Spatial Variations in Geometries of Polygonal Faults Due to Stress Perturbations and Interplay with Fluid Venting Features

Abstract

High-resolution 3-D seismic data from offshore Angola is used to document spatial variations in the planform geometry of polygonal faults (PFs) hosted in a succession of Neogene-Quaternary hemipelagite deposits. We show that the regionally isotropic PF network becomes anisotropic around pockmarks, salt stocks and salt withdrawal basins. Four main patterns are observed: ladder patterns composed of long (first-order) and short (second-order) faults which are orthogonal to each other, radial patterns around salt stocks, concentric patterns around pockmarks and in salt withdrawal basins, and a hybrid form of radial and concentric fault patterns where pockmarks occur on diapir flanks. Fluid venting structures such methane-related carbonates and chimneys stem from PF intersections. The linear-to-elliptical planform geometry of fluid venting structures is controlled by the local state of stress around PFs. The transition between radial faults and background PF occurs at 0.5-1 stock radius from the salt contact and is interpreted as a marker of the diapirs hoop stress field. Our work highlights the sensitivity of polygonal fault systems to perturbations in the regional stress state, which further controls the location and geometry of fluid venting structures.