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    geologic structures

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    geologic structuresup until now, we have focused our attention mostly on flat-lying rocks, i.e. sedimentary or volcanic layers

    not all layers on Earth are flat-lying: geologic structures

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    geologic structuresdynamically produced patterns or arrangements of rock or sediment that result from forces acting within the Earth

    structuresyield information about these forces

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    geologic structuresproduced as rocks change shape or orientation from

    applied stress-- force / area

    structural geologyis study of shapes, arrangements and

    relationships among rocks and stresses that deform them

    rocks, like the fence below, deformin response to stress

    can bend or break

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    geologic structuresbending or breaking of rocks yield different structures

    bending: folds breaking: faults

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    stressis force/area--hitting with a hammer

    geologic structures -- key concepts

    importance of area:think of difference

    betweenstanding on water bedin high heels or sneakers

    3 types of stress:1) compression:pushed together2) tension:pulled apart3) shear stress:moved horizontally past each other

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    strainis change in shape or volume arising from stress

    geologic structures -- key concepts

    rock flattens afterbeing hit with hammer

    stressand resulting strain1)compression:shortening2) tension:stretching

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    3) shearing stress:shear strain

    strainis change in shape or volume arising from stress

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    translation(no change in shape)

    rotation(no change in shape)

    distortion(shape changes)

    time 1 time 2

    volume change(dilation, contraction)

    4 responses possible to stress

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    from: Davis and Reynolds, 1996

    dilation translation

    rotation distortion

    another way to look at it:

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    geologic structuresgeologic structures reflect the type of stress appliedand its rate of application in addition to the phyiscal

    properties of the rocks or sediments

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    how rocks respond to stressrocks behave as elastic, brittle, or ductilebodies depending on

    amount and rate of stress application

    type of rock

    temperature & pressure

    elastic: rock returns

    to orginal shape whenstress is removed

    (think rubber band)

    brittle: rock breaksat yield point

    (strength of rock overcome)lower T and P

    rock under compression (break)

    ductile: rock flowsat yield point

    (no continuous break)

    higher T and P

    rock under compression(change in shape)

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    factors that affect deformation of rock

    lithostatic pressure: is weight of overlying rock

    (lithos = rock)

    heat: causes atomic bonds to weaken --temperatures low at shallow depths (brittle)

    --temperatures high at great depths (ductile)

    time: allows stress to be applied slowly or quickly--initial stress may not be enough, but

    will be given enough time (sagging bookshelves)--stress applied quickly (snap stick) (brittle)--stress applied slowly (bend stick) (ductile)

    composition: controls rock response to stress--minerals, weaknesses in rock, fluids in pores, etc.

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    to understand deformationneed to know orientations

    of deformed rocks

    orientation of geologic structures

    deformation is easiest to see

    in sedimentary rocks

    i.e. layers are not flat,but are tilted or dipping

    geologists measure orientation

    of layers in outcrops

    outcrop: where bedrock isexposed at the surface

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    orientation of geologic structuresrock layers are planes - use 2 lines to define plane in space

    find 2 lines that can be used and easily measured

    1) strike: intersection of plane with horizontal plane (e.g. sea level)--gives direction of plane relative to northi.e. N60ENorth

    60

    strike

    2)dip: angle that plane

    is inclined relative to the horizontal planee.g. 45 to SE

    45

    Earths surface

    dip

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    orientation of geologic structures

    measure strikewith a compass

    measure dipwith an inclinometer

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    map

    cross-sectionprofile along vertical plane

    --allows one to seestructure in subsurface--

    NS strike

    30 dip to W

    30

    orientation of geologic structuresplot strike and dip

    of layer

    on a geologic map

    strike and dip symbol

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    types of geologic structuresfolds

    wavelike bends in layered rock represent ductile deformation

    form during compression

    fold divided into two limbsby its axial plane

    limbshingeis where fold curves

    hinge

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    types of geologic structures

    folds

    can arch upward or downward

    anticline:upward arching fold

    syncline:downward arching fold

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    types of geologic structuresfolds

    have different geometries

    open folds:gently dipping limbs

    isoclinal folds:parallel limbs

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    types of geologic structuresfolds

    have different geometries

    overturned folds:limbs dip in same direction

    recumbent folds:limbs are parallel and horizontal

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    types of geologic structuresfolds

    have different orientations

    plunging folds:hinge is not horizontal

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    from: http://www.stmarys.ca/academic/science/geology/structural/

    folds occur at all scales from < mm to mountains

    from http://www.eos.duke.edu/geo41/geo41.htm

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    from: http://earth.leeds.ac.uk/learnstructure/index.htm

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    Appalachian Mountains

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    Arkansas

    Ouachitas

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    types of geologic structuresdomes and basins

    domes:layers dip away from

    central point (look at symbols)

    basins:layers dip towardcentral point (look at symbols)

    outcrop patterns form rings

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    Ozark dome

    bright pink areain

    Missouri andNorthern

    Arkansas

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    geologic structuresfolds -- nticlinesand synclines-- produce characteristic

    pattern, or superposition, of layers

    note:1 (oldest) - 5 (youngest) layers below

    top or middle(hinge)

    of nticlineat surfacehas oldestage rocks

    (3)and limbsat surface

    haveyoungest

    (4)

    top or middle(hinge)

    of synclineat surface

    has youngestage rocks

    (5)and limbsat surface

    have oldest(4)

    12

    34

    5

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    faults

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    Back to geologic structuresbending or breaking of rocks yield different structures

    breaking: faults (discuss now)bending: folds (already discussed)

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    how rocks respond to stressrocks behave as elastic, brittle, or ductile bodies

    brittle: rock breaksat yield point

    (strength of rock overcome)lower T and P

    FAULTS

    rock under compression (break)

    ductile: rock flowsat yield point

    (no continuous break)higher T and P

    FOLDS

    rock under compression(change in shape)

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    geologic structuresfractures: cracks in rocks

    joint -- fracture along which no movement has occurred

    very common;do not reflect much strain

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    geologic structures

    joints: form perpendicular to tensiondirection

    cracks open to form joints

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    joints control erosion and can make spectacular landscapeswater gets into the openings

    Bryce Canyon

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    geologic structuresfractures: cracks in rocks

    fault -- fracture along which movement has occurred; considered tiveif motion

    has occurred < 11,000 years

    recognize by juxtaposition ofdifferent types of rocks

    or offset layers

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    faults: have noticeable movement across them

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    fault zones: can be wide with crushed rock in them

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