Model of the Nazca slab constrained by continent-scale tomography
Model of the Nazca slab constrained by continent-scale tomography reveals that the slab penetrates into the mantle unimpeded by the 660 km discontinuity, only to stagnate near 1000 km depth. Note the newly mapped lower-mantle flat slab, which extends 2000 km beneath the Amazon basin. Credit: Portner et al. [2020], Figure 10d
Source: Journal of Geophysical Research: Solid Earth

Tomographic images of oceanic plates as they dive into the mantle are iconic illustrations of the potential of seismology to reveal Earth’s inner workings. Local and regional seismic studies constrain subducting slab morphology [Hayes et al. 2018] in the shallow mantle, which are important for understanding earthquake hazards, volatile cycling, and deformation, topography and volcanism in the overriding plate. Global tomographic studies, on the other hand, constrain slab morphology in the mid and lower mantle with remarkable consistency (e.g. Ritsema and Lekić, 2020), informing tectonic reconstructions as well as mantle properties and dynamics.

Using P-wave travel-time measurements at over 1,000 seismic stations across South America, Portner et al. [2020] image structure beneath the continent in unprecedented detail and extent. Their continent-scale model helps bridge the gap between the two approaches, extending resolution into the shallow lower mantle.

The model reveals along-strike variations in Nazca slab dip and temperature, mapping the slab’s apparently unimpeded penetration into the lower mantle. Once it enters the lower mantle, the slab appears to stagnate near 1000 kilometers depth, flattening and extending eastward for as much as 2000 kilometers beneath the Amazon basin. In addition to transforming our view of the Nazca slab, the findings of this paper hold important implications for the origin of deep seismicity and for models of tectonic evolution of South America.

Citation: Portner, D. E., Rodríguez, E. E., Beck, S., Zandt, G., Scire, A., Rocha, M. P., et al. [2020]. Detailed structure of the subducted Nazca slab into the lower mantle derived from continent‐scale teleseismic P wave tomography. Journal of Geophysical Research: Solid Earth, 125, e2019JB017884.

—Vedran Lekic, Associate Editor, JGR: Solid Earth

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