Antithetic fault linkages in a deep water fold and thrust belt

Deep water fold and thrust belts consist of both forethrusts and backthrusts that can link along strike to form continuous folds in the overburden. The interaction of faults of opposing dip are termed ‘antithetic thrust fault linkages’ and share the common feature of a switch in vergence of overlying hangingwall anticlines. Using three-dimensional seismic data, on the toe-of-slope of the Niger Delta, linkages are classified into three distinct structural styles. This preliminary classification is based on the vertical extent of faulting within a transfer zones relative to the branch line of the antithetic faults. The stratigraphic level of the lateral tip of the fault, the shape of lateral tip region of a fault plane and the stratal deformation within the transfer zones is also distinctive in each type of fault linkage. A Type 1 linkage comprises faults that overlap exclusively above the level of the branch line. A ‘pop-up’ structure forms within the transfer zone with sediments below remaining planar. The lower tip lines of faults climb stratigraphically towards the linkage zone creating asymmetric, upward-tapering lateral tip regions. In Type 2 linkages fault overlap occurs lower than the level of the branch line such that lateral fault tips are located within the footwall of the counterpart fault. Faulting is thus limited to the deeper section within the transfer zone and creates unfaulted, symmetric, bell-shaped folds in the overburden. Upper tip lines of faults lose elevation within the transfer zone creating asymmetric, downwards-tapering lateral tip regions. In Type 3 linkages both faults continue above and below the branch line within the transfer zone resulting in cross-cutting fault relationships. Horizon continuity across the folds, through the transfer zones, varies significantly with depth and with the type of fault intersection.