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. sub-parallel 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 as enigmatic, seaward-dipping, supra-salt rollover that contains a post-Albian sedimentary succession that is up to 9 km thick. Due to its unique geometry, size, and counter-regional aspect, the origin and evolution of the Albian Gap has been the ...  more

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. sub-parallel 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 as enigmatic, seaward-dipping, supra-salt rollover that contains a post-Albian sedimentary succession that is up to 9 km thick. Due 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; i.e. did it form due to 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 due to 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 which thus permits, for the first time, a detailed, quasi-3D structural analysis of the entire Albian Gap. In this study we focus on: i) the evolution of base-salt relief and the original salt thickness variations; and ii) 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. Seaward-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.