Investigation of Anomalous Travel Times in the Central Andes: Possible Evidence for a Lithospheric Root Trapped Above a Flat Slab

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Studies of continental deformation at convergent plate boundaries are of fundamental importance for advancing our understanding of how continents evolve, how natural resourses are concentrated, and how natural hazards such as earthquakes and volcanoes occur. Because of the broad spectrum of processes represented and the abundance of data available, the Andes mountains along the western coast of South America provide one of the best venues to investigate such continental deformation. This project seeks to take advantage of the extensive archive of seismic data recorded over the past several decades in South America to investigate how deformation is occurring beneath a particular part of the high Andes close to the border between Chile and Argentina where the subducting Nazca plate appears to flatten under the overriding South American lithosphere. This region is of particular interest because previous studies identified a seismic signal indicating unusually high elastic wavespeeds that suggest that the South American lithosphere is thickening above the flat slab and deforming it in the process. While such thickening is expected in this type of convergent margin, its existence above a flat slab challenges our current understanding of the physics and chemistry involved in such processes. This project will develop and apply state-of-the-art seismic imaging methodologies to an assembled dataset to allow inferences about what this feature is and how it is influencing the deformation of the both the slab and continent, and by extension how it affects the evolution of South America and the generation of earthquakes and volcanos in the region. This project involves an extensive international collaboration between students and senior scientists in the US, Chile, and Argentina. This project will support the work of graduate students who are both female and a member of an underrepresented demographic in STEM research. It will also provide semester-long research opportunities for 2-3 undergraduate students. A primary emphasis of this project is strengthening international research ties between the US, Chile and Argentina, as in involves highly collaborative work between scientists and students at RPI, the Universidad de Chile and Santiago, and the National University of San Juan in Argentina. Flat slabs and lithospheric roots are both fundamental features in the growth and evolution of continental lithosphere at Andean margins, but rarely, if ever, are they considered interreleated phenomena. The mantle above flat slabs is presumed to be cold and static, while root formation is generally believed to be the province of the ostensibly more dynamic environment of an asthenospheric mantle wedge. Based on previous work the investigators have done in this area with aftershocks of the 2015 Illapel earthquake, there is evidence that the part of the Pampean flat slab beneath the high Andes hosts such a root. Moreover, the team hypothesizes that the higher density of this root is causing the underlying “flat” lithosphere of the subducted Nazca plate to deflect downwards locally by 15-20 km. The identification of a lithospheric root above the Pampean flat slab would force us to rethink how roots evolve in a compressive tectonic environment. Several factors that are commonly cited to be critical to root formation, including a hydrated mantle wedge and vigorous corner flow within that wedge would be greatly reduced, if not completely absent, above a flat slab. Additionally, an extant lithospheric root is a relatively rare phenomenon; they often make their presence known only by their removal. Hence, the positive identification of an existing root presents a rare opportunity to investigate one in situ. While previous studies of this area hint at the existence of such an entity, for the most part it has escaped detection due to a combination of insufficient resolution or sampling and the potential misclassification of signals as outliers in generic background models. Preliminary analysis suggests that current datasets have been underutilized. The international team plans to generate enhanced catalogs of events, and then apply a combination of standard (e.g., arrival time tomography, attenuation tomography, anisotropy from shear wave birefringence) and more advanced (e.g., coda migration, anisotropic receiver functions, joint inversions) analysis techniques to this data to identify and characterize the hypothesized root.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.