Trench-parallel shortening in the forearc caused by subduction along a seaward-concave plate boundary: Insights from analogue modelling experiments

• Seaward-concave trench can generate trench-parallel shortening in forearc.
• Shortening is located in forearc near symmetry axis of plate boundary.
• Trench-parallel shortening only between frontal cordillera and trench.

Three-dimensional thermo-mechanical analogue experiments are employed to test the hypothesis that oceanic subduction along a seaward-concave plate boundary can generate trench-parallel shortening in the forearc near the axis of curvature. The model deformation is analyzed with a Particle Imaging Velocimetry (PIV) system that allows for comparison of forearc deformation along the oblique limbs of the curved plate boundary and near the axis of curvature. Moreover, PIV allows for separation of the trench-parallel and trench-perpendicular components of strain, regardless of trench orientation. The resulting deformation maps show a remarkable symmetry and indicate drag of the forearc above the interplate coupling area towards the axis of curvature. Trench-perpendicular profiles show that along the oblique limbs of the plate boundary, the forearc is submitted to trench-normal shortening and trench-parallel shearing but not trench-parallel shortening or extension. This contrasts with the situation near the axis of symmetry where the forearc is submitted to trench-parallel and trench-perpendicular normal shortening, but is not sheared. The experimental results confirm that trench-normal thrusts observed in the fore-arc of the Central-Andes can be a mechanical consequence of subduction along a seaward-concave plate boundary if the degree of interplate coupling is large.