| The study on dynamics and deep-seated factors controlling the intracontinental orogeny is the leading edge of scientific research in earth sciences nowadays. Tianshan Mountains, which is far from the plate collision zone, is one of the components of typical intracontinental-centra Asia orogeny in the world. The converging and subduction of the Tarim and Kazakstan blocks under the Tianshan Mountains, which are located at the either side of Tianshan Mountains, may be the intracontinental orogenic impetus of Tianshan Mountains. At present the volcanic activities and frequent piedmont seismicities of the South Tianshan Mountains imply that the converging and subduction of the Tarim and Kazakstan blocks continue, and it may suggest the existence of a large-scale subduction of the continent in front of the South Tianshan Moutains. The subduction of Tarim block (or the overriding of Tianshan Mountains) may cause the formation of a large-scale foreland basin in front of Tianshan Mountains and has sediments with ten thousand meters since the Cenozoic Era, which are the conditions of forming large oil and gas fields.The lithospheric structure in the junction zone of South Tianshan Mountains and Tarim basin records many important information about the formation and evolution of the continental crust. There are rich energy resource and solid mineral deposits in this region. For a long time the scholars from all over the world try to uncover the formation and evolution of Tianshan Mountains with keen interesting, look for and explore the resource of it to serve the human being. In earth sciences, the deep seismic reflection profile is acknowledged as an effective approach to study the lithospheric structure in detail in the world.The deep seismic reflection profile showed in this paper goes across the junction zone of the southwest Tianshan Mountains and Tarim basin. The raw seismic data of the profile is acquisitioned by the Institute of Geology, Chinese Academy of Geological Sciences in 2007, working together with the Exploration Subsidudy Company of Sinopec Corporation and the University of South Carolina in U.S.A. Based on a comprehensive analysis of the characteristics of the raw seismic data in the junction belt of basin and range, the difficulties of data processing and the shortcomings of the conventional seismic data processing,1 use the high resolution structure targetted processing method, prestack migration in time domain and prestack depth migration during the data processing.There are three key steps during the high resolution structure targetted processing. Firstly, remove noise combining with different domains and raise the signal and noise rate of the data, then solve the surface statics problem in the junction belt of basin and range using the tomography statistics, and finally obtain the consistent of the amplitude and frequency bandwidth using swing compensation on surface consistent and amplitude consistent deconvolution. The three, steps lay a solid foundation for the rest processing of the seismic data.The prestack Kirchhoff migration in time domain and prestack depth migration are used in the data processing. Those prestack migrations in both time and depth domains based on flat datum provide wonderful results. A proper data processing flow of prestack migration is summarized for the seismic data from the junction belt of the basin and range.It’s difficult to obtain proper image due to the rough surface in the junction belt of basin and range, especially in the deep seismic reflection detection. Conventional migration methods are not suitable to be used in the data processing. However, prestack migrations in both time and depth domains based on flat datum can obtain acceptable results, because the wavefields at both the common reflection points and the common imaging points can realize in-phase stack, which is better than common middle point stack. Therefore, the prestack migrations can be used to solve the imaging problem in the junction belt of basin and range.Through the researchs of high resolution structure targetted processing and prestack migrations, I obtain the deep seismic reflection profile in pseudo-depth domain. It shows the fine lithospheric structure in the junction zone of South Tianshan Mountains and Tarim basin, and the quickly collisional orogenic deep processes of Tianshan Mountains since the Cenozoic Era. The interpretations of the deep seismic reflection profile in the pseudo-depth domain are as follows:There are two clear layers in the crust under Tarim basin. There are huge deposits and stable, consistent crystalline basement in the upper crust, which provide material base for hydrocarbon generation and preservation. Compared with the stable deposits under Tarim basin, the strata in the up and middle of the crust under the South Tianshan Mountains become fractured due to strong reformation, the reflections of those strata are weak and discontinuous. A series of southward thrust structures suggest that the overriding of the southwest Tianshan toward the basin affects the whole sedimentary cover, the closer it overrides to the Tianshan Mountains, the deeper the deformation of the thrusting structure is. The strong reflection of the stratigraphy sedimentary records the structural slip planes in different depths, the upthrust nappe structure causes the stack of the strata along the slip planes of structures and increases the thickness of the crust.The lower crust under Tarim basin wedges and subducts into the South Tianshan Mountains and stops at the Kashi uplift. Cross the wedge structure, a series of northward reflections record the trails of the Tarim basin subducting into that under South Tianshan Mountains, those trails close to the Moho are very clear.The depth of the Moho changes from 55 Km in the south to 61 Km in the north, and the Moho are separated into three platforms with different depths. The middle platform has slightly upwarping and the Moho under the South Tianshan Mountains is 6 km deeper than that under the Tarim basin on the whole.The lithospheric mantle clearly records the trail of the lithospheric structure under Tarim basin thrusting into that under Tianshan Mountains. The refelctions of northward thrusting structure under the Tarim basin around 26 to 27 seconds are clear. The similar structural reflection in the lithospheric mantle extends continually toward Tianshan Mountains.In summary, the subduction of lithospheric structure under Tarim basin into that under Tianshan Mountains is the impetus of quickly uplifting of Tianshan Mountains since the Cenozoic Era. The upper crust under Tianshan Mountains thrusts into Tarim basin, and it increases the thickness of the crust, which is mainly happened in the sedimentary covers. Howerer, the thickness of the lower crust under Tianshan Mountains does not considerably increase, that is, there is no mountain root under Tianshan Mountains. Therefore, the thrust thin-skinned nappe structure may be produced by a new orogenic mechanism of Tianshan Mountains since the Cenozoic Era. The deep seismic reflection profile implies the collisional orogenic deep processes of Tianshan Mountains in detail. |
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