L10-mountain-deformation_2020

Chapter 11: Mountains and Deformation

1. Learning Objectives

  • Describe the two types of deformation and the conditions under which they occur.

  • Relate the different types of stress to the resulting type of faulting.

  • Identify fault motion and the corresponding fault type.

  • Understand faulting types associated with convergent and divergent boundaries.

2. Deformation

2.1 Definition

  • Deformation is the alteration of rocks due to applied forces, leading to bending, breaking, shortening, stretching, and shearing.

2.2 Changes in Rock Characteristics

  • Deformation modifies the physical appearance and structure of rocks, making changes observable.

  • Types of Rock Changes:

    • Displacement: Movement of rock blocks.

    • Rotation: Changing the orientation of rock units.

    • Distortion: Alteration of shape without change in volume.

3. Styles of Deformation

3.1 Ductile Deformation

  • Occurs under high temperature and pressure, at slow deformation rates.

  • Rocks undergo permanent changes without breaking.

3.2 Brittle Deformation

  • Occurs under cold conditions, at high stress, and lower pressure.

  • Results in fractures or faults in rocks.

3.3 Factors Affecting Deformation Behavior

  • Temperature: Higher temperatures allow for ductility.

  • Pressure: Higher pressures decrease brittleness.

  • Deformation Rate: Faster deformation typically leads to brittle behavior.

  • Mineralogical Composition: Different minerals respond variously to stress (e.g., quartz vs. clay).

4. Types of Stress

4.1 Compression

  • Causes shortening and thickening of rock layers.

  • Results in folding and reverse faults.

4.2 Tension

  • Causes elongation and thinning of rocks.

  • Leads to normal faults and rift structures.

4.3 Shear

  • Causes lateral movement within rocks.

  • Creates strike-slip faults, displacing layers horizontally.

5. Fault Motion

5.1 Fault Classification

  • Faults are classified based on geometry:

    • Normal Fault: Occurs due to tension, hanging wall moves down.

    • Reverse Fault: Caused by compression, hanging wall moves up.

    • Thrust Fault: A type of reverse fault with a gentle slope.

    • Strike-Slip Fault: Horizontal movement parallel to fault line (left or right lateral).

6. Fault Recognition

6.1 Key Structures

  • Fault Trace: Surface expression of a fault.

  • Footwall: Block beneath the fault.

  • Hanging Wall: Block above the fault.

  • Drag Fold: Deformation resulting from motion along a fault.

7. Generating Folds

7.1 Fold Geometry

  • Axial Plane: Divides the two sides of a fold.

  • Hinge: The line along the fold where curvature is greatest.

  • Limb: The sides of the fold that taper away from the hinge.

8. Types of Folds

  • Anticlines: Upward-arching folds.

  • Synclines: Downward-troughing folds.

9. Orogeny

  • Refers to the processes that create mountains.

9.1 Types of Orogenic Processes

  • Convergent Boundaries: Ocean-continent and continent-continent collisions causing mountain formation.

  • Isostasy: Balance between buoyancy of the lithosphere and the denser mantle.

  • Continental Rifting: Involves thinning of the lithosphere and formation of rift valleys.

10. Case History: The Appalachians

  • Historical geologic events and processes that shaped the Appalachian Mountains, including rifting and orogeny throughout geological periods.

11. Summary

  • Types of Deformation: Brittle vs. Ductile Responses.

  • Stress Types: Compression, Tension, Shear, and Pressure.

  • Fault Types: Normal, Reverse, Thrust, Strike-slip.

  • Mountain Building Processes: Associated with convergent and divergent boundaries.