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Tectonic Stresses Large Scale Strain of the Crust - Geologic Structures
• Inner core: Solid iron
• Outer core: Liquid iron, convecting (magnetic field)
• Mantle (Asthenosphere) : Solid iron-magnesium silicate, plastic, convecting
• Crust (Lithosphere): Rigid, thin 5-30km
Crust: Rigid, Thin
Mantle: Plastic, Convecting
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Tectonics and Structural Geology
Tectonic Stresses resulting from Internal Energy (heat driving convection) Strains (deforms) the Mantle and Crust
Bends Rocksductile strain (Folds)
•Breaks Rock•brittle strain (Joints)
•Moves large blocks •Faults
Releases energy Earthquakes
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Stresses at Plate Boundaries
•Divergent (Tensional)
|•Convergent (Compressional) |•Transform (Shear)
e.g., Pacific NW
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Geological Structures
•Different stresses result in various forms of strain (geologic structures)• Folds (compressive stresses
may cause ductile strain)• Faults (Any type of stress
may cause brittle strain. The type of fault depends on the type of stress)
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Geological Structures
• Fault: a discontinuity surface across which there has been shear displacement
• Hangingwall: the wall and body of rock above an inclined fault
• Footwall: the wall and body of rock beneath an inclined fault
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Geological Structures
• Normal Fault: hanging wall moved downwards compared to the footwall
• Reverse Fault: hanging wall moved upwards compared to the footwall
• Thrust fault: low angle reverse fault
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Geological Structures
• Strike Slip Fault: displacement parallel to the fault plane.
• Can be left or right handed!
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Strike and Dip
•Define and map the orientation of planar features• Bedding planes (sedimentary rocks)• Foliation• Joints• Faults• Dikes • Sills• Ore Veins
Fig. 10-4, p. 221
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Strike and Dip
•Strike: The line of intersection between the plane and a horizontal surface
•Dip: Angle that the plane makes with that horizontal plane
Fig. 10-4, p. 221
Strike and Dip Map Symbol
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Fold Terminology
•Hinge: the greatest curvature of a folded surface
•Axial plane: a planar surface defined by the successive positions of fold hinges
•Plunge
Axis Axis
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Defining Fault Orientation
•Strike of fault plane parallels the • fault trace• fault scarp
•Direction of Dip of the fault plane indicates the Hanging wall block
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Fault:
•Movement occurring along a discontinuity•Brittle strain and subsequent movement as a result
of stress•Fault terminology
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Faults
•Fault: When movement occurs along a discontinuity
•Fault type depends on the type of stress
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Horsts and Grabens
•Older Rocks are exposed along the ridges formed by the horsts
•Younger rocks lie beneath the grabens•Sediment fills in the linear valleys
HorstGraben
HorstGraben
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Nevada• “Washboard
topography” is the result of Horsts and Grabens
• A.k.a, Basin and Range• E.g., Humbolt Range• E.g., Death Valley
(Graben)
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Reverse and Thrust Faults
•Compressive stress causes the hanging wall to move upward relative to the foot wall Reverse Fault
•At convergent plate boundaries ancient rocks can be thrust over younger rocks Thrust Fault
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