Geology 111G/Lecture 14

Structural Geology

Stress and Strain

Brittle Strain

Plastic Strain

Folds-Features of brittle strain

Features of Brittle Strain

Joints

Faults

I. Structural geology: The study of rock deformation. Features of rock deformation are collectively referred to as structure or structural features.

II. Stress and Strain.

A. Stress: Force applied to a body divided by the area over which force operates.

B. Strain: Effect of applying stress to a body, as manifested by a change in shape. Strain is proportional to stress; that is, a large stress results in more change in shape.

1. Elastic strain: Recoverable strain; when stress is removed, object regains original shape. Example is stretching a rubber band.

2. Plastic strain: Permanent strain; when stress is removed, object remains strained or deformed.

3. Brittle deformation: If stress exceeds the strength of a rock, the rock will break or fracture.

a. Joint: A rock fracture across which there has been no movement.

b. Fault: A rock fracture across which there has been movement or displacement.

C. Types of Stress.

1. Tension: A stretching stress. Rocks are comparatively weak in extension; therefore, they break and fault easily.

2. Compression: A squeezing stress. Rocks are comparatively strong in compression, which means they tend to bend before they break.

3. Shear: Stress operates in opposite directions across the body.

III. Folds. Bent layers of rock representing elastic or plastic strain as a result of compressive stress. Folds range widely in size, from microscopic structures to extremely large features the size of mountains.

A. Bedding attitude. In order to discuss folds, we need to understand how planes represented by rock layers are measured in space.

1. Dip: Maximum angle of inclination of a bedding plane or layer.

2. Strike: The direction of a horizontal line on the dipping surface.

B. Types of folds.

1. Monocline: Double flexure or bend connecting strata at one level with strata at another level.

2. Anticline: Arched strata or layers, with the limbs of the fold dipping away from each other.

a. Axial plane: Imaginary plane that bisects the fold.

b. Axis: Intersection of that plane with a layer of rock in the fold.

3. Syncline: Trough-like fold with strata or limbs dipping toward one another.

C. Fold form. Describes general shape of fold.

1. Symmetrical: Each half of fold is the same.

2. Asymmetrical: One limb is shorter.

3. Overturned: One limb is upside down.

4. Recumbent: Axial plane is horizontal; fold appears to be on its side.

5. Isoclinal: Limbs are parallel; thus, fold is very tight.

IV. Features of Brittle Deformation.

A. Joints. Fracture surfaces in rocks with no movement along surface; no displacement. Joints usually form sets that are essentially swarms of fractures parallel to one another. Intersection joint sets are called joint systems.

B. Faults. Fractures in rock where rocks on each side of the break have moved relative to one another. Faults are classified according to the apparent movement of one block relative to the other.

1. Dip-slip faults: Displacement is in the direction of dip of the fault plane. The blocks on either side of the fault are named for their position relative to the dipping fault plane. Footwall block: Block beneath the fault. Hanging-wall block: Block above the fault.

a. Normal fault: Hanging-wall block appears to have moved down.

b. Reverse fault: Hanging-wall block appears to have moved up. A thrust fault is a special case of a gently dipping to flat reverse fault.

2. Strike-slip faults: Displacement is parallel to the strike of the fault.

a. Left-lateral: Opposite block appears to have moved left as one looks across the fault.

b. Right-lateral: Opposite block appears to have moved right as one looks across the fault.

3. Grabens and Horsts. A graben is a structural trough or down-thrown block between two normal faults. A horst is an upthrown block between normal faults.

4. Transform faults. Strike-slip faults, often with large horizontal displacements, which terminate segments of mid-ocean spreading ridges. The motion of blocks across the faults is opposite to the apparent displacement of the ridge segments that it connects because those ridge segments produce oceanic lithosphere.