Cold Tibetan Lower Crust from the Eclogite-Granulite Seismic Thermometer

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.