Amazonia, Landscape and Species Evolution: A Look into the Past, 1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing SIX The Nazca Ridge and uplift of the Fitzcarrald Arch: implications for regional geology in northern South America Nicolas Espurt 1 , Patrice Baby 2 , Stéphane Brusset 2 , Martin Roddaz 2 , Wilber Hermoza 3 and Jocelyn Barbarand 4 1 Université Paul Cézanne, Aix en Provence, France 2 Université de Toulouse, France 3 REPSOL-YPF, Madrid, Spain 4 Université Paris Sud, France Abstract The Fitzcarrald Arch corresponds to a NE–SW-trending widespread dissected relief of about 400,000 km 2 located to the east of the sub-Andean thrust front. Analyses of drainage maps demonstrate that this regional uplift controls the modern Amazonian drainage network. During the Miocene, the Fitzcarrald Arch did not exist and this area corresponded to the subsiding foredeep of the Amazonian foreland basin. Analyses of the Miocene deposits outcropping within the Fitzcarrald Arch area indicate that deposition was controlled by tidal current and consisted of tide-dominated deltas and estuaries. The tidal Miocene deposits were followed by fluvial conglomerate and sandstone sediments, probably Pliocene to Pleistocene in age. Modelling of the vitrinite reflectance data shows that the uplift of the Fitzcarrald Arch began in Pliocene times; this date is further confirmed by sedimentary evidence such as provenance and a change of palaeocurrent directions from the Pliocene and Pleistocene record. Geophysical data show that the Fitzcarrald Arch uplift superimposed on the eastern part of the Nazca Ridge flat slab segment. The flat subduction process linked to the buoyancy of the Nazca Ridge has been active since ~4 Ma. Thus, the coincidence of the initiation of flat slab subduction with the Pliocene uplift of the Fitzcarrald Arch implies that the flat subduction of the Nazca Ridge is the cause of uplift. The Nazca Ridge flat subduction is the process responsible for the modern configuration of the Amazon drainage basin and may be one of the decisive factors that triggered large-scale modification of the Amazonian landscape inducing drastic biota changes in the Amazonian basin during the last 4 million years. Introduction The Fitzcarrald Arch is a major geomorphic feature of about 400,000 km 2 in the landscape of the Amazon drainage basin. It divides the northern Amazonian foreland basin from the southern Amazonian foreland basin, both of which are driven by Andean oro- genic loading (Roddaz et al. 2005). The Fitzcarrald Arch makes the Amazon foreland basin an atypical foreland basin, not like the four- component model of DeCelles & Giles (1996) (see also Chapter 5). The causes of the appearance of Fitzcarrald Arch regional uplift are the subject of active discussion. Some authors have proposed alternative hypotheses for the Fitzcarrald Arch uplift: surface expression of inherited Paleozoic heterogeneities (Jacques 2003; Carlotto et al. 2007), simple mega-fan geometry (Picard et al. 2008) or a relatively higher stress regime above a subduction zone (Dumont 1996). Based on geomorphic, geophysical and geody- namic evidence, Espurt et al. (2007) have demonstrated that the high elevation of the Fitzcarrald Arch was caused by the flat slab subduction of the aseismic Nazca Ridge below northern South America. These authors estimated the uplift of the Fitzcarrald Arch to be no older than Pliocene (Espurt et al. 2007). This chapter reviews the criteria that permitted Espurt et al. (2007) to link the subduction of the aseismic Nazca Ridge and the uplift of the Fitzcarrald Arch. We also constrained better spa- tial and time relationships between the Nazca Ridge subduction and the Fitzcarrald Arch uplift in the Amazonian foreland basin, Hoorn_ch06_Final.indd 89 Hoorn_ch06_Final.indd 89 10/24/2009 11:03:32 Shob 10/24/2009 11:03:32 Shob