汤加俯冲带上地幔剪切波速度和方位各向异性结构

doi:10.19657/j.geoscience.1000-8527.2024.101

摘要/Abstract

摘要：

位于西南太平洋的汤加俯冲带发育有世界上最典型的“沟-弧-盆”体系,该区域不仅具有全球最快的板块俯冲速度,还发育扩张速率最快的弧后盆地——劳盆地。汤加俯冲带东北侧的萨摩亚地幔柱也与该俯冲系统间存在着强烈的相互作用,这使得该区的地幔动力过程更加复杂。虽然前人已在此进行了许多地球物理研究,但汤加俯冲带上地幔的精细三维结构,尤其是各向异性结构特征仍不清晰。为此,本文利用20~150 s周期范围内的远震基阶瑞利波的振幅和相位数据开展了方位各向异性层析成像研究,以期进一步约束汤加地区上地幔速度结构。研究结果认为:(1)萨摩亚地幔柱物质向南流入劳盆地主要发生在小于50 km的深度范围内;(2)劳盆地北部地幔楔中主要呈现东西向的地幔流动,而南部则存在南北向的地幔流动,这可能是地幔物质被动流动以适应俯冲板块不对称回卷的结果;(3)俯冲板片内主要呈现近南北向的快波方向,这可能与俯冲相关的正断层有关;(4)海沟外侧软流圈中存在平行海沟的横向地幔流动,这可能是软流圈物质受俯冲板块回卷挤压产生的。

关键词: 各向异性层析成像, 地幔对流, 俯冲板片, 汤加俯冲带, 劳盆地

Abstract:

The Tonga Subduction Zone, located in the southwestern Pacific, is characterized by the global most typical trench-arc-basin system. It not only has the fastest plate convergence rate globally but also features the fastest back-arc basin, the Lau Basin. The Samoan mantle plume, situated on the northeastern side of the Tonga Subduction Zone, interacts strongly with the subducting system, making the mantle dynamics in this region even more complex. Many previous geophysical investigations of the region have been conducted, but the fine three-dimensional structure of the upper mantle, especially its anisotropic structure, is still not very clear. Here we apply azimuthal anisotropy tomography using amplitude and phase data of teleseismic fundamental-mode Rayleigh waves at periods of 20-150 s to further constrain the upper mantle structure beneath Tonga. Our results show the following features: (1) The southward inflow of the Samoan mantle plume material into the Lau Basin mainly occurs at depths less than 50 km. (2) There is a west-east mantle flow beneath the northern part of the Lau Basin, while a south-north mantle flow exists beneath the southern part, possibly resulting from passive mantle flow to accommodate asymmetric rollback of the subducting slab. (3) Within the subducting slab, the fast-velocity direction is nearly north-south, possibly caused by the presence of subduction-related normal faults. (4) In the asthenosphere beneath the outer-rise region, there is a lateral mantle flow parallel to the trench, probably formed because the asthenospheric material being extruded by the rollback of the subducting slab.

Key words: anisotropic tomography, mantle convection, subducting slab, Tonga subduction zone, Lau basin

中图分类号:

赵迪, 刘鑫, 赵大鹏. 汤加俯冲带上地幔剪切波速度和方位各向异性结构[J]. 现代地质, 2025, 39(01): 1-7.

ZHAO Di, LIU Xin, ZHAO Dapeng. Shear-wave Velocity and Azimuthal Anisotropy in the Upper Mantle of the Tonga Subduction Zone[J]. Geoscience, 2025, 39(01): 1-7.

图/表 15

图1 汤加—劳—斐济地区构造背景图红色和黄色三角形分别表示全新世和更新世火山;紫色线表示俯冲板片上表面等深线;红色线表示弧后扩张中心;黑色锯齿线代表汤加海沟;黑白箭头分别表示太平洋板块的绝对和相对澳大利亚板块的运动速度

Fig.1 Tectonic setting background in the Tonga-Lau-Fiji region

图2 研究使用的地震台站分布图正方形表示陆地台站;圆形表示海底地震仪;其余图例与图1相同

Fig.2 Distribution of seismic stations used in this study

图3 瑞利波数据处理示例 (a)地震台站和远震事件位置示意图;(b)三分量地震波形;(c)高斯滤波前(红色线)、后(黑色线)各周期瑞利波,紫色线为包络线;(d)包络线归一化振幅图,粉色菱形代表选取的群速度到时

Fig.3 An example of measuring Rayleigh-wave data

图4 研究使用的1088个远震事件分布(a)及统计(b)示意图

Fig.4 Distribution of 1088 teleseismic events used in this study

图5 瑞利波相速度层析成像相关参数 (a)阻尼折中曲线,红色数字表示本研究采用的阻尼系数;(b)平滑折中曲线,红色数字表示本研究采用的平滑系数;(c)平均相速度频散曲线迭代图,灰色线表示源自经修正的AK135模型的初始相速度频散曲线,红色线表示经十次迭代后的最佳一维频散曲线;(d)各向同性(Iso)和各向异性(Ani)反演前后残差的均方根变化图

Fig.5 Parameters of Rayleigh-wave phase velocity tomography

图6 剪切波速度层析反演前后残差分布图 (a)反演前后C0残差分布图,蓝色和红色分别表示反演前后,下同;(b)反演前后各向异性参数 ( A C c o s 2 φ + B C s i n 2 φ)残差分布图;(c)反演前后C0残差均方根变化图;(d)反演前后各向异性参数残差均方根变化图

Fig.6 Distributions of residuals after the Vs tomographic inversion

图7 最佳平均相速度频散曲线可靠性检验黑线表示最佳平均相速度频散曲线;白线表示源自AK135模型的初始频散曲线

Fig.7 Reliability test of the optimal average phase-velocity dispersion curve

图8 瑞利波方位各向异性相速度层析成像的结果平面图各子图右下角标注了对应周期的平均相速度;底图中红色和蓝色分别表示相对低和高的瑞利波各向同性相速度;白色短棒的方向指示快波方向,短棒的长短表示各向异性的振幅大小;其余图例与图1相同

Fig.8 Map views of Rayleigh-wave phase-velocity azimuthal anisotropy tomography

图9 瑞利波方位各向异性相速度层析成像的恢复分辨率测试 (a1)-(a8) 输入模型;(b1)-(b8) 输出模型;底图中红色和蓝色分别表示相对低和高的瑞利波各向同性相速度;白色短棒的方向指示相速度快波方向,短棒的长短表示各向异性的振幅大小;其余图例与图1相同

Fig.9 Restoring resolution test(RRT)for the Rayleigh-wave phase-velocity azimuthal anisotropy tomography

图10 剪切波方位各向异性速度层析成像的结果平面图底图中红色和蓝色分别代表相对低和高的剪切波各向同性速度扰动;白色短棒的方向指示剪切波速度的快波方向,短棒的长短表示各向异性的振幅大小;紫色线表示对应深度的俯冲板片上表面位置;灰色锯齿线表示洋底年龄等时线;紫色剪头表示太平洋板块运动方向;各深度对应的平均剪切波速度标示在子图右下角

Fig.10 Map views of the Vs azimuthal anisotropy tomography

图11 剪切波方位各向异性速度层析成像的结果剖面图剖面图横轴含义是距汤加海沟的距离,向东为正,向西为负,纵轴表示距地表深度;图中红色和蓝色分别代表相对低和高的剪切波各向同性速度扰动;红色线表示源自Slab 2模型[39]的俯冲板片上表面位置;白色圆圈表示剖面两侧25 km距离范围内的地震事件;黑色线表示源自CRUST 1.0模型的莫霍面位置;地形图上红色三角形、红色箭头和黑色倒三角分别表示活火山、弧后扩张中心和汤加海沟;剖面位置如右下角插图中蓝色线所示;插图中正方形代表本研究使用的地震台站,插图中的其他图例与图1相同

Fig.11 Vertical cross-sections of the Vs azimuthal anisotropy tomography

图12 剪切波方位各向异性速度层析成像的棋盘格分辨率测试结果平面图 (a1)-(a8) 输入模型;(b1)-(b8) 输出模型;图中红色和蓝色分别代表相对低和高的剪切波各向同性速度扰动;白色短棒的方向指示剪切波速度的快波方向,短棒的长短表示各向异性的振幅大小;其余图例与图1相同

Fig.12 Map views of a checkerboard resolution test (CRT) for the Vs azimuthal anisotropy tomography

图13 剪切波方位各向异性速度层析成像的棋盘格分辨率测试结果剖面图图中均为输出模型,输入模型如图12所示;红色和蓝色分别代表了低和高的剪切波各向同性速度扰动;此图中的剖面位置与图11中的不同

Fig.13 Vertical cross-sections of a checkerboard resolution test (CRT) for the Vs azimuthal anisotropy tomography

图14 剪切波方位各向异性速度层析成像的恢复分辨率测试平面图 (a1)—(a8) 输入模型;(b1)—(b8) 输出模型;底图中红色和蓝色分别代表相对低和高的剪切波各向同性速度扰动;白色短棒的方向指示剪切波速度快波方向,短棒的长短表示各向异性的振幅大小;其余图例与图1相同

Fig.14 Map views of a restoring resolution test(RRT)for the Vs azimuthal anisotropy tomography

图15 汤加—劳—斐济地区上地幔构造特征示意图 NWLSC,西北劳扩张中心;LETZ, 劳伸展变形带;CLSC, 中劳扩张中心;ELSC, 东劳扩张中心;VFR,瓦鲁法脊;FRSC, 弗努阿雷裂谷和扩张中心

Fig.15 Schematic diagram of the upper mantle structure beneath the Tonga-Lau-Fiji region

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