We thank Encinas and Finger (2007) for their comments on our paper (Clift and Hartley, 2007), as they add detail to the model we presented, while leaving intact the essential idea of Neogene tectonic erosion along the Andean margin being followed by accretion and underplating since 2 Ma. Our analysis and erosion rate estimates hinge on paleobathymetry data that were compiled from a series of published studies and enhanced by field observations by Hartley. In essence, the Comment suggests that some of the Upper Miocene and Pliocene sections were actually deposited in much deeper water than we calculated, and consequently, that subsidence rates, and thus rates of tectonic erosion, would have been much higher at that time. However, we did not incorporate some of the data referred to because they are not peer reviewed or published in the open literature (Achurra, 2004; Achurra et al., 2003), or they are not still in press. In any case, Achurra et al. (2003) stated that the sediments in the Caldera basin were hemipelagic, but this does not imply a specific water depth. Dark shales and diatomites are often deposited in deep water, but are not restricted to such zones, and their accumulation is also dependent on paleoceanographic conditions. More robust benthic foraminifer studies (Resig, 1990) from Lima Basin, further northwest, indicate that water depths on the Peruvian shelf have been approximately constant since the Eocene, interrupted by periods of emergence, but not bathyal conditions. The seismic stratigraphy of the Lima basin shows gradual trenchward tilting during the Cenozoic (Clift et al., 2003), consistent with gradual erosion and subsidence increasing toward the trench, but not with the proposed wild fluctuations of the coast suggested here. Regarding the study of Ishman et al. (2003), these authors indicated that there were some bathyal sediments exposed on the Mejillones Peninsula in northern Chile. While this may be true, the majority of the Neogene in that region was deposited in shelf depths and strongly supports an approximately stationary shoreline (Hartley et al., 2000).
Even if the “deep water facies” of the Peruvian basins (Dunbar et al., 1990) are truly bathyal as suggested, then this would have the effect of increasing the rate of subduction erosion prior to inversion, and the degree of underplating since that time. This does not change the essential reconstruction of a switch in margin tectonics at 2 Ma. This switch was preceded by a period of slower tectonic erosion than is typical since the Mesozoic. Indeed, even if the shallow-water sediments exposed 40 km from the coast noted by Encinas et al. (2006) mark the landward limit of tectonic erosion, this would still preclude the rapid trench retreat rates of ~3 km/m.y. noted in many active margins (Clift and Vannucchi, 2004). We note that since 2 Ma, continued tectonic erosion in the trench region (Laursen et al., 2002; von Huene and Ranero, 2003) during basin inversion requires a steepening of the forearc taper, not the margin-scale accretion proposed by Encinas and Finger. Current evidence points to synchronous accretion and erosion.
Encinas and Finger seem to be confused regarding the 170 km retreat of the coast at 33–35°S. We did not suggest that this retreat had to be caused by shallowing of the subducting slab, although this certainly happens (Kay et al., 2005) and may well have affected the forearc vertical tectonics. We only indicated that slab shallowing may play a part, as well as tectonic erosion, in driving arc retreat in this region. Separation of the two processes is difficult using the data at our disposal in this study.