Geophysical-Geochemical Modeling of Deep Crustal Compositions: Examples of Continental Crust in Typical Tectonic Settings and North China Craton

The chemical composition of the deep continental crust is key to understanding the formation and evolution of the continental crust. Constraining the chemical composition of present-day deep continental crust is, however, limited by indirect accessibility. This paper presents a modeling method for constraining deep crustal chemical structures from observed crustal seismic structures. We compiled a set of published composition models for the continental crust to construct functional relationships between seismic wave speed and major oxide content in the crust. Phase equilibria and compressional wave speeds (V-P) for each composition model were calculated over a range of depths and temperatures of the deep crust. For conditions within the alpha(a)-quartz stability field, robust functional relationships were obtained between V-P and major oxide contents of the crust. Based on these relationships, observed V-P of the deep crust can be inverted to chemical compositions for regions with given geotherms. We provide a MATLAB code for this process (CalcCrustComp). We apply this method to constrain compositions from deep crustal V-P of global typical tectonic settings and the North China Craton (NCC). Our modeling results suggest that the lower crust in subduction-related and rifting-related tectonic settings may be more mafic than platforms/shields and orogens. The low V-P signature in the deep crust of the NCC can be explained by intermediate crustal compositions, higher water contents, and/or higher temperatures. The chemical structure obtained by this method can serve as a reference model to further identify deep crustal features.

Automated summary

The chemical composition of the deep continental crust is crucial for understanding its formation and evolution, but direct measurements are limited. This paper presents a modeling method to constrain deep crustal chemical structures based on observed seismic structures. By establishing functional relationships between seismic wave speed and major oxide content, the V-P of the deep crust can be inverted to obtain chemical compositions. The results from applying this method suggest that the mafic content in the lower crust is higher in subduction and rifting settings compared to platforms/shields and orogens. The obtained chemical structure can serve as a reference model for identifying deep crustal features.