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Passive margins evolved from a confined orogen: comparison of both sides of the South Atlantic (D.R. Congo - east Brazil)
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  • Gerben Van Ranst,
  • Tiago A. Novo,
  • Daniel Baudet,
  • Antônio Carlos Pedrosa-Soares,
  • Luc Tack,
  • Johan De Grave
Gerben Van Ranst
isni|0000000120697798|Universiteit Gent

Corresponding Author:[email protected]

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Tiago A. Novo
isni|0000000121814888|Universidade Federal de Minas Gerais
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Daniel Baudet
Musée royal de l'Afrique centrale
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Antônio Carlos Pedrosa-Soares
isni|0000000121814888|Universidade Federal de Minas Gerais
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Luc Tack
isni|0000000121556508|Koninklijk Museum voor Midden-Afrika
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Johan De Grave
isni|0000000120697798|Universiteit Gent
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Abstract

The evolution and denudation histories of passive margins around the world has been subject to many studies involving thermochronology over the last thirty years. This, mostly with emphasis on margins showing high elevation, low-relief plateaus. Thermochronology, and even more, multi-method thermochronology has hereby been proven to be an ideal tool to reveal information concerning the tectonic evolution of passive margins. One outcome returning from these studies, is the evidence of tectonic activity (exhumation and denudation and/or reburial) after continent break-up and the formation of the passive margin. The processes attributed to this tectonic activation are diverse, including flexural riftflank uplift, magmatic underplating, continental and oceanic far-field stress and significant climate change as the continental break-up progresses. Many uncertainties concerning the geodynamic context of (elevated) passive margins however still remain, as is discussed in a recent review by Green et al. (2018). The Araçuaí – West Congo orogen (AWCO) was formed during the Pan-African – Brasiliano orogeny (late Neoproterozoic to early Cambrian), confined in an embayment of the São Francisco – Congo craton (SFCC). During the Early Cretaceous (~130 Ma), the AWCO separated, due to the opening of the South Atlantic Ocean and the break-up of Gondwana. Today we find the eastern part of the AWCO on the African continent as the West Congo Belt (Angola, D.R. Congo, Congo Brazzaville and Gabon), and its western counterpart, the Araçuaí orogenic belt, on the South American continent (east Brazil). The Araçuaí orogen and the West Congo Belt are both bordered by the São Francisco craton to the west and the Congo craton to the east, respectively. The unique setting of this region resulted into two passive margins, with a range of structures inherited from its Proterozoic formation history, edged by cratonic domains. Furthermore, these margins have dissimilar morphology, the Brazilian margin having the typical elevated, low-relief morphology, and the Congolese margin close to sea-level. In this study we will compare the Araçuaí and the West Congo Belt in terms of their thermal history. We analysed apatite fission tracks in basement samples from transects on both side of the South Atlantic (east Brazil – D.R. Congo). The Brazilian samples reveal AFT mean ages between c. 100 and 50 Ma and mean track lengths in range 12 to 14 μm, with a majority over 13 μm. We interpreted a major denudation event around 80 Ma, which is in agreement with other studies from east Brazil. This is often correlated and attributed to a drastic change in spreading geometry of the South Atlanic around ~75 – 80 Ma. For the Congolese AFT ages, we expect at least ages related to the opening of the South Atlantic (c. 130–100 Ma). If the 80 Ma event in Brazil is linked to geometry changes in the South Atlantic, this is also expected to be found in the West Congo Belt.