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The eastern Andean slope at 37º S was formed by the Malargüe fold and thrust belt (Kozlowski et al., 1993), whose orogenic front exhumed a lower angular unconformity between Eocene terms of the Pircala Formation and late Oligocene to early Miocene rocks of the Palauco Formation and an upper uncorformity between Palauco Formation and a Neogene sedimentary cover (Tristeza Formation) and Pliocene to Quaternary volcanic beds of the Payenia volcanic field. In order to constraint the relative importance between these different contractional pulses that led to the formation of the orogenic front at these latitudes, a sedimentological and petrographical study was carried out through profiles in the Neogene sequences (Tristeza Formation) of the Pampa de Carrizalito syncline eastward the Sierra de Reyes anticline (Figure 1) plus a series of U/Pb datings in detrital zircons. The Sierra de Reyes anticline is a basement structure that was produced by tectonic inversion of Late Triassic normal faults (Kozlowski et al., 1993; Zamora and Zapata, 2005; Giambiagi et al., 2009). Three contractional stages have been related to exhumation in the area (Late Cretaceous, late Eocene and late Miocene), based on different studies (Cobbold and Rosello 2003; Orts and Ramos 2006; Tunik et. al., 2010). The Upper Cretaceous contractional stage affected just the westernmost zone and probably did not reach the present orogenic front where this study is hosted. The second contractional event took place at the present orogenic front, although this event is just associated with a 50 100 m thick shales and sandstones of the Pircala Formation (Kozlowski et al., 1987). The third mountain building episode registered at the orogenic front developed in late Miocene times and is associated with synorogenic sedimentation. We have focussed our attention in this last episode to compare this sedimentary record and evaluate total denudation. Historically the Tristeza Formation hosted in the Pampa de Carrizalito depocenter was assigned to the late Miocene, based on their occurrence on top of the Palauco Formation whose upper part was dated in 18,12 ± 0,24 Ma (Ar-Ar; Silvestro and Atencio, 2009). Two profiles located in both syncline flanks (A A and B B; see figure 1 for location) were done in order to constrain the geometry, thickness variation next to the orogenic front, and detrital compositional changes, of main sequences. Based on the sedimentological and petrografical analysis plus the results of U/Pb dating, 6 different units (a,b,c,d,e and f in A A profile) could be identified in a series of terms characterized by contrasting sedimentary sources. In detail basal sections (a+b units) are mainly composed of ignimbrites and volcaniclastic beds containing basalt clasts coming from the underlying Palauco Formation; then there are sandstone clasts on top of the previous units (c unit), followed by beds with limestone detritus (d unit), then gypsum clasts (e unit) and finally shale clasts (f unit) (Figure 2). The uppermost section contains clasts of polycrystalline quartz associated with a lithic metamorphic source. This same compositional variations are also recognized in the eastern profile (B-B; see figure 2), with the only exception of the lowest volcaniclastic and the gypsum clast horizons. The lower sandstones are derived from the erosion of the Neuquén Group, while the limestone fragments are related to the erosion of the Mendoza Group, and the gypsum clasts to the Auquilco Formation. Finally, the shales are related to the Bardas Blancas Formation. All these rocks are in a reverse order represented in the eastern Sierra de Reyes flank. This implies that the succession hosted in the Pampa de Carrizalito syncline constitutes a typical unroofing sequence: At the time when the Sierra de Reyes was uplifted its exhumation led to the deposition of a sequence characterized by a variable clastic composition. Finally, metamorphic clasts indicate that the foreland area was uplifted and exhumed at this time, implying the cannibalization of the foreland basin (Figure 2). Since the geochronological dating of detrital zircons is a valuable tool in provenance analysis of sedimentary basins, we have obtained several U-Pb (LA-ICP-MS) ages from six samples in the late Miocene Tristeza Formation. Analyzed detrital zircons in unit b yielded prominent U/Pb age peaks for the early Jurassic (ca. 190-180 Ma) and Permo-Triassic (ca. 270-260 Ma). In addition, minor peaks appear with Paleozoic ages (Devonian and Carboniferous). Equivalently, the detrital grains from units d to e are characterized for prominent peaks located in the early Jurassic and Permo-Triassic, but in addition they have an important group of early Cretaceous ages (ca. 98 Ma). Subordinate peaks appear in the Carboniferous, Devonian, and Neoproterozoic-Paleozoic. The U-Pb age spectra of the younger unit (f) showed significant changes in the pattern of detrital zircon ages respect to the previously described units. In the former, the early Cretaceous zircons are absent, and the Jurassic and Permo-Triassic zircons become less important, although several populations of zircons of Mesoproterozoic, Neoproterozoic and early Paleozoic ages begin to dominate, with subordinate Paleoproterozoic ages (ca. 2200 Ma). The obtained detrital zircon ages are clearly in line with the results of the microscopic and macroscopic detrital analyses. The main characteristics can be summarized as follows: An igneous source (volcanic and plutonic) in the basal half of the sequence that is probably coming from the Jurassic to Cretaceous Andean arc roots, located to the west in the main Andes. Additionally a volcanic component is associated with the Permian to Triassic Choiyoi Group. The origin of the Carboniferous and Devonian zircons without a clear neighbor source remains more controversial. Finally, is worth noting that at the top of the sequence microscopic evidence of a metamorphic source appears. Equivalently, the ages obtained are consistent with a source area formed during the Grenville, Pampean, and Famatinian cycles. The San Rafael-Las Matras block, located about 200 km east of this Neogene synorogenic depocenter is the best candidate as a source area. The fact that an entire unroofing sequence is registered in a Neogene depocenter next to the orogenic front implies that complete exhumation of the Sierra de Reyes started about this time, and therefore that previous phases are negligible in the study region. We can conclude that the Neogene contractional stage was the responsible for the exhumation of the orogenic front at these latitudes and created the present topography. These provenance studies show that the wedge top foreland basin was cannibalized during the exhumation of the last stages of the sedimentary record when the San Rafael-Las Matras block was exhumed to the east in the broken foreland final stage.