Modulation of intra-oceanic trench bending and along-trench thermochemical transport by mantle toroidal flow

Oceanic trench bending over 1000 km occurs globally, with notable along-trench variations in mantle wedge thermochemistry. However, the mechanisms influencing both shallow trench geometry and deep mantle state remain unclear. Here we focus on the Mariana Trench as a representative case. Our 3D model successfully replicates its curved geometry. Modeling results further reveal that extensive intra-oceanic trench retreat and bending occur when the incoming plate features sharp density variations along the trench accompanied by a large buoyant segment (e.g., the Caroline Plate). In this scenario, significant dynamic pressure fluctuations develop in the mantle wedge, generating vigorous toroidal flow extending >1000 km from the buoyant segment, even under non-linear mantle rheology. The model-predicted toroidal flow transports warm mantle materials, reshaping local topography, rejuvenate the mantle wedge, and heating the slab surface to facilitate volatile release. These experiments highlight toroidal flow as a key regular of both large-scale trench bending and along-trench thermochemical variations.