Raising the Roof of the World: Intra-Crustal Asian Mantle Supports the Himalayan-Tibetan Orogen

The Himalayan-Tibetan orogen formed via the ongoing collision of India and Asia. Its colossal elevations stem from buoyant crustal roots that doubled in thickness during continental collision, widely believed to result from Indian crust under-thrusting its Asian counterpart and Asian crustal thickening. However, a single crustal layer of up to ∼70–80 km thickness conflicts with experimental-rheological and observational constraints, especially if related to the vertical juxtaposition of Indian and Asian crusts. For instance, crustal thickness above ∼40 km implies reduced strength of the continental lithosphere, which may become unable to sustain a plateau the size of Tibet throughout much of the Cenozoic. In addition, the geochemistry and association of ultramafic xenoliths and K-rich magmas from Southern Tibet indicates the presence of mantle material between ∼50 and 80 km depths. The mechanisms controlling the rise and persistence of Earth's highest orogen, therefore, remain enigmatic. Here, new fully-coupled numerical petrological-thermomechanical geodynamic models reconcile the wealth of available structural and petrological constraints through viscous underplating of Indian crust beneath Asian lithosphere, which together supply buoyancy and strength to raise and support the Himalayas and Tibet. We further convert our geodynamic models into receiver functions and shear waves velocity maps that match, to a first order, the available geophysical data from the Himalayas and southern Tibet. We propose that viscous underplating of Indian crust beneath Asian lithosphere, not crust, forms the overall architecture of the Himalayan-Tibetan orogen.