Year

2022

Degree Name

Bachelor of Science (Honours)

Department

School of Earth, Atmospheric and Life Sciences

Advisor(s)

Nicolas Flament

Abstract

Subducted slabs of oceanic lithosphere are key drivers of mantle convection, representing the cold limbs of mantle convection. Seismic tomography has long been the leading tool to study subducted slabs, where present-day high seismic velocity structures are interpreted as subducted slabs. The present-day mantle structure can then be used to explore subducted slab sinking history by linking subducted slabs to paleosubduction remnants on the surface, such as magmatic arcs. In this process, subducted slabs are assumed to sink vertically at a constant rate. Numerical models of mantle convection provide a tool which can reconstruct mantle structure through time, making it possible to investigate the temporal and spatial evolution of subducted slab sinking dynamics. The present-day mantle structure predicted by convection models can be compared to mantle structure imaged by seismic tomography to validate the convection models. In this study, three palaeogeographically constrained mantle convection models, differing only in the reference frame of the boundary plate tectonic model, were compared to a set of seismic tomography vote maps. This comparison tested how appropriately the thermal structure and flow field of the convection model matched to the seismic tomographic image of the mantle. A no-net-rotation reference frame was found to produce the model with the best match to tomography. It was only slightly higher than a reference frame optimised by minimised net rotation, trench migration, plate speeds, and matching of hotspot tracks, with both offering significant improvements over a palaeomagnetic reference frame. Subducted slab sinking rates in the no-net-rotation model are 1.3 cm/yr, matching those found in seismic tomography. Model slabs better match to tomography where lateral moiv v tion is relatively minor (<1 cm/yr average), supporting the vertical sinking assumption of tomographic studies. It was found that vertical sinking rates were dominantly controlled by convergence rates and that the reference frame has a major impact on subduction location and subducted slab lateral motion, making the plate tectonic boundary condition a major factor in subducted slab behaviour. The lateral motion of slabs in the mantle was found to likely be controlled by surface plate motions and lateral viscosity variations.

FoR codes (2020)

370604 Geodynamics, 370511 Structural geology and tectonics, 370609 Seismology and seismic exploration

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.