Passive margins evolved from a confined orogen: comparison of both sides
of the South Atlantic (D.R. Congo - east Brazil)
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.