Structural Characteristics and Genesic Mechanism of Antiformal Negative Flower Structures: Insights from the Doseo Basin, Central-West African Rift System

Unlike classical positive and negative flower structures, antiformal negative flower structures represent a unique structural type developed under strike-slip tectonic inversion regimes. Rarely reported in previous studies, the genesic mechanism of such structures remains poorly understood, particularly the mechanical reasons that inhibit the inversion of pre-existing normal faults under compressional conditions. Based on detailed interpretation of high-resolution seismic data, this study systematically investigates the tectonic setting, structural characteristics, and formation mechanism of antiformal negative flower structures in the Doseo Basin of the Central–West African Rift System, and discusses their implications for the structural evolution of strike-slip inversion basins and the development of hydrocarbon traps.

The Doseo Basin is located within the basin-controlling domain of the Central-West African Rift System and experienced two major tectonic episodes: transtension during the Early Cretaceous and intense tectonic inversion during the Eocene. These tectonic events resulted in the development of multiple types of inversion-related structures, including fault-associated, thrust-related, fold-dominated, and antiformal negative flower structures. Antiformal negative flower structures are mainly developed within the central low-relief uplift belt of the basin. In planar view, these structures are arranged in en echelon with a NWW-SEE trend, whereas in cross-section they are characterized by an antiformal uplift controlled by a set of normal faults. During the inversion stage, the pre-existing normal faults were not reactivated to reverse faults; instead, the strata experienced pronounced compressional arching. Notably, the spatial extent of the anticlinal uplift closely coincides with the distribution of the normal faults. Genetic analysis indicates that under the Early Cretaceous transtensional stress field, basement weak zones were reactivated, leading to the formation of normal faults and the initial development of negative flower structures. During this stage, the scale, vertical extent, and activity intensity of the normal faults were established. During the Eocene tectonic inversion, regional transpressional stress was superimposed on the negative flower structure system. However, constrained by two key factors, the relatively high mechanical stability of the early transtensional structures (related to fault cementation and lithological properties of surrounding rocks) and high dipping of the normal faults, the inversion-stage stress failed to reach the critical threshold required for fault polarity reversal. Instead, it was only sufficient to induce compressional arching of the strata, ultimately resulting in antiformal negative flower structures characterized by the preservation of pre-existing normal faults combined with an antiformal uplift.

This study demonstrates that the preservation of normal faults is jointly controlled by insufficient inversion-stage stress and the mechanical stability of pre-existing transtensional fault systems. These findings expand current genetic models of structural styles in strike-slip inversion basins and provide new geological constraints for structural interpretation and hydrocarbon trap prediction in the Doseo Basin and other analogous basins.