STRATIGRAPHIC ANALYSIS OF THE NEOGENE CACHEUTA BASIN: A RECORD OF OROGENIC EXHUMATION AND BASIN INVERSION IN THE SOUTH CENTRAL ANDES

Neogene synorogenic strata in the south-central Andes (28-35S) provide a sensitive record of the structural evolution of the Andean orogenic system and may be utilized to constrain the chronology of deformation and pattern of orogenic exhumation. The precise succession of thrust belt propagation and foreland basin development is a matter of debate. Analysis of the Cacheuta basin at the latitude of Mendoza, Argentina (33S) provides critical constraints on basin evolution and orogenic exhumation.

The Cacheuta basin is subdivided into five distinct formations [Divisadero Largo, Mariño, La Pilona, Tobas Angostura and Río de los Pozos] that reflect episodic sediment flux produced during eastward thrust belt propagation. Stratigraphic analysis, U/Pb geochronology, conglomerate clast counts, and (U-Th)/He analyses constrain basin evolution and subsequent inversion. A ca. 17.9 Ma U/Pb age from a volcanic tuff near the base of the succession constrains initial basin subsidence to >18 Ma, and suggests that previous magnetostratigraphic age constraints require revision. Provenance analysis identifies episodic sediment flux from the Cordillera Costal (JuraCretaceous zircon), Cordillera Principal (distinctive Jurassic conglomerate, Cretaceous fossiliferous limestone, and Tertiary hornblende andesite clasts) and the Cordillera Frontal (PermoTriassic clasts and detrital zircon). Results from the overlapping U-Pb crystallization ages and (U-Th)/He ages on detrital zircon confirm that the Permo-Triassic Choiyoi Group of the Cordillera Frontal has remained at upper crustal levels (<6 km) since emplacement. Evidence of sediment input to the Cacheuta basin from the Precordillera is equivocal at this time.

Preliminary detrital apatite (U-Th)/He analyses suggest an estimated cooling age of approximately 6 Ma, which is interpreted to reflect basin uplift and inversion in the late Miocene.