Mantle dynamics of the Andean Subduction Zone from continent-scale teleseismic S-wave tomography

Abstract

Summary The Andean Subduction Zone is one of the longest continuous subduction zones on Earth. The relative simplicity of the two-plate system has makes it an ideal natural laboratory to study the dynamics in subduction zones. We measure teleseismic S and SKS travel-time residuals at > 1,000 seismic stations that have been deployed across South America over the last 30 years to produce a finite-frequency teleseismic S-wave tomography model of the mantle beneath the Andean Subduction Zone related to the Nazca Plate, spanning from ∼5° N to 45° S and from depths of ∼130 km to 1,200 km. Within our model, the subducted Nazca slab is imaged as a fast velocity seismic anomaly. The geometry and amplitude of the Nazca slab anomaly varies along the margin while the slab anomaly continues into the lower mantle along the entirety of the subduction margin. Beneath northern Brazil, the Nazca slab appears to stagnate at ∼1,000 km depth and extend eastward sub-horizontally for > 2,000 km. South of 25° S the slab anomaly in the lower mantle extends offshore of eastern Argentina, hence we do not image if a similar stagnation occurs. We image several distinct features surrounding the slab including two vertically-oriented slow seismic velocity anomalies: one beneath the Peruvian flat slab and the other beneath the Paraná Basin of Brazil. The presence of the latter anomaly directly adjacent to the stagnant Nazca slab suggests that the plume, known as the Paraná Plume, may be a focused upwelling formed in response to slab stagnation in the lower mantle. Additionally, we image a high amplitude fast seismic velocity anomaly beneath the Chile trench at the latitude of the Sierras Pampeanas which extends from ∼400 km to ∼1000 km depth. This anomaly may be the remnants of an older, detached slab, however its relationship with the Nazca-South America subduction zone remains enigmatic.

Adjoint Tomography of South America based on 3D Spectral-Element Seismic Wave Simulations

“…15, profile H) shows this deep, flat slab segment to be around 1,000 km long, and it seems to be descending into the lower mantle in two separate parts. A very similar image was obtained by Rodríguez et al (2021) (their Fig. 8b, our Fig.…”

“…12). Here we show some of the main features of the subducted Nazca plate as imaged in SAAM23 and compare them with previous models (e.g., Li et al, 2008;Simmons et al, 2012;Obayashi et al, 2013;Portner et al, 2020;Rodríguez et al, 2021;Gao et al, 2021).…”

“…Further South, around 30 • S and in the transition zone, the Nazca Slab has been imaged mainly as a relatively flat high-velocity anomaly along and near the 650 km discontinuity, suggesting either a diminished slab pull (stagnation) just above or just below the transition zone or reflects a change in convergence rate or westwards trench migration (Rodríguez et al, 2021). Our model (Fig.…”

“…16a). Rodríguez et al (2021) interpret this behavior as the result of spatio-temporal changes in the rate of westwards trench migration rather than as a change in slab pull with depth. This behavior favors the hypothesis that the absolute motion of slabs is mainly vertical downwards, and the apparent dip of the slab at shallow depths is the result of the westwards trench migration, as the absolute South American motion is towards the NW.…”

“…North of about 20 • S, several tomographic models image the Nazca Slab crossing the transition zone and continuing into the lower mantle, such as the global models LLNL-3Dv3 (Simmons et al, 2012), GAP-4 (Obayashi et al, 2013), SPani-P,S (Tesoniero et al, 2015), DETOX-P1 (Hosseini et al, 2020), as well as the regional models SAM-P-2019 and SAM-S-2020 (Portner et al, 2020;Rodríguez et al, 2021), as shown in Fig. 15.…”