The thermal structure of orogenies is subject to wide debate and remains largely uncertain, especially in the lower crust. Eclogite created due to the subduction of the cold Indian crust under Tibet has been widely accepted as the cause of the ‘Moho doublet’, a pair of positive lower crustal wave speed increases in the Tibetan lower crust. Previous studies have used the northward extent of the Moho doublet as a structural marker for the northern extent of the Indian plate. Here, we offer an alternative interpretation: as the Indian eclogite further subducts, the cold eclogitized crust warms into granulite facies and the northern extent of the Moho doublet records the eclogite-granulite transition. This interpretation also implies that the northward extent of the Moho doublet map acts as a seismic thermometer. We calibrate this thermometer using thermodynamic modeling of two endmember average lower crustal compositions. Sensitivity tests to show receiver functions can observe an eclogite doublet >=10 km in thickness in the lower crust. We map the northern extent of the Moho doublet by compiling and calculating receiver functions for all available seismic stations and compare our results with other thermal indicators for the Tibetan lower crust. The presence of a Moho doublet >200 km north of the Bangong–Nujiang suture suggests that Indian crust has subducted further than previously hypothesized. Along-strike variation in the extent of regional cold lower crustal temperatures (<800±100°C at 65–70 km) indicate significant along-strike thermal and tectonic variability.

The chemical composition of the deep continental crust is key to understanding the formation and evolution of the continental crust. However, constraining the chemical composition of deep continental crust is limited by indirect accessibility. Here we present a modeling method to constrain deep crustal chemical structures from observed crustal seismic structures. We first compile a set of published composition models for the continental crust. Phase equilibria and compressional wave speeds (VP) are calculated for each composition model at a range of pressure and temperature (278–2223 MPa, 50–1200°C). Functional relationships are obtained between calculated wave speeds and crustal compositions at pressure and temperature conditions within the alpha(α)-quartz stability field. These relationships can invert observed seismic wave speeds of the deep crust to chemical compositions in regions with given geotherms (MATLAB code is provided). We apply these relationships to wave speed constraints of typical tectonic settings of the global continental crust and the North China Craton. Our method predicts that the lower crust in regions with thin- (e.g., rifted margins, rifts, extensional settings, and forearcs) or thick-crust (e.g., contractional orogens) is more mafic than previously estimated. The difference is largest in extensional settings (52.47 ± 1.18 and 51.11 ± 1.61 vs. 59.37 wt. % SiO2). The obtained 2-D chemical structure of the North China Craton further shows features consistent with the regional tectonic evolution history and xenoliths. The obtained chemical structure can serve as a reference model from which chemical features in the deep crust can be recognized.