利用接收函数散射核方法探测中国东北地区地幔转换带界面三维形态

Topography of mantle transition zone discontinuities beneath Northeast China imaged by receiver function with Ps scattering kernel

地幔转换带是上下地幔物质运移和能量交换的必经通道,其速度结构和上下界面的起伏能够为认识地幔内部的温度和物质变化、地幔对流模式等地球演化相关的科学问题提供关键约束.本文利用布设在中国东北地区的高密度固定台网和流动台阵所记录的远震体波接收函数,采用Ps散射核叠前深度偏移成像方法,获得了台站下方地幔转换带界面及其内部速度间断面的三维精细图像.研究结果表明:西北太平洋俯冲板片上下界面在地幔转换带内清晰可见,在高纬度(44°N)区域存在约30°的倾角;660-km间断面的深度起伏具有明显的分区性,在与俯冲板片相交处以西200~300 km,界面出现约20~40 km的下沉,而长白山和龙岗火山的西北区域存在约5~15 km的抬升,分别与板片滞留引起的低温异常和局部热物质上涌相对应;410\|km间断面的起伏形态较复杂,在大部分区域观测到大于10 km的下沉,且表现出明显的区域性横向变化,与深俯冲动力学背景下冷的温度异常造成的影响不一致.我们认为板片俯冲、停滞和海沟后撤过程中引起的地幔转换带物质异常、含水状态及分布的变化是显著改变410\|km间断面形态的主要原因.本文获得的高精度地幔转换带界面三维形态为更好地认识东北亚地区俯冲板片在地幔物质分布和能量交换中的作用提供了重要参考.

Mantle transition zone connects Earth′s upper and lower mantle and holds key information on the compositional and thermal states at depths.The depth variation,i.e.topography,of mantle transition zone discontinuities implies regional changes of temperature and/or composition because of their well-established relationship to the solid phase transitions between high-pressure polymorphs of olivine.Based on the teleseismic receiver functions collected from densely deployed permanent network and two temporary arrays in Northeast China,we imaged the transition zone structures using Ps scattering kernel.The results show clear boundaries of the Northwest Pacific slab at the transition zone depth,with a dip angle of about 30°.We observed 20~40 km depression of the 660-km discontinuity near where prior tomographic studies suggested subducted slab.Such depression may be explained by the lower mantle temperature caused by the cold Pacific plate.Beneath the northwest of Changbaishan and Longgang Volcano,the 660-km discontinuity is elevated by 5~15 km,indicating high temperature anomalies that may come from upwelling.In contrast,the topography of the 410-km discontinuity reveals a widely spreading 10 km depression in most areas,and it shows obvious regional lateral variation.Such large-scale depression suggests that compositional changes introduced from subduction,such as basalt fraction and water content,also play a role in controlling the 410-km discontinuity topography.The high-resolution three-dimensional topography of the mantle transition zone obtained in this study will motivate further investigations on understanding the transformation of mantle material distribution and energy exchange by subduction plates in Northeast Asia.