Kattenhorn, S.A (1999)
Characteristics and Evolution of Segmented Normal Faults in 3-D
The 3-D geometries of normal faults defining the Wytch Farm oilfield, southern England, have been interpreted from a high quality 3-D seismic reflection survey. In the deepest portions of the survey (~1600m depth), where faults were active the longest, the faults are laterally continuous. Higher in the section, younger faults form isolated segments. The deeper faults may thus have formed through the linkage of initially isolated segments. This hypothesis is supported by multiple maxima in the fault surface slip distributions, indicating individual segment nucleation locations. Multiple slip maxima are restricted to specific stratigraphic depths, predominantly in sandstone or limestone, indicating significant lithologic control on fault nucleation. As a result, laterally-segmented fault systems dominated, forming composite fault surfaces as the segments linked together along their lateral edges. Subtle indicators in the seismic data allow the determination and corroboration of such linkages.
Fault interactions play a major role in the definition of tipline shapes. Steep tiplines occur where the through-going fault of a conjugate pair scissors from one fault to the other, or where mechanical interaction arrests the growth of laterally overlapping faults. Horizontal upper and lower tiplines developed as a result of interaction between vertically stepping faults, with vertical offsets occurring across thick shale horizons. Shaley lithologies exert a major control on fault growth, often impeding vertical linkage. Consequently, predominantly lateral propagation and segment linkage produced very long, rectangular faults. Fault seal continuity may thus be expected to extend a great distance along strike, while being more discontinuous down the fault dip as a result of vertical steps. For earthquake-producing normal faults, such a geometry introduces vertical barriers to rupture propagation and may contribute to why aftershock distributions tend to spread out further laterally than vertically.
Viewed from directly above, the spatial arrangement of faults in the fault system defines a systematic quilt-like patchwork geometry in 3-D space, with each fault filling a hole in the patchwork. This pattern of faulting is consistent irrespective of fault dip direction, and implies that the 3-D extent of propagation of each fault is greatly controlled by the geometries and arrangement of other faults in the patchwork. Each individual fault formed through the linkage of numerous smaller segments, thus defining a secondary patchwork system for each composite fault surface. Fault evolution is thus governed by a hierarchical system of segmentation that develops composite faults through segment linkage and controls the relative arrangements of composite faults in 3-D space. These insights into 3-D fault geometry may be useful for the development of fault evolution models and for understanding earthquake rupture propagation, mechanical interaction between faults, leakage points in hydrocarbon reservoirs, and complex slip distributions.