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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 5-30km 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 4 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) 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 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 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