Download Crustal thickening and lateral extrusion during the Indo

Tectonophysics 465 (2009) 128–135
Contents lists available at ScienceDirect
Tectonophysics
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o
Crustal thickening and lateral extrusion during the Indo-Asian collision:
A 3D viscous flow model
Youqing Yang ⁎, Mian Liu
Dept. of Geological Sciences, University of Missouri, Columbia, MO 65211, USA
a r t i c l e
i n f o
Article history:
Received 26 March 2008
Received in revised form 21 October 2008
Accepted 3 November 2008
Available online 12 November 2008
Keywords:
Crustal thickening
Lateral extrusion
Indo-Asian collision
Himalayan–Tibetan plateau
Asian tectonics
a b s t r a c t
The ∼ 2000 km crustal shortening from the Indo-Asian collision in the past 40–70 million years has been
accommodated mainly by crustal thickening in the Himalayan–Tibetan Plateau and lateral extrusion of
blocks of Asian continents. However, the spatiotemporal evolution of crustal thickening and lateral extrusion,
hence the far-field impact of the Indo-Asian collision on Asian tectonics, remains controversial. Here we
present a 3D viscous flow model that simulates the partitioning of crustal material between thickening and
lateral extrusion during the Indo-Asian collision. The model assumes conservation of crustal mass, and is
constrained by the history of the Indo-Asian plate convergence and by the present-day topography of the
Himalayan–Tibetan Plateau. The results show that much of the early collision was absorbed by crustal
thickening within the Himalayan–Tibetan plateau. However, lateral extrusion of crustal material has
gradually become dominant in the past ∼ 10–20 Myr, indicating increasing influence of the Indo-Asian
collision on Asian tectonics.
Published by Elsevier B.V.
1. Introduction
The Indo-Asian collision in the past ∼40–70 Myr has caused about
2000 km crustal shortening (Yin and Harrison, 2000). Some collision
was accommodated by crustal thickening in the Himalayan–Tibetan
Plateau and surrounding regions (England and Houseman, 1986); the
rest may be accommodated by lateral extrusion of blocks of Asian
continent along numerous strike-slip fault systems (Tapponnier et al.,
1982; Peltzer et al., 1989; Replumaz and Tapponnier, 2003) and by
ductile lower crustal flow out of Tibet (Clark and Royden, 2000) (Fig. 1).
How the shortened crust was partitioned between crustal thickening
and lateral extrusion during the Indo-Asian has important implications
for the diffuse Asian tectonics, yet the answers from previous studies are
inconclusive. In a continuum mechanical model that approximates the
Asian continent as a viscous thin sheet indented by a rigid Indian plate,
England and Houseman (1986, 1988) have shown that the collision is
accommodated mainly through crustal thickening in central Asia, mostly
within the Himalayan–Tibetan plateau. Conversely, using an analogue
experiment of indentation of a plasticene Asian continent, Tapponnier
and co-workers show far-reaching influence of the collision through
lateral extrusion of blocks of Asian continent (Tapponnier et al., 1982).
Subsequent studies have narrowed the gap between these end member
models (Houseman and England, 1993), and the perceptions of how
crustal thickens and extrudes have evolved. The lateral extrusion could
occur in forms of both discrete translation of crustal blocks and distributed
⁎ Corresponding author.
E-mail address: [email protected] (Y. Yang).
0040-1951/$ – see front matter. Published by Elsevier B.V.
doi:10.1016/j.tecto.2008.11.002
extrusion of crustal material (Kong and Bird, 1996), or by lateral flow of
lower crustal material out of the collision zone (Clark and Royden, 2000).
On the other hand, the Himalayan–Tibetan Plateau may have risen in
discrete steps (Tapponnier et al., 2001). Nonetheless, the partitioning of
strain between crustal thickening and lateral extrusion, and more
importantly, the spatiotemporal evolution of such strain partitioning,
remain controversial (Tapponnier et al., 2001; Wright et al., 2004).
The tectonic implications can be profound. When most of the IndoAsian collision is absorbed by crustal thickening in the Tibetan Plateau,
its impact would be limited to the plateau and surrounding regions
(England and Molnar, 1997). Conversely, when the collision is
accommodated mainly by lateral extrusion of Asian continent, its
impact on Asian tectonics would be immediate and far reaching. Hence
how the crustal shortening is partitioned between crustal thickening
and lateral extrusion controls both the rise of the Himalayan–Tibetan
Plateau and Cenozoic tectonics in much of the central and eastern Asia.
In this study we first estimate the amount of crustal shortening and
the portion accommodated by crustal thickening in the Himalayan–
Tibetan plateau, based on the history of the Indo-Asian collision and the
present crustal volume in the plateau. We then use a three-dimensional
(3D) viscous flow model to simulate the spatiotemporal portioning of
the shortened crust between crustal thickening and lateral extrusion.
2. The amount of crustal shortening and thickening
The relatively well-defined plate convergence history and the
present-day crustal volume of the Himalayan–Tibetan Plateau permit
some estimation of the portioning of crustal mass between thickening
Y. Yang, M. Liu / Tectonophysics 465 (2009) 128–135
129
Fig. 1. Topography and simplified tectonic boundaries of the Himalayan–Tibetan Plateau and surrounding regions. The inset shows the movement of the Indian plate relative to stable
Eurasian. The fault zones bounding the plateau, together with the dashed lines, enclose the area of present Himalayan–Tibetan Plateau over which crustal volume in Fig. 5 is
calculated. Black dots shows the locations used in Fig. 6. H stands for Hindu-Kush; K for Kunlun, and G for Gonghe.
and lateral extrusion during the Indo-Asian collision (Dewey et al.,
1989; Le Pichon et al., 1992; Molnar et al., 1993). If we use the presentday Himalayan–Tibetan Plateau as the control volume, we can
estimate how much crustal material has been squeezed into this
control volume since the beginning of the collision, and how much of
the added crustal mass has been retained for plateau building. The rest
must then be moved out of the control volume by lateral extrusion or
other processes (see below).
The crustal volume of the present-day Himalayan–Tibetan Plateau
is given by its surface area and the crustal thickness. At present the
plateau may be defined along the Altyn Tagh fault (ATF) to its north,
the Main Boundary Thrust (MBT) to its south, the Pamir plateau to its
west, and the Longmenshan thrust and the Xiaojiang–Red River Faults
to its east (Fig. 1). The area of the plateau so defined is ∼ 3 × 106 km2.
The crustal thickness within the Himalayan–Tibetan Plateau may be
constrained by gravity and seismic results, which indicate that the
plateau is generally in the Airy-type of isostasy (Houseman and
England, 1996; Fischer, 2002). Seismic tomography and deep seismic
sounding show that the Moho reaches 70–74 km beneath the Tibetan
plateau (Li et al., 2006; Sun and Toksoz, 2006). Receiver function
analyses show that the crust thickens from 50 km beneath the Qaidam
basin to 80 km beneath the south Tibet (Kind et al., 2002; Shi et al.,
2004). However, the coverage of seismological studies is insufficient to
constrain the crustal thickness of the entire plateau. So we use the
elevation of the plateau and assume the Airy isostasy to estimate the
crustal thickness; the total crustal volume of the present-day
Himalayan–Tibetan Plateau thus calculated is 2.0 × 108 km3, assuming
1) uniform crustal and mantle density of 2800 kg/m3 and 3300 kg/m3,
respectively, and 2) initial elevation near the sea level (this serves as
the upper bound for crustal thickening within the plateau).
This crustal volume of the present Tibetan Plateau includes the initial
crustal mass, Vo, and crustal thickening from the collision, Vt (Fig. 2). Vo
would be 0.9 × 108 km3 if the initial crustal thickness was 30 km, or
1.35 × 108 km3 if the initial crustal thickness was 45 km. The initial crustal
thickness is poorly constrained, but likely lies between 30 and 45 km, a
range encompassing the crustal thickness of the India shield and most
lowland Asian continent. Accordingly, 1.1–0.65× 108 km3 of crustal mass
(Vt in Fig. 2) has been added to build today's Tibetan Plateau.
The total volume of the crustal material pushed into the control
volume by the Indian indenter during the collision (Vin in Fig. 2) may
be estimated from the collisional history between the Indian and
Eurasian plates, which is relatively well constrained from marine
magnetic anomalies (Acton, 1999; Schettino and Scotese, 2005)
(Fig. 1). Because the continental margin of the Indian plate before
the collision is not well constrained, we define the northern edge of
the Indian intender along the MBT, the current collision front.
Similarly, we assume the Altyn Tagh fault to be the northern boundary
of the collision zone. In doing so, we have lumped the effects of
continental margins of the Indian and the Asian plates, both poorly
constrained, into the uncertainties of the initial crustal thickness.
Uncertainties of other parameters, such as the inception time of the
collision, probably have less impact than that of the initial crustal
thickness. Assuming the crust was initially 30–45 km thick, we
estimate that the volume of crust that has entered the control volume
since 50 Ma is 1.8–2.7 × 108 km3.
Given that Vt = 1.1–0.65 × 108 km3, or 61–24% of the Vin, the crustal
mass entered the control volume, the rest 39–76% of Vin must be
Fig. 2. Conceptual model for crustal mass balance. The control volume is defined by the
present-day Himalayan–Tibetan crust, consisting of Vo, the initial crustal volume, and
Vt, the crustal volume added to the plateau by collision. Vin and Vout are the volumes of
crustal mass entering and leaving the control volume, Vin − Vout = Vt.
130
Y. Yang, M. Liu / Tectonophysics 465 (2009) 128–135
Fig. 3. Finite element mesh and boundary conditions of the numerical model. The Indian
indenter moves northward at 40 mm/yr near the western corner and 51 mm/yr at the
eastern corner, with rigid transformation and rotation along the whole edge. The
dashed line shows the location of the Indian intender at present.
accommodated by lateral extrusion and other processes, which may
include crustal thickening in the surrounding areas and erosion.
In this calculation we assumed mass conservation of the crustal
material. Some studies have found evidence for ecologites in the bottom
of crust beneath southern Tibet (Jackson et al., 2004; Hetenyi et al., 2007).
The potential eclogitization may significantly reduce crust volume in old
orogenic belts; however, eclogitization generally takes hundreds of
million years, so the amount of crustal mass affected by eclogitization in
young orogens like the Tibetan plateau is likely insignificant (Fischer,
2002). Similarly, although igneous intrusion or underplating may add
crustal mass, their impact is likely small when compared with the large
uncertainty in the pre-collision configuration of Indian and Tibetan
continental boundaries and initial crustal thickness.
3. The 3D finite-strain flow model
To simulate the spatiotemporal partitioning between crustal
thickening and lateral extrusion during the Indo-Asian collision, we
developed a 3D viscous flow model (Fig. 3). This model approximates the
Asian continent as a power-law viscous plate indented by a stiff Indian
plate, similar to previous viscous thin-sheet models (England and
Houseman, 1988). The model can include vertical variations of rheology.
For this study we included a relatively weak lower crust 20-km below
the surface to simulate the first-order effect of lower crustal flow
(Royden, 1996; Clark and Royden, 2000). We also considered first-order
rheological contrasts among the Indian indenter, the Himalayan–
Tibetan orogenic zone, and the stiff Tarim and the South China blocks
(Fig. 3). The major fault zones bounding these tectonic units are
represented in the model with simplified geometry (Fig. 1) and
simulated as weak zones using a special fault element (Goodman
et al., 1968). The faults within the interior of the Tibetan plateau are not
included. Their role is probably largely confined to the plateau (Kirby
et al., 2007). The evolution of the finite strain is calculated using the
finite element method (Yang et al., 2003).
The model includes only the crust, which is our primary concern
here. The crustal material subducted into the mantle, if happened, is
likely minor (Houseman et al., 2000). Deformation in the mantle has
no major effects on the crustal volume of the Tibetan Plateau, although
buoyancy forces in the mantle may have contributed to the elevation
of the central-northern Tibet (England and Houseman, 1989; Molnar
et al., 1993). Nonetheless, most part of the high Tibetan Plateau is
isostatically compensated by crustal roots (Fischer, 2002).
The geological boundary conditions are simplified in the model
(Fig. 3). The southern boundary moves northward at 40 mm/yr at the
western end of the Indian indenter front, with a 0.22°/Myr counter
clockwise rotation around a pole at the western end of the indenter. This
is an approximation of the trajectories of the Indian plate derived from
marine magnetic anomalies (Patriat and Achache, 1984; Ali and
Aitchison, 2005). The western side of the model domain is fixed in the
normal direction and free-slip in the tangential direction, reflecting
geological conditions imposed by the Pamir plateau. The Himalayan–
Tibetan Plateau is bounded by the Tarim block to the north and the South
China block to the east, both assumed fixed in the model. Doing so is
justifiable because their motion is insignificant in comparison to that of
the Indian Indenter in the Cenozoic (Replumaz and Tapponnier, 2003).
More importantly, it is the relative motion between the Indian plate and
these bounding blocks that controls crustal shortening during the
collision. On the surface and bottom of the model domain, the Winkler
spring mattress (Desai, 1979) is applied to simulate topographic loading
and isostatic restoring force (Williams and Richardson, 1991).
In this model, the Indian indenter bulldozes crustal material in the
collisional zone, causing both crustal thickening in its front, and lateral
extrusion in the form of crustal flow away from the collisional zone.
Because of the rigid blocks surrounding the Himalayan–Tibetan
Plateau, the main exits for the lateral extruding crustal mass are on
the southwestern corner where crustal mass flows into Indochina, and
on the northeastern side of the plateau (Clark and Royden, 2000).
Accordingly, we used viscous-piston elements on these boundaries
(indicated by pistons in Fig. 3), where the viscous resistance is
proportional to the flow velocity at the boundaries. When the
resistance coefficient is set to zero, most excessive crustal material
would flow out of the collision zone. With a resistance coefficient of
2.8 × 106 Pa s/m or higher, these boundaries act like retaining walls
that trap all crust material within the plateau. We adjusted the viscous
resistance of these elements to preserve the right amount of crustal
material required by that of today's Himalayan–Tibetan Plateau. In
most cases the required resistance coefficient is 5 × 103 Pa s/m at the
Indochina boundary and 2.8 × 105 Pa s/m on the Gobi boundary.
The initial configuration of the collision zone is constructed by
restoring the location of the Indian indenter relative to the Asian
continent (Fig. 3). Given the uncertain initial crustal thickness of the
collision zone (Yin and Harrison, 2000), we tested different values
ranging from 30 km to 45 km in the model.
The model assumes the rheology of the power-law fluid. We
explored different rheology with the power index varying between 1
and 10. The optimal models have a power index of 3 and effective
viscosity of 6 × 1022 Pa s (defined at the strain rate of 10− 15 s− 1) for the
upper curst (top 10 km), 4 × 1022 Pa s for the middle crust (10–20 km
depth), and 1 × 1019–4 × 1022 Pa s for the lower crust, within the
collision zone. For the Indian indenter, the Tarim block, and the South
China block, we used a high effective viscosity of 4 × 1023 Pa s for the
entire crust. The boundary faults are simulated as 20-km wide zones
of a relatively low viscosity, 3 × 1019 Pa s in most cases. Increasing the
viscosity of the fault zone would simulate strong coupling across the
fault. The fault zone would be effectively shut off (fully locked), if its
viscosity takes the value same as that of the ambient crust. Geological
studies indicate that the Altyn Tagh fault and the Longmenshan fault
were active at the early stage of the Indo-Asian collision (Yin et al.,
2002; Burchfiel et al., 2008), so we included these faults from the
beginning of the modeling. On the other hand, the Main Boundary
Thrust was probably formed quite late, but is included from the
beginning. This simplification may lead to overestimate of the role of
strike-slipping during the early stages of collision.
4. Model results
Based on the history of plate convergence, we conducted a series of
forward models to simulate the partitioning of crustal material between
Y. Yang, M. Liu / Tectonophysics 465 (2009) 128–135
Fig. 4. Snapshots of the simulated Indo-Asian collision and the rise of the Himalayan–Tibetan Plateau. For this case the viscosity is 1 × 1021 Pa s for the lower crust in the orogenic zone; other rheological values are given in the text. All fault zones
bounding the Tarim and Shichuan-South China block, and the Main Boundary Fault are simulated with a 20 km wide zone of low viscosity (1 × 1020 Pa s). Viscous resistance at the exit boundaries (shown by pistons in Fig. 3) is proportional to the
velocity at the boundaries; the resistance coefficient is 5 × 103 Pa s/m at the Indochina boundary, 2.8 × 105 Pa s/m on the Gobi boundary. The inset in (c) compares the predicted topography (thick lines) with that of present Tibetan plateau.
131
132
Y. Yang, M. Liu / Tectonophysics 465 (2009) 128–135
Fig. 5. (a) Crustal volume telescoped by the Indian indenter and that absorbed by crustal
thickening within the Himalayan–Tibetan Plateau. Gray bands show the range with
different initial crustal thickness (marked by the numbers in kilometer). (b) Estimate of
eroded crustal material from the Himalayan–from Metivier et al. (1999). (c) The
proportion of crustal thickening, lateral extrusion, and erosion, averaged over 5-Myr
bins, in accommodating the crust telescoped by the Indian indenter, calculated with a
35-km thick initial crust in the collisional zone.
thickening within the Himalayan–Tibetan Plateau and lateral extrusion.
The acceptable models should reproduce the present-day crustal
volume and general topography of the Himalayan–Tibetan Plateau.
Fig. 4 shows snapshots of one of the simulations. As the Indian plate
collided with the Asian continent, crust was thickened near the Indian
indenter and then spread northward, as in previous thin-sheet models
(England and Houseman, 1986; England and Houseman, 1988). Because
of the collisional zone narrows to its west (a shorter than average
distance between the western Indian indenter and the Tarim block), the
western portion of the Himalayan–Tibetan Plateau rises faster and
higher, and spreads eastwards. Lateral extrusion, in the form of crustal
flow out of the collisional zone, depends on the crustal viscosity and the
boundary conditions. We found that flow in the weak lower crust is
instrumental in reproducing the strikingly flat Tibetan plateau (Fig. 4c),
as suggested by Zhao and Morgan (1987).
In the model the Indian indenter moves at a constant rate of 40–51
mm/yr, thus its 2700-km long northern edge moves 3.7–5.4 km3 of
crustal material per year toward today's Himalayan–Tibetan Plateau,
depending on the assumed initial crustal thickness (30–45 km
in the model). Over the past 50 Myr, this process has squeezed out
1.8–2.7 × 108 km3 crustal mass (Fig. 5a), as in the aforementioned
estimation. The portion accommodated by crustal thickening in the
present Tibetan Plateau by a given time during the collision can be
calculated by volume integration over the area enclosed by the Pamir,
the ATF, the Longmenshan Fault and the MBT, whose positions were
traced in the model. The total amount of the added crustal mass for
plateau building, however, is constrained by today's crustal volume
(2.0 × 108 km3). Depending on the initial crustal thickness, in the
model the viscous resistance along the “exit” boundaries (shown as
pistons in Fig. 3) is adjusted to retain the right amount of crustal mass.
We found that, within the range of 30–45 km thick initial crust, crustal
thickening absorbed much of the collision in the early history of the
collision, but its role has gradually declined with time. The relative
proportion between crustal thickening and lateral extrusion depends
on the initial crustal thickness. A thicker initial crust would require
less crustal thickening within the Himalayan–Tibetan Plateau, and
consequently more lateral extrusion in the later periods, but the same
time-dependent trend of partitioning between thickening and lateral
extrusion are independent of the initial crustal thickness.
The influx of crustal material not accommodated by plateau building
must be absorbed by lateral extrusion, either by translation of
continental blocks along strike-slip faults or by crustal flow out of the
Tibetan Plateau, and by other processes, mainly erosion. This part has
increased with time (Fig. 5a). The contribution from erosion is difficult to
determine, but some rough estimates may be derived from Cenozoic
sediments in basins around central Asia (Metivier et al., 1999) (Fig. 5b).
Subtracting the contribution from erosion gives the estimate of lateral
extrusion. Fig. 5c shows the proportion of crustal thickening, lateral
extrusion, and erosion at selected time intervals from a model with
35 km initial crustal thickness. Given the uncertainties of initial crustal
thickness and the rate of erosion, Fig. 3c is meant only to illustrate the
general trend of partitioning among the various processes. The outflow
of crustal mass was less than 13% of the inflow in the first 10 Myr but
over 100% in the last 5 Myr. The negative contribution from crustal mass
added to the plateau in the past ∼5 Myr means that the total crustal
volume of the Himalayan–Tibetan Plateau, as defined in Fig. 1, is
shrinking as the indentation continues to shrink the surface area
enclosed by those boundary faults in Fig. 1. However, the loss of surface
area and crustal volume does not necessarily mean lose of crustal
thickness and elevation. Because within the plateau, either adding new
crustal mass from outside or having internal shortening can both lead to
crustal thickening and hence uplift.
The predicted uplift history of the plateau is shown in Fig. 6.
Everywhere within the Tibetan Plateau, the uplift accelerated with
time. The uplift history varies from place to place; it was fastest in
the western part and slowest in the northeastern part of the plateau.
Nonetheless, most part of the plateau rose above 3 km about 10–20 Myr
ago, broadly consistent with the timing of initiation of the widespread
north-trending grabens in the Tibetan Plateau (Coleman and Hodges,
1995; Yin, 2000).
5. Discussion
The 2000–2500 km northward motion of the Indian plate relative
to Eurasian plate in the past 40–70 Ma implies that up to 2.7 × 108 km3
crustal material between the two plates has been telescoped. About
61–24% of this crustal mass have been accommodated by thickening
within the Himalayan–Tibetan Plateau, the rest has to be moved out of
the collision zone, either by lateral translation of blocks of the Asian
continent or by crustal flow out of the Himalayan–Tibetan Plateau, and
by other processes such as erosion.
Y. Yang, M. Liu / Tectonophysics 465 (2009) 128–135
133
Fig. 6. Predicted uplift history of selected locations (shown in inset) in the Tibetan Plateau.
The current crustal motion in the Himalayan–Tibetan Plateau and
surrounding regions are shown by the Global Positioning System
(GPS) measurements (Fig. 7). The GPS velocities represent mainly
short-term strain rates, and it is unclear how the crustal motion varies
with depth. Nonetheless, if we take the GPS velocities as an
approximation of the average motion of the whole crustal blocks,
we may estimate the crustal mass flux in and out of the Himalayan–
Tibetan Plateau enclosed by the simplified boundaries in Fig. 7.
Through each boundary segment the mass flux is given by the product
of the length of the segment, the local crustal thickness, and the
velocity of crustal motion normal to the segment, extrapolated from
the GPS data. We found that the total influx of crustal material from
the Himalayan front is 3.3–3.7 km3/yr, depends on the average crustal
thickness of the Indian indenter; the total efflux from other sides of
the Tibetan Plateau is 54–70% of the influx, close but lower than our
estimated rate of extruding crustal mass (Fig. 5c). One possible
explanation is the faster lower crustal flow than at the earth's surface.
In our model the rates of lower crustal flow can be orders of
Fig. 7. GPS velocity relative to stable Eurasia (data from Zhang et al. (2004)). The error ellipses are 95% confidence intervals. The line segments enclose the plateau over which the
influx and efflux of crustal material are calculated. See text for details.
134
Y. Yang, M. Liu / Tectonophysics 465 (2009) 128–135
magnitude higher that the surface velocity. Royden and co-workers
appealed to such lower crustal flow to explain the rise of eastern Tibet
where surface shortening is insufficient (Clark and Royden, 2000;
Royden et al., 2008).
The spatiotemporal partitioning of the telescoped crustal material
is simulated in a viscous flow model, constrained by 1) the history of
the Indo-Asian convergence, and 2) the topography (or the crustal
volume) of the present Himalayan–Tibetan Plateau. Satisfying these
constraints requires trade-offs between free or less constrained model
parameters. For example, weakening the boundary fault zones in the
model would enhance lateral extrusion, but then higher viscous
resistance must be applied at the Indochina and the Gobi boundaries
(indicated by pistons in Fig. 3) to retain enough crustal material to
reproduce the present Tibetan topography.
Our finite strain model is a first-order approximation. Many
parameters are poorly constrained and may change the details of the
model predictions. One major uncertainty is the initial crustal
thickness of the collision zone. Some part of the Tibetan Plateau
probable stood high with a thick crust at the beginning of the IndoAsian collision (Murphy et al., 1997; DeCelles et al., 2002; Rowley and
Currie, 2006; Yin, 2006). In this case less of the telescoped crustal
material would be needed to build the present Tibetan Plateau,
consequently more lateral extrusion would be predicted. The effects of
many other uncertainties, such as the initial geometry and structure of
the margins of the Indian intender and the Asian continent, as well as
the paleo-positions of the Indian indenter (Ali and Aitchison, 2005),
can be lumped into the large range of initial crustal thickness explored
in the model. As shown in Fig. 5a, while the details vary with the
different initial crustal thickness, the general trend of partitioning of
crustal mass between thickening and lateral extrusion remains the
same.
Other uncertainties include the boundary conditions. In the model
we hold both the South China and the Tarim blocks fixed, although
they both have moved away from the Indo-Asian collisional zone
through the Cenozoic. However, their moving histories are not well
constrained. Besides, their motion is of second order relative to that of
the Indian indenter, thus their impact on the model is limited. Erosion
is not included in the model, and the estimated rates of erosion are
associated with large intrinsic uncertainties (Metivier et al., 1999). We
included erosion in Fig. 5 to show the complete partitioning of the
telescoped crustal material. Additionally, some of the lower crust of
the Indian plate may have experienced eclogitization and subducted
into the mantle (Le Pichon et al., 1992). The amount, while hard to
constrain, is unlikely to be significant to alter the general results in this
study.
With all the uncertainties, the model consistently predicts that most
of the Indo-Asian collision was accommodated by crustal thickening
within the Himalayan–Tibetan Plateau during the early stages of the
collision. However, the role of crustal thickening declines with time, and
is gradually replaced by lateral extrusion of crustal material. The
underlying physics for this trend is straightforward. In the model, the
driving force for lateral crustal flow is the gradient of gravitational
potential energy between the Tibetan Plateau and the surrounding
lowland. As the Tibetan Plateau rises, the gravitational force increases,
hence the lateral crustal extrusion intensifies (England and Molnar,
1997; Kong et al.,1997). Furthermore, crust thickening increases the flow
channel of the weak lower crust, hence promoting lateral extrusion.
The increasing role of lateral extrusion is generally consistent with
initiation of widespread north-trending grabens in south and central
Tibet (Armijo et al., 1986; Blisniuk et al., 2001), which has been
explained either as gravitational spreading of the elevated Tibetan
Plateau (Dewey, 1988; Liu and Yang, 2003) or as a consequence of
lateral extrusion of the Tibetan crust (Yin, 2000). Note that in our
model lateral extrusion is defined as crustal material flowing out of
the presently defined Tibetan Plateau. Thus it includes both lateral
translation of rigid blocks of Asian continents along strike-slip faults,
and ductile flow of crustal material that may have contributed to
crustal thickening in other parts of Asia. We cannot distinguish these
two processes from modeling. It is conceivable that, when the Tibetan
crust was relatively thin and strong during the early stages of collision,
crustal flow is ineffective and block motion along strike-slip faults was
the main form of extrusion. This may help explain early fault slips
along some of the faults within and around the plateau, such as the
late Oligocene to early Miocene slip along the Red River fault (Gilley
et al., 2003). As the crust become thicker and presumably weaker,
ductile flow in the lower crust would become more effective (Clark
and Royden, 2000). The increasing role of lower crustal flow is
consistent with late Miocene and younger uplift in eastern Tibet
(Royden et al., 1997; Schoenbohm et al., 2006). Rapid changes of
lithospheric rheology and boundary conditions, such as delamination
or convective downwelling of mantle lithosphere under Tibet
(England, 1993), could cause departure from the gradual change of
crustal thickening and lateral extrusion predicted in this model
(Tapponnier et al., 2001).
The existence of a weak, ductile lower crust under the Tibetan
Plateau and its role in the plateau building have been challenged by
recent results of consistent GPS velocities and the fast polarization
direction of split SKS waves (Flesch et al., 2005; Wang et al., 2008). The
apparently vertical coherence of deformation near surface and in the
uppermost mantle has been interpreted as evidence for a strong
crust–mantle mechanical coupling that is unfavorable for the
existence of a weak lower crust (Bendick and Flesch, 2007; Wang
et al., 2008). We suggest that the vertical coherence of lithospheric
deformation within the Tibetan Plateau may simply result from the
tectonic boundary conditions. The Tarim block and the South China
blocks are rigid structures with high seismic velocities extending to
N200 km depth (Liu and Jin, 1993; Huang and Zhao, 2006), with the
Indian plate indenting from the south and southwest, the crustal and
mantle mass is forced to extrude southwestward and northeastward
(Fig. 1). Hence the coherent surface and mantle deformation cannot
exclude the existence of a weak lower crust. Royden et al. (2008) have
shown clearly reduced S wave velocities in the lower crust under
eastern Tibet that are indicative of a weak lower crust.
The increasing role of lateral extrusion may also explain tectonics
in many regions in central and eastern Asia where early Cenozoic
tectonics show no clear link to the Indo-Asian collision (Northrup
et al., 1995), whereas the impact of the collision in these regions is
clear today (Avouac and Tapponnier, 1993; Zhang et al., 2003).
6. Conclusions
Major conclusions we may draw from this study include the
following:
1) In Indo-Asian collision has telescoped up to 2.7 × 108 km3 crustal
material between these two plates. About 61–24% of the shortened
crustal material has been accommodated by crustal thickening
within the Himalayan–Tibetan Plateau; the rest has been accommodated by lateral extrusion, either in form of lateral translation of
blocks of the Asian continent or by ductile flow in the lower crust,
and by other processes such as erosion.
2) Most of the Indo-Asian collision was accommodated by crustal
thickening within the Himalayan–Tibetan Plateau during the early
stages of collision. The plateau rose from south to north at the
beginning, and then from west to east. Most part of the Tibetan
Plateau likely reached an elevation of 3 km or more about
15 million years ago.
3) As the Himalayan–Tibetan rises, lateral extrusion has gradually
become the dominant way to accommodate the Indo-Asian collision
in the past 10–15 Ma. The increasing role of lateral extrusion implies
broader and stronger impact of the Indo-Asian collision on Asian
tectonics.
Y. Yang, M. Liu / Tectonophysics 465 (2009) 128–135
Acknowledgements
This work was supported by NSF grants EAR0207200 and 0710354,
and grant 40228005 of the National Science Foundation of China. We
thank two anonymous reviewers and editor Sandiford for helpful and
constructive comments.
References
Acton, G.D., 1999. Apparent polar wander of India since the Cretaceous with
implications for regional tectonics and true polar wander. In: Radhakrishna, T.,
Piper, J.D.A. (Eds.), The Indian Subcontinent and Gondwana: A Palaeomagnetic and
Rock Magnetic Perspective. Mem. Geol. Soc. India, pp. 129–175.
Ali, J.R., Aitchison, J.C., 2005. Greater India. Earth-Science Reviews 72 (3–4), 169–188.
Armijo, R., Tapponnier, P., Mercier, J.L., Han, T.L., 1986. Quaternary extension in Southern
Tibet: field observations and tectonic implications. J. Geophys. Res. 91, 13803–13872.
Avouac, J.P., Tapponnier, P., 1993. Kinematic model of active deformation in Central Asia.
Geophys. Res. Lett. 20 (10), 895–898.
Bendick, R., Flesch, L., 2007. Reconciling lithospheric deformation and lower crustal flow
beneath central Tibet. Geology 35 (10), 895–898.
Blisniuk, P.M., et al., 2001. Normal faulting in central Tibet since at least 13.5 Myr ago.
Nature (London) 412 (6847), 628–632.
Burchfiel, C., et al., 2008. A geological and geophysical context for the Wenchuan
earthquake of 12 May 2008, Sichuan, People's Republic of China. GSA Today 18 (7),
4–11. doi:10.1130/GSATG18A.1.
Clark, M.K., Royden, L.H., 2000. Topographic ooze: building the eastern margin of Tibet
by lower crustal flow. Geology (Boulder) 28 (8), 703–706.
Coleman, M., Hodges, K., 1995. Evidence for Tibetan Plateau uplift before 14 Myr ago
from a new minimum age for east–west extension. Nature 374, 49–52.
DeCelles, P.G., Robinson, D.M., Zandt, G., 2002. Implications of shortening in the
Himalayan fold-thrust belt for uplift of the Tibetan Plateau. Tectonics 21 (6).
Desai, C.S., 1979. Elementary Finite Element Method. Prentice-Hall, Englewood Cliffs, N. J.
434 pp.
Dewey, J.F., 1988. Extensional collapse of orogens. Tectonics 7, 1123–1139.
Dewey, J.F., Cande, S., Pitman, W.C., 1989. Tectonic evolution of the Indian–Eurasia
collision zone Ecol. Geol. Helv. 82, 717–734.
England, P., 1993. Convective removal of thermal boundary layer of thickened
continental lithosphere: a brief summary of causes and consequences with special
reference to the Cenozoic tectonics of the Tibetan Plateau and surrounding regions.
Tectonophysics 223, 67–73.
England, P.C., Houseman, G.A., 1986. Finite strain calculations of continental deformation 2. Comparison with the India–Asia collision. J. Geophys. Res. 91, 3664–3676.
England, P., Houseman, G., 1988. The mechanics of the Tibetan plateau. Phil. Trans. R.
Soc. Lond. A327, 379–413.
England, P.C., Houseman, G.A., 1989. Extension during continental convergence with
special reference to the Tibetan plateau. J. Geophys. Res. 94, 17,561–17,597.
England, P., Molnar, P., 1997. Active deformation of Asia: from kinematics to dynamics.
Science 278, 647–650.
Fischer, K.M., 2002. Waning buoyancy in the crustal roots of old mountains. Nature 417
(6892), 933–936.
Flesch, L.M., et al., 2005. Constraining the extent of crust–mantle coupling in central
Asia using GPS, geologic, and shear wave splitting data. Earth Planet. Sci. Lett. 238
(1–2), 248–268.
Gilley, L.D., et al., 2003. Direct dating of left-lateral deformation along the Red River
shear zone, China and Vietnam. J. Geophys. Res. 108 (B2).
Goodman, R.E., Taylor, R.L., Brekke, T.L., 1968. A model for the mechanics of jointed rock.
American Society of Civil Engineers, New York, pp. 637–659.
Hetenyi, G., et al., 2007. Density distribution of the India plate beneath the Tibetan
plateau: geophysical and petrological constraints on the kinetics of lower-crustal
eclogitization. Earth Planet. Sci. Lett. 264 (1–2), 226–244.
Houseman, G., England, P., 1993. Crustal thickening versus lateral expulsion in the
Indian–Asian continental collision. J. Geophys. Res. 98 (7), 12233–12249.
Houseman, G., England, P., 1996. A lithospheric-thickening model for the Indo-Asian
collision. In: Yin, A., Harrision, T.M. (Eds.), The Tectonic Evolution of Asia. Cambridge
University, pp. 3–17.
Houseman, G.A., Neil, E.A., Kohler, M.D., 2000. Lithospheric instability beneath the
Transverse Ranges of California. J. Geophys. Res. 105 (B7), 16237–16250.
Huang, J., Zhao, D., 2006. High-resolution mantle tomography of China and surrounding
regions. J. Geophys. Res. 111 (9), B09305. doi:10.1029/2005JB004066.
Jackson, J.A., Austrheim, H., McKenzie, D., Priestley, K., 2004. Metastability, mechanical
strength, and the support of mountain belts. Geology 32 (7), 625–628.
Kind, R., et al., 2002. Seismic images of crust and upper mantle beneath Tibet: evidence
for Eurasian plate subduction. Science 298 (5596), 1219–1221.
Kirby, E., et al., 2007. Slip rate gradients along the eastern Kunlun fault. Tectonics 26 (2).
Kong, X., Bird, P., 1996. Neotectonics of Asia: thin-shell finite-element models with
faults. In: Yin, A., Harrison, T.M. (Eds.), The Tectonic Evolution of Asia. World And
Regional Geology Series. Cambridge University Press, pp. 18–36.
135
Kong, X., Yin, A., Harrison, T.M., 1997. Evaluating the role of preexisting weakness and
topographic distributions in the Indo-Asian collision by use of a thin-shell
numerical model. Geology 25, 527–530.
Le Pichon, X., Fournier, M., Jolivet, L., 1992. Kinematics, topography, shortening, and
extrusion in the India–Eurasia collision. Tectonics 11 (6), 1085–1098.
Li, S.L., Mooney, W.D., Fan, J.C., 2006. Crustal structure of mainland China from deep
seismic sounding data. Tectonophysics 420 (1–2), 239–252.
Liu, F., Jin, A., 1993. Seismic tomography of China. In: Iyer, H.M., Hirahara, K. (Eds.), Seismic
Tomography: Theory and Practice. Chapman and Hall, New York, pp. 299–310.
Liu, M., Yang, Y., 2003. Extensional collapse of the Tibetan Plateau: results of threedimensional finite element modeling. J. Geophys. Res. 108 (8), 2361. doi:10.1029/
2002JB002248.
Metivier, F., Gaudemer, Y., Tapponnier, P., Klein, M., 1999. Mass accumulation rates in
Asia during the Cenozoic. Geophys. J. Int. 137 (2), 280–318.
Molnar, P., England, P., Martinod, J., 1993. Mantle dynamics, uplift of the Tibetan plateau,
and the Indian monsoon. Rev. Geophys. 31 (4), 357–396.
Murphy, M.A., et al., 1997. Did the Indo-Asian collision alone create the Tibetan plateau?
Geology 25 (8), 719–722.
Northrup, C.J., Royden, L.H., Burchfiel, B.C., 1995. Motion of the Pacific plate relative to
Eurasia and its potential relation to Cenozoic extension along the eastern margin of
Eurasia. Geology 23 (8), 719–722.
Patriat, P., Achache, J., 1984. India–Eurasia collision chronology has implications for
crustal shortening and driving mechanism of plates. Nature (London) 311 (5987),
615–621.
Peltzer, G., Tapponnier, P., Amijio, R., 1989. Magnitude of late Quaternary left-lateral
displacement along north edge of Tibet. Science 246, 1285–1289.
Replumaz, A., Tapponnier, P., 2003. Reconstruction of the deformed collision zone
between India and Asia by backward motion of lithospheric blocks. J. Geophys. Res.
108 (6), 2285. doi:10.1029/2001JB000661.
Rowley, D.B., Currie, B.S., 2006. Palaeo-altimetry of the late Eocene to Miocene Lunpola
basin, central Tibet. Nature 439 (7077), 677–681.
Royden, L., 1996. Coupling and decoupling of crust and mantle in convergent orogens:
implications for strain partitioning in the crust. J. Geophys. Res. 101, 17,679–17,705.
Royden, L.H., et al., 1997. Surface deformation and lower crustal flow in Eastern Tibet.
Science 276, 788–790.
Royden, L.H., Burchfiel, B.C., Hilst, R.D.v.d., 2008. The geological evolution of the Tibetan
Plateau. Science 321, 1054–1058.
Schettino, A., Scotese, C.R., 2005. Apparent polar wander paths for the major continents
(200 Ma to the present day): a palaeomagnetic reference frame for global plate
tectonic reconstructions. Geophys. J. Int. 163 (2), 727–759.
Schoenbohm, L.M., Burchfiel, B.C., Chen, L.Z., 2006. Propagation of surface uplift, lower
crustal flow, and Cenozoic tectonics of the southeast margin of the Tibetan Plateau.
Geology 34 (10), 813–816.
Shi, D., et al., 2004. Detection of southward intracontinental subduction of Tibetan
lithosphere along the Bangong–Nujiang suture by P-to-S converted waves. Geology
32 (3), 209–212.
Sun, Y.S., Toksoz, M.N., 2006. Crustal structure of China and surrounding regions from
P wave traveltime tomography. J. Geophys. Res. 111 (B3), 12.
Tapponnier, P., Peltzer, G., Le Dain, A.Y., Armijo, R., Cobbold, P., 1982. Propagating
extrusion tectonics in Asia; new insights from simple experiments with plasticine.
Geology (Boulder) 10, 611–616.
Tapponnier, P., et al., 2001. Oblique stepwise rise and growth of the Tibet Plateau.
Science 294 (5547), 1671–1677.
Wang, C.Y., Flesch, L.M., Silver, P.G., Chang, L.J., Chan, W.W., 2008. Evidence for
mechanically coupled lithosphere in central Asia and resulting implications.
Geology 36 (5), 363–366.
Williams, C.A., Richardson, R.M., 1991. A rheological layered three-dimensional model of
the San Andreas Fault in central and southern California. J. Geophys. Res. 96,
16597–16623.
Wright, T.J., Parsons, B., England, P.C., Fielding, E.J., 2004. InSAR observations of low slip
rates on the major faults of western Tibet. Science 305 (5681), 236–239.
Yang, Y., Liu, M., Stein, S., 2003. A 3-D geodynamic model of lateral crustal flow during Andean
mountain building. Geophys. Res. Lett. 30 (21), 2093. doi:10.1029/2003GL018308.
Yin, A., 2000. Model of Cenozic east–west extension in Tibet suggesting a common
origin of rifts in Asia during the Indo-Asia collision. J. Geophy. Res. 105 (9),
21745–21759.
Yin, A., 2006. Cenozoic tectonic evolution of the Himalayan orogen as constrained by
along-strike variation of structural geometry, exhumation history, and foreland
sedimentation. Earth-Sci. Rev. 76 (1–2), 1–131.
Yin, A., Harrison, M., 2000. Geological evolution of the Himalayan–Tibetan orogen. Ann.
Rev. Earth Planet. Sci. 28, 211–280.
Yin, A., et al., 2002. Tectonic history of the Altyn Tagh fault system in northern Tibet
inferred from Cenozoic sedimentation. Geol. Soc. Am. Bull. 114 (10), 1257–1295.
Zhang, Y.Q., Ma, Y.S., Yang, N., Shi, W., Dong, S.W., 2003. Cenozoic extensional stress
evolution in North China. J. Geodyn. 36 (5), 591–613.
Zhang, P.Z., et al., 2004. Continuous deformation of the Tibetan Plateau from global
positioning system data. Geology 32 (9), 809–812.
Zhao, W.-L., Morgan, W.J., 1987. Injection of Indian crust into Tibetan lower crust;
a two-dimensionalfinite element model study. Tectonics 6 (4), 489–504.