The Mount Irene Shear Zone (MISZ) in Fiordland, New Zealand, is a low-angle extensional mylonitic structure that has contributed to exhumation of a deep section of a Cretaceous magmatic arc. The dominantly metasedimentary hanging wall last equilibrated at ~5.9 kbar, 603¿C, whereas the metadioritic footwall, correlated with the Western Fiordland Orthogneiss (WFO), equilibrated at ~8.5 kbar, 722¿C, during retrogression associated with the MISZ. Displacement across the MISZ is also implied from differences in deformational episodes, metamorphic fabrics, and the presence/absence of partial melt products. A late stage synkinematic granitoid dike swarm intrudes the MISZ, but the dikes are invariably truncated and sheared within the basal calc-silicate mylonites of the hanging wall. U/Pb dates from one such dike demonstrate that juxtaposition of the footwall against the hanging wall was effectively complete by ~108 Ma. This date makes the MISZ the earliest Cretaceous extensional shear zone yet established in Fiordland and indicates that the switch from a convergent to an extensional tectonic regime on the paleo-Pacific arc margin of Gondwana occurred between 111 and 108 Ma. Therefore there was only 8 Myr between final intrusion and high-pressure metamorphism of the WFO and the initiation of extensional exhumation of the lower crust of this magmatic arc.