Using silicone slabs as a model analogue for lithospheric plates subducting into a box of glucose syrup, as an analogue of the mantle, we explore the subduction of continental lithosphere in a context of intercontinental collision. The continental indenter pushed by a piston, reproducing the collision, attached to a dense oceanic plate, subducts to two-thirds of the depth of the mantle box. We show that, surprisingly, the continental plate attached to the back wall of the box subducts, even if not attached to a dense oceanic slab. The engine of this subduction is not the weight of the slab, because the slab is lighter than the mantle, but the motion of the piston, which generates horizontal tectonic forces. These are transmitted to the back wall plate through the indenter and the upper plate at the surface, and by the advancing indenter slab through the mantle at shallow depth. We define this process as collisional subduction occurring in a compressional context. The collisional subduction absorbs between 14% and 20% of the convergence, and represents an unexplored component of collisional mass balance. The transmission of tectonic forces far from the collision front favors the formation of a wide plateau. Our experiments reproduce adequately the amount and geometry of the Asian lithosphere subduction episodes inferred during the collision, leading us to conclude that it reproduces adequately the physics of such process.

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