Abstract
In this thesis, I aimed at searching for new ways of constraining paleo-geographic, -atmosphere and -sea level reconstructions, through an extensive investigation of mantle structure in seismic tomographic models. To this end, I explored evidence for paleo-subduction in these models and how this may allow for a new avenue towards
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reconstructing the plate tectonic history of our planet. The major findings can be summarized as follows: Initially 28 slabs were characterized in terms of ranges of depth and timing of subduction. This effort led to 1) a determination of the sinking rates of slabs as a novel implicit observation of the rate of lower mantle flow; 2) to constrain how far back in time ‘mantle memory’ of past subduction extends, being a maximum of 250-300 Myr; and 3) to test whether slab remnants of paleo-subduction can be used as a constraint on absolute plate position yielding a new approach to constrain absolute plate reconstructions by ‘slab-fitting’ reconstructions. This methodology is applied to the mantle underneath the Pacific Ocean. Geological evidence from east Asia suggested that accreted exotic terranes with Lower Mesozoic volcanic arcs should have originated from central Panthalassa Ocean locations, yet how these fitted in plate kinematic reconstructions remained poorly constrained. Remnants of paleo-subduction were identified in the mantle, revealing the location of Mesozoic intra-Panthalassa subduction zones and shedding light on the plate tectonic configuration of the Panthalassa Ocean. Subsequently we interpret paleo-subduction zone configurations at a series of depth slices of the mantle and calculate total slab, and hence subduction zone length versus depth. This provided a first-order estimate of global subduction zone length for the last 235 Myr. This was used to compute volcanic degassing of CO2 from subduction and ridge spreading and served as input into a carbon-cycle climate model. The new degassing estimate resulted in an improved fit between modelled atmospheric CO2 and proxy data. In addition, it provided a good first-order fit of 87Sr/86Sr ratios from marine carbonates, a record of paleo-seawater composition. This correlation between the 87Sr/86Sr record and plate tectonic activity was improved by correcting for weathering of continents and used to calculate global ocean floor production rates to compute a eustatic sea level curve. We compare this curve with previous sea level and flooded shelf area curves derived from sequence stratigraphy and from plate motion models. Aside from providing a novel method of estimating sea level variation as a function of global plate tectonic activity, it extended sea level curves back to ~840 Myr. The identification and geological interpretation of slabs is further expanded to 94 slabs. This compilation is named ‘Atlas of the Underworld’, which provides a complete overview of the present interpretation of slab remnants by the scientific community and ourselves. The slabs' subduction age-depth data provided new average and in situ sinking rates. Slab deceleration in the top of the lower mantle is modelled as well as the required associated slab thickening or buckling. Instead of stagnated slabs, we observe that all slabs sink further towards the core-mantle-boundary.
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