Collaborative Research: NorthEast China Extended seiSmic Array (NECESS Array): Deep Subduction, Mantle Dynamics, and Lithospheric Evolution beneath Northeast China

沿中国东北东部边界，俯冲的太平洋板块接近600公里深。东北地表是一个板块内大陆岩浆活动和伸展活动活跃的地区。东北基底岩由太古宙克拉通和古生代褶皱带并置组成。中国东北扩展地震台阵（NECESSArray）项目是一项跨国合作的地震实验，在东北地区部署了280个宽带地震仪，研究地幔过渡带的板块行为、板块内大陆岩浆活动和构造运动的成因以及发生实质性变化的古代太古代岩石圈的演化。该实验将与来自中国地震局（CEA）、东京大学地震研究所（ERI）和日本东北大学地震和火山爆发预测研究中心的科学家以及美国该提案的项目负责人合作进行。总共，中国将提供140个宽带地震仪的数据，日本提供40个，我们正在申请资金部署100个仪器。阵列中仪器的最大间距约为80公里，孔径约为1000公里乘1200公里，部署时间为两年。通过我们的合作，我们将把所有可用的地震成像技术应用到我们收集的数据中，以开发中国东北地区从地壳到1000公里深度的地幔地震图像。地壳和地幔的地震图像将包括一个三维P和S速度模型，其尺度长度约为50公里，反射率剖面和各向异性图。地震结果将用于回答一些问题，包括：1)俯冲的太平洋板块是否位于660公里的边界上？2)如果板块是平的，那么在过渡带内，它在东北下方向西延伸了多远？3)板块是否在某一时刻下沉到下地幔中，如果是，它是如何下沉的？4)东北地区下地幔顶部是否存在其他俯冲带所观察到的反射体，如果存在，它们在矿物学和化学上与俯冲板块有何关系？5)是否有迹象表明下地幔顶部存在阻碍板块流动的屏障？6)俯冲板块上地幔流动与东北地区活动构造（中朝边界岩浆活动、蒙中边界岩浆活动和隆升活动、松辽盆地内伸展活动）的关系如何？7)太古代地幔岩石圈在俯冲和碰撞过程中是如何被改造和变形的？8)太古宙地幔岩石圈是否真的发生了分层，还是仅仅发生了水平的改造和位移？NECESS项目是地震学的，但我们的结果应该对地幔过渡带俯冲过程提供约束。这反过来又将为矿物物理学家和地球动力学家研究地幔中的对流及其矿物学和化学成分提供基本的限制。浅层地幔的地震图像将以前所未有的分辨率覆盖如此大的区域，并将包括经历活跃构造过程的区域以及经历改造的太古代岩石圈。因此，我们的结果将对地球化学家理解旧岩石圈改造具有重要意义，并有助于更好地理解通过对捕虏体和地震观测结果的研究进行的岩石测量之间的关系，以及板内构造的原因。

Along the eastern boundary of Northeast China the subducting Pacific plate approaches depths of 600 km. The surface of Northeast China is a region of active intraplate continental magmatism and extension. The basement rock of Northeast China is composed of an Archean craton juxtaposed with a Paleozoic foldbelt. The NorthEast China Extended SeiSmic Array (NECESSArray) project is a multi-national collaborative seismic experiment to deploy 280 broadband seismometers across Northeast China to study slab behavior in the mantle transition zone, the cause of intraplate continental magmatism and tectonics, and the evolution of ancient Archean lithosphere that has undergone substantial modification. The experiment will be done in collaboration with scientists from the China Earthquake Administration (CEA), the Earthquake Research Institute (ERI) of Tokyo University and the Research Center for Prediction of Earthquakes and Volcanic Eruptions of Tohoku Univeristy, in Japan and the PI's of this proposal from the United States. In total, the Chinese will provide data from 140 broadband seismometers, the Japanese 40, and we are requesting funding to deploy 100 instruments. The maximum spacing of instruments in the array will be about 80 km with an aperture of roughly 1000 km by 1200 km and a duration of deployment of two years. Through our collaborations we will apply all available seismic imaging techniques to the data we collect to develop a seismic image of the mantle beneath Northeast China from the crust to depths on the order of 1000 km. The seismic image of the crust and mantle will include a three-dimensional P and S velocity model with scale lengths of roughly 50 km, a reflectivity profile, and a map of anisotropy. The seismic results will be used to answer a number of questions including:1) Does the subducted Pacific plate lie flat on the 660 km boundary?2) If the plate is flat, how far does it extend to the west beneath Northeast China within the transition zone?3) Does the plate at some point sink into the lower mantle and if so how does it do this?4) Are there reflectors in the top of the lower mantle beneath Northeast China as observed in other subduction zones, and if so how are they related to subducted slab in terms of mineralogy and chemistry?5) Is there any indication of a barrier to slab flow within the top of the lower mantle?6) What is the relationship of mantle flow above the subducted slab to the active tectonics of Northeast China ( magmatism along the North Korea-China border, magmatism and uplift along the Mongolian-China border, and extension within the Songliao basin)? 7) How has the Archean mantle lithosphere been modified and deformed due to the subduction and collision history it has experienced?8) Has Archean mantle lithosphere truly been delaminated or has it only been modified and displaced horizontally?The NECESS project is seismological but our results should provide constraints on the processes involved in subduction in the mantle transition zone. This in turn will provide fundamental constraints for mineral physicists and geodynamicists studying convection in the mantle as well as its mineralogy and chemical composition. The seismic images of the shallow mantle will be at an unprecedented resolution for such a large area and will include regions undergoing active tectonic processes as well as Archean lithosphere that has undergone modification. Thus our results will be important for geochemists understanding of old lithosphere modification and lead to a better understanding of the relationship between rock measurements made through studies of xenoliths and seismic observables as well as the cause of intraplate tectonics.