Dual structure of poloidal and toroidal flow under the Cocos subduction zone

•Whether poloidal or toroidal flow dominates the sub-slab mantle is hotly debated.•We present evidence that they coexist under the Cocos subduction zone.•The entrained, poloidal layer suggests coupling across the lithosphere-asthenosphere boundary. How the underlying mantle responds to the subductio...

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Bibliographic Details
Published in:Earth and planetary science letters 2021-07, Vol.565, p.116911, Article 116911
Main Authors: Peng, Cheng-Chien, Kuo, Ban-Yuan, Tan, Eh
Format: Article
Language:English
Subjects:
asthenosphere
Cocos subduction zone
mantle flow
seismic anisotropy
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Summary:•Whether poloidal or toroidal flow dominates the sub-slab mantle is hotly debated.•We present evidence that they coexist under the Cocos subduction zone.•The entrained, poloidal layer suggests coupling across the lithosphere-asthenosphere boundary. How the underlying mantle responds to the subduction of an oceanic plate remains controversial. Numerical models demonstrate the presence of 3D flow in the sub-slab mantle with a poloidal component induced by slab entrainment and a toroidal component driven by slab rollback. These flows produce plate–motion–parallel and trench-parallel fast directions of seismic anisotropy, respectively, allowing them to be distinguished via the anisotropy pattern. However, seismology studies have so far documented the dominance of either, but not both, type of flow in most subduction zones, evoking debates concerning the flow mechanisms. We employed source-side anisotropy technique to map the sub–slab mantle of the Cocos subduction zone. The observed anisotropy pattern can be best explained by a plate–motion–parallel anisotropy layer, roughly 130 km thick, underlain by a trench-oblique anisotropy regime. We interpret this phenomenon as an entrained poloidal flow transitioning to a rollback–induced toroidal flow modified to suit the regional tectonics. This finding reconciles the current debate over which flow prevails behind a subduction zone. The upper, entrained layer in our model extends below the lithosphere–asthenosphere boundary previously defined in the vicinity of the middle America trench, suggesting strong coupling along the lithosphere–asthenosphere boundary in this region.
ISSN:0012-821X
1385-013X
DOI:10.1016/j.epsl.2021.116911