Volumetric contraction during the compaction of mudrocks: a mechanism for the development of regional-scale polygonal fault systems

This paper describes the geometry and strain characteristics of a complex system of small extensional faults affecting Lower Tertiary mudrock-dominated successions throughout the central North Sea Basin. Structural mapping using three-dimensional seismic data shows that the fault trace geometry is polygonal. The shallow origin of the faults is confirmed by the recognition of growth sequences developed in their hangingwalls. Line balancing techniques were used to measure the extensional strain in two survey areas. This was found to be radially isotropic in the map plane. Extension in any line of section was found to vary from 6 to 19%. Since the deformation is clearly layer-bound and there is no evidence for displacement transfer to basement structures, it is argued that the only explanation for this apparent extension is by layer-parallel volumetric contraction. This is believed to occur in response to fluid expulsion from the mudrocks during early compaction. The conditions for failure may be achieved through increased pore fluid pressure or through tensile stresses generated as a result of pore fluid loss, or a combination of these two processes. Far-field tectonic stresses are not considered to be responsible for the formation of this fault system.