Structural Geology:
Deformation of Rocks

Geology 200

Geology for Environmental Scientists

Major Concepts

• Folds in rocks range from microscopic to hundreds of kilometers long.
• Faults are fractures along which displacement has occurred.
• Joints are fractures where there has been no displacement.
• Rocks deform when applied stress exceeds rock strength. Deformation may be ductile flow or brittle fracture.

Folds

• anticlines and synclines - plunge out eventually, don’t go on forever
• domes and basins - very broad features within continental interiors
• complex folds - the result of very ductile behavior

Faults: 3 major categories

• normal faults - caused by extension or tension
• thrust faults and reverse faults - caused by compression
• strike-slip or tear faults - caused by lateral shear

Strike and Dip

• Describes the orientations of bedding planes, faults, or joints.
• Strike - compass bearing of a horizontal line on a plane
• Dip - the inclination of a plane from the horizontal. Dip direction is always at a right angle to the strike.
• See Fig. 7.5 in your book.

Fold Nomenclature

• anticlines - rocks older in the center
• synclines - rocks younger in the center
• limbs
• hinge and hinge plane or axial plane
• plunge

Domes and Basins

• Very large features with gentle dips, usually only visible on geologic maps (too large to recognize on the ground).
• Restricted to continental interiors.
• Examples include the Michigan Basin, Illinois Basin, Cincinnati Arch, Nashville Dome, Black Hills, etc.

Complex Folds

• Formed by intense deformation in mountain ranges.
• Usually the result of multiple episodes of folding.
• The folds themselves may be folded and they are often recumbent.
• Can be at any scale from outcrop to map scale.

Types of Thrust Faults

• Fault bend folds - fault forms before the fold; deformation restricted to the hanging wall
• Fault propagation folds - fault forms along with the folding; deformation in both the hanging wall and foot wall

Joints

• Fractures in rocks with no appreciable displacement. They are very common in all rocks.
• Such joints are often zones of weakness where later faults could develop.
• Fluids can travel along joints: water, petroleum, pollution
• valuable metal ores may form along joints from hydrothermal solutions.

Causes of Rock Deformation

• Stress - pressure placed on rocks
• Strain - deformation of the rock
• Strength - rock resistance to deformation
• Brittle deformation - the rocks break or fracture. Occurs at low temperatures and low pressures.
• Ductile deformation - the rocks bend or flow. Occurs at higher temperature and pressures.

Strain indicators

• fault offset
• tension gashes
• fractures
• ductile flow