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Analysis and Significance of Ductile Deformation in the Volume Surrounding Two Seismically Imaged Normal Faults

Long, Jonathan J.1; Imber, Jonathan 1
1 Earth Sciences, Durham University, Durham, United Kingdom.

Previous studies of outcrop scale normal faults show that fault tip lines are characterised by embayments and lobes that develop due to bifurcation of tip lines during fault propagation. 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 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 fault traces. Areas of abnormally high dip with respect to the regional tilt were automatically identified and mapped onto horizon surfaces. 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. 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, ductile displacement is mapped to extend at least 400 m past the mapped lateral fault tip. 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.

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.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009