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(A) (B) orientation in the eastern part of China and its coincidence with
absolute plate motion (APM), which is assumed to be coupled
38°N with the underlying asthenosphere, these studies suggest that the
WNW-ESE to NW-SE fast direction anisotropy is produced by the
36°N motion of asthenospheric flow.
34°N In our results, some individual values and station averages
show such a fast polarization direction, and two-layer model
32°N fitting also suggests a lower layer with a N120°E- to N130°E–
trending fast polarization direction, which is parallel to the
30°N 116°E 118°E 120°E 122°E 124°E direction of Pacific plate subduction (Fig. 2B) calculated from
116°E 118°E 120°E 122°E 124°E HS3-Nuvel1A (Gripp and Gordon, 2002). We interpret this fast
direction as the asthenospheric flow induced by the subduction
(C) (D) of the western Pacific or Philippine plates (see Fig. 4B). Global
and regional seismic tomography shows that the subducting
GSA TODAY | FEBRUARY 2015 (E) (F) western Pacific slab becomes stagnant in the mantle transition
zone under eastern China and that there are extensive low-
Figure 2. (A) Individual splitting measurements plotted at each station. The velocity anomalies in the upper mantle (Huang and Zhao, 2006).
orientation and length of the bars correspond to the fast direction and delay Mantle convection beneath the overriding plate may be induced
time, respectively. (B) Anisotropy map of eastern China presenting the by deep slab dehydration (Zhao et al., 2007). Both the low
averaged splitting measurements (red bars). Previous results are plotted in velocity and thinness of the lithosphere imply a hot mantle
blue. APM—absolute plate motion; NCB—North China block; QLDB— beneath eastern China (Zhao et al., 2007). The development of
Qinling-Dabie orogenic belts; SCB—South China block; SULU—Sulu LPO becomes much easier under the shear flow in the upper
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 (Karato et al., 2008). Several teleseismic shear wave splitting
distribution for individual and station average, respectively. studies have shown a NW-SE–trending fast polarization direc-
tion at many stations in eastern China and have suggested that
DISCUSSION this feature is caused by the subducting Pacific or Philippine
In our individual measurements (see Figs. 2A, 2C, and 2E), slab-induced flow (Liu et al., 2008).
most results were characterized by E-W–trending fast polariza- The ENE-WSW- to E-W–trending fast polarization direction
tion direction and delay times of 1.2 s. In addition, some anisotropy in our study region has been noted at several stations
measurements show ENE-WSW- or NW-SE–trending fast polar- in previous studies (Chang et al., 2009; Zhao et al., 2007). This
ization directions. Similar features can also be found in station study, based on dense stations and 5–6 years of data, shows this
average values (see Figs. 2B, 2D, and 2F), with most averages fast direction to be characteristic of most individual measure-
having ENE-WSW- to E-W (N070°E to N110°E) trending fast ments and station average values. Two-layer model fitting in this
polarization direction and delay times of 1.0 s. However, some study also suggests that the upper layer is characterized by an
averages show a NW-SE (N110°E to N150°E) trending fast polar- ENE-WSW- to E-W–trending fast polarization direction. This
ization direction. direction differs from the direction of plate motion and astheno-
spheric flow and is considered to have been generated by litho-
Previous studies (Chang et al., 2009; Zhao et al., 2007; Zhao et al., spheric deformation. From the late Mesozoic through the
2013; Zhao and Xue, 2010) have shown a WNW-ESE- to NW-SE– Cenozoic, deformation of the eastern Asian continent was domi-
trending fast polarization direction at most stations in the eastern nated by WNW-ESE–trending extensional tectonics leading to
part of China. Based on the wide distribution of the WNW-ESE 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).
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