Lecture 5

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1 Structural Geology: Deformation and Mountain Building 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 5-30km Mantle: Plastic, Convecting 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 Folds and Faults (Palmdale, Ca) Kaynasli, Turkey Stresses at Plate Boundaries • Divergent (Tensional) | • Convergent (Compressional) | • Transform (Shear) e.g., Pacific NW 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 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 22 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 Horst Graben 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 [...]... 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) 17 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... positions of fold hinges • Plunge Axis 20 Plunging Anticline, Colorado 21 Domes and Basins 22 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... 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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