Sub-slab seismic anisotropy and mantle flow beneath the Caribbean and Scotia subduction zones: Effects of slab morphology and kinematics

Abstract

Subduction systems are vitally important to plate tectonics and mantle convection, but questions remain about many aspects of subduction dynamics, particularly the nature of sub-slab mantle flow. Observations of seismic anisotropy can shed light on the pattern of mantle flow in subduction systems, but major uncertainties remain regarding the interpretation of sub-slab anisotropy. Here, we present measurements of shear wave splitting due to anisotropy in the sub-slab mantle beneath the Caribbean and Scotia subduction zones. These subduction systems are morphologically similar, with high arc curvature and short arc length, but there are differences between them in terms of their kinematic and tectonic settings. We apply the source-side shear wave splitting technique to direct teleseismic S phases originating from slab earthquakes. We restrict our analysis to seismic stations at which we have examined the receiver-side splitting in detail to ensure accurate corrections for anisotropy in the upper mantle beneath the station. We observe a well-defined pattern of trench-parallel fast directions (ϕ) in the northern half of the Caribbean subduction zone, with a transition to dominantly trench-perpendicular ϕ at the southern end. There is more scatter in the measurements for Scotia, but we observe generally trench-parallel ϕ in the northern portion of the subduction system, with a mix of trench-parallel, -perpendicular, and -oblique fast directions to the south. Our preferred interpretation of these splitting patterns is that they reflect 3-D return flow of the sub-slab mantle due to trench rollback. In both systems, we infer that sub-slab flow is being driven from south to north. Beneath Scotia, this is likely driven by differential migration of the Scotia trench along strike. Beneath the Caribbean, we hypothesize that mantle flow around the southern edge of the slab is inhibited by the presence of the South American continental keel, enabling trench-perpendicular stretching in the sub-slab mantle and forcing mantle flow to escape to the north. The similarities and differences we observe between the two systems yield insight into the relative contributions of slab morphology and plate kinematics in controlling mantle flow beneath subducting slabs.