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Structural Geology: Deformation and Mountain Building. 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.
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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 Tectonic Stresses Large Scale Strain of the Crust - Geologic Structures Crust: Rigid, Thin Mantle: Plastic, Convecting
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 energyEarthquakes Tectonics and Structural Geology
Divergent (Tensional) | Convergent (Compressional) | Transform (Shear) e.g., Pacific NW Stresses at Plate Boundaries
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)
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
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
Geological Structures • Strike Slip Fault: displacement parallel to the fault plane. • Can be left or right handed!
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
Strike and Dip Strike and Dip Map Symbol • 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
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
Brittle Strain Joints • When shallow crust is strained rocks tend to exhibit brittle strain
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
Fault: • Movement occurring along a discontinuity • Brittle strain and subsequent movement as a result of stress • Fault terminology
Fault: When movement occurs along a discontinuity Fault type depends on the type of stress Faults
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 Horst Horst Graben Graben
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)
Horst and Graben, Nevada Horst Graben Humboldt Range, Northern Nevada
Horst and Graben, Nevada Horst Graben Humboldt Range, Northern Nevada
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
Thrust Fault: Glacier NP, Montana Old Younger
Strike Slip Faults • Physiographic Features