STRUCTURAL GEOLOGY

Structural Geology

• What is Structural Geology

Review
Stress, Strain, Rupture

• Types of Pressure
• Confining pressure (Confining stress)
• Directed pressure - Stress
• tension - pull apart
• compression -push together
• shear - scissor action
• How do rocks respond to stress?
• Strain - deformation - elastic and plastic
• Brittle fracture - rupture, breakage

Earthquakes
How do rocks respond to stress? 
Stress and Strain

Structural Geology
Rock Behavior

• Rock Behavior - ductile vs. brittle
• Ductile - rocks bend, fold; get ductile behavior:
• at higher temperature
• at higher confining pressure
• if rocks contain water
• if stress is applied over long period of time
• depending on nature of the material
• shale more ductile than limestone

Laboratory Experiments with Marble Folds

• bending of horizontal features
• common with layered rock
• occur on a scale of mm to km
• "tightness" varies depending on deformation

Folds (cont.)

• How does a geologist recognize folds in the field?
• see an outcrop
• measure orientation of beds and infer from changes in orientation
• Measuring bed orientation
• orientation of 3 points relative to horizontal plane, or
• Strike and dip

Strike and Dip

• Strike - orientation of a line formed by the intersection of the bed with the horizontal plane
• Dip - angle between horizontal plane and bed
• note: dip direction will be at right angles to strike

Folds

• Cross-section and map-view of fold, with strikes and dips marked

Geologic Map

• Colors distinguish different formations
• Ex: Purple - Cook Mountain Formation

Characterizing Folds (cont.)

• Fold Anatomy
• Axial Plane - surface which divides a fold as symmetrically as possible
• Hinge line may plunge into the earth (plunging fold)
• Plunge - angle of hinge line to horizontal plane
• Trend - compass orientation of line

Characterizing Folds

• Syncline
• Younger beds in core of fold
• Often "U"-shaped, but not always
• Anticline
• Older beds in core of fold
• Often "A"-shaped, but not always
• Monocline
• step-like bend in an otherwise gently dipping or horizontal bed

Characterizing Folds (cont.)

• Other types of folds
• Asymmetrical
• Overturned
• Limb tilted more than 90º from horizontal
• Recumbent
• Axial plane horizontal, or nearly horizontal

Characterizing Folds
Summary

Characterizing Folds (cont.)

• Other Features
• Domes - circular or elliptical anticline (Ex: Black Hills, SD)
• Basin - circular or elliptical syncline (Ex: Michigan Basin)

Joints and Faults

• Products of brittle behavior
• Joint - fracture in rock along which no appreciable relative displacement has occurred
• Fault - fracture in rock along which there has been displacement.

Joints

• Joints
• may be due to compression, tension or shear stress
• tension joints form a regular pattern at right angles to tension
• joints occur in sets, not alone
• Joint sets have a common origin
• usually consist of parallel joints

Faults

• Faults - must be able to prove displacement for a fracture to be a fault
• Evidence of faulting
• crushed rock in fracture
• fault breccia
• mylonite - fine grained, recrystallized
• fault gouge - fine grained, soft
• slickenslides - polished and striated surfaces, formed by friction
• Displacement of bed (or man-made feature like a fence)
• Fault drag (drag folding) - folding due to drag on a bed

Faults (cont.)

• Types of Faults
• Dip-slip - displacement up or down dip of fault plane

Dip-slip Faults

• Normal fault - hanging wall block moves down; tension
• Reverse fault - hanging wall block moves up; compression

Normal or Reverse fault?

Dip-Slip (cont.)

• Thrust fault (Fig. 12-18) - low angle reverse fault
• results from compression

Strike-slip Faults

• Strike-slip - displacement parallel to strike of fault plane; movement is horizontal
• Left lateral fault
• Right lateral fault

Faults (cont.)

• oblique-slip - displacement is a combination of dip-slip and strike-slip motion

Faults and Plate Boundaries

• Divergent
• tension
• normal faulting
• Convergent (Subduction zones)
• compression
• reverse and thrust faulting
• Transform
• shear
• strike-slip