The enigma of the Albian Gap: spatial variability and the competition between salt expulsion and extension

The Albian Gap is a uniquely large (up to 65 km wide and >450 km long), enigmatic salt-related structure in the Santos Basin, offshore Brazil. It is located near the basin margin and trends NE (i.e. subparallel to the Brazilian coastline). The gap is characterized by a near-complete absence of Albian strata above depleted Aptian salt. Its most remarkable feature is an equivalently large, equally enigmatic, basinward-dipping, supra-salt rollover that contains a post-Albian sedimentary succession that is up to 9 km thick. Owing to its unique geometry, size and counter-regional aspect, the origin and evolution of the Albian Gap has been the centre of debate for >25 years. This debate revolves around two competing models; that is, did it form as a result of thin-skinned (i.e. supra-salt) extension, or progradational loading and salt expulsion? The extension-driven model states that the Albian Gap (and overlying rollover) formed as a result of post-Albian gravity-driven extension accommodated by slip on a large, counter-regional, listric normal fault (the Cabo Frio Fault). Conversely, the expulsion-driven hypothesis states that the Albian Gap was established earlier, during the Albian, and that post-Albian deformation was controlled by differential loading, vertical subsidence and basinward salt expulsion in the absence of significant lateral extension. This study utilizes a large (c. 76 000 km2), dense (4–8 km line spacing), depth-migrated, 2D seismic dataset that fully covers and thus permits, for the first time, a detailed, quasi-3D structural analysis of the entire Albian Gap. In this study we focus on (1) the evolution of base-salt relief and the original salt thickness variations and (2) the geometry of the post-Albian rollover, and its related faults and salt structures. To constrain the kinematics of the Albian Gap, and how this relates to the evolution of the base-salt relief, we also apply novel structural restoration workflows that incorporate flexural isostasy, in addition to a detailed, sequential reconstruction of the intra-gap rollover sequences. Our results show that the geometry and kinematics of the Albian Gap vary along-strike, and that both post-Albian extension and expulsion play a significant role in its evolution. Basinward-dipping growth wedges, salt rollers and listric normal faults record extension, whereas sigmoidal wedges, halokinetic sequences and upturned near-diapir flaps, the latter two associated with large diapirs bounding the downdip edge of the gap, record basinward salt expulsion and inflation. Where the Albian Gap is relatively wide (>50 km), these processes alternate and operate at approximately equal proportions. Our results are consistent with the amount of basinward translation inferred from the analysis of ramp–syncline basins located downdip on the São Paulo Plateau. Our results seemingly reconcile one of the longest-running debates in salt tectonics, as well as having more general implications for understanding the regional kinematics and dynamics of salt-related structures in other salt basins, in particular the controls on the development of large, counter-regional faults. Supplementary material: Uninterpreted versions of the seismic sections are available at https://doi.org/10.6084/m9.figshare. c.5023088