Markedly different cooling histories for the hanging- and footwall of the Vari detachment on Syros and Tinos islands, Greece, are revealed by zircon and apatite fission-track data. The Vari/Akrotiri unit in the hangingwall cooled slowly at rates of 5–15 °C Myr−1 since Late Cretaceous times. Samples from the Cycladic blueschist unit in the footwall of the detachment on Tinos Island have a mean zircon fission-track age of 10.0 ± 1.0 Ma, which together with a published mean apatite fission-track age of 9.4 ± 0.5 Ma indicates rapid cooling at rates of at least ~ 60 °C Myr−1. We derive a minimum slip rate of ~ 6.5 km Myr−1 and a displacement of >~ 20 km and propose that the development of the detachment in the thermally softened magmatic arc aided fast displacement. Intra-arc extension accomplished the final ~ 6–9 km of exhumation of the Cycladic blueschists from ~ 60 km depth. The fast-slipping intra-arc detachments did not cause much exhumation, but were important for regional-scale extension and the formation of the Aegean Sea.