D.C.P. Peacock

Glossary of normal faults

Increased interest in normal faults and extended terranes has led to the development of an increasingly complex terminology. The most important terms are defined in this paper, with original references being given wherever possible, along with examples of current usage.

Nucleation and growth of strike-slip faults in limestones from Somerset, U.K.

Abstract Small-scale structures along strike-slip fault zones in limestones exposed around the Bristol Channel, U.K., suggest that pressure solution plays a key role during fault nucleation and growth. Incipient shear zones consist of enechelon veins. The first generation of solution seams form due to bending of the intact rock (bridge) between overlapping veins. As the bridge rotates, slip occurs along the seams, linking the veins, causing cm-scale calcite-filled pull-apart structures to form and allowing fault displacement to increase. A second generation of solution seams forms at the tip of the sliding seams. As displacement increases further, causing larger rotation, slip also can occur along these second-generation solution seams, producing the third generation of solution seams as well as tail cracks (pinnate veins) at their tips. These three generations of solution seams all contribute to the formation of individual fault segments. Fourth and fifth generations of solution seams occur within larger-scale contractional oversteps between side-stepping fault segments. The oversteps are breached by slip along these localized solution seams, eventually leading to the formation of a distinct through-going fault with several metres of displacement. The initial enechelon veins, solution seams of various generations and tail cracks progressively fragment the fault-zone material as fault slip accumulates. Slip planes nucleate on these pre-existing discontinuities, principally along the clay-enriched, weaker solution seams. This can be observed at a variety of scales and suggests that Mode II shear fracturing does not occur as a primary fracture mechanism, but only as a macroscopic phenomenon following Mode I (veins and tail cracks) and anti-mode I (solution seams) deformation. It appears that solution seams can play a similar role to microcracks in localizing a through-going slip plane. This micromechanical model of faulting may be applicable to some other faults and shear zones in host rocks which are prone to pressure solution.

Strain and stress

Structural analyses of specific features in naturally deformed rock consist of geometric observations (e.g. shape), kinematic measurements (e.g. strain), and dynamic models (e.g. stress). Although analytical definitions clearly distinguish strain and stress, common usage of the terms tends to blur the conceptual diAerence. Strain and stress do not have a simple cause-and-eAect relationship. The fundamental diAerence between strain and stress is that strain terms reflect descriptive interpretations of what movements produced a structure, while stress terms reflect genetic interpretations of why the structure formed. This descriptive vs genetic distinction has several implications. First, kinematic analysis is less speculative and more directly related to observations than dynamic analysis. Second, kinematic analysis is less computationally and analytically intensive than dynamic analysis. Third, kinematic analysis is amenable to more intuitive, but shallower, understanding than dynamic analysis. The most useful terminology communicates this conceptual framework through clear and accurate use of terms for strain, stress, and related concepts. A variety of examples illustrate the descriptive and genetic usage of strain and stress terminology. # 1999 Elsevier Science Ltd. All rights reserved.

Active relay ramps and normal fault propagation on Kilauea Volcano, Hawaii

Individual segments of the Koae Fault System, Hawaii, show four patterns of structures around fault tips, and these are inferred to represent four evolutionary stages of fault growth. These are: (1) monoclinal bending occurs, probably above a steeply dipping fault at depth, (2) cracks develop in the hinges of the monocline, (3) throw starts to develop on the cracks when they reach a width of about 3 m, which is probably when they link downwards to the normal fault, and (4) rollover and related cracks develops in the hanging wall as throw increases. Widths of monocline- and fault-related cracks obey a power-law distribution, with a 3 m upper cut-off, beyond which the monocline- and fault-related cracks develop a throw and become faults. Relay ramps are common within the highly segmented Koae and Hilina active normal fault systems. Three distinct geometries of relay ramps can be identified at Kilauea Volcano, and these are inferred to represent the following three evolutionary stages of relay ramps. (1) Where the bounding faults understep, the relay ramps have a gentle dip, and a set of en echelon cracks may cut across the relay ramp; these cracks suggest that the two understepping faults connect into a single fault at depth. (2) The dip of the relay ramp increases as the faults overstep. Connecting faults start to cut across the relay ramp. (3) When the relay ramp is breached by the connecting fault, a single, irregular fault is produced. Cracks or small breaching faults across a relay ramp suggest the bounding faults are connected at depth, and suggest that the bounding faults may both slip during an earthquake event.

The temporal relationship between joints and faults

Abstract Examples are presented of three temporal relationships between joints and faults: joints that pre-date faults; joints that are precursors to, or synchronous with, faults; and joints that post-date faults. Emphasis is placed on strike-slip faults in carbonate beds, but other examples are used. General rules are given for identifying the three temporal relationships between joints and faults. Joints that formed before faults can be dilated, sheared or affected by pressure solution during faulting, depending on their orientation in relation to the applied stress system. Faulted joints can preserve some original geometry of a joint pattern, with pinnate joints or veins commonly developing where faulted joints interact. Joints formed synchronously with faults reflect the same stress system that caused the faulting, and tend to increase in frequency toward faults. In contrast, joints that pre- or post-date faults tend not to increase in frequency towards the fault. Joints that post-date a fault may cut across or abut the fault and fault-related veins, without being displaced by the fault. They may also lack dilation near the fault, even if the fault has associated veins. Joints formed either syn- or post-faulting may curve into the fault, indicating stress perturbation around the fault. Different joint patterns may exist across the fault because of mechanical variations. Geometric features may therefore be used in the field to identify the temporal relationships between faults and joints, especially where early joints affect or control fault development, or where the distribution of late joints are influenced by faults.

Joints in the Mesozoic sediments around the Bristol Channel Basin

Abstract Analysis has been carried out at four locations on the edges of the Bristol Channel Basin to illustrate the later phases of deformation of a sedimentary basin, and to illustrate the control on joint patterns of subtle changes in the stress system. The characteristics of the joints are described and influences on joints are determined, including the roles of faults, folds and beds. There is a low coefficient of correlation between joint spacing and bed thickness, except in very thin limestone beds, which have a high density of joints. The lengths and spacings of earlier joint phases are usually greater than those of later phases. Later joints normally abut against earlier joints. The joints abut the latest faults but are not displaced by them, so the joints post-date the main Alpine contraction. The joints formed in five main phases during reduction of the Alpine stresses. Phase 1 joints are sub-parallel to the regional compression direction (160–180°). Phase 2 joints are perturbed by faults, often curving towards points of stress concentrations along the faults. Phase 3 joints are sub-parallel to the earlier E–W-striking fold axes. Phase 4 joints are cross-joints, and phase 5 joints form polygonal patterns within joint-bound blocks. Phases 2 and 3 do not occur in the absence of faults and folds, and correspond with a reduction in horizontal compression and an increase in the importance of local factors. Phases 4 and 5 occur at all locations.

Selective reverse-reactivation of normal faults, and deformation around reverse-reactivated faults in the Mesozoic of the Somerset coast

Abstract Normal faults exposed in the Triassic–Jurassic limestones and shales of the Somerset coast were formed during the Mesozoic development of the Bristol Channel Basin. Reverse-reactivation of some of these normal faults occurred during Late Cretaceous to Early Tertiary north–south contraction. The contraction is also evident from thrusts and conjugate strike-slip faults. Preferential reactivation of the normal faults is attributed to: (1) decreased fault-plane friction, (2) domino block rotation, (3) displacement magnitude, and (4) fault connectivity. The geometries of overlapping and underlapping zones in reactivated fault zones are dependent on the existing structural geometry. Two distinctive styles of displacement accommodation occur between reverse-reactivated normal faults: (1) formation of a network of strike-slip faults, conjugate about NNE–SSW, and (2) oblique steeply-dipping reverse faults. Interaction between strike-slip and an existing fault is dependent on whether the normal fault was reactivated. The range of structures related to the north–south contraction has been incorporated into a single deformation model, controlled by the northwards movement of the hanging wall of the Quantocks Head Fault. Pure dip-slip movement occurred in the centre of its curved fault trace, with a sinistral component at the western tip, and a dextral component of displacement and strike-slip block rotations occurred at the eastern tip. Shortening of these blocks was achieved through development of a strike-slip fault network and NW-striking thrusts. In an underlap zone, loading of the footwall by the hanging wall block modified the local stress system to allow formation of oblique, steeply-dipping reverse faults.

Linkage and evolution of conjugate strike-slip fault zones in limestones of Somerset and Northumbria

Conjugate strike-slip fault zones that cover metre-scale areas at Beadnell, Northumbria, and Kilve, Somerset, were initiated as conjugate vein arrays. Early conjugate faults are linked by the propagation of one fault that eventually by-passes the other fault. A model for the development of strike-slip faults is presented, using fault and vein geometries and the position of damage zones with respect to the master faults as an indication of the propagation direction. This model includes the evolution of networks from (1) the initial random development of vein arrays, to (2) the isolated development of several unconnected conjugate fault segments that pass into vein arrays, through (3) the intersection of a conjugate set of master faults and linkage with minor antithetic faults, and the formation of new vein arrays with extensional geometries after a linked network of faults is established, to (4) breaching of intersection points by dominant faults, and finally (5) the propagation towards oversteps that are breached to form a through-going fault. The geometry of the active structures simplifies with time, as strain is localised along the master fault, but the complexities are preserved in the fault walls.

Use of curved scanlines and boreholes to predict fracture frequencies

Abstract We advance the method of Hudson and Priest (Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts 20 (1983) 73–89) to develop a method for a curved scanline to be used to predict the numbers of fractures that would be observed in any direction. When sampling along a scanline, the probability of intersecting a fracture is influenced by the relative orientations of the fracture and of the scanline at that location. This sampling bias can be minimised by the use of the Terzaghi correction, w =(cos χ ) −1 , where χ is the angle between the scanline and the normal to the fracture. These corrected frequencies are used to simulate fracture frequencies for all other orientations by doubly-correcting the data. Modelled fracture frequency is contoured on a graph of simulated scanline plunge against simulated scanline azimuth. This method is based upon the assumption that the data collected along the scanline is representative of the fracture population when the Terzaghi correction has been applied. A graph of cumulative frequency of fractures against distance along a scanline provides a simple method for determining whether the scanline crosses differently fractured areas. Frequencies are corrected for dip, strike, and both dip and strike, with data from homogeneously fractured areas plotting as straight lines. These frequencies can be normalised for ease of comparison.

The World’s biggest relay ramp: Hold With Hope, NE Greenland

Abstract Fault interaction in the Hold With Hope region of NE Greenland occurs between basin-margin faults that have a separation of about 100 km, with the relay ramp covering an area of about 25u2008000 km 2 . This structure is therefore much larger than previously described relay ramps, showing that interaction between normal faults can occur over large areas and can control deformation across a region. The Western Fault Zone links north and eastwards with the Hochstetters Forland Fault via the Gauss Halvo Fault. These faults that control the relay ramp have kilometre-scale throws, juxtaposing Pre-Caledonian basement against Upper Palaeozoic and Mesozoic cover. The relay ramp initiated during the Devonian, but was at least partially breached at the end of the Devonian or beginning of the Carboniferous. Beds in the relay ramp are tilted towards the footwall, this tilt being similar to the results of recent numerical models of interacting normal faults. The relay ramp is affected by faults that are synthetic to, and that link, the basin-margin faults. These breaching faults suggest that stresses can interact over distances of at least 100 km. This model explains variations in the depth of the Moho across Kong Oscar Fjord. The basin-margin faults may be linked at depth, passing down into a relatively shallow detachment, or into a lower-crustal shear zone. Alternatively, the faults may not be directly connected at depth, but pass down into a zone of distributed ductile deformation.