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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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