Vol. 45 No. 10
SCIENCE IN CHINA (Series D)
October 2002
Pr esent-day cr ustal movement and tect onic defor mat ion in
China continent
WANG Qi (王 琪)1 , ZHANG P e izhe n (张培震)2 , NIU Zhijun (牛之俊)2 ,
J . T. F reymue lle r3 , LAI Xi’
an (赖锡安)1 , LI Ya nxin (李延新)4 ,
ZHU Wenya o (朱文耀)5 , LIU J ingna n (刘经南)6 , R. Bilha m 7 & K. M. Lars on8
1. Ins titu te of Se is molog y, Ch in a S eis molog ica l Bu re au, Wuha n 430 071 , Ch in a;
2. Ins titu te of Geo lo gy, China Se is molog ica l Bu re au, Beijin g 1 00 029 , Ch in a;
3. Ge op hys ical Ins titu te , Un ivers ity of Alas ka Fa irb an ks AK 99 775 , USA
4. Firs t Ce nte r fo r Crus tal Defo rma tio n Mo nito ring , Chin a S eis molog ica l Bu re au , Tian jin 30 018 0, China;
5. Sh ang ha i As tron omica l Ob s erva tory, Ch in es e Aca de my of Scien ces , Sha ng hai 2 00 03 0, Ch in a;
6. Sch ool o f G eo scie nce an d Surveyin g E ng in ee ring , Wuh an Univers ity, Wu han 4 300 70 , China
7. Dep artment of Ge olog ica l Scie nces an d CIRE S, Un ivers ity o f Colorad o, Bou ld er CO 80 309 , USA;
8. Depa rtme nt of Aero s pa ce E ng inee rin g Scien ces , Unive rs ity of Co lo ra do, Bo ulder CO 80 30 9, USA
Corre s pon de nce s ho uld be a dd re s s ed to Zha ng Pe izh en (email: p eizhe n@p ub lic3. bta .ne t.cn)
Receive d De cembe r 14 , 2 001
Abstract
Velocity fie ld o f China co ntinen t con stra ined by Glob al P os itio ning Sys te m (GP S) reve als both continuo us a nd blo ck-like s tyle s o f de fo rmatio n. Co ntin uous de fo rmatio n commonly
ch aracterizes a ctively d efo rming moun ta in range s s uch as the Tians ha n Mo un ta in, Qilia n Mou ntain,
and Tibet. The block-like movemen t o ften repres en ts de forma tio n in the te ctonically s table regio ns
su ch as O rdos , South China and Tarim blo cks . G PS mea su rements indicate 5.1 2.5 mm/a leftlateral s trike -s lip rate alo ng the Altun fault. Eas tward con ve rgence a long the Lon gmens han fault is
les s than 6.7 3.0 mm/a. South China mo ves 11 14 mm/a e as twa rd co mpared with the stab le
Eura sia . Thes e lo w s lip ra tes do n ot imp ly ra pid ea stward extrus ion of China contine nt p redicted b y
th e mod el of “
co ntinen ta l e xtru sio n”
. It appe ars that “
cru stal th ickenin g”mode l more pro pe rly describ es b oth con tin uous and b lock-like s tyle s of deformation in Chin a co ntinen t.
Keywor d s: Global Positionin g System, p resen t-day cru stal movement, active cr ustal blo ck , lithosp h er ic dyna mics .
Widespread Cenozoic tectonic deformation in Asia and creation of the Tibet Plateau are the
grandest products of the collision and subsequent penetration of India into Eurasia. The geodynamics driving this deformation is still enigmatic. Existing theoretic attempts can be grouped into
two end-member models. In one model, continents deform continuously in a diffused fashion
commonly distributing in vast region[1
3]
. In the other, continents deform as a number of rigid
blocks that past one another along narrow boundaries and their kinematics can be described by the
same rules applied to oceanic plates [4, 5]. The first important step to understand the dynamics of
these processes is to obtain a kinematic description of deformation over the entire region of China
continent. GPS provides a powerful means to directly measure the kinematic pattern of presentday crustal deformation[ 6, 7] .
866
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Dat a and analysis
Since the late 1980s, several regional GPS networks for active tectonic studies were estab-
lished in China continent[ 8
20 ]
. All of these networks were surveyed with campaign mode at one to
two-year interval and have been conducted on a scale ranging from several hundred kilometers to
a few thousand kilometers. Each individual network was originally designed to address local
problems other than large-scale deformation of Asia, and therefore cannot merge together seamlessly, due to different data analysis strategies involved, to yield a uniform velocity field[ 21]. The
only realistic approach for a large-scale GPS solution to date is to merge the observations from
regional networks into a coherent solution. We adopt the following strategy to deduce disparate
data sets observed at different epochs and regions into an integrated solution of station motions on
a common reference frame at continental scale.
In the first step, regional GPS data were combined with continuous tracking data from IGS
stations using the GIPSY/OASIS-II software [2 2]. For data observed between 1991 and 1994, we
use a global solution strategy in which parameters associated with GPS satellite orbits are estimated together with station coordinates, phase ambiguities and other modeling station clocks and
troposphere behavior based on a well-distributed set of global IGS stations [12 , 23 ]. For data observed
from 1995 to 1999, a regional solution strategy was adopted, using JPL’
s no-fiducial orbits and
satellite clocks[ 24]. Only a subset of IGS stations was used. For all daily solution we follow a “
non[24
]
fiducial”strategy similar to that described by Heflin et al. . A loosely constrained solution for
the station positions is obtained each day.
In the second step, all of the loosely constrained solutions are transformed into the ITRF97
reference frame [25 ] by estimating the seven-parameter transformation that minimizes misfits of the
common stations between them. The quality of transformation depends on accuracy of the coordinates and velocities of the reference stations, and on their geographic distribution. Therefore we
select those stations that have at least five-year data span in Asia as well as well-distributed global
stations chosen by Larson et al. [12] .
In the final step, daily solutions are combined together to determine station velocities and
station coordinates in 1995 by the standard weighted least squares adjustment from which an inte grated velocity field can be derived. Furthermore we define a Eurasia-fixed reference frame by
subtracting the Eurasia plate movement predicted by the NNR-NUVEL-1A model[ 26–28 ] from GPS
station motion on the ITRF97 reference frame. The residual station velocities are deformation
relative to stable Eurasia continent. The aggregate velocity solution (fig. 1) represents the first
synthesis of independent geodetic networks available throughout Asia, and provides the most
coherent image of ongoing crustal deformation in this region to date.
2
Velocit y field of pr esent-day crusta l movement and active crusta l blocks
Late Quaternary active faults are distributed over a vast region of China continent with a full
spectrum of styles and structural orientations. The upper crust of China continent has been sliced
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Fig . 1. GPS velocity vectors (mm/a) relative to the s tab le Eurasia in China and its vicinity.
into a number of blocks by major active faults[29
31]
. There is very little deformation in the interior
of an active crustal block. Deformation commonly takes places along boundaries of active crustal
blocks. Devastating earthquakes mostly occur along the major boundaries of active crustal blocks.
Each crustal block has its own style, rate, and history of deformation.
GPS stations located in the territory of Nepal move mostly northward with a little eastward
component relative to stable Eurasia (fig. 1). Slip rates of these stations are commonly in the range
of 35 –42 mm/a which is in a good agreement with those obtained by Bilham et al.[ 10] and Larson
et al.[1 2]. This velocity represents continuous penetration of Indian sub-continent into Eurasia after
collision between them about 50 Ma ago.
The southern Tibet has been subjected to east-west extension. A series of north-trending
normal faults and grabens characterize the extension[ 32, 33 ]. GPS measurements indicate that the
Lhasa block moves to NE30
47 , at a rate between 27 and 30 mm/a (fig. 1). The rate of ex-
tension between Shiquanhe in the west and Lhasa in the east is measured to be 14.5 mm/a. This
rate is larger than (10±5) mm/a geological rate [3 2], but is less than (18
rate
[34]
9) mm/a seismological
. Since there may also be some extension east of Lhasa and west of Shiquanhe where there
is no GPS station available, this rate may be a minimum. Thus, we think the rate of extension in
southern Tibet may be in the range of 15
20 mm/a .
The central Tibet is cut by several NWW-trending left-lateral strike slip faults. These faults
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SCIENCE IN CHINA (Series D)
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divide the central Tibet into four active crustal blocks. From south to north, they are the Qiangtang
block, the Kunlun block, the Qaidam block, and the Qilianshan block (fig. 1). There are only five
GPS stations in the Qiangtang block, and all of these five stations show relative motion of N60 E
at rate of (28
5) mm/a. Unfortunately there is no GPS station in the Kunlun block. Stations in the
Qaidam block show similar direction of movement to the Qiangtang block, but the rates decrease
to 12 14 mm/a (fig. 1). Northward to the Qilianshan block, most stations move to NEE-NW
direction, and the rates decrease to 7
14 mm/a. It appears that different active crustal blocks
within the Tibet Plateau behave differently.
Widespread active faulting and folding, uplifting of mountain range, and high seismicity
within the belt attest to continued crust shortening throughout the Tianshan. Velocity profile across
the Tianshan between 81°
E and 85°
E shows progressive decrease from 18.9 mm/a south of the
Tianshan in Tarim basin to 17.0 mm/a within the southern foreland, and to 12.9 mm/a in the
northern foreland of Tianshan (fig. 1). Total shortening rate along this longitude across Tianshan is
~8 mm/a which is not significantly different from ~6 mm/a east of this profile given by Avouac et
al. [ 5]. Farther east, the rate of crustal shortening across the Tianshan decreases to about 2 mm/a. In
the western Tianshan, Abdrakhmatov et al. [9] estimated crustal shortening rate to be about
20 mm/a and Wang et al. [17 ] obtained the similar result. The crustal shortening across the Tianshan
shows a tendency of eastward decrease, which is consistent with that inferred from theoretical
modeling and geological investigations.
The North China region includes the Ordos block and North China Plain. Despite variations
in rate and direction of each individual station, GPS measurements in the North China Plain exhibit a relatively homogeneous strain field with most stations moving at rates between 8 and 13
mm/a, and in orientations varying from 90°to 110°relative to stable Eurasia [13, 15]. According to
Shen et al.[ 15], the two obvious velocity gradients are located along a fault zone north of Beijing
with 3 4 mm/a left-lateral displacement and across the Shanxi graben with 3 4 mm/a southeastward extension. A prominent difference of GPS result from geological and seismological
studies is the lack of an extension component across the entire basin or individual faults. This
deviation may imply that these faults were in the state of low activity in the past decade, or the
current deformation in the North China Plain is no longer characterized solely by extension.
The South China region has been geologically and seismologically stable and behaves as a
coherent block. Molnar and Gibson[ 35 ] used Very Long Baseline Interferometry (VLBI) measurements to infer that the South China block moves to 116°at a rate of (8±
0.5) mm/a. Shen et al.[15]
and Chen et al. [2 0] also concluded that the South China block moves south-southeastward as a rigid
block at a rate of 6 10 mm/a based on motions of less than 5 GPS stations. Based on 14 GPS
stations distributed over the block (fig. 1), we obtain a velocity of 11
14 mm/a with respect to
stable Eurasia in the orientations of NE90° 120°
. Thus, the South China block indeed behaves as
a rigid block without internal deformation[1 5, 2 0, 3 5].
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The western Yunnan and eastern Sichuan active crustal block is one of the most seismically
active regions in the world (fig. 1). The northeastern boundary of the block is the left-lateral Xianshuihe and the Xiaojiang faults, which offset ridges and valleys with average slip rates of 10
15 mm/a [3 6]. The southwest boundary of the block is the right-lateral Red River fault, and its
average long-term slip rate is 7 8 mm/a. Active tectonic studies indicate that the block has a
tendency of moving to the southeast, but the movements may not be uniform. GPS stations in the
northern part of the block, especially along the Xianshuihe fault, move in the direction of ~120 .
Stations in the southern part of the block, however move to ~N160
E. The block itself exhibits a
clockwise rotation in the background southeast movement (fig. 1).
3
Two different styles of defor mation of the active crusta l blocks
China continent consists of two kinds of active crustal blocks. The first behaves as a relative
rigid block without or with only little internal deformation and seismicity, referred to as the coherent block. Internal deformation and seismicity characterize the second kind of blocks although
magnitude of deformation cannot be compared with those along the block boundaries, and we call
this kind of block the deforming block.
The typical coherent blocks include the Tarim, the Ordos, and the South China blocks. GPS
stations within these blocks show no relative motion for each other. Blocks inside the Tibet Plateau also show coherent movement (fig. 1). There are six stations in the Lhasa block. Directions of
motion change in the range of N30 E to N47 E, and the rates vary from 27 to 29 mm/a. Five
GPS stations in the Qiangtang block move in the direction of N62 E to N72 E at rates of 27 to
33 mm/a. In the Qaidam block, slip vectors in the three GPS stations change from N57.8 E to
N58.2 E at rates of 11.9 to 14.8 mm/a. Although there are differences in both direction and rate
for stations within each block, yet the amounts of the differences are much smaller than those
across different blocks.
Deformation occurs in the interior of the deforming blocks. The Tianshan and the Qilianshan
blocks are typical examples. Active tectonics in the Qilianshan region is dominated by northeastward convergence of northeastern margin of the Tibetan Plateau. The convergence has been partitioned into crustal shortening along the NWW-trend Qilianshan range-front thrust faults and leftlateral strike-slip faults along the crest of the Qilianshan. Neotectonic studies indicate that the
entire Qilianshan region including the Hexi Corridor, and a foreland basin of the Qilianshan has
subjected to progressive crustal shortening[ 37 ]. GPS measurements show the similar pattern of
present-day deformation (fig. 2). Velocity vector in the southern Qilianshan block is the largest,
and progressively decreases northward across the Qilianshan. From station HAER to MINQ across the Qilianshan block the total crustal shortening is (6.0 2.0) mm/a constrained by our GPS
measurements.
The causes of the two kinds of active crustal blocks may result from differences in structure,
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Vol. 45
material, thermal and mechanical environment
of the block itself. The coherent active crustal
block commonly has large strength and low
heat flow such as the Tarim, the Ordos, and the
South China blocks. Rheological flow in the
low crust and upper mantle drives the overlain
upper crustal blocks to move coherently. Significant deformation occurs solely along the
block boundaries. The deforming block commonly has less strength and higher heat flow
Fig. 2 . Velocity profile across the Qilianshan block showing
continuous deformation.
than the coherent blocks. This kind of blocks
only appears in a certain locality of tectonic
settings. The Qilianshan block is located in the leading edge of the northeastward progression of
Tibet Plateau. Internal deformation is caused by combination of northeastward compression from
the Tibet Plateau in the south and the resistance of the stable Alashan block in the north. The Tianshan block is flanked by the stable Tarim and Junggar blocks on its both sides. Its internal deformation probably results from northward compression from the Pamir and Tibet Plateau, and resistance from the stable Siberia. It appears that coherent movement of crustal block represents
common behavior of upper crustal deformation, and the deforming block only happens in certain
tectonic settings.
4
Cont inenta l defor mation and dr iving mechanism of active crust al blocks
Why is present-day deformation of China continent characterized by movement of active
crustal blocks? What drives this kind of deformation? In the following we address these questions
in terms of crustal dynamics.
Continental lithosphere is characterized by rheological stratification with various strengths [3 8].
Deformation in the brittle upper crust is governed by frictional sliding where faults developed.
The strength of upper crust increases with depth. Ductile deformation occurs in the low crust
where its strength decreases with depth. Ductile deformation also dominates mechanical process
in the lithospheric mantle in which rheological strength also decreases with depth[ 2, 39 ]. The brittle
upper crust has been cut into blocks of different sizes and shapes during the long-term geological
evolution. Two models have been proposed to describe geodynamic processes that drive movement and deformation of the active crustal blocks. In one model, the major fault zones separating
different active crustal blocks are viewed as lithospheric shear zones. Deformation in the upper
part of the shear zone is characterized by brittle failure governed by friction law. In the low part of
the shear zone deformation is dominated by rheological flows[4 0] . Most of deformation concentrates along the block boundaries. Stress is transmitted through interaction of the blocks. Forces in
the block boundaries drive movement of the blocks. This style of deformation can be attributed to
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CRUSTAL MOVEMENT & TECTONIC DEFORMATION
871
[4 , 5]
the model of “
continental extrusion”
. In the other model, faults in the block boundaries pene-
trate only through the brittle part of the upper crust and are diffused into rheological flow in the
low crust or upper mantle. Rheological flow in the low crust and upper mantle drives the upper
brittle crust to move. The “
crustal thickening”model represents this style of deformation[1–3] . Most
of the theories and hypothesis on continental dynamics can be attributed to these two models.
According to “continental extrusion”model, late Cenozoic deformation of China continent is
characterized by strike-slip along major faults and rapid eastward movement of crustal blocks [4, 41] .
Eastward “
extrusion”of blocks east of the Tibet Plateau should take up about 50% of total relative
motion between India and Eurasia[ 41]. The Altun fault along the northern margin of the Tibet Plateau should have 20
30 mm/a left-lateral strike-slip movement [42 ]. The crustal shortening per-
pendicular to the proposed direction of “
extrusion”along the Longmenshan fault should be in the
order of 20
30 mm/a. According to “
crustal thickening”model, crustal shortening and thicken-
ing characterize continental deformation. Strike-slip is viewed as a minor process in the late stage
of continental deformation[1 ]. Eastward motion of crustal blocks east of the Tibet Plateau should
not exceed 20% of the relative motion between India and Eurasia. The left-lateral displacement
should not exceed 10 mm/a along the Altun fault and the crustal shortening should not exceed 10
mm/a across the Longmenshan fault. Quantitative determination of the velocity along these faults
and blocks would resolve that of the two dynamic models describing continental deformation
more properly. GPS observation is surely a key to provide the information.
GPS network and stations have been set up since 1994 to study present-day slip rates of the
Altun fault[1 5, 18, 20 ]. Measurements indicate that stations located south of the fault move to directions of N18°
E to N35°
E. The closer to the fault, the more they orient to the north (fig. 1). Stations
north of the fault show that the farther away from the fault the more they trend northward. This
deflection toward the fault manifests dragged deformation due to left-lateral shear along the Altun
fault. Fig. 3 shows velocity parallel to the strike of the Aljin fault with respect to stable Eurasia.
Stations south of the fault move 5 8 mm/a parallel to the fault (N80 E), and stations north of the
fault move 0 —4 mm/a parallel to the fault. The velocity contrast between the south and north
walls of the fault gives a left-lateral strike-slip rate of (5.1 2.0) mm/a. This rate is significantly
smaller than 20
30 mm/a obtained by Peltzer et al. [4 2], but similar to (9
5) mm/a GPS meas-
urement by Bendick et al. [1 6] and 3 5 mm/a geological rate given by Chinese geologists[ 43].
There are GPS networks across the Longmenshan fault in Sichuan Province that show almost
negligible amount of crustal shortening across the fault[ 11, 15, 20]. Our data also provide constraint s.
There are 13 GPS stations being located in the tectonically stable South China block. These stations move at rates of 11 to 14 mm/a. Velocities increase to 19 21 mm/a across the Longmenshan fault in the western Sichuan and eastern Yunnan block (fig. 4). The velocity contrast across
the fault can be expressed as (6.7 3.0) mm/a. This rate is larger that that given by King et al.[ 11]
and Chen et al. [2 0], but is significantly smaller than the rate predicted by “
continental extrusion”
872
SCIENCE IN CHINA (Series D)
Fig. 3. Velocity profile across the Altun fault sho wing leftlateral slip along the fault.
Vol. 45
Fig. 4. Velocity profile across eastern margin of the Tibet
Plateau.
model.
The relatively low slip rates along the Aljin fault and the Longmenshan fault as well as the
velocity of the South China block do not support the dynamic model of “
continental extrusion”
which predicted that slip rates tripled the observed ones. Rheological flow in the low crust and
upper mantle drives active crustal block in the upper brittle crust to move and to deform.
5
Conclusions
Velocity field revealed by GPS measurements shows that present-day deformation of China
continent is characterized by active crustal blocks. Some blocks are stable with little or no internal
deformation. Others show internal deformation. In any case, the magnitude of deformation in the
block interior is much less than that along the block boundaries. The observed slip rates along the
margins of Tibet Plateau are significantly less than those predicted by “
continental extrusion”
model. Rheological flow in the low crust and upper mantle may play an important rule in controlling deformation of upper crust of China continent.
Ack nowledgemen ts We deeply express sincere thanks to our colleagues fro m Chin a Seis mological Bureau, Chinese
Academy of Sciences, and National Bureau of Surveying and Mapping, Wuhan University and Chang’an University, and staffs
from University of Colorado Boulder and Un iversity of Alaska Fairbanks, USA for performing the GPS field measurement. We
thank Peter Molnar for his guidance in continental d ynamics . We also thank Shen Zhengkang at UCLA and Fang Peng at UCSD
fo r their helps . This work was supported by the National Natural Science Foundation of China (Grant No. 4982 5104), National
Key Basic Res earch Program (Grant No. 19980407 ), National Climbing Project and National Major Scientific Infrastructure
Program, and National Science Foundation granting to J.T.F., R.B. and K.M.L. in USA.
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