The geometry of polygonal fault systems in Tertiary mudrocks of the North Sea

Polygonal networks of extensional faults have recently been identified deforming large volumes of Tertiary mudrocks in the North Sea. The main distinctive feature of this fault system is a near equal distribution of fault strike orientations and polygonal organisation of the fault trace segments in plan view. Detailed fault mapping using three-dimensional seismic reflection datasets from the central North Sea show that the complex three-dimensional geometries are primarily a function of intersections between fault surfaces oriented in all directions in space. Within stratigraphic units or layers the deformation is distributed as opposed to the clustered deformation of more co-linear tectonic fault arrays where the strain is concentrated into zones or bands at a variety of scales. End member map patterns with curved, rectilinear or irregular polygonal geometries are identified that encompass the variability observed on over 30 fault maps made at different stratigraphic levels within the three-dimesional seismic datasets. ain classes of intersections between fault segments are defined providing a descriptive geometric framework for polygonal fault systems. The geometry of the intersection types and their geometrical stability in three-dimensions is discussed with reference to examples from the seismic datasets. shapes are irregular and, in detail, shape is largely a function of intersection relationships with neighbouring faults. Faults can be planar or listric, and can have triangular shapes that may taper upwards or downwards. They frequently exhibit complex irregular upper tips. Completely isolated non-connected faults are very rare within the three-dimensional datasets studied.