基于岩石圈厚度和地幔横向黏度变化的地幔对流模型重估云南地区剪切波各向异性源的深度

doi:10.13745/j.esf.sf.2020.6.28

摘要/Abstract

摘要：

在已有模型的基础上,考虑岩石圈厚度和软流层横向黏度的变化,本文建立了更接近地球实际情形的地幔对流模型,然后重新推测了导致云南地区剪切波各向异性的软流层源的深度。结果表明:岩石圈厚度和软流层横向黏度变化对云南地区的软流层各向异性源的深度及软流层的变形程度和机制具有重要影响;软流层各向异性对云南西南部区域、东部区域北纬26°N以南和四川盆地及其西缘的剪切波分裂具有明显的贡献,它们分别位于90~180、170~330和200~320 km深度;在云南西南部区域和东部区域北纬26°N以南,导致剪切波分裂的软流层可能处于大剪切变形状态,主要受地幔流动方向/流动平面模式控制,而四川盆地及其西缘的则处于小剪切变形状态,主要受应变模式的控制。

关键词: 软流层各向异性, 地震各向异性, 地幔对流, 软流层变形, 地幔动力学

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

中图分类号:

朱涛, 马小溪. 基于岩石圈厚度和地幔横向黏度变化的地幔对流模型重估云南地区剪切波各向异性源的深度[J]. 地学前缘, 2021, 28(2): 284-295.

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.

图/表 6

图1 研究区域的剪切波分裂测量中快波方向的误差不大于10的结果[4,5,6,7,8,9,10,11,12,13]和岩石圈厚度分布[20,32] 黑色实线方向和长短代表剪切波分裂的快波极化方向和时间;白色圆点代表剪切波分裂测量的台站;粉色实线代表断裂;绿色实线将研究区域分为4个区:西北部(NWYN)、西南部(SWYN)、东部(EYN)和四川盆地及其西缘(SB)。

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).

图2 模型预测的(a)地幔对流速度方向和(b)地幔最大水平拉张速率方向与快波方向之间的平均角度差异

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

图3 全球地震速度参考模型PREM、IASP91和AK135的剪切波各向异性强度随深度的变化(据文献[44,45,46])

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

图4 几个地震S波速度模型在不同深度上的速度异常图中括号内的数值分别为色标的最小值(Min)和最大值(Max),如(-8,8)表示Min为一8且Max为8。

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.

图5 几个地震S波速度模型与TX2011在不同深度上速度异常的相关性

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

图6 不同地震速度模型预测的地幔对流速度方向(a,c,e,g,i)和地幔最大水平拉张速率方向(b,d,f,h,j)与观测结果(图1)之间的平均角度差异随深度的分布 (a,b)、(c,d)、(e,f)、(g,h)和(i,j)分别代表云南西北部、西南部、东部区域北纬26°N以南、东部区域北纬26°N以北和四川盆地及其西缘的结果。

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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