P and SH velocity structure in the upper mantle beneath Northeast China: Evidence for a stagnant slab in hydrous mantle transition zone
P and SH velocity structure in the upper mantle beneath Northeast China: Evidence for a stagnant slab in hydrous mantle transition zone
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中国东北地区上地幔P和SH速度结构：含水地幔过渡带存在静止板片的证据
DOI：
10.1016/j.epsl.2013.02.026
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发表时间：
2013-04
影响因子：
5.3
通讯作者：
Yuen, David A.
中科院分区：
文献类型：
--
作者：
Li, Juan;Wang, Xin;Wang, Xiujiao;Yuen, David A.
利用发生在俄罗斯-中国边界附近的三次深部地震的高密度区域体波，研究了中国东北和北中国克拉通北部地幔过渡带的S波和P波的速度结构，西北太平洋板块被成像为亚水平位于660公里不连续面上。在11-16.5°的距离范围内，S-P走时残差沿震中距离呈递增趋势，表明MTZ存在速度异常。我们寻求最简单的模型来解释观察到的宽带波形和受限方位扇区三倍频的相对旅行时。SH和P数据都表明，在一个略低而宽的660公里的不连续面上有一个∼140±20公里的高速层。过渡带深部∼的剪切速度需要降低2.5%，以补偿AB和CD三分支间较大的相对时间，以及S-P走时残差的增加趋势。整体上，MTZ具有低剪切速度和高Vp/Vs比值的特点。H2O含量为0.2-0.4wt%的富水地幔过渡带可能解释了观测到的Vp和Vs速度结构之间的差异。我们的结果支持粘性为主的滞流板的情况，其厚度增加了∼140公里，这是由于上、下地幔之间的巨大粘性差异造成的。水的补充和向东的海沟退缩可能促进了中国东北俯冲的太平洋板块的停滞。
Using high-dense regional body waves for three deep earthquakes that occurred around Russia–China border, we investigate both S and P wave velocity structures in the mantle transition zone beneath Northeast China and northern part of North China Craton, where the northwestern Pacific plate is imaged to subhorizontally lie above the 660-km discontinuity. We observe an increasing trend of S–P travel time residuals along the epicentral distance within a distance range of 11–16.5°, indicating a velocity anomaly in MTZ. We seek the simplest model that explains the observed broadband waveforms and relative travel times of triplication for a confined azimuth sector. Both SH and P data suggest a ∼140±20km high velocity layer lying above a slightly depressed and broad 660-km discontinuity. Shear velocity reduction of ∼2.5% in the deeper part of the transition zone is required to compensate for the significantly large relative time between AB and CD triplicate branches and the increased trending of S–P travel time residuals as well. The MTZ, as a whole, is featured by low shear velocity and high Vp/Vsratio. A water-rich mantle transition zone with 0.2–0.4wt% of H2O may account for the discrepancy between the observed Vpand Vsvelocity structures. Our result supports the scenario of a viscosity-dominated stagnant slab with an increased thickness of ∼140km, which was caused by the large viscosity contrast between the lower and upper mantles. The addition of water and eastward trench retreat might facilitate stagnation of the subducting Pacific slab beneath Northeast China.
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