The definition of a geological fault and why most dictionaries get it wrong.

The definition of a geological fault, and why most dictionaries get it wrong

The most important structures for any mineral explorer to understand are faults.

But what exactly, is a fault? To geologists the answer seems so obvious that few of them (or even many structural geology textbooks) ever bother with a definition. And when they do, they very often get it wrong. When thinking of a fault, geologists usually have in mind a planar dislocation where the rocks on either side have slid past each other with the direction of movement lying in the plane of the fault itself.  Most geological dictionary or structural geology textbooks (or at least the half-dozen that I have checked) reflect this same misunderstanding with a definition that specifies a direction of movement along the plane of the fault.  They also frequently specify that faults are “brittle fractures”, and that fault movement must always be “observable”.

The authoritative Glossary of Geology[1] with 36,000 defined terms defines a fault as:

A fracture, or zone of fractures, along which there has been displacement of the sides relative to one another parallel to the fracture. (my italics)

The Oxford Dictionary of Geology and Earth Sciences[2] offers this:

Approximately plane surface of fracture caused by brittle failure and along which observable relative displacement has occurred between adjacent rocks. (my italics)

Hobbs, Means & Williams in their well-known (and otherwise impeccable) structural geology textbook[3] provide this definition:

A (fault is a) planar discontinuity between blocks of rock that have been displaced past one another in a direction parallel to the discontinuity. (my italics)

Neville J Price, in his 1964 textbook on rock mechanics (4), offers this:

A fault is a fracture which exhibits obvious signs of differential movement on either side of the plane.” (my italics)

and…

 Joints are cracks and fractures in rock along which there has been extremely little or no movement. (my italics)

The definition of a geological fault in Wikipedia [5]:

A fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement (my italics)

Or this one, perhaps the worst of all, from the internet here:

 if rocks on both sides of the (fault) plane have moved relative to each other, parallel to the plane of the fault (faults are shear fractures)…Joints, if there is no component of displacement parallel to the (fault) plane (joints are extension fractures). (my italics)

All these definitions are inadequate, in fact, downright wrong, as I hope to show below. I have placed the problematic words of the definitions in italics. They are the words that define fault movement as taking place parallel to (or along) the fault plane. They are words that require the fault mechanism to be brittle (or a “fracture” which implies the same thing). They are words that require fault movement to be greater than some vaguely-defined (“obvious”, “significant”, “observable”) and arbitrary minimum value.

If all the criteria of these definitions were strictly applied, they would exclude almost all structures that geologists normally understand by the term fault and make it impossible to understand and interpret the multitude of secondary structures that occur within a fault, and provide a means of interpreting its history and movement direction. But more importantly, from the point of view of the exploration geologist, these definition make it impossible to understand and predict the emplacement of the epigenetic mineral veins in which they are interested.

All of which begs these questions:

If the rocks on either side of a fracture plane have moved relative to each other in a direction other than parallel to the plane of fracture – is it a fault?

If the amount of fault movement is not significant (whatever that means)  - is it a fault?  

If no observable displacement has taken place across a fault – is it a fault?

If a fault formed through ductile, rather than brittle, deformation processes – is it a fault?

The answer to all these questions is YES.

Discussion on Fault Movement

Fault Movement Vectors (FMVs) define the relative movement that has taken place across a fault.  FMVs are the direction of movement of any point on one side of a fault with respect to any point on the other side. FMVs can be shown as two parallel arrows pointing in the direction of relative movement – one for the rocks on either side of the fault. They may point towards each other. They may point away from each other. They may lie at any angle to the plane of the fault. The plane of the FMVs is any plane that includes the line of the arrows.

If the FMVs are parallel to the fault plane, the fault has formed through the mechanism of simple shear: that is, the two sides of the fault have moved laterally past each other. If the arrows are normal (at right angles) to the fault, then the fault has formed by the mechanism of pure shear.  Pure shear may be compressional or extensional. In compression (where the FMVs point towards each other) there must have been a reduction in the volume of the affected rocks. In extension (where the FMVs point away from each other) there must have been an increase in the volume of the affected rocks.

Extensional faults are of vital interest to the exploration geologist because the extension provides the space, and the driving mechanism, for emplacement epigenetic mineral veins – an important source of ore.

A more detailed discussion on the dynamics of fault formation can be found in my earlier blog post here, entitled: The movement of faults.

Movement of faults fig 1 (2)

The diagram shows a series of two-dimensional slices through rocks affected by different dynamic styles of faulting. The red opposed arrows are the Fault Movement Vectors and indicate the direction of net movement of any point on either side of the fault trace. The sections are all in the plane of the FMVs. Where the vectors are parallel to the fault plane (as in a), the opposed blocks have moved laterally past each other: sinistral if to the left (as shown) or dextral if to the right. However, FMVs can lie at any angle to the fault plane. Where FMVs point towards each other, and not parallel to the fault plane, the stress state is known as transpression; if the arrows point away from each other and are not parallel to the fault plane (b), the stress state is known as transtension. Two end member states, either of pure compression or pure extension, occur where the vectors are normal to the fault (b & c). There is a continuum between these different styles of fault movement, not just between different faults, but within any one fault at different places and at different times during its formation. Click for a larger, sharper image.

Simple shear and pure shear faults are end members of a continuum of styles of displacement across a fault. Even where simple shear might be the predominant mechanism, different parts of the fault will locally exhibit the effects of pure shear. Conversely, in dominantly pure shear structures, there will be zones where the structures observed formed through the mechanism of simple shear. If the FMV arrows lie some angle between 0⁰ and 90⁰ to the fault plane, then fault deformation took place by some combination of simple shear and pure shear mechanisms.

Marker Bed Movement Vectors (hereafter, MMVs) indicate the apparent displacement, on any section across a fault, of a marker surface intersected by it. The plane of the MMVs is thus, by definition, always parallel to the fault plane.  However, the displacement of a single plane across a fault is incapable of fully defining the relative movement of the rock bodies on either side of the fault – only the relative displacement of a point on either side of the fault can do that. When viewed on a random surface – such as an outcrop face or a geological section or a map, intersected marker beds with different orientation may give different MMVs across the same fault. Depending on the angle which the line of intersection of bed and fault makes with the Fault Movement Vectors, there may be no lateral displacement at all of the marker bed at all. Where there is a displacement, MMVs are commonly shown on a section or a plan by a pair of opposed arrows, but these arrows are only equivalent to FMV arrows in the special case where the section is parallel to the FMVs.

When viewing a fault on the FMV plane, all marker beds will show the same amount and sense of displacement irrespective of their orientation. Conversely, if differently oriented beds show the same amount and sense of displacement, then the plane on which that displacement is viewed must be the plane of the FMVs.

Many geologists, including the writers of some geological textbooks and dictionaries, confuse FMVs and MMVs, but FMVs are the more fundamental measure of fault movement.

Discussion on Joints

The amount of movement that has taken place across a fault can vary through several orders of magnitude – from a fraction of a millimeter to hundreds of kilometers. Small, locally developed, brittle fracture surfaces across which insignificant displacement has taken place are called joints. The suggestion by Neville Price (see his definition above) that there may have been no movement across a joint is of course nonsense: if there had been no movement, there would have been no fracture. By “insignificant” I mean difficult or impossible to see with the naked eye. Joints are presumably the category of fracture which the lexicographers sought to exclude from their definition of faults by their requirement that displacement be “significant”, “observable” or “obvious”. But Joints form in the same way as faults and should be regarded as a sub-category of brittle faulting. There is no logic for imposing an artificial division between faults and joints based on some arbitrarily defined amount of movement. However, the term joint, even if ill-defined, remains a useful field term for describing arrays of small-scale brittle fractures, but joints are micro-faults, nonetheless.

Discussion on Brittle and Ductile Deformation

The method of deformation that enables fault movement can be either brittle or ductile, or, more typically, some combination of brittle and ductile. Almost all faults show evidence for both styles of deformation either at different places within the same fault zone, and/or at different times during its formation.

Faults where the dominant deformation mechanism is brittle typically consist of tabular arrays of close-spaced, sub-parallel anastomosing fractures separating slices of lesser deformed, or undeformed, rock.

In faults in which the dominant deformation mechanism is ductile, the strain is more uniformly and smoothly spread across across the width of the fault.  Ductile faults are often described as ductile shear zones, but they are faults, nonetheless.

So here is my definition of a fault:

A fault is a restricted tabular zone of high strain with relative displacement of the rocks on either side.

Postscript

Since writing this post, I have located yet another definition of a fault.

From www.britanica.com :

Fault in geology, a planar or gently curved fracture in rocks of the Earth’s crust, where compressional or tensional stresses cause relative displacement of the rocks on either side of the fracture.

This definition, at least, avoids avoids defining a fault movement as being along the fault plane, but manages to suggest, incorrectly, that brittle “fracture” is the only allowable mechanism. Also, by specifying deformation in terms of a stress state (abstract force which has to be deduced), rather than a strain (physical reality which can be observed), the Britanica definition is neither particularly helpful nor informative.

**********

[1] J A Jackson & R L Bates (eds), 1980: Glossary of Geology. Published by the American Geophysical Institute, 2nd Edition, 1980.

[2] Michael Allaby 4th Ed. 2013 online version. DOI: 10.1093/acref/9780198839033.001.001

[3] Hobbs B E, Means W D & Williams P F, 1976: An outline of structural geology. John F Wiley and Sons, 571p.

(4) Price, Neville J: 1964. Fault and joint development in brittle and semi-brittle rocks. Pergamon Press, 176p.

[5] Accessed July 2020

 

 

Comments are closed.