In their Comment of our Geology paper (Espurt et al., 2007), Clift and Ruiz (2008) argue that: 1) the flat-slab subduction of the Nazca Ridge is unlikely to have produced uplift of the Fitzcarrald Arch in the Amazonian retroforeland basin (using geologic data from the forearc area); 2) tectonic indentation, differential erosion, shear stress along the subduction zone, or inherited heterogeneities from a Permo-Triassic rift could also contribute to the uplift of the Fitzcarrald Arch; and 3) thrust reactivation of Paleozoic structures would be the “most likely cause of uplift and active tectonism in the Arch.”

Clift and Ruiz suggest that the observed uplift in the ridge-collision zone is temporarily “high in the outer forearc, but decreases rapidly landward” and refer to several works from the forearc zone (see references in Clift and Ruiz). Clift et al. (2003) deal with the tectonic erosion of the Peruvian forearc by subduction of the Nazca Ridge in the Lima basin. From seismic reflection and well data coupled with age and paleowater depth, Clift et al. (2003) showed that the effects of the Nazca Ridge have been predominant near the trench (tectonic erosion of ~3 km), but decrease rapidly onshore (~130 km from the trench axis). In a recent study, Clift and Hartley (2007) present data from above the present-day Nazca Ridge segment (Pisco basin) from backstripped analysis and coastal morphology that shows small Pleistocene uplift (~120 m) at ~160 km from the trench. Similar observations are found in the northern and central Chilean forearc and in the Costa Rica forearc. Nevertheless, we would like to clarify that the geodynamic setting of the Peruvian forearc zone is not comparable to that of the Amazonian retroforeland basin. The Peruvian forearc basin is situated above the “normal” 30°-dipping portion of the Nazca plate (Gutscher et al., 1999), and this is the case for the other forearc basins cited by Ruiz and Clift. Uplift decrease coincides with the end of the “normally” dipping slab, which becomes horizontal below the continental lithosphere at ~150 km from the trench, and these studies do not deal with the effect of the ridge subduction within the Andes and associated retro-basins. Several authors have pointed out the imprints of the Nazca Ridge flat subduction in the Andean Cordillera. For example, McNulty and Farber (2002) emphasized recent extensional collapse in relation to the Nazca Ridge flat subduction, and Rousse et al. (2003) have demonstrated through paleomagnetic studies that Neogene counterclockwise rotations in the Eastern Cordillera were a result of the southward migration of the Nazca Ridge.

Clift and Ruiz also argue that the Fitzcarrald Arch is related to the “eastern Andes” indenter on the basis of Marques and Cobbold's (2006) model experiments. In fact, Marques and Cobbold's models deal with the development of tectonic salients—acting as indenters—as a function of local elevation, inducing transfer zone in the foreland. A simple examination of the tectonic map (see our Figure 2A in Espurt et al., 2007) reveals that the regional Fitzcarrald Arch uplift is precisely situated in a re-entrant and cannot be interpreted in any way as formed in front of an indenter.

Clift and Ruiz suggest that a transversal E-W cross section is needed to exclude thrust faulting as a mechanism to produce the uplift of the Fitzcarrald Arch. Unfortunately, such data are not yet available in the area. Perhaps we were not clear enough; the synthetic NW-SE profile of the Fitzcarrald Arch shows Paleozoic structures incorporated in the regional bulge (see our Figure 3 in Espurt et al.). These structures are unconformably overlain by undeformed Cretaceous strata which preclude reactivation of the Paleozoic structures. Reactivated Paleozoic structures are effectively observed in the Amazonian foreland basin, but they are localized to the north of the Fitzcarrald Arch uplift (see our Figure 2A in Espurt et al.). In any event, it should be stressed that the Fitzcarrald Arch regional arch is a large-scale, very low amplitude (~500 m), large half-wavelength (>500 km) bulge, one order of magnitude greater than the structures observed in the Eastern Cordillera, and we are not aware of such large and very low amplitude bulges formed by thrust-related processes anywhere in the world.

Finally, the argument for the horizontal reconstruction of the eastward continuation of the Nazca Ridge beneath the South American lithosphere is based on a seismic gap observed at the Subandes-Amazonian foreland boundary (Gutscher et al., 1999; Hampel, 2002). A cluster of deep (~660 km) seismic events are recorded beneath the Brazilian part of the Fitzcarrald Arch but are concerned with subduction processes at the upper/lower mantle interface (Okal and Bina, 1994). Therefore, no evidence of a crustal seismicity, which may correlate the tectonic reactivation postulated by Clift and Ruiz, can be found in this area.

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