自20世纪90年代首次探测到410-km间断面上覆低速层以来,全球多个俯冲带和大陆克拉通地区都陆续发现了该低速层结构.对其特性及形成机理的探讨是深部地幔结构、物性和动力学研究的热点问题.本文聚焦于西北太平洋俯冲地区410-km间断面上覆低速层的探测及特性研究上.通过对发生于日本北海道地区两个中等深度地震区域波形资料的分析,利用三重震相波形拟合方法获得了我国东北及日本海西北部下方410-km间断面附近的P波速度结构.速度模型明确显示,410-km间断面上方存在厚～47±14 km,异常值～2％的低速层,横向展布近700 km.结合区域地震层析成像、矿物高温高压物理实验及动力学模拟结果,我们否定了“从下至上”的上涌热物质导致410-km间断面上覆低速层的模型;认为较老且快速俯冲的太平洋板块在地幔过渡带顶部脱水导致硅酸盐矿物的部分熔融,由于熔体密度较大能够稳定存在于410-km间断面之上,从而产生了观测到的横向展布较广的410-km间断面上覆低速层结构.%Since the first detection of the low-velocity layer atop the 410-km discontinuity in 1990s,the seismic structure has been revealed in many subduction zones and continental craton regions.Its characteristics and mechanism of formation are among one of the hottest topics in studies of deep mantle structure,physical properties of mantle material,and geodynamic processes.We focus on the detection and feature investigation of the low-velocity layer atop the 410-km discontinuity in the northwestern Pacific subduction zone in this study.By modeling P triplicate waveforms from two intermediate-depth earthquakes,which are recorded by regional network stations,we obtain the P wave velocity structure near the 410-km discontinuity beneath northeast China and northwest area of Japan Sea.A low-velocity layer with thickness ～47 ± 14 km and ～2％ velocity anomaly is detected above the Mantle Transition Zone,covering a lateral range around 700 km.Combined with results from regional seismic tomography,high pressure and high temperature experiments and geodynamic simulations,we regard the "bottom-to-up" upwelling model an unlikely mechanism for the observed low-velocity layer;instead,we propose that the low-velocity layer may represent the partially melted silicates caused by dehydration of the cold and fast subducting Pacific slab near the 410-km discontinuity.The higher density makes it stable at the 410 km depth and produces the observed widely distributed low-velocity layer.
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