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:  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 can mimic tectonic structures    Meteor impacts Landslides Structures produce by gravitational forces

3-Dimensional Objects

► Visualization of 3-Dimensional Objects

Structural Geology

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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

Applications of Structural Geology

► Environmental 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 – Hand specimens and outcrops  Foliation, Folds, Faults ► Macroscopic – Mountainside to map levels  Basins, domes, Metamorphic Core Complexes

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

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

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Reviewing basic atomic structure

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Atomic 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

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Reviewing basic atomic structure

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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

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Radioactivity

► ► Spontaneous changes (decay) in the structure of atomic nuclei

Types of radioactive decay

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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

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Types of radioactive decay

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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

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Types of radioactive decay

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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

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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

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Radiometric dating

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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

Using radioactivity in dating

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Radiometric dating

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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.

Mineral

Zircon Garnet Rutile Muscovite K-spar Biotite Hornblend e Biotite Muscovite

Geochronology

Syste m

U-Pb U-Pb U-Pb Rb-Sr Rb-Sr Rb-Sr K-Ar

Daughter

207, 206 207, 206 207, 206 Pb Pb Pb 87 Sr 87 Sr 87 Sr 40 Ar

Blocking T ºC

>800 700-725 550-650 300 480 K-Ar K-Ar 40 Ar Ar 300 350

Geochronology

► Uranium-Lead Method (U-Pb)    Most reliable technique for rocks Ages exceed 10 million years Use of Zircons for dating 238 U 235 U 232 Th 206 Pb (half-life = 4.5x10

9 yrs) 207 Pb (half-life = 0.7x10

9 yrs) 208 Pb (half-life = 1.4x10

9 yrs)

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.

87 Rb 87 Sr + ß – (half-life = 48.8x10

9 yrs)

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) 40 K 40 Ca + ß – (half-life = 1.2x10

9 yrs) 40 Ar

Geochronology

► Argon-Argon ( 40 Ar 39 Ar)    Samples must be irradiated to convert 39 K to 39 Ar 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 147 Sm 143 Nd (half-life = 106x10 9 yrs)

Rock Cycle