Subduction zone cross sections

Earthquake locations : black = teleseismic est. gray = local-array est.

red line = top of slab seismicity blue line = center of slab seismicity

Juan Fernd.

Non-ideal Subduction

Oblique subduction causes strike-slip faulting in upper pate. Typically, about half of the transform component is partitioned onto the strike-slip system

Plateau subduction (i.e., subduction of thick ocean crust) causes subducted slab to flatten, oftentimes right against the overriding plate, which causes increased coupling and continental thrusting. (slab flattening happens because slab weight is reduced by inclusion of low-density crust.)

Subduction near slab edge allows flow around the slab edge, which results in slab rollback.

…and Plateau Subduction

Subduction Near Slab Edge

Oblique Subduction…

Continental thrusting Arc not like ping pong ball Subduction

Backarc spreading strong weak {

+ _

Slab Dynamics Gravity pulls the slab straight down (old slab is heavier)

Flow-induced pressure difference holds slab up (fast-subducting slab has greater pressure difference)

Old, slow slab tends to roll back, causing back arc extension Young, fast slab tends to advance, causing back arc compression

mantle evacuated

mantle “wind”

Subduction Primer

neutral

compressional

tensional

BACK

backarc thrusting

Marianas Type

Chilian Type

low Vsub/age

backarc extension

high Vsub/age

old young 10

Slab age (m.y.)

Subd

uctio

n ve

loci

ty (

cm/y

r

(numbers give max earthquake magnitude)

Subduc&onPrimer.ppt

Paleozoic thrus&ngTheAntlerorogeny(370m.y.ago)endedthepassive&mesofoftheAtlan&c‐typemargin,whenthedeepwatersedimentsofthemiogeoclinewerethrustovertheshallowwatersediments(ontheRobertsMountainthrustfault),andavolcanicarcarrived(fragmentsofwhicharefoundwithintheKlamathandSierraNevadaMountains).Themainelementsareshowntotheright;thedetailsbeyondthisareamaIerdebate.Belowistheclassicstory,whichhasavolcanicarcapproachingfromthewest,whichbothclosestheseawaybetweenthearcandNorthAmericaandcausesvolcanisminthearc.Thedockingofthearc

drivesthrus&ng.TheAntlerthrustsheetridesoverthedownwarpedmargin(foredeep)ontheRobertsMountainthrust(RMTabove,showninbluebelow).

AproblemwiththismodelisthepresenceoftheHavallahBasinbetweenthethrustzoneandthevolcanicarc(thatissupposedtobecausingthethrus&ng).

Twosuggestedsolu&onsare:1)Atransformfault

APENNINEOROGENY,ITALY

TheSonomaorogeny(250m.y.ago)followedtheAntlerorogeny,andifmostnotableforbeingsosimilartotheAntler.

TheAncestralRockyMountainswerecreatedbyamysteriousorogenythatoccurredinbetween,atabout300m.y.ago.

juxtaposedunrelatedelementsatalater&me(seeabovefigure),and2)TheAntlerorogenywasmoreliketheApennineorogenycurrentlyac&veinItaly.Thisisthesubjectofthe3/31in‐classexercise.

4 Thissec&onstolenfrom:hIp://ocw.mit.edu/ans7870/12/12.114/f05/gallery/lec3final/pages/lec3photo1.html

LateDevonian/EarlyMississippianAntlerOrogeny.

Above:theshearedandfoldeddeepwatersedimentsofthedistal(farwest,deepsea)Antlermarinebasinthrustoverandplacedontopoftheshallowwaterdepositsofthecon&nentalmargin.Itisuncertainwhatprocesscausedthesedeepwatersedimentstothrustupandoverthecon&nentaledge,extendingfromsouthernCaliforniatoAlaska.WehaveagoodideathattheywerenottransportedmuchmorethanafewhundredkilometersasthesedimentscontainfossilsofNorthAmericanaffinitybut,theylackvolcanicandArcmaterials.Tounderstandthestoryofwhathappenedwhen,werelyonthesedimentsdepositedduringtheorogeny.TheAntlerforedeep(betweenthethrustfrontandthemiogeocline)recordsthesedimentsderivedfromthemountainbuildingeventthatoccurredasthedeep‐watersedimentoverrodetheshallowsediments.Intheforedeep,weseebothsedimentsderivedlocallyoffthetopographicallyhighAntlerthrustsheetandthosefromthelongtravelingcon&nentalrivers.TheAntlersediments,transportedfromthewestarecomposedofconglomera&cdepositscomposedofcherts,shalesandotherlow‐grademetamorphicrocks.Thisisthefirst&meweseesedimentsderivedfromthewest‐‐afeaturethatindicatesthefirsttectonicac&vityandmountainbuildinginthewest.Transportedfromtheeast,wefindfiner‐grained,quartz‐richsedimentsthathavetraveledgreatdistances(duringtransport,othermineralsareerodedaway,weatheredordeposited,leavingquartzgrainspreferen&allycarriedoverlargedistances).

ThisisaviewoftheRobert’sMountainthrustcontactupclose.AbovethecontactweseetheEarlyMissisppianconglomeratesandbelowweseeathickbandofshearedrocks.

Toconstraintheageofathrustfaultlikethis,youneedtoknowtheageoftheyoungestrocksinthefootwallandtheoldestageofthesedimentsproducedbythedeforma&on(theoldestageoftheforedeepsediments).FortheRobertsMountainthrust,weknowthatthefaultwasac&veoverroughly10millionyears.

Thisisathicksec&onontheeasternedgeoftheforedeepwherecon&nentalsedimentsandcarbonatescon&nuedtodobedeposited.HerethereisnorecordoftheAntlerforedeepororogeny.

Ifwemovewestfromthepreviousslideweseethecon&nentalmarginsedimentssourcedfromtheeastinter‐fingerwiththeconglomeratessourcedfromtheAntlerOrogenyintheforedeep.Inthecentralpartoftheforedeep,thesedimentsare5‐6kmthick,butifyougoeast,outoftheforedeep,towardthecon&nentallyderivedsediments,youfindforthesame&meperiodsedimentsthatare300‐400mthick.

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Timingofdeposi&onaleventsaroundthe&meoftheAntlerOrogeny(datesareapproximate).ArcVolcanismcon&nuedthroughoutthe&meodtheAntler.SedimentfoundintheAntlerAllochthonexperiencedac&vedeposi&onun&l~360Ma.Thefillingoftheforedeepthenstarts~355Ma,sugges&ngthatthematerialhadbeenthrustoverthesinkingcon&nentalmargin.By~350Ma,wefindsedimentsoverlappingtheAntlerOrogeny,sugges&ngthatithadbeensignificantlyeroded.Notethatdeposi&oncon&nuesintheHavallahbasineastofthearcandwestofthedevelopingAntlerMountainbelt,sugges&ngtherewasnoarccollisionduringAntler&me.

AviewofearlyPennsylvaniansedimentsdepositedovertheerodedmountainsoftheAntlerOrogeny.ThissuggeststhatthetopographycreatedbytheAntlerwaserodedtolowrelief,andthattheAntlerOrogenywascompletebytheearlyPennsylvanian.

FoldedcarbonatesthatweredepositedintheHavallahbasinwestoftheAntlerOrogenyatthe&methatdeforma&onwasoccurring.Thissuggeststhattherewasnomajorcon&nentalorislandarccollisionthatcreatedtheAntlerOrogeny.

AnarcofthesameageastheAntlerorogeny,nowfoundintheKlamathMountains,wasac&vesomewherewestofNorthAmerica.Thisimageshowsashesandsedimentsassociatedwiththisarc.

Althoughthesevolcanicallyderiveddepositswerecreatedatthesame&meastheAntler,therearenoevidenceofashesorvolcanicfragmentsintheAntler‐relatedsediments,sugges&ngthattheArcsweres&llfarawayfromtheedgeofNorthAmericaduringAntler&me.

Studentcontempla&ngtherocksfurtherwestoftheKlamatharc.ThisistheoceanflooruponwhichtheArcwasdeposited.Thestructurallylowestrockswefindaretectonicallylayereddunite(olivine‐rich)andperido&te(olivine‐pyroxene)mantlerocksfrombelowtheoceancrust.

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