Shallow Fault Systems Of Thrust Anticlines Responding To Changes In Accretionary Prism Lithology (Nankai, Se Japan)

Three-dimensional (3D) pre-stack depth migrated seismic data are used to analyse the geometry and growth of shallow faults associated with tectonic shortening in four (4) prominent thrust anticlines off Nankai, SE Japan. The four thrust anticlines show a trenchward increase in horizontal shortening and deform the seafloor at present. They shortened the overburden strata by 7143 m in the Late Quaternary, reflecting a horizontal shortening of 32.9% in response to plate subduction. A significant number of closely spaced and segmented fault arrays is observed in their hinge regions. Vertically segmented fault arrays with local throw maxima between 5 and 14 m relate to the existence of more competent (strong) intervals, or layers. Incompetent (weak) intervals record relatively small throw values between 2 and 5 m. We show that the presence of closely spaced, segmented fault arrays at shallow stratigraphic levels can have a significant impact on local stress distribution, controlling near-seafloor strain in accretionary prisms as Nankai's. The observed mechanical layering is likely to continue at depth to control the accumulation of tectonic stress in faults posed to reactivate during seismic events.