View
1
Download
0
Category
Preview:
Citation preview
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.
1
4
32
5
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.
5 6
87
Recommended