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EnglandGeologySyllabus dot point

How do you recognise and classify folds, faults and joints, and read them on maps?

Folds, faults and joints: fold elements (limb, axial plane, hinge) and types (anticline and syncline, symmetric, asymmetric, overturned, recumbent, monocline); fault types and the stress they record (normal from tension, reverse and thrust from compression, strike-slip and tear from shear); dip-slip versus strike-slip movement; throw, heave and the fault plane; joints as fractures with no displacement; and reading these structures on geological maps and cross-sections.

A focused answer to the Eduqas Geology statement on folds, faults and joints. Covers fold elements and types (anticline, syncline, symmetric, asymmetric, overturned, recumbent, monocline), fault classification and the stress each records, throw and heave, joints as undisplaced fractures, and how to read these structures on maps and cross-sections for Components 1 and 3.

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

Eduqas wants you to recognise and classify the two main ways rocks deform permanently: folds (the ductile response, mostly to compression) and faults (the brittle response). You must label the fold elements (limb, hinge, axial plane), name the fold types (anticline, syncline, symmetric, asymmetric, overturned, recumbent, monocline), classify the faults by the stress they record (normal, reverse, thrust, strike-slip), measure their displacement (throw and heave), and recognise joints as fractures with no displacement. The skill is then applied: reading these structures on geological maps and cross-sections in Components 1 and 3.

The answer

Fold elements

A fold is a buckle in originally flat layering. The elements you must be able to label on a diagram are:

The orientation of the axial plane is what separates the fold types, so learn it as the key reference line.

Fold types

  • Anticline. An up-arched fold (convex up). After erosion, the oldest rocks are exposed in the core and the beds dip away from the axis on both sides.
  • Syncline. A down-warped fold (concave up). The youngest rocks are in the core and the beds dip towards the axis.
  • Symmetric fold. The axial plane is vertical and the two limbs dip at equal angles in opposite directions (mirror images).
  • Asymmetric fold. The axial plane is tilted, so one limb is steeper than the other.
  • Overturned fold. The axial plane is so inclined that both limbs dip in the same direction, and one limb has been rotated past vertical (the beds in it are upside down).
  • Recumbent fold. The axial plane is nearly horizontal, so the fold lies on its side. These form under intense compression, for example in nappes during continental collision.
  • Monocline. A local step-like flexure where otherwise horizontal beds bend down (or up) in one direction only, often draped over a fault or basement block at depth.

The progression from symmetric, to asymmetric, to overturned, to recumbent tracks increasing compression rotating the axial plane from vertical towards horizontal.

Fault types and the stress they record

A fault is a fracture along which the rocks have moved. The block above the inclined fault plane is the hanging wall; the block below is the footwall. The type records the stress:

  • Normal fault. Hanging wall moves down relative to the footwall; records tension (extension), lengthening the crust. Found at constructive margins and rifts.
  • Reverse fault. Hanging wall moves up relative to the footwall; records compression, shortening the crust. Found at destructive margins.
  • Thrust fault. A low-angle reverse fault (a shallow fault plane); records strong compression and can push older rocks over younger ones in fold-and-thrust belts.
  • Strike-slip (tear) fault. The blocks slide horizontally past each other along a near-vertical plane; records shear, at conservative margins (the San Andreas).

Dip-slip versus strike-slip, throw and heave

The direction of movement on the fault plane splits faults two ways:

  • Dip-slip movement is up or down the dip of the fault plane (normal and reverse faults).
  • Strike-slip movement is horizontal, along the strike of the plane (tear faults).

The displacement of a dip-slip fault is described by two components measured on the cross-section:

Joints

A joint is a fracture in rock with no displacement across it (the two sides have not moved relative to each other), unlike a fault. Joints form in sets (parallel families) from cooling contraction (columnar joints in basalt), unloading as overlying rock is eroded, or tectonic stress. They are important because they control permeability (fluid flow), weathering and the strength of a rock mass.

Reading the structures on maps and cross-sections

The outcrop pattern on a map records the structure:

  • Folds give repeated, mirror-image outcrop patterns. The oldest beds in the core mark an anticline, the youngest a syncline.
  • Faults appear as a line that offsets (displaces) the outcrops of the beds it cuts; the size of the offset reflects the throw.
  • Joints are usually too small to map but appear in field sketches and rock descriptions.

On a cross-section you draw the beds at their dip, fold them to match the dips on either side, and offset them across any fault by the throw and heave.

Examples in context

Example 1. Recumbent folds and thrusts in the Alps. Continental collision produced such intense compression that the rocks folded into near-horizontal recumbent folds (nappes) and were carried over younger rocks along low-angle thrust faults, stacking the crust and shortening it by tens of kilometres, the structural signature of a convergent margin.

Example 2. A graben in a rift. Where the crust is pulled apart by tension, a central block drops down between two outward-dipping normal faults to form a graben (rift valley), as in the East African Rift; the hanging walls have moved down, lengthening the crust, the opposite of the Alpine compression.

Try this

Q1. State which rocks (oldest or youngest) lie in the core of a syncline, and which way the beds dip. [2 marks]

  • Cue. The youngest rocks lie in the core, and the beds dip inwards towards the axis.

Q2. A reverse fault records which type of stress, and how does its hanging wall move? [2 marks]

  • Cue. Compression; the hanging wall moves up relative to the footwall, shortening the crust.

Q3. Distinguish a joint from a fault. [2 marks]

  • Cue. A joint is a fracture with no displacement (the sides have not moved); a fault is a fracture with displacement (the rocks have moved and the beds are offset).

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20196 marksDescribe the differences between a normal fault, a reverse fault and a strike-slip fault, naming the type of stress that produces each and the relative movement involved.
Show worked answer →

A levels-of-response answer; take each fault in turn, defining the movement by the hanging wall and then the stress.

Normal fault
The fault plane is inclined and the hanging wall (the block above the plane) moves down relative to the footwall (the block below). This lengthens and thins the crust, so it records tensional (extensional) stress, found at constructive margins and rifts. The movement is dip-slip (down the dip of the plane).
Reverse fault
The hanging wall moves up relative to the footwall. This shortens and thickens the crust, so it records compressional stress, found at destructive (convergent) margins. A low-angle reverse fault (a shallow dip) is a thrust fault, which can carry older rocks over younger ones. The movement is again dip-slip.
Strike-slip (tear) fault
The two blocks slide horizontally past each other along a near-vertical plane, with little vertical movement. This records shear stress, found at conservative margins (for example the San Andreas Fault). The movement is strike-slip (along the strike of the plane), not dip-slip.

Markers reward the correct hanging-wall movement and matching stress for the normal and reverse faults, the horizontal shear movement for the strike-slip fault, and the dip-slip versus strike-slip distinction.

Eduqas 20214 marksA normal fault has a fault plane dipping at 60 degrees and a measured throw (vertical displacement) of 30 m. Calculate the heave (horizontal displacement) of the fault.
Show worked answer →

Use the right-angled triangle formed by the throw, the heave and the displacement along the fault plane.

The relationship. For a fault plane dipping at an angle δ\delta, the throw TT (vertical component) and the heave HH (horizontal component) are the two legs of a right-angled triangle, related by

H=Ttan⁡δH = \frac{T}{\tan\delta}

because tan⁡δ=T/H\tan\delta = T/H (the throw is opposite the dip angle and the heave is adjacent to it).

Substitute.

H=30tan⁡60∘=301.732≈17.3 mH = \frac{30}{\tan 60^{\circ}} = \frac{30}{1.732} \approx 17.3\ \mathrm{m}

Answer. The heave is about 17.3 m17.3\ \mathrm{m}. (A steep fault gives a heave smaller than its throw; a shallow fault would give a heave larger than its throw.)

Markers reward the correct relationship (H=T/tan⁡δH = T/\tan\delta), the substitution of the 60∘60^{\circ} dip and 30 m30\ \mathrm{m} throw, and the answer of about 17.3 m17.3\ \mathrm{m}.

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