Fault Lines

Normal faults generally occur in places where the lithosphere is being stretched. Consequently they are the chief structural components of many sedimentary rift basins (e.g. the North Sea) where they have major significance for hydrocarbon exploration. They can also be found in deltas, at the rear edges of huge gravitation slumps and slides. Normal faults can show different geometries – and a few are shown here. In some situations the faults can become gently dipping at depth so that they have a spoon (or listric) shape. Other normal faults are found in batches, dipping in the same direction, with rotated fault blocks between. These are termed domino faults. Although most active normal faults can be shown to dip at angles steeper than 50 degrees, there are examples of very low-angle normal faults. These are often termed “detachments” – although this is a pretty vague term! Detachments show gentle dips and often expose high grade metamorphic rocks in their footwalls. These footwalls can be termed metamorphic core complexes. Normal faulting is now thought to be an important way in which metamorphic rocks come to be at the earth’s surface today.

Thrusts are reverse faults and commonly dominate the structure of collision mountain belts. Some thrusts have moved a long way – many mountain belts have thrusts that have moved many tens of kilometers. The photograph above shows one such structure from the Alps – which carries basement of the Mont Blanc massif onto Jurassic sediments. Many thrusts can be shown to follow so-called staircase trajectories. Otherwise, explore the nature of thrust systems by selecting from the icons. The material introduces concepts used in the Leeds first year structure course but some aspects are suited to higher level studies.

Strike-slip faults include some of the world’s most famous – or infamous structures, including the San Andreas Fault system and the North Anatolian Fault system. Both of these are renowned for devastating earthquakes. Strike-slip faults are those where the relative displacement is parallel to the strike of the fault. Strike-slip fault zones are commonly, but by no means exclusively, steep and can be rather difficult to recognize on cross-sections. 

All structures form in response to forces acting on rocks – and these give rise to stresses. In almost all geological situations stresses are always compressive but vary in different directions. We can evaluate the stress state in terms of the orientation and magnitudes of the so-called three principal stresses – which each act at 90 degrees to each other (i.e. they are orthogonal). Conventionally these are denoted using the Greek letter “sigma”. Patterns of conjugate faults – provided they formed together – can be related to the orientations of the principal stress axes.

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