Geological Structures: Folds, Faults, and Deformation

Geological Deformation: Types and Structures

Types of Material Deformation

• Elastic Deformation: Material changes shape when subjected to stress but recovers its original shape when stress ceases.
• Plastic Deformation: Material deforms when subjected to stress and does not regain its original shape when the stress stops.
• Fracture: The effort causes the material to lose its internal cohesion and break.

Strike and Dip

• Strike: The angle measured in a horizontal plane between true north and the strike line of a geological feature (e.g., a bedding plane or fault plane).
• Dip: The angle between a geological surface (e.g., a bedding plane or fault plane) and a horizontal plane. It is measured perpendicular to the strike and indicates the direction a ball would roll if placed on the stratum.

Elements of a Fold

• Axial Plane: A plane that divides the fold into two halves as symmetrically as possible.
• Hinge: The area of maximum curvature within a fold.
• Fold Axis or Hinge Line: The intersection of the axial plane with the hinge.
• Limbs: The sides of the fold located on both sides of the hinge.
• Core: The innermost zone of a fold.

Types of Folds

According to the Relative Age of Materials

• Anticline: A fold where the oldest materials are found in the core.
• Syncline: A fold where the youngest materials are found in the core.

According to the Position of its Axial Plane

• Upright (Symmetrical) Fold: A fold whose axial plane dips 90°.
• Inclined Fold: A fold whose axial plane dips between 10° and 85°.
• Recumbent Fold: A fold whose axial plane dips below 10°.
• Overturned Fold: A fold whose axial plane has rotated beyond 90° from the vertical position, causing one limb to be inverted.

According to its Symmetry

• Symmetric Fold: A fold where the axial plane divides the fold into two symmetrical halves.
• Asymmetric Fold: A fold where the axial plane divides the fold into two clearly non-symmetrical halves.

Faults

Elements of a Fault

• Fault Plane: The fracture surface along which displacement of one block relative to another has occurred.
• Fault Blocks (or Lips): The two blocks of rock that have been separated by the fracture.
• Fault Displacement (or Throw/Heave): A measure of the relative displacement produced along the fault.
• Fault Orientation: The strike and dip of the fault plane.

Types of Faults

• Normal Fault: A fault where the hanging wall (the block above the fault plane) moves down relative to the footwall (the block below the fault plane). The fault plane typically dips towards the down-dropped block.
• Reverse Fault: A fault where the hanging wall moves up relative to the footwall. The fault plane typically dips towards the up-thrown block.
• Strike-Slip Fault (or Tear Fault): A fault where the relative displacement of the blocks is primarily horizontal, parallel to the strike of the fault plane.

Geological Processes: Internal and External

• Internal Geological Processes: Processes originating from thermal energy within the Earth, often aided by gravity (e.g., plate tectonics, volcanism, earthquakes).
• External Geological Processes: Processes generated by solar energy and aided by gravity (e.g., weathering, erosion, deposition).