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Evolution of the Miocene Manantiales Basin, South-Central Andes: Evidence for Strongly Coupled Rapid Basin Subsidence and Exhumation


The Manantiales Basin of the south-central Andes (~ 32.15°S) is a retroarc foreland basin developed in front of the east-vergent La Ramada fold-thrust system in Miocene time. Recent geologic mapping, stratigraphic and structural studies and integrated geochronologic and thermochronologic analyses resolve aspects of basin stratigraphic architecture and provide critical constraints on the subsidence and exhumation patterns of the basin. Manantiales Basin strata have traditionally been assigned to the Chinches Formation, including a basal succession referred to as the Chocolate Arenites. New zircon U/Pb geochronologic constraints demonstrate these basal strata are Eocene in age, and represent an older stratigraphic succession. The overlying Chinches Formation contains >3000 m of coarse clastic fluvial strata and intercalated lacustrine deposits. The initiation of subsidence associated with Chinches deposition is firmly pinned to Early Miocene (16.84+/-0.23 Ma) by zircon U/Pb from a welded lapilli tuff that directly overlies rhyolite of the subjacent Choiyoi Group. Previous workers have used a Maximum Depositional Age (MDA) to suggest subsidence began at ~22 Ma. However, the recognition of a basal ~17 Ma unconformity and the presence of ~21-22 Ma zircon in the basal welded tuff suggest this MDA is not a valid depositional age. Zircon fission track from volcanic tuff and magnetostratigraphic constraints suggest the uppermost strata in the Chinches Formation is ~10 Ma. Available chronologic constraints suggest a minimum subsidence rate of >400 m/Ma. However, new apatite (U-Th/He) thermochronology suggest that the original stratigraphic thickness is ~5000 m thick, implying a significantly higher subsidence rate. Apatite (U-Th/He) thermochronology from both the Eocene succession and the Miocene Chinches Formation suggest the Eocene strata and the lower ~2000 m of the Chinches Formation reached temperatures high enough to reset the apatite thermochronometer prior to ~9 Ma, implying stratigraphic or structural burial to at least ~3 km by this time. The entire stratigraphic succession underwent rapid exhumation between 9-7 Ma, presumably in response to propagation of the La Ramada thrust system to the east. These data imply exceptionally high subsidence rates along the leading edge of the fold and thrust belt beginning in the late early Miocene, followed by rapid basin inversion and cannibalization in late Miocene time.