Impact of tectonic rafts’ gravitational instability on fault reactivation and geometry

Abstract

The downslope gravitational instability of tectonic rafts can deform large volumes of supra-salt strata on continental margins. Detailed mapping of fault throw and geometry using three-dimensional (3D) seismic reflection data from the salt-rich Espírito Santo Basin (SE Brazil) shows that the complex fault geometries observed are primarily due to downslope gravitational instability of tectonic rafts. Three main tiers of faults were identified and their geometry analysed in detail. Tier 1 faults are associated with the initial stage of fragmentation of supra-salt strata into individual blocks, or rafts, which are separated by listric (roller) faults and associated minibasins. These faults comprise closely spaced normal faults that resulted from outer-arc stretching of strata overlaying discrete tectonic rafts. The faults accommodate a significant part of the bending strain occurring in the hanging-wall blocks of roller faults, thus forming a curved-polygonal planform geometry adjacently to large roller faults. Tier 2 faults are associated with a renewed stage of downslope gravitational gliding of tectonic rafts. The progressive bending of overburden strata during this stage led to the progressive faulting of overburden strata by these faults. The faults comprise closely spaced normal faults with a restricted range in fault strikes, forming a rectangular-polygonal planform geometry over rollover anticlines. Tier 3 faults are associated with a later stage of diachronous grounding of tectonic rafts. Differences in the degree of diachronous grounding of tectonic rafts are responsible for the development of these faults and subsequent shortening of the overburden strata. These faults comprise densely spaced set of normal faults with diverse range of fault strikes, forming an irregular-polygonal planform geometry over rollover anticlines. The interpreted fault tiers show fault throw maxima ranging from 50 m to 60 m, spacings of 180–420 m and trace lengths of 120–650 m. The interpreted fault tiers were reactivated and their growth is characterized by fault segment linkage. The diachronous downslope translation and grounding (welding) of tectonic rafts on sub-salt strata was an important process controlling the degree of deformation off Espírito Santo. Successive episodes of fault reactivation, and the subsequent generation of salt welds below tectonic rafts, potentially formed large conduits favouring fluid migration and the generation of hydrocarbon traps in supra-salt strata.