Deformation

• Deformation is a general term that refers to all changes in the original form and/or size of a rock body
  
• Stress – The Cause of Deformation
  
  • Geologic forces at work: Plate tectonics
    
  • Stress = force/area
    
  • Types of stress:
    
    • Compressional
      
    • Tensional
      
    • Shear
      
• Components of deformation:
  
  • Relocation
    
  • Tilting
    
  • Changing shape
    
  • Faulting
    
  • Folding
    
• Strain
  
  • Change in shape due to deformation
    
  • Types
    
    • Stretching
      
    • Shortening
      
    • Shear
      
• Most crustal deformation occurs along plate margins
  
• Types of deformation
  
  • Brittle
    
  • Ductile
    
• Factors that influence the strength of a rock
  
  • Temperature
    
  • Pressure
    
  • Rock type (composition)
    
  • Rate of deformation
    
  • Time
    

Folds

• Folds = Rocks bent into a series of waves
  
• Most folds result from compressional forces which shorten and thicken the crust
  
• Types of folds
  
  • Anticline – upfolded, or arched, rock layers
    
  • Syncline – downfolded rock layers
    
  • Anticlines and synclines can be
    
    • Symmetrical - limbs are mirror images
      
    • Asymmetrical - limbs are not mirror images
      
    • Overturned - one limb is tilted beyond the vertical
      
    • Where folds die out they are said to be plunging
      
  • Plunging folds
    
  • Other types of folds
    
    • Dome
      
      • Circular, or slightly elongated
        
      • Upwarped displacement of rocks
        
      • Oldest rocks in core
        
    • Basin
      
      • Circular, or slightly elongated
        
      • Downwarped displacement of rocks
        
      • Youngest rocks in core
        
    • The Black Hills of South Dakota are a large dome
      
    • The bedrock geology of the Michigan Basin
      

Faults

• Faults
  
• Faults are fractures (breaks) in rocks along which appreciable displacement has taken place
  
• Types of faults
  
  • Dip-slip fault
    
    • Movement along the inclination (dip) of fault plane
      
    • Parts of a dip-slip fault
      
      • Hanging wall – the rock above the fault surface
        
      • Footwall – the rock below the fault surface
        
    • Concept of hanging wall and footwall along a fault
    • Types of dip-slip faults
      
      • Normal fault
        
        • Hanging wall block moves down
          
        • Associated with fault-block mountains
          
        • Prevalent at spreading centers
          
        • Caused by tensional forces
          
        • Fault block mountains produced by normal faulting
          
      • Reverse and thrust faults
        
        • Hanging wall block moves up
          
        • Caused by strong compressional stresses
          
        • Reverse fault - dips greater than 45º
          
        • Thrust fault - dips less than 45º
          
• Strike-slip faults
  
  • Dominant displacement is horizontal and parallel to the trend, or strike
    
  • Transform fault
    
  • Large strike-slip fault that cuts through the lithosphere
    
  • Often associated with plate boundaries
    
• Joints
  
  • Fractures along which no appreciable displacement has occurred
    
  • Most are formed when rocks in the outer-most crust are deformed
    
    

Mountain belts

• Orogenesis refers to processes that collectively produce a mountain belt
  
• Three mechanisms for mountain building
  
  • Ocean-Ocean and Ocean-Continental Convergence
    
  • Continental-Continental Convergence
    
  • Continental Accretion
    
• Ocean-Ocean and Ocean-Continental Convergence
  
  • Aleutian-type mountain building
    
    • Where two oceanic plates converge and one is subducted beneath the other
      
    • Volcanic island arcs forms
      
    • Found in shrinking ocean basins, such as the Pacific
      
    • e.g. Mariana, Tonga, Aleutian, and Japan arcs Formation of a volcanic island arc
      
  • Andean-type mountain building
    
    • Oceanic-continental crust convergence
      
    • e.g. Andes Mountains
    • Types related to the overriding plate
      
      • Passive margins
        
        • Prior to the formation of a subduction zone
          
        • e.g. East Coast of North America
          
      • Active continental margins
        
        • Subduction zone forms
          
        • Deformation process begins
          
        • Continental volcanic arc forms
          
        • Accretionary wedge forms
          
        • Examples of inactive Andean-type orogenic belts include Sierra Nevada Range and California's Coast Ranges
          
        • Orogenesis along an Andean-type subduction zone
          
• Continental collisions
  
  • Where two plates with continental crust converge
    
  • e.g., India and Eurasian plate collision
    
  • Himalayan Mountains and the Tibetan Plateau
    
  • Plate relationships prior to the collision of India with Eurasia
    
  • Position of India in relation to Eurasia at various times
    
  • Formation of the Himalayas
    
• Continental accretion
  
  • Third mechanism of mountain building
    
  • Small crustal fragments collide with and accrete to continental margins
    
  • Accreted crustal blocks are called terranes
    
  • Occurred along the Pacific Coast
    
  • Distribution of modern day oceanic plateaus and other submerged crustal fragments
    
  • Accreted terranes along the western margin of North America
    
• Buoyancy and the principle of isostasy
  
  • Evidence for crustal uplift includes wave-cut platforms high above sea level
    
  • Reasons for crustal uplift
    • Not so easy to determine
      
  • Isostasy
    
    • Concept of a floating crust in gravitational balance
      
      • When weight is removed from the crust, crustal uplifting occurs (isostatic adjustment)
        
    • The principle of isostasyErosion and resulting isostatic adjustment of the crust
      

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