Another challenging aspect relates to determining the timing of the initiation of contractional tectonics and the onset of Andean growth in the region. In the Eastern Principal Cordillera, the CMS is overlain by sedimentary and volcanic rocks of the Trapa Trapa Formation. The age of this formation is also discussed (fig. 4). Jordan et al. (2001) assigned it to the late early Miocene, while Flynn et al. (2008) proposed a late Miocene age. The strata within the CMS and Trapa Trapa formations exhibit significant deformation, indicating that contractional tectonics commenced either during or after the deposition of these units. Jordan et al. (2001), in their analysis of seismic data from the Ro Lileo area, suggested that tectonic inversion in the region began during or after the deposition of the Trapa Trapa Formation, as they identified potential synorogenic strata within this unit. Lower to middle Miocene nonmarine clastic deposits are also present in the Agrio and Chos Malal Fold and Thrust Belts (FTBs) to the east, and these have been interpreted as products of contractional tectonics, based on the presence of syncontractional features (e.g., V. A. Ramos, 1998; Zamora Valcarce et al., 2006). Due to the disparity in reported ages for the CMS and Trapa Trapa formations (figs. 4, and 5, and table 1), the correlation of these units with the Miocene deposits of the FTBs remains uncertain. Consequently, deciphering the tectonic evolution of the Andean Cordillera in this area, encompassing both the Principal Cordillera and the FTBs, during the Neogene has proven elusive. This uncertainty also hinders comparisons of the tectonic evolution of this region with other areas in the Andes of south-central Chile and Argentina.