Structural Geology

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Structural Geology

• Spring 2003

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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.

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Plate Tectonics
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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

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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

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Tectonic Structures

• Ductile deformation produces
• Folds
• Ductile Faults
• Cleavages
• Foliation

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Tectonic Structures

• Brittle Deformation
• Certain types of folds
• Brittle Faults
• Joints

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Nontectonic Structures

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

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3-Dimensional Objects

• Visualization of 3-Dimensional Objects

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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

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Applications of Structural Geology

• Engineering Issues
• Bridges
• Dams
• Power Plants
• Highway Cuts
• Large Buildings
• Airports

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Applications of Structural Geology

• Environmental Issues
• Earthquake hazard
• Location of landfill sites
• Contamination cleanup
• Distribution of groundwater
• Mineral exploration

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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

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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

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Scale and Folds
Figure 1-6
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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

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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.

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Plate Tectonics

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

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Geochronology

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

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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

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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

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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

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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

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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

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Common Types of Radioactive Decay
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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

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A radioactive decay curve
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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

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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.

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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
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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)

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Uranium-Lead Method
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Uranium-Lead Method
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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)

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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

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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

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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)

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Rock Cycle
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