Spatial and mechanical controls on normal fault populations

Characteristics of normal faults have been measured from outcrops in western Sinai and northern Britain, and from the subsurface in the North Sea basin. Fault damage zone widths and the spatial distribution of minor structures around faults have been related to variation in throw along a fault surface. Secondary shear fractures, including faults and granulation seams, are most common within damage zone envelopes, in the immediate footwall and hangingwall, in the extensional field of major normal faults. Shear fractures are less common in the contractional field of normal faulted rock volumes. Fault zone thickness measurements from outcrop indicate scale invariance over several orders of magnitude within the same location and lithology. Fault zone thickness is linearly correlated to fault throw. There is a strong mechanical and lithological control on fault zone thickness. Coarse sandstones have thicker fault zones for a given throw, due to the processes of strain hardening and cataclasis, compared with argillaceous rocks which are prone to strain softening. Fault spacing in siliciclastic rocks can show scale invariance over several orders of magnitude. The influence of grain size and layer thickness on fault growth can lead to either non-fractal behaviour or changes in the power law for fault characteristics such as throw, thickness and spacing.