Themes of the Day:

• Strike and Dip (Leftovers)
• Folds
• Faults

Strike & Dip

• Strike - the intersection of a horizontal plane with a planar feature in a rock (such as sedimentary bedding or metamorphic foliation) - measured in the horizontal plane as the azimuth or bearing of the strike line
• Dip - oriented perpendicular to strike - the directly downhill direction - measured in the vertical plane as the angle from the horizontal down to the dipping plane
• Strike and dip are measured in the field and used to describe the attitude of geologic features on maps

Folds

• Plastic deformation - more common deep in the crust or as a result of low strain rates
• Best seen in stratified rocks
• Parts of a Fold:
  • Axial Plane
  • Limbs
  • Axis
  • Fold Nose or Hinge
• Basic Types of Folds:
  • Anticline - concave down - oldest strata exposed at the axial plane
  • Syncline - concave up - youngest strata exposed at the axial plane
  • GigaPan: Calico Folds, Mojave Desert, California
  • GigaPan: Bear Valley Folds, near Shamokin, Pennsylvania
  • Monocline - flat-ramp-flat geometry
  • Structural Dome - anticlinal cross section in all directions - oldest rocks exposed in the core - e.g., Black Hills, South Dakota
    Sinclair/Grenville/Teapot Dome, WY - Topographic Map - Google Earth - Sinclair Oil
  • Structural Basin - synclinal cross section in all directions - youngest rocks exposed in the core - Michigan Basin
• Other Descriptors:
  • Open, Tight, and Isoclinal Folds
  • Symmetrical, Asymmetrical, Overturned, and Recumbant Folds
  • Plunging folds - when the axis of a fold is not horizontal - Valley and Ridge, PA; Sheep Mountain Anticline, WY

Faults

• Brittle deformation - more common shallow in the crust or as a result of high strain rates
• Joints are fractures that have no offset - "Joints Relieve Stress"
• Fault Types:
  • Normal Faults - dipping fault planes, dip-slip movement, hanging wall moves down with respect to the footwall - thinning and extension of the crust - associated with divergent plate boundaries - Rooks County Normal Fault GigaPan - detachment faults or horst & graben structure - A horst is a horst, of course of course!
  • Reverse Faults (Thrust Faults)- dipping fault planes, dip-slip movement, hanging wall moves up with respect to the footwall - thickening and compression of the crust - Thrust Faults have shallow dips (< 30^o) - associated with convergent plate boundaries - Example: Keystone Thrust, NV
  • Strike-slip (transform) faults - vertical fault planes, strike-slip movement, right-lateral and left-lateral senses of movement - example: San Andreas Fault, CA
  • Oblique-slip - some combination of dip-slip and strike-slip - resulting from transtension or transpression - associated with transform plate boundaries