Re-estimating the depth of shear wave splitting anisotropy in the Yunnan region by using a mantle convection model based on lithospheric thickness and lateral mantle viscosity variations

doi:10.13745/j.esf.sf.2020.6.28

Abstract

Abstract:

In the present paper, we propose a more realistic mantle convection model by introducing the lithospheric thickness and lateral asthenospheric viscosity variations into a previous model and re-estimate the depth of shear wave splitting (SWS) anisotropy. Our results indicate that the variations greatly affected the depth of asthenospheric source responsible for SWS anisotropy and the intensity and mechanism of asthenospheric deformation in the Yunnan region. Asthenospheric anisotropy obviously contributed to a SWS residing at 90-180, 170-330, and 200-320 km depths in southwestern Yunnan, eastern Yunnan south of 26°N, and the Sichuan Basin and its western margin, respectively. Asthenosphere responsible for a SWS in southwestern Yunnan and eastern Yunnan south of 26°N likely experienced large shear deformation and was primarily controlled by mantle flow-direction/flow-plane mode; whereas asthenosphere resposible for a SWS in the Sichuan Basin and its western margin likely experienced small shear deformation and was mainly controlled by strain mode.

Key words: asthenospheric anisotropy, seismic anisotropy, mantle convection, asthenospheric deformation, mantle dynamics

CLC Number:

ZHU Tao, MA Xiaoxi. Re-estimating the depth of shear wave splitting anisotropy in the Yunnan region by using a mantle convection model based on lithospheric thickness and lateral mantle viscosity variations[J]. Earth Science Frontiers, 2021, 28(2): 284-295.

Figures/Tables 6

Fig.1 Results of shear wave splitting measurements that have errors ≤10° in the fast polarization direction (data adapted from [4-13]), and variation of lithospheric thickness (adapted from [20, 32]) in the study area. Symbols: Dark bar—fast polarization direction and time of shear wave splitting; White dot—shear wave splitting stations; Pink line—fault. The study area is divided into four subregions by green lines: northwestern (NWYN), southwestern (SWYN) and eastern (EYN) Yunnan, and Sichuan Basin and its western margin (SB).

Fig.2 Variations of the mean angular difference between the (a) mantle convective velocity direction or (b) mantle maximum elongation direction and the fast polarization direction with depth for different subregions

Fig.3 Plots of shear wave anisotropy vs. depth according to the global reference models PREM, IASP91 and AK135. Adapted from [44,45,46].

Fig.4 Velocity anomalies at different depths from six seismic S-wave velocity models. Numbers in each bracket indicate the minimum (Min) and maximum (Max) values of the color bar. For instance, (-8, 8) means Min is -8 and Max 8.

Fig.5 The correlations between the velocity anomalies from six seismic S-wave velocity models and those from TX2011 at different depths

Fig.6 Plots of the mean angular difference between the mantle convective velocity direction (a, c, e, g, i) or mantle maximum elongation direction (b, d, f, h, j) predicted by five seismic models and the observed fast direction (see Fig.1) vs. depth. (a, b), (c, d), (e, f), (g, h) and (i, j) are the results for the NWYN, SWYN, EYN26S, EYN26N and SB subregions, respectively.

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