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$ard!ocation of land,ll sites5ontamination cleanup&istribution of groundwater+ineral e#ploration-cale in -tructural 7eology (icroscopic < =eed magni,cation Foliation, +icro folds (esoscopic < *and specimens and outcrops Foliation, Folds, Faults (acroscopic < +ountainside to map levels 9asins, domes, +etamorphic 5ore 5omple#es =on'penetrative structures < not present on all scales Faults )solated folds enetrative structures < found on any scale that we chose to study -laty cleavage Foliation -ome folds parallelsimilar?displacementthrowHeave Fault offsets - the jargonExercise 3: interpret the faultsfolds exist at all scales from < mm to entire mountain rangesw! do folds exist"what do they tell usabout kinematicsfrom# tt$#%%eart&leeds&ac&u'%learnstructure%index&tm(ot from# tt$#%%www&stmar!s&ca%academic%science%geolog!%structural%ow 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# tt$#%%eart&leeds&ac&u'%learnstructure%index&tmc!lindrical#move (aton$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$les of folds witinge lines andaxial surfacesfrom# /avis and 0e!nolds1 1226to name fold1 need to 'now facing4 to name fold1 need to 'now facing4!oungerolderolder!ounger!ounging direction!ounging direction s!ncline)s!nform wit normal facing* s!nformal anticline)s!nform wit reverse facing1 aging li'e anticline* antiformal s!ncline)anticline wit reverse facing*)aging li'e s!ncline*anticline )antiform wit normal facing*all from# tt$#%%eart&leeds&ac&u'%learnstructure%index&tmclassification of folds# fold shapes !cylindrical vs" non#cylindrical$fold facing !upward and downward#facing$fold sa$e in $rofile $lanefold orientation )di$ of axial $lane*all from# tt$#%%eart&leeds&ac&u'%learnstructure%index&tmu$rigtinclinedinclinedrecum(entfold orientation# fold axis ori9ontalfrom# tt$#%%eart&leeds&ac&u'%learnstructure%index&tmfrom# tt$#%%eart&leeds&ac&u'%learnstructure%index&tmfrom tt$#%%www&stmar!s&ca%academic%science%geolog!%structural%from# tt$#%%eart&leeds&ac&u'%learnstructure%index&tmif 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$lunge of fold axisfrom# tt$#%%eart&leeds&ac&u'%learnstructure%index&tm 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# tt$#%%eart&leeds&ac&u'%learnstructure%index&tm