Granitoid tracers of early Indian crust underthrusting and Moho doubling beneath southern Tibet

Southern Tibet is marked by an ∼15−20-km-thick high-velocity layer in the lowermost crust at depths of 50−70 km, forming a distinct seismic Moho doublet. However, the origin of this geophysical anomaly remains elusive. Here, we use pressure-sensitive geochemical proxies and isotopic fingerprints to show that 60 Ma and 49 Ma granitoids, collected from the same outcrop (<5 m apart) in the southwestern Gangdese batholith, formed through anatexis of juvenile and ancient continental crust at depths of ∼40−50 km and 55−65 km. However, their mantle-like zircon δ18O values argue against the prevailing hypothesis linking the isotopic shift in Gangdese granitoids to input from Indian or Tibetan supracrustal components. Given the minimal post-collisional upper-crustal deformation in southern Tibet, we propose that the Indian lower crust likely decoupled from its upper crust and underthrust beneath Tibet shortly after the onset of India-Asia collision, leading to the development of the Moho doublet much earlier than previously thought.