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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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