How an ultra-low velocity zone in the top asthenosphere deflects mantle flow around the subduction zone: Insights into the evolution of congested plate margins

Roberta Carluccio

The ingestion of buoyant material embedded in the oceanic plate is a characteristic feature of many congested subduction zones. At these margins, it is the competition between positive and negative buoyancy that results in emergent complex structures. In this study, we report a striking spatial coincidence with the presence of a relict oceanic plateau (OPl) and observations of an ultra-low-velocity zone (ULVz) in the uppermost asthenosphere along some segments of the Circum Pacific Belt. We compile these observations and employ three-dimensional subduction models to explore the influence of each of these factors (ULVz and OPl), in isolation and together, on deformation patterns of slab, trench migration, and seismic anisotropies around the subduction zone. Our results show that the presence of a large and uneven ULVz underneath the base of an oceanic lithosphere decouples the motion of the tectonic plates above from the flow in the mantle beneath resulting in slab and trench asymmetric deformation, and plate motion parallel flow in the subslab. Conversely, when an ULVz is not modelled, a larger degree of plate-mantle coupling reflects in the trend for the slab and trench to roll back and retreat, respectively, whilst the flow in the slab behind the trench aligns parallel to the direction of trench motion. If both an ULVz and OPl are considered more complex geological structure can emerge. Asymmetric trench and slab deformations and seismic anisotropy signatures can be obtained considering along trench variation in ULVz and/or OPl width – both having the capacity to localise strike-slip deformation. A similar setting, with asymmetric patterns of subslab seismic anisotropies, and slab and trench migrations styles can be found in subduction regions such as the Cocos-Nazca and Hikurangi- Kermadec among others, where plates move faster than their trenches retreat, consistent with geophysical and geological observations of an ULVz and OPl.

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