How Rocks DeformBrittle-Ductile BehaviorFaulting and Folding

Deformation of Rocks

Stress and Strain

• The keys to understanding any deformation are stress (the cause) and strain (the effect)

Compression

• Rocks are squeezed or compressed by forces directed toward one another.

• Rocks are shortened by folding or faulting

Plate Boundary: Convergence Zones

Tension

• Rocks are lengthened or pulled apart by forces acting in opposite directions

• Rocks are stretched and thinned

Plate Boundary: Divergence Zones

Shear

• Forces act parallel to one another but in opposite directions

• Results in displacement of adjacent layers along closely spaced planes

Plate Boundary: Transform Faults

Relationship between stress and strain

Strain

Stress

Elastic behavior

Ductile behavior

X

Fracture, breaks

Permanent strain

Rock

Rubber band

Relationship between stress and strain

Strain

Stress

Brittle behavior:Very little ductile deformation before fracturing

X

Fracture

X

Ductile behavior:Extensive ductile deformation before fracturing

Ductile Brittle

Ductile BehaviorFolding of Rocks

Brittle BehaviorFaulting of Rocks

What controls brittle vs. ductile?

– Rate of deformation (fast = brittle)– Rock strength (strong = brittle)– Temperature (cold = brittle)– Confining pressure (shallow = brittle)

• Just remember deeper = ductile– Near surface= rocks are brittle– At depth= rocks are ductile

What controls brittle vs. ductile?

Rate of deformation (strain rate)

Low strain rates Ductile (Mantle Convection)

High strain rates Brittle (Earthquake waves)

Yield stressElastic limit

Effects of Temperature and Strain Rate

Brittle-Ductile Transition

Mantle

Crust

surface

Low TemperatureLow Pressure 15-20 km

Brittle

DuctileHigher TemperatureHigher Pressure

Limits the depths of earthquakes

schematic strength profile

through continental lithosphere

StrainS

tress Yield

strength=0

T=1300 C

Lithosphere-Asthenosphere

Deformation in Progress

Abrupt Movement along Faults

Uplifted sea floor at Cape Cleare, Montague Island, Prince William Sound. Uplift about 33 ft

Gradual Movement: Perspective view of the Los Angeles region with superimposed InSAR( Interferometric Synthetic Aperture Radar) measurements of ground motions between May and September 1999. Large regions of metropolitan Los Angeles are rising and falling by up to 11 cm annually, and a large portion of the city of Santa Ana is sinking at a rate of 12 mm per year.

upliftsubsidence

LA

SA

Past Deformation: Folding

Large scale and small scale folds

Folding: large and small scale

Past Deformation: Faulting

Large scale and small scale

Strike and Dip

Measuring Deformation in the Rocks Strike & Dip

Faults

• Fractures along which there is relativerelative motion parallel to the fracture

• The fracture is called the fault plane– Vertical motion (dip-slip) – horizontal (strike-slip). – Most faults have a combination of

both types of motion (oblique).

Types of Faults

Classified according to:

Dip of faultDirection of relative movement

Normal Fault (dip-slip)

Normal Faulting

Hanging wall

Foot wall

Tetons – fault range scale

Basin and Range

Normal FaultingHorst-Graben Structures

Death Valley, CA

Reverse Fault (dip slip)

> 45° dip

Reverse Faults

Thrust Fault (dip-slip)

< 45° dip

Older rocks

Younger rocks

Thrust Fault

Thrust Faults. Snake Range, Wy

Strike-Slip Fault (horizontal motion, no vertical motion)

Strike-Slip Fault

San Andreas Fault

• Transform plate boundary (Pac / N.A.)

• System of right lateral faults

Offset Streams (San Andreas Fault)

A pair of streams that has been offset by right-lateral slip on the San Andreas fault (lineament extending from left to right edge of photograph). View northeastward across fault toward the Temblor Range. Photograph by Sandra Schultz Burford, U.S. Geological Survey.

Strike-slip fault

Off-set stream

Right-lateralStrike-slipStress: shear

Types of FoldsDuring mountain building or compressional

stress, rocks undergo ductile deformation to produce folds

anticline

syncline

Types of Folds

Anticline: Warped upwards. Limbs dip outward. When eroded, oldest rocks crop out in the center (assuming everything is right-side-up).

Syncline: Warped downwards. Limbs dip inward. When eroded, youngest rocks crop out in the center (assuming everything is right-side-up).

Basins and Domes resemble anticlines & synclines vertical motions instead of lateral motions

Stress, Strain & Plate Tectonics

• Plate collisions (convergent margins)– Compressive strsses– Folds & reverse faults

Stress, Strain & Plate Tectonics• Divergent plate boundaries

– Tensional stresses– Normal faults

Stress, Strain & Plate Tectonics

• Transform plate boundaries– Shear stress– Transform faults