SAPHERE · Seismic Signature of Plumes and the Heat Budget of Earth’s Mantle: Bridging the Gap Between Theoretical High-Performance Computations and Observations

Seismic Signature of Plumes and the Heat Budget of Earth’s Mantle: Bridging the Gap Between Theoretical High-Performance Computations and Observations

Summary description of the Project objectives The primary goals of SAPHERE were: 1) to improve conceptual models of the general circulation of Earth's mantle, 2) to setup and assess a new multi-disciplinary approach that can be used to test geodynamic hypotheses directly against seismic observations, 3) to constrain the relative importance of thermal anomalies and variations in chemical composition in generating seismic heterogeneity and 4) to estimate the heat and related mass flux that is transported by the hot upwelling plumes through the Earth's lower mantle and the boundary to the upper mantle. Description of the work performed since the beginning of the project The first part of SAPHERE was focused on the amelioration of global mantle circulation models (MCMs), which here was done using computational resources offered through the DECI-5 call of the DEISA consortium (Distributed European Infrastructure for Supercomputing Applications). More specifically, we studied the effect of different absolute reference frames, which are needed to reconstruct tectonic plate motions in the geologic past, on lower mantle structure. To this end, we ran high-resolution mantle circulation models in which we prescribe the location of plate boundaries and the directions of motion, as well as their evolution over time as surface boundary conditions. We tested five different reference frames looking at the correlation between the lower mantle structure predicted from each simulation and a tomographic model, additionally accounting for the effects of limited resolving power of seismic data. In parallel to the investigation of the effects of reference frames, we worked on the coupling of the two rather different simulation techniques, for which we made use of a published MCM [Schuberth et al. 2009] to be able to run first simulations of wave propagation in order to assess geodynamic models. Doing so, we could establish an efficient coupling of the two simulation types, which then allowed us to directly link different geodynamic input parameters entering the MCMs to corresponding synthetic seismic data. We explored this new approach and tested the assumption of isochemical whole mantle flow with strong core heating directly against the statistics of observed traveltime variations of both P-and S-waves. Using a spectral element method, we simulated 3-D global wave propagation for periods down to 10 seconds in synthetic 3-D elastic structures derived from our geodynamic model. Seismic heterogeneity was predicted by converting the temperature field of a high-resolution mantle circulation model into seismic velocities using thermodynamic models of mantle mineralogy. Being based on forward modelling only, this approach avoids the problems of limited resolution and non-uniqueness inherent in tomographic inversions while taking all possible finite-frequency effects into account. Capturing the correct physics of wave propagation allowed us to perform a consistent test of the assumption of high core heat flow against seismic data. Description of the main results achieved so far In a first part of our project, we have tested five alternative absolute plate motion models based on several lines of data including hotspot tracks displaying age progression, and assuming either fixed or moving hotspots as well as palaeomagnetically-based reference frames with true and apparent polar wander corrections, subduction reference frames and hybrid reference frames. We found that at long wavelengths, MCMs with reconstructions based on hotspot reference frames provide the highest correlation with tomography, and we will concentrate on these models in future when running 3-D global wave propagation simulations. In a second part of the, we used the joint forward modelling approach to test the assumption of a large thermal gradient in D''and corresponding high core heat flow against the statistics of long-period P-and S-wave traveltime variations. The statistics of long-period body wave traveltime observations show a markedly different behaviour for P-and S-waves: the standard deviation of P-wave delay times stays almost constant with turning depth, while that of the S-wave delay times increases strongly throughout the mantle. Surprisingly, synthetic traveltime variations computed for the isochemical mantle circulation model reproduce these different trends. This is not expected from a ray-theoretical point of view and highlights the importance of finite-frequency effects. Most important, the large lateral temperature variations in the lower mantle related to strong core heating are able to explain most of the standard deviation of observed P-and S-wave delay times. This is a strong indication that seismic heterogeneity in the lower mantle is likely dominated by thermal variations on the length-scales relevant for long-period body waves. Expected final results and their potential impact and use The results of our effort will be of great relevance to future geophysical, geologic as well as geochemical deep Earth studies, as we have provided further constraints on mantle dynamics, especially its thermal state, which has strong implications for building the next generation conceptual models of Earth's mantle. Better knowledge of the dynamic behaviour is important as the flow drives plate tectonics and controls the way the Earth looses its heat. Thus, it is a crucial factor in simulations of the geodynamo and of the thermal history of the Earth. Together with our new constraints on global absolute reference frames, this will not only affect studies of Earth's deep mantle and the geodynamo, but will also bear on our understanding and modelling of plate tectonics, which is of great societal importance. For example, the continuous drift of tectonic plates relative to each other results in a build up of stress at the plate boundaries. This stress can eventually exceed the yield stress of rock thus leading to (often disastrous) earthquakes. References 1. Schuberth, B. S. A., Zaroli, C. and G. Nolet, (2012), Synthetic seismograms for a synthetic Earth: long-period P-and S-wave traveltime variations can be explained by temperature alone, Geophys. J. Int., 188 (3), 1393–1412, doi: 10. 1111/j. 1365-246X. 2011. 05333. x. 2. Schuberth, B. S. A., H.-P. Bunge, G. Steinle-Neumann, C. Moder and J. Oeser, (2009), Thermal versus elastic heterogeneity in high-resolution mantle circulation models with pyrolite composition: High plume excess temperatures in the lowermost mantle, Geochem. Geophys. Geosyst., 10 (1), Q01W01, doi: 10. 1029/2008GC002235

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