关键词: 深反射地震特征, 地壳结构, 地球动力学

Abstract: The Songliao basin located in Northeast China is a large Mesozoic rift basin. A lot of work has been done on the geological characteristics, formation and evolution of the basin, as well as the accumulation regularity of oil and gas in this basin. However, the question about the formation of the basin is still a point at issue, and has attracted the great attention of many geologists. Various models have been proposed so far for the formation of the basin, such as continent rift, back arc rift, and pull-apart models. Basing on deep seismic reflection data, this paper discusses the type of the basin and the geodynamic cause of its formation. Deep seismic reflection profiling has revealed that the crust of the Songliao basin is characterized by both layered and fault-block structures. The crust can be divided into three layers: the upper crust, middle crust and lower crust. Parallel reflections in shallow layer and oblique reflections at above 6s twt represent the upper crust. The strong reflection belt below the upper crust represents the detachment zone between the upper and middle crust. Most of the faults in the upper crust terminate in this zone. The analysis shows that the faults in the upper crust are extensional syn sedimentary faults formed during late Jurassic to early Cretaceous and thrust system formed in late Paleozoic era. The middle crust at about 68s twt not only has a lot of discontinuous parallel reflections and rhombic reflections, but also a small quantity of "crocodile mouth" reflectors. The former represents extensional reformation formed in late Mesozoic era, and the later represents compressional structure formed in late Paleozoic era. The middle-crust is also a low-velocity and high conductivity layer. The lower crust is located at about 8～11s twt, and is characterized by rhombic reflection events. The Moho below the lower crust appears as a strong reflection belt, beneath which the layer is transparent. The reflection structure of the Songliao basin is similar to that of typical rift basin. Boundary faults between blocks were the passage of upward transferring of hot fluid from the middle lower crust or mantle to the basin. "Mushroom cloud" reflections at the deep part of the boundary faults might be the reflections of hot fluid diapir. Mirror symmetry between the down-warped basin and mantle uplift is the main evidence for the rift-origin of the Songliao basin. Deep seismic reflection data indicate that the main factor at depth that affected the subsidence of the basin was not only the uplift of the mantle, but also the "low velocity-high conductivity" layer in the upper-middle crust and hot fluid diapirs. The former caused the subsidence in later stage, while the later caused the subsidence in early stage. Regional geology and geophysical data indicate that this area is located in back-arc formed by the subduction of the Kula-pacific plate into the Eurasia plate in Mesozoic. Mesozoic volcanic rock system in this area is mainly of CA series, with a few exception of a small amount of A and Th type. Geochemical analysis reveals that the volcanic rocks here are characterized not only by high SiO₂ and K₂O contents and enrichment in LREE, but also by EMII type mantle, indicating back arc environment disturbed by plate subduction. However, seismic tomography data have revealed that the Songliao basin is located in disturbed area of continental mantle, rather than back-arc area. It is suggested, therefore, that formation and evolution of the Songliao basin was the result of the subduction of Kula-Pacific plate into the Eurasian plate. The plate subduction has led to the uplift and diapir of the mantle, and the detachment of the crust. Strong crust-mantle interaction has led to the delimitation and extension of the crust, resulting in the formation of sandwich structure and large scale subsidence of the crust. It was the afore mentioned factors that caused the formation of mirror symmetry between the downwarping part of the basin and the uplifting part of the mantle, as well as the dissymmetry between the fault depression and the uplifting part of the mantle. The former was the result of large scale hot subsidence of the crust caused by the mantle cooling, while the later was resulted from simple shearing deformation of the middl-upper crust caused by the detachment of middle crust from the upper crust. The intense crust-mantle interaction was the major factor controlling the formation of no-granite basin w ith sandw ich structure and hot fluid diapirs at depth, and it may play an important role in the accumulation of deep-seated fluid and oib gas in the basin.

Key words: deep seismic reflection, crustal structure, basin geodynamics, no-granite basin