Tomographic evidence for wholesale underthrusting
of India beneath the entire Tibetan plateau
Hua-wei Zhou*, Michael A. Murphy
Department of Geosciences, University of Houston, Houston, TX 77204-5007, USA
Received 15 April 2004; accepted 26 April 2004
Abstract
We analyzed a global tomographic model for the Tibet–Himalayan collision zone, which indicates that the Indian lithospheric slab has
been subducted subhorizontally beneath nearly the entire Tibetan plateau to depths of 165–260 km. Tibetan velocity structure is low in its
crust and high in its lithospheric mantle at depths between 75 and 120 km. We interpret an asthenospheric layer positioned above the
subducted Indian slab at depths between 120 and 165 km beneath the Tibetan plateau. Beneath the central portion of the plateau a low-
velocity anomaly exists from the crust down to 310 km depth, indicating mantle upwelling through a weakened part of the subducted slab.
We present a model, which explains that, the uplift history and low relief of the Tibetan plateau is a result of subhorizontal subduction and
heating of Indian lithosphere that is separated from Tibetan lithosphere by a thin channel of asthenosphere. Two predictions made by our
model are: (1) the amount of shortening in the Himalayas is equivalent to the amount of underthrusted Indian mantle lithosphere; and (2) a
young mantle geochemical signature should be present along the entire southern portion of the Tibetan plateau.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Seismic tomography; Tibetan plateau; Subduction; Underthrusting; India; Himalaya
1. Introduction
Continent–continent collision leading to orogenesis,
which may ultimately result in the development of super-
continents, is a fundamental tectonic process that has
significantly affected the character of Earth’s surface for at
least the past one billion years. In order to characterize the
dynamics of continent–continent collisions, it is essential to
define its present geometry and physical state. This is
especially true for the largest active continent–continent
collision zone on Earth, the Himalayan–Tibetan orogen.
The northward convergence of India into Asia over the past
50 Ma (Patriat and Achache, 1984; Dewey et al., 1988) is
intimately linked to the creation of the Tibetan plateau and
has impacted the tectonic framework throughout Asia
since Cenozoic time (Molnar and Tapponnier, 1975;
1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jseaes.2004.04.007
* Corresponding author. Tel.: C1-713-743-3424; fax: C1-713-743-
9164.
E-mail address: [email protected] (H.-w. Zhou).
Yin and Harrison, 2000). The challenge of unraveling the
evolution of the Tibet–Himalayan orogen, and specifically
the uplift history of the Tibetan plateau, has been
significantly bolstered by the collection of a wide variety
of geologic and geophysical information. Nevertheless,
unresolved issues remain regarding the mechanisms by
which the convergence between India and Asia was
accommodated and the Tibetan plateau was uplifted.
Models that explain the Cenozoic uplift of the Tibetan
plateau can be differentiated from one another by the
predictions they make regarding the lithospheric structure
beneath the Tibetan plateau: (a) complete underthrusting of
the Indian plate (Argand, 1924; Powell and Conaghan,
1973; Powell, 1986) or portions of it beneath Tibet
(DeCelles et al., 2003); (b) lithospheric thickening by
distributed thrust faulting (Dewey and Burke, 1973; Chang
et al., 1986; England and Houseman, 1989) and attendant
convective removal of overthickened mantle lithosphere
(England and Houseman, 1989; Houseman and Molnar,
2001); and (d) intracontinental subduction along reactivated
Journal of Asian Earth Sciences 25 (2005) 445–457
www.elsevier.com/locate/jaes
Fig. 1. Map view of P1200 tomographic model (Zhou, 1996) for Tibet region at depth range of 165–260 km. Red lines are major faults and red dotted
lines are major suture zones in Tibet. IYSZ, Indus-Yalu suture zone; BNS, Bangong-Nujiang suture; JS, Jinsha suture. Purple crosses are earthquake
foci. Purple star is the location of the destructive earthquake near Jamnagar, India, on 26/01/01. Dark gray curve delineates the inferred northern
margin of the subducted Indian lithospheric slab in this depth range. Light gray curve outlines the high-velocity portion of the Tibetan lithospheric
mantle above 120 km depth.
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457446
sutures (Meyer et al., 1998). A necessary first step towards
assessing the merits of these models is a thorough
description of the geometry and physical state of the
lithospheric structure in this region.
This paper provides new insight into the lithospheric
structure beneath the Tibetan plateau based on analyzing a
high-resolution global tomographic model (Zhou, 1996).
Two surprising results are the presence of a slab-shaped
high seismic velocity anomaly beneath nearly the entire
Tibetan plateau that we interpret to be subducted Indian
lithospheric mantle, and an overlying wedge-shaped, slow
seismic velocity anomaly interpreted as an asthenospheric
layer (Fig. 1). Not only does this seismic structure set
boundary conditions for geodynamic modeling of the Tibet–
Himalayan collision zone, but also supports an inference for
the cause of the destructive earthquake that occurred in
western India on 26/01/01.
2. Observations from a global tomography
Seismic tomography provides a snapshot of the deep
Earth structure as expressed by its seismic properties, based
on traveltimes of seismic waves propagating from earth-
quake or man-made sources to seismological stations. In
deriving tomographic models at regional scales, data from
sources or receivers outside the model area are usually used
to enhance the data coverage. Since there are few
seismological stations in Tibet, for example, it is necessary
to use outside sources and stations that have rays traversing
beneath Tibet. A dilemma in using raypaths partially outside
of the model area is that the gain in data quantity is at the
expense of bringing potential contaminations from outside.
However, this dilemma can be overcome by using a
global model that leaves no outside areas, if the model has
high enough resolution (Zhou, 1996; Bijwaard et al., 1998).
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457 447
In this study, we analyze a high-resolution global tomo-
graphic model called P1200 that describes the P-wave
velocity variations from the surface down to 1200 km depth
(Zhou, 1996), using cell size of 18 laterally and 35–50 km
vertically. Although lateral heterogeneities below 1200 km
depth could have small effects on steep-dipping features at
shallow depths, most velocity anomalies interpreted in this
paper are of subhorizontal geometry and therefore not
affected much by deep mantle anomalies through deep-
diving rays.
Fig. 2. Depth slices of velocity variations of the P1200 model (Zhou, 1996). The
75–120 km slice the high-velocity portion of the Tibetan lithospheric mantle is o
slices the inferred margin of the subducted Indian lithospheric mantle is indicate
Fig. 2 shows nine depth slices of the P1200 model
from the surface down to 410 km depth across the
Himalayan–Tibetan collision zone. The abundance of
earthquakes in the region and its surrounding areas
helps to constrain the velocity anomalies. The images
on the depth slices are dominated by coherent velocity
anomalies greater than 108!108 in lateral dimensions,
although there are also smaller anomalies. The resolution
of the data has been tested using checkerboard impulse
resolution tests (Zhou, 1988; Zhou and Clayton, 1990).
coastline is shown in green, and 4-km elevation is shown in black. On the
utlined by a shadowed light gray curve. On the 165–210 and 210–260 km
d by a shadowed dark gray curve. Other legends follow that of Fig. 1.
Fig. 3. Resultofcheckerboard resolution tests at six depth ranges, as indicated in the lower-left cornerof eachpanel. The lateraldimensionof the impulses is38!38.
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457448
As shown in Fig. 3, except some corner areas at shallow
depth ranges, excellent results are achieved throughout the
region at a resolution of 38 laterally and about 150 km
vertically.
The good resolution result as shown in Fig. 3 is largely
due to the use of global data set that contains many rays with
neither sources nor stations inside Tibet, such as earth-
quakes in central Asia that were recorded by Chinese
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457 449
stations. The resolution tests suggest that the velocity
anomalies interpreted in this paper would be reliable if the
signal-to-noise ratio of the data is also sufficient. One
concern is from mantle anomalies below 1200 km depth.
For the subhorizontal, high-amplitude, and shallow struc-
tures interpreted in this paper, however, the contamination
from mantle anomalies below 1200 km depth is relatively
small. The processing of the ISC traveltime data for the
tomographic inversion included corrections for station
statics, hypocentral re-determination, and use of summary
ray culling (Zhou, 1996). The model was constrained by the
multi-scale inversion that improves the handling of uneven
ray coverage. Nevertheless, to reduce the bias of our
interpretation, the full array of depth slices and cross-
sections are presented here.
As shown in Figs. 1 and 2, low velocities characterize the
thick crust of Tibet from the surface down to 75 km depth.
In the 0–35 km depth slice two low-velocity zones extend
from the Tien Shan through western Tarim to western Tibet,
and from the eastern edge of Tibet southwards to the Gulf of
Thailand. These anomalies persist in the second depth slice,
which reveal two additional low velocity patches; one in
central Tibet between 82 and 938E, and the other beneath
western Mongolia and the Qilian Shan. The slow anomaly in
the central Tibet continues in deeper slices to at least
310 km depth, and is narrowing with depth to be within 84
and 918E around 300 km depth. In the 165–260 km depth
range, the region is actually faster than the layers’ average
values, but slower than the rest of the high-velocity slab.
We interpret the extremely coherent, slab-shaped high-
velocity anomaly in the four depth slices from 75 to 260 km
in Fig. 2 as Indian mantle lithosphere. On the 75–120 km
depth slice, for example, the appropriate shape of a
combined India and Tibet is discernable as the largest
high-velocity anomaly. In contrast, on the 120–165 km
depth slice, there is a northwest-trending low-velocity
anomaly from Tarim to central Tibet. This slow anomaly
appears to be connected with the low-velocity anomaly in
the Bay of Bengal. At the same depth range to the
southwest, a northwest-trending high-velocity feature
stretches from the Aral Sea to central India, and another
high-velocity anomaly exists from eastern Tibet to the
Burma arc. On the 165–210 and 210–265 km depth slices,
the Indian lithospheric mantle is the most coherent high-
velocity anomaly that extends northwards beneath nearly
the entire Tibetan plateau. Results from these two depth
slices have been stacked vertically to produce the depth slice
of the subducting Indian slab shown in Fig. 1. In the deepest
three depth slices of Fig. 2 the coherent high-velocity
anomaly in India and Tibet disappears, and the largest high-
velocity anomaly on these slices is north of 408N.
A comparison between the depth slices in Fig. 2 reveals
that the high-velocity Indian lithospheric mantle anomaly is
shifted north–northeasterly as it plunges deeper, manifest-
ing a subduction pattern. As seen in cross-sections A–A1,
B–B1, and D–D1 in Fig. 4, the high-velocity Indian slab
starts to dip towards the Tibetan plateau hundreds of
kilometers south of the Himalayas. Beneath the Tibetan
plateau a low-velocity layer, which we interpret as a thin
asthenospheric layer, exists between the high-velocity
Tibetan lithospheric mantle and the subducted high-velocity
Indian slab. Cross-section C–C1 traverses through the
anomalous low-velocity area in central Tibet and will be
compared later with another study. At lower-mantle depths
beneath India (cross-sections A–A1, B–B1, and D–D1)
there is a large, high-velocity anomaly that has been
interpreted as the subducted Tethyan oceanic slab (Van der
Voo et al., 1999). A complete set of cross-sections is shown
in Fig. 5, with 11 dip sections (E–E1 to O–O1) and three
strike sections (P–P1 to R–R1) with respect to the orogenic
front in the central Himalaya.
A general interpretation can be drawn from the depth
slices and cross-sections. The Indian lithospheric slab is
clearly subducted horizontally beneath the Tibetan plateau.
Above the subducted Indian slab, the Tibetan lithosphere
has an abnormally low-velocity crust and a high-velocity
lithospheric mantle above 120 km depth. In the depth range
of 120–165 km a low-velocity layer exists which we
interpret as an asthenospheric layer between the Tibetan
lithosphere and the subducted Indian slab. The subducted
Indian slab is very strong west of 858E and east of 938E
(Fig. 1). In central Tibet, between 85 and 938E and north of
the Indus-Yalu suture zone, there is a low-velocity area
within the subducted slab. This area is part of a low-velocity
anomaly that extends vertically from 35 km down to at least
310 km in depth. Interestingly, many previous studies in
Tibet were conducted in the area over this anomaly, and
have found features such as low seismic velocities and
indications of partial melting and a mantle-originated
geochemical anomaly (McNamara et al., 1994; Nelson et
al., 1996; Kind et al., 1996; Turner et al., 1996; Owens and
Zandt, 1997; Yuan et al., 1997; Makovsky and Klemperer,
1999; Kosarev et al., 1999; Hoke et al., 2000).
3. Comparison with previous studies
The unique nature of Tibetan plateau has attracted a large
number of previous seismologic studies. During the 1990’s,
for instance, there were findings of high S-wave velocities
under westernmost Tibet and the Karakorum, and low
velocities under central Tibet (Molnar, 1990; Woodward and
Molnar, 1995), low Pn velocity region in north central (Zhao
and Xie, 1993; McNamara et al., 1997), and high P-wave
velocities beneath southern Tibet (Pandey et al., 1991).
The deep seismic velocity structure analyzed in this paper is
consistent with many of the previous studies. For instance,
the lower mantle high-velocity anomaly beneath northern
India as shown in Fig. 4 is similar to the shallow portion of a
large high-velocity belt mapped by Van der Voo et al. (1999).
These investigators interpreted it as a subducted Tethyan
oceanic slab that extends laterally from the Mediterranean
Fig. 4. Four great-circle cross-sections of P-wave lateral velocity variations at locations shown in the insert map. Small purple crosses are earthquake foci. The
star in A–A1 denotes the destructive earthquake near Jamnagar, India. Shown above each section is topographic profile with a 20:1 vertical exaggeration.
Dashed lines in magenta are elevations of 0, 2, and 4 km.
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457450
Sea to the Andaman Sea and deepens to at least 2000 km in
depth. The low-velocity anomaly mapped and interpreted as
the signature of the Deccan plume by Kenneth and
Widiyantoro (1999) can also be seen in the 35–75 km
depth slice in Fig. 2, though the anomaly occurs slightly to
the south in the P1200 model. From the two depth slices by
these authors at 100 and 250 km depths, one can infer similar
patterns of the Indian slab as shown in the corresponding
depth slices in Fig. 2. In fact, the high-velocity Indian
slab in Fig. 1 matches well with the configuration of
Fig. 5. Fourteen great-circle cross-sections of P-wave lateral velocity variations at locations shown in the insert map. The lateral dimension is latitude for
sections E–E1 to O–O1, and longitude for sections P–P1 to R–R1. See caption of Fig. 4 for other legends.
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457 451
the high-velocity anomaly in the 200 km panel of Plate 1 by
Bijwaard et al. (1998). The S-wave velocity map at 100 km
depth by Villasenor et al. (2001) also show that the high-
velocity Indian slab anomaly extends into the western part of
Tibet and that the central part of Tibet has a large low-
velocity region as seen in Fig. 1 in this paper.
Zhou et al. (1996) conducted a careful modeling study of
differential residual sphere data for four Tibetan earth-
quakes. Their best fitting model consists of a narrow, nearly
vertical, high velocity slab extending to at least 400 km
beneath the southern Tibetan plateau. As shown in Figs. 2
and 5, the P1200 model also contains a high velocity slab at
similar location, though the slab in the P1200 model
stays above 300 km and extends northward subhorizontally.
Fig. 6 shows the predicted differential residual spheres from
the P1200 model for the same four events (Zhou et al.,
1996). The level of fitness with the differential residual
sphere data is compatible between predictions by the P1200
model in Fig. 6 and that by the model of Zhou et al. (1996).
Hence a subhorizontal Indian slab offers a compatible
possibility for the differential residual sphere data.
Before the 1990’s most seismologic studies of the
Tibetan deep structure were aimed at constraining large-
scale features using stations mostly outside the plateau
(Roecker, 1982; Ni and Barazangi, 1983; Jobert et al., 1985;
Molnar, 1988). In the last decade, there have been several
deployments of seismologic arrays, such as the PASSCAL
experiment (McNamara et al., 1994; Owens and Zandt,
1997) and the INDEPTH II/GEODEPTH experiment
(Nelson et al., 1996; Kind et al., 1996; Yuan et al., 1997;
Fig. 6. Differential residual sphere predictions from the P1200 model for four events studied by Zhou et al. (1996). Each event pair is indicated below each
sphere. The differential traveltime residuals are plotted as a function of azimuth along the circumference and takeoff angle along the radius. The blue stars
indicate relatively fast arrivals in seconds, and the red crosses indicate relatively slow arrivals.
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457452
Makovsky and Klemperer, 1999) across the central and
southern Tibet. Interestingly, these studies were conducted
within the area over the low-velocity anomaly shown in
Figs. 1 and 2. Our study demonstrates that this low-velocity
feature extends down to at least 310 km depth. This feature
is consistent with the presence of volcanic activity (Turner
et al., 1996) and indications of partial melting in the
lower crust (Owens and Zandt, 1997; Nelson et al., 1996;
Kind et al., 1996).
Although the P1200 model is of lower resolution
compared to that of regional array studies, the former has
a much broader spatial coverage. Therefore, a comparison
between the two may assist in extrapolating the array data
laterally. Cross-section C–C1 in Fig. 4 features a compari-
son with the receiver function image of the eastern Tibetan
plateau by Kosarev et al. (1999), who found that on this
traverse the Moho depth reaches to its maximum depth of
80 km near 308N latitude, and becomes shallower toward
the north. Cross-section C–C1 shows a low-velocity
anomaly in the top 100 km depth range beneath most of
the Tibetan plateau. The lower boundary of this slow
anomaly follows closely to the interpreted Moho profile
from the receiver function study. The lower boundary also
reaches its deepest point near 308N and becomes shallower
toward the north.
An alternative explanation for the first-order seismic
structure beneath Tibet (high-velocity zones in the east,
west, and south and a slow-velocity zone underneath
north-central Tibet) is that it reflects temperature vari-
ations in the Tibetan upper mantle associated with small-
scale mantle convection (Molnar, 1990; Woodward and
Molnar, 1995; McNamara et al., 1997; Pandey et al.,
1991). As expressed by Molnar (1988) north-central Tibet
may reflect a region where previously thickened mantle
lithosphere is being convectively thinned and is flanked on
the east and west by downwelling mantle lithosphere.
Since subduction of the Indian plate is occurring, mantle
convection beneath parts of Tibet must certainly be
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457 453
occurring also. Nevertheless, the geometry and position of
the Indian slab places limits on where convection may be
occurring. More conservative interpretations than ours
indicate that the Indian slab shallowly dips beneath Tibet
w350 km north of the Indus-Yalu suture zone (Owens and
Zandt, 1997; Kosarev et al., 1999). Although, as
mentioned earlier, these studies were conducted in the
central portions of the Tibetan plateau, extrapolating this
position to the west, places the Indian slab in a region
where downwelling of Tibetan mantle lithosphere is
inferred (Zhou et al., 1996). Another short-coming of the
mantle lithosphere convection model is that it does not
provide an explanation for a young mantle geochemical
signature observed in geothermal spring samples (Hoke et
al., 2000) nor early Miocene high-K volcanic rocks in
southwest Tibet (Miller et al., 1999).
4. Implications
Much of the Tibetan plateau is flat with an average
elevation of 5 km (Fielding et al., 1994), and its crust is
about twice as thick as the crust elsewhere (Jobert et al.,
1985; Molnar, 1988). The seismic images analyzed in this
study require that any theory seeking to explain the uplift
history of the Tibetan plateau include the existence of Indian
lithospheric slab beneath the western and eastern portions of
the Tibetan lithosphere. Less clear in the seismic images is
the presence of low-velocity channel, interpreted as
asthenosphere, sandwiched between the Indian and Tibetan
lithosphere.
Our proposed interpretation of the lithospheric structure
is similar in various aspects to other previously published
models involving underthrusting of Indian lithosphere
beneath Tibet (Argand, 1924; Powell and Conaghan,
1973; DeCelles et al., 2003; Chemenda et al., 2000). We
discuss below testable predictions made by our model
concerning the geologic evolution of the Himalaya and
Tibetan plateau.
4.1. Comparison with crustal shortening estimates
Assuming that a lithospheric-scale detachment separ-
ating Tibetan lithosphere from Indian lithosphere exists
along strike of the orogen (Zhao et al., 1993), our
interpretation that Indian mantle lithosphere has been
underthrust northward beneath Tibet to the Jinsha suture
(Fig. 7) predicts the magnitude of crustal shortening of rocks
within the Himalaya. On the 165–210 and 210–265 km
depth slices (Fig. 2), the Indian lithospheric mantle is the
most coherent high-velocity anomaly that extends north-
wards beneath nearly the entire Tibetan plateau to
approximately 368N in latitude. This latitude is, for the
western Tibet, around its boundary with the Tarim
Basin and, for the eastern Tibet, around its boundary
with the Qaidam basin. Zhu and Helmberger (1998) found
a 15–20 km step change in Moho depth between the thick
Tibetan crust and relative thinner Qaidam basin crust.
The above model places the Indian lithospheric slab
570 km north of the Indus-Yalu suture zone in southwest
Tibet (Fig. 7). DeCelles et al. (1998) calculated w228 km of
horizontal shortening of the Subhimalaya and Lesser
Himalaya, and later refined this estimate with detailed
mapping along the Seti River corridor to 460 km (DeCelles
et al., 2001). A minimum-shortening estimate for the
Dadeldhura thrust and Main Central Thrust is 117 km
(DeCelles et al., 2001). Along the Seti and Karnali river
corridors considerably more shortening is required if both
thrusts reached as far south as the Dadeldhura synform
(DeCelles et al., 2001; Upreti and Le Fort, 1999). Srivastava
and Mitra (1994) estimated between w193 and 260 km on
the Main Central Thrust and Almora thrust (equivalent to
the Dadeldhura thrust) in northern India. Combining the
shortening estimates for the Subhimalaya and Lesser
Himalaya along the Dadeldhura–Baitadadi road transect
(DeCelles et al., 1998) (Fig. 7b), the Main Central Thrust
and Almora thrust (Srivastava and Mitra, 1994), and the
Tethyan fold–thrust belt and Indus-Yalu suture zone
(Murphy and Yin, 2003), yields a total horizontal shortening
estimate across the central Himalaya in western Nepal and
southwest Tibet of 597–664 km. Alternatively, combining
the shortening estimates along the Seti River corridor
(DeCelles et al., 2001) with those in the Tethyan fold–thrust
belt and Indus-Yalu suture zone yields a total shortening
estimate of 763 km, which is clearly a minimum estimate
since internal deformation of the metamorphic rocks in the
High Himalaya is not accounted for. This amount of crustal
shortening is sufficient to explain Indian mantle lithosphere
to latitude of the Jinsha suture separating the Qiangtang
terrane from the Songpan Ganzi–Hoh Xil terrane (Yin and
Harrison, 2000) (Fig. 7). A similar conclusion was reached
by DeCelles et al. (2003) who used shortening estimates
from several parts of the Himalaya to suggest that portions
of Indian lithosphere have been underthrust beneath Tibet to
368N latitude. What is different about the two models is in
the interpretation of the lithospheric structure beneath north-
central Tibet. In the model by DeCelles et al. (2003) the
Indian mantle lithosphere has been delaminated leaving
behind lower crust that was previously beneath the Greater
Himalaya crystalline sequence. In our model, the Indian
mantle lithosphere has subducted at steeper angle in north-
central Tibet, and is separated from the shallowly subducted
slab to the west and east by lithospheric tears.
4.2. Late Cenozoic mantle-derived magmatism
in southern Tibet
The presence of a thin channel of asthenosphere between
underthrusted Indian mantle lithosphere and Tibetan litho-
sphere provides a mechanism to generate mantle-derived
melts beneath the Tibetan plateau. Our interpreted seismic
structure predicts that a young mantle geochemical
Fig. 7. (a) Tectonic map of the Tibet–Himalaya collision zone modified from Yin and Harrison (2000). (b) Schematic cross-section (A–A 0) across Tibet–
Himalayan orogen showing tectonic interpretation based on surface geology and tomography results from this study. Indian mantle lithosphere beneath central
Tibet is interpreted to be underthrust at a steeper angle than in the western and eastern portions. AKMS, Ayimaqin-Kunlun Mutztagh suture; BNS, Bangong-
Nujiang suture; IYS, Indus-Yalu suture; JS, Jinsha suture; MFT, Main Frontal thrust; MBT, Main Boundary thrust; MCT, Main Central thrust; STD, South
Tibet detachment; GCT, Great Counter thrust; ATF, Altyn Tagh fault; LM, lithospheric mantle.
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457454
signature exists in along the entire southern portion of the
Tibetan plateau, which differs from other models that
restrict this signature to northcentral Tibet (Molnar et al.,
1993). Two studies have essentially tested this prediction.
Miller et al. (1999) conducted a major and trace element
study of volcanic rocks in southwest Tibet. Their results
indicate that mantle-derived high-K, calc-alkaline magma-
tism did not end until 17 Ma. A recent 3He study (Hoke et
al., 2000) of geothermal spring samples from the Tibetan
plateau, which included samples from southwest Tibet,
argues for degassing of volatiles from young mantle-derived
melts intruded into the crust. Their results indicate the
presence of mantle-derived helium exists along the entire
southern margin of the Tibetan plateau adjacent to the
Indus-Yalu suture zone. Combining the results from both
these studies indicate that conditions appropriate for melting
of mantle lithosphere persisted in southern Tibet since the
Miocene.
4.3. Thermal effect on buoyancy of Indian lithosphere
The coherent high-velocity Indian slab anomaly exists
beneath most of the Indian subcontinent and nearly the
entire Tibetan plateau. This indicates that the Indian
lithosphere is strong due to its great thickness, which is
more than 200 km at its core beneath northern India. In
contrast, the Tibetan lithosphere has a very thick and low
velocity crust and a thin lithospheric mantle. Although the
thickness and velocity of the Tibetan crust may have been
altered during the post-collision period, the Tibetan slab was
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457 455
most likely warmer than the Indian slab prior to their
collision given that it has been a locus of deformation since
the Mesozoic (Yin and Harrison, 2000). The Indian
lithospheric slab is twice as thick as that of an oceanic
slab, but not as cold as the latter. The seismic velocity of
oceanic slabs is 3–5% higher than the ambient region at
shallow depths (Zhou and Clayton, 1990; Van der Hilst et
al., 1991), while the Indian slab is only 2–3% higher near
surface. In fact, as shown in cross-sections B–B1 and D–D1
in Fig. 4, the slab appears losing its amplitude of high-
velocity as it subducts to the north, suggesting the slab may
be heating up. This implies that, though the cold continental
Indian slab could subduct, its negative buoyancy is far less
than that of oceanic slabs. The Indian slab possibly reached
zero buoyancy at a depth around 210 km, as indicated by the
subhorizontal geometry of the subduction trajectory (Figs. 4
and 5). Further convergence of the two plates is interpreted
to have resulted in underthrusting-type horizontal subduc-
tion. As the subducted or underthrusted slab is heated up, it
is predicted to change to positive buoyancy that will
effectively lift the overlying Tibetan lithosphere and
asthenosphere. Heating of the Indian slab is certainly
enhanced by the slow convergence rate of w 20 mm/yr
between the two plates (Bilham et al., 1997). Hence, we
hypothesize that the support to the Tibetan plateau to
maintain its high elevation is most likely driven, at least
partially, by the buoyancy of the heated subducted Indian
slab. The much more yielding asthenospheric layer at 120–
165 km depth may provide a cushion to help maintain the
flatness of the plateau, similar to the mechanism suggested
by models involving hydraulic uplift by injection of Indian
continental crust into a fluid-like Tibetan lower crust (Zhao
and Morgan, 1987) or crustal thickening by lower crustal
flow (Royden et al., 1997).
We explore this hypothesis using a simple thermal model
by placing a slab of constant initial temperature into a
mantle of constant temperature. With time the temperature
Fig. 8. Time required for a lithospheric slab to lose half of its initial
temperature difference from that of ambient mantle, assuming thermal
diffusivity kZ1 mm2 sK1.
of the slab will approach to that of the mantle. Fig. 8 shows,
for different slab thickness, the time required for the slab to
lose one-half of its initial temperature difference from the
mantle. Since the Indian slab is a continental plate, its
buoyancy with respect to the mantle should turn from
negative to positive after losing a portion of its temperature
difference with respect to the mantle. Although we do not
know how much the portion is for the Indian slab, Fig. 8
predicts the trend. For instance, we estimate it took 36 Ma
for a 140-km-thick slab to lose half of its initial temperature
difference. This estimated time for the slab to switch from
negative to positive buoyancy, and therefore assist in
supporting the Tibetan lithosphere, is reasonable with
previous estimates on the timing of plateau uplift (Harrison
et al., 1995; Garzione et al., 2000).
4.4. The low-velocity anomaly beneath central Tibet
An intriguing feature in the tomographic model is a large
low-velocity anomaly north of the Indus-Yalu suture zone
and between 85 and 938E beneath the Tibetan plateau. This
anomaly can be traced from the crust down to at least 310 km
depth, though at depths of 75–120 and 165–260 km (Fig. 2) it
is a relatively slow area within the fast slab anomalies. This
implies that, though mantle upwelling might be occurring in
this area, the slab might just be weakened with fractured or
faulted zones. We suggest that the origin of the weakened
zone of the slab may be due to segmentation or fingering of
the slab during subduction (Fig. 7b). Alternatively, it may
have been weakened by mantle convection (Houseman and
Molnar, 2001) or slab break-off of Greater Indian lithosphere
(DeCelles et al., 2003). In response to the stresses caused by
subduction and/or uplifting of the heated Indian slab, the
fractured zone in the slab and over-pressure of the astheno-
spheric layer could produce seismic anisotropy observed
over this area (McNamara et al., 1994). It is fortunate that
previous geophysical and geochemical studies conducted
within this anomalous area have the opportunity to observe
the signature of the deep mantle. However, it is unfortunate
that, if this anomaly truly exists, we cannot generalize the
lithospheric structure beneath the Tibetan plateau based on
findings in this area alone.
4.5. Inference for the cause of a destructive intraplate
earthquake
The core of the Indian slab is a coherent high-velocity
anomaly as thick as 200 km, such as that seen beneath
northern India in cross-section B–B1 in Fig. 4. The coherent
high-velocity anomaly of the slab and its apparent bending
far south of the Himalayan thrust front indicates that the slab
is quite strong and rigid. An implication of this is that stress
generated by the resistance to the subduction due to
buoyancy of a heated slab can be carried along the slab.
The Mw7.5 destructive earthquake that occurred on
26/01/01 near Jamnagar, India, is located at the thinnest
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457456
point of the high-velocity slab layer (Fig. 4, A–A1). The
hypocenter of this intraplate earthquake is located at the
position where the slab just starts to thicken northwards
toward the subduction zone. The thinning of the slab at this
location is partially due to the low-velocity anomaly that
was interpreted as the signature of the Deccan plume
(Kenneth and Widiyantoro, 1999). According to the USGS
rapid moment tensor solution, the first focal plane of the
double-couple solutions (2928 strike, 368 dip, 1368 slip) of
this thrust event faces the direction of N228E, which is
parallel with cross-section A–A1. Thus, we propose that this
destructive earthquake is due to a failure of the Indian
lithosphere at its weakest point in response to the stress
originated from its subduction beneath Tibetan plateau.
5. Conclusions
Establishing the 3-D deep structure of the crust and
mantle is key to understanding the formation of the Tibetan
plateau. Our analysis of a high-resolution global tomo-
graphic model indicates the existence of a high-velocity
subducted Indian lithospheric slab beneath nearly the entire
Tibetan plateau. These results, coupled with existing field-
based crustal shortening estimates suggest that uplift of the
Tibetan plateau was assisted by subhorizontal subduction of
Indian lithosphere. In addition, a low-velocity astheno-
spheric layer might exist between the Tibetan lithosphere
and the subducted Indian slab, though this notion is highly
interpretive. The heating of the subducted continental slab
and the presence of the overlying asthenospheric layer likely
play a major role in the development and maintenance of the
high elevation and flatness of the Tibetan plateau. Our
interpretation is that, the uplift history and low relief of the
Tibetan plateau is a result of subhorizontal subduction and
heating of Indian lithosphere that is separated from Tibetan
lithosphere by a thin channel of asthenosphere. Two
predictions made by our model are: (1) the amount of
shortening in the Himalayas is equivalent to the amount of
underthrusted Indian mantle lithosphere; and (2) a young
mantle geochemical signature should be present along the
entire southern portion of the Tibetan plateau. The Tibetan
crust is not completed cut off from mantle flows beneath the
subducted slab. There is a large low-velocity anomaly north
of the Indus-Yalu suture zone between 85 and 938E that
extends from the crust all the way down to at least 310 km
depth beneath the plateau. This low-velocity anomaly is
indicative of mantle upwelling through a weakened zone of
the subducted slab. Interestingly, many previous studies,
including recent seismic array experiments, were conducted
within this anomalous region. At its strongest core west of
858E, the Indian slab begins its downward bending hundreds
of kilometers south of the Himalayas. Thus, the Indian slab
is probably quite rigid and the stress caused by the resistance
to the subduction could be carried along the Indian slab.
This scenario suggests that the destructive earthquake that
occurred on 26/01/01, a thrust event located at the weakest
point of the Indian slab, is likely a direct consequence of
subduction of the Indian slab beneath Tibetan plateau.
Acknowledgements
The authors thank S.A. Hall, K. Burke, K. Cooper, and J.
D’Andrea-Kapp for helpful discussions.
References
Argand, E., 1924. La tectonique de l’ Asie. Proc. 13th Int. Geol. Cong. 7,
171–372.
Bijwaard, H., Spakman, W., Engdahl, E.R., 1998. Closing the gap between
regional and global travel time tomography. J. Geophys. Res. 103,
30055–30078.
Bilham, R., Larson, K., Freymueller, J., Project Idylhim members, 1997.
GPS measurements of the present-day convergence across the Nepal
Himalaya. Nature 386, 61–64.
Chang, C., Chen, N., Coward, M.P., Deng, W., Dewey, J.F., Gansser, A.,
Harris, N.B.W., Jin, C., Kidd, W.S.F., Leeder, M.R., Li, H., Lin, J., Liu, C.,
Mei, H., Molnar, P., Pan, Y., Pan, Y., Pearce, J.A., Shackleton, R.M.,
Smith, A.B., Sun, Y., Ward, M., Watts, D.R., Xu, J., Xu, R., Yin, J.,
Zhang, Y., 1986. Preliminary conclusions of the Royal Society and
Academia Sinica 1985 Geotraverse of Tibet. Nature 323, 501–507.
Chemenda, A., Burg, J.-P., Mattauer, M., 2000. Evolutionary model of the
Himalaya–Tibet system; geopoem based on new modelling, geological
and geophysical data. Earth Planet. Sci. Lett. 174, 397–409.
DeCelles, P.G., Gehrels, G.E., Quade, J., Ojha, T.P., Kapp, P., Upreti, B.N.,
1998. Neogene foreland basin deposits, erosional unroofing, and
kinematic history of the Himalayan fold-thrust belt, western Nepal.
Geol. Soc. Am. Bull. 110, 2–21.
DeCelles, P.G., Robinson, D.M., Quade, J., Copeland, P., Upreti, B.N.,
2001. Stratigraphy, structure, and tectonic evolution of the Himalayan
fold–thrust belt in western Nepal. Tectonics 20, 487–509.
DeCelles, P.G., Robinson, D.M., Zandt, G., 2003. Implications of
shortening in the Himalayan fold–thrust belt for uplift of the Tibetan
plateau. Tectonics 21 doi:10.1029/2001TC001322.
Dewey, J.F., Burke, K., 1973. Tibetan, Variscan and Precambrian basement
reactivation: products of continental collision. J. Geol. 81, 683–692.
Dewey, J.F., Shackleton, R.M., Chang, C., Sun, Y., 1988. The tectonic
evolution of the Tibetan Plateau. Philos. Trans. R. Soc. London Ser., A
327, 379–413.
England, P., Houseman, G., 1989. Extension during continental conver-
gence, with application to the Tibetan Plateau. J. Geophys. Res. 94,
17561–17579.
Fielding, E., Isacks, B., Barazangi, M., Duncan, C., 1994. How flat is
Tibet?. Geology 22, 163–167.
Garzione, C.N., Quade, J., DeCelles, P.G., English, N.B., 2000. Predicting
paleoelevation of Tibet and the Himalaya from delta (super 18) O vs.
altitude gradients in meteoric water across the Nepal Himalaya. Earth
Planet. Sci. Lett. 183, 215–229.
Harrison, T.M., Copeland, P., Kidd, W.S.F., Lovera, O.M., 1995.
Activation of the Nyainqentanghla shear zone: implications for uplift
of the southern Tibetan Plateau. Tectonics 14, 658–676.
Hoke, L., Lamb, S., Hilton, D.R., Poreda, R.J., 2000. Southern limit of
mantle-derived geothermal helium emissions in Tibet: implications for
lithospheric structure. Earth Planet. Sci. Lett. 180, 297–308.
Houseman, G., Molnar, P., 2001. Mechanisms of lithospheric rejuvenation
associated with continental orogeny, in: Miller, J.A., Holdsworth, R.E.,
Buick, I.S., Hand, M. (Eds.), Continental Reactivation and Reworking
Geol. Soc. Lond. Spec. Publ., 184, pp. 13–38.
H.-w. Zhou, M.A. Murphy / Journal of Asian Earth Sciences 25 (2005) 445–457 457
Jobert, N., Journet, B., Jobert, G., Hirn, A., Zhong, S.K., 1985. Deep
structure of southern Tibet inferred from the dispersion of Rayleigh
waves through a long-period seismic network. Nature 313, 386–388.
Kenneth, B.L.N., Widiyantoro, S., 1999. A low seismic wavespeed
anomaly beneath northerwestern India: a seismic signature of the
Deccan plume?. Earth Planet. Sci. Lett. 165, 145–155.
Kind, R., Ni, J.F., Zhao, W., Wu, J., Yuan, X., Zhao, L., Sandvol, E.A.,
Reese, C., Nabelek, J., Hearn, T., 1996. Evidence from earthquake data
for a partially molten crustal layer in southern Tibet. Science 274,
1692–1696.
Kosarev, G., Kind, R., Sobolev, S.V., Yuan, X., Hanka, W., Oreshin, S.,
1999. Seismic evidence for a detached Indian lithospheric mantle
beneath Tibet. Science 283, 1306–1309.
Makovsky, Y., Klemperer, S.L., 1999. Measuring the seismic properties of
Tibetan bright spot: evidence for free aqueous fluids in the Tibetan
middle crust. J. Geophys. Res. 104, 10795–10825.
McNamara, D.E., Owens, T.J., Silver, P.G., Wu, F.T., 1994. Shear wave
anisotropy beneath the Tibetan Plateau. J. Geophys. Res. 99, 13655–
13665.
McNamara, D.E., Walter, W.R., Owens, T.J., Ammon, C.J., 1997. Upper
mantle velocity structure beneath the Tibetan Plateau from Pn travel
time tomography. J. Geophys. Res. 102, 493–505.
Meyer, B., Tapponnier, P., Bourjot, L., Metivier, F., Gaudemer, Y.,
Peltzer, G., Guo, S., Chen, Z., 1998. Crustal thickening in Gansu-
Qunghai, lithospheric mantle subduction, and oblique, strike–slip
controlled growth of the Tibet plateau. Geophys. J. Int. 135, 1–47.
Miller, C., Schuster, R., Klotzli, U., Frank, W., Purtscheller, F., 1999. Post-
collisional potassic and ultrapotassic magmatism in SW Tibet:
geochemical and Sr–Nd–Pb–O isotopic constraints for mantle source
characteristics and petrogenesis. J. Petrol. 40, 1399–1424.
Molnar, P., 1988. A review of geophysical constraints on the deep structure
of the Tibetan Plateau, the Himalaya, and the Karakorum and their
tectonic implications. Phil. Trans. R. Soc. Lond. Ser. A 326, 33–88.
Molnar, P., 1990. S-wave residuals from earthquakes in the Tibetan region
and lateral variations in the upper mantle. Earth Planet. Sci. Lett. 101,
68–77.
Molnar, P., Tapponnier, P., 1975. Cenozoic tectonics of Asia: effects of a
continental collision. Science 189, 419–426.
Molnar, P., England, P., Martinod, J., 1993. Mantle dynamics, the uplift of
the Tibetan Plateau, and the Indian monsoon. Rev. Geophys. 31, 357–
396.
Murphy, M.A., Yin, A., 2003. Structural evolution and sequence of
thrusting in the Tethyan fold–thrust belt and Indus-Yalu suture zone,
southwest Tibet. Geol. Soc. Am. Bull. 115, 21–34.
Nelson, K.D., Zhao, W., Brown, L.D., Kuo, J., Che, J., Liu, X.,
Klemperer, S.L., Makovsky, Y., Meissner, R., Mechie, J., Kind, R.,
Wenzel, F., Ni, J., Nabelek, J., Chen, L., Tan, H., Wei, W., Jones, A.G.,
Booker, J., Unsworth, M., Kidd, W.S.F., Hauck, M., Alsdorf, D.,
Ross, A., Cogan, M., Wu, C., Sandvol, E.A., Edwards, M.A., 1996.
Partially molten middle crust beneath southern Tibet: synthesis of
project INDEPTH results. Science 274, 1684–1688.
Ni, J., Barazangi, M., 1983. High-frequency seismic wave propagation
beneath the Indian Shield, Himalayan arc, Tibetan Plateau, and
surrounding regions: high uppermost mantle velocities and efficient
Sn propagation beneath Tibet. Geophys. J. R. Astron. Soc. 72, 665–689.
Owens, T.J., Zandt, G., 1997. Implications of crustal property variations for
models of Tibetan plateau evolution. Nature 387, 37–43.
Pandey, M.R., Roecker, S.W., Molnar, P., 1991. P-wave residuals at
stations in Nepal: evidence for a high velocity region beneath the
Karakorum. Geophys. Res. Lett. 18, 1909–1912.
Patriat, P., Achache, J., 1984. India–Eurasia collision chronology has
implications for crustal shortening and driving mechanism of plates.
Nature 311, 615–621.
Powell, C.M., 1986. Curvature of the Himalayan arc related to Miocene
normal faults in southern Tibet. Geology 14, 358–359.
Powell, C.M., Conaghan, P.G., 1973. Plate tectonics and the Himalayas.
Earth Planet. Sci. Lett. 20, 1–12.
Roecker, S.W., 1982. Velocity structure of the Pamir–Hindu Kush region:
possible evidence of subducted crust. J. Geophys. Res. 87, 945–959.
Royden, L.H., Burchfiel, B.C., King, R.W., Wang, E., Chen, Z., 1997.
Surface deformation and lower crustal flow in eastern Tibet. Science
276, 788–790.
Srivastava, P., Mitra, G., 1994. Thrust geometries and deep structure of the
outer and lesser Himalaya, Kumaon and Garhwal (India): implications
for evolution of the Himalayan fold-and-thrust belt. Tectonics 13, 89–
109.
Turner, S., Arnaud, N., Liu, J., Rogers, N., Hawkesworth, C., Harris, N.,
Kelley, S., VanCalstren, P., Deng, W., 1996. Post-collision, shoshonitic
volcanism on the Tibetan plateau: implications for convective thinning
of the lithosphere and the source of oceanic island basalts. J. Petrol. 37,
45–71.
Upreti, B.N., Le Fort, P., 1999. Lesser Himalayan crystalline nappes of
Nepal: problems of their origin, in: MacFarlane, A., Sorkhabi, R.B.,
Quade, J. (Eds.), Himalaya and Tibet: Mountain Roots to Mountain
Tops Geol. Soc. Am. Spec. Pap., 328, pp. 225–238.
Van der Hilst, R., Engdahl, R., Spakman, W., Nolet, G., 1991. Tomographic
imaging of subducted lithosphere below northwest Pacific island arcs.
Nature 353, 733–739.
Van der Voo, R., Spakman, W., Bijwaard, H., 1999. Tethyan subducted
slabs under India. Earth Planet. Sci. Lett. 171, 7–20.
Villasenor, A., Ritzwoller, M.H., Levshin, A.L., Barmin, M.P.,
Engdahl, E.R., Spakman, W., Trampert, J., 2001. Shear velocity
structure of central Eurasia from inversion of surface wave velocities.
Phys. Earth Planet. Interiors 123, 169–184.
Woodward, R.L., Molnar, P., 1995. Lateral heterogeneity in the upper
mantle and SS–S traveltime intervals for SS rays reflected from the
Tibetan Plateau and its surroundings. Earth Planet. Sci. Lett. 135, 139–
148.
Yin, A., Harrison, T.M., 2000. Geologic evolution of the Himalayan–
Tibetan orogen. Annu. Rev. Earth Planet. Sci. 28, 211–280.
Yuan, X., Ni, J., Kind, R., Mechie, J., Sandvol, E., 1997. Lithospheric and
upper mantle structure of southern Tibet from a seismological passive
source experiment. J. Geophys. Res. 102, 27491–27500.
Zhao, W.L., Morgan, W.J., 1987. Injection of Indian crust into Tibetan
lower crust: a two-dimensional finite element model study. Tectonics 6,
489–504.
Zhao, L.-S., Xie, X., 1993. Lateral variations in compressional velocities
beneath the Tibetan Plateau from Pn traveltime tomography. Geophys.
J. Int. 115, 1070–1084.
Zhao, W., Nelson, K.D., Che, J., Borwn, L.D., Xu, Z., Kuo, J.T., 1993.
Deep seismic reflection evidence for continental underthrusting beneath
southern Tibet. Nature 366, 557–559.
Zhou, H., 1988. How well can we resolve the deep seismic slab with
seismic tomography?. Geophys. Res. Lett. 15, 1425–1428.
Zhou, H., 1996. A high-resolution P-wave model for top 1200 km of the
mantle. J. Geophys. Res. 101, 27791–27810.
Zhou, H., Clayton, R.W., 1990. P and S wave travel-time inversions for
subducted slab under the island arcs of the northwest Pacific.
J. Geophys. Res. 95, 6829–6852.
Zhou, R., Grand, S.P., Tajima, F., Ding, X.-Y., 1996. High velocity zone
beneath the southern Tibetan plateau from P-wave differential travel-
time data. Geophys. Res. Lett. 23, 25–28.
Zhu, L., Helmberger, D.V., 1998. Moho offset across the northern margin of
the Tibetan Plateau. Science 281, 1170–1172.
Top Related