Mantle heterogeneity across the Andean subduction zone from finite-frequency teleseismic S-wave tomography

The Andean subduction zone spans approximately 7,000 km of the western South America margin. Previous seismic imaging work and studies of the Wadati-Benioff zone have shown the structure of the Nazca plate exhibits significant variability along strike. Imaging work has mainly focused on relatively small sections of the slab and surrounding mantle, giving an incomplete view of the complex dynamics of the subduction zone. Herein, we invert for the shear wave velocity structure of the whole subduction zone, using data recorded by more than 1,000 stations deployed across the continent. Our model provides a high-resolution image on the continent-scale which shows the variability within the slab and surrounding mantle. The model shows a continuous Nazca slab from the top of our model, 100 - 150 km depth, into the lower mantle from 5 to 45°S. The Nazca slab penetrates into the lower mantle in most regions where we have good resolution. North of the Bolivian orocline, the slab is observed to stagnate at 1000 km depth beneath northern Brazil. This contrasts with the slab in the south, where it does not appear to stagnate and the slab anomaly is relatively weaker in amplitude and less coherent.

The mantle surrounding the slab also appears heterogeneous in both magnitude and polarity of velocity perturbations. In the region of the Bolivian orocline and southern edge of the Peruvian flat slab, a slow velocity anomaly (as low as -9% dVs) is imaged above the Nazca slab extending from the mantle transition zone to the top of our model. The slab anomaly beneath this slow also weakens in perturbation, however remains constant in P-wave tomography models. This is suggestive of a relatively high Vp/Vs within the slab, and may indicate a difference in hydration of the Nazca slab and overlying mantle along strike, where the central and northern segment is more hydrated than the southern segment, where the mantle is observed to be relatively faster. The overlying low velocity zone above the slab may also be indicative of fluids coming off the slab from dehydration.