| The Longxian-Qianyang basin is located on the southwestern margin of the Ordos block in the western section of the North China plate.The basin is controlled by the Longxian-Baoji fault and is distributed from northwest to southeast.The Longxian-Qianyang basin is located in the east and west of the mainland of China.Study on the crustal structure and structural characteristics of the Qinghai-Tibet Plateau uplift and growth process,strengthen the understanding of the tectonic evolution of the east-west and north-south megastructure tectonic belts on the Chinese mainland,and It is of great significance to further study the seismic activity law of Fenwei earthquake zone and north-south seismic zone.For many years,it was constrained by the thick layer of loess in the deep coverage area,and there was no clear understanding of the basement structure and basement fault under the cover of the loess in this study area,which also affected the understanding of the structural evolution of the area.In this paper,based on the working idea of combining drilling data with geophysical exploration in the study area,the basic time limit of the crustal structure and tectonic evolution of the LongxianQianyang basin and the long-range effects of the Qinghai-Tibet Plateau uplift on the northeastern margin of the Tibetan Plateau are The response of this area has reached the following conclusions:1.Regional gravity anomaly trend inversion results and high-precision Bouguer gravity anomaly profiles reveal the basement burial depth of the underground Cenozoic in the study area,ascertain the ups and downs of the basement surface,indicating the Longxian-QishanMazhao fault and other areas Structural features of important internal faults.Controlled source audio-frequency magnetotelluric sounding method inversion of resistivity profile shows the lateral change of morphology and underground electrical properties of the underground bedrock surface in the study area,indicating the existence of underground morphology and hidden faults in different eras,clearly reflecting the faults The stratigraphic structure of the upper and lower plates.The basement morphology of the Longxian-Qianyang basin presently presents the structural characteristics of "south,deep,north,shallow,concave and convex distribution",and the bottom interface of the Cenozoic in the study area shows a gradual upward trend from south to north,and the south of the Paleozoic top boundary fluctuates.Larger and relatively flat in the north.2.Since the Neogene,under the influence of the northeast-trending extrusion dynamics of the Qinghai-Tibet Plateau,the southwestern margin of Ordos,where the study area is located,has entered the stage of the neotectonic movement.The north-south fault of the Qinling Mountains and the Haiyuan fault has undergone a left-slip strike to absorb The eastward extrusion stress of the Qinghai-Tibet Plateau led to the further development of the Ordos rift basin.The Longxian-Qianyang rift basin was formed in the Early Pleistocene.In the late Early Pleistocene,river terraces began to form in the Qianhe River Valley.This indicates that the Qinghai-Tibet tectonic system has begun to dominate the tectonic movement in the southwestern margin of Ordos.Moreover,the ages of the five uplifts of the Qianhe fifth-level terrace corresponded to several stages of the Qinghai-Tibet Plateau uplift.At the same time,the Longxian-QishanMazhao fault controlling the northern boundary of the Qianhe River was a left-slip normal fault.The latest research results show that passive sliding is the result of the Tibetan Plateau squeezing northeast.Therefore,the formation and evolution of the Qianhe sixth-level terrace is a response to the long-range effect of the massive expansion of the Qinghai-Tibet Plateau uplift. |
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