Systematics of Faults and Fault Arrays

Reactivation involves the accommodation of geologically separable displacement events (intervals >1 Ma) along pre-existing structures. The definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings. In neotectonic environments, reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime. This mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors. Four sets of criteria may be used to recognize reactivation in the geological record: stratigraphic, structural, geochronological and neotectonic. Some structural criteria may not be reliable if used in isolation to identify reactivated structures. Much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend, dip or three-dimensional shape of structures. Numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long-term and may be invoked to explain reactivation. The collage of fault-bounded blocks forming most continents therefore carries a long-term architecture of inheritance which can explain much of the observed complexity of continental deformation zones.

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The recognition of reactivation during continental deformation

Holdsworth

Butler

Roberts

1997

JGS

254

120

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Assessment of the effects of sub-seismic faults on bulk permeabilities of reservoir sequences

Walsh

Watterson

Heath

et al. 1998

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Single-phase horizontal bulk permeabilities for 3 km × 3 km volumes of varying thickness of a typical Brent permeability sequence have been calculated, both before and after faulting by a range of sub-seismic fault arrays, with maximum fault displacements of 20m – 2m. The models incorporate realistic juxtaposition geometries across fault surfaces. Results are expressed in terms of fractional bulk permeabilities ( Kf ), i.e. ratio of bulk permeability of faulted model and of pre-faulting model. Fault and fault array variables modelled and tested were fault density, spatial distribution, orientation distribution and fault zone permeability relative to the host rocks, expressed as transmissibility factor ( Tf ). Realistic fault zone thicknesses were incorporated by use of a scaling factor. Low, moderate and high fault densities have significant and markedly different effects on Kf whereas the effects of spatial and orientation distribution variations are slight except at very low Tf values ( Tf < 0.001). The relative insignificance of fault spatial distributions is due to closer fault spacing resulting in locally high hydraulic gradients which increase flow through fault surfaces unless these surfaces have very low Tf values. Prediction of fault zone hydraulic properties remains the most important factor contributing to modelling uncertainties.

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Geometric controls on the evolution of normal fault systems

Walsh

Childs

Meyer

et al. 2001

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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.

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Systematics of Faults and Fault Arrays

Cited by 3 publications

References 68 publications

The recognition of reactivation during continental deformation

The recognition of reactivation during continental deformation

Assessment of the effects of sub-seismic faults on bulk permeabilities of reservoir sequences

Geometric controls on the evolution of normal fault systems

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