Diagram showing high velocity slabs in the lower mantle beneath South America's present position and profile showing the westward motion of South America.
Panel (a) High velocity slabs in the lower mantle, deeper than 900 km: Central America (CAM), Western Cordillera (WC), and Trans-Andean (TA) beneath South America's present position (gray color). These slabs may be sunken remnants of an intra-oceanic, west-dipping subduction (black barbed line) when South America was 1500 km away (light green) next to Africa at 140 Ma. Panel (b) Profile along the red line in (a) showing the westward motion of South America and the flip from west- to east-dipping around 85 Ma when the continent collides with the island arc. Slabs in the transition zone (between 410 and 670 km) get folded and sink at about 10 mm/yr. Credit: Mohammadzaheri et al. [2021], Figure 16
Source: Journal of Geophysical Research: Solid Earth

High velocity slabs deeper than 1,000 kilometers have been imaged beneath the Amazon by various tomographic studies and have been interpreted as a continuation of the present Nazca slab. Mohammadzaheri et al. [2021] propose a new interpretation of these slab pieces deeper than about 900 kilometers. Geodynamic and plate reconstruction analyses of a new global P-wave tomography model (DETOX-P1, based on both travel time data and multi-frequency waveform picks) suggest that these 900-1800 kilometer deep high-velocity anomalies are actually remnants of a west-dipping intra-oceanic subduction zone during late Jurassic and Early Cretaceous times when South America’s paleo-position was near Africa, before the start of the present, east-dipping Andean subduction around 85 million years ago. This gives support to the hypothesis that slabs in the lower mantle sink vertically with implications on models of plate motion reconstructions.

Citation: Mohammadzaheri, A., Sigloch, K., Hosseini, K., & Mihalynuk, M. G. [2021]. Subducted lithosphere under South America from multifrequency P wave tomography. Journal of Geophysical Research: Solid Earth, 126, e2020JB020704. https://doi.org/10.1029/2020JB020704

—Marcelo Assumpção, Associate Editor, JGR: Solid Earth

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