Abstract
Deformation in the Earth’s crust and the occurrence of earthquakes, volcanism and georesources are caused by processes occurring in the Earth’s interior. Geodynamical models are the only way to study the interplay between the Earth’s (deep) interior and its surface as many of these processes occur on timescales from millions
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to billions of years. Therefore, novel geological or geophysical observations provide a constant means to improve these models and thereby our understanding of the dynamic solid Earth. This thesis focusses on the interaction between the Earth’s mantle and the tectonic plate motion on top by using novel kinematic observations obtained through plate-tectonic reconstructions based on geological data. One of these observations is the inferred average slab sinking rates, which are obtained by the correlation of the geological record with tomographically imaged slabs in the mantle. These illustrate the deceleration of slabs from plate motion speeds in the upper mantle to an average sinking rate in the lower mantle. Our models with slab sinking rates of 10-15 mm/a have a very limited bulk mantle motion, of only a few mm/a. In these models the preservation of large unmixed zones in the mid-mantle occurs even after 1000 Ma of simulated subduction evolution and mantle convection. We use configurational entropy to quantify and map the mixing of different compositions in our mantle convection models. This entropy may be used to analyse, with a single global measure, the mixed state of the mantle and compare various geodynamic models with each other. Furthermore, the local entropy may be useful to validate geodynamic models against local anomalies in the mantle from seismological or geochemical observations. The deceleration of slabs from subducting plate velocities to the lower mantle average slab sinking rates occurs by thickening or buckling of the subducting plate. High-resolution kinematic reconstructions of the Indian Plate revealed rapid oscillations in plate motion of up to 50 % in 2-3 Ma. Our regional geodynamic models show that buckling of slabs can cause such oscillations in plate motion speed before eventual transfer of slabs in the lower mantle. The amplitude and period of such oscillations depend on the average subducting speed. This shows that tectonic plates are generally driven by subduction, or a ‘slab pull’ force, in the top few hundred kilometres of the mantle. However, plates without an attached slab, and therefore lacking such a force, are not mantle-stationary. We use 3D-geodynamic models of an oceanic plate, surrounded by mid-oceanic ridges, that is influenced by the plate motion of the plates on either side of those ridges. These models show that the centre oceanic plate slowly trails the faster moving neighbouring plate and whose plate motion is increased by viscous coupling below the mid-oceanic ridges. This thesis shows that geodynamic models, when tuned to specific kinematic observations of plate motion, are useful to study the processes driving tectonic plates and motion in the Earth’s mantle thereby using, and improving insights from the various disciplines that study the solid Earth.
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