S063-0006
Using Plumes as Indicators for Mantle Flow in the South America Subduction Zone
Wednesday, 16 December 2020
Poster
Wenjie Lei, Princeton University, Geosciences, Princeton, NJ, United States, Youyi Ruan, Brown University, Providence, RI, United States, Ebru Bozdag, Colorado School of Mines, Golden, CO, United States, Daniel B Peter, ETH Zurich, Zurich, Switzerland and Jeroen Tromp, Princeton University, Princeton, NJ, United States
Abstract:
The South America subduction system is one of the largest subduction systems on the Earth. In 1994, Russo et al. revealed the anisotropy and strain field of the mantle beneath the Nazca Plate using shear-wave splitting data. Their results provide evidence for horizontal trench-parallel mantle flow beneath the subducting slab. Since then, many studies try to hunt for additional evidence or provide further explanations for the mantle flow from various perspectives. In addition, Russo et al. discovered a few anomalous locations with distinct shear-wave splitting patterns, attributed to the effects of slab morphology or slab tears. Another heated discussion focuses on the Peruvian and Pampean flat slabs. The Peruvian flat slab is one of the longest flat slab sections discovered to date, expanding over 1500 km in width and 200-300 km in the direction of subduction. Many theories have been proposed to explain the cause of flat slabs, yet none of them reached universal acceptance.
In our most recently published global adjoint tomography model, GLAD-M25, we examined South America and found structures that may provide explanations for some of the phenomena mentioned above. We observe evidence of horizontal trench-parallel flow in the form of slow anomalies right beneath the Peruvian flat slab in the upper mantle. This flow may provide the extra buoyancy that supports the Peruvian flat slab. The origin of this slow anomaly may be traced to the nearby Galapagos hotspot, indicating a different mantle flow direction than proposed in Russo et al. study. We discovered several tears in the slab, located at depths of 410 km and 660 km, which are well correlated with oceanic ridges on the Nazca Plate. Those tears may serve as a conduit that transfers mantle material from the Pacific side of the slab to the Atlantic side, which may modulate shear-wave splitting measurements, in accordance with Russo’s study.
Many fascinating questions remain to be addressed for this subduction system. However, our global model shows complex interactions between the subducted plate and the underlying mantle, providing a more comprehensive view at the whole mantle.
