Thin-skinned extensional salt tectonics, counterregional faults, and the Albian Gap of Brazil

Abstract

Thin-skinned extension detached on salt is mostly well understood. Faults may dip basinward or landward (counterregional) and intersect the top salt or equivalent weld at cusps/rollers. The polarity of faulting may switch over time, and with sufficient extension, the oldest suprasalt strata separate into rafts with intervening younger depocenters. Salt may break through extensionally thinned overburden to grow as passive diapirs, which can widen during ongoing extension but collapse if the salt supply cannot keep pace with extension. Counterregional normal faults are the most debated structures as they may form by: extension, with the footwall moving basinward relative to the hanging wall; or progressive basinward salt expulsion but only vertical movement of the overburden. Extensional counterregional structures are characterized by: (i) strata that predominantly thicken into the bounding structures, but with areas of basinward thinning due to fault/salt geometry and displacement transfer; (ii) highly elongate plan-view geometries; and (iii) the presence of coeval distal translation and contraction. The most controversial counterregional system is the Albian Gap of the Santos Basin, Brazil. We use the criteria above to argue that it formed by post-Albian thin-skinned extension, with little/no extension during the Albian. This contrasts with the Campos Basin along strike to the NE, where Albian and later extension were accommodated on basinward-dipping faults, and is explained by the rift architecture and consequent base-salt dip. The Aptian salt dipped basinward in the Campos Basin and extension was therefore triggered by Albian gravity gliding. However, the base salt dipped landward over a large area of the northern and central Santos Basin, so that Albian gravity gliding was minimal and Late Cretaceous progradational loading drove gravity spreading and the development of the counterregional system. Accommodation for sediment within the Albian Gap was provided in part by stretching and consequent thinning of the underlying salt.