Intermediate- and small-scale topography of mantle discontinuities and implications for mantle convection

Topographic features of the 410-km and 660-km discontinuities are consequences of thermal and/or chemical variations in the mantle transition zone, and they are critical for our understanding of its thermo-chemical nature. Seismic body waves permit us to map these topographic features at the intermediate (100 km, about the thickness of an oceanic slab) and small (10 km, about the thickness of oceanic crust) scales. Among these body waves, PP precursors and Ps receiver functions have been widely used to constrain intermediate-scale topography on mantle transition zone discontinuities. The PP precursor method has the advantage of providing reasonably global data coverage, but the estimated depths and velocity contrasts of the 410km and 660km discontinuities can be biased by the effects of near-surface complexities. We investigate these effects and propose a method to correct these biases. Ps receiver functions are particularly useful for assessing intermediate length-scale variations in discontinuity topography in beneath regions with good station coverage. We develop an array-based Ps migration method and apply it to data from the the USArray to image the 660km and 410km discontinuities. This method allows us to image these topographic features with better resolution than in some previous studies. Detecting small-scale topography in the mantle transition zone is a challenging task. We introduce our recent work on back-scattered P'●660●P' waves, and discuss the constraints they can provide on the small-scale topography of the 660-km discontinuity. The interpretation of these features requires input from mineralogy, petrology and geodynamics.