Tectonic Setting of the Tordillo Formation in the Aconcagua Fold-and-Thrust Belt

Abstract

At the northwestern Mendoza province, the Mesozoic infill of the Neuquén Basin is tectonically repeated in the Aconcagua fold-and-thrust belt. Particularly, the Tordillo Formation (commonly associated with the Kimmeridgian) represents a local low stand period of sea level, with mainly alluvial and fluvial sediments. Toward the western sector of the belt, it interfingers with volcanic and volcaniclastic materials and presents a marked increase in the thickness. This unit was studied in two localities at the Blanco River valley, at the undeformed sector and over the second thrust that produces a second repetition of the Upper Jurassic and Lower Cretaceous sequences in the Aconcagua fold-and-thrust belt. This transect exposes facies variations and a significant increase in thickness to the west. Additionally, provenance analysis and paleocurrent directions indicate that the sediment supply was located to the E-SE, and that the underlying units were exhumed at the time of deposition of the Late Jurassic red beds. A consistent thickness increment of the Upper Jurassic deposits to the west through the Aconcagua fold-and-thrust belt suggests that sedimentation was controlled by NNW-directed structures. This is also supported by facies analyses that demonstrate high topographic breaks affecting a smooth westdipping fluvial ramp toward the volcanic arc. These features support an extensional setting for the deposition of the Tordillo Formation at the latitudes of the Aconcagua fold-and-thrust belt, as other authors have proposed for the Malargüe fold-and-thrust belt to the south. Plate tectonic reconstructions suggest trench rollback during this time previous to the westward migration of the South American plate, which is consistent with the back-arc extension proposed in the previous works.