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