We report the discovery of a detachment fault within reach of structural investigation in a shield volcano (Piton des Neiges, La Réunion hotspot). The detachment, exposed by erosion, is located between the top of an extinct gabbroic magma chamber and a cover of debris avalanche deposits. The fault simultaneously concentrated ductile and brittle deformation toward the sea, hydrothermal alteration in the greenschist facies, and repeated sill injections of basic magmas. Our study of this exceptional detachment exposure suggests that two mechanisms participate in volcano flank failure: hydrothermal alteration, likely sustained by the heat of cooling sills, induces slow inter-intrusion deformation, whereas sill injections in the fault may trigger rapid cointrusive slip. This twofold process of destabilization might currently occur on some active volcanoes such as Piton de la Fournaise.