Slabs and shear wave reflectors in the midmantle

An extensive analysis of multiple ScS reverberation data sets sampling regions of active or recent subduction reveals evidence of midmantle reflections apparently stemming from fragments of subducted slabs or from the interaction between subducted slabs and the surrounding mantle. Transition zone reflectors are detected beneath, the southwest Pacific Ocean and Melanesia with mean depths of ∼850 and 1100 km in the majority of source-receiver corridors crossing the study area. The observations are most spatially coherent beneath the Coral and Tasman Seas. Coincident observations of the two reflectors along most seismic corridors suggests (but does not mandate) the existence of two distinct reflectors rather than bimodal depth distribution of a single reflector or distributed scatterers. Beneath North America, reflectors at depths of ∼1380 and 1530 km are seen in the midcontinent region; further east, the reflections are shallower, with depths near 940 and 1130 km. Unlike the southwest Pacific, the reflectors are not paired in any of the individual source-receiver corridors. This and the depth variability of the observations indicate that the reflector (or reflectors) in the Americas is (are) fragmented from west to east (transecting source-receiver corridors). The impedance contrasts of these features rival that of the 660-km discontinuity, suggesting that individual fragments of the reflecting surfaces must be relatively continuous and flat from north to south (along individual corridors) to maintain a strong apparent impedance contrast. The reflections in both study areas are unlikely to be the result of slabs interacting with a chemical boundary layer or small-scale scatterers within the midmantle. More likely these reflectors result from a pressure-temperature-dependent phase transition within or around subducting slabs.