Rock Deformation: Folds, Faults, and Crustal Structures

Geological Deformation of Earth's Crust

The Earth's crust undergoes various types of deformation due to tectonic forces. These deformations can be categorized based on how the material responds to stress:

• Plastic Deformation: Undergoes permanent change, resulting in Folds.
• Elastic Deformation: Undergoes temporary change, returning to its original shape once stress is removed. No permanent deformation occurs.
• Brittle Deformation: Undergoes fracture, resulting in Faults and Joints.

Geological Folds: Structure and Classification

Folds are bends in rock strata or other planar structures, typically resulting from compressional forces.

Parts of a Fold

Flank:

Each of the areas that form the fold, extending outwards from the hinge.

Hinge:

The line joining the two flanks, representing the zone of maximum curvature of the bend.

Axial Plane or Surface:

An imaginary plane formed by the union of all strata hinges within the fold.

Fold Axis:

An imaginary line formed by the intersection of the axial plane with a horizontal plane.

Limb:

The area where the fold's curvature decreases and the strata become relatively straight.

Types of Folds

1. Classification by Arrangement of Layers:

• Anticline: The oldest materials are located in the core of the fold, with layers dipping away from the hinge.
• Syncline: The youngest materials are located in the core or center of the fold, with layers dipping towards the hinge.
• Monocline: A simple bend in otherwise horizontal or uniformly dipping rock layers, having only one limb.

2. Classification by Symmetry:

• Symmetrical: The angle between the two limbs and the horizontal plane is approximately the same.
• Asymmetrical: The two limbs have markedly different slopes or angles relative to the horizontal.

3. Classification by Axial Plane Orientation:

• Upright (Recto): The axial plane is vertical.
• Inclined (Bent): The axial plane is at an angle to the vertical.
• Recumbent (Lying): The axial plane is almost horizontal.

Geological Faults: Structure and Classification

Faults are planar fractures in rock bodies where there has been significant displacement as a result of rock-mass movement.

Parts of a Fault

Fault Blocks (or Lips):

Each of the rock masses separated by the fault plane.

Downthrown Block:

The fault block that is in a relatively lower position compared to the other.

Upthrown Block:

The fault block that remains in a relatively higher position compared to the downthrown block.

Fault Plane:

The planar surface of rupture along which the displacement has occurred.

Displacement (or Throw):

The magnitude of movement or offset along the fault plane.

Horizontal Displacement:

The removal of one block over another measured horizontally, perpendicular to the lateral displacement.

Vertical Displacement (Throw):

The vertical distance separating the two fault blocks.

Net Slip:

The total displacement, which is the vector sum of the horizontal and vertical components. Often, striations (called slickensides) can be seen on a fault plane, indicating the direction of the net slip.

Strike-Slip Displacement (or Lateral Slip):

The displacement along the fault plane measured horizontally, parallel to the strike of the fault.

Types of Faults

• Normal Fault: The hanging wall (downthrown block) moves down relative to the footwall (upthrown block). Its origin is from tensional (distensive) forces, leading to an increase in area.
• Reverse Fault: The hanging wall (upthrown block) moves up relative to the footwall (downthrown block). They originate from compressional forces, leading to a decrease in surface area.
• Vertical Fault: A fault with a near-vertical fault plane and primarily vertical displacement. These are actually very rare in pure form.
• Strike-Slip Fault: Characterized by horizontal movement along the fault plane, with little to no vertical displacement.
• Rotational (Scissor) Fault: The movement is produced by a rotation about an axis. The magnitude of displacement varies along the fault plane.