Description: Obliquely convergent subduction margins commonly exhibit strain partitioning leading to trench-parallel material transport in the forearc. If deformation is localised, large margin-parallel strike-slip faults may form (e.g. in Peru, North-Chile or Sumatra). These faults are typically located far arcward from the deformation front (100-300km) in the internal part of the forearc. To date sandbox experiments, however, show strain partitioning in the deforming accretionary prism only. We therefore designed a series of scaled 3D sandbox experiments to monitor the deformation of the rearward region of the forming wedge. We further test how deformation is dependent on modes of shear force transmission at the base of the wedge. The latter is modelled by shifting the S-line relative to the deformation front. The S-line is the location on the plate interface landward of which basal shear forces are not further transmitted into the overlying wedge. Such a singularity is inescapable in analogue or numerical modelling and may have its natural analogy in the line of intersection (transition zone) of the subducting slab with some brittle-ductile boundary in the continental wedge. Analysis of the 3D surface displacement using an image correlation technique (PIV: particle imaging velocimetry) is used to evaluate material transport and to differentiate kinematic domains on the forming sand wedge. First results for a setting with shear force being transmitted along the entire backstop base show that most of the oblique convergence is accommodated in the fore- and back-thrust system. With proceeding convergence, a fault system evolves on the arcward side of the accretionary wedge, formed by the surface traces of back-thrusts. The main back-thrust is reactivated with a noticeable strike-slip component at the end of each fore-thrust forming cycle. Finally, the back-thrust becomes inactive and a new, more favorably oriented fault is formed in the backstop. This shows that the seemingly decoupled rearward region of the forearc system accommodates a trench parallel displacement component.------------------------