The Global Consequences of a Weak Upper Asthenosphere on Subduction Dynamics

A range of geophysical studies has recently brought considerable attention to the detailed structure of the asthenosphere at subduction zones. These studies report an abrupt seismic velocity decrease and high electrical conductivity increase. This appears to be localised in a thin layer at the base of the subducting lithosphere at different convergent margins in the Pacific plate. This layer (WL) has been interpreted as being weaker and slightly more buoyant than the underlying asthenospheric mantle. Despite these observations, the role of a WL on subduction dynamics remains unclear, and whether this phenomenon represents a global or a regional feature has yet to be investigated. The ability to detect and distinguish whether these narrow channels constitute a global or a regional feature at the base of the lithosphere has been hindered by the complicated nature of the lithosphere-asthenosphere boundary and by the trade-off between resolving power and regional extent of seismic studies.

Here, we use 3D numerical models of subduction to explore the role of a weak upper asthenosphere for the evolution and deformation of subducting slabs, and on the partitioning of subduction motions and deformation flow. We aim to better understand the coupled dynamics of subduction and mantle flow deformation with the aim of reconciling surface observations and numerical models. In our models, we vary the physical properties and geometrical extent of the WL. We focus our study on plate convergence and trench migration motions in addition to components of the deformation flow such as vorticity, horizontal and vertical divergence.

Our results show that the presence of a weak upper asthenosphere alone has the potential to affect subducting plate dynamics and improves our numerical ability to match with natural observations of subduction motions and deformation flow on a global scale. We use these results to build a regime diagram that shows the effect of WL on the dynamics of subducting plates. Our models suggest that regardless of the geometrical extent of this feature, this mechanism represents a pertinent advancement in our modelling ability toward matching natural observations on a global scale.