Crustal Structure and Its Tectonic Responses During Mesozoic and Cenozoic in Beijing‐Tianjin‐Tangshan and Its Adjacent Region

Abstract

AbstractTaking 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 areas by integrating crustal velocity structure from eight wideangle 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 depths of upper crust, mid‐crust and lower crust. These images show that uplift and depression spreading along nearly EW and NE directions dominate in the study region. Faulted‐swells and faulted‐ depressions controlled by NE faults are present too. The formation of geological structure is closely related to tectonic movement that occurred since the Mesozoic in the study region. The authors inferred that there were at least three stages of intense tectonic movements in the Mesozoic and Cenozoic in the study region. Combining with regional tectonic researches, the authors conclude that the nearly EW uplifts and depressions formed in the Triassic and the NE‐trending uplifts and depressions formed in the Jurassic. The faulted‐uplifts and faulted depressions controlled by NE faults emerged in the 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 has low velocity in uplifted areas, whereas its thickness decreased and has high velocity 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 movements, thus affecting the occurrence of present earthquakes in the study region.