Back-arc rifting initiated with a hot and wet continental lithosphere

How back-arc rifting is initiated is studied by inferring the tectonic force and strain rate regimes from published subsidence data for nine back-arc basins in the European Tethyan belt and in the western Pacific. The results show that rifting only begins after some particular magnitude of tectonic force is reached. This implies that subduction has already progressed to a point such that the total negative buoyancy of the slab becomes so large that the relevant tectonic force exceeds its bending strength. Once back-arc rifting has been initiated, the force required to maintain it gradually decays. This indicates that the processes governing back-arc extension are fast enough that lithosphere weakening due to an increase in geothermal gradient exceeds strengthening due to the thermal relaxation and crustal thinning. This is achieved, given currently accepted estimates for the maximum force (= 7 TN/m), if the thickness of the thermal lithosphere is significantly less than 125 km and if the lithosphere has a wet rheology, meaning that a weak continental lithosphere is required for the initiation of back-arc rifting. The presence of significant strength in the uppermost mantle is not precluded, but it is not required.