Detailed Structures Of The 660-km Discontinuity And D" Discontinuity In The Mantle

As the links between upper-lower mantle and core-mantle respectively,mantle transition zone(MTZ)and the D"layer are the key to understand the composition and the evolution of the whole mantle and the geodynamics of the deep subduction and mantle convection.Seismic tomography studies have revealed that plate subduction and the mantle plume can be blocked when they reach the bottom of MTZ.Some slab can penerate into the lower mantle and may finally reach the D" layer at the lowermost mantle and interact with Large Low-Shear Velocity Provinces(LLSVPs).Based on resolving the velocity structure of the 660-km discontinuity and the D"discontinuity,our studies are essential for predicting the composition of the mantle and understanding dynamic process of deep subduction.We employed S-to-P(SdP)conversions to constrain the sharpness of 660-km discontinuity beneath the Izu-Bonin subduction zone.The records show two clear SdP arrivals and our forward modeling further suggests the 660-km discontinuity is sharp(?5 km in transition width)due to the dissociation of ringwoodite into perovskite plus magnesiowustite.And a diffusive discontinuity(-20 km in transition width)at a depth of?720 km is required to explain the second SdP arrival,which may be attributed to the majorite-perovsikite transformation associated with subducted basaltic crust.We analyzed the Scd(the lower mantle triplication)and SKS-SKKS-SKKKS phases sampled from the eastern margin of the Perm Anomaly-a Low-Shear Velocity Province(LSVP)beneath Rusia and found evidences for the coexistence of the D"discontinuity,seismic anisotropy and high-velocity anomaly.The D" discontinuities reside in a depth ranging from 250-300 km above core-mantle boundary(CMB)with a 2?3%S-wave velocity jump and it is most likely attributed to perovskite to post-perovskite transformation in conj unction with the presence of relic slab materials(likely related to the subducted Mongol-Okhotsk and Paleotethyan oceanic lithosphere).As a result,our observations thus favor the deep subduction of slabs,at least in regional areas and dynamically link it in addition to the Low-Shear Velocity Province with deformation taking place in D".