Heterogeneity at the top of the Earth's transition zone from "lower Upper Mantle reflectivity"

We document pronounced reflectivity from depths around the top of the mantle transition zone below Siberia and North America based on high resolution, explosion seismic profiles. This lower Upper Mantle Reflectivity indicates the presence of heterogeneity in the form of seismic scattering bodies. The top of the transition zone (the 410 km discontinuity) is usually considered as a sharp boundary at a relatively constant depth. Reflections from this boundary are often interpreted as strong seismic phases, with high amplitude and good correlation. However, interpretation of seismic sections from Eurasia, based on PNE shots, indicates the existence of two interfaces around depths of 350 and 410 km. There is strong seismic reflectivity between the two reflections, which indicates pronounced heterogeneity. This configuration has been observed in reversed sections, however with some amplitude difference between the two directions. Hence, there is indication that the 400 km discontinuity is not always sharp and that there is a transition zone in the depth interval from ~350-420 km depth in some parts of Eurasia. The reflection pattern around the top of the transition zone appears analogous to the pattern observed in crustal record sections for reflections from the lower crust and Moho, apart from the obvious differences in frequency content, offset and traveltime. Finite-difference modelling of the reflection pattern, indicates typical scale lengths of 10-30 km by 5-15 km in a 50-90 km thick depth interval. The observed heterogeneity may be explained by the phase transformation of pyroxenes into the garnet mineral majorite, or it may indicate changes in the chemical composition (to a high Fe content) in the lower Upper Mantle.