Abstract - The growth of normal fault arrays is most conveniently examined in basins where sedimentation rates were higher than fault displacement rates. In these basins fault growth histories are recorded by thickness and displacement variations within syn-faulting sequences and can be reconstructed using displacement backstripping methods. Using data from the Inner Moray Firth, a sub-basin of the North Sea, and from the Timor Sea, offshore Australia, the growth history of normal fault systems is shown to be characterised by progressive strain localisation.
The growth of normal fault systems in the Inner Moray Firth and in the Timor Sea has several features in common:
(i) Fault displacement rates correlate with fault size, where size is measured in terms of either displacement or length.
(ii) Large faults have higher displacement rates than smaller faults throughout the growth of the fault system.
(iii) Smaller faults have higher mortality rates and large faults grow at their expense.
(iv) Fault size populations vary from continuous power-law distributions at earlier stages of growth, through to non-power-law distributions as the system matures. As a fault system evolves, large faults progressively dominate, with small faults occupying a size range that is progressively distinct from that of the large faults.
(v) The progressive localisation of displacement and strain onto large faults, is accompanied by an increase in the connectivity of large faults, at the scale of observation.
These characteristic features of fault system growth support the case for the existence of long-range correlations between different faults within a system throughout its growth. Because of the pronounced geometric controls, in particular fault connectivity, demonstrated by the overall growth of these systems, we attribute the preferential growth of larger faults also to geometric factors (i.e. fault size, connectivity, orientation), rather than to differences between the fault rock mechanical properties of individual faults. A subordinate role for fault rock mechanical properties is supported by numerical models that reproduce the main characteristics of fault system growth, without differences between the mechanical properties of faults. Mechanical weakness is an important characteristic of faults at the early, fault localisation, stages of system growth. Thereafter, for a given fault system, the fault rock mechanical properties of different faults may be of significance on short, i.e. seismic cycle, time scales, but are not considered to exert the prime control over fault system growth on geological time scales.
Abstract of talk given to:
The nature of the tectonic significance of fault zone weakening, London, 8-9 March 2000