四川盆地—大巴山结合带地壳构造特征:深反射地震约束的重磁解释

大巴山位于四川盆地北部、秦岭南缘,以发育大规模的中生代逆冲推覆构造为世人瞩目,其地壳尺度结构与构造特征对于理解扬子克拉通和华北克拉通的碰撞过程有重要意义.本文基于深反射地震剖面和宽角反射与折射地震数据的约束,结合地质与钻井资料,通过重磁场分析拟合解释,对四川盆地北部—大巴山地壳尺度的构造进行综合研究,建立了地壳断面结构与构造模型.模型显示,四川盆地北部基底和沉积盖层变形差异较大,且上下地壳具有解耦性,上地壳向北延伸至紫阳断裂一带;而下地壳与上地壳拆离,向北延伸远至安康断裂一带.研究区莫霍面起伏较大,自南向北先加深后抬升,在大巴山下出现Moho的构造叠置,这种现象源于大陆地块(扬子克拉通)下地壳向大巴山—秦岭造山带下的俯冲.类似的现象也出现在西昆仑山下,即塔里木盆地下地壳向西昆仑山下俯冲,俯冲板片前缘出现Moho的叠置.

Crustal structure of Daba Shan and adjacent terranes revealed from high-resolutiondeep seismic-reflection profiling and potential field modelling

The Daba Shan thrust belt of central China known for its large-scale Mesozoic thrust nappe structure has been active since Mesozoic,which is important for understanding the collision process between north and south China.Restricted by high-resolution seismic-reflection profiling,wide-angle reflection and refraction seismic data,based on gravity surveying,and magnetic observations,combined with geology and drilling data,we obtain the crustal-scale structure from the northern Sichuan basin to Daba Shan.The model shows the different deformation between the basement and sediments in the northern Sichuan Basin,where the upper crust is decoupled from the lower crust.It is obvious that the upper crust extends to the Ziyang Fault while the separated lower crust extends further to the Ankang zone.We also the root zone of the Daba Shan thrust nappe structure extends to a depth of 27 km.Moreover,our study indicates the depth of Moho vary from the south to the north,and we propose the phenomenon resulted from the subduction crust of Sichuan basin which caused the duplex of Moho below the Daba Shan.A similar phenomenon is also found under the western Kunlun Mountains,where the lower crust of the Tarim Basin has subducted and the Moho overlap appeared on the front edge of the subduction plate.