Crustal Velocity Structure from the Northern Margin of the North China Craton to the Southern Margin of the Siberian Plate

Abstract

The survey of seismic refraction and wide-angle reflection from Zhangjiakou to China-Mongolia border is about 600 km long, which begins from the North China Plate, crosses Xing'an Mongolian orogenic belt and ends at the Siberian plate. The recordings of seismic wave field are composed of 8 shootings. Each shooting is generated from an explosion of 1.5 tons and received by 300 detectors along the survey. The qualities of received seismic wave field of P-wave are good for most of the shootings. The data collected from the experiment of seismic refraction and wide-angle reflection have been processed, analyzed and modeled. There are six seismic phases in the data: the arrivals of refracted and reflected seismic waves from the sediments and basements (Pg), the arrival of reflected seismic wave from the bottom of the upper crust (P2), the arrival of reflected seismic wave from the inner of middle crust (P3), the arrival of reflected seismic wave from the bottom of middle crust (P4), the arrival of reflected seismic wave from the inner of lower crust (P5) which exists only in the area between Xianghuang Qi and Sonid Youqi, and the arrival of reflected seismic wave from the Moho (Pm). The velocity distribution of crust is obtained using the seismic dynamics ray method (seis88). The results show that (1) there are local high velocity anomalies in the upper crust between the North China Plate and Inner Mongolia orogenic belt, as well as in the upper crust between the Inner Mongolia orogenic belt and Siberian plate, and there are also a lot of Paleozoic granite and ultramafic rock which are exposed at the surface of those places. (2) In the middle and lower crusts, the velocity of seismic wave in the northern margin of the North China plate is big, which is in the range of 6.3∼6.7 km/s, and in the southern margin of the Siberian Plate the velocity is relatively small and it is in the range of 6.1∼6.7 km/s. The velocity variation between the margins of the two areas is small. The reason for the velocity variations in different regions is that there exit the rugged undulation interfaces in middle and lower crusts, the splicing and substance exchanges between different land and sea blocks, which are caused by the crustal shortening and orogenic uplifting in the Inner Mongolia orogenic belt. (3) The Moho varies gently both in the south of F2 fault and in the north of F4 fault, and the depth of Moho is in the range of 40∼42 km. There exist double Moho boundaries between F2 and F4 faults. The Moho1 uplifts obviously, its depth is 33.5 km and the layer velocity is in the range of 6.6∼6.7 km/s. The Moho2 concaves downward obviously, the deepest boundary of it reaches 47 km and the layer velocity is in the range of 6.7∼6.9 km/s, which may be caused by the material exchange between the crust and mantle. Based on the characteristics of the Moho boundaries, the authors consider that the south area of double Moho boundaries is the northern margin of the North China plate, and the north area of them is the southern margin of the Siberian plate, the final position of suture between the North China Plate and Siberian plate locates in the area between Chifeng fault (F2) and Solon Obo-Ar Horqin Qi fault (F4).