Download Tectonic and Regional Structural Geology

Structural Geology
Spring 2003
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.
Plate Tectonics
Tectonic Structures
► Most
structures are driven by the forces of
Plate Tectonics
► The kinds of structures are determined by:

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

Temperature and pressure
Composition
Layering
Anisotropy or Isotropy of the layers
Amount of fluids present
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
Tectonic Structures
► Ductile




deformation produces:
Folds
Ductile Faults
Cleavages
Foliation
Tectonic Structures
► Brittle
Deformation
 Certain types of folds
 Brittle Faults
 Joints
Nontectonic Structures
► Nontectonic
structures
structures can mimic tectonic
 Meteor impacts
 Landslides
 Structures produce by gravitational forces
3-Dimensional Objects
► Visualization
of 3-Dimensional Objects
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
Applications of Structural
Geology
► Engineering


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Issues
Bridges
Dams
Power Plants
Highway Cuts
Large Buildings
Airports
Applications of Structural Geology
► Environmental

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
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
Issues
Earthquake hazard
Location of landfill sites
Contamination cleanup
Distribution of groundwater
Mineral exploration
Scale in Structural Geology
► Microscopic
– Need magnification
 Foliation, Micro folds
► Mesoscopic
outcrops
– Hand specimens and
 Foliation, Folds, Faults
► Macroscopic
– Mountainside to map levels
 Basins, domes, Metamorphic Core Complexes
Scale in Structural Geology
► Non-penetrative
scales
structures – not present on all
 Faults
 Isolated folds
► Penetrative
structures – found on any scale that
we chose to study
 Slaty cleavage
 Foliation
 Some folds
Scale and Folds
Figure 1-6
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
Fundamental Concepts
► Pumpelly’s
Rule – Small structures are a key
to and mimic the styles and orientations of
larger structures of the same generation
within a particular area.
Plate Tectonics
► Driving
Mechanisms
 Convection
 Push-Pull Theory
► Plate
Boundaries
 Divergent
 Convergent
 Transform
Geochronology
► Absolute
Age Dating
► Review of atomic structure
► Most useful isotope decay processes
Using radioactivity in dating
► Reviewing
►Atomic
basic atomic structure
number
 An element’s identifying number
 Equal to the number of protons in the atom’s
nucleus
►Mass
number
 Sum of the number of protons and neutrons in an
atom’s nucleus
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
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
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
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
Common Types of Radioactive
Decay
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
A radioactive decay curve
Using radioactivity in dating
► Radiometric
►Principle
dating
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
Using radioactivity in dating
► Radiometric
►Sources
dating
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.
Geochronology
Mineral
Zircon
Garnet
Rutile
Muscovite
K-spar
Biotite
Hornblend
e
Biotite
Syste Daughter
m
U-Pb 207, 206Pb
U-Pb 207, 206Pb
U-Pb 207, 206Pb
87Sr
Rb-Sr
87Sr
Rb-Sr
87Sr
Rb-Sr
40Ar
K-Ar
K-Ar
40Ar
Blocking T
ºC
>800
700-725
550-650
300
480
300
Geochronology
► Uranium-Lead
Method (U-Pb)
 Most reliable technique for rocks
 Ages exceed 10 million years
 Use of Zircons for dating
238U
235U
232Th
206Pb
(half-life = 4.5x109yrs)
207Pb (half-life = 0.7x109yrs)
208Pb (half-life = 1.4x109yrs)
Uranium-Lead Method
Uranium-Lead Method
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)
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
40K
+ ß–
(half-life = 1.2x109yrs)
40Ar
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
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)
Rock Cycle