Mantle transition zone variations across Central South America
Dotzer, Alec S.
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Seismic velocity discontinuities beneath South America have been imaged between 3◦N and 38◦S latitude using both the receiver function (RF) method and the SS bounce point function method (SdS). We analyzed 5,499 Ps RFs and 1,696 Sp RFs from 181 broadband stations using seismic data collected over approximately two decades to image the Moho, LAB, 410 km discontinuity (410d), 660 km discontinuity (660d), and subducting Nazca slab. An additional 2,850 SdS functions recorded by the US transportable array were used to fill in data gaps where RF stations had not been deployed. We developed a modified H-k method that traces the moveout curves for converted phases (Ps, PPs, and PSs) to modify the GyPSuM velocity model to generate a local 3D velocity model for depth conversions. We focus our interpretations along an EW transect of central South America at -20◦ latitude from approximately -35◦ to -74◦ longitude. The top of the Nazca slab appears to penetrate through the 410d boundary around 55◦W longitude. The subduction angle of the slab increases near 50◦W longitude and later flattens beneath southeast Brazil at a depth of approximately 588 km. An increase in mantle transition zone (MTZ) thickness of approximately 30 km is observed within the subduction zone region and is directly related to the cold Nazca slab penetrating into the MTZ. We identify 22 km of MTZ thinning beneath eastern Brazil near a low-velocity anomaly that previous work interpreted as the fossil conduit of the Tristan da Cunha Plume. The temperature anomaly required to cause the change in MTZ thickness in the region of this low-velocity anomaly zone is 140 to 251◦C. This inferred temperature anomaly associated with the change in MTZ thickness implies that the low velocity anomaly here is primarily thermal in origin and not chemical as previously implied. We believe this low-velocity anomaly is caused by localized upward convection that breaks through the subducting slab. Additional discontinues are observed at depths of approximately 300 and 520 km.