In a recent contribution to Geology, Espurt et al. (2007) advanced a new model to explain the presence of a major structural high, the Fitzcarrald Arch, in the Amazonian foreland basin. This model implies much greater coupling between subducting and overriding plates than previously suspected, and has implications for the tectonics of active margins far from the trench. In Espurt et al.'s interpretation, the presence of the Fitzcarrald Arch can be related to subduction of the buoyant Nazca Ridge in the presence of flat-slab subduction. However, evidence from the Andes and other active margins worldwide indicates that their model is unlikely to be correct.

The Fitzcarrald Arch is 650 m high and extends ~750 km from the trench. This degree of relative uplift is at odds with the general observation that in ridge-trench collision zones temporary uplift is high in the outer forearc but decreases rapidly landward. The vertical tectonic response to Nazca Ridge collision is well known from studies of multi-channel seismic profiles and Ocean Drilling Program (ODP) drill sites from the Lima Basin, Peru. These data suggested up to 3 km of uplift close to the trench during the initial collision ~11 Ma, but this value decreased rapidly onshore (Clift et al., 2003). ODP Site 679 showed that the shelf was uplifted by <400 m, and possibly as little as 200 m, during ridge subduction. This value was attained ~130 km from the trench axis, much closer than proposed for the Fitzcarrald Arch.

Constraints are also available from the onshore sedimentary sequences of the Pisco Basin, which presently overlies the crest of the subducting ridge. A backstripped analysis of the stratigraphy and terracing of the coastal zone, aimed at isolating the tectonic component of uplift in the collision area, indicates only ~120 m of post–2 Ma uplift ~160 km from the trench (Clift and Hartley, 2007). Furthermore, the fact that similar Pleistocene uplift is seen along much of the northern Andean margin suggests that much of that value is not linked to ridge subduction. Comparison with other arc-ridge collisions supports the idea that the temporary uplift does not extend far landward from the trench. In Costa Rica, subduction of Cocos Ridge deforms the forearc, but onshore uplift is minimal (Vannucchi et al., 2006). On the Andean margin, collision with the Juan Fernandez and Iquique ridges results in dramatic uplift of the outer forearc regions, but little vertical motion onshore or even in the coastal zones (Laursen et al., 2002; von Huene and Ranero, 2003). As in the Nazca Ridge subduction, these regions are also affected by flat-slab subduction.

Espurt et al. used an orogen-parallel seismic line to argue that the Fitzcarrald Arch was not formed by thrust faulting and is tectonically inactive. However, an across-strike seismic profile would be needed to exclude thrust faulting as a mechanism, and this is suggested by both shallow (<70 km) earthquakes (Engdahl et al., 1998) and a tilted fault block topography. Espurt et al. noted that the southern arch is characterized by radial drainage networks. However, the Subandean zones and Eastern Cordillera also exhibit congruent geomorphological patterns with NW-SE–oriented promontories. Indeed any topographic profile in the eastern Andes between 12°S and 13.5°S would yield a similar pattern to the Fitzcarrald Arch, suggesting a common mechanism for the uplift of the whole region. Marques and Cobbold (2006) modeled the effects of tectonic indenters as causing transfer zones to develop along indenter sides. The eastern Andes can be considered as an intender into the Amazon foreland (Carlotto, 1998), with its core located in the Eastern Cordillera. The area of the Fitzcarrald Arch marks a change in orogenic strike from NNW-SSE to NW-SE going south, and is a large-scale transfer zone, which could have generated arch uplift. Additional mechanisms also contributing to uplift include differential erosion, shear stress along the subduction zone, and inherited heterogeneities from a Permo-Triassic rift. Espurt et al. reported that the Neogene is partially eroded between the Mashansha and Panguana drill sites. We note that this eroded region closely overlies a Paleozoic structure with similar orientation. Reactivation of these structures is the most likely cause of uplift and active tectonism in the arch. Although it is not yet clear what processes control Andean morphology south of the Fitzcarrald Arch, it seems unlikely to be subduction of the Nazca Ridge.

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