The first EarthByte seminar of the new year will be held via Zoom at 11am–12pm Wednesday 2nd February (Sydney time). This week’s speaker will be Roberta Carluccio of the EarthByte Group. The Zoom meeting can be accessed here: https://uni-sydney.zoom.us/j/85723453817. Full details of the seminar are below:
The influence of a sub-lithospheric decoupling layer on subduction zone surface motion, mantle circulation and slab dynamics
The 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. Recent geophysical observations report an abrupt seismic velocity decrease and electrical conductivity increase directly beneath certain segments of the subducting Pacific lithosphere. This inferred sub-lithospheric weakness may represent the channelised base of the subducting oceanic lithosphere and is predicted to lubricate the base of subducting plates enforcing the decoupling at the LAB. It, therefore, has significant implications for our understanding of subduction dynamics and plate tectonics. However, little is known about the geodynamic impact of this feature on subducting slab dynamics and large-scale mantle flow.
In this study, we use subduction numerical models where we vary the physical properties of this layer and address its impact on slab deformation style including subduction surface motions, induced mantle flow and patterns of seismic anisotropies. 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 flow as it splits into its poloidal and toroidal components. Additionally, a very thin and weak sub-lithospheric layer promotes the development of complex patterns of mantle anisotropy around the subduction zone.
These findings contribute to the resolution of the subduction geodynamics debate about whether an SLL can substantially influence the dynamics of sinking plates and three-dimensional flow around the subduction zone. By acting as a slippery base for the motion of the subducting lithosphere, it sharpens the decoupling between the strong tectonic plates and the weaker mantle underneath. This has significant consequences on plate velocities and deformation, as well as on the mantle flow nature and evolution around the subduction zone. We suggest that a decoupling thin layer embedded beneath the base of a subducting plate can significantly influence patterns of slab geometries, trench migrations and seismic anisotropy observed in subduction regions, such as Izu-Bonin-Mariana, Central America, and Kamchatka-Aleutian among others, with broader implications for the evolution of complex plate margins.
