Ch. 10 - Crustal Deformation

Class: GEOL-101

Notes:

10.1 How Rocks Deform

Describe the three types of differential stress and name the type of plate boundary most commonly associated with each.

Deformation and geologic structures

Presentation content:

• Deformation is a general term that refers to all changes in the shape or position of a rock body in response to stress
  • Most occurs at or near plate boundaries
• Rock or geologic structures are the features that result from forces generated by the interactions of tectonic plates
  • Folds, faults, and joints
  • Foliation and rock cleavage

Notes from the lecture:

• Change could be folding, or it can be a change in position, maybe from horizontal to some angle
• Typically occur near a plate boundary (large amount of stress)
• Plates colliding = stress (squeezing a body of rocks)

Stress

Presentation content:

• Stress: The Force That Deforms Rocks
  • When stresses acting on a rock exceed its strength, the rock will deform by flowing, folding, fracturing, or faulting
  • The magnitude is a function of the amount of force applied to a given area
    • Uniformly in all directions = confining pressure
      • Does not change the shape or orientation of a rock body
    • Unequally in different directions = differential stress
• Types of stress
  • Compressional stress squeezes a rock and shortens a rock body
  • Tensional stress pulls apart a rock unit and lengthens it
  • Shear stress produces a motion similar to slippage that occurs between individual playing cards when the top of the stack is moved relative to the bottom

Notes from the lecture:

• Equal amount of stress in any direction = confining pressure
  • This won't change the shape of the rock
• Differential stress = one direction being pressed more than the others
  • This is what deforms a rock
• Compression
  • It would be related to a convergent plate boundary
• Tensional stress
  • Horizontal axis is being pulled apart (vertical axis may be applying a lot of pressure)
  • Could be the result of a divergent plate boundary
• Shear stress
  • Sliding and tearing

Strain

Presentation content:

• Strain: A Change in Shape Caused by Stress
  • Strained bodies lose their original configuration during deformation

Notes from the lecture:

• Shear stress = sliding a deck of cards
• Fossils deform because of that shear stress

Elastic, Brittle, and Ductile Deformation

Presentation content:

• Elastic deformation: The rock returns to nearly its original size and shape when the stress is removed
  • When stress is applied gradually, rocks initially respond by deforming elastically
• Once the elastic limit (strength) of a rock is surpassed, it either bends/folds (ductile deformation) or breaks (brittle deformation)

Notes from the lecture:

• Elastic:
  • Think of a rubber band, after stress, it rebounds to its original shape
  • The rock is able to kind of heal or recover
• Elastic limit: surpassing the strength of the rock
  • This is where permanent deformation appears

Factors That Affect Rock Strength

Presentation content:

• Temperature: Higher temperature rocks tend to deform by ductile deformation whereas cooler rocks tend to deform by brittle deformation
• Confining pressure: Confining pressure squeezes rocks, making them stronger and harder to break—these tend to undergo ductile deformation
• Rock type:
  • Crystalline igneous and some metamorphic rocks, composed of minerals with strong chemical bonds, generally experience brittle deformation
  • Sedimentary and metamorphic rocks with zones of weakness generally experience ductile deformation
• Time: Forces applied gradually over a long period generally result in ductile deformation

Notes from the lecture:

• Time is related to chemical rate at which pressure/temperature can change

Ductile vs. Brittle Deformation

Presentation content:

• Ductile Versus Brittle Deformation and the Resulting Rock Structures
  • Most rocks exhibit brittle behavior in the upper 10 kilometers of the crust
    • Joints (brittle) are cracks in the rocks resulting from the rock being stretched and pulled apart
    • Faults are fractures in the rocks where rocks on one side of the fault are displaced relative to the rocks on the other side of the fault
    • Folds (ductile) are evidence that rocks can bend without breaking
      • Usually the result of deformation in high-temperature and pressure environments

Notes from the lecture:

• Rocks are brittle at these pressures.

• Compression:
  • You will get reverse faults
  • As you ascend with depth you will see folding
• Tension:
  • At shallow depths you see normal faulting
  • At deeper levels you start to see stretching
• Shear:
  • Strike-slip faulting at shallow levels
  • Sharing occurs at greater depths

10.2 Folds: Rock Structures Formed by Ductile Deformation

List and describe five common folded structures.

Characteristics of folds

Presentation content:

• Most folds result from compressional stresses that result in a shortening and thickening of the crust

• Each rock layer is bent around an imaginary axis: the hinge line

  • Hinge lines can be horizontal or inclined

• The axial plane is a surface that connects all hinge lines of the folded strata

  

Notes from the lecture:

• Axial plane (blue) is the hinge line, the axis in which the rock will be folded up

Anticline and Synclines

Presentation content:

• Anticlines are upfolded or arched sedimentary layers
  • Oldest strata are in the center
• Synclines are downfolded or troughs of rock layers
  • Youngest strata are in the center

Notes from the lecture:

• Synclines (folded upward) makes a U form

Anticline and Synclines

Presentation content:

• Depending on their orientation, anticlines and synclines can be described as:
  • Symmetrical — the limbs of the fold are mirror images of each other
  • Asymmetrical — the limbs of the fold are not identical
    • Overturned (recumbent) — one or both limbs are tilted beyond vertical
    • Plunging — the axis of the fold penetrates the ground

Notes from the lecture:

• You can see the hinge line and all different levels of sedimentary rocks
• Prolific oil and gas region
• As you move away there are younger and younger rocks

Domes and Basins

Presentation content:

• Domes are upwarped circular features
  • Oldest rocks are in the center
  • Can form due to intrusion of a laccolith
• Basins are downwarped circular features
  • Youngest rocks are in the center
  • Can form from subsidence of large sedimentary basins

Notes from the lecture:

• Basin: youngest strata in the center, and older and older away from it.

• Domes are the other way around

  
  

Monoclines

Presentation content:

• Monoclines are large, steplike folds in otherwise horizontal sedimentary strata
  • Uniquely coupled with faults
  • As blocks of basement rocks are displaced upward, the ductile sedimentary strata drape over them

10.3 Faults and Joints: Rock Structures Formed by Brittle Deformation

Sketch and briefly describe the relative motion of rock bodies located on opposite sides of normal, reverse, and thrust faults as well as both types of strike-slip faults.

Faults

Presentation content:

• Faults are fractures in rocks along which displacement has occurred
• Sudden movements along faults are the cause of most earthquakes

Notes from the lecture:

• San Andreas Fault: building up pressure and eventually it would slip and release that pressure

Dip-Slip Faults

Presentation content:

• Dip-slip faults occur when movement is parallel to the inclination (dip)
  • The hanging wall is rock surface above the fault
  • The footwall is the rock surface below the fault
• There are two general types of dip slip faults
  • Normal—hanging wall moves down
  • Reverse—hanging wall moves up

Normal Faults

Presentation content:

• Normal faults are characterized by the hanging wall moving down relative to the footwall
  • Associated with tensional stress as the rocks pull apart, lengthening the crust (divergent boundaries)

Fault-block mountains

Presentation content:

• Larger scale normal faults are associated with fault-block mountains
  • Example: Basin and Range Province
  • Uplifted blocks are called horsts
  • Down-dropped blocks are called grabens

Notes from the lecture:

• First compressional, and now extensional

Reverse faults

Presentation content:

• Reverse faults are characterized by the hanging wall moving up relative to the footwall
• Associated with compressional stress as the crust shortens

Notes from the lecture:

• Doubling of the sedimentary extension (doubling of the strata)

Thrust faults

Presentation content:

• Thrust faults have an angle less than 45º, so the overlying plate moves almost horizontally
  • Most pronounced along convergent plate boundaries
  • Example: Glacier National Park

Notes from the lecture:

• Rocks are experience enough differential stress but they are brittle so they break
• Because of friction, it does not fall over easily, it starts to fold up instead

Strike-slip faults

Presentation content:

• Strike-slip faults are characterized by displacement that is horizontal and parallel to the strike of the fault
• Types of strike-slip faults
  • Right-lateral—As you face the fault, the opposite side of the fault moves to the right
  • Left-lateral—As you face the fault, the opposite side of the fault moves to the left
• Large strike-slip faults that cut through the crust to accommodate plate motion are called transform faults
• Most continental transform faults consist of a zone of roughly parallel fractures
  • San Andreas Fault

Notes from the lecture:

• There is sliding against one another
• We categorize faults based on the motion of the plate relative to position.
• San Andreas Fault is the most popular example of strike-slip faults

Oblique-slip faults

Presentation content:

• Oblique-slip faults exhibit both a strike-slip and a dip-slip movement

Notes from the lecture:

• If extensional it pulls apart and creates a little valley
• If its compressional it builds large mountains

Fault Scarps

Presentation content:

• Vertical displacement along faults may produce long low cliffs called fault scarps

Notes from the lecture:

• there is often displacement around faults
• 8-9ft of vertical displacement in this example (New Zealand)
• Very large impact

Slickensides

Presentation content:

• On some fault surfaces the rocks became highly polished and striated (grooved) as crustal blocks slid past each other
• These surfaces are called slickensides

Notes from the lecture:

• See the evidence where there is actually friction as the rock slip

Joints

Presentation content:

• Joints are fractures in a rock where there has been no rock movement
  • One of the most common rock structures
  • Most joints appear in parallel groups
  • Produced when rocks in the outermost crust are deformed and experience brittle failure

Notes from the lecture:

• breaking of the rocks with no displacement
• The rock behaves brittle (creates several little breaks)
• Sometimes the size of canyons, created and then weathered through time

10.4 Mapping Geologic Structures

Explain how strike and dip are measured and what these measurements tell geologists about the orientations of rock structures located below Earth’s surface.

Identify the dominant rock

Presentation content:

• A geologist identifies and describes the dominant rock structures in a region
  • Using outcrops of exposed bedrock
  • Work is now aided by
    • Aerial photography
    • Satellite imagery
    • Global positioning systems (GPS)
    • Seismic reflection profiling

Strike and Dip

Presentation content:

• Sedimentary rocks that are inclined or bent indicate that the layers were deformed following
• Strike
  • The compass bearing of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane
  • Generally expressed as an angle relative to north
• Dip
  • The angle of inclination of the surface of a rock unit or fault measured from a horizontal plane
  • Includes both an inclination and a direction toward which the rock is inclined
  • Always at a 90º to the strike

Notes from the lecture:

• Measure strike with a compass or other methods

Fault orientation

Presentation content:

• Fault orientation is described by strike and dip
  • Strike is the direction of a horizontal line on the inclined surface
  • Dip is the angle of inclination of that surface measured from the horizontal

End of Chapter 10 - Concept Checks

10.1 How Rocks Deform

1. List the three types of differential stress and briefly describe the changes they can impart to rock bodies.
2. Explain how strain differs from stress.
3. How is brittle deformation different from ductile deformation?
4. List and describe the four factors that affect whether a rock deforms in a brittle or ductile manner.

10.2 Folds: Rock Structures Formed by Ductile

1. Sketch and distinguish between anticlines and synclines and between domes and basins.
2. The Black Hills of South Dakota is a good example of what type of geologic structure?
3. Where are the youngest rocks in a structural basin found: near its center or near its margin?
4. Describe how a monocline forms.

10.3 Faults and Joints: Rock Structures Formed by Brittle Deformation

1. Contrast the movements that occur along normal and reverse faults. What type of stress is responsible for each kind of fault?
2. How are reverse faults different from thrust faults? In what way are they similar?
3. Describe the relative movement along a strike-slip fault.
4. How are joints different from faults?

10.4 Mapping Geologic Structures

1. Describe how the dip of an incline
2. Why are geologic maps useful?