How the Relationship Between Subduction and Mantle Dynamics Shapes Slab Evolution at Mid-Mantle Depths

Advances in seismic tomography have revealed that the Earth’s mantle is more complex than previously thought and hosts a plethora of slab morphologies. These morphologies can be broadly grouped into three main types; penetrating slabs, deflected slabs and broken, or orphan, slabs. Slab orphaning is a newly discovered phenomenon whereby slabs break directly at mid-mantle depths. This produces a flattened parent slab above 660 km and an orphan slab below it. As the orphan slab slowly sinks towards the core-mantle boundary, subduction continues through the lateral motion of the parent above 660 km. In nature, the Tonga, Arabian, Japan and Central American slabs are possible candidates for slab orphaning. Orphaning has significant implications for the interpretation of slab remnants and their inferred ages, with consequences for tectonic reconstructions. The subduction of slabs, however, does not take place in isolation and slab dynamics must be influenced to some degree by the overriding plate. The nature of the overriding plate plays a major role in the evolution of deep slab morphologies at mid-mantle depths. 2-D numerical simulations of subduction indicate that the presence of continental lithosphere at subduction zones produces markedly different slab behaviour at depth. In particular, a continental overriding plate results in bigger orphans and encourages the mid-mantle penetration of slabs that are otherwise inclined to flatten. It is therefore clear that, in contrast to the slabs of the upper mantle, the deeper slab morphologies are the result of a complex interaction between the overriding plate forcing and the changes in the relative strength ratios of the slab and mantle.