The influence of a sub-lithospheric decoupling layer on subduction zone surface motion, mantle circulation and slab dynamics

© 2020 Roberta CarluccioThe lithosphere consists of a strong surface layer fragmented in a series of major and minor tectonic plates moving over a weaker and more buoyant layer of asthenospheric material. The boundary between the lithosphere and asthenosphere, the LAB, is a critical element of plate tectonics as it mechanically decouples the motion of strong tectonic plates from the ow in the underlying weaker mantle. Recent geophysical observations report an abrupt seismic velocity decrease and electrical conductivity increase beneath certain segments of the subducting Pacific plate, which may represent the channelised base of the subducting oceanic lithosphere. This inferred sub-lithospheric weakness is predicted to lubricate the base of subducting plates and enforce the decoupling at the LAB, with potential implications for our understanding of subduction geodynamics. However, little is known about the impact of this feature on subduction zone dynamics. This research work for the first-time compiles worldwide observations of a very thin and weak sub-lithospheric layer (SLL) embedded beneath the base of a descending oceanic lithosphere and computationally simulates SLL effects on subducting plate and mantle dynamics. In the last few decades, the rapid progress in technology has contributed to increasing complexity of geological observations and capacity of computational infrastructures. With direct geological observations limited in both time and space scales, computational modelling offers the extraordinary opportunity to test different hypotheses for Earth's evolution through geologic time and ongoing processes, whilst promoting the development of multidisciplinary approaches. This study, therefore, employs subduction numerical models and addresses the SLL impact on slab deformation style including subduction surface motions, induced mantle ow and patterns of seismic anisotropy. Our results show that the physical properties of the sub-lithospheric layer fundamentally control the partitioning of surface motions between trench migration and horizontal plate velocities, but also the partitioning of the induced mantle ow as it splits into its poloidal and toroidal components. A regional SLL mechanism firstly provides a novel explanation for the high convergence and low trench migrations rates observed globally at natural subduction zones. Then, an SLL also delivers an alternative answer for plate margins characterised by some degree of toroidal ow and for the apparent lack of a direct link between buoyantly driven convection (which drives only vertical and divergent motion) and the generation of toroidal energy (which corresponds to strike slip motion and plate spin). Additionally, a very thin and weak sub-lithospheric layer promotes the alignment of the maximum strain axes in the direction parallel to plate motion within the oceanic lithosphere and mantle directly beneath it, and in the direction parallel to the trench deeper in the sub-slab mantle. As a result, these findings contribute to the resolution of the slab anisotropy debate on whether an SLL can substantially influence three-dimensional ow around the subduction zone. These outcomes reveal that a very weak and thin sub-layer embedded in the Earth's uppermost asthenosphere impacts the complexity of current natural subduction systems, for which little predictive and holistic modelling exists. By acting as a slippery base for the motion of the subducting lithosphere, this layer sharpens the decoupling between the strong tectonic plates and the weaker mantle underneath. This has significant consequences on slab subduction style and patterns of seismic anisotropy as observed in several subduction regions, such as Izu-Bonin-Mariana, Central America and Kamchatka- Aleutian among others, where the presence of this layer is confirmed by geophysical studies. This thesis work ultimately aims to provide a novel conceptual framework to reinterpret the geological record of complex plate-margins, with broader implications for better understanding the evolution of the Earth's dynamic system