K/Ar dating on llite in Upper Cretaceous low-grade metamorphic pelites in the Torres del Paine area was used to set new time constraints on the development of the Patagonian retroarc fold-and-thrust belt (FTB) caused by the subduction of the Antarctic Plate beneath the South American Plate. The combined use of illite crystallinity (Kübler Index), polytype quantification and K/Ar dating of illite fractions (<0.2, <2 and 2-6 µm) allowed to distinguish four distinct periods of illite growth based on their K/Ar ages and degree of regional metamorphism: (1) early Cenomanian (98 Ma) illite crystallization, (2) widespread early Campanian (ca. 80 Ma) diagenetic illite growth under anchizonal metamorphic conditions, (3) a significant period of illite formation in the early Paleocene (ca. 60 Ma), and (4) a late stage of illite growth in the early Eocene (55-46 Ma) under epizonal conditions. The earliest indication for the emergent FTB formation in the hinterland is documented in a metapelitic clast (14-9, <2 µm) within the Upper Cretaceous Cerro Toro conglomerate which yields a K/Ar cooling age of 98.3±1.2 Ma and an epizonal KI value of 0.24 ∆°2Θ. After a certain period of geological quietness an interval of major thrusting and uplift occurred between ca. 60 and 46 Ma. The east dipping Rio Nutria and Rio Rincon thrusts record the onset of thrust and fold activity which can be placed close to 60 Ma. They also mark the frontal thrust towards the less deformed Magallanes foreland basin. In the western part of the internal domain, widespread fault and thrust activity of the frontal wedge and associated thermal overprint continued and is recorded until 46 Ma by K/Ar illite cooling ages. The flexural subsidence that is driven by the thrust sheet loading in the internal domain was responsible for the eastward migration of the foreland depocenter and the rapid increase of sedimentation rate along the monoclinal belt. No Miocene thrusting nor uplift event has been recorded by K/Ar illite dating in the study area.