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.
The goal of the project entitled" Northeast China Extended Seismic Array (NECESS Array); Deep subduction, mantle dynamics, and lithosphere evolution beneath Northeast China" was to better understand what happens to subducted lithosphere when it encounters the boundary between the upper and lower mantle near 660 km depth in the Earth. A secondary goal was to understand the evolution of the lithosphere in Northeast China which has exhibited widespread magmatism and extension during the past 100 Ma. This region was chosen to examine deep subduction because, based on deep seismicity, it is known that old slabs that subducted beneath Japan are encountering the upper-lower mantle boundary on the eastern edge of Northeast China near the border with North Korea. To accomplish the goals of the project, in collaboration with Chinese and Japanese scientists we deployed 127 broadband seismometers throughout Northeast China for two years from August 2009 through August 2011. A number of seismic analysis techniques were used on the data to image the 3D seismic P and S wavespeeds in the mantle as well as the depths of subsurface boundaries such as the Moho and the 410 and 660 km depth discontinuities. Tomography results show slab piled up at 660 km depth over about 500 km in the horizontal direction but, unlike in older tomography images, they do not extend further in land. The 660 discontinuity, thought to be due to a temperature dependent phase transition, is observed to be depressed by about 30 km over a width of 200 to 500 km consistent with the seismic tomography results. Weak high velocity anomalies are observed beneath the piled slabs in the lower mantle but the signals are weaker than seen in other subduction zones. Our results show slab piling up in the upper mantle but likely ultimately sinking into the lower mantle although the weak signal we observe in the lower mantle is a puzzle. A second discovery of the project is that we observed a gap between the piled up slab between the north and south of our study area (fig. 1). We also observed anomalously slow seismic velocities in the gap as well as above the gap that connected to extremely slow shallow mantle found beneath Changbaishan volcano. Changbaishan volcano, on the China-North Korea border, is a large active volcanic complex that poses a grave hazard to both countries. We also found the 660 discontinuity is uplifted by about 8km within the gap between slabs indicating the anomalously slow mantle is warm. Our model for these observations is that anomalously warm mantle exists beneath the Pacific lithosphere and is carried downward during the subduction process. Where a gap opens within the slabs, the warm asthenosphere rises due to its buoyancy (fig. 2). The warm "plume" undergoes decompression melting in the shallow mantle resulting in the Changbaishan magmatism. Our model presents a new idea for the cause of this enigmatic volcanism. The shallow mantle and crust were also imaged throughout Northeast China. Regions of recent magmatism, like Changbaishan and within the Xing-An mountain range to the west, have slower shallow mantle S and P velocities than in the Songliao Basin, a region in the center of North East China that underwent extension in the Mesozoic time period. We find thin crust in the Songliao Basin but relatively high mantle velocities that may be due to cooling during the past 60-100 Ma since extension ended. Beneath the eastern edge of the Xing-An mountains bordering the western edge of the Songliao Basin, slow shallow mantle velocities are underlain by fast mantle near 400 km depth. We speculate that lithospheric delamination may be occurring here although further research is needed to confirm this.
