A series of 3-D asthenospheric-scale analogue models have been conducted in the laboratory in order to simulate the arrival of a spreading ridge at the trench and understand its effect on plate kinematics, slab geometry, and on the deformation of the overriding plate. These models are made of a two-layered linearly viscous system simulating the lithosphere and asthenosphere. We reproduce the progressive decrease in thickness of the oceanic lithosphere at the trench. We measure plate kinematics, slab geometry and upper plate deformation. Our experiments reveal that the subduction of a thinning plate beneath a freely moving overriding continent favors a decrease of the subduction velocity and an increase of the oceanic slab dip. When the upper plate motion is imposed by lateral boundary conditions, the evolution of the subducting plate geometry largely differs depending on the velocity of the overriding plate: the larger its trenchward velocity, the smaller the superficial dip of the oceanic slab. A slab flattening episode may occur resulting from the combined effect of the subduction of an increasingly thinner plate and the trenchward motion of a fast overriding plate. Slab flattening would be marked by an increase of the distance between the trench and the volcanic arc in nature. This phenomenon may explain the reported Neogene eastward motion of the volcanic arc in the Southern Patagonian that occurred prior to the subduction of the Chile Ridge.