Leonardo Muniz Pichel

Salt Tectonics in the Santos Basin, Brazil: the effects of translation, base-salt relief and the enigma of the Albian Gap

Salt-driven translation over variable base‐salt relief produces three‐dimensionally complex and multiphase deformation. Understanding how pre- and base-salt structures influence salt‐related deformation is important to constrain regional salt tectonic kinematics (i.e. direction, timing and magnitude of salt flow) and the distribution of salt tectonic domains. The São Paulo Plateau, Santos Basin, Brazil is characterized by a >2 km thick, Aptian salt deposited above prominent base‐salt relief. We use 3D seismic data, numerical and kinematic models to investigate how gravity‐driven translation above thick salt, underlain by variable base‐salt relief, generated a complex framework of salt structures and minibasins. We show that ramp‐syncline basins (RSBs) developed above and downdip of the main pre‐salt highs and record c. 30 km of post-Albian basinward translation. As salt and overburden moved downdip, salt flux variations over base‐salt relief resulted in the coeval development of extensional, contractional and load-driven salt structures in the São Paulo Plateau. This style of salt deformation and the magnitude of translation afforded an improved kinematic model for the enigmatic Albian Gap, located immediately updip of the São Paulo Plateau.

The Albian Gap is a uniquely large (up to 65 km wide and >450 km long) structure characterized by the near-complete absence of Albian strata above depleted Aptian salt, and a 8-9 km thick basinward-dipping post-Albian rollover. 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? We combine regional 2D PSDM seismic data and novel structural restoration workflows that incorporate flexural isostasy, in addition to a detailed, sequential reconstruction of the intra-gap rollover sequences. We 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. Where the Albian Gap is relatively wide (>50 km), these processes alternate and operate at approximately equal proportions. Our results seemingly reconcile one of the longest-running debates in salt tectonics, having also more general implications for understanding the regional kinematics and dynamics of salt tectonics in other basins, in particular the controls on the development of large, counter-regional faults and the effects of translation and pre-salt relief.