Class Notes - Deformation of the Crust

Review Questions

1. Review material presented during the first week of class. Where do Earthquakes tend to occur? ..... shallow Earthquakes ....... deep Earthquakes?

2. What kind of "action (pushing, pulling or sliding)" takes place at each type of plate margin?

Introduction

The San Andreas fault is perhaps the best known active feature of deformation in the United States. In the link given in the chapter title you will take a field trip along the fault in the San Francisco Bay area.

You should begin by reviewing the following terms that have be introduced in previous chapters:

• stress
  • compressive stress
  • tensile stress
  • shear stress

• strain
• elastic behavior
• plastic behavior
• ductile behavior
• brittle behavior

1. Material which fails while undergoing elastic behavior fails in a _______ fashion

   brittle
   plastic
   spastic
   

2. Material which fails while undergoing plastic behavior fails in a _______ fashion.

   brittle
   ductile
   spastic
   

   In addition, review the following stress-strain plot. Remember that not all materials behave in the same way. Under a fixed stress one rock may behaving plastically, another elastically and a third may fail in a brittle fashion.

   Orientation of Features

   The principal of original horizontality states that sedimentary rocks tend to be nearly flat lying (or conform to the slope of the basin they are deposited in). In mountainous regions rocks which are inclined at a large angle to the horizontal are common. In between the time of deposition and observation forces acting within the Earth have changed the orientation of this unit.

   When two planes intersect a line is formed. If one plane is the inclined rock layer and the other is a horizontal plane representing the surface of the Earth the direction of the line formed by their intersection is called the strike of the unit. Strike is measured with respect to true North. The dip is the maximum angle of inclination of an inclined plane and is measured perpendicular to the strike.

   Think of the ground as our reference plane. Imagine moving this plane parallel to itself until it intersects the roof line. The intersection of the two planes defines the strike of the house. The roof dips in opposite directions away from the roof line. This roof is symmetrical - both sides dip an equal amount in opposite directions. What would the symbols look like from a roof that dipped more to the right than the left? What if the roof was slanted in only one direction? (a monocline).

   Folds

   Folds form in rocks that have undergone a permanent change in their orientation - plastic flow. Up folds are anticlines and down folds are synclines . Monoclines are simple bends or flexures in otherwise horizontal rocks.

   1. Look at the two folds in the illustration at the beginning of this chapter. These are:

      monoclines
      synclines
      anticlines
      homoclines
      

      

      

      

      

   2. Take four pieces of paper. Label the bottom one A, the next one B and so on. In an undisturbed sequence which one represents the oldest rock layer?

      A
      B
      C
      D
      

   3. Now fold these units into an anticline with a vertical axial plane and limbs dipping in equal but opposite directions. Imagine that erosion levels off this structure and you walk across it. Where are the oldest rocks?

      in the center or core
      on the limbs
      

   4. If you make a syncline and erode it, where are the oldest rocks?

      center or core
      limbs
      

      If the limbs of a fold dip away from each other but at different angles the fold is asymmetrical. Make an asymmetrical anticline with your sheets of paper. If both limbs of a fold dip in the same direction, the fold is said to be upside down or overturned. If the axial plane is horizontal, the fold is said to be recumbent.

   5. Create a recumbent fold. Imagine that erosion has leveled the structure so that the top half is eroded away. What has happened to bed A?

      Bed A is on the bottom of the stack
      Bed A is at the top of the stack
      

      Thus, the principle of superposition seems to have been violated since A, the oldest bed is now at the top of the remaining section. Remember, however, that there was a stipulation that the section has not been disturbed and the formation of an overturned fold is quite a disturbance! When geologists were first mapping complicated deformed areas it was not uncommon for the stratigraphy to have been reversed in the sense that if they failed to recognize that the section was overturned, they would have argued that unit A was the youngest in the area.

      

   6. In the map above North is to the top. The strike of the beds is about:

      north-south
      east-west
      about 45 degrees west of north
      about 45 degrees east of north
      

   7. The structure represented is a (an):

      symmetrical syncline
      asymmetrical syncline
      symmetrical anticline
      asymmetrical anticline
      

   8. The oldest unit is _______.

      A
      B
      C
      D
      

   Faults

   A fracture is a break in rock material along which there has been no movement. A fault is a fracture along which movement has taken place. The fault plane separates the area into two blocks. Note that the orientation of the fault can be recorded by giving its strike and dip.

   It is difficult to determine the absolute sense of motion and the geologist often has to work with relative motion. In general, when a fault occurs one side of the fault plane may move parallel to the dip (dip slip), parallel to the strike of the faulty (strike slip), or it may move in an oblique way (a sense of motion with components parallel to both the dip and strike of the fault).

   Imagine that you are standing on the fault plane. The block at your feet is the foot wall and the block above your head is the hanging wall.

   

   Dip slip faults are classified as follows:

   • Tension - Normal - HW down and FW up

   • Compression:
     • Reverse - HW up and FW down - dip of fault plane is above 45 degrees.
     • Thrust - HW up and FW down - dip of fault plane is less than 20 degrees.

   • Shearing - Strike Slip - motion is in the horizontal - If the block across the fault plane has moved to the right the fault is right lateral.

     Use this animation to review different types of faults and then take the following quiz.

   Sketch each fault and label. Show the sense of motion with arrows.

   

   1. In figure I a ________ fault is depicted (assume this is a dip slip fault)

      reverse
      thrust
      normal
      strike slip
      

      Note (double headed arrow) that unit A is thicker due to the faulting.

   2. The fault in Figure Ia above was produced by:

      compression
      tension
      shear
      

   3. The faults in figure II are:

      normal
      reverse
      both normal and reverse
      

   4. Structure C is a:

      horst
      graben
      ophiolite
      

   5. Structure D is a:

      horst
      graben
      ophiolite
      

   Listric ("spoon-shaped") faults are curved faults with the concave side facing up. In order to avoid creating a huge hole in the ground, the hanging wall must deform as it moves. Locate the hanging wall and the footwall. Note that this is a Normal Fault as the hanging wall is moving down relative to the foot wall moving up. Sometimes these are called Growth Faults. Look at the following animation.

   Think about Listric Faults. As material accumulates the hanging wall continues to move down relative to the footwall. Thus, the basin deepens with time. This model can explain the formation of several km of shallow water sediments in sedimentary basins. At first this seems impossible, since the sediments at the bottom of the sedimentary pile cannot form a number of kilometers below sea level and still be classified as "shallow-water." View the animation for a solution! Note that this process leads to a new type of unconformity call a progressive unconformity. There is no significant angle between ajdacent beds but the bed at the bottom of the succession is at an angle to the bed deposited at the top of the succession.

   Permeability Barriers

   The flow of fluids within the Earth requires the presence of connected pore spaces (measured by the permeability). Folds and faults can produce permeability barriers and trap fluids.

   

   The Teton Mountains in Wyoming were uplifted as a result of a normal fault which is exposed at the base of the range on its eastern side. Uplift amounts to more than 7,000 feet from the Jackson Hole valley floor to the top of the Grand Teton. Much of the sculpturing is the result of Pleistocene glaciation.

   Mountains

   Mountains can form in a variety of ways:

   • Uplifted granitic plutons

   • block faulting - Basin and Range : horst and grabens - bounded by inward dipping normal faults. Grabens are the down dropped blocks and Horsts are the uplifted blocks

   • Folded Mountains
     • oceanic - continental plate boundaries - subduction complex

     • continent - continent plate boundary

     • ocean - ocean plate boundary - island arcs - may be welded onto a continental mass.

   Each continent possesses at lead one or more areas of exposed ancient (Precambrian) rocks called a shield. These shields represent the stable portions of the modern continents.

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   Copyright by John C. Butler, July 29, 1995

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