Analysis and significance of ductile deformation in the volume surrounding two seismically imaged normal faults

Previous studies of small-scale (displacements < 10 cm) normal faults exposed in serial crosssections show that fault tip lines are characterised by embayments and lobes that develop due to bifurcation of tip lines during fault propagation. Similar processes have been hypothesised to occur during growth of faults with throws of tens of metres or more. However, the resolution of 3D seismic reflection data is limited and the tip lines of such faults cannot be resolved in any detail. This study uses a 3D seismic volume to map the horizon dip variations in the volume surrounding two overlapping, syn-sedimentary normal faults. Our assumption is that variations in the spatial distribution and intensity of ductile deformation expressed as changes in horizon dip enable us to quantify the amount of displacement that is accommodated by folding and/or sub-seismic scale faulting at and beyond the mapped fault tip lines.

Horizon dips were calculated along transects oriented normal to fault strike and spaced every 20m along the c. 3 km long mapped fault traces. Areas of abnormally high dip with respect to the regional tilt were automatically identified and mapped onto horizon surfaces (Fig. 1a). The vertical displacement due to ductile deformation (horizon rotation) was calculated for each transect and combined with measured fault throws. The combined displacement / length profiles resemble that of a single fault, although the maximum total displacement is greater than that predicted by mapping fault throw alone (Fig. 1b). These observations suggest that our initial assumption and methodology to identify regions of fault-related ductile deformation are valid.

Our results suggest that an irregular zone of ductile deformation surrounds the mapped faults. In particular, the lateral tip point of the mapped fault system lies at least 300 m beyond the position that would be predicted by extrapolating the fault displacement gradient. The zone of ductile deformation above the upper tip line of one of the seismically imaged faults is characterised by the presence of three en-echelon monoclines. Summation of the vertical displacements across these folds again gives rise to a smoothly varying displacement profiles. We interpret these monoclines to have either developed above three overstepping, en-echelon sub-seismic scale fault segments or to represent coherent, en-echelon sub-seismic scale fault zones. These interpretations are both consistent with previously hypothesised fault tip line bifurcation growth models (Fig. 2).

More generally, our method can be used to map the 3D distribution and intensity of fault-related ductile deformation. However, outcrop observations are required to confirm the precise nature of this apparently continuous deformation.