Timing and thermal evolution of fold-and-thrust belt formation in the Ultima Esperanza District, 51°S Chile: Constraints from K-Ar dating and illite characterization

Abstract

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.