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Fossilized lithospheric deformation revealed by teleseismic
shear wave splitting in eastern China
GSA TODAY | FEBRUARY 2015 Xiaobo Tian, State Key Laboratory of Lithospheric Evolution, Understanding the global-scale velocity field associated with
Institute of Geology and Geophysics, Chinese Academy of convection in Earth’s mantle is important to constrain plate
Sciences, Beijing 100029, China, txb@mail.iggcas.ac.cn; and driving forces, lithospheric deformation, and the thermal and
M. Santosh, School of Earth Sciences and Resources, China compositional structure of the mantle (e.g., Hager and O’Connell,
University of Geosciences, 29 Xueyuan Road, Beijing 100083, China 1981; Bull et al., 2010; Flament et al., 2014). Seismic anisotropy has
been widely employed to gain insights on regional mantle flow
ABSTRACT patterns and mantle dynamics (e.g., Silver, 1996; Savage, 1999;
Long and Becker, 2010; Díaz and Gallart, 2014). When a shear
Global mantle convection significantly impacts the processes at wave propagates through an anisotropic region of the upper
Earth’s surface and has been used to gain insights on plate driving mantle, it undergoes shear wave splitting and the quasi-shear wave
forces, lithospheric deformation, and the thermal and composi- polarizations, and the delay time between them can be used to
tional structure of the mantle. Upper-mantle seismic anisotropy constrain the geometry of mantle deformation. Anisotropy
has been widely employed to study both present and past defor- describes a medium that has a different elastic property when
mation processes at lithospheric and asthenospheric depths. The measured in different directions. Seismic waves in an anisotropic
eastern China region was affected by extreme mantle perturbation medium travel at different velocities depending both on their
and crust-mantle interaction during the Mesozoic, leading to propagation and polarization (vibration) directions. The existence
large-scale destruction of the cratonic lithosphere, accompanied of seismic anisotropy indicates an ordered medium. In the middle
by widespread magmatism and metallogeny. Here we use tele- to lower crust and the upper mantle, the order is produced
seismic shear wave splitting measurements to evaluate the litho- primarily by the lattice preferred orientation (LPO) of anisotropic
sphere and upper mantle deformation beneath this region. Our minerals in response to finite strain. In the middle to lower crust,
results from some of the individual and station averages show the preferred orientations of biotite and amphibole are expected
WNW-ESE- to NW-SE–trending fast polarization direction, to be the major cause of seismic anisotropy (Barruol and
similar to those observed in eastern Asia in some previous studies, Mainprice, 1993). The seismic anisotropy in upper mantle rocks is
consistent with the direction of Pacific plate subduction during attributed mainly to the LPO of olivine (e.g., Silver, 1996), which
the Cenozoic. This feature suggests that the asthenospheric flow is the most abundant and deformable mineral in the upper
beneath the eastern China region is influenced by the subduction mantle. Seismic anisotropy is a powerful tool for imaging the style
of the western Pacific or Philippine plate. However, most of our and geometry of crust and mantle deformation. For example,
data show E-W- or ENE-WSW–trending fast polarization direc- olivine LPO can be produced by ongoing deformation and flow of
tion, which is inconsistent with subduction from the east. The the asthenospheric mantle (Kaminski et al., 2004). However,
seismic stations in this study are located near the Qinling-Dabie- anisotropy in the lithosphere is also generated from past and
Sulu orogenic belt, which formed through the collision between present deformational events. It has been demonstrated that
the North and South China blocks during the Late Paleozoic– earlier orogenic processes can imprint the lithosphere with a crys-
Triassic, and the anisotropy with an E-W- or ENE-WSW–trending tallographic fabric that remains stable and frozen, even after
fast polarization direction parallel to the southern edge of the thermal relaxation, for as long as 2.5–2.7 b.y. (Silver and Chan,
North China block suggests lithospheric compressional deforma- 1988). For mapping the seismic anisotropy of the upper mantle at
tion due to the collision between the North and South China a horizontal scale of several hundreds of kilometers, surface waves
blocks. Although the deep root of the craton was largely destroyed are effectively employed; at shorter length scales of a few tens of
by cratonic reactivation in the late Mesozoic, our results suggest kilometers, seismic anisotropy can be measured through the split-
that the “fossilized” anisotropic signature is still preserved in the ting of teleseismic core-shear waves (e.g., Savage, 1999).
remnant lithosphere beneath eastern China.
Eastern China includes the eastern parts of the North China
INTRODUCTION block (NCB) and South China block (SCB), which constitute two
of the major continental blocks of the Eurasian continent (Fig. 1).
The dynamics of Earth’s interior, particularly global mantle The Triassic collision between these two blocks generated the
convection, significantly impact the processes at Earth’s surface. Qinling-Dabie-Sulu orogenic belt and associated ultra-high
GSA Today, v. 25, no. 2, doi: 10.1130/GSATG220A.1.
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