CHAPTER 10: Folds, Faults and Rock Deformation
1. Figure 10.6: Rocks that were originally deposited in horizontal layers can subsequently deform by tectonic forces into folds and faults. Folds constitute the twists and bends in rocks. Faults are planes of detachment resulting when rocks on either side of the displacement slip past one another.
How Rocks Become Deformed
Three Types of Tectonic Forces
1. Figure 10.6: There are basically 3 types of tectonic forces that can deform rocks. The type of strain (deformation) that develops in a rock depends on the tectonic force.
(a) Compressive forces squeeze and shorten a body.
(b) Tensional forces stretch a body and pulls it apart
(c) Shearing forces push different parts of a body in opposite directions
2. The type of deformation experienced by a rock body depends largely on the type of force exerted.
(a) Fig. 10.6a: Compressive forces generate folding and faulting as a consequence of shortening. Compressive forces are common along convergent plate boundaries resulting in mountain ranges.
(b) Fig. 10.6b: Tensional forces cause stretching and thinning of the rocks, usually accompanied by tensional faults. Tensional forces common along extensional plate boundaries such as mid-ocean ridges.
(c) Fig. 10.6c: Shearing forces cause rocks to slide horizontally past one another such as along transform plate boundaries to produce extensive fault systems.
What Determines Whether a Rock Bends or Breaks?
1. Figure 10.7: Another factor that determines how a rock deforms is confining pressure, which is like the pressure you feel when you dive deep underwater. Under confining pressure, forces push against a body in all directions. In effect, the body is squeezed into itself.
2. Confining pressures within the earth are caused by the weight of the overlying rock pushing downward and from all sides. Drillers experience great problems with confining pressure. Holes drilled within the earths crust tend to remain open at shallow depths, but at greater depths holes tend to squeeze shut due to the increase in confining pressure.
3. Fig. 10.7b: When an external force is applied to buried rocks under low confining pressure, such as near the surface of the earth, the rock typically deform by simple fracturing. This is known as brittle deformation.
4. Fig. 10.7c: At higher confining pressures, a similarly directed external force will cause the deeply buried rock to actually flow and deform without fracturing. This is known as ductile deformation and the rock is said to behave plastically.
5. Rocks under low confining pressures near the earths surface therefore generally deform through fracturing and faulting. Rocks deep within the crust under high confining pressures deform by folding.
Brittleness and Ductility
1. Figure 10.7: Rocks are defined as brittle or ductile on the basis of the way they are deformed by forces.
2. In brittle deformation, a continuous, force is applied to a rock. As the force is gradually increased, little change occurs in the rock until suddenly it fractures.
3. In ductile deformation, a gradually increasing force will cause the rock to undergo smooth and continuous plastic deformation. The rock will contort and change shape without fracturing.
4. The type of rock also determines the type of deformation. Under similar confining pressures, halite (rock salt) is more susceptible to ductile deformation than is granite, which will more likely fracture.
5. Igneous and metamorphic rocks tend to be stronger and thus resist deformation to a greater extent than sedimentary rocks.
Measuring Strike and Dip
1. Figure 10.4: The orientations of rock layers, folds, fractures and faults can all be measured in three dimensional space using strike and dip.
2. The strike of a surface is the direction of a line formed by the intersection of a rock layer with a horizonal surface. The strike is described in terms of direction such as N 10o W.
3. The dip is measured at right angles to the strike and is a measure of the angle at which the surface tilts relative to a horizontal surface. The dip is indicated in terms of angle and direction (e.g. 35o E).
Types of Folds
1. Figure 10.9: Folds are a result of ductile deformation of rocks in response to external forces.
2. Layered rocks folded into arches are called anticlines whereas troughs are referred to as synclines.
3. Figures 10.10 & 10.11: The two sides of a fold are referred to as limbs. The two limbs come together to form an imaginary line called the fold axis. The direction in which the fold axis points indicates the strike of the fold.
4. Fig. 10.16a: A dome is an anticlinal structure where the flanking beds encircle a central point and dip radially away from it.
5. Figure 10.16b: A basin is a synclinal structure appearing as a bowl-shaped depression where rock layers dip radially towards a central point.
6. Figure 10.5: The eroded surface of a fold appears as a series of bands of different rocks. Rock bands appearing on one side of the fold axis are duplicated on the other side. For basins and domes, strata exposed at the surface form concentric circles around a central point (Figure 10.16).
7. Figure 10.5: For anticlines, the surface rock exposures become progressively older towards the fold axis.
8. Fig. 10.18: Synclines show the opposite trend. Rock exposures become progressively younger towards the axis of synclines.
9. Rock layers dip away from the fold axis in anticlines, but dip toward the fold axis in synclines.
1. Figure 10.10: A fold can be divided by an imaginary surface called the axial plane. The axial plane divides a fold as symmetrically as possible. The line formed by the intersection of the axial plane with the beds define the fold axis.
2. Figure 10.10: The axis of a fold can be horizontal. If the axis is not horizontal, the structure is said to be a plunging fold.
3. The plunge of a fold can be described as the angle a fold axis makes with a horizontal surface. The axis of a plunging fold can therefore be described as having a certain strike (e.g. N 10o W) and plunge (e.g. 20o NW). Unlike dipping beds, the plunge of a fold axis is in the same direction as the strike of the axial plane.
4. Figure 10.12: Folds can be classified by their geometry with respect to their axial plane.
(a) Symmetrical Folds: Axial plane is vertical an beds dip at approximately the same angle, but in opposite directions, on either side of the plane.
(b) Asymmetrical Folds: Axial planes are inclined and one limb of the fold dips more steeply than the opposite limb, but still in opposite directions.
(c) Overturned Folds: Axial plane is inclined and both limbs of the fold dip in the same direction.
5. In general, the greater asymmetry in the fold, the more intense the deformation.
6. Figure 10.14: When folds plunge into the earth, they essentially disappear from the surface. The curved strata comprising a plunging fold form a horseshoe or hairpin pattern on the surface where they plunge into the earth.
7. For anticlines, the horseshoe or hairpin shape closes in the direction that the anticline plunges.
8. For synclines, the horseshoe or hairpin-shape opens in the direction that the syncline plunges.
9. Figure 10.5: In the field, a geologist can reconstruct the geometry of folds by:
(a) measuring the strike and dip of various strata exposed in outcrops
(b) noting which direction the beds become younger
(c) measuring any structural deformations within the rocks.
(d) Once this information is obtained, the geologist can employ the principles of geometry and trigonometry to determine the orientation of the axial plane and also whether the fold plunges. If the fold plunges, then the plunge of the fold axis can also be determined using geometry, trigonometry and field measurements.
Joints and Faults
Rocks that undergo brittle deformation tend to fracture into joints and faults.
1. Figure 10.20: A joint is a crack in a rock along which no appreciable movement has occurred. Strata on one side of the joint align with strata on the other side.
2. Joints can form as a result of expansion and contraction of rocks. Expansion can occur if erosion strips away the overlying rocks to exhume once deeply buried rocks. Release of confining pressure causes the exhumed rock to expand and fracture, thereby producing joints.
3. Joints aid in weathering by providing channels where water and air can reach deep into the formation.
1. Figure 10.22: A fault is a plane of dislocation where rocks on one side of the fault have moved relative to rocks on the other side. Strata on one side of the fault plane are typically offset from strata on the opposite side.
2. Figure 10.6: Faults can form in response to any one of the three types of forces: compression, tension and shear: The type of fault produced, however, depends on the type of force exerted.
3. A fault plane divides a rock unit into two blocks. One block is referred to as the hanging wall, the other as the footwall.
(a) The hanging wall is the block of rock above an inclined fault plane.
(b) The block of rock below an inclined fault plane constitutes the footwall.
4. Figure 10.22a: If the hanging wall slips downward relative to the footwall, the fault is defined as a normal fault.
5. Figure 10.25: Normal faults result from tensional forces and typically form rift valleys. The down-faulted block in a rift valley is called a graben while the uplifted block is referred to as a horst.
6. Figure 10.22c: Shear forces typically produce strike-slip faults where one block slips horizontally past the another. In other words, slippage is parallel to the strike of the fault.
7. Figure 10.22b: Compressional forces typically push the hanging wall upward relative to the footwall, producing a reverse fault.
8. Figure 10.23: A reverse fault in which the dip of the fault plane is so small as to be almost horizontal is called a thrust fault. In thrust faults, the hanging wall moves almost horizontally over the footwall.
9. Figure 10.22d: Oblique faults occur where there is both a strike-slip and dip-slip component to the fault.