Using combined paleostress reconstruction and slip tendency for reconstructing the brittle structural history of a complex strike-slip fault system: Fault-controlled origin and evolution of the “Pool” area between Kinshasa and Brazzaville

Reconstructing the brittle structural history of a complex strike-slip fault system remains a challenging process in paleostress reconstructions. Here, we investigated the small-scale brittle structures such as shear fractures and tension joints which are well developed in the Early Paleozoic Inkisi red sandstones in the “Pool” region of Kinshasa and Brazzaville, along the Congo River. The fracture network affects the horizontally bedded sandstones with alternating cross-bedded, horizontally bedded and massive layers. The fractures are particularly dense and of various orientation in the rapids of the Congo River just downstream Kinshasa and Brazzaville. They control the channels of the Congo River in its connection to the Atlantic Coast.

A total of 1150 factures have been measured and assembled into a single data file, processed using the Win-Tensor Program. They contain only a limited number of kinematic indicators for slip sense (displaced pebbles, irregularities on striated surfaces, slickensides) or extension (plume joints). Before interactive fault-slip data separation into subset and stress inversion, a kinematic data analysis evidenced at least three different phases of brittle deformation, each starting by the formation of plume joints and evolving into a strike-slip fault system. We used the principle of progressive saturation of the rock mass by the apparition of new faults or the reactivation of already existing ones during the successive brittle stages. We combined the stress inversion of fault-slip data, fault-slip tendency analysis and data separation in order to obtain well-separated data subsets, each characterized by its own paleostress tensor. The total data set can be explained by the action of 4 different brittle deformation and related paleostress stages, all of strike-slip type. There possible age is estimated from stratigraphic relations and the known geological history of the area.

The oldest stage developed in intact rock under NW-SE horizontal compression, probably before the Jurassic unconformity that affects the entire Congo Basin. It generated dominantly N-160°E striking left-lateral faults. The second stage generated dominantly new N050°E striking right-lateral faults, at a high angle from the ones of the previous stage, under NE-SW horizontal compression. They are estimated to be related to ridge push forces from the opening of the Atlantic Ocean during the Oligocene. The third stage, which corresponds to N-S horizontal compression, generated additional N030°E and N340°E conjugated fractures and reactivated the preceding fracture networks. A fourth and relatively minor system was also identified with WNE-ESE horizontal compression but its chronological relation with the other ones is not clear.