Structural Analysis of Multiple Phases of Gravity Deformation in the Foz Do Amazonas Basin Using Seismic Interpretation and Experimental Modeling

The tectonic evolution of passive margins affected by gravity-driven shale tectonics can occur through episodic events, reflecting critical stages of pore-fluid overpressure in weak lithological levels, which can lead to gravitydriven deformation (gravity gliding/spreading) of the overlying sedimentary package above multiple décollements levels. Based on preliminary results of seismic interpretation, experimental models were developed to replicate possible scenarios for the structural evolution of the gravitational deformation that have affected the marine sequences of the Foz do Amazonas Basin. The resulting structural setting reminds in many aspects the gravity-driven features described in the basin. An initial gravity gliding phase of a flat-layered model resulted in an upslope set of extensional faults and a downslope gravitational fold-and-thrust belt. During the second deformation phase, the gravity spreading of progradational wedges over the sliding sheet previously deformed by gravity gliding was able to promote the<br>gradual reactivation of the compressive belt either in compression or sometimes reactivated in extension, as the shelf progrades. In all experiments, two décollements levels acted at some time or place supporting the gravitational deformation. The mechanical behavior of these décollements (silica microspheres layers) varies in time and space based on the magnitude of pore-fluid overpressure and, thus, resembles the behavior of overpressured shales.