切穿兴蒙造山带的深断面——岩石圈电性结构及改造模式

兴蒙造山带自早古生代形成稳定的佳蒙地块以来, 受到古亚洲洋闭合、蒙古—鄂霍茨克洋闭合, 尤其是太平洋板块俯冲构造事件的影响.为了揭示兴蒙造山带岩石圈电性结构及改造模式, 本文基于横切额尔古纳地块、兴安地块、松辽地块及佳木斯地块约1300 km的大地电磁测深剖面, 对81个长周期测点进行非线性共轭梯度二维反演, 获得了兴蒙造山带东西向完整的电性结构模型.结果显示: 对于岩石圈尺度的结构, 不同构造单元具有明显的横向差异性, 额尔古纳地块岩石圈整体呈低阻特征; 兴安地块岩石圈呈高阻特征; 松辽地块的松辽盆地和小兴安岭岩石圈分别呈低阻-高阻特征; 佳木斯地块岩石圈呈高、低阻交替的特征.在软流圈尺度, 兴安地块和松辽地块存在大规模高导异常C2、C3, 呈"U"型状与上方岩石圈相连, 连接处对应东北地区盆山结合部位或主要的缝合带.研究表明兴蒙造山带岩石圈遭受了强烈的改造作用, 结合区域构造背景, 本文提出"自下而上"的岩石圈改造模式: 在地幔转换带处水平滞留的太平洋板块持续扰动兴蒙造山带下方的软流圈, 导致软流圈物质的上涌以及古缝合线的活化, 形成的薄弱带为软流圈物质的侵入提供了有利通道, 太平洋板块的俯冲深刻影响着兴蒙造山带岩石圈的演化过程.

The deep section cutting through the Xing'an Mongolia Orogenic Belt: Lithospheric resistivity structure and modification model

Since the formation of the stable Jiameng Block in the early Paleozoic, the Xing'an-Mongolia Orogenic Belt has been affected by the closure of the Paleo-Asian Ocean, the closure of the Mongolian-Okhotsk Ocean, especially the subduction of the Pacific Plate. In order to reveal the lithospheric electrical structure and the model of the modification, a lithospheric resistivity structure model is obtained by using Two-dimensional (2D) inversion (nonlinear conjugate gradient) on a 1300 km long period magnetotelluric profile containing 81 sites from Erguna Block, Xing'an Block, Songliao Block to Jiamusi Block. The resistivity model shows that there are significant differences in lithospheric structure, the lithosphere of the Erguna Block is characterized by low resistivity. The lithosphere of Xing'an Block is characterized by high resistivity, and the lithosphere of Songliao Block presents a low-high resistivity corresponding to the Songliao Basin and the Lesser Xing'an Range. The lithosphere of Jiamusi Block is characterized by high and low resistivity. The "U"-shape high conductivity anomalies C2 and C3 are distributed at the asthenosphere of Xing'an Block and Songliao Block, which are connected with the lithosphere. The junction corresponds to the basin-mountain junction or the sutures in Northeast China. The results show that the lithosphere of the Xing'an-Mongolia Orogenic Belt has been strongly transformed. Combined with the tectonic environment of this area, we proposed the lithospheric transformation model from deep to shallow. The Pacific plate retained horizontally in the mantle transition zone continuously disturbed the asthenosphere beneath the Xing'an-Mongolia Orogenic Belt, resulting in the upwelling of asthenosphere materials and the reactivation of ancient sutures. The weak zone formed provided a channel for the invasion of asthenosphere materials. The subduction of the Pacific plate has a profound influence on the evolution of the Xing'an-Mongolia Orogenic Belt.