Rock Deformation: Folds and Faults

• Elastic deformation allows a rock to return to its original shape after stress is removed.
• Plastic deformation permanently changes a rock's shape.
• Fracture occurs when a rock breaks under stress.

Folds

• Folds are created when compressive stresses cause rocks to crumple.
• A monocline is a simple bend in rock layers.
• Anticlines are upward-arching folds where rock layers dip away from the center.
• A dome is an upward, circular structure formed by folding.
• Synclines are downward-bending folds.
• A basin is a downward, circular structure formed by bending.
• Fault-bend folds form where the dip of a fault changes.
• Detachment folds form where the amount of slip along a fault changes.
• Fault propagation folds form where the dip and slip of a fault change.

Faults

• Faults form when rocks fracture under sufficient stress.
• A joint is a fracture with no significant movement on either side.
• Slip is the distance rocks move along a fault.
• In a normal fault, the hanging wall moves down relative to the footwall.
• In a reverse fault, the footwall moves down relative to the hanging wall.
• A strike-slip fault is a dip-slip fault with a vertical fault plane dip.
• Converging continental plates create mountain ranges through upward smashing.
• Tensional stress pulls the crust apart, causing it to break into blocks that slide up or down along normal faults.
• Gravity regimes are dominated by normal faults.
• Thrust regimes are dominated by thrust faults.
• Wrench regimes are dominated by strike-slip faults.
• A horst is a block uplifted between two normal faults that dip away from each other.
• A graben is a block down-dropped between two normal faults that dip towards each other.
• A half-graben has a fault on only one side of the down-dropped block.
• Synthetic faults dip in the same direction as the main fault.
• Antithetic faults dip in the opposite direction of the main fault.
• A klippe is an isolated remnant of a hanging wall completely surrounded by the footwall due to erosion.
• A fenster (or window) is an area of exposed footwall surrounded by the hanging wall.
• A décollement is a flat, extensive surface with a large amount of movement.
• Allochthonous rocks formed in a different environment than the underlying footwall.
• Autochthonous refers to the footwall beneath a thrust fault that has not been transported.
• An imbricate fan is a series of thrust faults branching upward from a single flat fault surface.
• A duplex forms when faults merge into flats both upward and downward.
• A tectonic wedge is formed by opposing thrust faults that meet in the subsurface.
• A triangle zone is a triangular section cut off by a third thrust fault.
• Transtension combines strike-slip motion with extension, leading to crustal thinning.
• Transpression combines strike-slip motion with shortening, leading to crustal thickening.