The Weihe Basin is the main component of the extrusion and escape shear zone between the ancient North China craton block in Ordos and the ancient Yangtze platform in Sichuan Basin, and carries the dynamic transmission from the main power source of the Qinghai-Tibet Block in the west to the North China and South China regions in the east. The basin itself plays multi roles in the east-west and north-south tectonic movement, and is an excellent site for studying the structural interlacing, dynamic transformation and transmission. At the same time, Weihe Basin is also a famous strong earthquake zone in China. Historically, there was a strong earthquake of magnitude 8 1/4 occurring in Huaxian County in 1556, causing huge casualties and property losses. In view of the special geological structures and the characteristics of modern seismicity activities in the Weihe fault-depression zone, it is necessary to carry out fine three-dimensional velocity structure detection in the deep part of Weihe Basin and its adjacent areas, so as to study the relationship between velocity structure and geological structural units and their evolution process, as well as the deep medium environment where earth￣quakes develop and occur.
We investigate the S-wave velocity structure beneath Weihe Basin and its adjacent regions based on continuous background noise data and teleseismic data recorded by 257 broadband stations in Shaanxi Province and its adjacent regions and China Seismological Science Array Exploration Project, and by adopting seismic surface wave inter-station method and background noise cross-correlation method, a total of 10 049 fundamental-mode Rayleigh surface wave phase velocity dispersion curves in the periods of 5~70s are obtained. Firstly, using the average dispersion curve in this study area, we obtain the one-dimensional average S-wave velocity structure model of the study area, and then we apply the ray-tracing surface-wave-dispersion direct inversion method to obtain the S-wave velocity structure of the crust and uppermost mantle (3~80km) beneath Weihe Basin and its adjacent regions. The test results of a 1°×1° grid checker board show that the recovery is good, except for the areas east of 111° and south of 32° of the study area, where there is almost no resolution. The imaging results show that the velocity structure beneath each tectonic unit in the study area has a certain distribution rule, and there is a good correlation between surface geological structure and deep velocity structure.
Based on the analysis of velocity slices at different depths and S-wave velocity structures of three profiles, and combined with existing geological structures, geophysics and other deep exploration research results, we obtain the following knowledge and conclusions:1)The thick sedimentary layer covering the top of Weihe Basin is the cause of low velocity anomaly in its shallow crust, the middle and upper crust of the basin are of low velocity structure, and the low-velocity zone extends about 25km, the Moho interface uplifts abruptly relative to both the Ordos Block and the Qinling orogenic belt on opposite sides, and high-speed materials from the upper mantle intrude into the lower crust, which may be related to the underplating of mafic-ultramafic materials from the upper mantle in Mesozoic-Cenozoic period; 2)The south Ordos Block is not a homogeneous whole, the low-velocity structure of the shallow crust in southern Ordos Block is thin in east and thick in west, which may be related to the overall tilting of the Ordos Basin since the Phanerozoic, as well as the differential uplift and strong and uneven denudation of the Ordos Block since the Late Cretaceous. The crustal structure of the south Ordos Block is relatively simple and homogeneous. There is no significant low-velocity structure in the curst of the block, which shows that the low-velocity structure in the crust does not penetrate the whole Ordos block. We speculate that the southern Ordos Block still maintains the stable craton property, and has not been reformed significantly so far; 3)The variation characteristics of deep structure of the Qinling orogenic belt reflect the deep crustal structure and tectonic deformation characteristics of the orogenic belt which are strongly reformed by land-land collision and suture between North China plate and Yangtze plate, intracontinental orogeny, uplift of Qinghai-Tibet Plateau and its northeastern expansion since the Late Hercynian-Indosinian period. The deep structure beneath the eastern and western Qinling orogenic belt is different and has the characteristics of segmentation. The low-velocity anomaly at the bottom of the lower crust of the orogenic belt may be affected by tectonic activities such as uplift and outward extension of the NE Tibetan plateau, and the analysis considers that there is little possibility of the existence of lower crustal circulation channel for the eastward flowing of Tibetan plateau materials in the Qinling orogenic belt. However, since the maximum depth from the inversion of this paper is 80km, which is located at the top of the upper mantle, our results cannot prove that there exists a mantle flow channel for the eastward flow of Tibetan plateau material beneath the Qinling orogenic belt.

This paper presents the results of detailed paleomagnetic study on the ophiolite of Danfeng group in Qinling erogenic zone. The paleomagnetic pole position and paleolati-tude of Danfeng group have been located on the basis of the origin and formation age of remanence,as well as the impact of metamorphism and deformation on the characteristic component. The origin and evolution of Danfeng group have also been discussed.

A M 6.9 earthquake of 1981 occurred in the Daofu region in western Sichuan Province,an epicetre being siruated about 2km southeast of Daofu with an intensity of 8.On the basis of field investigations,including structural conditions,recent crustal movement and tectonic stress fields of the seismic region alongwith ground fractures,isoseis-mals,ground motions,aftershocks,focal mechanism solutions and geodetic observations,the Authors incline to consider that the event in question is caused by sinistral motion of the Xianshuihe fault under the action of nearly EW-trending tectonic stress field.Also discussed in this paper are locking of the fault and migrating of earthquakes along the northern segment of the fault.

Based on the data collected in the recent years by the scientific expedition in the Qinghai-Xizang region, the principal structural features of Himalayas and Qinghai-Xizang Plateau are discussed from viewpoint of plate tectonic theory.In the light of a lot of data, such as on the geological history, the sedimentary characteristics, the distribution of Gondwana sedimentary facies, ophiolite suite, melange and exotic blocks, the crustal deformation nature, the regularity of magmatic activity and paired metamorphic belt on both sides of Yarlung Zangbo River, this work deals with the pattern and nature of plate motion, makes a subdivision of Himalayan orogeny and gives a preliminary analysis of its character as well.A possible model on formation of Qinghai-Xizang Plateau by multi-underthrusting and multi-matching is presented and it is suggested that the Bangong Co-Dongqiao tectonic belt can probably serve as the northern boundary of Gondwana land. And we propose a new mechanism that the Yarlung Zangbo Jinag tectonic belt was opened in Mesozoic and closed in Early Tertiary.