# Geometry and Kinematics of the Central Fault Zone, Fula Sag, Central Africa Shear Zone
## 7 Discussion  
### Kinematics of the Study Area  
The evolution of the Fula sag is subject to the break-up of the Atlantic Ocean, the northward movement of the Indian Plate, and the openness of the Red Sea.  
The first rifting of the Fula sag was mainly influenced by Atlantic opening [45; 46; 47; 48]. When the shear movement extended north-eastward out of the Congo Craton in the right-lateral strike-slip CASZ, the tips of the CASZ were changed into an extensional stress field, inducing the development of two NS-striking boundary faults in the Fula sag (Figure 1a faults 1 and 2), and a near NS-tectonic striking (Figures 11a and 12a). During this period, the CFZ was not developed (Figure 9).  
The second rifting of the Fula sag is mainly influenced by the rapid opening of the Indian Ocean, which then created a near-NE-striking stress field in central Africa and caused the Fula sag to enter the second rifting [50; 61; 62]. Combined with the authors' previous study [20], it can be found that the second rifting not only caused the development of the southwestern fault-array-and-ramp zone and the CFZ in a near-perpendicular stress field (Figure 11b), but also sustained the activity of the east-west boundary faults (Figure 1a faults 1 and 2). The tectonic pattern of the Fula sag's \"overall NS-trending, internal tectonic NW-trending\" was also finalized during this period (Figure 12b). The activity intensity of the CFZ was the strongest in the second rifting (Figure 9). The fault segments were in the isolated growth stage during the second syn-rift and partially entered the soft-linkage stage during the second post-rift.  
Figure 11: Evolution of the main faults in the Fula sag. The CFZ faults depicted in Figure 1b represent F1, F3, and F2 from south to north. The black arrows represent the extension direction during the corresponding periods. Noted that during the first rifting, the Fula sag experienced extension in the east-west direction, whereas in the subsequent two rifting phases, the extension direction shifted to the northeast direction.  
The third rifting of the Fula sag is mainly subject to the separation of the Red Sea. When the Red Sea Rift began to expand into the newborn ocean during the Pliocene [49], an east-to-west weakening, near-NE-striking extensional stress field was forming within the African plate [50; 51]. Its main impact on the Fula sag is the formation of the main faults in the northeastern fault-array-and-ramp zone, while the rest of the main faults in the sag continued to be active (Figures 11c and 12c). The activity intensity of the CFZ was then the weakest (Figure 9). The fault segments here eventually formed full-grown faults during the third rifting.  
In a nutshell, the first rifting was mainly influenced by the Atlantic Ocean opening, which led to the development of two boundary faults in the Fula sag and the Fula sag began to form a near north-south-trending tectonic pattern. The CFZ was not developed during this period. The second rifting is influenced by the opening of the Indian Ocean. The development of the southwestern fault-array-and-ramp zone and the CFZ began. The Fula sag, therefore, formed the tectonic pattern of \"overall north-south-trending, internal tectonic north-west-trending\". The activity intensity of the CFZ was then the strongest and the fault segments entered the soft-linkage stage. The third rifting is influenced by the separation of the Red Sea and the formation of the northeastern fault-array-and-ramp zone began. The activity intensity of the CFZ was then the weakest. The fault segments went through the hard-linkage stage and eventually formed full-grown faults. Finally, it should be noted that during the last two rifting cycles, the opening of the Fula sag under the NE direction introduces a left-lateral strike-slip kinematic regime to the NS-striking boundary faults.