Abstract
Abstract. The North China Craton (NCC) is one of the oldest cratons in the world. Currently, the destruction mechanism and geodynamics of the NCC remain controversial. All of the proposed views regarding the issues involve studying the internal density structure of the NCC lithosphere. Gravity field data are among the most important data in regard to investigating the lithospheric density structure, and gravity gradient data and gravity data each possess their own advantages. Given the different observational plane heights between the on-orbit GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) satellite gravity gradient and terrestrial gravity and the effects of the initial density model on the inversion results, sequential inversion of the gravity gradient and gravity are divided into two integrated processes. By using the preconditioned conjugate gradient (PCG) inversion algorithm, the density data are calculated using the preprocessed corrected gravity anomaly data. Then, the newly obtained high-resolution density data are used as the initial density model, which can serve as constraints for the subsequent gravity gradient inversion. Several essential corrections are applied to the four gravity gradient tensors (Txx, Txz, Tyy, Tzz) of the GOCE satellite, after which the corrected gravity gradient anomalies (T′xx, T′xz, T′yy, T′zz) are used as observations. The lithospheric density distribution result within the depth range of 0–180 km in the NCC is obtained. This study clearly illustrates that GOCE data are helpful in understanding the geological settings and tectonic structures in the NCC with regional scale. The inversion results show that in the crust the eastern NCC is affected by lithospheric thinning with obvious local features. In the mantle, the presented obvious negative-density areas are mainly affected by the high-heat-flux environment. In the eastern NCC, the density anomaly in the Bohai Bay area is mostly attributed to the extension of the Tancheng–Lujiang major fault at the eastern boundary. In the western NCC, the crustal density anomaly distribution of the Qilian block is consistent with the northwest–southeast strike of the surface fault belt, whereas such an anomaly distribution experiences a clockwise rotation to a nearly north–south direction upon entering the mantle.
Highlights
The North China Craton (NCC, Fig. 1) is an outstanding example of a craton that has undergone both reconstruction and destruction
In the central NCC, it is easier to determine the center of the density anomaly distribution in the southern, middle and northern parts of the Taihang Orogenic belt at 42–80 km, as these areas have different regional block features of the density anomaly
For depths of 60–80 km, low-density residuals are found in the Bohai Bay area in the eastern NCC, while widely distributed low-density residuals are seen in the Yinshan orogenic belt
Summary
The North China Craton (NCC, Fig. 1) is an outstanding example of a craton that has undergone both reconstruction and destruction. Using the Bouguer gravity anomaly, Wang et al (2014) obtained the three-dimensional density structure of the NCC lithosphere through the algebraic reconstruction inversion method, which was constrained by seismic travel time. Using the preprocessed data of the GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) satellite gravity gradient anomaly, Tian and Wang (2018) constructed a threedimensional density structure within the depth range of 0– 120 km in the NCC lithosphere, during which the density variation induced by the temperature differences was incorporated. The amplitude of the gravity gradient data declines rapidly with increasing depth in the field source, which demonstrates that the gravity gradient anomaly is applicable to the highfrequency signal information of the shallow structure, characterized by short wavelengths. Sequential inversions of the gravity and gravity gradient data are able to achieve mutual supplementation, which is favorable to enhance the reliability of the inversion result and obtain more reasonable analyses of the inversion solutions
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