GEY 101 - Introductory Geology: Exploring Planet Earth
Deformation

Structural geologists study the architecture and processes responsible for deformation of Earth’s crust

A working knowledge of rock structures is essential to our modern way of life

Deformation is a general term that refers to all changes in the original form and/or size of a rock body

Most crustal deformation occurs along plate margins

Deformation involves:

Force - that which tends to put stationary objects in motion or changes the motions of moving objects

Concepts

• stress - causes strain (deformation), the force applied to a volume of rock.
• strain - change in shape caused by stress, the deformation caused by stress, strain can take place by folding or faulting.

Types of stress

• tension <---- -----> : Tension is the stretching of rocks.
• compression -----> <------ : Compression is the squeezing together of rocks.
• shear stress is pulling in opposite directions to provide a rolling stress.

Strain

Elastic deformation - the rock returns to nearly its original size and shape when the stress is removed Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation)

• types
  • brittle - fracture, break; If rocks are deformed quickly or are hard, they will Fracture
  • ductile - bend, fold (plastic deformation); Plastic Deformation when rocks are soft enough of are stressed slowly enough they will deform plastically and flow.
• controlling factors
  • temperature
  • confining pressure
  • time
  • rock type

Mapping geologic structures When conducting a study of a region, a geologist identifies and describes the dominant rock structures
Usually only a limited number of outcrops (sites where bedrock is exposed at the surface) are available
Work is aided by advances in aerial photography, satellite imagery and Global Positioning Systems (GPS)
Describing and mapping the orientation or attitude of a rock layer or fault surface involves determining the features; Strike and Dip tell us the orientation of rocks.

Strike (trend)

• The compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane
• Generally expressed an an angle relative to north

Dip (inclination)

• The angle of inclination of the surface of a rock unit or fault measured from a horizontal plane
• Includes both an of inclination and a direction toward which the rock is inclined

Ductile Deformation -- Folds

During crustal deformation rocks are often bent into a series of wave-like undulations called folds

Characteristics of folds

• Most folds result from compressional stresses which shorten and thicken the crust
• Parts of a fold
  • Limbs - refers to the two sides of a fold
  • Axis - a line drawn down the points of maximum curvature of each layer
  • Axial plane - an imaginary surface that divides a fold symmetrically

• Anticline - upfolded or arched rock layers
• Syncline - downfolds or troughs of rock layers

  Depending on their orientation, anticlines and synclines can be described as Symmetrical, asymmetrical, recumbent (an overturned fold), or plunging

  • Asymmetrical folds are when one side is steeper than the other.
  • Overturned folds are where one side is tipped underneath.
  • Plunging folds. are where one end of the fold is diving into the ground.

• Monoclines - large, step-like folds in otherwise horizontal sedimentary strata; Monoclines are often the result of movement along buried faults
• Dome
  • Upwarped displacement of rocks
  • Circular or slightly elongated structure
  • Oldest rocks in center, younger rocks on the flanks
• Basin
  • Circular or slightly elongated structure
  • Downwarped displacement of rocks
  • Youngest rocks are found near the center, oldest rocks on the flanks

  Circular outcrop patterns are typical for both domes and basins; Domes and Basins are round folds.

Faults are fractures in rocks along which appreciable displacement has taken place

Sudden movements along faults are the cause of most earthquakes

Classified by their relative movement which can be:
Horizontal, vertical, or oblique

Brittle deformation - faults

• basics
  • hanging wall -- The hanging wall is the overhanging side of a fault.
  • footwall -- The footwall is the one that you would be walking on.
• Dip Slip faults move in the dip direction, generally up and down.
• tension - hanging wall moves down
  • normal faults form in tensional areas where the earth is stretched.
  • Accommodate lengthening or extension of the crust
  • Most are small with displacements of a meter or so
  • Larger scale normal faults are associated with structures called fault-block mountains
  • Detachment fault is a nearly horizontal fault at depth where other faults sole into
  • Horsts and Grabens
• compression - hanging wall moves up; Accommodate shortening of the crust
  • reverse faults - steep fault, dips greater than 45o ; form in compressional regimes
  • thrust faults - gently dipping fault, dips less then 45o; form in compressional regimes
• shear
  • strike slip faults move generally sideways
  • Dominant displacement is horizontal and parallel to the strike of the fault
  • Types of strike-slip faults
    • Right-lateral - as you face the fault, the block on the opposite side of the fault moves to the right
    • Left-lateral - as you face the fault, the block on the opposite side of the fault moves to the left
  • Transform fault
    • Large strike-slip fault that cuts through the lithosphere
    • Accommodates motion between two large crustal plates
    • The San Andreas fault system is a major transform fault

Joints or Fractures

• Joints are among the most common rock structure
• Technically, a joint is a fracture with no movement
• Most occur in roughly parallel groups
• Significance of joints
  • Chemical weathering tends to be concentrated along joints
  • Many important mineral deposits are emplaced along joint systems
  • Highly jointed rocks often represent a risk to construction projects

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