摘要
中国东北地区在古生代期间以众多微陆块的拼合以及古亚洲洋的闭合为特征,其后又经历了中-新生代太平洋构造域及中生代蒙古—鄂霍茨克构造域的叠加与改造,以致东北地区的构造行迹显得极为复杂,而大兴安岭重力梯级带及其西部地区构造演化是否与西太平洋俯冲有关仍然存在争议.本研究利用分布于中国东北、华北地区以及韩国、日本等部分台网所接收的近震与远震走时数据获得了中国东北地区壳幔精细的三维P波速度结构.成像结果显示,太平洋板块持续西向俯冲,俯冲板片的前缘停滞在大兴安岭—太行山重力梯度带以东区域的地幔转换带之中;长白山火山区上地幔存在着显著的低速异常体,推测西太平洋板块的深俯冲脱水导致了上地幔底部岩石的熔点降低,从而形成了大范围的部分熔融物质上涌.通过分析上地幔的速度结构,我们认为由于太平洋板块的大规模西向深俯冲,在大地幔楔中发生板片脱水、低速热物质上涌等复杂的地球动力学过程;俯冲板片前缘带动上地幔中不均匀分布的地幔流强烈作用于上部的岩石圈,这对东北地区深部壳幔结构乃至大兴安岭重力梯级带的形成、演化有着重要的影响.
The Paleozoic tectonic evolution of Northeast(NE)China was dominated by theamalgamation of multiple microcontinental massifs and the closure of the Paleo-Asian ocean,whereas in the Mesozoic and Cenozoic,NE China experienced the overprinting of circum-Pacific and Mongol-Okhotsk tectonic systems,which finally resulted in the very complex tectonic features of NE China.In addition,there exists a big dispute on whether the tectonic evolution of Great Xingan gravity lineament(GXGL)and the region on its west was affected by the subduction of the Pacific plate.In this study,we determined a 3-D P-wave structure down to 1000 km depth beneath NE China using agreat number of high-quality arrival-time data of local,regional and teleseismic events recorded by dense seismic networks in NE China,North China,South Korea and Japan.Our tomographic images show some new features in the subsurface of this region.The subducting Pacific slab is imaged clearly by high-velocity anomalies extending from the Japan Trench to the deep GXGL,stagnating in the mantle transition zone.The Changbaishan intraplate volcano is underlain by a significant low-V anomaly from the mantle transition zone and a big mantle wedge(BMW)is present up to the surface,which may reflect that the deep subduction dehydration of the Western Pacific plate resulted in lower melting point of the rock in the upper mantle,and further caused large-scale upwelling of partially molten material.Complex geodynamic processes occur in the BMW due to large-scale deep subduction of the Pacific Plate,such as the deep dehydration process of the subducting slab and upwelling of low-velocity and hot materials.The uneven mantle flow in the upper mantle driven by the subducting slab front poses strong action on the upper lithosphere,which has an important influence on the tectonic evolution of crust and upper mantle structure beneath NE China and even on the formation and evolution of the GXGL.