Controls on the Dynamics of Subducting Slabs in a 3-D Spherical Shell Domain
Creators: name: Chen, Fangqin
Publisher: name: The Australian National University
Year: 2023

description: Subduction zones delineate tectonic plate boundaries where one plate descends beneath another into the underlying mantle. Subduction is responsible for many of Earth's most distinctive geological features, including mountain belts, volcanic island arcs and deep-sea trenches. It has long been recognised that the shape of subduction zones is influenced by Earth's sphericity, but the effects of sphericity are regularly neglected in numerical and laboratory studies that examine the factors controlling subduction dynamics: most existing studies have been executed in a Cartesian domain, with the small number of simulations undertaken in a spherical shell incorporating plates with an oversimplified rheology, limiting their applicability. In light of this, there are currently many outstanding questions relating to the key controls on the dynamics of subduction. For example, do predictions from Cartesian subduction models hold true in a spherical geometry? When combined, how do subducting plate age and width influence the dynamics of subducting slabs, and associated trench shape? How do relic slabs in the mantle feedback on the dynamics of subduction? These questions are of great importance to understanding the evolution of Earth's subduction systems, but remain under-explored. In this thesis, I target these questions through a systematic geodynamic modelling effort. I examine simulations of multi-material free-subduction of a visco-plastic slab in a 3-D spherical shell domain. I first highlight the limitation(s) of Cartesian models, due to two irreconcilable differences with the spherical domain: (i) the presence of sidewall boundaries in Cartesian models, which modify the flow regime; and (ii) the reduction of space with depth in spherical shells, alongside the radial gravity direction, which cannot be captured in Cartesian domains, especially for subduction zones exceeding 2400 km in width. I then demonstrate how slab age (approximated by co-varying thickness and density) and slab width affect the evolution of subducting slabs, using spherical subduction simulations, finding that: (i) as subducting plate age increases, slabs retreat more and subduct at a shallower dip angle, due to increased bending resistance and sinking rates; (ii) wider slabs can develop along-strike variations in trench curvature due to toroidal flow at slab edges, trending toward a 'W'-shaped trench with increasing slab width, and (iii) the width effect is strongly modulated by slab age, as age controls the slab's tendency to retreat. Finally, I show how remnant slabs in the mantle impact on subduction dynamics and the evolution of subduction systems. I find that negatively buoyant remnants drive downwelling mantle flow that can enhance the sinking velocity of nearby slabs. Such interactions may explain the poor correlation between subducting plate velocities and subducting plate age found in recent plate reconstructions. The location of remnants further influences trench motion: the trench advances towards a remnant positioned in the mantle wedge region, whereas remnants in the sub-slab region enhance trench retreat. I find that the subducting slab and remnant rotate towards each other, reducing the distance between them. In this process, the trench develops along-strike variations in shape that are dependent on the remnant's location.