Influence of plate motion reference frame on lowermost mantle structure

Project Description

Accurate knowledge of the spatial pattern of heat flow across the core-mantle boundary (CMB) and its evolution over geologic eras is of fundamental importance for better understanding the long-term behaviour of the geodynamo and the temporal variability of the reversal rate of Earth's magnetic dipole field. On such long time scales of tens to hundreds of millions of years, variations of CMB heat flow are controlled by mantle convection. However, many models of mantle dynamics are still largely qualitative in nature to date, and a data-driven quantitative understanding of the evolution of buoyancy forces and temperature variations is still lacking. One major challenge is to constrain both length-scales and magnitudes of the thermal and compositional heterogeneity in the mantle through seismic observations. Furthermore, mantle flow in the geologic past needs to be modelled in one way or the other incorporating Earth observations. The main goal of this project is to generate quantitative and robust predictions of lower mantle thermal evolution based on compressible high-resolution mantle circulation models (MCM) with 410 million years of plate motion history. This is equivalent to two mantle overturns, and the time span of geologically-informed structure above the CMB will thus cover at least one full superchron cycle. Our goal will be achieved through an integrated multi-disciplinary forward-modelling framework for the generation and assessment of Earth models. To estimate uncertainties in lower mantle evolution, we will employ systematic variations of the underlying plate motion absolute reference frame. Appraisal of the MCMs will be done on the one hand through geodynamic-tomographic model comparisons. More important, we will assess the robustness of each model by predicting seismic data for direct comparison to Earth observations. Long-period normal mode data are particularly suited in this context, as they provide global constraints, and splitting function coefficients show high sensitivity to variations in absolute reference frame. All components of the multi-disciplinary approach are in place required to perform a systematic examination of the influence of the absolute reference frame as well as slab sinking velocity through different choices of viscosity profile. Our project will have a strong impact on seismology and geodynamics as well as on tectonic modelling, geochemistry and, in particular, geodynamo simulations. Our high-resolution MCMs provide constraints on CMB evolution in the early and mid Mesozoic, a period of hyper-activity of the geodynamo. This makes them complementary to computationally expensive adjoint models, which provide information on CMB evolution for the past ~100 million years. The quantitative estimates of mantle thermal evolution generated in this project can in the context of DeepDyn be directly linked to geodynamo models and thus be used to generate synthetic time-series of Earth's magnetic field behaviour.