Structural Geology: Deformation and Mountain Building

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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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Folds and Faults (Palmdale, Ca)

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Kaynasli, Turkey

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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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Stikes and Dips are used to identify geologic structures

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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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Anticline (fold)15

Syncline (fold)16

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Plunging Anticline

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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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Plunging Anticline, Colorado

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Domes and Basins

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Brittle Strain Joints

• When shallow crust is strained rocks tend to exhibit brittle strain

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Sheet Joints

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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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Normal Faults28

Normal Faults, Horsts and Grabens

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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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Horst and Graben, Nevada

Humboldt Range, Northern Nevada

Graben

Horst

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Horst and Graben, Nevada

Humboldt Range, Northern Nevada

Graben

Horst

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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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Thrust Fault: Glacier NP, Montana

Old

Younger

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Strike Slip Faults

•Physiographic Features

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