In their article, Clift and Hartley (2007) propose the existence of two alternating modes of subduction erosion for north-central Chile and Peru: one fast, with steady-state retreat from 150 to 20 Ma, and the other slow, with erosion constrained only in the trench domain from 20 Ma onward.
The proposed model is intriguing, as it reveals that subduction erosion is not always by steady-state evolution with mass removal along the entire subduction channel and whole-scale landward retreat. However, we disagree with the timing proposed for these events because we consider that some of the data used in their analysis are incorrect.
Essential in Clift and Hartley's model are paleobathymetric data used to reconstruct vertical motions of the studied Chilean and Peruvian basins. In their study, they indicate that all Neogene sedimentation was limited to the shelf or coastal environments. However, other studies have presented data indicative of bathyal deposition. Achurra (2004) reports that benthic foraminifers indicate lower-middle bathyal (1500–2000 m) deposition in the Caldera basin during the late Miocene and at least part of the Pliocene. His interpretation is supported by the occurrence of manganese nodules formed in a hemipelagic environment (Achurra et al., 2003). Benthic foraminifers also indicate bathyal (500 m) deposition during part of the Pliocene in the Mejillones basin (Ishman et al., 2003). For their analyses of the Peruvian basins, Clift and Hartley extracted data from Dunbar et al. (1990) and Tsuchi (1992). However, none of these authors uses benthic foraminifers in their studies, nor do they make a detailed sedimentological analysis that allows accurate paleobathymetric interpretations. Furthermore, Dunbar et al. (1990) report a temporal shift from nearshore to deeper water organic-rich facies in the Neogene successions of Peru. They do not provide a precise quantification for their “deeper facies,” but the inclusion of diatomites and abundant radiolarians strongly suggest it is bathyal.
Clift and Hartley make mention of Miocene shallow-marine successions in central Chile (33°–35°S). However, data derived from benthic foraminifers and supported by sedimentological and ichnological studies indicate lower bathyal deposition (>2000 m) in this area during the late Miocene to early Pliocene, followed by nearshore sedimentation during the late(?) Pliocene and subsequent emersion and uplift of these successions (Encinas et al., 2006; Finger et al., 2007; Encinas et al., 2007). Correlative units in south-central Chile (~37°–43°S) show a similar history (Finger et al., 2007; Encinas et al., 2007) and indicate that basin subsidence and uplift were related to regional tectonics that affected the entire forearc.
Clift and Hartley suggest that the 170 km of eastward arc displacement during the Neogene calculated by Kay et al. (2005) at 33°–35°S is the product of decreasing subduction angle rather than trench retreat. However, the onset of major coastal subsidence for this area during the Neogene is a consistent argument favoring subduction erosion as the cause of arc motion.
One of the most important assumptions in Clift and Hartley's model is that the position of the coast has remained nearly stationary since ca. 20 Ma. One piece of the evidence for this is the occurrence of Upper Cretaceous to Miocene shallow marine and continental strata exposed along the coast of central Chile (33°–35°S). However, if presently exposed Neogene strata in the Caldera basin and central Chile were deposited at depths of ~2000 m, then the paleocoast must have been located tens of kilometers to the east. In fact, the easternmost occurrence of shallow marine Pliocene successions in central Chile is ~40 km from the present coast (Encinas et al., 2006). Furthermore, marine sequences in central Chile are punctuated by major unconformities between the Upper Cretaceous, Eocene, and Neogene (Gana et al., 1996). These have been recognized in wells drilled on the continental shelf of south-central Chile (~36°–40°S), some located 40 km east of the present coast (Mordojovich, 1981). Whereas the unconformities represent regressive intervals of uplift, nondeposition, and erosion, the position of the paleocoastline must have oscillated widely.
In conclusion, we share Clift and Hartley's point of view that there are alternating periods of fast and slow tectonic erosion, but we do not agree that a slow pace characterizes the last 20 m.y. Major subsidence of the coastal area between ~20–11 to 4–2 Ma (ages may vary depending on the area) indicate subduction erosion with mass removal along the entire subduction channel, from trench to coast, with eastward displacement of the coastline and a rapid erosion mode. In our opinion, the slow erosion mode did not commence until sometime within the last ~4–2 m.y.