Tectono-structural evolution and comparative brittle deformation in the West Congolian Supergroup (foreland of the belt) and the Inkisi Group on both sides of the Congo River, Republic of the Congo and the Democratic Republic of the Congo

Abstract

Previous studies on deformations in the western border of the Congo Basin have neglected the utilization of modern methods and the inclusion of a post-Pan-African tectonic perspective in their interpretations. This present study aims to address this gap by characterizing the deformations and late tectonic evolution, notably post-Pan-African, in the foreland of the West-Congo Belt, as well as in the Inkisi Group, situated in the Republic of Congo and the Democratic Republic of Congo. This research also encompasses an examination of seismicity in the Gulf of Guinea region. Focal mechanism analysis indicates that earthquakes in the Gulf of Guinea are influenced by three stress regimes: north-south compression along the continental margin, northeastsouthwest compression within the continent, and east-west compression in the Congo Basin. Utilizing these stress regimes, we have computed the slip tendency of fault systems in the Inkisi Group, the West-Congo Belt, and the Souanké Massif. Slip tendency analysis reveals that high-dip strike-slip/normal faults-oriented NNW-SSE and NNE-SSE have a significant potential for reactivation in the continental margin regime, while thrust faults and low-dip normal faults oriented NW-SE and NE-SW are more likely to be reactivated within the continental context. The Inkisi Group and the foreland of the West-Congo Belt exhibit nearly similar tectonic styles but have distinct tectonic origins. The Inkisi Group has experienced four phases of progressive deformation. The first phase (D1), marked by NO-SE compression, coincides with the Gondwanide orogeny during the PermianTriassic. The second phase (D2), characterized by ONO-ESE compression, results from intraplate compression at the end of the Cretaceous during the Santonian. The third phase (D3), associated with N-S compression, is linked to intraplate compression between the African and Eurasian plates during the Maastrichtian. Finally, the fourth phase, marked by ENE-OSO compression, results from the intraplate propagation of compressive stresses due to the Oligocene opening of the Atlantic Ocean, which also influenced the capture of the Congo River. Meanwhile, the foreland of the West-Congo Belt unveils a folded thrust belt structure resulting from four phases of deformation. The initial phase (D0), characterized by ENE-WSW compression, gave rise to the belt itself and its foreland, featuring thrusts and ductile-brittle shear zones, and resulted from the collision between the São Francisco and Congo cratonic blocks. The second, third, and fourth phases are characterized by brittle strike-slip deformations resulting from the same tectonic events described in phases D1-D2, D3 and D4 of the Inkisi Group, and are associated with the current stress field of seismic activity in the Gulf of Guinea.

Keywords: West-Congo belt, Inkisi Group, strike-slip faults, folds, thrusts, earthquakes.