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

Abstract

<p>The Albian Gap is an enigmatic salt-related feature in the Santos Basin, offshore Brazil. It is a uniquely large, up to 65 km wide and >450 km long structure, located in the updip portion of the basin and trending NE (i.e. sub-parallel to the coast). The gap is characterized by the near-complete absence of Albian strata above depleted Aptian salt. Its most remarkable feature is an equivalently large, post-Albian seaward-dipping rollover that is up to 9 km thick. Due to its unique geometry, size, and counter-regional aspect, the Albian Gap has been the centre of debate for >25 years. This debate revolves around two competing models for its origin and evolution; i.e. did it form due to thin-skinned extension, or progradation loading and expulsion? The extension-driven model invokes that the rollover and the Albian Gap formed due to post-Albian gravity-driven extension associated with a large, counter-regional, listric normal fault, the Cabo Frio Fault. Conversely, the expulsion-driven hypothesis suggests 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 without significant lateral extension. This study utilizes a large (c. 76,000 km<sup>2</sup>) and dense depth-migrated, 2D seismic dataset that covers and which thus permit a detailed, 3D structural analysis of the entire Albian Gap, focusing on i) base-salt relief and original salt thickness variations and ii) the geometry of the post-Albian rollover, and its related faults and salt structures. We also apply novel structural restoration workflows incorporating flexural isostasy, along with a detailed sequential reconstruction of the rollover sequences, to constrain the kinematics of the Albian Gap, and how this relates to base-salt relief. 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 on its evolution. Seaward-dipping growth wedges, salt rollers and normal listric faults evidence extension, whereas sigmoidal wedges, halokinetic sequences, and upturned near-diapir flaps, the latter two associated with inflated diapirs bounding the downdip edge of the gap, indicate 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 observed amount of basinward translation further downdip within ramp basins in the Sao Paulo Plateau and seemingly reconciles one of the longest-running debates in salt tectonics. Our results have 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, salt-detached, counter-regional faults.</p>