Polyphasic deformation during the evolution of the southern Central Andes: Insights from the La Ramada fold-thrust belt (32°-32°30′S)

Abstract

This work presents an update of the evolution of the La Ramada fold-thrust belt (~32◦S-32◦30′S), located in a remote area of the Principal Cordillera of Argentina, integrating two measured stratigraphic sections, novel field observations, and three new zircon U–Pb ages: two detrital and one igneous. Our new stratigraphic framework includes Paleocene to Miocene sedimentary and volcanic rocks, a period previously interpreted as a hiatus in the area. This sheds light on an enigmatic stage in the evolution of the Andes at these latitudes. We describe the Teatinos Formation, a sedimentary unit with a maximum depositional age of 11.2 ± 0.2 Ma at its base. These rocks were deposited in the Teatinos Basin, a newly defined piggyback basin developed within the Principal Cordillera, representing the innermost expression of the Miocene Andean foreland basin system at these latitudes. A new U–Pb crystallization age of 19.1 ± 0.5 Ma, obtained from a sill intruding ~900 m of volcanic rocks, confirms the presence of the late Eocene - early Miocene Abanico Formation in an anomalous position, up to 5 km east of the eastern boundary of the Abanico Basin. This boundary is represented in our area by the Major Andean Fault, a first-order structure that juxtaposes rocks of the western Principal Cordillera over the Teatinos Formation. We also present a new maximum depositional age of 145.2 ± 1.5 Ma from sedimentary rocks in the hanging wall of this fault. These are overlain by the Abanico Formation, defining a > 100 Myr hiatus compatible with the existence of a pre-Oligocene contractional event. We propose a polyphasic evolution for the La Ramada fold-thrust belt involving at least three contractional pulses: one pre-Oligocene and two during the Miocene (ca. 22–18 Ma and ca. 12–9 Ma).