Three-dimensional fault meshes and multi-layer shear in mass-transport blocks: Implications for fluid flow on continental margins

Submarine mass-transport deposits (MTDs) frequently include blocks of strata with variable deformation styles. In this paper, a 3D seismic volume from SE Brazil is compared with outcrop information from SE Crete to investigate multi-scale deformation patterns within mass-transport blocks. Block morphology in the two study areas relates to the orientation of internal faults and fractures, with wide chasms separating block fragments that opened along these planar features. Two families of sub-perpendicular faults associated with intra-block extension are observed at multiple scales. On seismic data, we show new evidence of intra-block horizontal shearing and thrusting. At outcrop, downslope verging folds, load casts and injectites are observed along poorly consolidated sandy intervals that are less than 1 m thick. Documented cross-cutting geometries suggest a syn-kinematic origin for these structures. By integrating the observations made at different scales we suggest the development of 3D meshes in mass-transport blocks, which are characterised by the intersection of faults and fractures with bed-parallel shear planes. These meshes greatly increase the permeability of individual blocks. The results in this work have implications for the analysis of fluid flow paths on continental margins, and show that mass-transport blocks can comprise important fluid bypass points when buried in low permeability strata.