Exhumation of the Precordillera and northern Sierras Pampeanas and along-strike correlation of the Andean orogenic front, northwestern Argentina

In NW Argentina (∼26°S), adjacent to the Puna Plateau of the central Andes, exhumation and deformation propagated from west to east across the Puna Plateau and Eastern Cordillera during early-late Cenozoic time. Presently existing data do not indicate a clear eastward younging trend in exhumation in the Precordillera and northern Sierras Pampeanas at the southeastern flank of the Puna Plateau at ∼28°S. In this study, we mapped an ∼80-km-wide transect at a latitude of ∼28°S in the Sierra de Las Planchadas and the Fiambalá Basin. Apatite fission-track (AFT) ages from six samples from thrust fault hanging walls are between 20 and 14 Ma, and apatite helium (U-Th)/He ages from five samples range from 21 Ma west of the Sierra de Las Planchadas to 2 Ma in the Fiambalá Basin. Several samples record mixed apatite (U-Th)/He ages and pre-Cenozoic AFT ages, indicating partial resetting through the 120-60 °C temperature window and suggesting a depth of exhumation between ∼6 and 3 km. Thermal modeling of AFT and (U-Th)/He ages indicates cooling from ca. 22 Ma until 2 Ma and a younging trend to the east. These ages are consistent with previously published AFT ages in the Fiambalá region and AHe ages in the Eastern Cordillera to the northeast and suggest Miocene exhumation and deformation in the Precordillera at 28°S and the Eastern Cordillera at 26°S. From a kinematic standpoint, the region of the Precordillera at 28°S may be considered a continuation to the south of the Eastern Cordillera at 26°S, implying a single continuous SW-NE-striking deformation front along this portion of the central Andes. Detrital zircon U-Pb ages from Upper Miocene strata in the Fiambalá Basin suggest that Permian-Miocene rocks in the Sierra de Las Planchadas are the primary source of the Miocene basin fill. This is consistent with previously published eastward-directed paleocurrent data and other provenance proxies and indicates that uplift of the Sierra de Las Planchadas began no later than ca. 9 Ma.