Scale-relationships and geometry of normal faults reactivated during gravitational gliding of Albian rafts (Espيrito Santo Basin, SE Brazil)

Three-dimensional (3D) seismic-reflection data from SE Brazil are used to investigate the geometry and significance of normal faults reactivated during gravitational gliding of Albian rafts. Late Cretaceous gravitational instability in the study area led to the fragmentation of Albian strata in individual rafts separated by listric (roller) faults and associated sub-basins (Stage 1). Renewed gravitational gliding caused Albian rafts to be translated downslope until salt welds were formed and Albian strata became grounded over pre-salt sequences (Stage 2). Diachronous grounding promoted the reactivation of faults and folds, which at places developed into pop-up structures and broad anticlines sub-parallel to the strike of individual rafts (Stage 3). In this paper we present a new methodology to identify and date reactivated (i.e. shortened) fault families. Statistical analyses of six main fault families confirm that fault reactivation was diachronous in the study area, and its cessation depended on: i) the complete grounding of Albian rafts or, ii) gravitational stabilisation of the rafts imposed by sediment loading and cessation of tectonic tilting of the slope. The results of this study show that tectonically generated pulses of compression and uplift, commonly related to the Andean Orogeny, are not the only mechanisms deforming post-salt overburden units in proximal extensional-dominated regions of the southeast Brazilian margin. This is a crucial piece of information for geodynamic reconstructions and hydrocarbon exploration, as the close control of Albian rafts on fault reactivation induced a continuum of overburden deformation in the Espيrito Santo Basin, rather than discrete episodes of inversion.