CONTROLS ON THE GEOMETRY OF POLYGONALLY ORGANISED DEFORMATION

Polygonally organised deformation is seen at all scales in terrestrial geology from mud-cracks and cooling joints, to kilometre-scale structures in the sediments in the North Sea. Extra-terrestrial examples include the giant polygons that have been imaged on the surface of Mars, in the Utopia Planita region, the polygonal fabrics in the lowland flow fields of Venus, and the chaos regions on Europa. Polygonal fracture/fault systems can be divided into two catagories: Layer-bound systems in which fracture/fault propagation direction is in the plane of the layer (e.g. mudcracks), and non-layer-bound systems such as columnar cooling joints. The principal geometrical characteristics of layer-bound polygonal fracture sets are a lack of preferred fracture orientations, many more tip–to-wall type intersections than tip-to-tip linkages and log-normal frequency/length distributions.

Layer-bound systems are considered in terms of the physical conditions and mechanisms that govern the development of deformation. Numerical simulations of polygonal fracture development show that layer thickness, material properties, coupling across layer boundaries, and stress/strain history are key parameters that affect polygon geometry and size. The extent to which these key factors can be constrained by plan-view observations of polygon geometries is discussed.