The transition along the strike of the Sunda subduction zone, from oceanic subduction in the west to subduction of continental Australian lithosphere in the east is envisioned as one of the canonical examples of the structural changes that take place within an overriding plate when a continental lithosphere wedge enters a subduction zone. Yet, the along-strike offset of the trench toward the Australian margin represents a structural response opposite to the predictions of numerical models. To understand this paradox, we analyse the morphotectonic evolution of Sumba island located at the transition from oceanic to continental Indo-Australian lithosphere subduction. Drainage evolution allows us to constrain the topographic evolution of the island since the Pliocene. Flights of uplifted coral reef terraces document Quaternary deformation. Focal mechanisms of shallow crustal earthquakes constrain the current stress field. Together, these data reveal that the island is affected by dextral en-échelon folding. Offshore, west of the island, reverse and strike-slip focal mechanisms evidence an active dextral transpressional zone. The emergence of the island and dextral shearing of the accretionary prism were triggered by subduction of the western lateral boundary of the Australian continental margin. We contend that the Plio-Quaternary tectonic evolution of the region, with transpression and migration of the trench toward the Australian margin is primarily dictated by shear stress transfer from the lower plate to the overriding plate, favored by strong interplate coupling, and by southwestward escape of the Savu-Sumba block following the impingement of the Australian continental margin against Timor island.