STRUCTURAL GEOLOGY

The Soul Would Have No Rainbow, If The Eyes Had No Tears.

Minquass

Structural GeologyDanny M. Vaughn, Ph.D.,

CMS

The study of how & why rocks fold, bend, & break.

Stress and Strain

• Stress – an applied force.

• Pressure – amount of force applied.

• Lithostatic • Directed

• Strain – a rock’s response to stress (deformation)

• Shape • Volume

Types of Strain

• Elastic deformation

• Brittle failure• Plastic

deformation

Temporary deformation (until the

elastic limit is surpassed)

Slow, continuous, low stressconditions.

• Permanent deformation that occurs when a rock breaks. • Occurs w/ low temperature,low, but sudden pressure changes.• Mineral bonds break along a zone of maximum stress.

Types of Strain

• Elastic deformation

• Brittle failure

• Plastic deformation

Types of Strain

• Elastic deformation

• Brittle failure• Plastic

deformation• Permanent deformation that occurs when a rock changes its shape without breaking.

• High temperature, high pressure conditions.

• Atoms move from maximum to lower stress areas.

Factors Affecting Rock Deformation

• Magnitude of stress

• Rate stress applied

• Temperature• Pressure• Composition

How much pressure & at what rate of speed the forceis applied.

What is magnitude?What does rate of stress mean?

Factors Affecting Rock Deformation

• Magnitude of stress

• Rate stress applied

• Temperature• Pressure• CompositionHigher temperatures &

pressure result in plasticdeformation.

Factors Affecting Rock Deformation

• Magnitude of stress

• Rate stress applied

• Temperature• Pressure• Composition• Time• Water weakens bonds creating

plastic behavior.• Mineralogy (bonds) can overridetemperature, pressure, & the rate of stress.• Pyroxene & garnet (strong).• Calcite & mica (weak).• Continuous low measures of stressover time result in plastic behavior.

Types of Stress

• Compression• Tension• Shear• Torque

Compass Directions

Bearing & Azimuth

Bearing – a quadrant system of angular measure.Azimuth – a continuous measure of angular measure.

Studying Deformed Rocks

Measuring Strike and Dip

Strike - The compass direction of the line that forms at the intersection of the plane and an imaginary horizontal planeDip - The angle at which a plane is inclined relative to the horizontal

Strike and Dip

Strike and Dip┤

Rule of Structure “V”

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Plastic Deformation - Folds

Photomicrograph of Folded Minerals

Types of Folds

• Anticlines

• Synclines

• Domes• Basins

Fold Symmetry

Fold Symmetry

Types of Folds

• Anticlines

• Synclines

• Domes• Basins

Asymmetric Folds

Recumbent Fold

Folding

Compressional forces (convergent plate boundaries)

Types of Folds

Landforms Following Erosion

When the angle of plunge of the fold is inclined along the axial plane, the fold is said to be plunging.

Valley & Ridge Physiographic ProvinceCentral Pennsylvania

The nose of the anticline is expressed as a long, tapering dip slope, while the nose of the syncline is typically blunt. Anticlines tend to be more cigar-shaped in body. The steeper escarpment faces to the outside of the structure in a syncline. The steeper escarpment faces the inside of the structure in an anticline. The oldest beds will be within the core (inside) of an anticline. The youngest beds will be within the core of a syncline.

Summary Statements

Valley & Ridge Physiographic ProvinceCentral Pennsylvania

Types of Folds

• Anticlines

• Synclines

• Domes• Basins

Types of Folds

• Anticlines

• Synclines

• Domes• Basins

Black Hills Dome

Brittle Deformation - Joints

Rock fractures with no evidence of movement

Brittle Deformation - Faults

Rock fractures with evidence of movement

Evidence of movement:• Displacement• Crushed rock along

fault plane• Slickensides

Faulting – fracturing w/ displacement due to one or more forms of stress.

Types of Faults

Strike-slip faults

Movement (slip) in the direction of

strike (horizontal)

Dip-slip faultsMovement (slip) in the direction of dip (vertical)

Types of Faults

Dip-slip faults

Normal Fault

Types of Faults

Dip-slip faults

• Normal fault

• Reverse fault

Hanging Wall

Foot Wall

Types of Faults

Horst & Graben

Types of Faults

Horst & Graben

Bells Canyon, Utah

Death Valley, California

Block Faulted Mountains

• Sierra-Nevada & Tetons.• Normal fault along base.• Tilted along a normal fault plane.• Intrusion of granitic magma.

Reverse Fault

Types of Faults

Dip-slip faults

• Normal fault

• Reverse fault

Hanging Wall

Foot Wall

Dip-Slip FaultsThrust Fault

Chief Mountain

Strike-Slip Fault

Types of Faults

Strike-slip faults

• Right-lateral

• Left-lateral

Types of Faults

Fault Planes

Plate Tectonics & Faulting

Plate Tectonics & Folding

Oceanic-Continental Collision

• Formed Rockies, Andes, Cascades,Sierra of Central America,

Plate Boundary Collisions

Continental-Continental Collision

• Results in folding, overthrusting, faulting, & uplifting w/ overturned folds (Nappes).

• Himalaya & Alps.

Oceanic-Oceanic Collision

• Results in volcanic island arcs –Indonesia, Japan, Philippines,Aleutians.

Orogenesis - Mountain Building  • Large scale crustal deformation.• Compressional forces, volcanism, capture of migrating terranes, intrusion of magma.

• Rocky Mountains (Laramide Orogeny, 40 million yr. ago). 

• Sierra Nevadas (Sierra-Nevadan Orogeny, 35 million yrs. ago), batholithic intrusions 130-160 million yrs. ago.

• Appalachians (Alleghany Orogeny, 250-300 millions yrs. ago w/ earlier episodes).

• Alps (Alpine Orogeny, 20-120 million yrs. ago w/ many earlier episodes).

Folds, Faults, & Fossil Fuels

What can you say about the general structure of the rocks in this photograph?

How did Mexican Hat form? Why does it still exist?

How do you explain the existence these isolated monuments?

Why are these rocks shaped and aligned as they appear in this photograph?

Syncline, ramp anticline, & thrust fault.Where are they?

What class of rocks are the light pink?Why are they not aligned?

What does this surface represent?

Striations, slickenside surface :A fault plane

What is structurally apparent about these rocks?

Strike/slip faults

What is structurally apparent about these rocks?

Reverse fault

Sketch this fold.