Do Stratigraphic Variations Influence Fault Zone Architecture in Deep-Water Fold-Thrust Belts?

Rob Butler¹, Bill McCaffrey¹, and Richard Morgan^2
1 University of Leeds, Leeds, United Kingdom
² VeritasDGC, Crawley, United Kingdom

In thrust system analysis, fault zones are commonly modelled as discrete surfaces, with surrounding rock deformation explained by combination of fault shape and displacement gradients. However, high resolution 3D seismic data from the compressional parts of gravitationally-driven fold belts on continental margins show more complex fault zone architectures. While in some settings stratigraphic cut-offs against inferred faults are crisply imaged, in others there are significant zones, up to 1km wide, within which there is substantial amplitude loss. In many instances these represent zones of closely-spaced subsidiary faults, separating fault-bounded slices of strata, together with distributed strain; adjacent stratal reflectors may be deflected into the fault zone. These non-discrete fault occurrences entail different predictions for structural development, the evolution of horizon dips (and hence the geometry of growth strata), and for the connectivity of the fault zone itself. We use insights from outcrop analogues (Marnosa Arenacea, northern Apennines of Italy; Gres du Champsaur, French Alps) to aid interpretation of thrust zones from deep-water Niger delta in 3D seismic volumes. The key feature controlling the width, structural complexity and arrangement of fault-bounded sandstone lenses in outcrop (and that may enhance vertical permeability in subsurface settings) is the propensity of buckling. This in turn is favoured by well-layered, alternating sand-shale sequences. The scale of buckling and cyclic thrusting and folding are strongly influenced by the gross stacking patterns of sand that in turn effect the fold amplification. Thus locally calibrated stratigraphic knowledge may be used to predict fault zone architecture and vice versa.