A ubiquitous low-velocity layer at the base of the mantle transition zone

Global stacks of receiver functions clearly exhibit the upper mantle stratification. Besides the most prominent seismic discontinuities, such as the Moho and the 410 and 660km discontinuities, a negative discontinuity is detected at a depth of similar to 600km, indicating a low-velocity layer at the base of the mantle transition zone. The slant-slack technique helps to identify the primary conversions from the multiple reverberations. Presence of the negative 600km discontinuity underneath both continent and ocean island stations, where the crustal thickness significantly differs, also precludes the possible cause of crustal reverberations. We conclude that the negative 600km discontinuity could be a global feature, possibly resulted from accumulation of ancient subducted oceanic crust. The X-discontinuity at similar to 300km depth is also observed in our global stacks, which can be explained by the coesite-stishovite phase transformation.