Mantle circulation models: Constraining mantle dynamics,testing plate motion history and calculating dynamictopography

Mantle circulation models are a modified class of mantle convection simulations assimilating recent plate motions as the surface velocity boundary condition. In this thesis, I present a suite of mantle circulation models assimilating the past 300 million years of tectonic history. By comparing model predictions of present day mantle temperature anomalies to mantle structure imaged by seismic tomography one can better understand the physical properties of Earth’s mantle. Given a mantle model with realistic physical properties, plate reconstructions can also be tested. Mantle viscosity is the most significant property affecting mantle circulation models. For subducted slabs to sink to depths predicted by tomography studies a lower mantle viscosity increase of around thirty times is required. For models with a factor of ten increase slabs do not remain at mid-mantle depths for long enough, while a factor of one hundred increase causes slab sinking rates too slow to match imaged tomographic anomalies. An endothermic phase changes could potentially layer mantle convection into two independent layers. In models assimilating plate motions, no model containing an endothermic phase change reaches a fully layered state, even with unrealistically large, negative Clapeyron slopes. The onset of plate tectonics could potentially break down a two-layered mantle into a partially layered state, similar to the present day mantle. Predictions of mantle heterogeneity from high-resolution, global mantle circulation models match well with complex mantle structure imaged by seismic tomography in the Tethys region. These models indicate that a more complicated history of subduction during the closure of the Neotethys Ocean is required to match the imaged mantle structure. Subduction is required in two locations, one at the Eurasian margin and a second behind a back-arc ocean opening in the Neotethys Ocean. Simultaneous subduction at both plate boundaries appears not to be necessary. Global mantle circulation models estimate long-wavelength dynamic topography with amplitudes of up to five kilometres. The largest amplitude signal of dynamic topography is at plate boundaries, suggesting that near surface density variations in the mantle contribute significantly to the dynamic topography signal. The five-kilometre amplitude of topography is larger than predicted elsewhere and is explained by the inclusion of near surface density variations, commonly ignored by other global calculations of dynamic topography. If dynamic topography is defined as ‘any topography arising from flow within Earth’s mantle’ then near surface density variations are significant to the dynamic signal. Predictions of dynamic topography from mantle circulation models reveal a dichotomy between continental and oceanic regions. Oceanic crust is a part of the mantle convection system and so predicted topography for ocean regions matches well with the expected depth versus age curve for oceanic crust. Continental regions are significantly subsided relative to oceans in the dynamic signal, suggesting that isostatic effects mask continental dynamic topography. When predictions of dynamic topography are corrected for isostatic effects and crustal thickness, an accurate estimate of Earth’s observed topography is generated. This work contributes to an on going debate on the nature of dynamic topography on global and regional scales.