Detailed Structure of the Subducted Nazca Slab into the Lower Mantle Derived From Continent‐Scale Teleseismic P Wave Tomography

Abstract

Nazca subduction beneath South America is one of our best modern examples of long‐lived ocean‐continent subduction on the planet, serving as a foundation for our understanding of subduction processes. Within that framework, persistent heterogeneities at a range of scales in both the South America and Nazca plates is difficult to reconcile without detailed knowledge of the subducted Nazca slab structure. Here we use teleseismic travel time residuals from >1,000 broadband and short‐period seismic stations across South America in a single tomographic inversion to produce the highest‐resolution contiguous P wave tomography model of the subducting slab and surrounding mantle beneath South America to date. Our model reveals a continuous trench‐parallel fast seismic velocity anomaly across the majority of South America that is consistent with the subducting Nazca slab. The imaged anomaly indicates a number of robust features of the subducted slab, including variable slab dip, extensive lower mantle penetration, slab stagnation in the lower mantle, and variable slab amplitude, that are incorporated into a new, comprehensive model of the geometry of the Nazca slab surface to ~1,100 km depth. Lower mantle slab penetration along the entire margin suggests that lower mantle slab anchoring is insufficient to explain along strike upper plate variability while slab stagnation in the lower mantle indicates that the 1,000 km discontinuity is dominant beneath South America.

The Deformational Journey of the Nazca Slab From Seismic Anisotropy

“…Comparison with 3‐D ray tracing demonstrates that rays are even more likely to be drawn into the slab than 1‐D predictions using IASP91 (Figure S4 and Text S3), similar to experiments by Portner et al (2020) (cf. Figure S4).…”

“…Previous studies were unable to make this distinction due to either a lack of downdip raypaths and/or an inability to assess slab traversing raypaths when only global tomographic (i.e., lower resolution) models are available (Foley & Long, 2011; Lynner & Long, 2015; Nowacki et al, 2015; Walpole et al, 2017; Wookey et al, 2002). Ferreira et al (2019) recently inferred global radial anisotropy of the ULM beneath slabs but with a lateral model resolution of ∼1,000 km and therefore unlikely to be able to pin‐point anisotropy within a relatively narrow slab (Portner et al, 2020). Third, anisotropy is detected from deep seismicity throughout the ASZ (Figure 3) suggesting the presence of a widespread intraslab mechanism, rather than a few localized pockets of anisotropy.…”

“…(a) Intraslab split (bars) and null (black diamonds) measurements ( 3) for earthquakes ≤410 km depth plotted at ray pierce points on top of the 410 km P wave tomography depth section (Portner et al, 2020). The orientation of individual splits indicates the fast direction ( ϕ ) projected on the downgoing ray.…”

Subducted Lithosphere Under South America From Multifrequency P Wave Tomography

“…Models PRI-P05 (Montelli et al, 2006) and LLNL-G3Dv3 (Simmons et al, 2012) show the Peruvian slab gap less clearly than other models, and they hint at southward slab continuation beneath the Atlantic, offshore Argentina. Similarly, the regional tomography by Portner et al, (2020) shows less of a slab gap than a thinning of fast anomalies. Additional tomography models (nine in total) are compared in Figures S12 to S14, at 400 to 600 km depth.…”

“…Only a few tomography studies in South America have been conducted at the continental scale, notably the surface-wave tomography of Heintz et al (2005) and the body-wave tomographies of Grand (1994), Ren et al (2007), and Portner et al (2020). Much of what is known about lower-mantle structure under South America has been learned from global-scale tomography models (e.g., Amaru, 2007;Bijwaard et al, 1998;Li et al, 2008;Lu et al, 2019;Montelli et al, 2006;Obayashi et al, 2013;Simmons et al, 2012).…”