Inferred Variations in Thermal and Rheological State of the Nazca Slab from High-Resolution Tomographic Imaging

The Nazca slab subducting beneath South America is the largest contiguous, actively subducting slab on earth, with significant along-strike variability in slab geometry, structure, and mechanical properties inferred from the irregular distribution of Wadati-Benioff zone earthquakes. Regional-scale seismic imaging studies have revealed more numerous variations in slab structure and behavior including the presence of gaps and tears as well as inconsistency with the slab's ability to descend into the lower mantle. While some of these observations can be directly attributed to heterogeneities in the incoming plate, such as the infrequent distribution of thickened ridges and fracture zones, others do not. This makes South America an ideal laboratory for investigating potential mechanisms for intraslab variability in structure and behavior. With this study, we combine data from more than 1,000 temporary and permanent seismic stations across South America in a single finite-frequency teleseismic P-wave tomographic inversion to image the majority of the Nazca slab in unprecedented detail from 100 km depth to >1,000 km depth.

Our results show prominent variations in slab character along the subduction margin. Our study confirms significant variability in slab dip angle, from sub-vertical dip near the Peruvian flat slab segment to 30° dip south of the Pampean flat slab segment, and the existence of a large slab gap near the Pampean flat slab. Our results additionally show significant variations in the seismic velocity of the slab, slab bending, and lower mantle penetration of the slab, with higher seismic velocities, higher degrees of bending, and slab penetration to the north of the Pampean flat slab than to the south. These differences may imply a fundamentally stronger, colder slab in the north that controls variable sinking dynamics along strike.