Chapter 27 Faults Fractures Fabrics and Deformation History

Year 2 · Structural Geology · G221-L3

Identify fault movement, joint sets, cleavage, foliation and lineation, and build a defensible relative deformation history.

Fold geometry and shortening in a schematic layered succession. Original Geology Hub teaching diagram.

This independent lesson follows topics in the supplied University of Peshawar BS Geology curriculum labelled session 2004–05 onward. It is a learning resource, not an official university course or a statement of current degree requirements.

Learning objectives

Distinguish separation on an exposure from true slip.
Describe fractures and penetrative fabrics systematically.
Use overprinting relations to construct a relative event sequence.

Fault movement in three dimensions

A fault is a surface or zone across which rocks have moved relative to one another. On an inclined fault the hanging wall lies above the surface and the footwall below it. In a normal fault the hanging wall moves down relative to the footwall; in a reverse fault it moves up. A thrust is a low-angle reverse fault. Strike-slip movement is mainly horizontal, and oblique slip combines strike and dip components.

Offset seen in a photograph is usually separation, not the total displacement vector. A dipping bed cut by a fault can show apparent horizontal offset even if movement was mostly vertical. True slip requires a line that was once continuous on both sides, such as a displaced piercing point, or independent kinematic evidence. Slickensides indicate a movement direction on the surface but may not uniquely establish its sense.

Fractures and fabrics

A joint is a fracture without appreciable shear displacement at the observation scale. Record orientation, spacing, persistence, aperture, infill and surface features. Parallel fractures form a set; intersecting sets make a fracture network. Extension fractures may open approximately perpendicular to the least compressive principal stress during formation, but later rotation or reopening can obscure that relationship. Do not interpret every joint as evidence of a single regional tectonic event.

Foliation is a broadly planar fabric, whereas lineation is a linear fabric. Cleavage is a tendency for a rock to split along repeated planes, commonly produced by aligned minerals or pressure-solution processes. Slaty cleavage, crenulation cleavage and compositional layering differ in origin and scale. Their relationship to bedding and fold hinges helps reconstruct deformation, but a cleavage plane is not automatically bedding. Measure each surface separately and label its identity.

Testing an event sequence

Overprinting relationships establish relative order. A vein folded with its host predates or accompanied folding; an undeformed vein cutting the fold may be younger. A fault that offsets cleavage postdates that cleavage at that locality. Some faults reactivate, and different mineral generations can occupy one fracture. Keep observations separate from inferred stages labelled D₁, D₂ and later events. Such labels are local working interpretations until regional correlations are justified.

Combine map patterns, cross-sections and stereonet plots. Check whether the same model explains repeated units, bedding orientations and fault intersections. If two models fit the evidence, identify the observation that would discriminate between them rather than concealing uncertainty under a more elaborate diagram.

Recover displacement from a piercing feature

A planar bed intersecting a fault produces a line, and that line may slide along itself without changing the apparent bed contact. A distinct feature such as the intersection of a dyke and a bed can instead provide a piercing point: matching its displaced positions constrains a displacement vector. Both sides must genuinely represent the same original feature. Apparent offsets of unrelated lenses or channel edges can create a false match. Draw the geometry before using a measured separation as a slip estimate.

Describe a fault surface in three dimensions

Fault strike and dip describe its plane, while a slip line on that plane has its own orientation. Rake measures the line’s angle within the plane from a stated strike direction. Predominantly dip-parallel slip gives a dip-slip fault; predominantly strike-parallel slip gives strike slip. An inclined slip line gives oblique movement. For an illustrative slip magnitude of 10 m and rake 30° from strike, the strike component is 10 cos 30° ≈ 8.66 m and the dip component is 10 sin 30° = 5 m. The signs still require an independently established movement sense. This decomposition does not determine vertical throw without the fault dip.

Use joint surfaces as observations

Spacing is measured perpendicular to successive fractures, while persistence describes their observable extent. Aperture records opening, and infill records material occupying the opening. Weathering can widen a joint after formation, so current aperture need not equal original opening displacement. Surface ridges or feather-like patterns may help investigate crack propagation, but their interpretation depends on preservation and geometry. Joint intersections control blocks and fluid pathways: two well-connected sets can behave differently from one set with the same average spacing. Record termination relationships rather than assuming every fracture penetrates the whole exposure.

Resolve more than one fabric generation

A rock may preserve bedding S₀, an earlier foliation S₁ and a later crenulation cleavage S₂. These labels are local descriptions of relative generations, not universal regional ages. Small folds of S₁ can have hinges parallel to a crenulation lineation, while the newer cleavage cuts the older planar fabric. In a sketch, draw the older surface continuously through the small folds and the newer surface separately. Then test whether veins, porphyroblasts or faults cross-cut one or both fabrics. An intersection lineation between bedding and cleavage is geometric; it need not be the same as a stretching lineation formed by elongated minerals. Measuring the two separately prevents an assumed common origin from replacing the actual evidence.

Worked activity

In an invented outcrop, bedding is folded, a cleavage is approximately parallel to the fold axial surface, and a quartz vein cuts both bedding and cleavage. A steep fault then displaces the vein by 25 cm on the exposure. Write a relative history.

Solution: sedimentation came first, followed by folding with cleavage development, then vein formation, then faulting. The 25 cm is observed separation unless a piercing line proves the slip vector. Cleavage timing remains an interpretation supported by geometry, not a direct absolute age.

Self-check

Can slickensides alone always give movement sense?
Answer
No. They give an orientation; sense needs reliable steps or other independent indicators.
What distinguishes a joint from a fault?
Answer
A fault has appreciable shear displacement; a joint lacks it at the stated scale.
Why measure bedding and cleavage separately?
Answer
They represent different surfaces and may record different processes.

Chapter 27 multiple choice questions

Choose one best answer for each question. Open each answer panel after making your choice.

Across an inclined fault, the block above the fault surface has moved downward relative to the block below. Which classification fits?
1. Reverse fault
2. Thrust fault
3. Normal fault
4. Pure strike-slip fault
Answer and explanation
C. Normal fault The block above the surface is the hanging wall. Its relative downward movement defines a normal fault.
A photograph shows a bed separated horizontally by 40 cm across a fault. What can be reported directly?
1. A true fault-slip magnitude of 40 cm
2. A slip rate of 40 cm per year
3. A vertical displacement of exactly 40 cm
4. An apparent separation of 40 cm on that exposure
Answer and explanation
D. An apparent separation of 40 cm on that exposure Separation on an exposure does not by itself determine the three-dimensional slip vector or a rate.
A fault offsets a quartz vein that cuts an older cleavage. Which relative order is supported?
1. Cleavage, then vein, then fault displacement
2. Vein, then fault displacement, then cleavage
3. Fault displacement, then cleavage, then vein
4. Vein and cleavage must have formed simultaneously
Answer and explanation
A. Cleavage, then vein, then fault displacement The vein cuts the cleavage, and the fault displaces the vein. Those cross-cutting relations establish the stated order.
Which field record best describes a joint set?
1. Only the colour of the host rock
2. Orientation, spacing, persistence, aperture and infill
3. An assumed movement rate without markers
4. Only the nearest fold’s age
Answer and explanation
B. Orientation, spacing, persistence, aperture and infill Those observations characterize fracture geometry and condition. A joint lacks appreciable shear displacement at the stated scale.
Bedding and slaty cleavage intersect at an outcrop. Which practice best preserves the evidence?
1. Average the two dips and call it bedding
2. Record only the surface that splits most easily
3. Measure and label the two surfaces separately
4. Treat the intersection as proof that both formed together
Answer and explanation
C. Measure and label the two surfaces separately Bedding and cleavage may record different processes and times. Separate measurements retain that distinction.

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