1. Geological structures1b
Fractures and Faults

2. Lecture guide Stress and strain Stages of deformation
Evidence of former deformation
Fracture of brittle rocks
Joints definition and importance
Fault geometry and nomenclature
Definition and classification
Rocks produced by faulting (fault rocks)
Features associated with fault planes

3. Stress and strain
Rocks are constantly subjected to forces causing them to bend, twist, or fracture.
They deform or strain (change shape or size). The forces that cause deformation are referred to as stresses.
To understand rock deformation we must first explore stress and strain.
The process of forming a mountain not only uplifts the surface of the crust, but also causes rocks to undergo deformation, a process by which rocks squash, stretch, bend or break in response to squeezing, stretching or shearing. Deformation produces geologic structures including joints (cracks), faults, folds (bends or wrinkles) and foliation (layering resulting from the alignment of mineral grains or the creation of compositional bands). Geologists refer to the change in shape that deformation causes as strain. Deformation includes one or moe of the following: a change in location (“translation”), a change in orientation (“rotation”) and a change in shape (“distortion”).

4. Stress and strain
Among the stresses that deform rock is confining stress (isotropic).
3 kinds of differential stress (anisotropic) occur.
Tensional stress (or extensional stress), which stretches rock;
Compressional stress, which squeezes rock;
Shear stress, which result in slippage and translation
Stress is a force applied over an area. One type of stress that we are all used to is a uniform stress, called pressure. A uniform stress is where the forces act equally from all directions. In the Earth the pressure due to the weight of overlying rocks is a uniform stress and is referred to as confining stress.  If stress is not equal from all directions then the stress is a differential stress.
We distinguish among different kinds according to how the rock changes shape. If a layer of rock becomes longer, it has undergone “stretching” , but if the layer becomes shorter, it has undergone “shortening”. If a change in shape involves the movement of one part of a rock body past another so that angles between features in the rock change, the result is called “shear strain”. Tensional stress (or extensional stress), which stretches rock; Compressional stress, which squeezes rock;Shear stress, which result in slippage and translation.

5. Illustration

6. Stages of deformation
A change in shape, size or volume is referred to as strain.
When stress is applied to rock, the rock passes through some stages of deformation.

7. Illustration
Rocks can temporarily change shape when subjected to force (push, pull or shear), developing an elastic strain and then change back when the force causing it is removed. But rocks can also develop a permanent strain in two fundamentally different ways.

8. Behaviour of material under stress
Brittle materials have a small to large region of elastic behavior, but only a small region of ductile behavior before they fracture.
Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture.
nDuring brittle deformation, a material breaks into two or more pieces; while during ductile deformation, a material changes shape without breaking.
Joints and faults can be said to be brittle structures while folds and foliations are ductile structures. What actually happens is that during elastic deformation, bonds stretch and bend, but do not break.
During brittle deformation, many bonds break at once so that rocks can no longer hold together, while during ductile deformation, some bonds break but new ones quickly form, so that rocks do not separate into pieces as they change shape.

9. Illustration
Brittle materials have a small to large region of elastic behavior, but only a small region of ductile behavior before they fracture.
Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture.

10. Factors involved in material behaviour
Temperature - At high temperature, materials will behave in more ductile manner. At low Temperature, materials are brittle.
Confining Pressure - When high, materials are less likely to fracture because the pressure of the surroundings tends to hinder the formation of fractures. At low confining stress, material will be brittle and tend to fracture sooner.
Strain rate - Strain rate refers to the rate at which the deformation occurs (strain divided by time). High strain rates material tends to fracture. Low strain rates, ductile behavior is favored.
Composition - Minerals, like quartz, olivine, and feldspars are very brittle. Others, like clay minerals, micas, and calcite are more ductile
Why do rocks of the Earth sometimes deform in a brittle manner and sometimes in a ductile manner? The behaviour of a rock depends on:
Temperature: Warm rocks tend to deform in a ductile manner while cold rocks tend to deform in a brittle manner.
Pressure: Pressure effectively prevents rock from separating into fragments.
Deformation rate: A sudden change in shape causes brittle deformation, while a slow change in shape causes ductile deformation.
Composition: Some rocks types or minerals are softer than others; for example, quartz, olivine, and feldspars can deform in a brittle manner while others like clay minerals, micas, and calcite are more ductile.
Considering that pressure and temperature both increase with depth in the Earth, geologists find that in typical continental crust, rocks behave in a brittle manner above about 10 – 15 km, while they are ductile below. Earthquakes in continental crust happen only above this depth because these earthquakes involve brittle breaking.
This is due to the chemical bond types that hold them together. Thus, the mineralogical composition of the rock will be a factor in determining the deformational behavior of the rock. Another aspect is presence or absence of water. Water appears to weaken the chemical bonds and forms films around mineral grains along which slippage can take place. Thus wet rock tends to behave in ductile manner, while dry rocks tend to behave in brittle manner.

11. Earth’s structure
Considering that pressure and temperature both increase with depth in the Earth, geologists find that in typical continental crust, rocks behave in a brittle manner above about 10 – 15 km, while they are ductile below. Earthquakes in continental crust happen only above this depth because these earthquakes involve brittle breaking.

12. Evidence of former deformation
In order to uniquely define the orientation of a planar feature we first need to define two terms - strike and dip.  For an inclined plane the strike is the compass direction of any horizontal line on the plane. The dip is the angle between a horizontal plane and the inclined plane, measured perpendicular to the direction of strike. 
In recording strike and dip measurements on a geologic map, a symbol is used that has a long line oriented parallel to the compass direction of the strike. A short tick mark is placed in the center of the line on the side to which the inclined plane dips, and the angle of dip is recorded next to the strike and dip symbol.

13. Evidence of former deformation
For beds with a 90˚ dip (vertical) the short line crosses the strike line, and for beds with no dip (horizontal) a circle with a cross inside is used.

14. Fracture of brittle rocks (Joints)
Definition: Fractures along which no appreciable or observable displacement has occurred.
Joints develop preferentially in brittle rather than ductile rocks
A group of parallel joints is called a joint set and several intersecting sets make a joint system
Often there exists in rocks natural cracks that result from brittle deformation. Geologists refer to such natural cracks as “joints”. Joints develop respond to tensional stress in brittle rock: a rock splits open because it has been pulled slightly apart. Some joints form when a rock cools and contracts; others develop when rock layers formerly at depth feel a decrease in pressure as overlying rock erodes away and thus change shape slightly; still others form when rock layers bend. Joints develop preferentially in brittle rather than ductile rocks. If groundwater seeps through joints for a long period of time, minerals like quartz or calcite can precipitate out of groundwater and fill the joint. Such mineral-filled joints are called veins and look like white stripes cutting across a body of rock. Some veins contain small quantities of valuable metals, like gold.

15. Importance of joints
Provide permeability for ground water migration as well as migration and accumulation of petroleum; also controls drainage patterns and shape coastlines. Due to passage provision, weathering can take place.
Controls mineralization, hence modern prospecting techniques include detailed fracture analyses
Useful in engineering works (major construction projects are affected by joint systems, since all joints are structural weaknesses, within rocks and allowances must be made for them in project planning)
Geotechnical engineers pay close attention to jointing when recommending where to put roads, dams and buildings.
Water flows much more easily through joints than it does through solid rock, so it would be a bad investment to situate a water reservoir over rock with closely spaced joints – the water would leak down into the joints. Also, building a road on a steep cliff composed of jointed rock could be risky, for joint-bounded blocks separate easily from bedrock, and the cliff might collapse.

16. Joint Types
Shrinkage joints: (caused by tensional forces as a result of drying out of sediments or cooling and contraction of igneous bodies; can result in a structure called columnar joints)
Sheet joints: a set of joints may develop which are more or less parallel to the surface of the ground, especially in plutonic igneous intrusions. They probably arise as a result of the unloading of the rock mass when the cover is eroded away
Tectonic joints: these arise as a direct result of folding or faulting in rocks.

17. Columnar joints

18. Fault: a fracture on which sliding has occurred

19. Fault geometry and nomenclature
Definition: A fault is a structure along which displacement has taken place.
A fault plane can be vertical, horizontal or at some angle in between whose orientation can be described by a strike and dip measurement.
If a fault plane (surface along which movement has taken place) is inclined to the horizontal, the rock mass above it is the hanging wall, below it is the footwall.
To describe the displacement, the nomenclature from miners who excavated shafts along fault zones is used. The rock surface above the fault is called the hanging wall. Rock surface below is the footwall. For any inclined fault plane, the block above the fault is the hanging wall block and the block below the fault is the footwall block.

20. Fault geometry and nomenclature

21. Fault Classification Geometry: dip-slip, strike-slip, oblique, etc.
Sense of movement: Normal, Reverse, Transform.
Responsible forces: Tension, Gravity and Compressional

22. Geometric and Kinematic Classification Of Faults
Faults may be classified according to the following categories:
Pattern of sets and systems
Relation to regional structure or topography
Direction of relative displacement

23. PATTERN OF SETS AND SYSTEMS
We may classify faults according to the pattern in which they occur.
Radial Faults- As their name implies, they are more or less radialy arranged around a present or former center of igneous activity. Faults that form cylindrical pattern around a central area are termed ring faults.
Parallel Faults- that are successively offset in a constant direction or along an arc are said to be arranged en echelon, a French term meaning in ladder-rung fashion

24. RELATION TO REGIONAL STRUCTURE OR TOPOGRAPHY
We may also classify faults by their geographic orientation. Thus one speaks of north-south, east-west trending faults etc. We may classify them according to their relations to other structures.
Longitudinal Faults- have their traces aligned more or less parallel to regional structural trends.
Cross or Transverse Faults- Transect the regional structure at large angles

25. DIRECTION OF RELATIVE DISPLACEMENT
Displacement along the Dip- A fault whose predominant component of displacements is along the line of dip is a Dip-slip Fault, which can be either normal or reverse.
A normal fault dips toward the block that seems relatively lowered, or the hanging-wall moves up with respect to the foot-wall.
Reverse Faults- Dips towards the block that seems relatively raised, or the hanging-wall moves up with respect to the foot-wall. This is also known as thrust fault.

26. Displacement along the strike- A faults whose predominant slip is along its strike is a strike- slip fault. Common partial synonyms of the strike-slip fault are wrench fault and transcurrent faults.

27. Faults can be divided into several different types depending on the direction of relative displacement. Since faults are planar features, the concept of strike and dip also applies, and thus the strike and dip of a fault plane can be measured. One division of faults is between dip-slip faults, where the displacement is measured along the dip direction of the fault, and strike-slip faults where the displacement is horizontal, parallel to the strike of the fault.
Dip Slip Faults - Dip slip faults are faults that have an inclined fault plane and along which the relative displacement or offset has occurred along the dip direction. Note that in looking at the displacement on any fault we don't know which side actually moved or if both sides moved, all we can determine is the relative sense of motion.

28. Geometry of displacement
Displacement is with respect to horizontal and vertical components are
Heave (horizontal)
Throw (vertical)
Throw is normally quoted rather than true displacement.

29. h α ds t

30. Sense of movement: Normal fault
Hanging block moves down relative to the footwall
Most normal faults are small, having displacements of only a meter or so.
Normal faults indicate the existence of tensional stresses that tend to pull the crust apart
Results in graben and horsts

31. Normal fault
Normal Faults - are faults that result from horizontal tensional stresses in brittle rocks and where the hanging-wall block has moved down relative to the footwall block.

32. Horsts and Grabens
Horsts & Grabens - Due to the tensional stress responsible for normal faults, they often occur in a series, with adjacent faults dipping in opposite directions. In such a case the down-dropped blocks form grabens and the uplifted blocks form horsts. In areas where tensional stress has recently affected the crust, the grabens may form rift valleys and the uplifted horst blocks may form linear mountain ranges. The East African Rift Valley is an example of an area where continental extension has created such a rift.

33. Normal fault

34. HORST AND GRABENS
Horsts & Grabens - Due to the tensional stress responsible for normal faults, they often occur in a series, with adjacent faults dipping in opposite directions. In such a case the down-dropped blocks form grabens and the uplifted blocks form horsts. In areas where tensional stress has recently affected the crust, the grabens may form rift valleys.

35. Reverse faults
Faults that result from compressional stresses in brittle rocks, where the hanging-wall block has moved up relative to the footwall;
Reverse faults (dips greater than 35˚);
Thrust faults (dips less than about 35˚) both occurring in compressional environments.

36. Reverse fault
Reverse Faults - are faults that result from horizontal compressional stresses in brittle rocks, where the hanging-wall block has moved up relative the footwall block

37. Reverse fault

38. Reverse fault

39. Thrust fault
A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 15o. Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result in older strata overlying younger strata.

40. Thrust fault

41. Transform fault
Strike Slip Faults - are faults where the relative motion on the fault has taken place along a horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip faults can be of two varieties, depending on the sense of displacement. To an observer standing on one side of the fault and looking across the fault, if the block on the other side has moved to the left, we say that the fault is a left-lateral strike-slip fault. If the block on the other side has moved to the right, we say that the fault is a right-lateral strike-slip fault. The famous San Andreas Fault in California is an example of a right-lateral strike-slip fault. Displacements on the San Andreas fault are estimated at over 600 km.
Transform-Faults are a special class of strike-slip faults.

42. Rocks produced by faulting
Fault breccia: coarse angular broken rock debris in zone along fault
Fault gouge: finely ground rock paste in thin zone along fault plane. These are more susceptible to erosion giving rise to depression often associated with fault outcrops.
Many faults are marked by a zone of broken and crushed rock fragments of varying sizes.
Fault breccia: coarse angular broken rock debris in zone along fault
Fault gouge: finely ground rock paste in thin zone along fault plane. Since such zones are normally softer and more easily eroded than unfaulted rock, they give rise to the marked topographic depressions often associated with fault outcrops.

43. Fault planes: Associated features
Slickensides: striated or shiny surfaces on a fault plane caused by rubbing or polishing action
Fault drag: disturbance and folding of rock near fault.
Slickensides: striated or shiny surfaces on a fault plane caused by rubbing or polishing action
Fault drag: disturbance and folding of rock near fault. This term should be used with caution since it suggests that the fold originated by dragging action of opposing fault block, whereas many flexures along fault planes result from an initial ductile strain that preceded fracturing.

44. Recognizing faults
The most obvious is the appearance of displacement or offset;
Displacement disrupts the layers in rocks, so that layers on one side of a fault are not continuous with layers on the other side;
Faults may also leave a mark on the landscape;
Faults surfaces and their borders typically look different from bedding planes.
Fault breccia
Fault gouge
Slickensides etc.
Play GEODE programme: Internal processes-Crustal deformation (1, 2 and 4).