Weak Tibetan mid-lower crust revealed by relocated shallow seismicity

The rise of the Tibetan Plateau and the Himalayan Orogen is subject to long-standing, unresolved discussion of possible geodynamic models. Although difficult to assess, the strength of the lithosphere and the depth variation of rheology are key parameters for discriminating between these models. The depth distribution of earthquakes defines the extent of the brittle seismogenic zones and provides first-order constraints on lithospheric rheology. Earthquake catalogues and recent seismological studies suggest that earthquakes occur at all depths in the crust and uppermost mantle of both the Tibetan Plateau and the Himalayas, but hypocentre location is highly uncertain due to poor station coverage. We demonstrate that seismicity is shallower than 25 km depth in the Tibetan Plateau whereas it covers the whole crust and uppermost mantle below the Himalayas, based on our relocation of local earthquakes recorded on a dense network, together with our compilation of earlier relocations in the region. We further interpret a 300–350 km long section of localized steep subduction of the Indian plate below the Himalayas between ∼85°E and ∼88.5°E with implications for regional deformation and seismicity. Our observations of relocated seismicity depth require completely different lithospheric strength profiles in Tibet and the Himalayas. Weak rheology of the mid-lower crust and mantle in the whole Tibetan Plateau facilitates deformation by ductile flow and buoyancy below a 25 km thick brittle upper crust, while strong crust and uppermost mantle support tectonic deformation in the Himalayas. This fundamental contrast explains the difference in deformation processes responsible for the uplift, compression and extension of the Tibetan Plateau and the Himalayas..