The lithosphere structure in the Hingmong-Jihei (Hinggan-Mongolia-Jilin-Heilongjiang) region, northeastern China

Abstract:The lithosphere beneath the Hingmong-Jihei (Hinggan-Mongolia-Jilin-Heilongjiang) region, northeastern China, is composed of the Ergun, Hinggan, Songneng and Jiamusi continental blocks and Mesozoic Wandashan accretionary complex. Nd isotope model ages indicate that: the Nd model age of the Jiamusi block is oldest, being 1500-2200 Ma; the age of the Ergun block comes next, being 1000-1600 Ma; and the Hinggan block and Songnen block have the same Nd model age, ranging from 500 to 1200 Ma. Geochemical tracing analysis indicates that the Nd isotope model age of Paleozoic supracrustal rocks is dominantly Mesoproterozoic, while that of Mesozoic granites is mainly Neoproterozoic. Therefore, it is concluded that the deep level of the crust is younger than the surficial level of the crust, indicating that the crust in the region has an age structure of being younger in the lower part and older in the upper part. The Os isotope analysis also indicates that the lithospheric mantle in this region also shows the younger character. The seismic (Vp) velocity structure shows that vertically the lithospheric structure in the region has the following two prominent features: (1) notably different from the traditional concept of the seismic lithosphere, the low-velocity zone of the lithospheric mantle has no persistent and continuous top interface which the low-velocity anomalous top is highly varied in depth and interlocks with the high-velocity anomalous bodies, and the low-velocity anomalies below some tectonic units may reach the Moho but the bottom interface occurring at 230-240 km depth is very persistent; and (2) the “overpass-type” velocity structure is manifested by the following: the velocity contours are generally distributed in a NE direction in the crust, in a NNW-NS direction in the lithospheric mantle and in a nearly E-W direction in the low-velocity anomaly asthenosphere.