9.2 Folds, faults, and joints

Earth's crust is constantly shaped by forces that bend, break, and crack rocks. These forces create folds, faults, and joints, which are key to understanding our planet's geological history and current structure.

Folds form when rocks bend, creating anticlines and synclines. Faults occur when rocks break and move, with types like normal, reverse, and strike-slip. Joints are cracks without movement, influencing rock strength and fluid flow.

Structural Geology: Folds, Faults, and Joints

Types of folds

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• Anticlines arch upward with oldest rocks in the core (Grand Canyon)
• Synclines curve downward containing youngest rocks in the core (Death Valley)
• Monoclines form step-like appearance with one-sided fold (Waterpocket Fold in Utah)
• Symmetrical folds have limbs dipping at equal angles from axial plane
• Asymmetrical folds feature limbs dipping at unequal angles from axial plane
• Overturned folds occur when one limb tilts beyond vertical
• Recumbent folds form with nearly horizontal axial plane
• Fold elements include:
  • Axial plane divides fold into two equal parts
  • Hinge line marks maximum curvature
  • Limbs form sides of the fold

Classification of faults

• Dip-slip faults involve vertical movement
  • Normal faults: hanging wall moves down relative to footwall, associated with extension (Basin and Range Province)
  • Reverse faults: hanging wall moves up relative to footwall, linked to compression (Rocky Mountains)
  • Thrust faults: low-angle reverse faults with dip < 45° (Appalachian Mountains)
• Strike-slip faults involve horizontal movement
  • Right-lateral (dextral) faults: right side moves toward observer (San Andreas Fault)
  • Left-lateral (sinistral) faults: left side moves toward observer (Garlock Fault)
• Oblique-slip faults combine dip-slip and strike-slip motion
• Transform faults occur at plate boundaries as special strike-slip faults (Mid-Atlantic Ridge)

Joints in rocks

• Joints form fractures in rocks without significant displacement
• Formation mechanisms include:
  1. Tensile stress
  2. Cooling and contraction
  3. Unloading (removal of overlying rock)
• Types:
  • Systematic joints: parallel sets with consistent orientation (columnar jointing in basalt)
  • Nonsystematic joints: irregular patterns and orientations
• Significance:
  • Influence rock strength and stability
  • Create pathways for fluid flow
  • Control weathering and erosion patterns (Giant's Causeway)
• Joint sets and systems:
  • Orthogonal: two sets at right angles
  • Conjugate: two sets intersecting at acute angles

Stress patterns and deformation

• Stress types shape geological structures:
  • Compressional stress causes shortening and thickening
  • Tensional stress leads to stretching and thinning
  • Shear stress produces lateral displacement
• Fold orientations: axial planes form perpendicular to maximum compressional stress
• Fault orientations reflect stress directions:
  • Normal faults: maximum principal stress vertical
  • Reverse faults: maximum principal stress horizontal
  • Strike-slip faults: maximum and minimum principal stresses horizontal
• Joint orientations often perpendicular to minimum principal stress
• Anderson's theory of faulting connects fault types to principal stress orientations
• Regional tectonic settings influence stress patterns:
  • Convergent boundaries: compressional stress, folding, reverse faulting (Himalayas)
  • Divergent boundaries: tensional stress, normal faulting (East African Rift)
  • Transform boundaries: shear stress, strike-slip faulting (Alpine Fault, New Zealand)
• Folds, faults, and joints serve as indicators of past and present stress fields