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(A) (B) orientation in the eastern part of China and its coincidence with
38°N
36°N
34°N
32°N
30°N 116°E 118°E 120°E 122°E 124°E
116°E 118°E 120°E 122°E 124°E
(C) (D)
GSA TODAY | FEBRUARY 2015 (E) (F)
Figure 2. (A) Individual splitting measurements plotted at each station. The
orientation and length of the bars correspond to the fast direction and delay
time, respectively. (B) Anisotropy map of eastern China presenting the
averaged splitting measurements (red bars). Previous results are plotted in
blue. APM—absolute plate motion; NCB—North China block; QLDB—
Qinling-Dabie orogenic belts; SCB—South China block; SULU—Sulu
orogenic belt. (C) and (D) are the histograms of the fast direction for individual mantle when the viscosity is reduced by high temperature
and station average, respectively. (E) and (F) show the splitting parameters
distribution for individual and station average, respectively.
DISCUSSION
In our individual measurements (see Figs. 2A, 2C, and 2E),
most results were characterized by E-W–trending fast polariza-
tion direction and delay times of 1.2 s. In addition, some
measurements show ENE-WSW- or NW-SE–trending fast polar-
ization directions. Similar features can also be found in station
average values (see Figs. 2B, 2D, and 2F), with most averages
having ENE-WSW- to E-W (N070°E to N110°E) trending fast
polarization direction and delay times of 1.0 s. However, some
averages show a NW-SE (N110°E to N150°E) trending fast polar-
ization direction.
Previous studies (Chang et al., 2009; Zhao et al., 2007; Zhao et al.,
2013; Zhao and Xue, 2010) have shown a WNW-ESE- to NW-SE–
trending fast polarization direction at most stations in the eastern
part of China. Based on the wide distribution of the WNW-ESE
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absolute plate motion (APM), which is assumed to be coupled
with the underlying asthenosphere, these studies suggest that the
WNW-ESE to NW-SE fast direction anisotropy is produced by the
motion of asthenospheric flow.
In our results, some individual values and station averages
show such a fast polarization direction, and two-layer model
fitting also suggests a lower layer with a N120°E- to N130°E–
trending fast polarization direction, which is parallel to the
direction of Pacific plate subduction (Fig. 2B) calculated from
HS3-Nuvel1A (Gripp and Gordon, 2002). We interpret this fast
direction as the asthenospheric flow induced by the subduction
of the western Pacific or Philippine plates (see Fig. 4B). Global
and regional seismic tomography shows that the subducting
western Pacific slab becomes stagnant in the mantle transition
zone under eastern China and that there are extensive low-
velocity anomalies in the upper mantle (Huang and Zhao, 2006).
Mantle convection beneath the overriding plate may be induced
by deep slab dehydration (Zhao et al., 2007). Both the low
velocity and thinness of the lithosphere imply a hot mantle
beneath eastern China (Zhao et al., 2007). The development of
LPO becomes much easier under the shear flow in the upper
(Karato et al., 2008). Several teleseismic shear wave splitting
studies have shown a NW-SE–trending fast polarization direc-
tion at many stations in eastern China and have suggested that
this feature is caused by the subducting Pacific or Philippine
slab-induced flow (Liu et al., 2008).
The ENE-WSW- to E-W–trending fast polarization direction
anisotropy in our study region has been noted at several stations
in previous studies (Chang et al., 2009; Zhao et al., 2007). This
study, based on dense stations and 5–6 years of data, shows this
fast direction to be characteristic of most individual measure-
ments and station average values. Two-layer model fitting in this
study also suggests that the upper layer is characterized by an
ENE-WSW- to E-W–trending fast polarization direction. This
direction differs from the direction of plate motion and astheno-
spheric flow and is considered to have been generated by litho-
spheric deformation. From the late Mesozoic through the
Cenozoic, deformation of the eastern Asian continent was domi-
nated by WNW-ESE–trending extensional tectonics leading to
the formation of Cenozoic intracontinental basins (Yin, 2010).
We therefore propose that the ENE-WSW- to E-W–trending fast
polarization direction represents a “fossilized” anisotropic
signature preserved in the lithosphere beneath eastern China.
Continental shortening and thickening accompany the
compressional tectonics associated with the convergence
between continents. The vertically coherent deformation
between crust and lithospheric mantle has been observed in
active tectonic regions (Silver, 1996). It is well established that
upper mantle minerals, especially olivine, are highly anisotropic
and develop LPO in response to finite strain, where the fast
direction is predominantly parallel to the strike of the orogenic
belt. This has been documented from modern orogenic belts
including the Zagros and Caucasus Mountains and the Alps
(Silver, 1996). In ancient orogens, the anisotropic signature of
deformation can remain “fossilized” in the lithospheric mantle
in cases where subsequent intense deformation has not erased
this record (Silver and Chan, 1991).