Origin of the largest South American transcontinental water divide

Interbasin arches between hydrographic systems have a heterogeneous geological origin, forming under the influence of several different geomorphological processes. Independent of the underlying processes, these arches compartmentalize present-day river basins, encompassing different water chemistries, habitat types, soil domains, potential energy and, on a geological/evolutionary time scale, aquatic life varieties in the ecosystem. Through most of its length, the water divide between the Amazonian, Paraná-Paraguay, and São Francisco river basins in central South America coincides with an Upper Cretaceous intracontinental igneous alkaline province. This magmatism, independent of its nature, caused intense crustal uplift and influenced hydrological networks at different scales: from continental-scale crustal doming to continental break-up, and finally to local-scale phenomena. The available ages for alkaline rocks indicate a well-defined time-interval between 72.4 to 91 Ma (concentrated between 76 and 88 Ma) period of uplift that contributed to large-scale drainage compartmentalization in the region. Here we show that uplift associated with intrusive magmatism explains the origin and maintenance of the divide between the Amazonian, Paraná-Paraguay, and São Francisco river basins.

to the origin of inland river basins are not well known 15 , and this holds true for the water divide comprising the Amazonian, Paraná-Paraguay, and São Francisco river basins. Geologically, the headwater streams of these continental-scale river systems are located at the margins of major South American cratons, namely the Amazonian, São Francisco, Rio de la Plata, São Luiz, and Luiz Alves cratons, which are surrounded by large ancient orogenic belts (Mantiqueira and Tocantins provinces) formed during the amalgamation of the Western Gondwana supercontinent in the Neoproterozoic 16 . Despite their orogenic origins, such ancestral mountains are too old to be directly associated with present-day landscapes or divides. However, a remarkable fact about Brazilian relief is the presence of Mesozoic summit surfaces at high altitudes 17 . Such flat tops on several high-relief relict topographic structures along the abovementioned water divide provide evidence of the long denudation history of an ancient Gondwanaland plateau 17 . The development of this Cretaceous mega-plateau of about 2000 m of topographic elevation 18 was coeval with local-scale volcanism, rifting, and uplifts 19 . Installation of the present-day observed drainages occurred alongside such mega-geomorphological dynamism.

Results
Current Amazonian-Paraná-Paraguay-São Francisco water divide. One of the most conspicuous characteristics of the current configuration of the Amazonian, Paraná-Paraguay, and São Francisco river basins is the ~2300-km-long, NW-SE-oriented water divide. This long trajectory over the Brazilian shield coincides with a remarkable geological feature -the Azimuth 125° lineament (Figs 1 and 2). This feature was first described as a succession of diamond deposits, located in Brazil, aligned from Abaeté (state of Minas Gerais) to Rio Machado Major geological features associated with the Azimuth 125° lineament: topographic high-relief consisting of water divides between the Amazonian, São Francisco (draining northward) and Paraná-Paraguay systems (draining southward). Intrusive alkaline complexes, paleo volcanoes, paleocurrents, and ages of alkaline intrusions are also plotted. Geological units correspond to sedimentary rocks deposited before uplift, (occurring both northern and southern to Azimuth 125° lineament, in black), and those deposited after uplift (restricted to northern or southern sides, in white). Area illustrated in Fig. 2  Age of the central South American river basins. Previous contributions dealing with the origins of the modern river system in the South American interior 1,2,23,24 agree with some basic points: (1) the present main water divides and basin architecture are Mesozoic in age; (2) major Jurassic-Cretaceous events, such as the break-up of the Gondwanaland, have a significant tectonic influence on the compartmentalization of present-day sedimentary and fluvial systems; and (3) the Andean chain significantly contributes to major hydrological changes; for example, Cenozoic deformations of the ancient post-Cretaceous paleo-plateaus were influenced by the geotectonic evolution of Andean foreland systems. Sedimentary records of intracratonic basins (and associated paleocurrent data) provide insights on the timing of spliting between adjacent fluvial systems. Along the Azimuth 125° lineament, the youngest shared sedimentary sequence between northern (Parecis) and southern (Paraná) intracratonic sedimentary basins is the Lower Cretaceous sandstone of the Botucatu Formation, located at the western limit of the azimuth between the upper Tapajós-Xingú and Paraguay river basins 25 . There is also no evidence of Mesozoic sediments of the Paraná basin (or Bauru basin) crossing the Canastra range 26 , the divide between the upper Paraná and São Francisco rivers. A compilation of available, mainly unpublished paleocurrent and provenance data [27][28][29][30][31] , for the late Cretaceous sedimentary units north and south of Azimuth 125° lineament shows a clear dispersion from this lineament, which behaved as a topographic high during sedimentation.
Relatively well-known major geological events, such as the opening of the South Atlantic Ocean (Jurassic to early Cretaceous) or the rise of the Andean chain (late Cretaceous to Cenozoic), can explain several aspects of the South American drainage evolution, particularly along the eastern, passive, rifted margin of the continent 23 as well as on the opposite convergent Andean margin 32 . However, the origin of the present-day N-S compartmentalization of the drainage network requires further explanation with respect to the underlying combination of mechanisms involved.
Heat source for intracontinental magmatic province formation. Heat source and faulting are important factors affecting the formation of intracontinental magmatic provinces, as here proposed to cause the formation of the long, South American transcontinental water divide. In this section, two proposed general alternative heat-source models are addressed: mantle plumes and tectonic reactivations.
Geologic, geomorphologic, and geochronologic evidence has been used to postulate that the alkaline rocks between Poços de Caldas (continental interior; Minas Gerais) and the Cabo Frio coast (Rio de Janeiro) have a WNW-ESE alignment and were emplaced during the displacement of the South American plate over the Trindade hot spot currently located at ~18°40′S in the Mid-Atlantic Ridge (mantle plume hypothesis) 33 . According to this view, during the Eocene, this supposedly existing hot spot probably moved to the eastern boundary (coast of Rio de Janeiro) of South America, causing important tectonic and magmatic events. This relative hot spot displacement has been considered to have caused the formation of the volcanic Vitória-Trindade chain, located off the eastern coast of Brazil, corresponding to the oceanic extension of the Azimuth 125° magmatic lineament. Furthermore, the genesis of the Poxoréu Igneous Province (Mato Grosso, western Brazil) has been also proposed to possibly be associated with a more intense lithospheric extension above the western margin of the postulated impact zone of the Trindade plume, permitting greater upwelling and melting farther to the west at ~84 Ma 34 . Therefore, according to this view, the Trindade plume was considered to possibly represent a super-plume with a diameter of ~1000 km, and the plume were thought to serve as heat sources for continental-interior igneous province formation. It is important to note, however, that the western end of the Vitoria-Trindade Chain is more than 280 km north of the southeastern end of the Azimuth 125° magmatic lineament. Moreover, the plume hypothesis has been criticized recently because geochemical data do not support that tholeiites from the Paraná Magmatic Province resulted from the Trindade plume 35 , and the oceanic crust was recently reactivated as well as subject to alternating compressive and extensional stresses associated with normal faulting and volcanism 36,37 . Several supposedly existing "hotspot tracks", such as the Vitória-Trindade chain, might reflect that the heat is derived from the accommodation of stresses in the lithosphere during rifting rather than continuous magmatic activity induced by mantle plumes beneath the moving lithospheric plates. Considering this view, regional thermal anomalies in the deep mantle, mapped using geoid and seismic tomography data, offer an alternative, non-plume-related heat source for the generation of intracontinental magmatic provinces 35 .
The distribution of alkaline occurrences along NW-SE-trending crustal discontinuities extending over 800 km and the nature of the magmatism as described above clearly indicate that deep lithospheric faults significantly controlled the tectonics of the alkaline provinces in the Azimuth 125° lineament 38 . Alkaline bodies were emplaced between 91 and 72.4 Ma (97 and 71.1 Ma including uncertainty), with a higher concentration between 76 and 88 Ma (Fig. 3). The distribution of age-dates of the alkaline rocks along the Azimuth 125° does not show any eastward-decreasing trend. Instead, the available ages indicate a relatively long magmatic activity (~12 Ma) that weakens the hypothesis of the action of a mantle plume. In fact, available age data indicate the occurrence of different phases of alkaline magmatism from Late Cretaceous to Paleogene 38 . Thus, the supposed "impact of the Trindade starting mantle plume head" 34 that developed at about 250 km west of the Poxoréu Igneous Province on intracontinental magmatic province formation has been perceived as "very improbable" 39 .

Discussion
Is there a link between drainage compartmentalization and uplift controlled by intrusive magmatism? The magnetic signature of the Azimuth 125° lineament indicates a set of linear features with regional continuity in the subsurface, characterized by a higher magnetic susceptibility compared with surrounding host rocks 40 . The importance of this lineament as a system of deep crustal discontinuities serving as the main conduit for several alkaline intrusions along the azimuth axis has been confirmed recently 40 . The injection of dike-forming magma into the faults of the lineament occurred during two or three tectonic events: (i) between 950 and 520 Ma at two Brasiliano orogeny cycles, older (950-650 Ma) and younger (ca. 700-520 Ma); (ii) at approximately 180 Ma, during the fragmentation of Gondwana; and (iii) at circa 90 Ma 40 . A compilation of the available ages of intrusions along Azimuth 125° indicates periods of intrusions, and consequently, uplifts and large-scale drainage compartmentalization between 91-72.4 Ma (Fig. 3, Table 1).
Low temperature thermochronology, including apatite fission track analysis (AFT) and a minor set of apatite U-Th/He dating (AHe), indicate that the onshore coastal region of SE Brazil experienced cooling, uplift and exhumation between 100 and 70 Ma 41 . Up to 3 km of denudation was inferred 42 , but this is significantly attenuated to the continental interior. Some alkaline rocks along the Azimuth 125° have deep sources (up to 100 and 150 km for kamafugites and kimberlites, respectively) 43 . The 3D inversion of magnetic data demonstrated that alkaline intrusions along Azimuth 125° are shallow 44 . A large number of occurrences have associated hypabyssal and/or volcanic (lavas) equivalents, or even rocks subject to phreatomagmatic interactions, indicating shallow or near surface emplacement and a very low, long-term denudation rate since the Late Cretaceous. Emplacement of intrusive bodies causes surface uplift, as observed in other regions of the world as forced folds with amplitudes related to intrusion thickness and length 45 . Some intrusions (Araxá, Catalão 1, Poços de Caldas, Serra Negra, Tapira) (Table 1) dragged the surrounding rocks, causing uplift. A conspicuous feature in the Araxá (see map in 46 ) and Serra Negra intrusions 47 is the presence of a ring of Precambrian schists and quartzites that surround the alkaline rock body. In Poços de Caldas 48 part of the roof (Early Cretaceous eolian sandstone) is preserved. Outcropping alkaline bodies show a maximum depth/major axis of 4.5/4.5 km for Araxá, 17/9 km for Tapira, 12-15/10 km for Serra Negra-Salitre and 5/5 km for Catalão 1, and alkaline bodies without surface manifestation show a minimum depth/major axis of 0.3-2/6 km for Pratinha and <2/14 km for Registro do Araguaia 44 . At the southwestern border of the Parecis Basin, along Azimuth 125°, a set of currently shallow intrusive bodies were identified from magnetic anomalies, having maximum length and thickness of approximately 11 and 3.6 km, respectively 49 . These dimensions suggest that, at the time of its placement, the surface of the terrain experienced a probable uplift of 0.1 to 1 km 45 . Although the minimum value was 100 m, this uplift is considered to be appreciable and is likely to have caused a change in the drainage network.
Here we show that uplift associated with late Cretaceous (91-72.4 Ma) intrusive magmatism explains the origin and maintenance of the present-day 2,300 km long, NW-SE-oriented water divide between the Amazonian, Paraná-Paraguay, and São Francisco river basins. Independent of the underlying mechanism (mantle plumes or tectonic reactivations), high cratonic topography arose from intracontinental magmatic activities in South America 19 . This scenario, along with several other completely different mechanisms (such as the Andean orogeny, large-scale foreland basins subsidence, marine incursions, the rise and disappearance of mega-wetlands, and erosive and tectonic headwater captures) illustrate the splendorous South American geodiversity acting on river basins throughout history.

Methods
Geological data were collected from the literature. Intrusive alkaline complexes (carbonatite, kimberlite, and syenite) were also mapped using CPRM data (Brazilian Geological Survey) available on http://geosgb.cprm. gov.br/. Mapping were performed using QGIS v2.18 (http://www.qgis.org). The ages of the alkaline rocks were obtained from different sources (listed in Table 1), and mainly comprise U-Pb, Ar-Ar and few K-Ar and Rb/Sr data.