Faults are much more complex and compound features that can accommodate large amounts of strain in the upper crust. The term fault is used in different ways, depending on geologist and context. A simple and traditional definition states:
A fault is any surface or narrow zone with visible shear displacement along the zone.
This definition is almost identical to that of a shear fracture, and some geologist use the two terms synonymously. Sometimes geologists even refer to shear fractures with millimeter- to centimeter-scale offsets as microfaults. However, most geologists would restrict the term shear fracture to small-scale structures and reserve the term fault for more composite structures with offsets in the order of a meter or more.
The thickness of a fault is another issue. Faults are often expressed as planes and surfaces in both oral and written communication and sketches, but close examination of faults reveals that they consist of fault rock material and subsidiary brittle structures and therefore have a definable thickness. However, the thickness is usually much smaller than the offset and several orders of magnitude less than the fault length. Whether a fault should be considered as a surface or a zone largely depends on the scale of observation, objectives and need for precision.
Faults tend to be complex zones of deformation, consisting of multiple slip surfaces, subsidiary fractures and perhaps also deformation bands. This is particularly apparent when considering large faults with kilometerscale offsets. Such faults can be considered as single faults on a map or a seismic line, but can be seen to consist of several small faults when examined in the field. In other words, the scale dependency, which haunts the descriptive structural geologist, is important. This has led most geologists to consider a fault as a volume of brittlely deformed rock that is relatively thin in one dimension:
A fault is a tabular volume of rock consisting of a central slip surface or core, formed by intense shearing, and a surrounding volume of rock that has been affected by more gentle brittle deformation spatially and genetically related to the fault.
The term fault may also be connected to deformation mechanisms (brittle or plastic). In a very informal sense, the term fault covers both brittle discontinuities and ductile shear zones dominated by plastic deformation. This is sometimes implied when discussing large faults on seismic or geologic sections that penetrate much or all of the crust. The term brittle fault (as opposed to ductile shear zone) can be used if it is important to be specific with regard to deformation mechanism. In most cases geologists implicitly restrict the term fault to slip or shear discontinuities dominated by brittle deformation mechanisms, rendering the term brittle fault redundant:
A fault is a discontinuity with wall-parallel displacement dominated by brittle deformation mechanisms.
By discontinuity we are here primarily referring to layers, i.e. faults cut off rock layers and make them discontinuous. However, faults also represent mechanical and displacement discontinuities. A kinematic definition, particularly useful for experimental work and GPS-monitoring of active faults can therefore be added:
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| Faults appear as discontinuities on velocity or displacement field maps and profiles. The left blocks in the undeformed map a) and profile (b) are fixed during the deformation. The result is abrupt changes in the displacement field (arrows) across faults. |
A fault is a discontinuity in the velocity or displacement field associated with deformation.
Faults differ from shear fractures because a simple shear fracture cannot expand in its own plane into a larger structure. In contrast, faults can grow by the creation of a complex process zone with numerous small fractures, some of which link to form the fault slip surface while the rest are abandoned.
Geometry of faults
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| Normal (a), strike-slip (sinistral) (b) and reverse (c) faults. These are end-members of a continuous spectrum of oblique faults. The stereonets show the fault plane (great circle) and the displacement vector (red point). |
Non-vertical faults separate the hanging wall from the underlying footwall. Where the hanging wall is lowered or down thrown relative to the footwall, the fault is a normal fault. The opposite case, where the hanging wall is up thrown relative to the footwall, is a reverse fault. If the movement is lateral, i.e. in the horizontal plane, then the fault is a strike-slip fault. Strike-slip faults can be sinistral (left-lateral) or dextral (right-lateral) (from the Latin words sinister and dexter, meaning left and right, respectively).
Although some fault dip ranges are more common than others, with strike-slip faults typically occurring as steep faults and reverse faults commonly having lower dips than normal faults, the full range from vertical to horizontal faults is found in naturally deformed rocks. If the dip angle is less than 30 the fault is called a low-angle fault, while steep faults dip steeper than 60 . Low-angle reverse faults are called thrust faults, particularly if the movement on the fault is tens or hundreds of kilometres.
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| Listric normal fault showing very irregular curvature in the sections perpendicular to the slip direction. These irregularities can be thought of as large grooves or corrugations along which the hanging wall can slide. |
A fault that flattens downward is called a listric fault, while downward-steepening faults are sometimes called antilistric. The terms ramps and flats, originally from thrust fault terminology, are used for alternating steep and sub-horizontal portions of any fault surface. For example, a fault that varies from steep to flat and back to steep again has a ramp-flat-ramp geometry.
Irregularities are particularly common in the section perpendicular to the fault slip direction. For normal and reverse faults this means curved fault traces in map view, as can be seen from the faults of the extensional oil field. Irregularities in this section cause no conflict during fault slippage as long as the axes of the irregularities coincide with the slip vector. Where irregularities also occur in the slip direction, the hanging wall and/or footwall must deform. For example, a listric normal fault typically creates a hanging-wall rollover.
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| The main faults in the North Sea Gullfaks oil field show high degree of curvature in map view and straight traces in the vertical sections (main slip direction). Red lines represent some of the well paths in this field. |
A fault can have any shape perpendicular to the slip direction, but non-linearity in the slip direction generates space problems leading to hanging or footwall strain.
The term fault zone traditionally means a series of sub-parallel faults or slip surfaces close enough to each other to define a zone. The width of the zone depends on the scale of observation – it ranges from centimetres or meters in the field to the order of a kilometre or more when studying large-scale faults such as the San Andreas Fault. The term fault zone is now also used inconsistently about the central part of the fault where most or all of the original structures of the rock are obliterated, or about the core and the surrounding deformation zone associated with the fault. This somewhat confusing use is widespread in the current petroleum related literature, so any use of the term fault zone requires clarification.
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| A horst (a), symmetric graben (b) and asymmetric graben (c), also known as a half-graben. Antithetic and synthetic faults are shown. |
Two separate normal faults dipping toward each other create a down thrown block known as a graben. Normal faults dipping away from each other create an up thrown block called a horst. The largest faults in a faulted area, called master faults, are associated with minor faults that may be antithetic or synthetic. An antithetic fault dips toward the master fault, while a synthetic fault dips in the same direction as the master fault. These expressions are relative and only make sense when minor faults are related to specific larger-scale faults.
Displacement, slip and separation
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| Illustration of a normal fault affecting a tilted layer. The fault is a normal fault with a dextral strike-slip component (a), but appears as a sinistral fault in map view (b, which is the horizontal section at level A). (c) and (d) show profiles perpendicular to fault strike (c) and in the (true) displacement direction (d). |
Displacement, slip and separation
The vector connecting two points that were connected prior to faulting indicates the local displacement vector or net slip direction. Ideally, a strike-slip fault has a horizontal slip direction while normal and reverse faults have displacement vectors in the dip direction. In general, the total slip that we observe on most faults is the sum of several increments (earthquakes), each with its own individual displacement or slip vector. The individual slip events may have had different slip directions. We are now back to the difference between deformation sensu stricto, which only relates the undeformed and deformed states, and deformation history. In the field we could look for traces of the slip history by searching for such things as multiple striations.
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| Classification of faults based on the dip of the fault plane and the pitch, which is the angle between the slip direction (displacement vector) and the strike. |
A series of displacement vectors over the slip surface gives us the displacement field or slip field on the surface. Striations, kinematic indicators and offset of layers provide the field geologist with information about direction, sense and amount of slip. Many faults show some deviation from true dip-slip and strike-slip displacement in the sense that the net slip vector is oblique. Such faults are called oblique-slip faults. The degree of obliquity is given by the pitch (also called rake), which is the angle between the strike of the slip surface and the slip vector (striation).
Unless we know the true displacement vector we may be fooled by the offset portrayed on an arbitrary section through the faulted volume, be it a seismic section or an outcrop. The apparent displacement that is observed on a section or plane is called the (apparent) separation. Horizontal separation is the separation of layers observed on a horizontal exposure or map, while the dip separation is that observed in a vertical section. In a vertical section the dip separation can be decomposed into the horizontal and vertical separation. Note that this horizontal separation is different from. These two separations recorded in a vertical section are more commonly referred to as heave (horizontal component) and throw (vertical component). Only a section that contains the true displacement vector shows the true displacement or total slip on the fault.
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| The relationship between a single fault, a mapped surface and its two fault cutoff lines. Such structure contour maps are used extensively in the oil industry where they are mainly based on seismic reflection data. |
A fault that affects a layered sequence will, in three dimensions, separate each surface (stratigraphic interface) so that two fault cut off lines appear. If the fault is non-vertical and the displacement vector is not parallel to the layering, then a map of the faulted surface will show an open space between the two cut-off lines. The width of the open space, which will not have any contours, is related to both the fault dip and the dip separation on the fault. Further, the opening reflects the heave (horizontal separation) seen on vertical sections across the fault.
Stratigraphic separation
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| (a) Missing section in vertical wells (well C) always indicates normal faults (assuming constant stratigraphy). (b) Repeated section (normally associated with reverse faults) occurs where the normal fault is steeper than the intersecting well bore (well G). |
Drilling through a fault results in either a repeated section or a missing section at the fault cut (the point where the wellbore intersects the fault). For vertical wells it is simple: normal faults omit stratigraphy (figure a), while reverse faults cause repeated stratigraphy in the well. For deviated wells where the plunge of the well bore is less than the dip of the fault, such as the well G (figure b), stratigraphic repetition is seen across normal faults. The general term for the stratigraphic section missing or repeated in wells drilled through a fault is stratigraphic separation. Stratigraphic separation, which is a measure of fault displacement obtainable from wells in subsurface oil fields, is equal to the fault throw if the strata are horizontal. Most faulted strata are not horizontal, and the throw must be calculated or constructed.
Credits: Haakon Fossen (Structural Geology)
