1 Structural Geology: Deformation and Mountain Building
2 Tectonic Stresses Large Scale Strain of the Crust - Geologic Structures Crust: Rigid, Thin • Inner core: Solid iron • Outer core: Liquid iron, convecting (magnetic field) • Mantle (Asthenosphere) : Solid iron-magnesium silicate, plastic, convecting • Crust (Lithosphere): Rigid, thin 5 -30 km Mantle: Plastic, Convecting
3 Tectonics and Structural Geology Tectonic Stresses resulting from Internal Energy (heat driving convection) Strains (deforms) the Mantle and Crust Bends Rocks ductile strain (Folds) • Breaks Rock • brittle strain (Joints) • Moves large blocks • Faults Releases energy Earthquakes
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5 Folds and Faults (Palmdale, Ca)
6 Kaynasli, Turkey
7 Stresses at Plate Boundaries • Divergent (Tensional) | • Convergent (Compressional) | • Transform (Shear) e. g. , Pacific NW
8 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)
9 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
10 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
11 Geological Structures • Strike Slip Fault: displacement parallel to the fault plane. • Can be left or right handed!
12 Stikes and Dips are used to identify geologic structures
13 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
14 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 Strike and Dip Map Symbol Fig. 10 -4, p. 221
15 Anticline (fold)
16 Syncline (fold)
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18 Plunging Anticline
19 Fold Terminology • Hinge: the greatest curvature of a folded surface Axis • Axial plane: a planar surface defined by the successive positions of fold hinges • Plunge Axis
20 Plunging Anticline, Colorado
21 Domes and Basins
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23 Brittle Strain Joints • When shallow crust is strained rocks tend to exhibit brittle strain
24 Sheet Joints
25 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
26 Fault: • Movement occurring along a discontinuity • Brittle strain and subsequent movement as a result of stress • Fault terminology
27 Faults • Fault: When movement occurs along a discontinuity • Fault type depends on the type of stress
28 Normal Faults
29 Normal Faults, Horsts and Grabens
30 Horsts and Grabens • Older Rocks are exposed along the ridges formed by the horsts Horst Graben • Younger rocks lie beneath the grabens • Sediment fills in the linear valleys
31 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)
32 Horst and Graben, Nevada Horst Graben Humboldt Range, Northern Nevada
33 Horst and Graben, Nevada Horst Graben Humboldt Range, Northern Nevada
34 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
35 Thrust Fault: Glacier NP, Montana Old Younger
36 Strike Slip Faults • Physiographic Features
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