How a Very Thin and Weak Sub-Lithospheric Layer Drives Mantle Flow Around the Subduction Zone: Insight Into the Evolution of Congested Subduction Zones.

Abstract:

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 suggest the presence of a very thin and weak sub-lithospheric decoupling layer beneath certain segments of the subducting Pacific plate; however, little is known about the lateral extent of this feature and its impact on three-dimensional slab dynamics and induced mantle flow. Here, we use three-dimensional buoyancy driven subduction numerical models where we vary the viscosity, thickness and width of this layer and address the impact on the partition of the induced mantle flow, surface motions and the relationship among them. Our results show that the physical properties of the upper asthenosphere fundamentally control the type of flow, slab shape and trench migration throughout the subduction zone. This significantly maximises the decoupling at the lithosphere-asthenosphere boundary hampering the alignment of the maximum finite strain in the direction parallel to trench in the oceanic plate. We suggest that the patterns of dominant trench perpendicular seismic anisotropies in the subslab, slab geometries and trench migration mechanisms observed in Central America and Kamchatka-Aleutian subduction zones are significantly affected by a decoupling thin sub-lithospheric layer at the base of the subducting plate with broader implication for the evolution of congested subduction margins.