The South China Block is formed by the assemblage of the Yangtze Craton and the Cathaysia Block in Neoproterozoic,with a disputed southwest boundary location.Based on a north-south orienting broadband seismic array crossing the Youjiang Basin and the East Sichuan fold belt,we obtained the variation of seismic upper mantle anisotropy beneath Southwest South China Block by SKS wave splitting measurements.The results show from 26°N to south,the fast wave polarization direction of Youjiang Basin is mainly E-W and NEE-SWW,with delay time varying between 0.5～2.5 s.The upper mantle anisotropy of the eastern Sichuan fold belt includes many null values and its fast wave polarization direction is dominantly E-W,with delay time varying between 0.5～1.5 s.In the transition zone of the Youjiang Basin and the East Sichuan fold belt,the upper mantle anisotropy is obviously weak.Combined with previous study of upper mantle velocity structure,we suggest the transition zone between East Sichuan fold belt and Youjiang Basin is the south boundary of Yangtze Craton where the extremely thick lithosphere of the East Sichuan fold belt thinned down to the south.The sharp lithospheric boundary change induced the upwelling of asthenosphere whose vertically motion resulted in the weak horizontal azimuth anisotropy above the area.
关键词
剪切波分裂/方位各向异性/上地幔变形/扬子克拉通边界
Key words
SKS wave splitting measurements/Azimuthal anisotropy/Uppermost mantle deformation/Margin of Yangtze Craton
维普
万方数据