Topographic construction along the northeastern Andes and the origin of the Transcontinental Amazon Basin

Abstract

Reconstructing the topographic evolution of mountains aids our understanding of the geodynamic processes that shape the Earth´s surface, its interaction with atmospheric circulation, and the ensuing development of ecosystems and biomes. One of the most biodiverse ecosystems on Earth lies in the Amazon basin (Antonelli et al., 2018), in particular along its western sector (e.g. Barthlott et al., 2007), including territories in Bolivia, Colombia, Ecuador, Peru, and western Brazil. There, available stratigraphic data document the existence of a mega-wetland (Hoorn et al., 2011; Wesselingh and Salo, 2006) between ~23 and ~11 Ma, before the incipient development of a transcontinental Amazon river flowing eastwards into the Atlantic Ocean (Figueiredo et al., 2009), which was fully connected with western Amazonia, as we know it today, by late Micoene-Pliocene (Hoorn et al., 2017; Hoorn et al., 2010; Latrubesse et al., 2010). Competing hypothesis on the geodynamic processes underlying such a shift include two end-member scenarios: (1) drainage reversal associated with dynamic subsidence due to sustained westward motion of the South American Plate (Shephard et al., 2010); and (2) surface processes and flexural isostasy associated to Andean uplift (Sacek, 2014). Testing the degree of contribution of these two end-member models rely on an accurate estimate of topographic construction, which is lacking in the northeastern Andes that constitute the Amazon watershed. Commonly, well documented Oligo-Miocene ages of rock exhumation in the northern Andes (Mora et al., 2013; Parra et al., 2009) have been mistakenly used as evidence of the presence of a high Andean range. The motivation of this proposal is to quantify the two components of rock uplift - surface uplift and exhumation - in the Andean watershed regions of northwestern Amazonia, including areas in southern Colombia, Ecuador, and Peru, using a multi-method approach that include detailed topographic surveying and geochronology of Quaternary fluvial terraces, stratigraphic surveying and provenance of Late Cenozoic strata, bedrock low- temperature thermochronometry in key areas lacking such information, and stable isotope paloaltimetry in Late Cenozoic strata. In a second stage, the project aims at studying the Andean uplift signal preserved Miocene and Quaternary Cenozoic deposits in western Amazonia using outcrop sections as well as by analyzing samples from drill cores collected in the framework of the collaborative TADP Project (Baker et al., 2015). So far, the drilling of a core in the state of Acre (Figure 1) is granted through funding by ICDP, and access to samples will be available (see Annex 2).Disentangling the two components of rock uplift - exhumation and surface uplift -, in a mountain range is challenging due to the rare coexistence of suitable proxies in nearby areas. Thermochronometry and sedimentary provenance data reveal that exhumation of the internal ranges of the northern Andes (the Central Cordillera in Colombia, the Cordillera Real in Ecuador, and the Eastern Cordillera in Perú) occur since late Cretaceous (Garver et al., 2005; Gomez et al., 2003; Louterbach et al., 2018; Parra et al., 2009; Spikings et al., 2001; Villagómez and Spikings, 2013). However, surface uplift is less well constrained, with studies in southern Colombia suggesting that major surface uplift occurred only after 6.4 Ma in the southern Eastern Cordillera (Anderson et al., 2016) and the central Eastern Cordillera (Hooghiemstra et al., 2006), despite exhumation being underway since ~30 Ma (Anderson et al., 2016; Horton et al., 2010; Mora et al., 2013; Parra et al., 2009). Farther to the south, there are no records available until southern Peru, where stable isotopic data suggesting that modern elevation in the western Cordillera were already attained by 19-16 Ma (Saylor and Horton, 2014). Here we aim at evaluating spatial trends in exhumation and surface uplift by (1) acquiring new a (AU)