The Nazca Ridge and Uplift of the Fitzcarrald Arch: Implications for Regional Geology in Northern South America

The Fitzcarrald Arch corresponds to a NE-SW-trending widespread dissected relief of about 400,000 km² 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.