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

Abstract

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 (VP) for each composition model were calculated over a range of depths and temperatures of the deep crust. For conditions within the alpha(α)-quartz stability field, robust functional relationships were obtained between VP and major oxide contents of the crust. Based on these relationships, observed VP 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 VP 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 VP 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. Plain Language Summary Understanding the formation and evolution of the continental crust requires knowledge of the chemical composition of the deep continental crust. Due to limited accessibility, it is difficult to determine the composition of deep crust. Here we present a method for estimating the composition of deep continental crust by combining published crust composition models, thermodynamic phase equilibrium calculations, and seismic wave speed calculations. Published model crustal compositions were used to establish functional relationships between crustal composition and seismic wave speed. Through the application of these functional relationships, it is possible to construct a chemical structure of the deep continental crust in regions subjected to seismic wave constraints. We apply this method to the global lower continental crust in typical tectonic settings and the North China Craton. The influence of crustal composition, geotherm, and water content on crustal VP is evaluated.