Folds1Fold Folds form from curving, buckling, and bending of
originally planar rock layers (e.g., beds, foliation) through
ductile deformation. Practically, folds are defned by the attitude
of theiraxis and/or hingeline, axial plane Folds occur in any
geologic layer such as bedding, lava fow layers, foliation. Folds
range in size from mm to km. Are a manifestation of ductile
deformation. i.e., form at depth where T,are high and fracturing
does not occur.2Single Folded Surface Hingeline !ines "oining
points of ma#imum (tightest) curvature n!ection lines !ines
connecting points of $ero curvature Half"#a$elength%distance
between two infection points Fold domains% are separated by the
infection lines These fold terminologies are only based on
geometry% %ntiform%&omains with upward closure (' curvature)
Synform%&omains with downward closure (( curvature) &eutral
fold%Fold that closes sideways3Single Folded Surface ' %nticline A
fold with the oldest rocks at the core (i.e., at the concave side).
Syncline A fold with the youngest rocks at the core. Note%)n
simply'folded areas, anticlines are generally antiformal, and
synclines are synformal. *owever, in refolded areas this is not
generally the case. %ntiformal syncline Synformal
anticline4HingelineA special fold a#is, connecting points of
ma#imum curvature on the folded surface *ingeline is a physical
line that can be marked on the folded layer with a pen and be
measured directly with a geologic compass5Folds in (ulti"layers)n a
multi'layered folded rock, every folded layer has its own hingeline
A plane that contains all these hingelines is called the hinge
plane (or hinge surface if curved)Hinge plane may or may not be
parallel to the axial plane6Fold %xis An imaginary line that
generates the fold if it is moved parallel to itself in space Fold
axis makes sense only for cylindrical folds +ost folds are
non'cylindrical at a large scale These can be broken into smaller
cylindrical parts (domains or segments) %xis (ln) is de,ned by the
intersection of the% A#ial plane (-.) (or a#ial plane cleavage if
it e#ists) with the folded layer (-.), i.e, (-.#-o) !imbs (-n) of
the fold (-n#-n), where n could be any generation (/, ., 0, 1)
7Fold Shape )ightness%)s de,ned by the interlimb angle nterlimb
angle%The angle between the tangents to the fold surface drawn
through the infection lines8Shape Interlimb anglegentle
180o-120oopen 120o-70oclose 70o-30otight 30o - 0oisoclinal
0oelastica Negative valuesP*)+,-. Folds and its kindsFolds result
from the plastic deformation of rocks at low strain'rates, usually
under elevated temperature and pressure conditions. Folds are
broadly subdivided into anticlines (upwards conve#) and synclines
(downwards conve#). /epartment 0f Petroleum technology1ni$ersity of
2arachiFold %ttitudeThe attitude of a fold is given by the attitude
of its%A#ial plane (strike, dip)*ingeline (trend, plunge) 2ertical
fold% vertical a#ial plane and vertical a#is 3pright plunging%
vertical a#ial plane, plunging a#is 3pright hori$ontal% vertical
a#ial plane, hori$ontal a#is )nclined plunging% inclined a#ial
plane, plunging a#is )nclined hori$ontal% inclined a#ial plane,
hori$ontal a#is 4eclined% plunging a#is trends along the dip of the
inclined a#ial plane 4ecumbent fold% hori$ontal a#is and a#ial
planeFolds are classifed based on the relati$e $alues of the dip of
the axial plane, and the plunge of the hingeline10)ypes of
Folds(onocline%A local steepening in otherwise uniformly dipping
strata.soclinal fold%!imbs are parallel to the a#ial
plane.3ecumbent fold%Fold with hori$ontal a#ial plane.5ommonly
isoclinal.Symmetric vs. asymmetric folds.11Symmetric FoldsThe
median plane and the axial plane are perpendicular, and the a#ial
plane divides the fold into mirror 6uarter wavesPolyharmonic
fold%Fold waves with two or more orders of wavelengths and
amplitude.!arge polyharmonic folds have parasitic (smaller)
fold.12-tructural 7eology-tructural geologists are concerned with
why parts of the 8arth have been bent into folds and others have
been broken by faults.+apping of these structures provides
important information to land managers and mineral
e#ploration.3nderstanding of these features help us understand the
dynamic 8arth.late TectonicsTectonic -tructures+ost structures are
driven by the forces of late TectonicsThe kinds of structures are
determined by%Temperature and pressure5omposition!ayeringAnisotropy
or )sotropy of the layersAmount of fuids presentTectonic
-tructures&uctile deformation produces%Folds&uctile
Faults5leavagesFoliation9rittle &eformation5ertain types of
folds9rittle Faults:oints-tructural 7eologySubdisciplines of
Structural 4eologyField 3elations +ake accurate geologic maps
+easure orientations of small structures to inform us of the shape
of larger structures -tudy the se6uence of development and
superposition of di;erent kinds of structures3ock (echanics <
the application of physics to the study of rock materials.)ectonic
and 3egional Structural 4eology < -tudy of mountain ranges,
parts of entire continents, trenches and island arcs, oceanic
ridgesApplications of -tructural 7eology8ngineering
)ssues9ridges&amsower lants*ighway 5uts!arge 9uildings
Airports8nvironmental )ssues8arth6uake ha$ard!ocation of land,ll
sites5ontamination cleanup&istribution of groundwater+ineral
e#ploration-cale in -tructural 7eology (icroscopic < =eed
magni,cation Foliation, +icro folds (esoscopic < *and specimens
and outcrops Foliation, Folds, Faults (acroscopic < +ountainside
to map levels 9asins, domes, +etamorphic 5ore 5omple#es
=on'penetrative structures < not present on all scales Faults
)solated folds enetrative structures < found on any scale that
we chose to study -laty cleavage Foliation -ome folds
parallelsimilar?displacementthrowHeave Fault offsets - the
jargonExercise 3: interpret the faultsfolds exist at all scales
from < mm to entire mountain rangesw! do folds exist"what do
they tell usabout kinematicsfrom#
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from#
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do te! form"wat does it mean for regional anal!sis"folded surface#
(asic geometric elementslim(ingetrougcrestinflection $oint# were
senseof curvature cangesinge line# werecurvature is
greatestenvelo$ing surface#defines limit of foldenvelo$ing surface
)s!nform* )connect trougs*envelo$ing surface)antiform*)connect
crests*s!nform# +valle!,antiform# +ill,(ot from#
tt$#%%eart&leeds&ac&u'%learnstructure%index&tmanticlinorium#
regionals!nclinorium# regionalc!lindrical fold#surface wra$s
$artwa! around c!linder-inge line is straigtonl! reall! exist at
outcro$ scale or lessingescurvilinear fold# curved inge
line)nonc!lindrical*note hinge line can moveparallel to itself and
stillbe within folded surface;generates foldare folds in $otogra$
c!lindrical"all from#
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(aton$arallel to $a$eronl! . as straigt inge linehinge lines not
always straightfrom# /avis and 0e!nolds1 1226from#
tt$#%%eart&leeds&ac&u'%learnstructure%index&tmwavelengtinterlim(
angleam$litudesome fold $arameters defined in te $rofile
$lane#interlim( angle gives 3ualitative estimate of intensit! of
folding- 4smaller te angle1 greater te intensit!wavelengt# distance
(etween inges of successive foldsam$litude# alf eigt of structure
measured from crest to trougfold $rofile $lane#$er$endicular to
inge line- reference $lanetrue shape of fold in 2Duse to descri(e#
5 ow tigt is fold 5 ow rounded is foldnote fold profile planenot
same as cross section--cross section an! vertical $lane-if inge
line $lunges )as ere*- cross section is not $rofile $lane(ot from#
tt$#%%eart&leeds&ac&u'%learnstructure%index&tmaxialsurfacefold
axis and axial surfacefold axis#line tat 6oins$oints of maximum
curvatureon folded surfacefor cylindrical folds: fold axis = hinge
lineaxial surface# imaginar! surfacetat 6oins inge lines of
differentfolded surfaces )la!ers*t!$icall! called axial $lane1 (ut
not alwa!s a $lanar surface fold axisintersection of axial surface
with groundoutcrop is axial traceall from#
tt$#%%eart&leeds&ac&u'%learnstructure%index&tmexam$les
of axial surfaces.# axial $lane/#sows axial tracefor
(otcross-sectionandma$ viewaxial trace is whatwe measurefrom# /avis
and 0e!nolds1 12267# curvi$lanar surface)s!stematic*8# curvi$lanar
surface)non-s!stematic*two exam$les of folds witinge lines andaxial
surfacesfrom# /avis and 0e!nolds1 1226to name fold1 need to 'now
facing4 to name fold1 need to 'now
facing4!oungerolderolder!ounger!ounging direction!ounging direction
s!ncline)s!nform wit normal facing* s!nformal anticline)s!nform wit
reverse facing1 aging li'e anticline* antiformal s!ncline)anticline
wit reverse facing*)aging li'e s!ncline*anticline )antiform wit
normal facing*all from#
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of folds# fold shapes !cylindrical vs" non#cylindrical$fold facing
!upward and downward#facing$fold sa$e in $rofile $lanefold
orientation )di$ of axial $lane*all from#
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orientation# fold axis ori9ontalfrom#
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fold axis is not ori9ontal1 folds are said to (e $lungingfold
orientation# $lunge of fold axisori9ontal# 0-10: sallow#
10-30:intermediate# 30-60:stee$#60-80:vertical# 80-20:$lunging
foldto classif! fold com$letel!1 use di$ of axial surface and$lunge
of fold axisfrom#
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from#van der ;lui6m and 10:* to envelo$ing surface# s!mmetric if
axial surface < 80: to envelo$ing surface# as!mmetric$arasitic
folds often as!mmetric- as!mmetr! de$ends on larger foldnote for
asymmetric folds% crest and hinge are not the sameall from#
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