Geology: Fault Analysis

Fault Analysis

• Fault types:
  • Normal faults: hanging wall moves down relative to footwall
  • Reverse faults: hanging wall moves up relative to footwall
  • Strike-slip faults: horizontal movement, no vertical displacement
  • Oblique-slip faults: combination of normal and strike-slip movement
• Fault orientation:
  • Strike: direction of fault plane intersection with horizontal surface
  • Dip: angle between fault plane and horizontal surface
  • Rake: angle between fault plane and horizontal surface in the direction of slip
• Fault kinematics:
  • Slip: amount of displacement along the fault plane
  • Slip rate: rate of displacement along the fault plane
  • Fault growth: increase in fault length or displacement over time

Fold Geometry

• Fold types:
  • Anticline: upward-convex fold, oldest rocks in center
  • Syncline: downward-convex fold, youngest rocks in center
  • Monocline: fold with a single limb, no axial plane
• Fold geometry:
  • Hinge: point of maximum curvature
  • Axis: imaginary line passing through the hinge
  • Limb: sides of the fold
  • Interlimb angle: angle between the two limbs
• Fold classification:
  • Symmetrical folds: equal limbs, axial plane bisects the fold
  • Asymmetrical folds: unequal limbs, axial plane does not bisect the fold
  • Recumbent folds: fold limbs parallel to the axial plane

Rock Deformation

• Types of deformation:
  • Elastic deformation: reversible, returns to original shape
  • Plastic deformation: irreversible, permanent change in shape
  • Brittle deformation: fracture and faulting
  • Ductile deformation: folding and flow
• Deformation mechanisms:
  • Dislocation creep: movement of defects in crystal lattice
  • Diffusion creep: movement of atoms through the crystal lattice
  • Grain boundary sliding: movement of grains past each other
• Deformation regimes:
  • Brittle regime: high stress, low temperature, and low pressure
  • Ductile regime: low stress, high temperature, and high pressure
  • Transitional regime: intermediate conditions, both brittle and ductile deformation occur

Fault Analysis

• Normal faults: hanging wall moves down relative to footwall
• Reverse faults: hanging wall moves up relative to footwall
• Strike-slip faults: horizontal movement, no vertical displacement
• Oblique-slip faults: combination of normal and strike-slip movement
• Fault orientation is described by strike, dip, and rake
• Strike: direction of fault plane intersection with horizontal surface
• Dip: angle between fault plane and horizontal surface
• Rake: angle between fault plane and horizontal surface in the direction of slip
• Fault kinematics involve slip, slip rate, and fault growth
• Slip: amount of displacement along the fault plane
• Slip rate: rate of displacement along the fault plane
• Fault growth: increase in fault length or displacement over time

Fold Geometry

• Anticline: upward-convex fold, oldest rocks in center
• Syncline: downward-convex fold, youngest rocks in center
• Monocline: fold with a single limb, no axial plane
• Fold geometry involves hinge, axis, limb, and interlimb angle
• Hinge: point of maximum curvature
• Axis: imaginary line passing through the hinge
• Limb: sides of the fold
• Interlimb angle: angle between the two limbs
• Folds can be classified as symmetrical, asymmetrical, or recumbent
• Symmetrical folds: equal limbs, axial plane bisects the fold
• Asymmetrical folds: unequal limbs, axial plane does not bisect the fold
• Recumbent folds: fold limbs parallel to the axial plane

Rock Deformation

• Elastic deformation: reversible, returns to original shape
• Plastic deformation: irreversible, permanent change in shape
• Brittle deformation: fracture and faulting
• Ductile deformation: folding and flow
• Deformation mechanisms include dislocation creep, diffusion creep, and grain boundary sliding
• Dislocation creep: movement of defects in crystal lattice
• Diffusion creep: movement of atoms through the crystal lattice
• Grain boundary sliding: movement of grains past each other
• Deformation regimes involve brittle, ductile, and transitional regimes
• Brittle regime: high stress, low temperature, and low pressure
• Ductile regime: low stress, high temperature, and high pressure
• Transitional regime: intermediate conditions, both brittle and ductile deformation occur

Definition and Types of Folds

• A fold is a bent or curved layer of rock formed due to tectonic forces
• There are two main types of folds: anticline and syncline
• Anticline: a fold that arches upwards, with the oldest rocks in the center
• Syncline: a fold that dips downwards, with the youngest rocks in the center

Fold Axis and Plane

• Fold axis: an imaginary line that runs along the center of the fold
• Fold plane: an imaginary surface that passes through the fold axis, perpendicular to the bedding planes

Fold Geometry Terminology

• Hinge: the point of maximum curvature on a fold
• Limb: the side of a fold that connects the hinge to the inflection point
• Inflection point: the point on a fold where the curvature changes direction
• Interlimb angle: the angle between the two limbs of a fold

Fold Classification

• Folds can be classified based on their style, orientation, and shape
• Examples of fold styles: tight, open, isoclinal, or recumbent
• Examples of fold orientations: upright, inclined, or recumbent
• Examples of fold shapes: symmetrical, asymmetrical, or irregular

Fold Analysis

• Fold profiling: the process of creating a 2D representation of a fold to analyze its geometry
• Stereo plotting: a method used to visualize and analyze the 3D geometry of a fold

Importance of Fold Geometry

• Fold geometry is crucial in understanding the tectonic evolution of a region
• It provides information on the orientation and magnitude of tectonic forces
• It helps in the exploration of natural resources, such as oil and gas, and mineral deposits