We investigate the conditions of hydrothermal alteration occurring along an active, seismogenic normal fault dipping <30¿ in the Moresby rift (Woodlark basin, Papua New Guinea). Petrological and fluid inclusion studies were conducted in the samples recovered from the Ocean Drilling Program leg 180 that cored the footwall, scarp, and hanging wall of the fault. Chemical analyses of the thermodependent Si-Al substitutions in chlorites show that synrift alteration is coeval with a temperature rise from 200¿C to 300¿C in the footwall of the detachment, whereas no thermal event is recorded in the hanging wall. In the same samples, fluid inclusions indicate that footwall rocks followed an isothermal exhumation path from 5--7 km to the subsurface. Taken together, the results strongly suggest that the +100¿C temperature rise results from the upward migration of H2O-NaCl fluids expelled by dehydrating metamorphic rocks from the lower crust. Fluid release began with the onset of detachment faulting and is still actively occurring nowadays. The fact that low-angle earthquakes also occur now at 5--7 km along the fault suggests that the dehydration of the metamorphic rocks and the subsequent circulation of hot hydrothermal fluids may be responsible for the weakening of the middle crust and the formation of a detachment in the Moresby rift.