Crustal structure and its tectonic response during Mesozoic and Cenozoic in Beijing-Tianjin-Tangshan and its adjacent region

According to the North China block as the demonstration zone of "Images of the Earth' s Crust & Upper Mantle (IGCP Project 474)", we calculated the average velocity and thickness of the upper, middle, and lower crust in Beijing-Tianjin-Tangshan (BTT) and adjacent regions by integrating crustal velocity structures from eight wide angle reflection seismic profiles in this region. Using Kriging interpolating technique, the authors have constructed many significant images such as average velocity, thickness and spatial variation of the bottom boundary depth of upper crust, mid-crust and lower crust. Those images show that uplift and depression spreading along near EW and NE directions are developed in the study region. Faulted uplift and faulted depression controlled by NE faults are developed too. The formation of geological structure is closely related to tectonic movement occurred since the Mesozoic in the study region. The authors inferred that there were at least three stages of intense tectonic movement in Mesozoic and Cenozoic in the study region. Combining with regional tectonic researches, the authors conclude that the near-EW uplift and depression were formed in the Triassic and the NE-trending uplift and depression were formed in Jurassic. The faulted uplift and faulted depression controlled by NE fault emerged in Cretaceous. Because the middle crust is a low-velocity layer and its deformation is mainly ductile, which is completely different from the upper and lower crust, it is thickened and its velocity decreased in uplifted areas, whereas its thickness decreased and velocity increased in depressions. The overall strength of the crust is reduced in places of thick middle crust, where faulting and deformation are easy to occur in later tectonic movement, thus affecting the occurrence of present earthquakes in the study region.