International Journal of Geosciences, 2011, 2, 631-639
doi:10.4236/ijg.2011.24064 Published Online November 2011 (
Copyright © 2011 SciRes. IJG
Regional Flow in the Lower Crust and Upper Mantle
under the Southeastern Tibetan Plateau
Zhi Wang1,2*, Runqiu Huang3, Jian Wang4, Shunping Pei5, Wenli Huang2
1State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology ,
Chengdu, China
2Key Laboratory of Earth Exploration and Information Techniques of Ministry of Education, Chengdu, China
3State Key Laboratory of Geohazards Preventi on and Geoe nvi ro n ment P rot ect i on , Chengdu, China
4Chengdu Institute of Geology and Mine ral Resource, Chengdu, China
5Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China
E-mail: *
Received July 9, 2011; revised August 24, 2011; accepted October 3, 2011
Seismic tomography reveals an “R-shape” regional flow constrained between the depths of 50 to 80 km in
the Southeastern Tibetan Plateau (STP) which demonstrates some of the differences revealed by the mag-
netotelluric (MT) soundings in some areas. The “R-shape” flow could be present in both the lower crust and
uppermost mantle, but not in the lower crust above the Moho discontinuity. Lateral flow has been imaged
under the Qiangtang and Songpan-Ganzi blocks while two channel flows have been revealed beneath the
south part of the STP with the eastward lateral flow from the Qiangtang block separating into two channel
flows. One branch turns southwards at the south Qiangtang block, along the Bangong-Nujiang fault reaching
to the Indochina block, and another is across the Songpan-Ganzi block (fold system) which then separates
into northward and southward parts. The northward branch is along the edge of the north Sichuan basin
reaching to the Qingling fault and the southward channel turns south along the Anninghe fault, then turns
eastward along the margins of the south Sichuan basin. Our study suggests that the crustal deformation along
the deep, large sutures (such as the Longmen Shan fault zone) is maintained by dynamic pressure from the
regional flow intermingled with the hot upwelling asthenosphere. The material in the lower crust and up-
permost mantle flowing outward from the center of the plateau is buttressed by the old, strong lithosphere
that underlies the Sichuan basin, pushing up on the crust above and maintaining steep topography through
dynamic pressure. We therefore consider that the “R-shape” regional flow played a key role in the crustal
deformation along the deep suture zones of the Bangong-Nujiang, the Longmen-Shan faults, and other local
heavily faulted zones.
Keywords: Regional Flow, Channel Flow, Crustal Deformation, Deep and Large Suture
1. Introduction
From a geological viewpoint the southeastern Tibetan
Plateau has a very complex structure and tectonics (Fig-
ure 1), where several tectonic blocks, including the Si-
chuan basin, the Songpan-Ganzi block, the Qiangtang
Block (including the Chuan-Dian Fragment) and the In-
dochina Block, are interacting with each other [1-6]. It is
a site of important processes including strong compres-
sional deformation with crust shortening and thickening,
and east-west crustal extension associated with the In-
dia-Asia collision and its abutment against the stable
Sichuan basin [1,3,4]. Previous studies have been made
of the crust and upper mantle structure in the STP using
various approaches. The Geological and Geodetic studies
show that the crustal blocks move clockwise along the
margins of the STP [7-11]. The local tomography models
show the three-dimensional (3-D) structure down to a
maximum of about 80 km depth because local earth-
quakes occur at focal depths shallower than 45 km under
this region [5,6,8,12]. Although teleseismic tomographic
studies imaged velocity anomalies in the upper mantle,
Figure 1. Topogr aphy and tectonic fr amework of the South-
eastern Tibetan Plateau. Blue solid lines indicate the deep,
large sutures and main tectonic boundari es. Blac k thin lines
show the active faults. Blue open circles show the locations
of the large historic earthquakes (M > 6.5) occurred from
1913 to 2008. Three blue arrows indicate the rotation direc-
tions of the Songpan-Ganzi fold system, the Qiangtang and
Indochina Blocks, respectively. Magnitude scale of the
earthquakes is shown on the left. A blue box in the inset
map shows the present study region.
the detailed structure in the crust was not imaged [13,14].
Tomographic and MT researchers revealed wide-scale
existence of lower crustal fluids in middle and south-
eastern Tibet. They suggest that uplift and crustal thick-
ening of the STP has been supported by the stable litho-
sphere of the Sichuan basin through the influx of the
crustal flow [6,15]. It should be noted that the existence
of lower crustal flow would be characterized as low
seismic velocity (Vp, Vs) and low electrical resistivity,
compared with a stable continental crust. Therefore, de-
termining high-resolution 3-D Vp and Vs models of both
the crust and upper mantle is crucial for a better under-
standing of the spatial distribution of the regional flow
beneath the STP.
The relationship between the Songpan-Ganzi fold sys-
tem (block) and the Longmen-Shan thrust belt deforma-
tion developing at a continental plate boundary, and the
reason for the development of an orogenic plateau away
from that boundary is still unclear in continental dynam-
ics. In order to investigate the features of the crustal and
upper mantle flow beneath the STP and its implication
on the crustal deformation along the deep, large sutures,
we determined the 3-D high resolution seismic velocity
in the crust and upper mantle using a large number of
P-wave and S-wave arrival times from both local and
teleseismic events. We believe that the Vp and Vs mod-
els, along with the previous electrical parameters, could
provide important evidence for a better understanding of
the spatial distribution of the flow materials in the lower
crust and uppermost mantle, as well as crustal deforma-
2. Data and Method
In this study we used 254 seismic stations for seismic
tomography (Figure 2(a)). These stations included four
temporal seismic arrays and four permanent seismic
networks. The temporal seismic arrays included the
Namche Barwa Tibet (XE) seismic array; the MIT-China
(YA) seismic array; the Namche Barwa-Tibet Pilot Ex-
periment (XC-2002); and the Tibetan Plateau Broadband
Experiment (XC-1991). The permanent networks are
deployed by the Sichuan and Yunnan Seismological Bu-
reaus (SYSB), the Institute of Geophysics of China
Earthquake Administration (IGCEA) and the Interna-
tional Seismological Center (ISC).
We collected a large number of P-wave and S-wave
arrival times from local earthquakes and teleseismic
events that occurred during the period from January 1985
to December 2007. Arrival times for local earthquakes
were collected according to the following criteria: earth-
quakes occurring in the study region with depths shal-
lower than 80 km and magnitudes greater than Mb 1.5;
earthquakes where epicentral distances were limited in
the range of 0˚ - 2˚ for P and S phases and 3˚ - 6˚ for Pn
and Sn arrivals. In the epicentral distance range of 3˚ - 6˚,
the Pn and Sn become the first arrivals and their arrival
times can be identified clearly from the P and S phases.
The travel time residuals from –3.0 s to +3.0 s for P and
Pn waves and from –4.0 s to +4.0 s for S and Sn phases
were used in the tomographic inversion. In total, 140,341
P and Pn phases and 114,268 S and Sn phases were se-
lected from 14,474 local earthquakes recorded by more
than eight seismic stations.
Because most of the local earthquakes occurred at a
focal depth shallower than 50 km (Figure 2(a)), it is dif-
ficult to image the deep structures in the lower crust and
upper mantle at high resolutions using the local arrival
time data only. We therefore collected the P-wave and
S-wave arrival times from teleseismic events to image
the upper mantle structure under the STP (Figure 2(b)).
We handpicked 16,382 P phases and 13,318 S phases
from the four portable seismic arrays along with 40 sta-
tions deployed by the SYSB. The selected teleseismic
events have a fairly complete azimuthal coverage (Fig-
re 2(b)). All the teleseismic events recorded by more u
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Figure 2. (a) 3-D hypocenter distribution of local earthquakes. Blue dots show the hypocenters of 14,474 local earthquakes
with magnitude larger than 1.5. Yellow triangles show the permanent stations deployed by the Seismological Bureaus of Si-
chuan and Yunnan Provinces. Green squares show the seismic station by the Institute of Geophysics of China Earthquake
administration. Red and blue triangles indicate the temporal seismic arrays. (b) Epicenter distribution of the 4782 teleseismic
events (red dots) used in this study. The concentric circles correspond to epicentral distance of 30˚, 60˚ and 90˚. The blue box
denotes the present study region.
than nine seismic stations are at epicentral distances be-
tween 30˚ and 90˚, thus removing the influences of com-
plex structures of the core-mantle boundary and the up-
per mantle outside of the study area. In total, 49,173
P-wave and 39,572 S-wave relative arrivals, including
the arrival time data from ISC were collected from the
4782 teleseismic events with magnitude (Mb) greater
than 5.5. The relative travel time residuals ranging from
–3.0 s to +3.0 s for P-wave and –4.0 s to +4.0 s for S-
wave are used in the tomographic inversion. In this study,
a joint tomographic inversion method [16] has been used
to analyze the local and teleseismic data simultaneously
for determining the 3-D Vp and Vs structures of the crust
and upper mantle in the STP. The absolute arrival time
data for local earthquakes are used for the hypocenter
location and tomogr aphic inversion simultaneou sly while
the relative travel time residuals for the teleseismic
events are used to invert to determine the upper mantle
structure. Thus the effects of the uncertainties in the
hypocenter parameters of the teleseismic events and the
effects of the structural heterogeneities outside of the
study region can be a v oided.
3. Resolution Analyses
Prior to seismic tomography, we conducted checkerboard
resolution tests (CRTs) to evaluate the resolution of our
3-D Vp and Vs models. Velocity perturbations of +/–2%
were assigned to the grid nodes adjacent to each other,
and then synthetic travel times were calculated for the
checkerboard model. Figure 3 shows the plan views of
the CRT results for Vp and Vs images at each depth,
indicating that the resolution of the tomographic images
is high at depths of between 25 and 90 km. The CRT
results for the Vp and Vs models are consistent with each
other (Figure 3). Although few earthquakes occurred in
the lower crust and no local earthquakes occurred in the
upper mantle under the study region, numerous rays of
head waves (Pn, Sn) from the crustal earthquakes were
refracted at the Moho discontinuity, allowing the near
vertical rays from the teleseismic events to sample the
lower crust and upper mantle. The checkerboard pattern
in the lower crust and uppermost mantle was generally
recovered, and therefore we believe that the seismic ve-
locity variations in the study region were resolvable fea-
4. Discussion
Figures 4 and 5 shows the vertical cross-sections per-
pendicular to the deep, large sutures of the Bangong-
Nujiang, the Xianshuihe, the Anninghe (Figure 4) and
Figure 3. Plan views of the checkerboard resolution tests
(CRTs) for the P and S wave tomography. The input am-
plitudes of the velocity perturbations are 2% at every grid
nodes. The layer depth is shown at the lower-left corner of
each map. The scale for the velocity perturbations (in %)
relative to the 1-D initial velocity model is shown at the
the Longmen-Shan fault (Figure 5). Strong low velocity
zones are clearly imaged beneath the deep, large sutures
at the depths of the lower crust and uppermost mantle.
The low velocity zone extends up to 100 km depth under
the Songpan-Ganzi and Qiangtang blocks (Figure 5 and
sections L1 and L2 in Figure 4). However, the extended
depth is shallower than 80 km in the Chuan-Dian frag-
ment (Figure 5 and profile L3 in Figure 4). Figure 6
indicates the plan view of the low velocity anomalies.
The depth of the low velocity zone varies from the lower
crust to the uppermost mantle. Two low velocity zones
are clearly imaged: one zone is along the Bangong-Nu-
jiang and the Nujiang sutures; another zone runs along
the Xianshuihe fault, the Anninghe fault and then turns
eastward along the edge portion of the south Sichuan
basin (Figures 4-6). The feature of the low velocity zone
along two channels imaged from the seismic tomography
is similar to that revealed by magnetotelluric soundings,
which is interpreted to be the low velocity zones with
high electrical conductivity as channel flow [15,17]. We
consider that the lateral low velocity zone under the
Qiangtang and Songpan-Ganzi blocks could be lateral
flow in the lower crust and uppermost mantle, and the
two branches of low velocity zones along the Nujiang
suture and the edges of the southwestern Sichuan basin
could be channel flow (Figure 6).
Our seismic study indicates that the low velocity zones
are not only constrained in the lower crust above the
Moho discontinuity bu t are also imaged in the uppermost
mantle (Figures 4-6), similar to mantle flow under a
continent determined through previous studies [18,19].
The higher conductivity anomalies found in the lower
crust and upper mantle are consistent with the Plio-
Pleistocene volcanic rocks associated with delimitation
and derivation of the upper mantle in northern Tibet [20,
21]. The high electrical conductivity of the upper man-
tle is consistent with a region of low seismic velocities
with high attenuation found in northern Tibet [22]. The
depth of the flow is different from those revealed by the
previous studies which suggest the channel flow is
along the Moho boundary in the lower crust [15,23,24].
The degree of rheological stratification which dictates
crust-mantle coupling strongly influences the behavior
of the lithosphere during deformation by controlling
how strain is vertically partitioned [2,25]. We consider
that the low velocity zones in both the lower crust and
up pe rm ost mantle as a reliable feature because: 1) a large
number of arrival times are used from both local and tele-
seismic events which effectively sample the lower crust
and uppermost mantle (Figure 2); and 2) the CRT results
show high resolution along the two deep, large sutures at
these depths (Figure 3). The previous studies suggest
that a weak lower crustal layer is unable to support
large topographic stresses and leads to a low relief pla-
teau. The identified features of the seismic and electri-
al properties are to be expected if a partial melt were c
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Figure 4. Vertical cross-sections of P- and S-wave velocity images along the lines shown on the up-right inset map. Red color
indicates low veloci ty while blue c olor denotes high velocity. The surface topography is shown on the top of each section. LB,
Lhasa Block. QTB, Qiangtang Block. SGB, Songpan-Ganzi block (fold system). CDF, Chuan-Dian Fragment. SCB, Sichuan
Basin. IC, Indochina. SB, South China block.
Figure 5. Vertical cross-sections of P- and S-wave velocity images along the lines shown on the up-left inset map. Red color
indicates low velocity while blue color denotes high velocity. The surface topography is shown on the top of each profile. SGB,
Songpan-Ganzi block (fold system). SCB, Sichuan Basin. LMS, Longmen-Shan. LB, Lhasa Block. CDF, Chuan-Dian Frag-
present. We would then suggest that the flow includes
both the lower crust and uppermost mantle as evidenced
by the seismic and magnetotelluric data.
Our seismic images show that lateral low velocity
anomalies are imaged beneath the Qiangtang and Song-
pan-Ganzi blocks while channel low velocity zones are
determined along the Bangong-Nujiang suture and the
margin areas to the southwestern Sichuan basin (Figure
6). The previous lower crustal flow model suggests that
after encountering the rheologically strong basement of
the Sichuan Basin along the edge portion of the north
Songpan-Ganzi block [23], crustal flow was diverted
northeastward beneath the edge of the northwestern Si-
chuan basin. This follows the rheologically “weak”
crustal corridor along the Paleozoic-Mesozoic Qinling
fault zone, which leads to the dextral strike-slip along th e
Longmen-Shan fault zone and related second order faults
[26]. The locations of the low velocity zones identified
from our seismic images show a good correlation with
he previous lower crustal flow model under the Song- t
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Figure 6. Left plan show ing contour of smoothed elevations of Tibe tan Plateau and surrounding re gions [23]. Dark gray and
light gray areas indicate regions of cold, strong, continental materials and intermediate strength, respectively. White areas
represent weak crustal regions. Right map showing low velocity anomalies (in %) along the depth between the lower crust
and uppermost mantle. Red color indicates the locations of the regional flow. Block dashed lines show the deep, large sutures
and the main tectonic boundarie s. Black arrows indicate the presumed directions of the re gional flow outward from the cen-
ter of the plateau. BNJF, the Bangong-Nujian suture. XSF, Xianshuihe suture. LMF, the Longmen-Shan suture. NJF, the
Nujiang suture. ANF, the Anninghe suture. RRF, the red river suture. QLF, the Qingling fault.
pan-Ganzi block and the center part of the Qiangtang
block. We thus consider that there is both lateral flow
and channel flow present under the north and south parts
of the STP, respectively.
Recent seismic imaging and crustal stress analysis
have revealed similar features of the crustal structure
beneath the STP [5,6,27]. The low velocity zones re-
vealed along the Moho discontinuity in the lower crust
and uppermost mantle beneath the STP (Figures 4-6)
show gene ra l agreem en t with the previous studies [2,5,12,
27]. The magnetotelluric profiles show the high conduc-
tivity anomaly at depths of between 40 and 80 km under
southeast Tibet [15,22,28,29], to represent the wide-
spread existence of fluids at these depths. Satellite mag-
netic studies and geophysical modeling imply the exis-
tence of partial melts along the Moho boundary. This is
because the elevated temperatures caused by radiogenic
heating are high enough to cause melting due to the rapid
variations of surface topography over a short horizontal
distance across the margins of the STP [4,30]. We inter-
pret the low velocity zones with high conductivity
anomalies as lateral flow under the north part and chan-
nel flow beneath the south part of the STP. These are
associated with the presence of fluids such as partial melt
or aqueous fluids from the lower crust and upper mantle
To explain the crustal deformation and widespread ex-
tensional tectonism, many mod els associated with crustal
thinning, mantle thinning or removal, and lower crustal
flow have been proposed. Based on our tomographic
images along with the results of the previous studies, we
propose a dynamic model for the margin regions of the
STP. The flow from central Tibet intermingles with the
hot upwelling asthenosphere from the upper mantle,
building up the tectonic stress beneath the margins of the
STP. The deeply rooted, craton-like lithosphere under the
Sichuan basin forms a sharply contrasted velocity struc-
ture across the margins from the basin to the mountains
[6]. The velocity anomalies revealed by this study, and
the spatial distribution of the crustal stress indicated by
the previous study, suggest that the crustal deformation is
strongly coupled with the motions of the crust and upper
mantle [27]. This leads to high pore pressure accumulat-
ing in the upper crust in the margins. A previous study
reveals the lower fault friction within the deep and large
sutures under the southeastern Tibetan margins, which
leads to negligible compression across the sutures [31].
Compressional or extensional deformation observed in
the surface geological record is considered to represent
changes in thickness to the entire crust or lithosphere
beneath the deforming region. However, if material is
added to the crust from mantle melts or from adjacent
undeformed regions by lower crustal flow, or if material
is lost from the lithosphere by mantle foundering, there
may be a deficit or excess of crust or mantle material
from what would otherwise be predicted from the de-
Copyright © 2011 SciRes. IJG
formation recorded in surface rocks. We conclude that
crustal deformation in the margin regions of the STP is
maintained by the tectonic stress from the fluid bearing
flow along the Moho boundary in both the lower crust
and the uppermost mantle. This is because the material
flowing outward from the center of the plateau is but-
tressed by the old and strong cratonic lithosphere beneath
the Sichuan basin [3,6], pushing up on the crust above
and maintaining steep topography through dynamic
pressure in the edge portions of the STP.
5. Conclusions
Our tomographic study indicates the existence of an “R-
shape” regional flow in the lower crust and uppermost
mantle beneath the STP. The spatial distribution of the
channel flows coincide well with the great deep sutures,
such as the Bangong-Nuj iang fault; the Xianshuihe fault;
the Longmen-Shan fault; and the Anninghe fault, indi-
cating the close relationship between the channel flow
and the deep fault structures. The crustal deformation
along the sutures, for example, along the Longmen-Shan
fault, is maintained by dynamic pressure from the re-
gional channel flow intermingling with the ho t upwelling
asthenosphere. The material in the lower crust and up-
permost mantle flowing outward from the center of the
plateau is buttressed by the old, strong lithosphere that
underlies the Sichuan basin, pushing up on the crust
above and maintaining the steep topography by dynamic
pressure. We thus suggest that the “R-shape” regional
flow plays a key role on crustal deformation along the
deep and large suture zones.
6. Acknowledgements
The arrival-time data used is provided by the Seismol-
ogical Bureaus of Sichuan and Yunnan provinces and
the Institute of Geophysics of China Earthquake Ad-
ministration. We thank the staffs of the temporal seis-
mic arrays and the IRIS data center for providing the
seismic data used in the present study. This work was
partially supported by the projects sponsored by NSFC
(40872148, 40974024, 40839909, 40972087, 41030426)
and the research grants (NCET-10-0887, KY TD201002,
2010JQ0033). We are also grateful to the Cultivating
Programme of excellent innovation team of Chengdu
University of Technology for supporting this work.
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