Discussion on a model for the structure and development of fault zones: reply



C. Childs, J. Watterson & J. J. Walsh

Reply - We thank Dr James for his positive comments on the model of fault zone development which we put forward. In describing this simple model we concentrated on the geometry of the main slip surfaces which border fault zones and placed less emphasis on the complexities of the internal geometries and distributions of fault rocks within fault zones (but see Childs et al. in press). We suggest that the fault surface bifurcation mechanisms outlined in our paper are the chief agencies by which wall rock is incorporated into a fault zone. Rock volumes incorporated into a fault zone by either of these processes may become intensely deformed, resulting in a fault zone comprising 'lenses' of fault rock of varying composition and intensity of deformation (Pettinga 1982). Repetition of these processes results in internally complex fault zones. Our comments on lithological juxtaposition maps of fault surfaces were restricted to the inadequacy of assuming a simple discrete slip-surface and James' reference to Tertiary faults reinforces rather than conflicts with these comments. The implications of complex internal fault zone structure for the prediction of fault sealing are clear.

Our model was developed for brittle faults which in some resepects contrast with the ductile faults or shear zones common in deformed soft-sediments (e.g. Weber et al. 1978). However, we believe the tip-line bifuration mechanism is applicable to both brittle and soft-sediment faults, although asperity bifurcation alone could result in the fault zone structures characteristic of the poorly consolidated sediments of the Tertiary deltas described by James. Our conception of the internal geometry of a fault zone is therefore very similar to that illustrated by James which shows a degree of complexity in soft-sediment faults similar to that observed in brittle faults. For example,

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Fig. 1 illustrates the internal complexity of a brittle fault zone which offsets a lithified sequence of alternating sandstones and shales and which demonstrates extreme spatial variation in fault rock type within a zone. Sandstone and shale within the fault zone occur as lenses which taper rapidly, demonstrating the variable partitioning of slip referred to by James and predicted by our model. Observations made at a point on, or cross-section through, the fault in Fig. 1 cannot be extrapolated for any significant distance along the fault surface. As pointed out by James, predictive modelling of such a fault zone is extremely difficult, if not impossible (Childs et al. in press). This point is further illustrated by the fault zone shown on Face C in Fig. 2,

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the fault zone comprises a 2m thickness of shale and shale gouge while 8m along strike, on Face B, the fault zone is composed primarily of sandstone. The only element of the fault zone which persists along the length of the outcrop is a layer of shale gouge, which has a minimum thickness of 2cm on Face B. Other outcrop observations (Weber et al. 1978; Lindsay et al. 1993) suggest that a recurring feature of fault zones is a high degree of continuity of shaley gouge or shale smear, which is crucial in determining the hydraulic properties of faults and provides the observational basis for existing empirical methods of fault seal prediction based on shale gouge calculations.


The Journal of the Geological Society 154, 366-368, 1997.