Characterization of Nazca slab and possible subducted aseismic ridges from seismic tomography

Journal Title
Journal ISSN
Volume Title

The South American Andean margin involves eastwards Farallon-Nazca oceanic plate subduction under western South America. Subduction of the Juan Fernandez, Nazca, and Carnegie aseismic ridges on the Nazca plate is thought to profoundly affect Andean tectonics but the downdip geometry of these subducted features is ambiguous. In this study, we attempt to trace subducted features (e.g., aseismic ridges) within the Nazca slab from recent regional P-wave tomography, SAM5_P_2019 (Portner et al., 2020), and adjoint SAAM23 (Ciardelli et al., 2022); results are compared to a suite of global models including MITP08 (Li et al., 2008). We map a 3D Nazca mid-slab model using GOCAD software and extract the tomographic velocities along the slab. Our mapped areas near the Juan Fernandez and Nazca ridges show sufficient resolution based on published resolution tests from Portner et al. (2020); SAAM23 shows resolution in shallower regions downdip of all three aseismic ridges. Our results show the Nazca slab downdip of the subducting Carnegie and Juan Fernandez ridges generally exhibits anomalous, slower velocity anomalies in SAM5_P_2019 tomography at > 150 km depths whereas the slab downdip of the Nazca ridge is generally fast. SAAM23 shows slow velocity perturbations downdip of all three aseismic ridges in this study. Extraction of seismic velocities to an alternative mid-slab model following Slab2 generally shows slow velocity perturbations along the Carnegie and Nazca ridges, but results are inconsistent. We compare our tomographic aseismic ridge locations to modeled hotspot tracks from various plate reconstructions and published studies. We conclude that the subducted Nazca ridge does not show anomalous velocities; however, the subducted Carnegie and Juan Fernandez aseismic ridges are possibly revealed by slower velocity anomalies at similar locations in both primary tomography. These anomalies could be warmer slab gaps or tears due to buoyancy contrasts between the aseismic ridges and surrounding Nazca slab.

aseismic ridges, tomographic velocities, resolution test, hotspot tracks.