Download Is the Asian lithosphere underthrusting beneath northeastern

Earth and Planetary Science Letters 428 (2015) 172–180
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Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Is the Asian lithosphere underthrusting beneath northeastern Tibetan
Plateau? Insights from seismic receiver functions
Xuzhang Shen a,∗ , Xiaohui Yuan b , Mian Liu c,d
a
Lanzhou Institute of Seismology, China Earthquake Administration, Lanzhou, 730000, China
Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, 14473 Potsdam, Germany
c
Department of Geological Sciences, University of Missouri, Columbia, MO 65211, USA
d
Key Laboratory of Computational Geodynamics, Chinese Academy of Sciences, Beijing, China
b
a r t i c l e
i n f o
Article history:
Received 11 May 2015
Received in revised form 16 July 2015
Accepted 18 July 2015
Available online xxxx
Editor: A. Yin
Keywords:
lithosphere–asthenosphere boundary
Northeastern margin of the Tibetan Plateau
Receiver functions
a b s t r a c t
Whether or not the Asian lithosphere has underthrusted beneath the Tibetan Plateau is important for
understanding the mechanisms of the plateau’s growth. Using data from the permanent seismic stations
in northeastern Tibetan Plateau, we studied seismic structures of the lithosphere and upper mantle across
the plateau’s northeastern margin using P and S receiver functions. The migrated P- and S-receiver
function images reveal a thick crust and a diffuse lithosphere–asthenosphere boundary (LAB) beneath
the Tibetan Plateau, contrasting sharply with the relatively thin crust and clear, sharp LAB under the
bounding Asian blocks. The well-defined LAB under the Asian blocks tilts toward but does not extend
significantly under the Tibetan Plateau; this is inconsistent with the model of Asian mantle lithosphere
underthrusting beneath the Tibet Plateau. Instead, our results indicate limited, passive deformation of the
bounding Asian lithosphere as it encounters the growing Tibetan Plateau.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
The formation of the Tibetan Plateau through the Cenozoic has
been driven by the northward moving Indian plate colliding with
the Eurasian plate (Molnar and Tapponnier, 1975; Dewey et al.,
1988; Royden et al., 2008; Yin and Harrison, 2000; Yin et al.,
2002). On the southern side of the plateau, the plate convergence
is marked by large-scale underthrusting of the Indian plate under
the Tibetan Plateau, as indicated by seismic images (e.g., Nábělek
et al., 2009; Nelson et al., 1996; Owens and Zandt, 1997), although
the northern limit of the underthrusting Indian plate remains debated (Nábělek et al., 2009; Zhao et al., 2010).
On the northern and the eastern sides, the Tibetan Plateau
is bounded by relatively stable blocks of the Asian lithosphere
(Fig. 1). How these Asian lithospheric blocks responded to the
Indo–Eurasian collision is critical for understanding the mechanisms of the growth of the Tibetan Plateau. Numerous models,
based on geological reconstructions and geophysical data, suggested for substantial underthrusting (subduction) of the Asian
lithosphere under the Tibetan Plateau (Tapponnier et al., 2001;
Yin et al., 2008a, 2008b; Kind et al., 2002; Zhao et al., 2011), sim-
*
Corresponding author.
E-mail address: [email protected] (X. Shen).
http://dx.doi.org/10.1016/j.epsl.2015.07.041
0012-821X/© 2015 Elsevier B.V. All rights reserved.
ilar to the underthrusting of the Indian plate beneath Himalaya.
Meyer et al. (1998) studied the growth of Tibetan Plateau based on
satellite image analysis and suggested that south-directed subduction in the northeastern Tibet may have resulted from Palaeozoic
and Mesozoic accretion on the southern margin of the Eurasian
plate. Tapponnier et al. (2001) proposed several possible onsets
of southward mantle underthrusting beneath northeastern Tibetan
Plateau. However, in contrast to the abundant seismic evidence
for the subducting Indian plate under southern Tibetan Plateau,
evidence for subduction of the Asian lithosphere under northern Tibet has been limited and inconclusive (Kind et al., 2002;
Zhao et al., 2011). Zhao et al. (2011) and Ye et al. (2015) reported evidence for possible southward underthrusting of Asian
lithosphere beneath central and northern Tibet based on receiver
functions of local temporary seismic array. However, the results
of receiver functions can be affected by station coverage, which
might lead to the overgeneralization of the lithosphere deformation. A seismic tomography study (Liang et al., 2012) observed
a low velocity upper mantle beneath northern Tibet, which argues against underthrusting of the Asian lithosphere. Furthermore,
whereas underthrusting of the Indian plate is marked by abundant
seismicity, no earthquakes with focal mechanisms consistent with
underthrusting of Asian lithosphere have been observed in northern Tibet.
X. Shen et al. / Earth and Planetary Science Letters 428 (2015) 172–180
Fig. 1. Map of topographic relief and seismic stations (blue triangles) used for this
study. Thick black lines denote the boundaries of major tectonic blocks (Deng et al.,
2003). Thin gray lines show the main faults. Green and red crosses represent the location of P-to-s and S-to-p pierce points at 100 km depth, respectively. Purple lines
are locations of receiver function profiles shown in Figs. 5–8. HXC: Hexi corridor;
EMT: eastern margin of the Tibetan Plateau. Inset map shows the study area. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
A key place to investigate the hypothesized underthrusting of
the Asian lithosphere is northeastern Tibetan Plateau, where the
collision-driven crustal shortening and uplift may have started as
early as Eocene (Clark et al., 2010; Yin, 2010; Yuan et al., 2013),
and active crustal deformation is indicated by GPS measurements
(Gan et al., 2007; Liang et al., 2013; Zhang et al., 2004), precise
leveling data (Hao et al., 2014), and intensive seismicity (Deng et
al., 2003; Liu et al., 2007). Bounded by the relatively stable Alxa,
Ordos, and Yangtze blocks (Fig. 1), northeastern Tibetan Plateau
is also the primary site of geological studies that led the models
of large-scale underthrusting of the Asian lithosphere under the
Tibetan Plateau, because the sequences of anticlinal thrust systems
at the plateau’s northern edges indicate detachment of the growing
plateau from the underlain Asian lithosphere (Meyer et al., 1998;
Tapponnier et al., 2001).
For various reasons, seismic studies in northeastern Tibetan
Plateau have been limited. In this work, we use the P- and
S-receiver functions, derived from data collected from four local
networks of permanent seismic stations (Fig. 1) which well cover
the north Tibet and the bounding Asian blocks, to image the lithospheric and upper mantle structures along and across the margins of northeastern Tibetan Plateau. Our results show no significant underthrusting of the Asian lithosphere beneath the Tibetan
Plateau.
2. Data and method
The seismic waveforms used in this study are from four seismic networks of 75 permanent stations distributed in the Gansu,
Qinghai, Ningxia and Sichuan provinces, China (Fig. 1). An instrument update in 2008 has equipped all stations with broad-band
seismometers, most of them are CMG-3ESPC made in UK.
We used the receiver function method to image the Moho and
LAB in the study area. Receiver functions, derived from analyzing
P-to-S or S-to-P wave conversions, have been proven an effective tool for detecting seismic discontinuities in the upper mantle (Langston, 1979; Farra and Vinnik, 2000; Yuan et al., 2006;
Li et al., 2004; Kumar et al., 2005). P receiver function (PRF) is good
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Fig. 2. Map of teleseismic events used in this study. The triangle is the centroid
location of the seismic network. Black squares and red circles are the events used
for calculation of P and S receiver functions, respectively. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
for detecting the Moho and upper mantle discontinuities, but can
be difficult for identifying the LAB because of the interference with
multiples from the Moho or shallow structures. For this reason,
S receiver function (SRF), although more difficult to analyze, is better suited for studying the LAB. Hence we used both the PRF and
SRF in our studies. The reverberations from shallow structures in
the PRFs are separated from the primary conversions in the SRFs.
The arrivals of the converted phases are earlier than the S phase,
whereas multiples arrive after the main phase. On the other hand,
the amplitude of S-to-P phase may be higher than the corresponding P-to-S phase, given the stronger anelastic attenuation of the S
waves than that of the P waves (Wittlinger and Farra, 2007).
The calculation of PRF is usually stable, and the results from different studies are generally repeatable, because the initial P wave
and its coda are relatively simple, and there are no signals before
the arrival of P wave. The signals around the S phase are more
complicated than P wave, so it is no easy way to separate S-to-P
phase from S phase. To ensure the objectivity and rationality of
SRF, we used the P-to-S phase from the Moho on PRFs as a criterion to evaluate the validity of SRFs.
For P receiver functions, records of teleseismic events with
Ms > 5.5 from January 2008 to April 2011, with epicentral distances in the range of 30–90 deg, are collected. The source parameters were taken from the US Geological Survey global catalog
(http://neic.usgs.gov/neis/epic/epic_global.html). Fig. 2 shows the
locations of these events. We selected records with high signal-tonoise ratio and clear onset of P-waves. The waveforms were rotated
from the north–east–vertical (N–E–Z) to the radial–transverse–
vertical (R–T–Z) coordinates using the back-azimuth. The threecomponent records were then cut in the time window of 20 s prior
to and 100 s after the P-arrival. We then constructed the receiver
functions by deconvolving the vertical component from radial component using an iterative approach (Ligorria and Ammon, 1999). A
low-pass Gaussian filter with half-width constant a = 2.5 was applied to smooth the PRFs. All traces are moveout corrected before
summation, using a reference slowness of 6.4 s/deg based on the
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X. Shen et al. / Earth and Planetary Science Letters 428 (2015) 172–180
Fig. 3. Moveout-corrected P (a) and S (b) receiver functions of station AXX (location in Fig. 1). Red color represents positive energy, and blue color represents negative. The
stacking result is shown in the top panels. PM s and SM p phases at about 6.5 s are P-to-S and S-to-P converted phases at the Moho, respectively. PL s and SL p mark the
converted phases at the LAB. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
IASP91 global reference model (Kennett and Engdahl, 1991). Fig. 3a
shows 128 P-receiver functions of station AXX (Fig. 1).
For S receiver functions, the records with epicentral distances
in the range of 60–85 deg were collected (Fig. 2). We first selected records with high signal-to-noise ratio and clear S phase,
and then applied a third order low-pass filter of 2 s to the original
waveforms. The N–E–Z components were rotated to R–T–Z coordinate system according to the theoretical back azimuth. R and Z
are then rotated into the P–SV system. In this step, series of incidence angles ranging from 0 to 60 deg with a 2-degree step are
tested. For each rotation test, the S receiver function is constructed
by deconvolving the SV component from P component. The deconvolution is performed in time domain as described by Kumar et
al. (2006). The time axis is reversed for comparison with the P receiver function. The optimal S receiver function for each event is
chosen by the criteria of minimum energy at the zero time. All S
receiver function traces for each station are corrected for distance
moveout as for P receiver functions. Fig. 3b shows 50 SRFs of station AXX.
In the same way, we obtained P and S receiver functions for all
the stations and migrated them to image the seismic structure in
the region. Generally, more data are available for calculating PRFs
than SRFs. In this study, 7316 P- and 2737 S-receiver functions are
calculated.
3. Results
The stacked P and S receiver functions of station AXX exhibit
prominent positive signals around 6 s and negative signals around
12 s (Fig. 3). The consistency of the Moho signals around 6 s on
both PRF and SRF indicates the stability and validity of SRF. The
strong negative phases on SRF around 12 s come from an interface of decreasing velocity, which conforms with the feature of
LAB. Similar negative signals are also shown in PRFs. Based on
the PRFs alone, it would be questionable to consider the negative
signals around 12 s as the P-to-S phase from the LAB, because
multiples on PRFs from interfaces within the crust may arrive
in this time window. But the consistent arrivals of positive and
negative signals on both P and S receiver functions suggest that
these signals indicate true interfaces rather than artifacts of multiples.
We stacked the moveout corrected P and S receiver functions
of all stations as a function of the longitude of P-to-S (or S-to-P)
pierce points at 100 km depth (Fig. 4). The S-to-P phases from the
Moho and LAB in S receiver functions are consistent with those in
P receiver functions, indicating that these phases are reliable and
stable.
To investigate the variations of LAB across the margins of the
Tibetan Plateau, we used the CCP (common conversion point)
stacking method to image the structures down to a depth of
250 km along three profiles (Fig. 1). The horizontal and vertical
step is 1 km. Receiver functions with the pierce points within one
Fresnel’s zone along the ray path were stacked to get the intensity
of the migration images. The AA profile (Fig. 5) goes from the Tibetan Plateau to the Alxa block. We picked the Moho depth in S
receiver functions (Fig. 5a), and copied it to the P receiver function image (Fig. 5b). The Moho depth shows crustal thinning from
∼70 km beneath Tibetan Plateau to ∼50 km beneath the Alxa
black. The largest change of the Moho depth occurs at ∼97.8◦ E,
near the topographic boundary of the Tibetan Plateau. Strong negative signals in SRFs between 70–150 km depth, interpreted as
caused by the LAB, are found beneath the Alxa block, but the LAB
is not so clear beneath the Tibetan Plateau. We sketched the contours of LAB with gray lines in Fig. 5a according to the prominent
negative signals. The sharp LAB under the Alxa block slightly dips
southward, but does not extend significantly into the mantle under
the Tibetan Plateau.
The BB profile (Fig. 6) goes through northeast Tibet to the Ordos block (Fig. 1). As in the AA profile, the Moho is determined
from the SRFs. It shows crustal thinning from the Tibetan Plateau
to the Ordos block and other terranes in the plateau’s east margin (EMT in Fig. 1a). Similar to the AA profile, the Moho depth
changes for ∼10 km beneath the topographic boundary of the Tibetan Plateau in both S receiver functions and P receiver functions
(Fig. 6). Besides the clear negative signals at 90–130 km depth,
strong negative signals are also found at 160–175 km depth in the
S receiver function image. These negative signals, interpreted to be
the LAB, stops roughly under the topographic boundary of the Ti-
X. Shen et al. / Earth and Planetary Science Letters 428 (2015) 172–180
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Fig. 4. Stacked P (a) and S (b) receiver functions of all stations in bins of longitude of pierce points at 100 km depth. PM s and SM p at about 6 s are P-to-S and S-to-P
converted phases at the Moho, respectively. PL s and SL p at about 11 s mark the converted phases at the LAB.
Fig. 5. (a) Migration images of S receiver functions along profile AA (location shown in Fig. 1). (b) Migration images of P receiver functions along the same profile. The Moho
and LAB are marked with black and gray dash lines, respectively. The topography along the profile is shown in the top panel. The thick black line marks the topographic
boundary of the Tibetan Plateau.
betan Plateau. Under the Tibetan Plateau, the LAB signals are weak
or absent.
The CC profile (Fig. 7) goes through the Tibetan Plateau and the
Yangtze block (Fig. 1). Similar to AA and BB profiles, the Moho is
deeper under the Tibetan Plateau and changes abruptly near the
topographic boundary of the plateau. The pronounced LAB signals
extend slightly into the mantle under the Tibetan Plateau, but the
scale is much more limited than the subduction of Asian litho-
sphere envisioned in previous models. The LAB signals under the
Tibetan Plateau are not clear, as in the other profiles.
In order to evaluate the errors of the CCP stacking, we performed binning stacks of the S receiver functions along the 3
profiles as Figs. 5–7. We calculated S-to-P piercing points at 100
km depth and stacked S receiver functions with piercing points
closer than 100 km from each profile by bins of piercing point
longitude of 0.15 deg (Fig. 8). The bootstrap resampling method
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X. Shen et al. / Earth and Planetary Science Letters 428 (2015) 172–180
Fig. 6. (a) Migration images of S receiver functions along profile BB (location shown in Fig. 1). (b) Migration images of P receiver functions along the same profile. The Moho
and LAB are marked with black and gray dash lines, respectively. The topography along the profile is shown in the top panel. The thick black lines mark the block boundaries.
Fig. 7. (a) Migration images of S receiver functions along profile CC (location shown in Fig. 1). (b) Migration images of P receiver functions along the same profile. The Moho
and LAB are marked with black and gray dash lines, respectively. The topography along the profile is shown in the top panel. The thick black line marks the topographic
boundary of the Tibetan Plateau.
X. Shen et al. / Earth and Planetary Science Letters 428 (2015) 172–180
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Fig. 8. Binning stacks of S receiver functions along the same profiles of Figs. 5–7. Profile locations are shown in Fig. 1. For each profile S receiver functions with piercing
points at 100 km depth closer than 100 km from the profile are stacked in bins of 0.15◦ piercing point longitude. Only amplitudes exceeding a standard deviation are plotted.
The Moho and LAB phases are marked. Topography along each profile is plotted on the top of each figure with tectonic units indicated. (For interpretation of the references
to color in this figure, the reader is referred to the web version of this article.)
(Efron and Tibshirani, 1998) is used to estimate the errors. In each
profile only the amplitudes larger than a standard deviation are
plotted, positive in red and negative in blue. Although for geological interpretation the binning stack profiles are only valid for the
depth range of around 100 km, they provide useful information to
assess the errors and reliability of the data. We picked the Moho
and LAB signals by coherent phases throughout the profiles. The
Moho remains clear throughout all the profiles, the LAB is more
visible in the eastern parts of the profiles in the Asian lithospheric
blocks.
4. Discussion
In this work we interpreted the negative velocity gradients in
the depth range of 70–150 km as the LAB. This is similar to the
LAB depths observed by receiver functions under many cratonic
regions, but shallower than the typical 200–250 km depths derived under cratons from tomographic results, where velocity gradient changes only slightly (Eaton et al., 2009; Abt et al., 2010;
Kumar et al., 2012; Rychert et al., 2005; Thybo, 2006; Nettles
and Dziewonski, 2008). Karato (2012) has explained this discrep-
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X. Shen et al. / Earth and Planetary Science Letters 428 (2015) 172–180
Fig. 10. Conceptual model of the lithospheric structures across the northeastern
margin of the Tibetan Plateau marked by sharp and clear LAB under the bounding Asian blocks and diffuse LAB under the plateau. The results show no large-scale
underthrusting of the Asian lithosphere under the Tibetan Plateau.
Fig. 9. RFs stacks in the Tibetan Plateau, the bounding Asian blocks, and the transition zone. (a) Pierce points of S100p in the Tibetan Plateau (blue), the transition
zone (red), and the Asian blocks (green). (b) Stacked SRFs of the three regions according to the location of piercing points of S100p phase. Signals from the Moho
and LAB are also marked. Num: the number of SRFs in each region.
ancy by attributing the larger velocity decreases in the shallower
mantle to greater temperature gradient there. With increasing
depth, temperature gradient decreases and the effect of pressure
start to dominate, leading to a minimum in seismic wave velocities. In this study, the LAB as shown by the strongest negative
signals on SRFs is 70–150 km deep; we also found other relatively weak negative signals, such as those around ∼170 km
depth in Fig. 6a. Using the results from surface wave dispersion
(Huang et al., 2003), An and Shi (2006) estimated the lithosphere
thickness to be about 120–200 km in our study area. The difference may arise from the fact that surface wave dispersion is
sensitive to the average of S velocity and relatively insensitive to
the sharpness of the LAB, while the receiver function method is
more sensitive to the velocity contrast.
Whether the true LAB is identified by the sharp velocity contrast in the receiver functions or by the greatest gradients of velocity drop in surface waves is not critical here. The most important
and robust feature in our results is the clear contrast of diffuse
LAB beneath the Tibetan Plateau and sharp LABs under the bounding Asian blocks. To better show this contrast, we divided the
moveout-corrected SRF into three regions: the Tibetan Plateau, the
transition zone, and the bounding Asian lithosphere, according to
the S-to-P pierce point at 100-km depth (Fig. 9a). The SRFs in each
region are stacked and the bootstrap resampling method (Efron
and Tibshirani, 1998; Shen et al., 2008) is used to estimate the error of the stacks. Amplitudes exceeding two times of the standard
deviation are shaded in color, which gives a 95% confidence detection level. Fig. 9b shows the stacked SRF for each region. The Moho
remains clear in all regions. The LAB is clearly indicated by the
negative signals around 11 s under the bounding Asian blocks, but
such signals are unclear under the Tibetan Plateau. Although the
number of the SRF in the Tibetan Plateau region is smaller than in
other two regions, the Smp phase from the Moho is equally clear
in all regions, confirming the validity of the comparison of the LAB
amplitudes.
The diffuse LAB under the Tibetan Plateau may imply small
temperature gradient between mantle lithosphere and the asthenosphere. Previous results from tomography with body wave
(Liang et al., 2012), surface wave (Fu et al., 2010) and Pn wave
(Liang and Song, 2006) revealed that the mantle lithosphere and
asthenosphere beneath eastern Tibet are hotter than the reference
values of Asian continent. These results, together with the receiver
functions from our study, do not support a large-scale underthrusting of the Asian lithosphere under the Tibetan Plateau.
No major underthrusting and deformation of the bounding
Asian lithosphere would limit the lateral expansion of the Tibetan
Plateau. Our results are consistent with the notion that the Tibetan Plateau has been largely confined between the indenting
Indian plate and the surrounding Asian blocks with limited lateral
growth (Yuan et al., 2013). Fig. 10 is a conceptual model based on
our data and our interpretation. In this model, relatively hot upper mantle beneath northeast Tibetan Plateau causes the diffuse
LAB. The hot Tibetan mantle was bounded by the cold and rigid
Asian lithosphere, which experienced only limited underthrusting
and deformation. Accordingly, the lateral expansion of the northeastern Tibetan Plateau has been limited.
5. Conclusions
We have imaged the Moho and LAB across the northeastern
margin of the Tibetan Plateau using teleseismic P and S receiver
functions. The consistence of the Moho signals in both P and S
receiver functions indicates the ability and reliability of the SRF
data in detecting the LAB. Our results show clear contrasts of the
lithospheric structures beneath the Tibetan Plateau and the surrounding Asian blocks. The LAB beneath the Tibetan Plateau is
diffuse, whereas it is sharp and clear beneath the Asian blocks.
The boundary of this contrasting LAB occurs roughly under the
topographic boundary of the Tibetan Plateau, indicating no signifi-
X. Shen et al. / Earth and Planetary Science Letters 428 (2015) 172–180
cant underthrusting of Asian mantle lithosphere under the Tibetan
Plateau, and a limited lateral expansion of northeastern Tibetan
Plateau.
Acknowledgements
This work is supported by the basic R&D fund of the Institute of Earthquake Science, China Earthquake Administration
(Grant 2014IESLZ03) and the National Natural Science Foundation of China (Grant 41274093). Waveform data for this study
are provided by Data Management Centre of China National Seismic Network at Institute of Geophysics, China Earthquake Administration (SEISDMC, doi:10.7914/SN/CB, Zheng et al., 2010). Figures were plotted with General Mapping Tools (GMT; Wessel and
Smith, 1995). We thank Dr. Kumar for helping us in data processing.
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