The Andean foreland of western Argentina (28¿S--33¿S) corresponds to back arc deformations associated with the ongoing flat subduction of the Nazca plate beneath the South American lithosphere. In this paper, short- and long-term tectonic regimes for this Andean region are investigated and compared to NUVEL-1 and the new GPS-derived velocity field for the Andes Mountains. From this comparison, it appears that (1) the slight convergence obliquity at the plate boundary is accommodated by the subduction zone itself, precluding any deformation partitioning at lithospheric scale, (2) clockwise rotation of σ1 and P axes within the Andean foreland of western Argentina suggests that Plio-Quaternary deformation partitioning may occur in the upper plate at crustal scale, and (3) comparison of seismic rates of shallow deformations determined from moderate to large earthquakes localized between 22¿S and 36¿S shows that the amount of shortening is about the same (2--4 cm yr-1) between the forearcs of the 30¿ dipping slab segments and the back arc of the flat segment. All together, those observations are in close agreement with the idea that interplate coupling in flat subduction settings may play a key role in the present-day transfer of plate boundary forces. Consequently, the Andean back arc of western Argentina might be regarded as an obliquely converging foreland where Plio-Quaternary deformations are partitioned between strike-slip and thrust motions that are localized in the east verging thin-skinned Argentine Precordillera and west verging thick-skinned Pampean Ranges, respectively.