ULTRAFAST UPWELLING BURSTING THROUGH THE UPPER-MANTLE

A high-resolution calculation of strongly time-dependent thermal conve ction in the upper mantle with non-Newtonian, temperature-dependent rh eology shows that, for an effective Rayleigh number of around 10(6), e xtremely fast upwellings, at times exceeding 10 m/yr, can be generated a few hundred kilometers below the lithosphere. There is a clear sepa ration of timescales between this fast jet and the more slowly convect ing mantle. Within this fast vertical shear layer is embedded a therma l boundary layer with a width of the order of 50 km. The development o f the fast non-Newtonian upwelling is characterized by the growth of t he plume head to a large enough size, before the plume takes off rapid ly at a depth of around 350 km. Upon impinging the base of the lithosp here, this fast plume thins the lithosphere and the flow then becomes a horizontally moving hot sheet, extending out for around 1000 km. Thi s scenario is found to repeat itself at the same location about 10 Myr after the first plume impingement.