Mid-mantle heterogeneities associated with Izanagi plate: Implications for regional mantle viscosity

We employed S-to-P converted waveforms to detect mid-mantle scattering beneath northeastern China and the adjacent Japan Sea. Broadband and short-period waveforms recorded by seismic arrays for eight moderate-sized deep earthquakes were analyzed using a non-linear array stacking technique, and mid-mantle scatterers within the depth range 930–1120 km were clearly revealed. The heterogeneities have an overall lateral extent of ∼ 800 km , and mostly occur within a region with a high velocity anomaly. The accumulation of MORB-like slab materials at mid-mantle depths might cause a different chemical composition than that of the surrounding peridotitic mantle. The spatial isolation of the heterogeneities from the stagnant Pacific slab suggests an origin related to the subduction of ancient Izanagi plate. In combination with the reconstruction history of plate motions, we estimate the viscosity of the topmost lower mantle to vary from 1.0 × 10 22 to 1.6 × 10 23 Pa s , which can be used as an independent constraint on the rheology of the lower mantle on a regional scale.