Fore-arc deformation at the transition between collision and subduction: Insights from 3-D thermomechanical laboratory experiments

Three-dimensional thermomechanical laboratory experiments of arc-continent collision investigate the deformation of the fore arc at the transition between collision and subduction. The deformation of the plates in the collision area propagates into the subduction-collision transition zone via along-strike coupling of the neighboring segments of the plate boundary. In our experiments, the largest along-strike gradient of trench-perpendicular compression does not produce sufficiently localized shear strain in the transition zone to form a strike-slip system because of the fast propagation of arc lithosphere failure. Deformation is continuous along-strike, but the deformation mechanism is three-dimensional. Progressive along-strike structural variations arise because coupling between neighboring segments induces either advanced or delayed failure of the arc lithosphere and passive margin. The modeling results suggest that orogenic belts should experience deeper subduction of continental crust and hence higher-pressure metamorphism where the two plates first collided than elsewhere along the plate boundary where collision subsequently propagated. Furthermore, during the initial stage of collision the accretionary wedge is partially subducted, which leads to lubrication of the interplate zone and a reduction of shear traction. Therefore, a large convergence obliquity angle does not produce a migrating fore-arc sliver. Rather, the pressure force generated by subduction of the buoyant continental crust causes fore-arc motion. It follows that convergence obliquity during collision does not yield trench-parallel deformation of the fore arc and its influence on the collision process is limited. However, convergence obliquity may control the geometry of the active margin during the oceanic subduction stage prior to collision, and inherited structures may influence the propagation mechanism.