Polygonal fault systems and channel boudinage: 3D analysis of multidirectional extension in analogue sandbox experiments

A systematic set of analogue models has been carried out to study the structural evolution of a sedimentary pile consisting of superposed brittle and ductile lithologies in a regional context of passive margins. The models are composed of silicone representing a ductile basal layer of evaporites or overpressured shales, and sand progressively sedimented on top to simulate brittle overburden sediments. Channels with various shapes are embedded in the silicone. Deformation is triggered by gravitational instability of the growing sedimentary pile and basal slope angles ranging from 0 to 5°. The main results follow.
• A polygonal fault pattern developed, due to the 3D boudinage of the overburden sand above the silicone layer during multidirectional extension.
• If a sandy channel was embedded in the ductile base, it was immediately cut by faults, which were perpendicular to the channel boundaries, no matter what the angle between channel and slope direction.
• The boundaries of the channel preferentially localised faults, which propagated through the sedimentary pile above. Ongoing deformation along these faults therefore strongly influenced further accumulation of sediments.