Mantle Structure beneath East Asia from Seismic Full-Waveform Inversion: Implications for Tibetan Plateau Growth and Western Pacific Subduction

We have successfully developed a continental-scale multiparameter full-waveform tomographic model for China and adjacent areas, employing over 500,000 unique source-receiver pairs. Our model makes possible comprehensive characterization of structural heterogeneities within the lithosphere, asthenosphere, and mantle transition zone beneath this large region.  Here, we provide detailed tectonic interpretations of observed shear-wave velocity anomalies in the lithosphere and upper mantle beneath the Tibetan Plateau that are related to the India–Asia collision, and the western Pacific subduction zone.

The tectonic evolution of the Tibetan Plateau has been influenced by continental collision and postcollisional convergence of Indian and Eurasian plates, both of which have undoubtedly imposed their imprints on the lithosphere and upper-mantle structures beneath the collision zone. However, the mode by which the Indian Plate has subducted beneath Tibet, and its driving forces, have been highly uncertain. Here, our seismic evidence reveals flat subduction of the Indian Plate beneath nearly the entire plateau at ~300 km depth, implying that the slab may have transitioned to positive/neutral buoyancy and is no longer capable of supporting steep-angle deep subduction. The horizontal distance over which the flat slab slides northward increases from west (where it collides with the Tarim lithospheric keel) to east (where it has resided approximately north of the Songpan-Ganzi Fold Belt beyond the Qiangtang Block). The Asian lithosphere is subducting beneath northeastern Tibet without colliding with the Indian slab. The low-velocity zone, with a thickness of 50 to 110 km, sandwiched between the Tibetan crust and Indian slab, is positively correlated with the high-elevation, low-relief topography of Tibet, suggesting partial melting of the uppermost mantle that has facilitated the growth and flatness of the plateau by adding buoyant material to its base. We propose that deep mantle convective currents, traced to the Réunion plume and imaged as large-scale low-velocity anomalies from the upper mantle under the Indian Plate downward toward the uppermost lower mantle under the Baikal-Mongolia Plateau, are the primary force driving the ongoing India–Asia postcollisional convergence.

The mechanism behind intracontinental rifting far from plate boundaries remains a central question in geodynamics. The Baikal Rift Zone (BRZ), situated within the Eurasian continental interior, provides a critical case to investigate whether such rifting is a passive response to far-field tectonic stresses or an active process driven by mantle upwelling. Full-waveform tomographic results reveal that westward subduction and stagnation of the Pacific slab within the mantle transition zone have generated a big mantle wedge beneath East Asia, facilitating the development of large-aspect-ratio convection cells. This system produces focused asthenospheric upwelling, seismically characterized by significant negative radial anisotropy from the vertical mantle flow directly located beneath the BRZ beyond the western edge of the flat slab. The process provides primary buoyant forces that drive domal uplift, crustal extension, and ultimately localizes strain to initiate and sustain the continental rupture. The BRZ is a modern archetype of mantle-driven lithosphere-scale continental fracturing.