Chapter 11 Folds, Faults, and Other Records of Rock Deformation

Geologic Structures - Faults, joints, folds. Bent, tilted, deformed rocks on all scales - microscopic to mountain sized

Why do we care?

• They give information on forces that lead to earthquakes, mountain building and plate tectonics.
• Also, many economic mineral resources (that is, deposits that can be mined profitably) are associated with certain kinds of geologic structures - for example, oil and natural gas are often found in anticlines

Geologic maps - rock types, ages of rocks, rock exposures, mineral deposits, strike and dip, and geologic structures all can be mapped on geologic maps. Faults and many rock formations can be defined as a plane in geometry. A plane can be uniquely defined by its strike and dip.

• Strike - line formed by intersection of a plane (such as a fault plane) with a horizontal plane. The strike of a plane is defined by the direction of that line, for example, due north, or 30 degrees west of north.
• Dip - angle down from the horizontal of the plane perpendicular to the strike.
• See the figure 11-4 from your text for examples.

Forces (stresses)-

• Compressive forces - pushing together (at convergent plate margins)
• Tensional forces - pulling apart (at divergent plate margins)
• Shearing forces - sliding past (transform plate boundaries)

Strain - response of a rock (or any material) to a stress or force acting on it

Types of strain (or deformation)

• Elastic - bounces back to same size and shape (like rubber, seismic waves)
• Brittle - fractures or breaks - (earthquakes)
• Ductile (or plastic) - deforms permanently, does not bounce back (like putty)

Factors influencing type of deformation that occurs (recall silly putty demonstration)

• Temperature and pressure (higher T and/or higher P - ductile)
• Rate of deformation (fast causes brittle deformation, slow causes ductile)
• Rock composition and mineralogy - quartz is strong, calcite and halite are weak. Igneous rocks are strong, sedimentary rocks are weaker

Faults and Joints - brittle response to stress

• Joint - a fracture in rock that has had no relative movement between one side and the other
• Fault - a fracture in rock along which movement has taken place
• Fault plane - the plane on which fault motion has taken place
• Hanging wall - the side of the fault that is above the fault plane
• Footwall - the side of the fault that lies below the fault plane

Types of faults

• Dip-slip faults - motion of fault is along (up or down) the dip of the fault plane
  • Normal faults (tensional stress, or horizontal extension) (Basin & Range Extension, Mid-ocean ridges, divergent plate margins).
  • Reverse faults or Thrust faults (compressional stress, convergent plate margins)
• Strike-slip faults - fault motion is along the strike of the fault plane (shearing stress, transform plate boundaries).
  • Right lateral vs. left lateral. If the opposite side of the fault appears to move to the right relative to where you are standing, it is a right-lateral strike slip fault. The San Andreas fault is a right lateral fault.

Folds - ductile (or plastic) response to forces (stress)

• Anticline - rocks are pushed up in the center (oldest rocks in center, dips are away from the center) - (make a drawing)
• Syncline - rock s are pushed down in the center, (youngest rocks are in the center, dips are toward the center) (make a drawing)
• Note how anticlines and synclines are recognized on a geologic map - by ages of rocks and the strikes and dips of the rocks
• Elements of a fold - axial plane, fold axis, limbs
• Structural basins and Domes - oil and gas, minerals, geothermal resources