LARGE-SCALE STRESS AND STRAIN PATTERNS CAUSED BY OCEANIC SUBDUCTION ALONG A SEAWARD-CONCAVE PLATE BOUNDARY: INSIGHTS FROM 2D NUMERICAL AND 3D THERMO-MECHANICAL LABORATORY EXPERIMENTS AND APPLICATION TO THE CENTRAL ANDES

The influence of plate boundary curvature on the large-scale stress and strain patterns caused by oceanic subduction in the overriding plate is explored using numerical and analogue modeling techniques. Numerical experiments reveal that trench-parallel compression is produced near the symmetry axis of a seaward-concave plate boundary if interplate friction is high or the subducting lithosphere has a low flexural rigidity. Trench-parallel compression is reduced along the oblique parts of the plate boundary. Both the stress conditions on the interplate zone and its 3D geometry control whether the sign of the trench-parallel stress in the centre of the curvature. Low dip angle and high convergence obliquity favor trench-parallel compression. In the central Andes, minor N-S shortening in the centre of the curvature and paleomagnetic rotations suggest that shear traction dominated during Cenozoic time. Laboratory experiments of oceanic subduction along a seaward-concave plate boundary investigate the large-scale 3D deformation pattern in the upper plate. Model deformation was restricted to the fore-arc domain because high friction was only imposed in the upper part of the interplate zone. Nevertheless, the large-scale deformation pattern shows characteristics, which fit observations in the Andes. Along the oblique section of the plate boundary, subduction produces trench-parallel shearing of the fore-arc towards the centre of the curvature, significant trench-perpendicular normal shortening but little to no trench-parallel normal shortening. Fore-arc deformation near the centre of the curvature includes the largest trench-perpendicular normal shortening, significant trench-parallel normal shortening but no trench-parallel shearing. Furthermore, trench-perpendicular normal shortening is shifted inland because of the trench-parallel normal shortening occurring in the frontal part of the fore-arc. This pattern is characteristic of the 3D deformation along seaward-concave plate boundary caused by to high interplate friction. It may extend to the arc/back-arc if the high interplate friction is extended further down the interplate zone.