Linked Extensional-compressional Tectonics in Gravitational Systems in the Equatorial Margin of Brazil

The Para-Maranhao and Barreirinhas basins in the equatorial margin of Brazil contain gravitational gliding systems composed of three structural domains: a proximal extensional, a distal contractional, and a transitional (or translational) domain between the two others. The main faults of these domains detach on a decollement surface of shales and marls, presumably overpressured. Several methods were applied to investigate these thin-skinned tectonics systems, including interpretation of seismic sections, physical modeling, numerical modeling, restoration of cross sections, and integration with field data. These methods indicated that thrusts developed in a classical backstepping sequence with younger thrusts developing in the hanging wall and with landward migration of depocenters through geologic time. Out-of-sequence thrusts were observed locally. The results of cross section restorations suggested that the total amount of shortening exceeded the total amount of stretching in the basal layers, close to the detachment surface, whereas stretching exceeded shortening in the upper layers. Our conclusions point out that gravitational gliding was caused by the combined effect of sedimentary loading, slope gradient, and probably, pore fluid overpressure, with gliding events being triggered by episodic reactivations of the intervening Romanche and Saint Paul fracture zones.