Neoproterozoic evolution of the basement of the South-American platform
Introduction
The Neoproterozoic era, encompassing a long time interval around ca. 500 Ma, has been considered as one of the most important stages of the continental crust evolution and the transition from the dominant processes of large continental masses during the Mesoproterozoic era to the present-day plate tectonics (e.g. Stern, 2005). The Neoproterozoic events and records are better preserved in southern hemisphere continents because these continents were components of Gondwana, a supercontinent formed during the Neoproterozoic era as the result of gradual fusion of many continental fragments inherited from the Rodinia breakup. The Gondwana amalgamation is especially well documented in the basement of the South American and African platforms.
Within the South American platform, two large domains stand out due to their general geologic and geotectonic features as well as for their role in the history of Gondwana. In the northernmost domain, the Amazonian region and surroundings appear as one of the largest and best-preserved descendants of Rodinia, the Amazonian craton (Fuck et al., 2008; Li et al., 2008). In this large domain (>4.3 × 106 km2) of remarkable pre-Neoproterozoic evolution, Neoproterozoic events and records are restricted as secondary and/or local. The Amazonian portion shows clear pre-Mesozoic drift links to the continents of the northern hemisphere (Almeida, 1978; Fuck et al., 2008; Li et al., 2008). The inferred links have been the subject of several correlation attempts for the Archean cratonic nuclei and Paleo- and Mesoproterozoic mobile belts that evaluated possible previous supercontinental arrangements (e.g. Rogers and Santosh, 2004).
In the central, eastern and southeastern portions of the South-American continent (the extra-Amazonian domain), the so-called “Brasiliano” structures date back to the Neoproterozoic age. These Brasiliano areas encompass a large fraction of West Gondwana, including cratons and mobile belts. In the extra-Amazonian domain, structures generated by Neoproterozoic fold belts are largely dominant, although cratonic nuclei (relatively smaller than the Amazonian craton) and other pre-Neoproterozoic basement inliers of different types (Brito Neves, 2003) occur and display varied degrees of reworking in the orogenic processes. This observation indicates that several segments of pre-Neoproterozoic basement were involved to some extent in the orogenic processes during the formation of the Neoproterozoic Brasiliano orogenic belts. The basement segments include Archean to Mesoproterozoic high- to medium-grade metamorphic rock units and their Paleoproterozoic to Mesoproterozoic cover rocks.
Within the context of the Neoproterozoic collage that gave birth to the West Gondwana record in South America, it is possible to identify four major structural provinces, all positioned between cratons (see Figs. 1 and 2): Borborema (northeast of the continent), Tocantins (central South America), Mantiqueira (south and southeast) and Pampean (southwest).
These structural provinces were firstly outlined in the work of Almeida et al. (1981) with the exception of the newly added Pampean province (Argentina). Although these provinces may be revised, divided or complemented in the future, the concept of four provinces is quite practical and useful for the purpose of the current work.
Many of the structural provinces of the extra-Amazonian domain continue into the continental platform and show correspondence with the African continent, thus reflecting the Pan-African structures (see Pankhurst et al., 2008 for recent reviews on this subject).
A large polycyclic NNE shear belt is exposed between the major domains discussed above. This important lineament, known as the Transbrasiliano lineament (Schobbenhaus et al., 1975), crosses the South American continent from NW Ceará to Argentina, abutting diagonally against the suture line between the South American platform and the North Patagonian terrane (Ramos, 2008b). From NW Ceará, the lineament extends to northwest Africa, where it is referred to as the Kandi lineament. Little is known of the role of the Transbrasiliano lineament, but Phanerozoic activity has been recorded within the large Paraná and Parnaíba intracontinental sedimentary basins as well as Neogene activity in the Pantanal basin and in the coastal areas of Ceará.
In this work, we stress the differences in framework, composition, and geologic–geochronologic evolution between the two large domains. Additionally, we present the geologic and geochronologic data obtained in recent decades, showing that diachronism is more the rule than the exception in the evolution of the Neoproterozoic mobile belts in South America. We attempt to identify the most important cycles of plate interactions based on available geochronologic data and note possible exceptions. It should be stressed that multiphase processes of continental assemblages are expected to occur, similar to those previously recorded in the African counterpart (see Meert, 2003; Hanson, 2003). The diachronism of the processes is demonstrated by comparing the age data from the different phases of a complete plate interaction cycle (rifting, convergence, accretion, collision, post-collision events etc., Wilson, 1965) within the provinces and inter-provinces in the South-American continent.
The analysis of the Brasiliano provinces of our continent and their surroundings, together with comparison with recent reviews of the African continent, also stresses the diachronic aspect of the mobile belt evolution that resulted in the amalgamation of West and East Gondwana.
Since the publication of the paper by Brito Neves and Cordani (1991) on the Neoproterozoic era in South America, much progress has been gained in the general geological knowledge of both domains, as evidenced by the large number of maps and papers subsequently published. Much of this knowledge was reviewed and synthesized in the books edited by Cordani et al. (2000), Bizzi et al. (2003), Mantesso-Neto et al. (2004) and Pankhurst et al. (2008), which are recommended and obligatory literature on this subject.
Section snippets
The Amazonian or “pre-Brasiliano” domain: N-NW region of the South-American continent
The Amazonian block is considered as the largest and best-preserved descendant of the Rodinia fission in South America (Fuck et al., 2008). Encompassing more than 4,300,00 km2, the Amazonian block provides an excellent record of Meso- and Neo-archean rock formation events that resulted in cratonic seed nuclei and mobile belts formed during the Paleoproterozoic and Mesoproterozoic eras (see reviews by Tassinari and Macambira, 2004; Cordani and Teixeira, 2007). The Archean seed nucleus is located
Taphrogeneses
Several Late Stenian and Early Neoproterozoic breakup processes were reported as predecessors of the “Wilsonian” cycles of the Brasiliano orogenies. However, the larger picture is still incomplete. A rough view of the general context is presented in Chart 1. These intraplate processes presumably operated initially within Rodinia, and afterward, in the smaller continental fractions and remnants of this supercontinent.
Attention should be focused on the fact that these breakup processes co-existed
The Tonian orogenic belts
Certain Early Neoproterozoic mobile belts were recognized in the three main Neoproterozoic structural provinces of the Brazilian territory; this is a relatively new finding and is far from being fully understood. With further investigations, their number may increase.
In the central domain of the Borborema province (the so-called Transversal Zone, Cariris Velhos terrains), its southern domain (Poço Redondo, northern region of the Sergipano belt) and in the Riacho do Pontal belt, various
Nature and role of the cratons and basement inliers
The paleogeography of the Neoproterozoic Brasiliano structural provinces is complex (Figs. 2 and 3) and this includes the nature and geometry of the original basins, particularly in terms of the form, number and dimensions of the basins as well as the inter-basin “highs”. A large collection of information is available on the nature of the basins and their tectono-stratigraphic assemblages as well as on the types of plate interactions and the resulting orogenic systems, the closing of which was
Tocantins and Pampean provinces (Chart 2)
Analysis of the available data from the Tocantins and Pampean provinces points out several similarities in the timing of the tectonic processes, suggesting that they may be a component of the same geotectonic context. Therefore, their current longitudinal separation might have been caused by such younger surface processes as erosion and sedimentation. From this point of view, the accretionary phenomena dated from the Late Ediacaran to the beginning of the Cambrian period stand out. In contrast,
Comparative study on the development of the Brasiliano provinces (Chart 6)
New geological maps were produced and new geological data were acquired over the last three decades. Additionally, geological knowledge was greatly improved by a wealth of new geochronological data, mainly due to new laboratories built in Brazil and to the introduction of new analytical methods (e.g., Sm–Nd, U–Pb TIMS, SHRIMP, and ICPMS-Laser Ablation). Combined with increased cooperation from international partners, these developments have led to a general improvement in geological knowledge
The Pan-African counterpart (Chart 7)
From the point of view of geotectonic evolution through geologic time, the panel on the Neoproterozoic Brasiliano mobile belts (encompassing the 850–480 Ma time span) shows a resemblance to the fold belts of the western portion of the African continent, which together with South America correspond to West Gondwana, as previously stated in this paper. Many of these African belts are the natural continuation of portions of the South American structural provinces.
However, with respect to the
Concluding remarks
The Neoproterozoic evolution of the extra-Amazonian domain of the South American platform is marked by a number of tectonic events as summarized in Chart 1, Chart 2, Chart 3, Chart 4, Chart 5, Chart 6 above.
The main breakup events took place during the Cryogenian era, between 850 and 740 Ma. Earlier records of breakup events most likely represent local intraplate events. Younger breakup data (ca. 680 Ma) recorded in SE Bahia of east Brazil do not appear to be related to mobile belt development.
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