Analysis of the relationships between strain, polarity and population slope for normal fault systems

The evolution of normal fault populations and their synthetic and antithetic sub-populations has been studied for a number of fault systems imaged from seismic reflection data. Relationships between fault strain and polarity and the slope of fault throw populations have been investigated by backstripping a fault system in the Inner Moray Firth, North Sea, and by comparison between throw populations for pre-faulting horizons from an additional 11 fault systems with variable extensional strains. Fault population slopes decrease with increasing strain reflecting strain localisation onto progressively fewer and larger faults. Synthetic and antithetic sub-populations also show an inverse relationship between slope and the strain they accommodate, but the slopes of antithetic sub-populations are independent of the total strain on the fault system. The slopes of antithetic sub-populations are higher than those of synthetic sub-populations from the same area and the difference in slope correlates with the polarity of the fault system, i.e. the proportion of the total strain accommodated by the two sub-populations. The data are consistent with progressive decrease in the slopes of sub-populations until the antithetic fault sets become inactive. The absence of a correlation between strain and polarity is attributed to the effects of fault system reactivation.