[S-IT16] Planetary cores: Structure, formation, and evolution

[SIT16-P05] Preliminary Core-Mantle Boundary Heat Flux map implied by deep seismic structures

*Takumi Matsunaga^1,2, John Hernlund^1,2, Christine Houser¹ (1.Earth-Life Science Institute, Tokyo Institute of Technology, 2.Departmment of Earth and Planetary Sciences, Tokyo Institute of Technology)

Keywords:Core-Mantle Boundary, ULVZs, Post-Perovskite, CMB Heat Flux

Seismology offers the best constraints on lowermost mantle structure, which may be interpreted using mineral physics theory and experiments. Accordingly, to obtain maps of the core-mantle boundary (CMB) region, seismological constraints and additional constraints such as the post-perovskite phase transition are useful for providing a glimpse into the processes occurring at this important frontier deep inside our planet. Here, we introduce a preliminary method to begin mapping patterns of deformation and heat flux at the CMB. Two strategies are used: (1) Estimation of the flow and geotherm beneath downwellings using a "double-crossing" of the post-perovskite transition, which offers joint constraints on temperature, heat flux, and strain rate. 2) Using the "thin layer approximation" for the equation of motion of a dense material embedded at the CMB representative of seismically observed Ultralow-Velocity Zones (ULVZ) the local poloidal mantle flow can be estimated beneath presumed upwellings. For simplicity, we used 1-D geotherm models for the double crossing, and 2-D analytical solutions for the ULVZ thin layer. Combined, these yield key insights into mantle flow and heat flux beneath both downwellings and upwellings just above CMB, including their geographic distribution. We also explore the range of uncertainty of key parameters and find that many ULVZ morphologies remain poorly constrained due to variations in seismological methodology and geographical coverage. To estimate a mean value of ULVZ strain rate and its uncertainty, we employed Monte Carlo sampling based on assumed uncertainties in input parameters. At downwellings we estimate that the heat flux falls in the range of 59-283 mW/m^2. Also, the inferred strain rate beneath upwellings based on ULVZ shape varies by orders of magnitude (10^-21-10^13 sec^-1)and we require better constraints on properties such as viscosity and density in order to provide more accurate constraints. In addition to these CMB heat flux conversions, we predicted post-perovskite transition area using "Vote Map" method, which is sensitive to consistent structures between seismic tomography models. Considering vertical difference of shear velocity larger than the difference of compressional velocity as post-perovskite transition, we suggest that there's 200 km vertical resolution difference on voted regions which indicates geothermal and/or transition zone difference in lateral. These tools represent a step toward construction of a CMB "tectonic map" using geophysical constraints that may be useful for comparison to a variety of other observations such as seismic anisotropy, and also useful for imposing on models of core dynamics (e.g., geodynamo) as a boundary condition.