Geometry and Kinematics of the Central Fault Zone, Fula Sag, Central Africa Shear Zone

Abstract

The Central Africa Shear Zone (CASZ) harbors abundant hydrocarbon resources within its Central Fault Zones (CFZs). The studies of CASZ have dominantly focused on the evolution and superimposition processes of prototype basins in CASZ. Meanwhile, research on the geometry and segmental growth of main faults in CFZs remains poorly understudied, which limits hydrocarbon exploration. In this paper, we focus on the CFZ of the Fula sag as an example of CASZ and utilize the 3-D throw mapping technique along with the maximum throw subtraction method to investigate its geometric and growth processes. Results show faults in the northern and central parts of the CFZ form multiple Y-shaped combinations, and a system of sub-parallel faults in the south forms the bookshelf faults. Meanwhile, the divergent overlapping transfer zone is identified in the CFZ. Our investigation found abrupt changes in throw-distance diagrams of main faults in the CFZ, which indicate that the main faults, F1, F3 and F2, are laterally segmented into 4, 4, and 3 segments, respectively. As an intracontinental passive rift basin, the Fula sag has undergone three major rifting cycles since the Early Cretaceous, triggered by the segmental expansion of the Atlantic Ocean, the rapid opening of the Indian Ocean, and the separation of the Red Sea. Our analysis also reveals that the main faults in the CFZ were primarily active during the second rifting, with the fault segments undergoing isolated growth, soft linkage, and eventually forming fully grown faults during the third rifting. We observe a significant decrease in activity intensity during the transition between the second and third rifting cycles. Our findings provide insights into the growth and activity of the CFZ faults, which are applicable to other CFZs of similar origin in rift basins, and provide suggestions for hydrocarbon exploration and production.