An analysis of upper mantle discontinuity structure using 3D synthetics and global network data

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Radioactive decay of naturally occurring isotopes (e.g., uranium, thorium and potassium) has been a major source of Earth's internal heat throughout geologic time. This heat escapes primarily by slow (inches per year) convection in Earth's mantle. These currents are responsible for geologic activity including mountain building, volcanic eruptions, and earthquakes. While plate tectonics is the surface expression of solid-state rock flow in the mantle, it is still uncertain whether separate convection cycles exist in the upper and lower mantle (i.e., layered convection) or whether convection encompasses the entire mantle (i.e. whole-mantle convection). An understanding of mantle convection is important for modeling the thermal evolution of our planet.The proposed work is focused on boundaries near 410 km and 660 km. At the 410-km and 660-km boundaries, the most abundant rock-forming minerals undergo phase transitions to denser structures. Since the 410-km and 660-km mineral phase transitions depend on the ambient mantle temperature, precise maps of the depths of the 410-km and 660-km transitions act as 'thermometers' of the mantle and can be used to infer the scales of mantle flow. To date, the most precise global maps of the depths of the 410-km and 660-km transitions have been derived from the processing of seismic wave reflections produced by thousands of earthquakes and recorded by global arrays of seismometers. However, the latest maps produced by various research groups remain inconsistent. They exhibit different predominance of long-wavelength versus short-wavelength variations and major anomalies (i.e., thick and thin transition zone regions) are located incoherently. We will address these inconsistencies by investigating artifacts due to systematic data processing errors and ignored effects from 3D wave propagation in the heterogeneous mantle. Using state-of-the-art software for 3D seismic wave propagation, we will estimate the uncertainties in the applied analytical procedures, and we will determine the expected 410-km and 660-km topography for a range of convection scenarios for the mantle.