Renata da Silva Schmitt

Cambrian orogeny in the Ribeira Belt (SE Brazil) and correlations within West Gondwana: ties that bind underwater

Abstract A 530–490 Ma tectono-metamorphic event, the Búzios orogeny, is recognized within the Ribeira Belt, along the coast of SE Brazil. Tectonic evolution started with a Late Neoproterozoic marine basin and volcanic activity at c. 610 Ma. The rocks in this basin were affected by high-grade metamorphism at c. 530 Ma, coeval with deformational phases D1–D2, which generated compressive low-angle tectonic structures with top-to-NW tectonic transport. Large recumbent folds with NW–SE axes parallel to the main stretching lineation formed during D3 as the Cabo Frio tectonic domain, the focus of this study, collided with the Oriental terrane to the NW. D4 sub-vertical shear zones are limited in extent. A new U–Pb age of 501±6 Ma is reported for zircon from an amphibolite-facies shear zone related to either D3 or D4. Post-tectonic 440 Ma pegmatites mark the final stage of tectono-magmatic activity. The Cabo Frio tectonic domain has African affinities and is exotic to the Ribeira Belt. Middle Cambrian deformational and metamorphic ages are also reported from the ‘Angolan’ Pan-African belt, the southern Kaoko and Damara belts in Namibia, and the Cuchilla Dionisio–Punta Del Este terrane in Uruguay. The occurrence of Cambrian metamorphic rocks along the present African and South American coastlines shows that Mesozoic rifting closely follows Palaeozoic sutures of West Gondwana.

O significado tectônico do Domínio Tectônico do Cabo Frio na margem Sudeste brasileira: a saga de uma margem continental retrabalhada do Paleoproterozoico ao Cretáceo

The Cabo Frio Tectonic Domain is composed of a Paleoproterozoic basement tectonically interleaved with Neoproterozoic supracrustal rocks (Buzios-Palmital successions). It is in contact with the Neoproterozoic-Cambrian Ribeira Orogen along the SE Brazilian coast. The basement was part of at least three continental margins: (a) 1.97 Ga; (b) 0.59 - 0.53 Ga; (c) 0.14 Ga to today. It consists of continental magmatic arc rocks of 1.99 to 1.94 Ga. Zircon cores show a 2.5 - 2.6 Ga inheritance from the ancient margin of the Congo Craton. During the Ediacaran, this domain was thinned and intruded by tholeiitic mafic dykes during the development of an oceanic basin at ca. 0.59 Ma. After the tectonic inversion, these basin deposits reached high P-T metamorphic conditions, by subduction of the oceanic lithosphere, and were later exhumed as nappes over the basement. The Cabo Frio Tectonic Domain collided with the arc domain of the Ribeira Orogen at ca. 0.54 Ga. It is not an exotic block, but the eastern transition between this orogen and the Congo Craton. Almost 400 m.y. later, the South Atlantic rift zone followed roughly this suture, not coincidently. It shows how the Cabo Frio Tectonic Domain was reactivated as a continental margin in successive extensional and convergent events through geological time.

How to make a transverse triple junction—New evidence for the assemblage of Gondwana along the Kaoko-Damara belts, Namibia

T-shaped orogenic triple junctions between mobile belts usually form in two unrelated stages by subsequent and oblique continental collisions separated by a significant time span. Besides these “oblique triple junctions”, another type, named “transverse triple junctions”, may exist. Such junctions are created by a more complex mechanism of partly contemporaneous convergence of three cratons in a restricted time frame, involving strike slip. The Neoproterozoic–Cambrian Kaoko-Damara junction between the Rio de la Plata, Congo, and Kalahari cratons in Namibia is an example of such a transverse orogenic triple junction, formed by at least four subsequent but partly related deformation events. Initial north-south convergence between the Congo and Kalahari cratons was followed by east-west collision of the Rio de la Plata and Congo cratons. Subsequently, the Kalahari and Congo cratons collided, contemporaneous with sinistral strike-slip motion between the Congo and Rio de la Plata cratons and with the intrusion of large granite-syenite plutons, probably associated with slab detachment aided by the strike-slip movement. Other examples of transverse triple junctions may exist in Gondwana. Transcurrent shear zones, some possibly nucleated on transform faults from the pre-collision setting, are essential for the formation of transverse triple junctions.

Suturing Gondwana in the Cambrian: The Orogenic Events of the Final Amalgamation

Gondwana was consolidated in the late Cambrian after 180 myr of tectonic convergence among Neoproterozoic paleocontinents and smaller fragments. We present a compilation of 55 orogens that record its final amalgamation. Collisional events are registered by the metamorphic peak assemblages and contractional deformational structures. Two main periods of orogenic activity are recognized. The first at c. 670–575 Ma includes few orogens (c. 15) but over larger areas. During this stage, the Saharan, West African, Sao Francisco-Congo and Paranapanema paleocontinents, along with the Arabian Nubian shield that consisted of juvenile Tonian terranes and some East African Orogen microcontinents, were accreted to form the proto-Gondwana core. The second stage, at 575–480 Ma, incorporated more orogens, c. 40, that sutured the Amazonia, Rio de La Plata, Kalahari, Dhawar, East Antarctica and Australian paleocontinents. The collisional orogen pattern throughout both western and eastern Gondwana is similar, indicating that although Gondwana was built up by the convergence of distinct paleocontinents, their approximation might be orchestrated by global geodynamics. In SW Gondwana, the opening of c. 610–570 Ma basins, some with oceanic crust, coincided with the suturing of the proto-Gondwana core. They were rapidly formed but even more quickly inverted during the second and last 575–480 Ma collisional stage, represented by the major east–west Kuunga and north–south South Atlantic belts. We propose that the 570–500 Ma collision of the Damara-Lufilian-Zambesi belt was coeval with the Cuchilla Dionisio-Saldania-Gariep-Dom Feliciano-Kaoko-Ribeira-Cabo Frio orogens. This South Atlantic orogenic system consists of the main SW Gondwana suture, reactivated 350 myr later to form the South Atlantic Ocean. The suture is preserved by old orogenic high-pressure and oceanic-derived rocks on the actual conjugate continental margins.

Estudo Magnetometrico da Região de Cabo Frio

O presente projeto visa estudar o terreno Cabo Frio e as rochas que o compoem, fazendo uso da magnetometria, que fornece informacoes sobre as estruturas em subsuperficie. Um estudo comparativo com o mapa regional aeromagnetico fornecera ainda subsidios que permitem verificar a causa das grandes anomalias magneticas observadas e que caracterizam o arcabouco estrutural deste terreno. Como a regiao de Cabo Frio e cortada por inumeros diques maficos que possuem uma grande susceptibilidade magnetica, e, por conseguinte um provavel alto valor der magnetizacao remanescente, foi realizado um primeiro perfil sobre um dique de diabasio de direcao NE-SW e 15 metros, para entender melhor como se comporta a anomalia magnetica sobre o campo regional, gerada por uma estrutura desse porte. A partir do conhecimento da forma da anomalia, pudemos interpretar melhor os dados obtidos nos demais perfis. Um segundo perfil foi tracado estrategicamente sobre uma area com diversas dessas estruturas aflorantes e nao aflorantes, observando assim, a variacao no campo das anomalias. Com os resultados obtidos, pudemos observar a grande influencia dessas estruturas igneas maficas, atribuindo a elas a causa de anomalias no campo magnetico da regiao, que se apresentam sob a forma dos lineamentos magneticos positivos de alta intensidade observados no mapa aeromagnetico.