A PRELIMINARY STUDY ON THE CRUSTAL VELOCITY STRUCTURE OF MAQIN—LANZHOU—JINGBIAN BY MEANS OF DEEP SEISMIC SOUNDING PROFILE

A 1000km long comprehensive geophysical profile was deployed along Marqin—Lanzhou—Jingbian for studying interaction between the Qinghai Xizang plateau block and the Ordos block and for studying the deep driving mechanism of tectonic deformation. A preliminary result is presented in this paper on the basis of deep seismic sounding (DS) data. The crust is clearly stratified. On the whole, it can be divided into two parts, i.e. the upper crust and the lower crust. Each on the two parts contains a number of secondary interfaces. The crust becomes gradually thicker from northeast to southwest mainly due to the variation in the depth of the lowr crust. The average crustal velocity becomes gradually lower from northeast to southwest—velocity is obviously low in the west of Zeku and in Haiyuan region. A number of low velocity layers exist in the west of Zeku, and another low velocity layer exists near Haiyuan. The number of reflecting horizons increases gradually from northeast to southwest. As seismic reflection waveform shows, Pc wave is very strong and lasts long at Haiyuan. Besides, the complexity coefficients of Pm wave in Haiyuan and in the west of Zeku are, much larger than those in other parts. The above results clearly show that anomalous structures exists in the crust and upper mantle in the west of Zeku and in Haiyuan region, which reflects the interaction between BayanHar block and Chaidamu block as well as between Qilian block and Ordos block. [