3.16 Oceanic Plateaus A.C.Kerr CardiffUniversity,Wales,UK 3.16.1 INTRODUCTION 537 3.16.2 FORMATION OF OCEANIC PLATEAUS 539 3.16.3 PRESERVATIONOFOCEANIC PLATEAUS 540 3.16.4GEOCHEMISTRY OF CRETACEOUSOCEANICPLATEAUS 540 3.16.4.1 GeneralChemicalCharacteristics 540 3.16.4.2 MantlePlumeSource Regions ofOceanic Plateaus 541 3.16.4.3 Caribbean–ColombianOceanic Plateau(, 90 Ma) 544 3.16.4.4OntongJavaPlateau(, 122 and , 90 Ma) 548 3.16.5THE INFLUENCE OF CONTINENTALCRUST ON OCEANIC PLATEAUS 549 3.16.5.1 The NorthAtlantic Igneous Province ( , 60 Ma to Present Day) 549 3.16.5.2 The KerguelenIgneous Province ( , 133 Ma to Present Day) 550 3.16.6 IDENTIFICATION OF OCEANIC PLATEAUS IN THE GEOLOGICAL RECORD 551 3.16.6.1 Diagnostic FeaturesofOceanic Plateaus 552 3.16.6.2 Mafic Triassic Accreted Terranesinthe NorthAmericanCordillera 553 3.16.6.3 Carboniferous to CretaceousAccreted Oceanic Plateaus inJapan 554 3.16.7 PRECAMBRIAN OCEANICPLATEAUS 556 3.16.8ENVIRONMENTAL IMPACT OF OCEANICPLATEAU FORMATION557 3.16.8.1 Cenomanian–TuronianBoundary (CTB)Extinction Event 558 3.16.8.2 LinksbetweenCTB Oceanic PlateauVolcanism andEnvironmentalPerturbation 558 3.16.9 CONCLUDING STATEMENTS 560 REFERENCES 561 3.16.1 INTRODUCTION knowledge ofthe oceanbasins hasimproved over the last 25years,many moreoceanic plateaus Although the existence oflarge continentalflood havebeenidentified (Figure1).Coffinand basalt provinceshasbeenknownfor some Eldholm (1992) introduced the term “large igneous considerabletime, e.g.,Holmes(1918),the provinces” (LIPs) asageneric term encompassing recognition thatsimilarfloodbasalt provinces oceanic plateaus,continentalfloodbasalt alsoexist belowthe oceans isrelatively recent. In provinces,andthoseprovinceswhich form at the early 1970s increasingamounts ofevidence the continent–oceanboundary (volcanic rifted fromseismic reflection andrefraction studies margins). revealed thatthe crust inseverallarge portions of LIPsaregenerallybelieved to be formed by the oceanfloorissignificantly thickerthan decompression meltingofupwellinghottermantle, “normal” oceanic crust,which is6–7kmthick. knownasmantleplumes. Although ideasabout Oneofthe first areasofsuch over-thickened crust hotpots andmantleplumeshavebeenaroundfor to be identified wasthe Caribbeanplate(Edgar almost 40years (Wilson,1963),itisonly inthe etal.,1971)which Donnelly (1973) proposed to be past 15years thatLIPshavebecomethe focus of an“oceanic floodbasalt province”. Theterm major research.Oneofthe mainreasons for the oceanic plateauwascoined byKroenke (1974), increased research activity intoLIPsisthe andwasprompted bythe discovery ofalarge area realization thatsignificant proportions ofthese ofthickened crust(. 30 km) inthe western Pacific LIPserupted overarelatively short time, often known asthe OntongJavaplateau(OJP). Asour less than2–3Myr (see reviewinCoffin,1994). 537 538 Oceanic Plateaus Thishasimportant implications for mantlepro- Pacific, whileinthe IndianOceanthe Kerguelen cessesandsource regions (Hart etal.,1992; Stein plateauwasdeveloping.Theareas,volume andHofmann,1994),aswell asenvironmental maximum thicknessesandagesofthe largerof effects on the globalbiosphere(e.g.,Caldeiraand theseplateaus aregiveninTable1. TheOntong Rampino,1990; Courtillot etal.,1996; Kerr, Javaisthe largest ofthe Cretaceous plateaus. 1998). Oceanic plateaus canalsobecomeaccreted Itcovers anarea of1.9 106 km 2 ,andhasan to continentalmargins,andithasbeenproposed estimatedtotalvolum£ eof4.4 107 km 3 thattheseplateaus havebeensignificant contri- (EldholmandCoffin,2000). Althou£ gh early butors to the growthofcontinentalcrust (e.g., seismic refraction datasuggested thatthe OJP Abbott,1996; Albarede, 1998). wasasthick as43km(Furomoto etal .,1976),a The most recent major phaseofoceanic morerecent synthesisbased on existingseismic plateauformation wasinthe Cretaceous when andnewgravity data(Gladczenko etal.,1997) the OntongJava, Manihiki, Hess Rise, andthe hasindicated the averagethickness to Caribbean–Colombianplateaus formed inthe be , 32 km. Figure1 Mapshowingall major oceanic plateaus,andotherlarge igneous provincesdiscussed inthe text (afterSaunders etal.,1992). Table1 Agesanddimensions ofJurassic–Cretaceous oceanic plateaus. Oceanic plateauMeanage Area Thickness range Volume (Ma) (106 km 2 ) (km) (106 km 3 ) Hikurangi early-mid Cretaceous 0.7 10–152.7 ShatskyRise1470.2 10–282.5 MagellanRise145 0.510 1.8 Manihiki 123 0.8 . 20 8.8 OntongJava121(90) 1.9 15–32 44.4 Hess Rise99 0.8 . 159.1 Caribbean88 1.1 8–20 4.4 SouthKerguelen110 1.0 , 22 6.0 CentralKerguelen/BrokenRidge 861.0 19–21 9.1 SierraLeoneRise , 73 0.9 . 10 2.5 MaudRise , . 73 0.2 . 10 1.2 AfterEldholm andCoffin(2000). Formation ofOceanic Plateaus 539 3.16.2 FORMATION OF OCEANIC PLATEAUS The production oflarge volumes(. 106 km 3 ) ofmelt inaperiodasshort as2–3Myr implies magmaproduction ratesupto25%higherthan thoseobserved atpresent-daymidoceanridges (Eldholm andCoffin,2000),andisgenerally believed to necessitateahigh flux ofhotter-than- ambient asthenospheric mantlebelowthese provinces(e.g.,McKenzie andBickle, 1988). Numericalandphysicalmodels show thatthis hottermantlecommonly takesthe form ofa mantleplumewhich ascendsbythermalbuoy- ancythrough the overlyingmantle(Loper,1983; McKenzie andBickle, 1988;Campbell etal., 1989; FarnetaniandRichards,1995). Physical constraints demandthatmantleplumesmust ascendfrom aboundary layerwithinthe Earth, eitherthe core–mantleboundary ( D 00)orthe 670 kmdiscontinuity. Large ascendingmantle plumesare, on average, 200 8 Chotterthanthe ambient uppermantle(McKenzie andBickle, 1988)andundergodecompression meltingas theyapproachthe baseofthe lithosphere. PhysicalmodelingexperimentsbyGriffithsand Campbell (1990) haveshown thatmantleplumes arelikely to ascendthrough the mantlefrom their source boundary layerinthe form ofalarge semi-spherical“head”fed from the source region Figure2 Cartoon to illustratehow mantleplumesare byanarrowerplumetail(Figure2). Alterna- believed to (a)risethrough the asthenospherefrom either tively,numericalmodelingbyFarnetaniand the 670 kmdiscontinuity or D 00 and(b)flattenalongthe Richards(1995)suggested thatplumeheads baseofthe lithosphereandundergodecompression startinginthe mantleonly riseabout three meltingto produce aLIP (afterSaunders etal.,1992). plumehead diameters beforespreadingout. In eithercase, asthe plumeapproachesthe baseof the lithosphere, itspreadsout overabroadly circulararea (whichcanbe asmuch as1000 km indiameter) andundergoesadiabatic decompres- sion,producingmelt overmostofthe area covered bythe flattened-out plumehead (Campbell andGriffiths,1990). Theamount of melt producediscritically dependent on the thickness ofthe preexistinglithosphere, since the baseofthe rigid, nonconvectinglithospherewill actasa“lid”onthe upwellingplumemantleand on the extent ofdecompression melting.Thus,a mantleplumeascendingbelow thick continental lithosphere( . 50km) will produce asmaller thickness ofmelt thanaplumewhich ascends Figure3 Schematic diagramshowinghow original beneathoceanic lithosphere( # 7km) (Figure3). lithospheric thickness andmantlepotentialtemperature Anothersignificant factor indeterminingthe affectthe amount ofmelt produced (melt thickness) and amount ofmelt generated byamantleplumeis how thesefactors relateto continentalfloodbasalts the temperatureofthe plume: generallythe (CFB),volcanic rifted margins (VRM),off –ridge and higherthe temperature, the moremelt will be ridge–centered oceanic plateaus (OP),andmidocean produced(Figure3). ridges(MOR). Theinitial 40 Ar/39Arstep-heatingagesfor LIPs supportmodels ofrapid formation anderuption, Caribbeanplateau,95–86Ma: Kerr etal.,1997a; ofteninless than2–3Myr (Richards etal.,1989). Sinton etal.,1998;Hauff etal.,2000b). Never- Asmoreage datahavebecomeavailable, awider theless,itstill appears thatsubstantialproportions age-range hasemergedfor someLIPs(e.g.,the oftheseprovinceswereformed overgeologically 540 Oceanic Plateaus short timeperiods(e.g.,the OntongJavaplateau accreted oceanic plateausections. For example, formed on two occasions:122^ 3and90^ 4Ma; the OntongJavaplateaucollided withthe Neal etal.,1997). westward-dippingSolomon Islandssubduction Theoretically,oceanic plateaus canform any- zoneat10–20 Ma, resultinginareversalinthe whereinthe oceanbasins; however,most oceanic polarity ofsubduction from west to east,and plateaus appeartohaveformed atornear the upliftandexposureofthe deepersections of midoceanridges(e.g.,Kerguelen,Manihiki and the plateauonthe Solomon Islands(Neal etal., OntongJava),i.e.,regions thatareconduciveto 1997; Petterson etal.,1999). Asecondexampleis voluminous decompression melting(Eldholm and the Caribbean–Colombianoceanic plateau,which Coffin,2000). Atfirst glance, itappears somewhat formed inthe Pacific at , 90 Ma (Sinton etal., coincidentalthatplumesofdeepmantleorigin 1998;Hauff etal.,2000a). Within10Myr the reach the baseofthe lithosphereatamidocean eastward-movingFarallon platehad broughtthe ridge.However,aspointed out bySaunders etal. southern portion ofthe plateauinto collision with (1996),alikely explanation for thisobservation continentalnorthwestern SouthAmerica,resulting isthatmantleplumescan“capture”oceanic inthe accretion ofslicesofthe plateauontothe spreadingcenters (cf.present-dayIceland). continentalmargin(Kerr etal.,1997b). This accretion wasaccompanied byback-steppingof the subduction zonewest ofthe accreted plateau 3.16.3 PRESERVATION OF OCEANIC slices. Shortly afterits formation the northern PLATEAUS portion ofthe plateaubegantomoveinto the The oldest insitu oceanic crustisJurassic inage proto-CaribbeanseawaybetweenNorthandSouth