ANDEAN UPLIFT AND FORELAND BASIN FORMATION IN WESTERN ARGENTINA, MENDOZA PROVINCE, 34°30'-35°30'S

Synorogenic molasse deposits were used to constrain the timing of foreland basin formation east of the Andean Cordillera Principal. Following Nullo et al. (2002), intrusive and volcanic deposits of the FTB are classified as Molle eruptive cycle (19 Ma to approximately 14 Ma), Huincan Andesite (14 Ma to 10 Ma), and La Brea Andesite (10 Ma to 5 Ma). Farellones suite volcanics and intrusives, similar in age and lithology to Huincan and La Brea Andesites, crop out west of the study area. Huincan Andesites intruded during the formation of the FTB and have been uplifted and eroded, and with few exceptions, leaving only the feeder dikes. Significantly younger La Brea Andesites were emplaced during the waning phases of FTB development. La Brea Andesite stocks are interpreted as cross-cutting FTB deformation of the Cretaceous platform sequence. Based on 40Ar/39Ar cooling ages, the oldest intrusives in this segment of the foreland are no older than 14 Ma.

Foreland basin fill at the leading edge of FTB deformation is comprised of the Agua de la Piedra and the Loma Fiera Fms. The Agua de la Piedra Fm was deposited in series of alluvial fans along the leading edge of the developing FTB. Air fall tuffs and lahar deposits make up the overlying Loma Fiera Fm. Clasts collected from the Rodados Lustrosos unit, at the base of the Agua de la Piedra Fm gave cooling ages as young as 12.8 Ma. Tuff clasts from the undeformed Loma Fiera Fm gave cooling ages of as young as 9.9 Ma.

In our tectonic history, arc-related volcanism begins in the FTB sometime after 14 Ma and continues until at least 5.6 Ma. The foreland basin fill begins with the deposition of the Agua de la Piedra Fm and associated volcaniclastic material from the Huincan Andesite by 12.8 Ma. By 10 Ma, with the emplacement and eruption of the La Brea Andesite and the deposition of the Loma Fiera Fm, deformation in the foreland is waning. These data constrain the initiation of sedimentation in the foreland basin between 34°30'-35°30'S to several million years younger than the Pinchiera-Ventana basin to the south, or the Alto Tunuyán basin to the north.