K-Ar dating on synkinematically formed illite in low-grade metamorphic pelites was used to set new time constraints on the development of the Patagonian retroarc fold-and-thrust belt 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) is a powerful tool in constraining the missing time gap between the emergent fold-and-thrust belt formation and early Miocene uplift of the Magallanes Basin. Four distinct periods of illite growth were identified 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) long-lasting burial illitization processes 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. Based on K-Ar fine fraction ages and Kübler index values, we propose to subdivide the internal domain along the Río Nutria and Río Rincon thrust into a thrust zone with maximum rock uplift in the west and a foreland-vergent imbricate thrust zone in the east.

The earliest indication for the emergent fold-and-thrust belt formation in the hinterland is documented in a metapelitic clast (sample 14-9, <2 µm) within the Upper Cretaceous Cerro Toro conglomerate, which yielded a K-Ar cooling age of 98.3 ± 1.2 Ma and an epizonal Kübler index value of 0.24 ∆°2Ɵ. After a certain period of geological quiescence, an interval of major thrusting and rock uplift occurred between ca. 60 and 46 Ma. The west-dipping Río Nutria and Río Rincon thrusts record the onset of fold-and-thrust belt activity, which can be placed close to 60 Ma. In the western part of the internal domain, thrusts remained active, and associated structural uplift is recorded until 46 Ma by K-Ar illite cooling ages. Flexural subsidence driven by thrust sheet loading in the internal domain was responsible for the eastward migration of the foreland depocenter and a rapid increase in sedimentation rate along the monoclinal belt.

Our results prove a synchronous onset of thick-skinned thrusting between 49°S and 55°S and suggest constant convergence rates of the Patagonian arc against the adjacent South American continental margin. Time-averaged exhumation rates along the Río Rincon anticline (54.6−22 Ma) and along the Río Nutria thrust (46.5−22 Ma) suggest rather low exhumation rates of 1.6−1.2 mm/yr (maximum). The low exhumation rates link back to constantly low subduction rates, resulting in a period of geological quiescence between 46.5 and 22 Ma.

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