Seismic velocity structures and detailed features of the D discontinuity near the core-mantle boundary beneath eastern Eurasia

Shear and compressional velocity structures and features of the D '' discontinuity near the core-mantle boundary (CMB) beneath eastern Eurasia are studied based on forward waveform modeling and differential-travel-time analysis of ScSH-SH, sScS-sS, PcP-P and pPcP-pP phases. High-quality differential-travel-time analysis reveals two high-velocity patches spanning a width of 3000 km and separated by a 300-km wide normal velocity gap. The maximum shear and compressional velocity perturbations reach 3.8% and 2.7%, respectively. Seismic data indicate that the D '' discontinuity emerges in the high velocity patches and disappears in the normal-velocity gap. Furthermore, the features of the D '' discontinuity, although constrained by rather sparse seismic data, vary from diffusive discontinuities in the eastern patch to sharp discontinuities in the western patch. The D '' discontinuities occur at about 270 km above the CMB in the eastern patch with a shear velocity jump of 3.0-5.0% and a transitional depth of at least 100 km, and at 250 km above the CMB in the western patch with an abrupt shear velocity jump of 5.0%. A diffusive P wave discontinuity is also observed in the eastern patch with the similar features of the S wave discontinuity. The inferred features of the D '' discontinuity can be explained by the post-perovskite transformation in the lower mantle. The observations that the D '' discontinuities occur only in the high-velocity patches may be due to the effect of the positive Clapeyron slope of the post-perovskite transition, and the varying transitional depths of the D '' discontinuity in the high-velocity regions may be caused by the compositional effects on the post-perovskite transition in the presence of strong variations of composition within the high-velocity anomalies. The high-velocity patches geographically correlate with the subduction of the Pacific and Indian plates in the past 30-60 Ma, and the normal-velocity gap falls into the subduction gap between the Pacific and Indian plates. These geographical correlations raise the possibility that the existence of the D '' discontinuity is intimately related to the presence of the past subduction material at the CMB, and varying features of the discontinuity further suggest existence of strong variations of composition within the subduction materials at the CMB. (C) 2011 Elsevier B.V. All rights reserved.