Lithospheric Structure Of Zoigê Basin-West Qinling Orogen And Its Geodynamical Process

The Zoigêbasin situated in the northern margin of the Songpan block is located in the northeastern margin of the Qinghai-Tibet plateau, whose lithosphere structure and relationship to the nearby orogenic belts reveal the evolvement process trending eastward and northeastward of the Qinghai-Tibet Plateau. And it also lies in the joint of the China continental east-west and south-north structures. Owing to its special geological location, the Zoigêbasin has been considered as an important sit to study the geodynamic process of the south-north conjunction and the east-west transformation. As a main body of the Songpan block, the Zoigêbasin is researched principally on account of the Songpan block being a virginal area for oil and gas exploration. The inner lithosphere structures and their attributes may reveal the structure relationship of mountain and basin, trail the geodynamic process of orogens and basins, and inform the existing of oil and gas basins, so it is necessary to disclose lithosphere structure. Deep seismic reflection is a most valid method in studying fine lithosphere structure and revealing some typical structure.A deep seismic profile across the Zoigêbasin and the whole the west Qinling orogen was carried out in the winter of 2004. Because of the complicated surface geological conditions and the unusual reflection with high dips and lateral variation in velocities, the data is characterized by the lower S/N and the anormally higher statics. The data processing is far more difficult and the interpretation of the imagines is also hard. Geoscientists face to the work how to process and imagine the data,reveal the geological deformation and the structure relationship of basin and mountain, and tell the evolution stories.The main work in the paper is to develop new techniques to process the data and do the interpretation of the data imagines. In the paper, the problem in the normal seismic process theory has been pointed out and the new algorithms to solve the problem have been developed. The ameliorated methods of normal movement and velocity analysis that were given in the paper, have been tested. The very good imaging results have been obtained in the processing of the deep seismic reflection data. Furthermore, the geological interpretation of the data has also been done in the paper. The paper gives partly the evolution process of the Zoigêbasin and the west Qinling oregen, and recovers partly the tectonic and structural formation of the Block.The paper is composed with three parts of the research work, the deep seismic data processing algorithm, the data preprocessing and processing work, and the imaging interpretation for the structure evolution and the structure relationship of the basin and the orogen.In the first part, the problems of using normal seismic data processing techniques in the deep seismic data have been analyzed. The new algorithms for NMO and velocity analysis based on topography are described and the effectiveness of the algorithms in the processing is proved with the synthetic models and the deep seismic data. In the second part, the processing flow and procedure are clarified. The processing data includes the synthetic models and the deep seismic data( line RH04-1-2)which acquired in the field. In the third part, the interpretation of the data is described for the geological structures formations and the evolutions of the Zoigêbasin and the west Qinling orogen. The structure relationship and the geodynamic process of formation of the Zoigêbasin and the west Qinling orogen are discussed in the paper based on the seismic stack data and some other geophysical data(such as the gravity data, the magnetic data, and the refraction data) and the geological data. Upon the interpretation of seismic imagines and some other studies, the paper gives some research conclusions as follows.(1)The boundary in depth between the Zoigêbasin and the west Qinling orogen is found on the south edge of Dashui Arc like structure, the exact location is in the CDP No.3790(receiver location No.4923.5). It goes to 7.65km south of the surface boundary.(2)The bottom of Triassic layer is found to be at 2.5s(TWT) on the seismic stack section. It is estimated to be 7.0km in depth with an average velocity of 6.0km/s. The basements of the Zoigêbasin and the west Qinling orogen belong to the same basement according to the analysis of seismic data reflection characteristics.(3)It looks similar for the reflection features of the basement, the upper crust, the lower crust and the Moho in the Zoigêbasin and the west Qinling orogen. This reveals that two geological units ever underwent the same evolution process in the geological stage.(4) The deep seismic reflection section show that there are some strong north-dipping reflections which afford reliable seismic evidences for the collision between the Zoigêbasin and the west Qingling orogen.The west Qinling orogen and the Songpan-Ganzi block moved northward and escaped eastward owing to the strong extrusion of Indian plate and Eurasian plate and the uplifting of the Qinghai-Tibet plateau. Movement eastward brought out thrust-pressure and strike-slip movement from northwest to southeast that resulted in the upwarp structure in the upper crust. Layer deformation occured mainly in the upper crust on account of the structural detachment layer at a depth of 18～21km in the crust, and the lower crust keep the trail of the thrust and subduction in Indiosinian Period.