Obliquity Variation and Polyphase Faulting During Rifting of the Outer High of the Santos Basin—Offshore Brazil

Abstract

Quantifying rift obliquity and coaxiality from seismic data remains intrinsically difficult in polyphase rifted margins, where fault populations of different ages, hierarchical orders and spatial scales are commonly analysed together. This practice can blur kinematically meaningful signals and leads to interpretations that are hard to compare across domains and through time. This study aims to (1) present a seismic‐based structural workflow that explicitly separates syn‐rift fault systems by relative chronology, hierarchical order and analytical scale prior to kinematic analysis, and (2) use that workflow to evaluate how rift obliquity and coaxiality vary through successive rifting stages. The approach integrates 3D seismic interpretation of fault geometry with stratigraphic truncation and cross‐cutting relationships. Fault systems and stratigraphic units are classified hierarchically and then analysed using kinematically constrained paleostress inversion. Rift obliquity ( λ ) is defined as the angular relationship between extension direction and rift trend, whereas rift coaxiality (Θ) expresses the degree to which successive deformation phases share a common principal deformation axis. The workflow documents a non‐monotonic evolution of rift obliquity across rifting stages. Early stretching is characterised by relatively high obliquity and low‐to‐moderate coaxiality, consistent with distributed deformation and strong influence of inherited structural fabrics. During the necking stage, deformation reorganises into a near‐orthogonal and highly coaxial configuration, interpreted as the most kinematically efficient phase of rifting on the outer proximal margin. As rifting progresses, obliquity decreases overall, but its expression becomes strongly dependent on scale and hierarchy: higher‐order fault systems preferentially record basinward trends, while lower‐order structures keep greater freedom to adapt locally through linkage, segmentation and relay development. Rift obliquity and coaxiality are not scale‐invariant properties. Basin‐scale kinematic trends may coexist with pronounced local variability, and non‐coaxial deformation can develop without major rotations of the regional stress field. By forcing an explicit separation of fault systems by time, hierarchy and scale, the proposed workflow provides a transferable framework for comparative kinematic analysis in other polyphase rifted margins where seismic interpretation is the primary constraint. Key limitations include uncertainties in plate‐kinematic reconstructions, seismic resolution, fault classification and the non‐uniqueness of palaeostress solutions; conversely, the main value of the method lies in resolving relative temporal trends and scale‐dependent behaviour rather than defining a single basin‐wide obliquity value.