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Hongyan Zhang, Institute of Crustal Dynamics,CSB (China)
Furen Xie, Institute of Crustal Dynamics,CSB (China)
Xiaofeng Cui, Institute of Crustal Dynamics,CSB (China)
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Based on the results of previous research and combining the latest geological and tectonic data, we studied the recent tectonic stress field in Xinjiang area and its variation characteristics by inversion of fault slip and focal mechanism, and discussed its dynamic background.
The main contents of the paper are as follows:
1. The basic characteristics of recent tectonic stress field in the Quaternary Period in Urumchi and adjacent areas. By the investigation of active faults and the measurements of slickensides of 17 spots, we got about 400 slickensides data. And by using the method of inversion of fault slip data, we analysed the recent tectonic stress field and obtained the parameters of 13 tectonic stress tensors. The inversion result shows that the recent tectonic stress field in the research area is characterized by near N-S maximum compressional stress, the stress regime is mainly of reverse slip, with some cases of strike slip.
2. Dividing the tectonic stress field of Xinjiang area and exactly delimiting the boundary of each stress district. On the basis of previous stress district division, and according to the consistency of principal stress direction, the similarity of stress regime, and the proximity of stress intensity, the stress orientations are inverted from more than 300 earthquake focal mechanisms during the period of 1931-2004, which are calculated using the step by step convergence method. Thus, we got the spatial distribution of principal stress axes in each new stress district and the temporal changes of stress orientations in the individual stress district. There are 5 first order and 3 second order tectonic stress districts in Xinjiang area. The characteristics of spatial distribution of principal stress axes indicate much difference in the three sub-districts. In Pamirs, the stress condition is complex and the principal stress axis is characterized by two dominant directions coexisting. And it shows a temporal variation characteristic in Jiashi. However, the direction of principal stress axes are relatively simple in Urumchi.
3. Dynamic analysis of tectonic stress field in Xinjiang area. According to the heterogeneity of the spatial distribution of recent tectonic stress field in Xinjiang area, try to analyse and explain this phenomenon in dynamics. There are three force sources acted on Xinjiang sub-block. The first is the strong acting force from colliding between Indian plate and Eurasian plate and passed by Qinghai-Tibet Plateau. The second is the pushing force from Siberian plate moving towards south. The effect of the both forces on Xinjiang sub-block is different, the former becomes gradually weak from south to north, and the latter is contrary. And the third is the force directly acted by Pamirs corner, it makes the spatial distribution of stress orientation change in sub-districts, and that is the reason why the principal stress axis is characterized by two dominant direction coexisting.
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