Effects of kinematic boundary conditions on trench curvature in a retreating subduction zone: insights from analog modelling

Subduction trench curvature is generally attributed to the Earth's sphericity. Although, observed trench curvature in Nature are usually larger than those imposed by Earth radius. Other factors potentially influencing curvature are the mantle flow (produced by some other geodynamic process), internal heterogeneities in the subducting plate (e.g. variations in plate age and consequently in its thickness), the presence of crustal features (e.g. seamounts or plateaus), the orientation of the main plate structures (parallel/perpendicular to the trench) and the geometry of the upper plate. The role of a weak lithosphere on a lateral side perpendicular to the strike of the retreating subduction, e.g. continent/transfer zones, is unexplored. We study the influence of this boundary condition in a retreating subduction setting by varying its kinematic nature using analog modelling. A continental block is considered in one side of the plate, and the kinematic nature of the contact with the oceanic plate is varied considering: 1) fully attached, 2) half attached, and 3) completely free. To first order, lateral boundary perpendicular to the trench introduces an asymmetry in the trench curvature. The magnitude of this asymmetry is proportional to the resistance of the transfer zone. This kinematic resistance slows down the subduction of the oceanic plate near the contact and enhances the subduction in the free lateral side, thus affecting the retreat velocities along the trench. This work has implications for present and geologic past subduction zones where asymmetric trench and laterally varying retreat velocities have been observed/postulated. This is a SUBITOP (674899-SUBITOP-H2020-MSCA-ITN-2015) contribution.