
continuous with Hindu Kush intermediate depth seismicity
[Billington et al., 1977; Pegler and Das, 1998; Pavlis and
Das, 2000], or within lithosphere subducted southward
beneath the Pamir and separate from the lithosphere containing Hindu Kush seismicity [Burtman and Molnar, 1993;
Chatelain et al., 1980; Hamburger et al., 1992; Koulakov
and Sobolev, 2006; Kumar et al., 2005; Mechie et al., 2011;
Negredo et al., 2007; Roecker et al., 1980]. Furthermore,
although subduction of Indian lithosphere may include oceanic
lithosphere that formed beneath the Tethyan Ocean and
north of the Indian subcontinent, the origin of the seismically
active materials at depth in the Pamir and Hindu Kush, as well
as their mechanical and chemical properties, is disputed
[Khalturin et al., 1977; Mechie et al., 2011; Mellors et al.,
2408
ISCHUK ET AL.: PAMIR KINEMATICS
Figure 1
2409
ISCHUK ET AL.: PAMIR KINEMATICS
1995; Roecker, 1982]. In fact, if the Pamir intermediate depth
seismicity occurs in lithosphere subducted from the north,
that lithosphere could have been continental in origin, with
subduction initiating in thinned continental crust [Burtman
and Molnar, 1993; Chatelain et al., 1980].
[4] With these similarities and differences between the
Pamir and Tibet in mind, we use GPS geodesy from the
Pamir and Hindu Kush regions to calculate a surface velocity
field. In the following descriptions and discussion, we consider the Hindu Kush range to extend eastward from northern
Afghanistan near Faizabad through northernmost Pakistan
and the Wakhan Corridor to a junction with the Karakorum
Range northwest of K2 (Figure 1). These surface velocities,
plus the distribution of seismic moment release from a
global catalog [Engdahl and Villaseñor, 2002], topographic
and gravitational potential energy gradients, and maps
and estimated rates of active faulting from surface geology
[e.g., Arrowsmith and Strecker, 1999; Chevalier et al.,
2011; Cowgill, 2010; Kuchai and Trifonov, 1977; Nikonov,
1974, 1977; Nikonov et al., 1983; Robinson et al., 2004,
2007; Ruzhentsev, 1963; Strecker et al., 1995; Trifonov,
1978] are then used to place bounds on the present-day
kinematics of crustal deformation in Central Asia.
2.
Methods
[5] We installed and collected data at 39 new geodetic
sites in Tajikistan from 2005 to 2011 (Table 1). Four of
these have recorded continuously since 2007, and the rest
were measured in campaign-style occupations of permanent
monuments. Campaign sites used in this work have a minimum of two 2 day occupations, and most sites have three
2 day observation epochs. Data were collected by Trimble
netRS instruments at continuous sites and Trimble R7 survey
instruments at campaign sites.
[6] These raw data were combined with observations from
nine other sites in India, Pakistan, and Afghanistan plus
19 sites in Kyrgyzstan with data available from the UNAVCO
archive from historical geodetic surveys [Abdrakhmatov et al.,
1996], and 16 IGS (International GNSS Service) sites for
loosely constrained daily position estimates calculated in
GAMIT. These loosely constrained solutions were then
combined with daily global solutions from the Massachusetts
Institute of Technology processing center to generate time
series using GLRED and GLOBK. We edited time series for
each station to remove outliers and to calculate appropriate site
weights to estimate a consistent velocity solution. Adjustments
from the ITRF08 were minimized to define the reference
frame. Finally, we averaged position estimates over 15 day
intervals and used these averages to estimate velocities in
the ITRF08 frame using GLOBK. The time series of position
averages are provided in the supporting information. In order
to account for temporal correlations in the displacement data,
which usually dominate the velocity uncertainties, we added
to each station a random-walk component determined by
fitting the daily time series to a Gauss-Markov process, as
described by Herring [2003] and Reilinger et al. [2006].
For the campaign stations, whose observations are too few to
allow a reliable estimate of the random walk, we used the
median noise value from the continuously recording stations.
We also calculated transformations from the ITRF08 reference
frame to a Eurasia-fixed frame by choosing sites located on
the stable Eurasian interior and by minimizing their velocities
(Table 1).
[7] We supplemented our regional solution with 21 additional velocities reported by Zubovich et al. [2010], primarily to add additional information about the velocity of the
Tarim Basin relative both to stable Eurasia and to the study
sites. In order to include these sites, we calculated a sixparameter (three rotations and three translations) transformation from the Eurasia-fixed frame used in Zubovich
et al. [2010] to our Eurasia-fixed frame using 57 common
stations, mainly global IGS sites.
[8] To estimate relative velocities between geographic
regions, we differenced the weighted mean velocities for
several sites in a region, rather than simply differencing
the velocity vectors of selected representative individual
sites. Differences in velocities among nearby sites, which
are comparable to uncertainties for each, make more
detailed analysis premature. For these weighted estimates,
the reported uncertainties are the square roots of the sums
of the variances of the weighted mean velocities. For example, to estimate the east-west extension within the Pamir, we
calculated the difference between the weighted average
eastward component of velocity of all sites in the western
Pamir and that of all sites in the eastern Pamir. Similarly,
the estimate of relative velocity between the eastern Pamir
and Tarim Basin is the difference between the average
velocity of all sites in the eastern Pamir and all sites in the
Tarim. The estimate of relative velocity across the western
Pamir margin is complicated by internal deformation within
the Tajik Depression. So, for that region, we use only the
three extreme easternmost Tajik Depression sites and compare their average westward components of velocity with
that of the four westernmost sites in the Pamir. Geographic
regions were defined using elevation, surface geology, and
velocity, so that defined regions reflect previous regional
geological interpretations and do not contain significant
(greater than 2s) internal relative velocities. Figure 2 shows
the northward and eastward velocities for all reported sites
and the regional averages color coded by these geographic
regions.
[9] We prefer weighted averaging of site velocities in this
way to a block-fitting algorithm because we are interested
both in regions whose internal strain rates are very low (such
as within the High Pamir) and in regions with systematic
Figure 1. (a) Regional velocities relative to Eurasia. Error ellipses show 95% confidence intervals. Vectors are color
coded by geographic region to correspond to Figure 2; open arrows are from Zubovich et al. [2010]. Red circles are earthquake epicenters from Engdahl and Villasenor [2002]. Faults discussed in the text are shown in black and labeled as follows:
MKT: Main Karakorum Thrust; TM: Tirich Mir Fault; SPT: South Pamir Thrust; DK: Darvaz-Karakul Fault; MPT: Main
Pamir Thrust. The transparent grey lines with labels correspond to the velocity transects in Figure 3. (b) As in Figure 1a,
for the Pamir and Tajik Depression. Some site names are omitted for clarity.
2410
ISCHUK ET AL.: PAMIR KINEMATICS
Table 1. Geodetic Velocities for Sites Used in This Analysis, in the ITRF08 Reference Frame and a Eurasia-Fixed Framea
Site
BJFS_GPS
WUHN_GPS
ULAB_GPS
IRKT_GPS
LHAZ_GPS
NRIL_GPS
PODG_GPS
IAOH_GPS
HYDE_GPS
RSCL_GPS
IISC_GPS
SELE_GPS
SHAS_GPS
SHMA_GPS
CHUM_GPS
POL2_GPS
STSA_GPS
KAZA_GPS
TUS4_GPS
KHA1_GPS
MADA_GPS
AKBA_GPS
SUU4_GPS
KRKA_GPS
UBLA_GPS
BZRA_GPS
AJLA_GPS
OTM4_GPS
TOR4_GPS
ABLA_GPS
ATSA_GPS
SKA4_GPS
KAKK_GPS
CHY4_GPS
OSHK_GPS
PAMA_GPS
BRDB_GPS
UZU4_GPS
KEN4_GPS
TALA_GPS
AKJ4_GPS
ABD4_GPS
MALA_GPS
DRJA_GPS
QLND_GPS
BOZ4_GPS
CHA4_GPS
MANM_GPS
SEDA_GPS
EMTA_GPS
ISHA_GPS
UCHU_GPS
KBU4_GPS
NCEG_GPS
ALA4_GPS
LHSQ_GPS
SHKA_GPS
KHAA_GPS
HRBA_GPS
KUM4_GPS
GARM_GPS
KKTL_GPS
KMRA_GPS
HINA_GPS
DUBA_GPS
OBGA_GPS
CHRA_GPS
LAKA_GPS
KBUL_GPS
GANA_GPS
TDPA_GPS
AINA_GPS
Longitude
Latitude
ITRF08 East
115.892
114.357
107.052
104.316
91.104
88.36
79.485
78.973
78.551
77.6
77.57
77.017
75.315
74.828
74.751
74.694
74.185
73.944
73.824
73.672
73.638
73.618
73.555
73.46
73.427
73.415
73.28
73.201
73.16
73.12
73.106
72.92
72.904
72.875
72.778
72.764
72.668
72.498
72.367
72.21
72.145
72.05
71.933
71.923
71.852
71.792
71.721
71.68
71.675
71.663
71.612
71.608
71.579
71.487
71.46
71.337
70.946
70.851
70.767
70.601
70.317
70.222
70.201
70.056
69.421
69.277
69.223
69.199
69.13
68.616
68.61
68.545
39.609
30.532
47.865
52.219
29.657
69.362
43.328
32.779
17.417
34.128
13.021
43.179
42.621
37.54
42.998
42.68
37.836
41.385
42.32
44.38
38.146
38.56
42.206
38.956
39.226
37.793
38.902
42.235
41.895
37.461
38.554
42.41
42.806
41.966
40.53
39.917
39.495
41.98
42.593
42.445
41.557
41.784
34.572
38.095
35.272
41.495
42.015
37.542
38.518
38.003
36.73
36.001
42.202
34.004
41.362
39.241
38.896
39.224
38.576
41.669
39.006
43.271
39.12
38.869
38.585
38.853
38.393
38.037
34.574
37.934
38.742
39.389
29.21
30.6
25.88
23.85
45.53
21.16
29.15
26.12
40.17
22.95
41.16
27.67
33.35
24.36
26.65
26.74
20.79
26.88
27.71
20.26
20.45
19.21
28.2
21.37
18.18
23.57
19.3
27
27.25
24.46
20.66
25.87
25.73
26.78
27.51
25.93
27.34
27.2
26.18
26.87
25.63
24.87
32.23
20.93
28.01
26.91
26.92
18.64
21.52
18.49
19.02
24.93
26.42
31.08
26.66
22.69
15.8
24.93
5.73
26.54
26.53
21.85
28.16
20.46
26.6
23.9
11.1
12.66
27.25
20.47
29.05
20.64
ITRF08 North
Eurasia-Fixed East
13.82
15.03
11.36
9.81
11.53
4.87
1.57
15.92
31.45
16.75
31.76
0.18
1.6
18.89
0.96
1.5
15.18
3.82
0.08
6.85
22.94
17.9
2.05
15.79
18.83
18.41
12.58
0.23
1.79
16.5
16.76
0.09
3.83
1.1
4.13
0.53
9.27
1.92
1.48
0.23
1.68
5.53
25.9
12.1
21.63
1.13
0.76
14.15
15.87
14.01
12.53
17.77
0.66
27.15
1.38
5.88
10.27
6.48
18.9
0.02
2.94
4
2.62
4.64
1.52
2.4
2.59
2.1
5.28
2.16
1.15
2.58
4.13
4.51
1
0.53
17.76
0.45
1.61
2.07
12.8
5.24
14.37
0.04
5.63
3.75
1.04
0.98
7.31
0.97
0.06
7.3
7.64
8.86
0.41
6.68
9.85
4.54
8.75
0.79
0.56
3.68
7.41
1.9
2
1.03
0.42
2.05
0.67
0.61
1.58
0.9
2.22
2.96
3.99
7.17
0.22
0.95
0.9
9.49
6.56
9.62
9.16
3.27
1.39
2.83
1.21
5.35
12.26
3.11
33.81
1.31
1.52
5.85
0.11
7.6
1.48
4.16
16.99
15.45
1
7.64
0.98
7.39
2411
Eurasia-Fixed North
3.08
4.56
2.32
1.34
17.01
0.07
4.16
18.38
33.8
18.86
33.86
2.13
0.08
20.28
0.41
2.86
16.41
4.98
1.21
7.94
24.03
18.98
3.11
16.83
19.86
19.44
13.57
0.74
2.75
17.45
17.71
0.99
4.73
1.99
4.99
0.33
10.11
2.71
0.72
0.95
2.38
6.21
26.55
12.74
22.26
1.74
1.35
14.73
16.45
14.59
13.09
18.33
1.21
27.68
1.9
6.37
10.66
6.85
18.56
0.28
3.17
4.2
2.82
4.8
1.52
2.36
2.54
2.04
5.2
1.95
0.94
2.81
East Sigma
North Sigma
0.29
0.41
0.34
0.2
0.23
0.21
0.13
0.55
0.86
0.55
0.36
0.25
0.78
0.84
0.45
0.31
1.3
0.14
0.86
3.6
1.51
1.35
0.65
1.32
1.61
1.48
1.34
0.74
0.68
1.48
1.35
0.87
3.66
0.73
0.63
2.45
2.98
0.68
0.92
0.12
0.71
2.43
1.31
1.56
0.76
0.72
0.71
0.54
1.64
1
1.22
0.8
0.74
0.75
2.07
1.39
1.4
0.94
1.39
0.86
0.5
3.86
3.1
1.54
1.42
2.67
1.3
1.26
5.15
1.44
1.24
3.68
0.34
0.32
0.37
0.17
0.23
0.31
0.11
0.38
0.61
0.29
0.21
0.24
0.61
0.91
0.42
0.18
1.39
0.11
1.09
3.91
1.62
1.46
0.71
1.41
1.77
1.59
1.44
1.32
0.74
1.57
1.44
0.94
3.95
1.27
2.29
2.65
6.65
0.72
1.17
0.11
0.76
2.59
0.62
1.68
0.81
1.07
1.05
0.39
1.76
1.08
1.06
0.87
0.92
0.54
2.1
1.5
1.52
1.02
1.47
1.09
0.37
4.14
1.83
1.67
1.58
1.85
1.37
1.33
6.15
1.51
1.93
3.92
Rho
0.022
0.019
0.007
0.011
0.016
0.023
0.013
0.001
0.001
0
0.006
0.004
0.001
0.01
0.002
0.004
0.004
0.016
0.008
0.02
0.012
0.007
0.012
0.016
0.004
0.004
0.012
0.01
0.018
0.007
0.009
0.014
0.011
0.007
0
0.015
0
0.008
0.007
0.022
0.01
0.031
0.002
0.02
0
0.008
0.006
0.001
0.005
0.097
0.002
0.013
0.006
0
0.002
0.003
0
0
0.023
0.012
0.002
0.011
0.012
0.005
0.017
0.004
0.017
0.01
0.002
0.012
0.027
0.021
ISCHUK ET AL.: PAMIR KINEMATICS
Table 1. (continued)
Site
Longitude
Latitude
ITRF08 East
68.532
68.371
68.123
67.113
66.991
66.989
66.885
64.634
38.334
39.084
37.562
24.931
30.166
30.163
39.135
25.209
21.11
26.88
23.1
33.83
21.05
21.05
27.68
22.35
LNSA_GPS
SRYA_GPS
SHTZ_GPS
KCHI_GPS
QTAG_GPS
QTIT_GPS
KIT3_GPS
ORMA_GPS
a
ITRF08 North
Eurasia-Fixed East
5.18
2.26
3.01
27.94
13.51
13.51
1.5
5.52
6.98
1.17
5.03
5.74
7.22
7.22
0.35
5.74
4.95
1.98
2.67
27.34
12.88
12.88
0.84
4.28
East Sigma
North Sigma
1.21
0.98
0.59
0.67
1.56
1.56
0.36
6.02
1.3
1.07
0.41
0.29
0.74
0.74
0.23
4.89
Rho
0.004
0.002
0.001
0.001
0
0
0.004
0.004
Uncertainties are 1-s.
patterns of deformation (such as within the Tajik Depression). We also consider the main structures in the region
and their kinematics to be too poorly known to allow
block-like domains to be defined sensibly.
3.
Eurasia-Fixed North
Results
[10] Maps of surface velocities relative to Eurasia (Figure 1)
with surface velocities classified by region (Figure 2) and
components plotted along three profiles (Figure 3) reveal
several aspects of the regional kinematics. First, four
regions have northward components of velocity greater than
10 mm/yr relative to stable Eurasia: northern Pakistan,
including both the Chitral Himalaya and the Hindu Kush;
the Pamir; the Tarim Basin; and the westernmost Tibetan
Plateau in Ladakh (Figure 2a). Sites in the Tajik Depression,
the South Tien Shan, and near the western end of the Tien
Shan in Kyrgyzstan move northward more slowly with
respect to Eurasia, from 3 mm/yr at the western end of the
Tien Shan to as much as 6 mm/yr in the South Tien Shan
and northernmost Tajik Depression. This faster movement
of the South Tien Shan than regions farther north reflects
both a counter-clockwise rotation of the Ferghana Valley
about an axis near its southwest end [Reigber et al., 2001;
Zubovich et al., 2010] and convergence between the South
Tien Shan and the Ferghana Valley. Northward components
of velocity in the Pamir also vary systematically with longitude, faster in the east and center than in the west (Figure 2a).
[11] North-south profiles across the western and central
Pamir (Figures 3a and 3b) suggest different distributions
of strain and accommodation of India’s convergence with
Eurasia. The western transect (Figure 3a) from Peshawar
(NCEG) across the westernmost Pamir (ISHA and MANM)
to the South Tien Shan (GARM, KHAA, and KUM4) shows
that 10 mm/yr of convergence is distributed across the Chitral Himalaya and Hindu Kush (Figure 2a) over a zone approximately 200 km wide, that 11 1 mm/yr occurs across
the northern margin of the Pamir and the Vakhsh Valley,
and that the remaining small amount of relative motion is accommodated north of the South Tien Shan. The eastern transect, from Leh, Ladakh (RSCL) across the eastern Pamir and
Alai Valley, shows no convergence between westernmost
Tibet and the eastern Pamir, but 15 1 mm/yr across the
eastern Alai Valley, with an additional 5 2 mm/yr of relative motion accommodated in the South Tien Shan and in
the Chatkal Ranges west of the Tien Shan. In both transects
across the Pamir, no significant shortening occurs internally
within the Pamir (Figures 2 and 3). A comparison of the two
transects indicates that although Leh and Peshawar are at
approximately the same latitude, their positions relative to
stable India are different. Peshawar has nearly the Indian
plate’s velocity, but Leh moves northward as much as
10 mm/yr more slowly, suggesting a N-S component of
shortening of at least 10 mm/yr in the Kashmir Himalaya
and Pir Pinjal [Bilham et al., 2011]. Thus, the main zone
of shortening across the Himalaya bends sharply southward
between Peshawar and Ladakh as do the primary faults
through the western Himalayan syntaxis.
Figure 2. (a) Northward and (b) eastward components of
velocity versus longitude of sites. Symbols are color coded
by geographic region, with the weighted mean velocities
used for relative velocity estimates shown as bold symbols
of the same colors. Error bars show 1s.
2412
ISCHUK ET AL.: PAMIR KINEMATICS
Depression suggests divergence between them at 2 1 mm/yr
(Figures 2b and 3c). Thus, the easternmost edge of the Tajik
Depression moves west at 7 2 mm/yr with respect to the
western edge of the Tarim Basin. The difference in eastward
weighted average velocities between the eastern and western
sides of the Pamir is 3 1 mm/yr (Figures 2b and 3c), much
of which is then recovered by shortening in the Tajik Depression. Averaging velocities of Pamir sites in two geographic
bins provide a minimum bound on the internal extension;
the difference in easterly components of velocity between
Khorog (MANM) and our easternmost site (SHMA) gives
internal extension of 6 1 mm/yr (Figure 3c). This estimate
omits some additional extension between the easternmost
Pamir and Tarim Basin suggested by observations of normal
faulting there [e.g., Arnaud et al., 1993; Brunel et al., 1994;
Chevalier et al., 2011; Tapponnier and Molnar, 1979;
Robinson et al., 2010].
[13] Significant shear does not appear to occur between
the Tarim Basin and the Pamir [Zubovich et al., 2010]
despite the presence of mapped strike-slip faults with
Quaternary offsets [Chevalier et al., 2011; Cowgill, 2010;
Robinson et al., 2007; Ruzhentsev, 1963; Sobel and
Dumitru, 1997]. We measure only 1 1 mm/yr of sinistral
relative motion between the average northward velocity
for all sites in the eastern Pamir and all sites in the western
Tarim, or less than 2 2 mm/yr along faults striking NW-SE.
In contrast, the difference between the average northward
components of velocities of sites in the western Pamir and in
the Tajik Depression shows that the Tajik Depression margin accommodates 10 1 mm/yr of sinistral shear, which
may be localized entirely on the Darvaz-Karakul Fault,
given geological evidence suggesting a similar rate [Kuchai
and Trifonov, 1977; Trifonov, 1978]. Some of this shear
may also be accommodated over a wider zone by counterclockwise rotations, as implied by paleomagnetic declinations from samples along the eastern margin of the Tajik
Depression [Bazhenov and Burtman, 1981, 1982, 1986,
1990; Burtman and Molnar, 1993; Thomas et al., 1994].
Rates of rotation for the Tarim Basin and Ferghana regions
from geodetic observations have been presented previously
[e.g., Reigber et al., 2001; Zubovich et al., 2010] and these
observations do not provide additional constraints.
4.
Figure 3. Profiles of northward components of velocity
with 1s error bars for (a) the Chitral Himalaya, Hindu Kush,
western Pamir, and western South Tien Shan, and (b) the
westernmost Tibetan Plateau, eastern Pamir, and Tien Shan.
The mean elevation of the profile is in black. (c) Profile of
eastward components of velocity with 1s error bars for the
Tajik Depression, Pamir, and westernmost Tarim Basin.
The solid black line is the mean elevation of the profile.
[12] Although north-south shortening occurs mainly in
localized zones on the margins of the Pamir, east-west
extension occurs both on the margins and within the high
terrain. The difference between weighted average eastward
components of velocity in the eastern Pamir and the westernmost Tarim Basin suggests that they diverge at 5 1 mm/yr
and that between the western Pamir and easternmost Tajik
Discussion
[14] The kinematic budget for Indian-Asian collision across
Central Asia differs from that of the Tibetan region to the east
in two ways. First, in Central Asia, most of the relative motion
between India and Eurasia is accommodated in relatively
localized deforming zones, with large convergence rates over
short baselines implying high strain rates; in Tibet, relative
motion is accommodated in both localized and broadly distributed deforming zones. From the Pakistani lowlands across
Central Asia to the Kazakh platform of Eurasia, this total
velocity is separated into ~10 mm/yr across the combined
Chitral Himalaya and Hindu Kush, ~15 mm/yr about the
Trans-Alai Thrust, and <5 mm/yr across the South Tien Shan
farther north. In contrast, across the central Himalaya and
Tibet, ~20 mm/yr is accommodated across the Himalaya,
~10 mm/yr by conjugate strike-slip faulting throughout the
Tibetan Plateau, and ~5 mm/yr across the Qilian Shan on the
northwestern edge of Tibet [Zhang et al., 2004].
2413
ISCHUK ET AL.: PAMIR KINEMATICS
[15] The Trans-Alai Thrust, a system of active Quaternary
faults, accommodates shortening on the north side of the
Pamir [Arrowsmith and Strecker, 1999; Nikonov, 1974,
1977; Nikonov et al., 1983; Strecker et al., 1995]. The southward dipping zone of intermediate depth seismicity projects
to the surface near the trace of Trans-Alai Thrust and its
westward continuation [e.g., Burtman and Molnar, 1993;
Hamburger et al., 1992]. Lower bounds on Quaternary slip
rates are consistent with the present-day geodetic observations [Mohadjer et al., 2010; Reigber et al., 2001; Zubovich
et al., 2010]. These observations combine to make a strong
case for subduction, or at least underthrusting, of lithosphere
beneath the Alay Valley under the northern edge of the
Pamir. That lithosphere presumably continues west and
underlies the Tajik Depression.
[16] The Tajik Depression is underlain by relatively thin
crust, which is overlain by ~10–15 km of Mesozoic and
Cenozoic sedimentary rock [Kulagina et al., 1974]. Near the
base of that sedimentary rock is a thin layer of Jurassic salt,
along which the overlying sedimentary rock detaches to form
folds throughout the Depression [e.g., Bekker et al., 1974a,
1974b, 1983; Nikolaev, 2002]. Bourgeois et al. [1997] estimated as much as 150 km of horizontal shortening across
the Depression. Moreover, Leith [1985] showed that the subsidence history of the basin resembled that associated with
cooling of the plate beginning in Jurassic time. He inferred
that crust had been stretched and thinned at that time. If
the crust beneath the Tajik Depression continued eastward
beneath the territory now occupied by the Pamir, then the
lithosphere there would have been thick, comparable to that of
oceanic lithosphere of the same age (~100 km), but capped by
thin crust (≤20–25 km). For these reasons, Hamburger et al.
[1992] and Burtman and Molnar [1993] inferred that the intermediate depth earthquakes occur in an eastward continuation
of the lithosphere that underlies the Tajik Depression.
[17] The other main contributor to the total displacement
budget in Central Asia is not, however, distributed shortening throughout the high plateau, as it is in Tibet, but a
second relatively localized zone of thrusting and shortening
across the Chitral Himalaya and Hindu Kush, intersecting
the surface between Malakand Pass (MALA) and Dir
(QLND) (Figure 1a). Making the simplest possible interpretation that this shortening is accommodated on a planar
thrust dislocation separating downgoing Indian lithosphere
from overriding Hindu Kush material, we may draw a plane
from this location to meet the top of the intermediate
depth seismicity (at 100 km) in the Hindu Kush. Doing so
constrains the dip of the top of Indian lithosphere to be
25 –30 , with a minimum total offset of ~500 km, more
consistent with previous inferences of northward subduction of Indian lithosphere [Koulakov and Sobolev, 2006;
Negredo et al., 2007; Priestley et al., 2008], than interpretations of Hindu Kush seismicity isolated in a relict piece of
lithospheric material [Pavlis and Das, 2000; Pegler and
Das, 1998] unconnected to surface tectonics. Furthermore,
attributing shortening in the Chitral region to convergence
between India and the Pamir requires a sharply bent geometry for the India-Eurasia subduction interface in map view
for it to be located far to the south of Ladakh in its eastward
extension. Whether the origin of either the north dipping
slab beneath the Hindu Kush or the south dipping slab beneath the Pamir is entirely continental, however, requires
additional data from seismology, xenolith studies, and other
sources. Existing tomography [e.g., Koulakov and Sobolev,
2006; Mechie et al., 2011; Negredo et al., 2007; Priestley
et al., 2008] does not have sufficient resolution to demonstrate continuity of a subducting slab.
[18] Although the shortening in Central Asia is localized
to two zones of relatively rapid convergence, this is not
the case with the east-west extension. The total rate of
east-west extension is at most 10 mm/yr from the easternmost edge of the Tajik Depression (thus ignoring shortening
therein) to the Tarim Basin. As this is less than half of the
total shortening rate across the whole region, it precludes
overall planar pure shear deformation. Moreover, the internal east-west extension is much greater than the internal
shortening within the Pamir region itself, since most, if
not all, of the total shortening occurs across the Pamir
margins. As appears to be the case in Tibet, the surface
area of the Pamir must be increasing. Insofar as these rates
apply to the entire lithosphere, the crust must be thinning.
Thus, to remain in isostatic equilibrium, the surface should
be subsiding [England and Houseman, 1989; Fleitout and
Froidevaux, 1982]. Given that the east-west extension rate
within the Pamir is close to the shortening rate across the
Tajik Depression, this thinning may be accommodated by
the advection of material from the Pamir into the Tajik
Depression and, to a considerably lesser extent, into the
Tarim Basin. We note too, however, that shortening across
the Tajik Depression seems to be thin skinned and limited
to the upper 10–15 km of sedimentary rock [Bekker et al.,
1974a, 1974b, 1983; Burtman and Molnar, 1993; Kulagina
et al., 1974], but extension within the Pamir must involve
much the crust, if not all of it, given the deep exhumation by
tectonic, not erosional processes [e.g., Ducea et al., 2003;
Hacker, 2005; Schmidt et al., 2011].
[19] Also, GPS velocities give no indication of shortening
on the eastern edge of the Pamir where it abuts against the
stable Tarim Basin. Yet, that margin reveals abundant
evidence of folding and thrust faulting in early Cenozoic
time [e.g., Bershaw et al., 2012; Yin et al., 2002]. Thus, it
appears that convergence at that margin has ceased. As the
northward movement of Tarim toward stable Eurasia at near
20 mm/yr cannot have been sustained for more than ~10 Ma
[Abdrakhmatov et al., 1996; Reigber et al., 2001; Zubovich
et al., 2010], we presume that the thrust faulting, folding,
and convergence along the eastern margin of the Pamir
ceased at approximately the same time.
[20] We may use these kinematic observations to place
some bounds on the magnitudes of the principal stresses
within the Pamir, assuming that shear stresses on horizontal
planes are negligible. Because there is negligible internal
north-south strain, the north-south compressive stress must
approximately equal the vertical compressive stress. The
east-west compressive stress, however, must be smaller
than either of these. This is consistent with the topographic
form of the region, where large basins east and west of the
Pamir allow accommodation space for lateral extrusion of
material. This is also consistent with limited focal mechanisms from the Pamir (www.globalcmt.org), most of which
show normal faulting on north-south trending planes and
therefore east-west extension [e.g., Burtman and Molnar,
1993; Strecker et al., 1995]. In fact, the absence of eastwest components of crustal shortening, except in the Tajik
2414
ISCHUK ET AL.: PAMIR KINEMATICS
Depression where salt allows lateral transport of overlying
sedimentary rock, might imply that the vertically averaged
east-west compressive stress is much smaller than the vertically averaged north-south compressive stress. At present, it
appears that the Pamir is both moving northward and
collapsing laterally, suggesting that it is unlikely to reach
the size of the neighboring Tibetan Plateau and that the
properties of the lithosphere or the boundary conditions also
differ from those of Tibet.
[21] Acknowledgments. This research was supported by the National
Science Foundation under grants EAR-0636080 and 0636092. The authors
thank two anonymous reviewers for their constructive efforts. We also
thank the Civil Defense Research Fund for logistic assistance. Some of
the GPS data from the Tien Shan were obtained with support from NSF
grants EAR-8915334, EAR-9117889, EAR-9614302, EAR-9708618, and
EAR-0636092 and NASA through grants NAG5-1941 and NAG5-1947.
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