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

These metamorphic rocks have been severely deformed and folded by the collision of tectonic plates in what is now Connecticut.

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

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!

Stikes and Dips are used to identify geologic structures

Strike and Dip Define and map the orientation of planar features Bedding planes (sedimentary rocks) Foliation Joints Faults Dikes Sills Ore Veins Figure 10.4: Strike and dip. The intersection of a horizontal plane (the water surface) and an inclined plane (the surface of any of the rock layers) forms a line known as strike. The dip of these layers is their maximum angular deviation from horizontal. Notice the strike and dip symbol with 50 adjacent to it, indicating the angle of dip. Fig. 10-4, p. 221

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 Strike and Dip Map Symbol Figure 10.4: Strike and dip. The intersection of a horizontal plane (the water surface) and an inclined plane (the surface of any of the rock layers) forms a line known as strike. The dip of these layers is their maximum angular deviation from horizontal. Notice the strike and dip symbol with 50 adjacent to it, indicating the angle of dip. Fig. 10-4, p. 221

Anticline (fold)

Syncline (fold)

Plunging Anticline

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

Plunging Anticline, Colorado

Domes and Basins

Brittle Strain  Joints When shallow crust is strained rocks tend to exhibit brittle strain

Sheet Joints

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 Figure 10.11a: Fault terminology.

Fault: Movement occurring along a discontinuity Brittle strain and subsequent movement as a result of stress Fault terminology

Faults Fault: When movement occurs along a discontinuity Fault type depends on the type of stress

Normal Faults

Normal Faults, Horsts and Grabens

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 Figure 10.15b: View of the Humboldt Range in Nevada, which is a horst bounded by normal faults. Graben Humboldt Range, Northern Nevada

Horst and Graben, Nevada Figure 10.15b: View of the Humboldt Range in Nevada, which is a horst bounded by normal faults. 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