Joshua Weber

Year

2020

Degree Name

Honours degree of Bachelor of Science

Department

School of Earth, Atmospheric and Life Sciences

Advisor(s)

Dr Nicolas Flament

Abstract

Through seismic imaging, varying geometries of slab subduction are noted throughout the mantle. While some slabs remain mostly unaffected when penetrating down to the lower mantle (e.g. Mariana plate/trench), others appear to flatten out and lie along the 660 km discontinuity (e.g. Izanagi plate). Penetrating slabs with moderate dip angles (e.g. South American plate/Peru-Chile trench) are also observed. The exact cause of these varying geometries is largely unknown. This study attempts to identify if a variation in slab geometry at the upper lower mantle transition zone is caused by an internal (e.g. phase change, viscosity structure, Rayleigh number) or external (trench migration) processes of subduction. The mantle flow regime of the mantle has also been assessed. Mantle convection models with tectonic surface boundary reconstructions were grouped together according to their specific input parameters and then had their present-day temperatures compared to seismic tomography. Matches and mismatches to tomography were analysed and quantified on global and regional scales. The presence of switching geometries throughout a slab’s evolution has led this study to favour a whole mantle convection regime with compositionally different layers. Results from comparisons of mantle flow models with tomography indicate that the presence of a phase change and a change in viscosity structure, two internal parameters, have little effect on the variation of slab geometry. More significantly, a change in the Rayleigh number caused major variations, with too low and too high a convection vigour resulting in a variation of predicted slab material between mantle flow models. The external parameter of surface boundary reconstructions was observed to be the main contributor to a high variation in predicted slab geometry. When modelled through time, both the internal and external parameters displayed the same level of variation seen at present day. In the evolution of subduction, each modelled slab location was seen to start as a steeply penetrating slab. Trench migration at the surface caused upper mantle material to migrate at a high rate, while material in the lower mantle moved at a lower rate, resulting in a decrease in the angle of slab penetration in the lower mantle. Trench migration appears to be the key factor in shaping slab geometry.

FoR codes (2008)

040402 Geodynamics

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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.