Diagrammatically explain the formation of normal fault, strike-slip fault and thrust fault with the help of stress ellipsoid. IAS 2024, 15 Marks

Introduction:

Faults are fractures in the Earth's crust where movement has occurred. There are three main types of faults: normal faults, strike-slip faults, and thrust faults. These faults are formed due to different types of stress acting on the rocks in the Earth's crust.

Formation of Faults: Normal Fault, Strike-Slip Fault, and Thrust Fault with Stress Ellipsoid

The formation of different types of faults can be explained using stress ellipsoids. These diagrams represent the principal stresses (σ1, σ2, σ3) acting on a rock body, where:

• σ1: Maximum principal stress.
• σ2: Intermediate principal stress.
• σ3: Minimum principal stress.

1. Normal Fault

• Formation: Occurs under tensional stress where rocks are pulled apart.
• Stress Ellipsoid:
  
  • σ1 is vertical, causing the crust to stretch.
  • σ3 (minimum stress) acts horizontally, perpendicular to the fault plane.
• Diagram Explanation:
  
  • Hanging wall moves downward relative to the footwall.
  • Fault plane dips steeply, typically between 50° and 70°.

2. Strike-Slip Fault

• Formation: Results from horizontal shear stress where two blocks move laterally past each other.
• Stress Ellipsoid:
  
  • σ1 and σ3 are horizontal.
  • σ2 is vertical, allowing for lateral movement.
• Diagram Explanation:
  
  • No vertical displacement, movement is horizontal.
  • Commonly categorized as:
    
    • Right-lateral (dextral): Block on the opposite side moves to the right.
    • Left-lateral (sinistral): Block on the opposite side moves to the left.

3. Thrust Fault

• Formation: Forms under compressional stress when the crust is pushed together.
• Stress Ellipsoid:
  
  • σ1 is horizontal, indicating strong compressive forces.
  • σ3 is vertical, leading to shortening and thickening of the crust.
• Diagram Explanation:
  
  • Hanging wall moves upward relative to the footwall.
  • Fault plane dips shallowly, typically less than 30°.

General Characteristics:

• Stress Orientation:
  
  • Normal fault: Tension dominates; σ3 horizontal.
  • Strike-slip fault: Shear dominates; σ2 vertical.
  • Thrust fault: Compression dominates; σ3 vertical.
• Geological Context:
  
  • Normal faults: Extensional regimes like rift zones.
  • Strike-slip faults: Transform plate boundaries (e.g., San Andreas Fault).
  • Thrust faults: Convergent boundaries and orogenic belts.

Diagrams

Conclusion:

Normal faults are formed by tensional stress, strike-slip faults are formed by shear stress, and thrust faults are formed by compressional stress. The stress ellipsoid can help us understand how these different types of faults are formed and how the rocks move along the fault planes.