We analyzed refraction and reflection seismic data covering the Central Lau Spreading Center (CLSC) and the Tonga volcanic arc at 18 °S to investigate tectonic inactivity in actively extending back-arc systems. Our P-wave travel time tomography shows a characteristic 6-8 km thick back-arc crust beneath and around the CLSC and a ~13 km thick arc-crust at the eastern end of our profie. Lower crustal velocities increase to 7.5 km/s towards the volcanic arc, marking the change from back-arc to arc crust. These high P-wave velocities can be explained by a high pyroxene continent in the lower crust originating from depleted metls. Our seismic reflection data show steep normal faults below the CLSC and volcanic structures closer to the volcanic arc, without a tectonically active zone (also knows as the ‘diffuse plate boundary’) in between, hence the Niuafo’ou and Tonga microplates should be treated as one tectonic plate with local zones that deform, but that are not fully developed plate boundaries. We build on the concept that cold, depleted mantle material may invade the back-arc domain, and argue that this invading mantle material cools the overriding plate with increasing distance from the trench leading to 1) relative strengthening of the overlying back-arc crust, b) relocalisation of extension and c) tectonic quiescence in previously active extending parts of the back-arc domain.