Download - Lect 18 Folds Faults 07

8/11/2019 Lect 18 Folds Faults 07

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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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have different geometries

open folds:gently dipping limbs

isoclinal folds:parallel limbs


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have different geometries

overturned folds:limbs dip in same direction

recumbent folds:limbs are parallel and horizontal


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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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San Andreas Fault

Garlock Fault

Garlock

San Andreas

from: http://www.geo.duke.edu/geo42/st.html

faults:examples

large faults


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faults:examples

offset ditchalong

San Andreas Faultin 1975


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geologic structurestypes of faults

use strike and dip to classify faults

dip-slip fault: slip parallel to dip (up or down plane)strike-slip fault:slip parallel to strike (horizontal)


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geologic structurestypes of faults

dip-slipis not unique (up or down?)

strike-slipis not unique (left or right?)

need another description

can label oppositesides of a

dip-slip fault--imagine standing ina hole along fault --

brown is blockabove your head--hanging wall--

pink is block below your feet --footwall--


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hanging wall

footwall

types of faults: dip-slip

reverse faultdip-slip fault with motionof hanging wall up

the fault plane

normal faultdip-slip fault with motionof hanging wall down

the fault plane


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types of faults: strike-slip

right lateral

stand on one side of faultand look across at the otherto label the type of fault

strike-slip faults are vertical

cannot use hanging wall and footwall

(one side is not above the other)

left lateral


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grabendowndropped

blockhorst

high blockbetween

grabens

form during tensionalstress (extension)

types of faults: normal(dip-slip)

extend crust (horizontal) thin crust (vertical)

think pulling taffy

hanging wallmoves down

fault plane


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normal faults


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develop where crust is stretchedby tensional stresses

form from high horsts with intervening low grabens

Basin and Range:western US

normal faults: fault-block mountains

topography of Nevada

horst horst

graben


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form during compressionalstress (shortening)

types of faults: reverse(dip-slip)

shorten crust (horizontal)

thicken crust (vertical)

hanging wallmoves upfault plane

thrust faultreverse fault thathas a shallow dip


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reverse fault


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thrust faults are low angle reverse faults,which have dips < 30

thrust faultsnorthern Rockies


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thrust faults


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normal vs. reverse faults: keyconcepts

original unfaulted rectangle

--has width AB--(distance between X and Y)

normalfaulted rectangle--has width AB--

(distance between X and Y)longer than original length AB

reversefaulted rectangle--has width AB--

(distance between X and Y)shorter than original length AB


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normal vs. reverse faults: keyconcepts

motion of hanging wall downfault plane leads to

omission of section, i.e. younger layers over older

(gray layer is missing alongyellow line)

motion of hanging wall upfault plane leads to

repetition of section, i.e. older layers over younger(gray layer is repeated along

yellow line)


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form during shearingstress

types of faults: strike-slip

faults are vertical: no hanging wall or footwall

neither shortening or extension occurs

offset surface features such as streams and valleys


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strike-slip fault: offset streams


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from: http://www.gps.caltech.edu/~sieh/research

from: Shelton

strike-slip fault: San Andreas


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have both strike-slip (horizontal) and dip-slip (vertical) motion

types of faults: oblique-slip

footwall

hanging wall

oblique slip

footwall

hanging wall

oblique slip


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geologic structures: importance for petroleumform traps

--locations where petroleum accumulates

trapin

anticline

gas and oilmigrate upthrough

permeablelayers andare trapped

byimpermeable

units


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eroded anticline will not serve as trapno impermeable cap to trap oil or gas


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geologic structures: importance for petroleum

trap alongfault

traps along sedimentary features

key is havingimpermeable unit

abovepermeable unit

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