Assignments

  1. Understanding Earth: Chapter 7- Deformation: Modification of Rocks by Folding and Fracturing and Chapter 21 - Landscapes: Tectonic and climate interactions
  2. film Earth Revealed: movies. (http://www.learner.org/resources/series78.html)  Requires Windows media Player.  Sign in and view #7 Mountain Building and #8 Earth's Structures
wikiglobe
Terms: stress, strain, deformation, tension, compression, shear, anticline, syncline, joint, fault, types of faults (dip-slip, strike-slip fault, normal, reverse, right and left lateral lateral), unconformities (disconformity, nonconformity, angular)

Structural geology is the study of the three-dimenional arrangement of rocks, fractures, and deformational fabrics in rocks.  Rocks that have been compressed and folded have a different structure than those that have not.  Flat-lying sedimentary rocks have a horizontal structure.  Rocks that have been deformed by compression, extension or shearing during tectonic activity will be folded and fractured.   Folds may be gentle, tight or overlapping depending on the extent of deformation.  Fractures, such as faults, many be large and displace the rocks on opposite sides, or they may be small and numerous like joints that displace nothing. Discontinuities in rocks called unconformities formed by erosion are another type of structure.  Structures record geologic events.  They also but also effect rocks erode and the the grain of a landscape.  A landscape underlain by horizontal sedimentary rocks  (i.e. Colorado Plateau) will have a distinctly different topography than one underlain by folded sedimentary rocks (ie.Appalachian Highlands) or faulted rocks (i.e. Basin and Range).

Review of stress and deformation

Types of stress that cause deformation (strain).

Deformation refers to any change in volume or shape caused by stress. Whether a volume and or shape change results is determined by the uniformity of the the applied stress. Changes in hydrostatic (uniform) pressure will cause volume changes, whereas unevenly applied stress (shear) will cause changes in shape resulting in thinning or thickening.

deformation

Behaviors of deforming solids

Depending on a number of factor a material may have different behaviors. Folding is the result of plastic deformation, faulting and jointing is caused by brittle deformation.

  1. elastic/ductile (recoverable) Deformation that is recovered with the stress is released.
  2. plastic (non recoverable flow) Permenant deformation caused by flow. Plastic deformation occurs through microscopic changes within and around mineral grains and may involve sliding along grain boundaries, pressure solution, rotation, dynamic recrystallization, and internal gliding.
  3. brittle (breakage) Permenant deformation caused by fracturing

Factors that govern how a material behaves in response to stress:

  1. inherent properties of the material
  2. temperature
  3. pressure
  4. time (rate at which the stress is applied)
  5. rate at which a stress is applied

Describing the orientation of a plane in space

dip: direction and amount of inclination
strike: azimuth of a horizontal line on the plane
symbol: strike and dip symbol 135 30 (135=strike, 30=amount of dip)
geologic maps

Geologic Structures

Fractures: faults and joints

Faults displace the rocks across the fracture whereas joints do not.  Rock along a joint may separate but do not slide along it in opposite directions.

fault example
Fault Joints

Simple classification of joints:

  1. Shear joints: form at an angle to the direction of maximum compressive stress
  2. Tension joints: form parallel to the direction of maximum compressive stress and pependicular to the direction of maximum extension

Classification of faults:

Dip slip fault: Motion is parallel to the dip of the fault plane. The classification is based on the relative motion of the hanging and footwall blocks. Dip slip faults typically the result of horizontal extension or compression.

  1. Normal faulting occurs with the hanging wall above the fault moves down relative to the footwall below the fault.  Normal fault are formed in response to stretching of the crust, such as along a divergent boundary. 
  2. Reverse faulting is when the hanging wall moves up.  Reverse fault are formed by compressing, such as along a convergent boundary.
  3. Thrust faults are low angle reverse fault.  They accommodate large amounts of crustal shortening during compression.
Dip slip faults in cross section
normal reverse thrust
1. Normal Faults
2. Reverse Fault
3. Thrust Fault

Strike-slip fault: Motion is largely parallel to the strike of the fault plane. Classification is based on the relative displacement of the block across the fault from the observer. Strike slip faults typically result from horizontal shear, particularly that occurring along a transform boundary, such as the San Andreas fault in California.

  1. left lateral
  2. right lateral

Oblique (diagonal) slip

related terms: hanging wall, foot wall, horst, graben, relative vs. absolute motion

Question: What tectonic environments are these faults associated with? Give examples.

Slide show of recent faults from the NOAA National Data Center

Folds

Criteria: Location of youngest beds relative to oldest beds

  • Anticline: The oldest beds are in the center.
  • Syncline: The youngest beds are in the center.
  • Note: Anticlines and synclines can have any orientation. However, in regions characterized by gentle deformation the strata in anticlines typically form arches, and in synclines form troughs.

    Criteria: Dip of the fold limbs relative to the axial plane

    • symmetrical fold: limbs dip equally in opposite directions (#1)
    • asymmetrical fold: limbs dip unequally in opposite directions (#2)
    • overturned fold: limbs dip unequally in same directions (#3)
    • isoclinal fold: limbs are parallel

    Criteria: Dip of axial plane

    • upright: axial plane is vertical (#1)
    • inclined: axial plane is inclined (#2 and #3)
    • recumbent: axial plane is horizontal (#4)

    Criteria: Inclination (plunge) of the fold axis (hinge)

    • plunging anticline or syncline: axis is plunging in one direction
    • doubly plunging anticline or syncline: axis is plunging in both directions
    • dome (circular anticline) and basin (circular syncline)

     

    Which of the above folds is an anticline? a syncline? In what direction are these folds plunging?

    Identifiy features A and B above. Explain your interpretation.

    Modified from text images (Earth Revealed) used with permission from McGraw Hill

    Unconformities - Erosional structures

    Unconformities are geologic structures created by erosion that are often difficult to distinguish from faults. An unconformity is an erosional surface between two or more bodies of rock. An unconformity is defined by the nature of the rocks that are juxtaposed.
    Type
    Description and Significance
    Illustration
    disconformity

    A subtile unconformity between parallel beds of sedimentary rock.

    Significance: The surface may represent only a few thousand or million years of erosion, such as what might accompany a tempory lowering of sea level.

    angular

    unconformity

    Flat lying strata overlie dipping or folded strata.

    Significance: The underlying strata were deformed and eroded before the overlying beds were deposited. The type of unconformity records a much longer period of erosion in addition to a deformational event.

    nonconformity

    Metamorphic or plutonic igneous rocks are overlain by sedimentary strata.

    Significance: Represents a very long period of erosion, perhaps several hundreds of millions of years, during which several kilometers of rocks were removed. Typically an entire mountain belt was created and deeply eroded prior to the depositon of the overlying strata.


    Types of mountains

    Volcanic Mountains
      Examples: Hawaiian Islands, Kilimanjaro
    Fault block moutains related to extension or foreland compression
      Formed in areas of extensional and compressional tectonic activity.Examples:
      • The mountains of the Basin and Range region of the United State are bound by normal faults created by crustal extension.
      • The Rocky Mountains are composed of fault blocks bound by reverse faults formed by compression.
    Complex (orogens) - formed along subduction and collision zones
      Formed along collision boundaries: Generally composed of both deformed, metamorphosed sedimentary rocks and igneous rocks. Thrust faulting prevalent.
      • oceanic-oceanic (Island arc)
      • continental-oceanic (Cordilleran -Type orogen)
      • continental-continental (Alpine-Type orogen)
      Major zones of active orogenic formation
      Mountain belts
      forelandthrust
      Cordilleran-type orogen

      Alpine-type orogen - Formation of the Himalayas and Tibetan Plateau

      (from Understanding Plate Motion - USGS)

     

    Puzzles and Quizzes

    Interactive crossword puzzle: Deformation and Geologic Structures

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