松辽盆地岩石圈减薄的深部动力学过程

Deep dynamic process of lithosphere thinning in Songliao basin

松辽盆地作为东亚裂谷系的一部分,与华北克拉通一起经历了中生代岩石圈减薄的重大地质事件.对大陆岩石圈-软流圈状态和构造的整体认识,是研究大陆岩石圈减薄深部动力学过程的关键.在获得过松辽盆地的106个宽频和30个长周期大地电磁测深数据的基础上,完成测点数据二维偏离度、构造走向等计算与分析,进一步采用非线性共轭梯度算法,对TE和TM模式数据进行二维联合反演,获得了沿剖面的壳-幔电性结构,并依此构建了松辽盆地壳-幔结构模型.研究结果表明:(1)大兴安岭地区岩石圈厚度约为160km,松辽盆地岩石圈厚度约为45km,张广才岭岩石圈厚度在70~100km之间,莫霍面与岩石圈底界面不呈镜像关系.软流圈整体表现为中、低阻异常,电阻率值在30Ωm左右,其形态呈西倾约30°的蘑菇状异常,指示了软流圈物质上涌的形式,有别于软流圈垂直上涌的传统认识.(2)松辽盆地深部存在双层高导异常(电阻率小于5Ωm),上层为壳内高导层,呈"蛇"状分布,推断为岩浆底侵区,下层为幔内高导层,呈"哑铃"状,为软流圈上涌区.软流圈内存在两个"哑铃"状中、高阻异常,推断为拆沉的岩石圈地幔.具有冷的、高密度的下降物质流的堆积以及拆沉块体下插到两侧山岭是促使大兴安岭与张广才岭在中生代伸展环境中快速隆升重要原因;(3)松辽盆地经历了岩石圈减薄事件,与大兴安岭岩石圈厚度相比,松辽盆地岩石圈厚度减薄了近100km,与东侧张广才岭相比减薄了70km,而与中生代华北地台100km的岩石圈厚度相比,减薄了近50km,其经历了岩石圈伸展期、裂解期、拆沉期和增长期的动力学过程.

As a part of the East Asian Rift System,Songliao basin experienced major geological events of Mesozoic lithospheric thinning together with the North China craton.The overall understanding of the state and structure of continental lithosphere and asthenosphere is the key tostudying the dynamics of continental lithosphere thinning.Based on the 106 broadband and 30 long period magnetotelluric sounding data from the Songliao basin,we complete the calculation and analysis of the data about two-dimensional deviation degree,the structural trend and so on.We further use the nonlinear conjugate gradient algorithm for 2 Dcombined inversion of the mode data of TE and TM to obtain the crust-mantle electrical structure along the profile,and then we are able to build crust-mantle structure model of the Songliao basin.The results are summarized as follows.（1）The lithosphere thickness is about 160 km in Da Hinggan Ling area,about 45 km in Songliao basin,and is between 70 and 100 km in the Zhang Guangcai Ling.The Moho and the bottom interface of lithosphere do not have a mirror image relationship.The overall property of the asthenosphere shows middle and low resistivity anomalies,and the resistivity value is about30Ωm,The abnormality shaped like a mushroom presents a westerly dip of 30°.It indicates an upwelling of the asthenosphere which is different from the traditional understanding of the vertical upwelling asthenosphere.（2）There are double high conductivity anomalies in the deep part of Songliao basin（the resistivity is less than 5Ωm）.The upper high conductivity layer is in the crust,which is snake-like distributed,and we infer that it is the bottom of the magma invasion zone.The lower high conductivity layer in the mantle is dumbbell shaped,and it is the asthenosphere upwelling area.The asthenosphere is characterized by two dumbbell shaped high resistivity anomalies,which are inferred to be the delaminated lithospheric mantle.The accumulation of cold,high-density descending flow of material and the insertion of deladimated blocks beneath the mountains on the two sides caused the rapid uplifting of the Da Hinggan Ling and Zhang Guangcai Ling in Mesozoic extensional environments.（3）The Songliao basin experienced lithospheric thinning event;compared with the Da Hinggan Ling,the Songliao basin lithosphere thinned near 100 km,or 70 km compared to the Zhang Guangcai Ling on the east,and compared to 100 km of the Mesozoic lithosphere in North China platform the thinning is near 50 km;it experienced a dynamic process of lithosphere extension,rifting,delamination,and growth.