Deformation of rocks
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Deformation of Rocks. How Rocks Deform Brittle-Ductile Behavior Faulting and Folding. 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.

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Deformation of Rocks

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Deformation of Rocks

How Rocks Deform

Brittle-Ductile Behavior

Faulting and Folding


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


Rock

Stress

Rubber band

Strain 

Relationship between stress and strain

Elastic behavior

Fracture, breaks

X

Ductile behavior

Permanent strain


Stress

Strain 

Relationship between stress and strain

Brittle behavior:

Very little ductile deformation before fracturing

X

X

Fracture

Ductile behavior:

Extensive ductile deformation before fracturing


Ductile

Brittle


Ductile Behavior

Folding of Rocks

Brittle Behavior

Faulting 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 stress

    Elastic limit

    Effects of Temperature and Strain Rate


    Brittle-DuctileTransition

    Limits the depths of

    earthquakes

    surface

    Brittle

    Low Temperature

    Low Pressure

    15-20 km

    Higher Temperature

    Higher Pressure

    Ductile

    Crust

    Mantle


    T=1300 C

    Yield

    strength=0

    Stress

    Strain

    Lithosphere-Asthenosphere

    schematic

    strength

    profile through

    continental

    lithosphere


    Deformation in Progress


    Abrupt Movement along Faults


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


    LA

    SA

    uplift

    subsidence

    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.


    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 relative 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 fault

    Direction of relative movement


    Normal Fault (dip-slip)


    Normal Faulting

    Foot wall

    Hanging wall


    Tetons – fault range scale


    Basin and Range

    Death Valley, CA

    Normal Faulting

    Horst-Graben Structures


    Reverse Fault (dip slip)

    > 45° dip


    Reverse Faults


    Thrust Fault (dip-slip)

    < 45° dip


    Thrust Fault

    Older rocks

    Younger rocks


    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-lateral

    Strike-slip

    Stress: shear


    anticline

    syncline

    Typesof Folds

    During mountain building or compressional stress, rocks undergo ductile deformation to produce folds


    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


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