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 themantle by subducting slabs. Here we use seismological measurements and global geodynamic models adapted to this unique region to investigate howmantle 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. The tectonic setting of the southeastern Caribbean involves subduction of oceanic lithosphere in opposite polarity beneath the Antilles arc and the Americas, which are linked by a transform plate boundary between continental South America and the Caribbean Plate (Fig. 1). The Cenozoic tectonic history has been primarily influenced by the Caribbean Plate migrating east relative to the Americas. About 55million years (Myr) ago, the Caribbean Plate collided with both the Bahamas bank and northern South America as it moved eastwards. The Caribbean Plate rotated clockwise as a result of to this collision, which initiated orogenesis in northern South America and development of the San Sebastian–El Pilar right lateral strike-slip system along the margin of northern South America to accommodate the sustained, east–west South American–Caribbean Plate motion. Just a few hundred kilometres south of this complex plate boundary system lies the Guyana Shield, a ∼1.7-billion-year-old craton, inferred to have a relatively rigid keel extending into the upper mantle (Fig. 1). Data from broadband seismic instrumentation from temporary deployments and permanent national networks in Venezuela and in the surrounding region allow for the investigation of lithospheric and upper-mantle structure beneath this complex plate boundary (Fig. 1). Rayleigh wave tomography reveals a linear shear-wave velocity change that parallels the dextral strike-slip fault system along the northern coast of Venezuela, imaging the differences between the South American continental