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Structural Geology

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Title: Structural Geology


1
Structural Geology
  • Spring 2003

2
Structural Geology
  • Structural geologists are concerned with why
    parts of the Earth have been bent into folds and
    others have been broken by faults.
  • Mapping of these structures provides important
    information to land managers and mineral
    exploration.
  • Understanding of these features help us
    understand the dynamic Earth.

3
Plate Tectonics
4
Tectonic Structures
  • Most structures are driven by the forces of Plate
    Tectonics
  • The kinds of structures are determined by
  • Temperature and pressure
  • Composition
  • Layering
  • Anisotropy or Isotropy of the layers
  • Amount of fluids present

5
Tectonic Structures
  • Time (or rate of change) is very importance
  • A rock may behave in a ductile or brittle fashion
    depending upon how quickly it is deformed

6
Tectonic Structures
  • Ductile deformation produces
  • Folds
  • Ductile Faults
  • Cleavages
  • Foliation

7
Tectonic Structures
  • Brittle Deformation
  • Certain types of folds
  • Brittle Faults
  • Joints

8
Nontectonic Structures
  • Nontectonic structures can mimic tectonic
    structures
  • Meteor impacts
  • Landslides
  • Structures produce by gravitational forces

9
3-Dimensional Objects
  • Visualization of 3-Dimensional Objects

10
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11
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12
Structural Geology
  • Subdisciplines of Structural Geology
  • Field Relations
  • Make accurate geologic maps
  • Measure orientations of small structures to
    inform us of the shape of larger structures
  • Study the sequence of development and
    superposition of different kinds of structures
  • Rock Mechanics the application of physics to
    the study of rock materials.
  • Tectonic and Regional Structural Geology Study
    of mountain ranges, parts of entire continents,
    trenches and island arcs, oceanic ridges

13
Applications of Structural Geology
  • Engineering Issues
  • Bridges
  • Dams
  • Power Plants
  • Highway Cuts
  • Large Buildings
  • Airports

14
Applications of Structural Geology
  • Environmental Issues
  • Earthquake hazard
  • Location of landfill sites
  • Contamination cleanup
  • Distribution of groundwater
  • Mineral exploration

15
Scale in Structural Geology
  • Microscopic Need magnification
  • Foliation, Micro folds
  • Mesoscopic Hand specimens and outcrops
  • Foliation, Folds, Faults
  • Macroscopic Mountainside to map levels
  • Basins, domes, Metamorphic Core Complexes

16
Scale in Structural Geology
  • Non-penetrative structures not present on all
    scales
  • Faults
  • Isolated folds
  • Penetrative structures found on any scale that
    we chose to study
  • Slaty cleavage
  • Foliation
  • Some folds

17
Scale and Folds
Figure 1-6
18
Fundamental Concepts
  • Doctrine of Uniformitarianism
  • Law of Superposition
  • Law of Original Horizontality
  • Law of Cross-Cutting Relationships
  • Law of Faunal Succession
  • Multiple Working Hypotheses
  • Outrageous Hypothesis

19
Fundamental Concepts
  • Pumpellys Rule Small structures are a key to
    and mimic the styles and orientations of larger
    structures of the same generation within a
    particular area.

20
Plate Tectonics
  • Driving Mechanisms
  • Convection
  • Push-Pull Theory
  • Plate Boundaries
  • Divergent
  • Convergent
  • Transform

21
Geochronology
  • Absolute Age Dating
  • Review of atomic structure
  • Most useful isotope decay processes

22
Using radioactivity in dating
  • Reviewing basic atomic structure
  • Atomic number
  • An elements identifying number
  • Equal to the number of protons in the atoms
    nucleus
  • Mass number
  • Sum of the number of protons and neutrons in an
    atoms nucleus

23
Using radioactivity in dating
  • Reviewing basic atomic structure
  • Isotope
  • Variant of the same parent atom
  • Differs in the number of neutrons
  • Results in a different mass number than the
    parent atom

24
Using radioactivity in dating
  • Radioactivity
  • Spontaneous changes (decay) in the structure of
    atomic nuclei
  • Types of radioactive decay
  • Alpha emission
  • Emission of 2 protons and 2 neutrons (an alpha
    particle)
  • Mass number is reduced by 4 and the atomic number
    is lowered by 2

25
Using radioactivity in dating
  • Types of radioactive decay
  • Beta emission
  • An electron (beta particle) is ejected from the
    nucleus
  • Mass number remains unchanged and the atomic
    number increases by 1

26
Using radioactivity in dating
  • Types of radioactive decay
  • Electron capture
  • An electron is captured by the nucleus
  • The electron combines with a proton to form a
    neutron
  • Mass number remains unchanged and the atomic
    number decreases by 1

27
Common Types of Radioactive Decay
28
Using radioactivity in dating
  • Parent an unstable radioactive isotope
  • Daughter product the isotopes resulting from
    the decay of a parent
  • Half-life the time required for one-half of the
    radioactive nuclei in a sample to decay

29
A radioactive decay curve
30
Using radioactivity in dating
  • Radiometric dating
  • Principle of radioactive dating
  • The percentage of radioactive atoms that decay
    during one half-life is always the same (50
    percent)
  • However, the actual number of atoms that decay
    continually decreases
  • Comparing the ratio of parent to daughter yields
    the age of the sample

31
Using radioactivity in dating
  • Radiometric dating
  • Sources of error
  • A closed system is required
  • To avoid potential problems, only fresh,
    unweathered rock samples should be used
  • Blocking Temperature The temperature below
    which a crystal lattice traps radioactive
    daughter products.

32
Geochronology
Mineral System Daughter Blocking T ºC
Zircon U-Pb 207, 206Pb gt800
Garnet U-Pb 207, 206Pb 700-725
Rutile U-Pb 207, 206Pb 550-650
Muscovite Rb-Sr 87Sr
K-spar Rb-Sr 87Sr
Biotite Rb-Sr 87Sr 300
Hornblende K-Ar 40Ar 480
Biotite K-Ar 40Ar 300
Muscovite K-Ar 40Ar 350
33
Geochronology
  • Uranium-Lead Method (U-Pb)
  • Most reliable technique for rocks
  • Ages exceed 10 million years
  • Use of Zircons for dating
  • 238U 206Pb (half-life 4.5x109yrs)
  • 235U 207Pb (half-life 0.7x109yrs)
  • 232Th 208Pb (half-life 1.4x109yrs)

34
Uranium-Lead Method
35
Uranium-Lead Method
36
Geochronology
  • Robidium-Strontium (Rb-Sr)
  • Most applicable in rocks over 100 million years
    old
  • Whole-rock ages are more reliable in Rb-Sr
  • No gaseous daughter elements
  • Principle source of error is later metamorphism
    and hydrothermal alteration.
  • 87Rb 87Sr ß (half-life 48.8x109yrs)

37
Geochronology
  • Potassium-Argon (K-Ar)
  • Used for rocks around 1 million years old
  • Ar is a gas and can be easily released from most
    rocks
  • Biotite, muscovite, hornblende retain argon
    better than other minerals
  • Low blocking temperatures (300ºC - 480 ºC)
  • 40Ca ß
  • 40K (half-life 1.2x109yrs)
  • 40Ar

38
Geochronology
  • Argon-Argon (40Ar-39Ar)
  • Samples must be irradiated to convert 39K to 39Ar
  • Can determine the cooling history of the rocks
  • Useful for determining the time of uplift,
    metamorphism, or emplacement of structures

39
Geochronology
  • Samarium - Neodynium (Sm-Nd)
  • Used mainly for dating ocean floor basalts
    because sea water is abundant in Sr but depleted
    in Nd
  • Therefore, can be used to determine contamination
    by sea water and hydrothermal alteration
  • 147Sm 143Nd (half-life 106x109yrs)

40
Rock Cycle
41
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