Abstract: Fault growth could be achieved by (1) synchronous increases in displacement and length or (2) rapid fault propagation succeeded by displacement-dominated growth. The second of these growth models (here referred to as the constant length model) is rarely applied to small outcrop-scale faults, yet it can account for many of the geometric and kinematic attributes of these faults. The constant length growth model is supported here using displacement profiles, displacement–length relationships and tip geometries for a system of small strike-slip faults (lengths of 1–200 m and maximum displacements of 0.001–3 m) exposed in a coastal platform in New Zealand. Displacement profiles have variable shapes that mainly reflect varying degrees of fault interaction. Increasing average displacement gradients with increasing fault size (maximum displacement and length) may indicate that the degree of interaction increases with fault size. Horsetail and synthetic splays confined to fault-tip regions are compatible with little fault propagation during much of the growth history. Fault displacements and tip geometries are consistent with a two-stage growth process initially dominated by propagation followed by displacement accumulation on faults with near-constant lengths. Retardation of propagation may arise due to fault interactions and associated reduction of tip stresses, with the early transition from propagation-to displacement-dominated growth stages produced by fault-system saturation (i.e. the onset of interactions between all faults). The constant length growth model accounts for different fault types over a range of scales and may have wide application.

Abstract: The boundaries between pairs of adjacent fault segments within normal fault arrays define a spectrum of structures, from relay ramps where the length of overlap between the fault segments is much larger than the separation, through low aspect ratio (overlap/separation) relay ramps and ultimately to underlapping fault segments. Where fault segments underlap, transfer of displacement between them is accommodated by a connecting monocline. When displacement increases and a through-going fault forms, relay ramps are preserved as fault-bounded zones of elevated bed dip and monoclines are preserved as areas of normal drag. Therefore, the orientation and magnitude of bed dips within and adjacent to a fault zone, and the numbers of segments seen on a cross-section through it, depend largely on the aspect ratios of relay ramps in the initial fault array. The aspect ratio of relay ramps varies between different fault systems. An analysis of the geometry of 512 relay ramps from 13 different fault systems suggests that the main controls on aspect ratio are the strength of the sequence at the time of faulting and the underlying structure.

Abstract Post-depositional normal faults within the turbidite sequence of the Late Miocene Mount Messenger Formation of the Taranaki Basin, New Zealand are characterized by granulation and cataclasis of sands and by the smearing of clay beds. Clay smears maintain continuity for high ratios of fault throw to clay source bed thickness ( c . 8), but are highly variable in thickness, and gaps occur at any point between the clay source bed cut-offs at higher ratios. Although cataclastic fault rock permeabilities may be appreciably lower ( c . two orders of magnitude) than host rock sandstone permeabilities, the occurrence of continuous clay smears, combined with low clay permeabilities (10s to 100s nD) means that the primary control on fault rock permeability is clay smear continuity. A new permeability predictor, the Probabilistic Shale Smear Factor (PSSF), is developed which incorporates the main characteristics of clay smearing from the Taranaki Basin. The PSSF method calculates fault permeabilities from a simple model of multiple clay smears within fault zones, predicting a more heterogeneous and realistic fault rock structure than other approaches (e.g. Shale Gouge Ratio, SGR). Nevertheless, its averaging effects at higher ratios of fault throw to bed thickness provide a rationale for the application of other fault rock mixing models, e.g. SGR, at appropriate scales.

Abstract The 500-km-long strike-slip North Island Fault System (NIFS) intersects and terminates against the Taupo Rift. Both fault systems are active, with strike-slip displacement transferred into the rift without displacing normal faults along the rift margin. Data from displaced landforms, fault-trenching, gravity and seismic-reflection profiles, and aerial photograph analysis suggest that within 150 km of the northern termination of the NIFS, the main faults in the strike-slip fault system bend through 25°, splay into five principal strands and decrease their mean dip. These changes in fault geometry are accompanied by a gradual steepening of the pitch of the slip vectors, and by an anticlockwise swing (up to 50°) in the azimuth of slip on the faults in the NIFS. As a consequence of the bending of the strike-slip faults and the changes in their slip vectors, near their intersection, the slip vectors on the two component fault systems become subparallel to each other and to their mutual line of intersection. This subparallelism facilitates the transfer of displacement from one fault system to the other, accounting for a significant amount of the NE increase of extension along the rift, whilst maintaining the overall coherence of the strike-slip termination. Changes in the slip vectors of the strike-slip faults arise from the superimposition of rift-orthogonal differential extension outside the rift margin, resulting in differential motion of the footwall and hanging-wall blocks of each fault in the NIFS. The combination of rift-orthogonal heterogeneous extension (dip-slip) and strike-slip, results in a steepening of the pitch of the slip vectors on the terminating fault system. Slip vectors on each splay close to their terminations are, therefore, the sum of strike-slip and dip-slip components, with the total angle through which the pitch of the slip vectors steepens being dependent on the relative values of both these two component vectors. In circumstances where interaction of the velocity fields for the intersecting fault systems cannot resolve to a slip vector that is boundary-coherent, either rotation about vertical axes of the terminating fault relative to the through-going fault system may take place to accommodate the termination of the strike-slip fault system, or the rift may be offset by the strike-slip fault system rather than terminating into it. At the termination of the NIFS, an earlier phase of such rotations may have produced the 25° anticlockwise bend in fault strike and contributed up to about one-third of the anticlockwise deflection in slip azimuth. On the terminating strike-slip NIFS, therefore, rotational and non-rotational termination mechanisms have both played a role, but at different times in its evolution, as the thermal structure, the rheology and the thickness of the crust in the rift intersection region have changed.

Abstract Quantitative analysis of faults within a South Australian polygonal fault system, interpreted from a 3-D onshore seismic survey, provides a basis for establishing their growth and linkage histories. The geometric characteristics of faults are consistent with an origin arising from the gravitational instability of an underlying low-density, overpressured, mobile layer. Fault size populations have scale-bound, non-power-law properties reflecting the thicknesses of the faulted and mobile layers and the strongly connected nature of the system. The spatial distributions of faults reflect the localization of conjugate faults at the top of the mobile layer and the scale of fault-bounded polygons. Displacement variations on faults show marked decreases at or adjacent to the top of the mobile layer and attest to its active role in faulting. The wide range of fault strike directions provides numerous fault intersections with high intersection angles (≥60°) forming triple-junctions at which fault linkage and capture occurred. Fault linkage and capture is attributed to a simple model in which continued movement on faults which share a mutual footwall is favoured and hanging wall faults are deactivated. The model involves thickening of the mobile layer within the footwalls of faults and thinning and eventual grounding of the overlying sequence, within their hanging walls.

Abstract The growth of normal fault arrays is examined in basins where sedimentation rates were higher than fault displacement rates and where fault growth histories are recorded by thickness and displacement variations within syn-faulting sequences. Progressive strain localization is the principal feature of the growth history of normal faults for study areas from the Inner Moray Firth, a sub-basin of the North Sea, and from the Timor Sea, offshore Australia. The kinematics of faulting are similar in both study areas. Fault displacement rates correlate with fault size, where size is measured in terms of either displacement or length. Small faults have higher mortality rates than larger faults throughout the growth of the fault system. Displacement and strain are progressively localized onto the larger faults at the expense of smaller faults at progressively larger scales. Strain localization and the preferential growth of larger faults are attributed to geometric factors, such as size and location, rather than to the mechanical properties of fault rock in individual faults. This conclusion is supported by numerical models that reproduce the main characteristics of fault system growth established from both study areas.