The SE Asian region is characterised by the active subduction of Cenozoic marginal basins. A new tomographic model interpreted in the light of geological data, provides details on the geometry of the oceanic lithospheric slabs subducted along the eastern margin of Sundaland. We present first evidences for a relatively continuous active margin which have extended from Taiwan to Java before the collision of the Banda bloc with Sundaland during the Miocene. The north dipping slab below Timor is interpreted as the result of the new subduction zone following this collision and connecting westward with the present Java-Sumatra trench. We also try to estimate the amount of shortening within the Sunda block, which was mainly accommodated by subduction of the SE Asia marginal basins, formerly opened within this part of Eurasia during the Palaeogene. Closure of the Sulu and Celebes basins is recent, whereas the subduction of the Proto-South China Sea is marked by a 300 km long slab below Borneo, but also eastward by a large velocity anomaly interpreted as a detached slab fragment of this basin sunk into the mantle.