Fluids in the upper crust can affect the strength properties, composition, and mineralization of rock units, but the source(s) of these fluids and their transport during deformation are not well understood. Stable isotopic studies of clay-rich rocks, particularly newly formed illitic clays, shed light on the source of geofluids involved during faulting and folding. Hydrogen isotopic (δD) measurements of fault gouge and folded clay-rich sediments from the Argentine Precordillera show meteoric, surface-sourced fluid involvement during Miocene faulting. The δD composition of neoformed clay minerals is between −70‰ and −90‰ ± 2‰. The associated composition of fluids ranges from −57‰ to −72‰ ± 10‰, matching values for a surface-derived source. Regional Miocene meteoric fluid infiltrated down into the evolving fold-and-thrust belt, accumulated in basin sediments, and was subsequently expelled by the overriding rock units during late Miocene shortening. A systematic increase in δD signature with deformation age is preserved that matches regional climatic changes during late Miocene aridification, cooling, and glacial expansion.

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