Radiogenic trigger for rifting and break-up of the continental lithosphere

Subduction of oceanic and continental crust during the closure of oceans frequently results in refertilization and enrichment of the orogenic, continental, lithospheric mantle in radioactive elements (Uranium and Thorium). Our results of conductive-convective thermal modelling indicate that subduction-/collision-related increased content of heat-producing elements causes a positive thermal anomaly, the magnitude of which increases with time as a result of the decay of radioactive elements. In this case, weakening of the lithosphere due to this thermal anomaly increase with time as well, providing favourable conditions for rifting at scales of million years (more than 50-200 Ma) after the ocean closure. Moreover, mantle upwelling flows have a tendency to move with time towards the thermally anomalous area of the lithospheric mantle. This migration of mantle upwellings can be responsible for an extensional stress regime that theoretically can trigger the break-up of the continental lithosphere in the place where it is thermally weakened. Thus, we propose a novel idea of a generally neglected or underestimated mechanism for weakening the continental lithosphere and allowing it to rift. Importantly, our thermal mechanism is time-dependent, assuming that high heat-producing lithospheric mantle, formed during ocean closure, weakens the lithosphere thermally and influences mantle-convection on a hundred-million-year-time scale, thus determining the location of rifting and providing an extensional driving force for the continental break-up. Besides, our results explain why rifts built on former orogens and continental break-up occurred along or near the former suture zones not immediately following orogeny but with a certain time delay. This time delay is dependent on the size of the anomalous lithospheric mantle block and/or concentration of the heat-producing elements in it.