Linking Slab Structure to Mantle Dynamics in the South American Subduction Zone

Abstract:

The South American subduction zone, where the oceanic Nazca plate subducts beneath the continental South America plate along a >7,000 km wide margin, hosts many of the world’s largest earthquakes, most destructive volcanoes, and broadest subducted slabs. The geometry and evolution of the Nazca slab is frequently linked to tectonic, magmatic, and seismic processes across the South American continent, but the nature of these interactions remains debated. Using seismic data from >1,000 seismic stations distributed across South America, we produce independent teleseismic P- and S-wave tomography models of the mantle beneath the entire South American continent from ~100-1,300 km depth that are the most detailed and comprehensive of the region to date. These models reveal the detailed geometry of the Nazca slab, which exhibits strong variations in slab dip, contortion, and stagnation, and highlights the slab’s interactions with the surrounding mantle dynamics. Among these interactions, our models show where a tear in the slab alters mantle flow patterns, where slab stagnation in the lower mantle triggers plume formation, where upwelling asthenosphere contributes to slab buoyancy, and where mantle viscosity variations inhibit slab sinking. Overall, these suggest strong linkages between the subducting slab and the surrounding mantle dynamics, and bring into question some models of the deep slab’s role in controlling tectonics in the overriding plate.