Abstract - There are two contrasting models for the formation, and eventual linkage, of segmented normal fault arrays. Over the past 10 years, the preferred model attributes their formation to the interaction of initially isolated normal faults. An alternative view is that segmented fault arrays form as by-products of the localisation and 3-D propagation of individual faults within heterogeneous rock volumes. The distinction between these two models is crucial as acceptance of one model over the other has a profound impact on our perception of fault growth and linkage. We suggest that, in most cases, attributing fault growth to the incidental overlap and linkage of previously isolated faults arises from a 2-D view of fault geometry, with the implicit assumption that fault growth approximates a 2-D process. This perspective places great significance on the 2-D linkage process, with hard-linkage providing the prime means of fault growth, without acknowledging the potential kinematic equivalence of hard- and soft-linkage (e.g. such as relay zones and associated relay ramps). The alternative view, that segments arise from the 3-D segmentation and bifurcation associated with the propagation of individual faults, acknowledges that on arbitrary inspection planes soft-linked segmented arrays may link into a single surface in 3-D or may evolve into a hard-linked array with increasing displacement. This model predicts instantaneous interaction, and kinematic coherence, of fault segments and attaches less significance to the progressive change from soft- to hard-linkage; segment linkage is a local response to high strains at segment boundaries (i.e. relay zones). Using examples of segmented fault arrays from outcrop, analogue and seismic datasets, we show that they arise from the propagation of individual faults and that the scale of segmentation can sometimes be related to the nature of the faulted sequence. These arrays form kinematically coherent systems that are equivalent to single faults, both in terms of aggregate displacement profiles and deformation of the surrounding rock volume. With continued fault growth subsequent hard-linkage is inevitable and whilst this linkage may have a profound effect on the appearance of a fault map, the impact of faults on fluid flow or the scatter on indiscriminate fault scaling plots, its impact on fault growth, such as displacement and mortality rates on geological time scales, may be rather limited.
Abstract of talk given to:
AGU Annual Conference, San Francisco, December 2003.