Connecting the Araçuaí and Ribeira belts (SE – Brazil): Progressive transition from contractional to transpressive strain regime during the Brasiliano orogeny

doi:10.1016/j.jsames.2018.06.005

Journal of South American Earth Sciences

Volume 86, October 2018, Pages 127-139

https://doi.org/10.1016/j.jsames.2018.06.005 Get rights and content

Abstract

Whether the Araçuaí and the Ribeira Neoproterozoic belts in southeast Brazil represent a continuous or two distinct orogenic belts is still a debated question. We compile existing geologic and geophysical data and argue that the two belts, in spite of differences in tectonic style and kinematics, should be considered as part of an orogenic continuity that formed during the mostly Late Proterozoic Brasiliano orogenic evolution. Structural mapping supported by Anisotropy of Magnetic Susceptibility data shows that the transition between the two belts is gradual, with a progressive change from a NE-trending subvertical foliation and subhorizontal stretching lineation in the Ribeira belt to gently dipping and less pronounced fabrics in the Araçuaí belt. The lineation progressively changes northward from NE to E-W, suggesting a transition from overall transcurrent to thrust kinematics, and the solid-state finite strain seems to get progressively higher into the Ribeira belt. Differences in tectonic style are explained by the southward termination of the rigid São Francisco craton, which caused oblique collision and lateral escape, as supported by numerical modeling. Shear-wave splitting measurements suggest that the transcurrent deformation in the Ribeira belt affected the entire lithosphere. In the transitional zone, the seismic anisotropy pattern is more complex and the delay time between the fast and slow shear-waves is smaller. These observations, together with a similar record of magmatism and timing of orogenic events and P-T conditions during peak metamorphism strongly support continuity between the Araçuaí and the Ribeira belts. This model is a "quasi-facsimile" of the Paleoproterozoic deformation that occurred in the Great Slave Lake area in Canada.

Introduction

Many orogens show evidence of transcurrent deformation along sections of the belt, often coeval with a large amount of magmatism and peak metamorphic conditions. Transcurrent deformation is, in most cases, associated with strain at the plate margins being partitioned into simple shear and pure shear components (e.g. Fitch, 1972; Sanderson and Marchini, 1984; Oldow et al., 1990; Fossen and Tikoff, 1993; Tikoff and Teyssier, 1994; Dewey et al., 1998). Several factors may explain the development of such transpressional deformation. The plate convergence vector, and thus the far-field forces, may be oblique to the plate boundaries, as is the case in Indonesia (e.g., Philippon and Corti, 2016), and strain will be partitioned to accommodate the obliquity. Alternatively, heterogeneities and anisotropy in the colliding plate margins may generate transpressional tectonics even when the convergence is normal to the plate boundaries (e.g., Tommasi and Vauchez, 2015 and references therein).

The Mantiqueira Province, as defined by Almeida et al. (1981), is the orogenic system along the Brazilian coast that extends from the southern border of the eastern São Francisco craton (16° S) to Uruguay (33ºS). It constitutes a more than 3000 km long NNE-trending Neoproterozoic orogenic system that recorded a protracted sequence of orogenic events related to the assembly of West Gondwana, and comprises, from north to south, the Araçuaí, Ribeira, and Dom Feliciano orogens (Fig. 1). In this contribution we explore the transition between two contiguous orogenic segments: the Araçuaí belt, which is characterized by orogen-normal shortening, and the Ribeira belt in which transpressional tectonics dominates. These two Brasiliano/Pan-African belts form a domain of orogenic deformation and magmatism from 17ºS to beyond 25ºS along the east coast of Brazil (Fig. 1).

Initially no distinction was made between these two belts, and the entire mobile belt was called Ribeira (Hasui et al., 1975). Then, Almeida (1977) introduced a division of the northern part of the orogen, which bounds the São Francisco craton, with the Araçuaí belt representing the westernmost metasedimentary rocks of the orogen and the Ribeira belt representing its eastern, internal domain. This distinction was mostly grounded on contrasted rock assemblages and geochemical characteristics, the boundary between the two belts was oriented ∼ NS parallel to the carton's eastern limit (e.g. Brueckner et al., 2000). Later on, the entire orogenic domain, East of the São Francisco craton, from 15ºS to 21ºS, has been considered as the Araçuaí belt (e.g., Trompette, 1994; Vauchez et al., 1994; Pedrosa-Soares et al., 2001, 2008; Heilbron et al., 2004). Regardless, both the Araçuaí and Ribeira orogenic belts are generally considered to have formed at convergent plate boundaries between ∼630 and 520 Ma during the Neoproterozoic Brasiliano or Pan-African convergence between the São Francisco and Congo cratons.

The Ribeira and Araçuaí belts display contrasting structural characteristics:

- In the Ribeira belt, steep lithospheric-scale orogen-parallel transcurrent shear zones associated with orogen-normal thrusting dominate the well-developed regional fabric, typical of a transpressional belt (Hasui and Oliveira, 1984; Trompette, 1994; Vauchez et al., 1994; Cunningham et al., 1998). Orogen-normal shortening possibly started slightly before the formation of transcurrent shear zones, but transcurrent shearing in the main shear zones occurred under peak metamorphic conditions (granulitic facies in the northern part of the Ribeira belt) similar to the pressure and temperature conditions of shortening tectonics (Egydio-Silva et al., 2002). In addition, the northwestern boundary of the shear-zone displays a gradual transition between transcurrent shearing and orogen-normal shortening. Combination of coeval orogen-parallel shearing and orogen-normal shortening, i.e., transpression, is also predicted in this part of the belt by numerical models (Vauchez et al., 1994) due to accommodation of the southward termination of the stiff São Francisco craton.

- Regional tectonics of the Araçuaí belt is dominated by westward thrusting of allochthonous units toward the São Francisco craton, and only subsidiary orogen-parallel transport. This contrast with the transpressive style of the Ribeira belt, together with the northward rotation of the orogenic fabric from NE to ∼ NS has raised the question whether the Araçuaí and Ribeira Neoproterozoic belts represent a continuous orogen or two distinct orogenic belts. A boundary between these two domains was suggested at the 21ºS parallel, i.e., about the termination of the São Francisco craton (e.g., Pedrosa-Soares and Wiedemann-Leonardos, 2000; Heilbron et al., 2004 and references therein). However, such an ∼ EW frontier between the two belts is not supported by recent geological and geophysical results. Instead, a gradual change in strain regime between these two belts agrees better with numerical modeling, mapping of foliation and lineation using the anisotropy of magnetic susceptibility (AMS) and seismic anisotropy measurements (Vauchez et al., 1994; Heintz et al., 2003; Egydio-Silva et al., 2005; Assumpção et al., 2006).

This paper aims to discuss the transition between the Araçuaí and Ribeira belts in the light of current geochronologic and structural data, numerical modeling and seismic anisotropy. We also discuss to what extent the structural differences are due to one single collision, or if the two belts experienced different orogenic histories.

Section snippets

Geologic setting

The Araçuaí and Ribeira orogenic segments (Fig. 2) are located in the northeastern and central parts of the Mantiqueira Province, respectively. Other Neoproterozoic Brasiliano belts, such as the Dom Feliciano belt in southern Brazil and Brasilia orogen south and west of the São Francisco craton, are somewhat older than the Araçuaí and Ribeira orogens, although with an overlap in age (Babinski et al., 1996; Silva et al., 2005; Philipp et al., 2016). The metamorphic peak in the Southern Brasilia

Geochronologic and thermal similarities between the Ribeira and Araçuaí belts

The geochronologic data available for Ribeira and Araçuaí belts, which can be separated into peak metamorphic and crystallization ages, are quite similar (Fig. 7). Pedrosa-Soares et al. (2001) consider that a continental magmatic arc was built in the core of the Araçuaí belt at ca. 625-590 Ma, and that the collision between the South American and African proto-continents occurred at 591-575 Ma. However, coupling U/Pb dating of zircon and structural analysis (Vauchez et al., 2007; Petitgirard et

The structural transition between Araçuaí and Ribeira belts

Comparing the structural geology of the Araçuaí and Ribeira belts reveals a gradual transition in tectonic style between these two belts, which is already reflected in the topographic expressions of the two domains (Fig. 8a). The Araçuaí belt shows foreland-directed thrusting with an anatectic core in which the tectonic fabric (especially lineations) displays lateral variations consistent with 3-D flow. In contrast, the central part of the Ribeira belt is characterized by a wide, well-defined

Discussion and conclusion

We have reviewed the structural situation at the transition between the Araçuaí and Ribeira belts, and shown that the two belts, despite their differences in tectonic fabric and strain regime formed concomitantly during the Neoproterozoic amalgamation of West Gondwana due to the convergence of the São Francisco and Congo cratons.

The structural geological map of the transition zone is mainly based on the Anisotropy of Magnetic Susceptibility data (Fig. 10, Fig. 11) and field observations (Fig. 8

Acknowledgements

The authors are grateful to Dr. Jeremie Lehmann and two others anonymous reviewers for very thorough reviews that permitted substantial improvements in the manuscript. We also thank CAPES for the financial support of the fieldwork. (Project CAPES-SIU: 003/2013. Integrated orogen-sedimentary basin studies. This work was supported by FAPESP projects 2013/19061-0, 2014/10146-5 (Geane Carolina Cavalcante) and 2015/23572/5 (Haakon Fossen).

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Citation Excerpt :
The Neoproterozoic–Cambrian Ribeira Belt is mostly interpreted as a NE-trending orogen that formed at the interface between the São Francisco, Paranapanema, Congo, Luís Alves and Rio de la Plata paleocontinents during West Gondwana assembly (Fig. 1A) (De Almeida et al., 1973; Campanha and Sadowski, 1999; Campos Neto, 2000; Heilbron et al., 2004, 2020; Faleiros et al., 2011; Cabrita et al., 2021). It comprises a series of tectonostratigraphic terranes (e.g., São Roque, Apiaí, Embu, Costeiro and Curitiba), generally dissected by crustal-scale NE-trending dextral shear zones developed under a transpressional system active between 900 Ma and 530 Ma (Egydio-Silva et al., 2002, 2005, 2018; Ribeiro et al., 2019, 2020b, 2020c; Conte et al., 2020; Cabrita et al., 2022; Faleiros et al., 2022). This terrane is bounded by the Taxaquara shear zone to the south and the Jundiuvira shear zone to the north (Fig. 1C).
New and compiled detrital zircon U–Pb ages from the southern Neoproterozoic–Cambrian Ribeira Belt, SE Brazil, demonstrate Laurentian affinity of the Embu Terrane which is statistically distinct from the adjoining Apiaí and São Roque terranes with cratonic affinity (e.g., São Francisco Craton). Zircon provenance results indicate that the type-area of the Embu Terrane is dominated by detrital zircon age modes at ca. 1200 Ma, 1400 Ma, and 1800 Ma, with maximum depositional age of ca. 1000 Ma. In contrast, the Apiaí and São Roque terranes are dominated by Paleoproterozoic detrital zircon ages (ca. 2200–2000 Ma age dominant component), with maximum depositional ages of ca. 1400 Ma and 1750 Ma, respectively. Multidimensional scaling (MDS) analysis of non-parametric similarity measurements on zircon age populations indicates for the first time that the Embu Terrane encompass two statistically distinct detrital zircon age spectra, which is also reflected in the metamorphic zircon age record. The statistical characterization of the Embu Terrane through populational metrics allow a quantitative comparison with surrounding tectonic domains and rock samples classified such as Embu-type. Our results clearly highlight the distinction between the statistically differentiated Embu Terrane from the Apiaí and São Roque terranes, supporting an allochthonous interpretation. In addition, we demonstrate that rocks samples previously classified as Embu-type are significantly dissimilar to the definition of Embu Terrane, failing to support alternative tectonic models (e.g., intracontinental evolution). Detrital zircon age spectra reveal that the Apiaí and São Roque terranes have similar zircon provenance to domains sourced from the São Francisco Craton, whereas detrital zircon populations from the Embu Terrane have greater affinity with SW Laurentia basins (and their inferred sediment sources), consistent with previous findings. Therefore, we interpret the Embu Terrane as a Rodinia descendant developed along the active margin of the SW Laurentia that collided with the Ribeira Belt during early Neoproterozoic (810–760 Ma).

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