Mantle flow deflected by interactions between subducted slabs and cratonic keels

Oceanic lithosphere is rapidly recycled into the mantle through subduction, an important part of the dynamic evolution of the Earth. Cratonic continental lithosphere, however, can exist for billions of years, moving coherently with the tectonic plates. At the Caribbean-South American Plate margin, a complex subduction system and continental transform fault is adjacent to the South American cratonic keel. Parallel to the transform fault plate boundary, an anomalous region of seismic anisotropy - created when minerals become aligned during mantle flow - is observed. This region of anisotropy has been attributed to stirring of the mantle by subducting slabs. Here we use seismological measurements and global geodynamic models adapted to this unique region to investigate how mantle flow, induced by subduction beneath the Antilles volcanic arc, is influenced by the stiff, deep continental craton. We find that three components - a stiff cratonic keel, a weak asthenospheric layer beneath the oceans and an accurate representation of the subducted slabs globally - are required in the models to match the unusual observed seismic anisotropy in the southeast Caribbean region. We conclude that mantle flow near the plate boundary is deflected and enhanced by the keel of the South American craton, rather than by slab stirring.