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(A) (B)
(C) (D)
Figure 3. Two-layer anisotropy fitting. Left and right maps
show apparent variations of the observed fast direction and
delay time (black circle with error bar) as a function of the
back-azimuth of the incoming wave, respectively. The best
fitting two layer model is shown as blue line. (A) and
(B) Station CSH; (C) and (D) Station ANQsd; (E) and
(F) Station NLA.
(E) (F)
Subduction of the western Pacific oceanic plate (including the A TriassictoLateJurassic
Izanagi plate) under eastern China could explain the “fossilized”
anisotropy in the continental lithosphere. Subduction of the NCB NSCB
western Pacific plate had started by 180 Ma (Maruyama et al.,
1997). If the western Pacific plate subducted as a flat-slab in a Oceaniclithosphere GSA TODAY | www.geosociety.org/gsatoday/
north or north-northwest direction under eastern Asia in the LPObylithosphericdeformation
Mesozoic, the overlying continental lithosphere might be expected Continentallithosphere
to have undergone north-south contraction. If so, an ENE-WSW-
to E-W–trending fast polarization direction anisotropy could have B LateCretaceoustopresent
been produced in the deforming lithosphere. However, neither
northward flat-slab subduction nor north-south contractional EChina W.Pacific
deformation induced by the flat-slab subduction has received
support from other studies in eastern China. On the other hand, if TLF SCB N
the western Pacific plate subducted at a steep angle in a west or
east-northeast direction in the Mesozoic, the asthenospheric flow NCB
resulting from slab retreat would also produce an ENE-WSW to
E-W fast direction anisotropy. After cooling, the fast direction LPOofolivinebyflow
anisotropy could be preserved in the lower part of the lithosphere. of the asthenospheric mantle
However, in our study region, the lithosphere is only 70–80 km
thick (Chen, 2010) and does not adequately explain the 1.0 s delay Figure 4. Cartoons illustrating a possible scenario for the origin of anisotropic
times in the lower part of the lithosphere. layering beneath eastern China. (A) Pervasive deformation occurred in the
lithosphere during the collision between the South China block (SCB) and
It is apparent that the crust overlying the lithospheric mantle North China block (NCB) from the Triassic to Late Jurassic. In response to the
lid, which is ~30 km thick in the study area (Xu et al., 2014), has a NNW-SSE convergence, the lithosphere was thickening and developing lattice
significant effect on the observed total delay time, possibly arising preferred orientation (LPO; shown as blue fusiform), the fast direction is
from the LPO in the middle to lower crust. Foliation planes predominantly parallel to the strike of collision belt. The thickened and
within the crust are usually horizontal, and the splitting of the anisotropic lithosphere was destroyed during the late Mesozoic, and only the
teleseismic shear wave with vertical propagation is expected to be shallow part of the anisotropic lithosphere survived. (B) Since the Late
small. However, dipping foliation planes in the middle to lower Cretaceous, lithosphere extension and asthenospheric flow were induced by
crust can be created in convergent plate margins (Barruol and the Pacific plate subduction beneath eastern China. The LPO of olivine (shown
Mainprice, 1993). Some studies (Okaya et al., 1995; Savage et al., as red fusiform) developed in the asthenospheric flow and trends NW-SE,
1996) have indicated that an ~10 km thickness of schistose rocks parallel to the direction of the subduction. TLF—Tan-Lu fault.
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