9.2 Folds, faults, and fractures

Rock deformation shapes Earth's crust through folds, faults, and fractures. These structures result from stress and strain, creating diverse landforms. Understanding their types and mechanisms is crucial for interpreting geological history and predicting future changes.

Folds form when rocks bend, creating anticlines and synclines. Faults occur when rocks break and move, classified as dip-slip or strike-slip. Fractures and joints are cracks without significant movement. These structures reflect the complex forces shaping our planet's surface.

Folds, Faults, and Fractures

Types of geological folds

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• Fold geometry determines the shape and orientation of rock layers
  • Anticlines arch upward with older rocks in the core (Grand Canyon)
  • Synclines arch downward with younger rocks in the core (Appalachian Mountains)
  • Monoclines form step-like folds with one steeply dipping limb and one nearly horizontal limb (Colorado Plateau)
• Fold orientation describes the inclination of the fold axis
  • Plunging folds have inclined fold axes relative to the horizontal plane
    • Plunge measures the angle between the fold axis and the horizontal plane (30° plunge)
  • Non-plunging folds have horizontal fold axes (Zagros Mountains, Iran)
• Fold symmetry relates to the dip angles of the limbs
  • Symmetrical folds have limbs dipping at equal angles on either side of the axial plane (Chevron folds)
  • Asymmetrical folds have limbs dipping at different angles on either side of the axial plane (Overturned folds)
• Fold tightness is determined by the interlimb angle
  • Gentle folds have interlimb angles greater than 120 degrees (Jura Mountains, Switzerland)
  • Open folds have interlimb angles between 70 and 120 degrees (Sheep Mountain Anticline, Wyoming)
  • Tight folds have interlimb angles between 30 and 70 degrees (Naukluft Nappe Complex, Namibia)
  • Isoclinal folds have interlimb angles less than 30 degrees (Gneiss terranes)

Classification of fault types

• Dip-slip faults involve predominantly vertical movement
  • Normal faults occur when the hanging wall moves down relative to the footwall
    • Form in extensional stress regimes (Basin and Range Province, USA)
    • Associated with horsts and grabens (East African Rift System)
  • Reverse faults occur when the hanging wall moves up relative to the footwall
    • Form in compressional stress regimes (Himalayan Mountains)
    • Thrust faults are low-angle reverse faults with a dip less than 45 degrees (Glarus Thrust, Switzerland)
• Strike-slip faults involve predominantly horizontal movement
  • Right-lateral (dextral) faults occur when the block on the opposite side of the fault moves to the right (San Andreas Fault, California)
  • Left-lateral (sinistral) faults occur when the block on the opposite side of the fault moves to the left (Dead Sea Transform, Middle East)
• Oblique-slip faults combine dip-slip and strike-slip movement (Denali Fault, Alaska)

Joints and fractures in rocks

• Joints are fractures in rocks along which no appreciable movement has occurred
  • Extensional joints form perpendicular to the minimum principal stress direction (Columnar jointing in basalts)
  • Shear joints form in conjugate pairs at an angle to the maximum principal stress direction (Conjugate joint sets in granites)
• Fractures are any breaks or discontinuities in a rock
  • Caused by mechanical stress, cooling, or unloading (Sheeted dike complexes)
• Joint sets are groups of parallel or subparallel joints with similar orientations (Orthogonal joint sets)
• Joint systems consist of two or more intersecting joint sets (Polygonal joint patterns)
• Columnar jointing forms polygonal fracture patterns during cooling of igneous intrusions or lava flows (Devil's Tower, Wyoming)

Stress-strain in rock deformation

• Stress is the force per unit area acting on a rock
  • Compressional stress leads to shortening and formation of folds and reverse faults (Folded sedimentary layers)
  • Tensional stress leads to extension and formation of normal faults and extensional fractures (Rift valleys)
  • Shear stress leads to lateral displacement and formation of strike-slip faults and shear fractures (Transform plate boundaries)
• Strain is the deformation of a rock in response to stress
  • Elastic strain is reversible deformation; rock returns to its original shape when stress is removed
  • Plastic strain is irreversible deformation; rock permanently changes shape when stress exceeds its yield strength
  • Brittle deformation occurs when rock fractures as stress exceeds its breaking strength
• Factors influencing rock deformation include:
  1. Composition - mineralogy and texture of the rock (Quartz-rich rocks are more resistant to deformation than clay-rich rocks)
  2. Temperature and pressure - higher temperatures and confining pressures promote ductile deformation (Metamorphic rocks)
  3. Strain rate - rapid strain rates favor brittle deformation, while slow strain rates favor ductile deformation (Fault gouges vs. mylonites)
• Deformation mechanisms involve:
  1. Elastic deformation - stretching of atomic bonds without permanent displacement
  2. Plastic deformation - movement of atoms or molecules within a crystal lattice
     • Dislocation creep involves the movement of linear defects (dislocations) through the crystal lattice (Quartz ribbons in mylonites)
     • Diffusion creep involves atom movement from areas of high stress to areas of low stress (Pressure solution in limestones)
  3. Brittle deformation - formation of fractures and faults (Breccias and cataclasites)