Download Evidence of active mantle flow beneath South China

GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 5137–5141, doi:10.1002/grl.50987, 2013
Evidence of active mantle flow beneath South China
Chun-Yung Wang,1 Lucy M. Flesch,2 Lijun Chang,1 and Tianyu Zheng3
Received 24 July 2013; revised 17 September 2013; accepted 23 September 2013; published 10 October 2013.
[1] The India-Eurasia collision is responsible for producing
the Himalayan Mountains and Tibetan plateau and has
been hypothesized to have significant far field influences,
including driving the Baikal rift and the eastward extrusion
of South China. However, quantification of lithospheric
buoyancy forces and integrated effect of tractions acting at
base of the lithosphere are unable to explain the observed
surface motions within South China. We present 198 new
SKS shear wave splitting observations beneath South China
and invert these data along with published GPS data to solve
for the subasthenospheric flow field beneath South China
to assess the role of small-scale convection here. We find a
15–20 mm/yr southwestward-directed mantle flow toward the
Burma slab. This flow is consistent with the mantle response
of slab retreat over the past 25 Ma, and counter flow due to
subduction of Burma/Sunda slabs demonstrating the
importance of localized mantle convection on present-day
plate motions. Citation: Wang, C.-Y., L. M. Flesch, L. Chang, and
T. Zheng (2013), Evidence of active mantle flow beneath South China,
Geophys. Res. Lett., 40, 5137–5141, doi:10.1002/grl.50987.
1. Introduction
[2] The collision between India and Eurasia has generated
the highest topography on the face of the Earth, a spatially
complicated surface deformation pattern, and has been
posited to be responsible for far-field deformation such as
the opening of the Baikal rift and extrusion of South China
[Avouac and Tapponnier, 1993]. The proliferation of GPS
observations over the past 15 years within this region has
allowed for the detailed knowledge of how relative motions
are accommodated across this broad zone of continental
deformation, and thus provide constraints for quantification
of the forces generating geologic features. These are generally
attributed to buoyancy forces resulting from density variations
within the lithosphere and the integrated effect of large-scale
tractions acting at the base of tectonic plates excited from
density variations within the deep mantle. Global models
have quantified the relative contributions of deviatoric stresses
associated lithospheric density variations and tractions acting
Additional supporting information may be found in the online version of
this article.
1
China Earthquake Administration, Institute of Geophysics, Beijing,
China.
2
Department of Earth, Atmospheric, and Planetary Sciences, Purdue
University, West Lafayette, Indiana, USA.
3
Chinese Academy of Sciences, Institute of Geology and Geophysics,
Beijing, China.
Corresponding author: L. M. Flesch, Department of Earth, Atmospheric,
and Planetary Sciences, Purdue University, West Lafayette, IN 47906, USA.
(lmfl[email protected])
©2013. American Geophysical Union. All Rights Reserved.
0094-8276/13/10.1002/grl.50987
at the base of the lithosphere driven by deeper mantle density
buoyancies [Ghosh and Holt, 2012; Lithgow-Bertelloni and
Guynn, 2004]. Regionally, several authors have modeled the
India-Eurasia collision accounting for the north-south convergence between the India and Eurasia balanced by the effects
of gravity acting on the elevated topography; however, no
study has been able to replicate both the eastward extrusion
of South China and curvature of velocities around the Eastern
Himalayan Syntaxis (EHS) without imposing an a priori velocity boundary condition [Flesch et al., 2001; Vergnolle et al.,
2007]. Likewise, it has been shown that the northward motion
of the Indian plate is connected to and driven by a large-scale
deep mantle convection cell [Becker and Faccenna, 2011];
however, the induced flow on the eastern side of the Burma/
Sunda slab has yet to be related to the surface motions in
South China and the EHS. The inability of either lithospheric
buoyancy forces or forces associated with large-scale mantle
convection to produce the observed surface motions in this
region [Becker and Faccenna, 2011; Flesch et al., 2001;
Ghosh and Holt, 2012; Vergnolle et al., 2007] necessitates
an additional set of forces acting within the EHS and South
China to generate the observed surface motions. Because the
structure of the lithosphere is known through seismic studies
[Li et al., 2008] and the deviatoric stress fields associated with
these density variations have been quantified through lithospheric models [Flesch et al., 2001; Ghosh et al., 2009], the
additional set of forces must be applied at the base of the
plate from a smaller-scale convection cell not resolved in
global convection models.
[3] Here we present 198 new SKS observations in South
China and combine them with published GPS and Quaternary
fault data to invert for motion of the subasthenospheric mantle
in order to assess the contribution from a smaller-scale convection cell to the surface velocity field in South China. Our model
predicts a southwestward directed mantle flow field beneath
South China at a rate of 15–20 mm/yr in a no-net-rotation
(NNR) absolute plate motion reference frame that is consistent
with the reorganization of the Burma/Sunda subduction margin
and slab retreat over the past 25ma, and/or counter flow due to
active subduction of Burma/Sunda slabs [Li et al., 2008;
Richards et al., 2007]. This mantle flow, if coupled to South
China, would generate a torque at the base of the lithosphere
providing the additional driving force needed to produce observed surface motions.
2. Inversion of SKS and GPS Data
[4] We determine the SKS fast axis azimuths and delay
times for 198 seismic stations in South China (Figure 1a)
where the fast polarization and delay times for each arrival
were determined following the method of Silver and Chan
[1991] and stacking procedure of Wolfe and Silver [1998]
(see online supplement). Seismic anisotropy, measured through
SKS shear wave splitting, develops when mantle minerals, most
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Figure 1. (a) The observed SKS splitting data used in this study [Wang et al., 2008; Flesch et al., 2005; Chang et al., 2011; Bai
et al., 2009; Huang et al., 2008], plotted over seismic velocity [Huang et al., 2003]. (b) The fit of the predicted anisotropy,
assuming the lithosphere is moving over a stationary asthenosphere in a no-net-rotation reference frame to the observed SKS
data. Plotted in the background is the magnitude of the surface strain rate field determined from Quaternary fault slip data
and GPS velocities. Data shown in Figure 1a that are located in region of surface strain rates higher than 10 × 10e 9/yr are
not used in the analysis and are not plotted in Figures 1b–f. (c) The model surface velocity field in a no-net-rotation reference
frame and the best fit rotation of the subasthenospeheric mantle. (d) Same as Figure 1b only for the subasthenospheric rotation
shown in Figure 1c. (e) Same as Figure 1c only excluding the SKS data in the Yangtze craton (red stars) from the inversion for
subasthenospheric flow. Background grid represents lithospheric thickness [An and Shi, 2006] and geologic provinces [Ren
et al., 1980] are denoted by black lines. (f) Same as Figure 1d for the best fit subasthenospheric rotation shown in Figure 1e.
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WANG ET AL.: ACTIVE MANTLE FLOW BENEATH SOUTH CHINA
Figure 2. The surface velocity field inferred from GPS and
Quaternary fault slip data (black vectors) in a no-net-rotation reference frame plotted with the predicted mantle velocity field (blue
vectors) in the study area of thin lithosphere. Background on the
continents is lithospheric thickness [An and Shi, 2006] and in
the oceans slab contours [modified from Richards et al., 2007].
noticeably olivine, undergo strain. Laboratory studies have
shown, for a dry olivine, the fast axis will align with the direction of maximum shear in a simple shear regime [Zhang
and Karato, 1995]. Thus, seismic anisotropy is an observation
of deformation of the deeper mantle [Silver and Chan, 1991]
and can be used to elucidate interaction of the deeper mantle
with the Earth’s surface. Anisotropic fabric can be formed
on (1) subvertical planes in the lithospheric mantle where
the direction of anisotropy correlates with the surface deformation inferred from GPS observations or, (2) on horizontal
planes in the asthenospheric mantle where the anisotropy
represents the differential motion between the lithosphere
and subasthenospheric mantle below [Silver, 1996] and measured SKS observations are most likely some combination
of the two sources. Previous studies of shear wave splitting
observations in the collision zone [Lavé et al., 1996; Davis
et al., 1997; Holt, 2000; Flesch et al., 2005; Wang et al.,
2008], which focused on Tibet and surrounding regions, identified a correlation between the surface deformation field and
SKS splitting observations illustrating that deformation in
the central Asian lithosphere is vertically coherent and further
argued that deviatoric stresses in the lithosphere were coupled.
The dramatic increase in SKS data beyond Tibet now allows
for the investigation of the lateral variation in both lithospheric
and lithosphere/mantle coupling throughout the India-Eurasia
collision zone.
[5] In order to achieve the above goal, we first determine a
model strain rate and velocity field (see online supplement,
Figure S1) using continuous spline functions to interpolate
between observed strain rate data [Haines et al., 1998] inferred
from Quaternary fault data [England and Molnar, 1997] and
GPS observations (see online supplement). Following the
methods of Silver and Holt [2002] and Wang et al. [2008], a
joint analysis of this surface deformation field and the mantle
deformation inferred from 198 new and previously published
SKS data (Figure 2a) [Bai et al., 2009; Chang et al., 2011;
Flesch et al., 2005; Huang et al., 2008; Wang et al., 2008] is
then performed in order to assess the interaction between the
surface and deeper mantle in South China.
[6] Unlike Tibet where the lithosphere is thick, deformation
rates are high, and anisotropy is located in the lithospheric
mantle [Lavé et al., 1996; Davis et al., 1997; Holt, 2000;
Flesch et al., 2005; Wang et al., 2008], South China has a
thin lithosphere on the order of ~80 km [An and Shi, 2006]
(Figures 1e and 1f) and deformation rates are less than
5 × 10 9/yr (Figure 1b). Thus, it is unlikely that these low
rates could generate an anisotropic fabric in the thin lithospheric mantle layer in South China capable of producing
delay times observed in the SKS splitting. Specifically,
travel time data of the shear waves (Sg, SmS) excited by
artificial sources [Carbonell et al., 2000; Satarugsa and
Johnson, 2000; Readman et al., 2009] estimated less than
2% crustal anisotropy, time delay of the shear wave splitting
in the upper crust is ~2.5 ms/km [Wu et al., 2008], and the
average crustal thickness in the South China is ~35 km [Li
et al., 2006], which corresponds to a delay time from the
crust on the order of 0.10 s. For an average mantle lithospheric
thickness in the South China of ~45 km [Huang et al., 2003;
Priestley et al., 2006; An and Shi, 2006], with the exception
of the western Yangtze craton, an anisotropy coefficient of
3% [Mainprice and Silver, 1993; Ben-Ismail et al., 2001],
and the average shear wave velocity of 4.60 km/s, the estimated maximum delay time for the lithospheric mantle here
is ~0.30 s, illustrating that the majority of the observed delay
time (~0.60 s) must be generated within the asthenosphere.
Indeed, a comparison of the predicted SKS observations from
the surface deformation field with SKS data, following the
method of Wang et al. [2008] used for Tibet, yields an average
misfit of 41° illustrating no correlation between present-day
surface deformation and anisotropy in South China. This
result and above calculations argue that the anisotropy here
is not located within the lithospheric mantle and must be
located within the asthenospheric mantle. The observed SKS
data here could also be an artifact of lithospheric fossil anisotropy from earlier deformation in South China’s long geologic
history; however, the thin mantle lithosphere in most of the
region would again not be able to produce the observed delay
times. The Yangtze craton maybe one exception; here the lithosphere reaches thicknesses of ~200 km and fossil anisotropy
present within the lithospheric mantle could generate the
observed delay times. We address this possibility below.
[7] Due to the above lack of evidence that the anisotropy
in South China is located within the lithospheric mantle, we
next explore the possibility that it is located within the asthenosphere and represents the differential motion between
the lithosphere and convecting mantle below. Because the
large SKS data set (Figure 1a) considered here includes
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WANG ET AL.: ACTIVE MANTLE FLOW BENEATH SOUTH CHINA
Tibet, a region that has been shown to contain lithospheric
anisotropy, we filter the existing SKS data by only using stations where the surface deformation is low and less than
10 × 10e 9/yr (Figures 1b and S1b) in order to analyze only
asthenospheric anisotropy. Additionally, because the data
within Yunnan exhibit the same general anisotropic trend
and have the same lithospheric thickness as regions of low
surface deformation, we add the SKS data in Yunnan back
to the data set considered for analysis.
[8] We first predict anisotropy directions under the assumption that the lithosphere is moving in an absolute reference
frame over a stationary asthenospheric mantle. We rotate the
continuous surface model velocity field into an NNR reference
frame [Kreemer and Holt, 2001], and assume that the velocity
at the surface of the lithosphere is equal to the velocity at the
base of the lithosphere, to generate shear between the lithosphere and asthenosphere and predict the resulting mantle
anisotropy at the splitting sites (Figure 1b). The comparison
produces an average misfit between the predicted and observed anisotropy of 22°, which is larger than the 10° uncertainty of the SKS data. Additionally, there is a systematic
clockwise rotation of the predicated anisotropy indicating
a nonstationary motion of the deeper mantle. Therefore,
we invert the surface velocity field (Figure 1c) in an NNR
reference frame with the subset of SKS data to invert for a
best fit rotation of the subastheospheric mantle (Figures 2c
and 2d). We use the method of Silver and Holt [2002]
who showed that for deforming regions a unique solution
of mantle flow can be determined unlike regions of undeforming lithosphere or single splitting observations, which
cannot yield mantle flow rate information or distinguish
between two directions of mantle flow 180° apart. We invert
for a single rotation of the subasthenosphere that produces a
differential flow with the lithosphere that best fits the SKS data
field (for details, see Silver and Holt [2002]). The flow field in
the mantle may indeed be more completed than our inversion;
however, a more complex treatment using splines or higherorder polynomials would over parameterize the solution since
the SKS observations and surface velocity field only vary
slightly spatially within South China. The best fit mantle rotation produces a southwestward-directed clockwise rotation of
the subasthenospheric mantle at a rate of 15–20 mm/yr directed toward the Burma/Sunda subduction zones and produces an average misfit of 15°. We tested different levels of
damping and an undamped inversion and all produced the
same subasthenospheric velocity field. The largest misfits
occur in the western Yangtze craton where the deformation
rates are low, fast axis orientations are highly spatially variable, and the lithosphere is anomalously thick compared to
the surrounding regions (Figures 2e and 2f). Anisotropy here
is most likely fossil from past geologic processes and has
been interpreted to be related to the presence of a preexisting
lithospheric structure that was recorded during the continentarc-continent collision in the early Neoproterozoic following the Grenvillian subduction of oceanic crust in the late
Mesoproterozoic [Charvet et al., 1996]. Therefore, it most
likely does not represent present-day mantle deformation.
This inference is further supported by the thicker lithosphere [An and Shi, 2006] that could produce large splitting
delay times and higher seismic velocity [Huang et al., 2003]
(Figure 1). However, we cannot rule out spatial variation in
SKS fast directions here is caused by asthenospheric flow
around the thicker Yangtze craton as has been observed in
other areas [Fouch et al., 2000; Miller and Becker, 2012].
As a result, we further filter the SKS data used in the analysis to remove all splitting stations within the Yangtze craton
(stars Figures 1e and 1f). The resulting inversion (Figures 1e
and 1f) using the smaller subset of SKS data produces a very
similar rotation of the subasthenospheric mantle as the
larger data set inversion, with the exception that the mantle
flow is directed slightly more southward and the average
misfit is further decreased to 12°, now on the order of the
uncertainty of the data.
3. Discussion
[9] Seismic tomography studies identify a ~60° dipping
Burma slab consistent with eastward subduction (Figure 2),
independent from Indian plate subduction [Ni et al., 1989]
that is currently retreating to the west [Li et al., 2008].
There is a zone of low P wave velocity anomaly below the
Red River fault region visible at 100 km depth extending to
300 km [Li et al., 2008; Wang et al., 2003] that provides seismic evidence for smaller-scale convective processes in the
upper mantle with warmer slower material drawn up over
the Burma slab due to its retreat.
[10] Furthermore, tectonic reconstructions of present-day
slab geometries along the Indo-Australian plate provide a detailed history of the Burma/Sunda subduction zone [Richards
et al., 2007]. The once continuous NW trending subduction
margin of the Indo-Australian plate experienced a subvertical
tear with the closing of the Tethyan Ocean and a subsequent
clockwise rotation of the Indo-Burma subduction (Figure 2).
Additionally, extension in the Andaman Sea [Raju, 2005] has
been hypothesized to have developed due to a westward slab
rollback/retreat of the Burma slab and southwest rollback of
the Sunda slab [Richards et al., 2007]. The rotation, rollback,
and retreat of the Sunda slab would draw warmer asthenospheric material southwestward toward the slab generating
a small-scale convection cell. The long history of Tethys subduction has been shown to drive a mantle scale convection
cell beneath India that is responsible for Indian plate motion
[Becker and Faccenna, 2011]. Here we map a mantle flow
beneath South China through a joint inversion of surface
velocity and SKS splitting data and link it to the history of
subduction of the Sunda/Burma arguing for a dynamic mantle convection cell beneath South China (Figure 2) similar to
that shown to exist beneath India [Becker and Faccenna,
2011]. Additionally, if this flow is coupled to the South
China lithosphere like the coupling observed beneath India
[Becker and Faccenna, 2011; Ghosh and Holt, 2012], it
would provide the missing force needed to drive surface
motions observed in South China and around the EHS.
4. Conclusions
[11] Both the original subset and smaller subset of SKS
data analyzed here produce a southwestward-directed clockwise flow beneath South China toward the Burma/Sunda
subduction zone at a rate of 15 mm/yr producing a differential
flow of ~35 mm/yr between the lithosphere and subasthenosphere (Figure 1e). This differential shear is sufficient to
generate the observed anisotropy in South China. Our predicted mantle flow is consistent with and could be excited
by: (1) counter flow generated by the active subduction of
the Burma [Satyabala, 1998] and Sunda slabs; and/or (2)
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westward slab roll back and retreat of the Burma and Sunda
slabs [Li et al., 2008; Richards et al., 2007]. The predicted
flow if coupled to the lithosphere would drive surface motions
and could be the “missing” force needed to drive surface
velocities in South China and around the EHS (Figure 2).
These results highlight the importance of small-scale convection cells, influenced by complex subduction system of SE
Asia, in the evolution of the India-Eurasia collision zone.
[12] Acknowledgments. This work was improved by the helpful reviews
from Karen Fischer and an anonymous reviewer. This work was supported by
NSF grant EAR-0609337 to LMF and National Natural Science Foundation
of China grants 41174070 and 91014006 to CW.
[13] The Editor thanks Karen Fischer and an anonymous reviewer for
their assistance in evaluating this paper.
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