Fault surface bifurcation

Fault surface bifurcation processes result in areas of a fault zone with paired bounding slip surfaces alternating with areas with only a single slip surface. Either laterally or up-/down-dip, the two slip surfaces of fault zone B may give way to a single slip surface. Similarly, it is unlikely that fault zone A is characterised everywhere by only a single slip surface. As... 

Fig. I. Cartoon illustrating the asperity bifurcation model of fault zone widening. An irregularity on a fault surface (grey fill) in (a) is sheared off by the formation of a new slip surface in (b). Subsequent fault movement may result in deformation of the newly formed slip surface bounded lens.

Asperity bifurcation is due to the shearing off of fault surface irregularities by the formation of new slip surfaces. These irregularities may occur anywhere on a fault surface and on any scale. Irregular... 

Fig. 2. Successive stages of the tip-line bifurcation process of fault zone widening and generation of paired bounding slip surfaces (see text). The tip-line of a fault surface (e), part of which is shown shaded in (a)-(d), propagates upwards through a rock volume. The area shown in (a)-(d) is indicated by the rectangle in (e). With fault growth the elliptical tip-line bounding the fault surface propagates radially to the successive positions, a-d, shown in (e). The lines labelled I-III in (a) indicate successive positions of the fault surface tipline.

The bifurcation mechanisms for formation of multi-slip fault zones suggest that maximum fault zone thickness will often correspond to the strike-normal distance between the traces of two overlapping slip surfaces (Fig. 2c). Fault overlaps and their breached equivalents occur on faults of all sizes as do, by implication, paired and multi-slip surface fault zones. Complex and paired slip surface fault zone structures will occur on scales below that resolvable by even high quality seismic data (lateral resolution is no better than 50-100 m at North Sea reservoir depths). The possible impact of sub-seismic complexity and paired slip surfaces on connectivity and sealing across faults offsetting an Upper Brent type sequence are briefly considered below. 

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