Mantle Transition Zone structure beneath the Eastern Block of North China Craton

We obtained the depths of the 410 and 660 km discontinuities beneath the Eastern Block of North China Craton (EBNCC) by using the common conversion point stacking of teleseismic P-to-S converted waves from 210 broadband stations. The stacking images reveal a prominent discontinuity structure of the upper mantle. The imaged 410 is sharp and coherent throughout the study region, while the 660 shows a complex character, multiple 660-km discontinuities are also found in the study area, which suggests a more heterogeneous thermal structure at the bottom of the transiztion zone. We pick up the apparent depths of the 410 and the 660 according to the maximum positive amplitudes around their nominal depths in the images, The transition zone thickness is determined simply by subtracting the depth of the 410 from that of the 660, The most striking feature is the clear 10-20 km thickening of the transition zone in the east compared with that in the west of the study region, which corresponds to 100-150 K temperature decrease according to Clapeyron slopes of the 410 and 660, possibly resulted from the stagnation of the west Pacific slab in the lower portion of the transition zone under the eastern part of EBNCC. We also find a correlated topography variation for the 410 and 660 km discontinuities in the southwest of the study area. The coincident deepening of both discontinuities suggests a slower upper mantle above the transition zone, which is likely associated with the lithosphere rejuvenation of the region.