Lateral Variations of Shear‐Wave Velocity in the D″ Layer Beneath the Indian‐Eurasian Plate Collision Zone

Seismic tomography has demonstrated that the shear‐wave velocity is relatively high over a 3,000‐km wide region in the lowermost mantle beneath southern and eastern Asia. This seismic anomaly demarcates the current position of slab remnants that may have subducted in the Cretaceous. To further characterize the seismic structure at smaller scales, we measure 929 residual travel time differences (δt) between the phases ScS and S using recordings of eight earthquakes beneath the Indian Ocean at stations from the Chinese Digital Seismic Network. We interpret variations of δt up to 10 s as due to horizontal shear‐velocity variations in D″ beneath northern India, Nepal, and southwestern China. The shear velocity can vary by as much as 7% over distances shorter than 300 km. Our observations provide additional observational evidence that compositional heterogeneity and possibly melt contribute to the seismic structure of the lower mantle characterized by long‐term subduction and mantle downwelling. Plain Language Summary Seismic tomography indicates that the seismic wave speed is relatively high in the lowermost mantle (i.e., the D″ region) beneath regions, such as eastern Asia, influenced by subduction since the Mesozoic era. Our new analysis of the propagation time of shear‐wave reflections off the outer core (i.e., the phase ScS) corroborates the result from seismic tomography that the shear velocity in D″ beneath eastern Asia is high overall. However, we also find that the shear velocity can vary by as much as 7% over distances shorter than 300 km within a region of D″ beneath northern India, Nepal, and southwestern China. This provides new evidence for the thermochemical nature of D″ beneath downwelling regions of the mantle. Key Points Using waveforms of S and ScS, we estimate the shear‐wave velocity (Vs) structure in D″ beneath the Indian‐Eurasian plate collision zone Vs is relatively high overall but we resolve horizontal variations by 3–7% over distances shorter than about 300 km Our models place new constraints on geodynamic scenarios for the generation of thermochemical heterogeneity in downwelling regions of D″