Large Igneous Provinces: Crustal Structure, Dimensions, and External Consequences

Home , Agulhas Basin, Broken Ridge, Deccan Traps, Discovery Seamounts, Hikurangi Plateau, Large igneous province

LARGE IGNEOUS PROVINCES: CRUSTAL STRUCTURE, DIMENSIONS, AND EXTERNAL CONSEQUENCES

Millard F. Coffin Olav Eldholm Institute for Geophysics Department of Geology Universityof Texas Universityof Oslo Austin Oslo, Norway

Abstract. Large igneous provinces (LIPs) are a conduction rate of the entire mid-ocean ridge system. The tinuum of voluminous iron and magnesium rich rock major part of the North Atlantic volcanic province lies emplacementswhich include continental flood basalts offshore and demonstrates that volcanic passive mar- and associated intrusive rocks, volcanic passive margins belong in the global LIP inventory. Deep crustal gins, oceanic plateaus, submarine ridges, seamount intrusive companions to continental flood volcanism groups, and ocean basin flood basalts. Such provinces represent volumetrically significant contributions to do not originate at "normal" seafloor spreading cen- the crust. We envision a complex mantle circulation ters. We compile all known in situ LIPs younger than which must account for a variety of LIP sizes, the 250 Ma and analyze dimensions, crustal structures, largest originating in the lower mantle and smaller ages, and emplacement rates of representatives of the ones developing in the upper mantle. This circulation three major LIP categories: Ontong Java and Ker- coexists with convection associated with plate tecton- guelen-Broken Ridge oceanic plateaus, North Atlantic ics, a complicated thermal structure, and at least four volcanic passive margins, and Deccan and Columbia distinct geochemical/isotopicreservoirs. LIPs episod- River continental flood basalts. Crustal thicknesses ically alter ocean basin, continental margin, and con- range from 20 to 40 km, and the lower crust is char- tinental geometries and affect the chemistry and phys- acterizedby high (7.0-7.6 km s-•) compressionalics of the oceans and atmosphere with enormous wave velocities. Volumes and emplacement rates de- potential environmental impact. Despite the impor- rived for the two giant oceanic plateaus, Ontong Java tance of LIPs in studiesof mantle dynamics and global and Kerguelen, reveal short-lived pulses of increased environment, scarce age and deep crustal data neces- global production; Ontong Java's rate of emplacement sitate intensified efforts in seismic imaging and scien- may have exceeded the contemporaneousglobal pro- tific drilling in a range of such features.

1. INTRODUCTION [McNutt and Fischer, 1987], megaswell [Vogt, 1991], superplume [Larson, 1991 a, b], rolling thunder [Prin- Large igneousprovinces (LIPs) are massive crustal gle, 1991], and supergreenhouse[Arthur et al., 1991]. emplacements of predominantly mafic (Mg and Fe LIPs are globally significant. After basalt and asso- rich) extrusive and intrusive rock which originate via ciated intrusive rock emplaced at spreading centers, processesother than "normal" seafloorspreading. As LIPs are the most significant accumulations of mafic physical manifestations of mantle processes, these material on the Earth's surface. They are commonly global phenomena include continental flood basalts, attributed to mantle plumes or hotspots [Wilson, 1963; volcanic passivemargins, oceanicplateaus, submarine Morgan, 1971, 1981] and presently account for be- ridges, seamountgroups, and ocean basin flood basalts tween 5% and 10% of the heat and magma expelled (Figure 1 and Tables 1 and 2). Study of LIP-forming from the mantle [Davies, 1988; Sleep, 1990]. However, processes has intensified over the past five years as these fluxes are not distributed evenly in space and new information has become available from seismic time; their episodicity punctuates the relatively steady tomography, geochemistry, marine geophysics, pe- state production of crust at seafloor spreading centers trology, and geodynamicmodeling, among other geo- (Figure 2) [Larson, 1991b]. LIPs therefore provide scientific disciplines [e.g., Coffin and Eldholm, 1991, windows into those regions of the mantle which do not 1992, 1993]. Furthermore, recent research has docu- generate normal mid-ocean ridge basalt. Seismic to- mented important temporal, spatial, and composi- mography suggests that velocity structure beneath tional similarities among various LIPs. These new mid-ocean ridges is different than that beneath hots- analyseshave produced fascinating, speculativeinter- pots [e.g., Dziewonski and Woodhouse, 1987], imply- pretations, headlined by bold terms such as superswell ing shallower, passive upwelling beneath ridges and

Copyright1994 by the AmericanGeophysical Union. Reviewsof Geophysics,32, 1 / February1994 pages 1-36 8755-1209/94/93 RG-02508515.00 Paper number 93RG02508 ele 2 ß Coffin and Eldholm- LARGEIGNEOUS PROVINCES 32, 1 / REVIEWSOF GEOPHYSICS

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TABLE 1. Large IgneousProvinces, Excluding Volcanic PassiveMargins

Abbreviations LIP in Figure 1 Type Reference

Agulhas Ridge AGUL SR IHO/IOC/CHS [1984] Alpha-MendeleyevRidge ALPH SR/OP* Weber and Sweeney [1990] Angola Plain ANGO ASSC Hinz and Block [1991] Austral seamounts AUST SMT Crough [1978] Azores AZOR SMT G. R. Davies et al. [1989] Balleny Islands BALL SMT Lanyon et al. [1993] Bermuda Rise BERM OP Detrick et al. [1986] Broken Ridge BROK SR MacKenzie [1984] Canary Islands CANA SMT Schmincke [1982] Cape Verde Rise CAPE OP Courtney and White [1986] Caribbean flood basalt CARI CFB/OBFB Bowland and Rosencrantz [1988] Caroline seamounts CARO SMT Mattey [1982] Ceara Rise CEAR OP Supko and Perch-Nielsen [1977] Chagos-LaccadiveRidge CHAG SR Duncan [1990] Chukchi Plateau CHUK OP Hall [1990] Clipperton seamounts CLIP SMT Mammerickx [ 1989] Cocos Ridge COCO SR Batiza [1989] Columbia River Basalt COLR CFB Reidel and Hooper [ 1989] Comores Archipelago COMO SMT Emerick and Duncan [1982] Conrad Rise CONR OP Diament and Goslin [1986] Crozet Plateau CROZ OP Goslin and Diament [1987] Cuvier Plateau CUVI OP Larson et al. [1979] Deccan traps DECC CFB Mahoney [1988] Del Carlo Rise DELC OP Goslin and Diament [1987] Discovery seamounts DISC SMT IHO/IOC/CHS [1984] Eauripik Rise EAUR OP Den et al. [1971] Eastern central North Atlantic ECNA ASSC K. Hinz (personal communication, 1991) East Mariana Basin EMAR OBFB Abrams et al. [1993] Emeishan basalts? CFB Chung and Jahn [1993] Etendeka ETEN CFB Cox [1988] Ethiopian flood basalt ETHI CFB Mohr and Zanettin [1988] Foundation seamounts FOUN SMT Mammerickx [ 1992] Galapagos/CarnegieRidge GALA SMT/SR Christie et al. [1992] Great Meteor-Atlantis seamounts GRAT SMT Tucholke and Smoot [1990] Guadelupe seamountchain GUAD SMT Batiza [1989] Hawaiian-Emperor seamounts HAWA SMT Clague and Dalrymple [1989] Hess Rise HESS OP Vallier et al. [1983] Hikurangi Plateau HIKU OP Wood and Davey [1994] Iceland/Greenland-ScotlandRidge ICEL OP/SR Vogt [1974] Islas Orcadas Rise ISLA SR LaBrecque and Hayes [1979] Juan Fernandez Archipelago JUAN SMT IHO/IOC/CHS [1984] Karoo KARO CFB Cox [1988] Kerguelen Plateau KERG OP Houtz et al. [1977] Line Islands LINE SMT Sandwell and Renkin [1988] Lord Howe Rise seamounts LORD SMT Wellman and McDougall [1974] Louisville Ridge LOUI SMT Lonsdale [1988] Madagascarflood basalts MAFB CFB Mahoney et al. [1991] MadagascarRidge MARI SR Sinha et al. [ 1981] Madeira Rise MADE OP Peirce and Barton [1991] Magellan Rise MAGR OP Winterer et al. [1973] Magellan seamounts MAGS SMT IHO/IOC/CHS [1984] Manihiki Plateau MANI OP Winterer et al. [1974] Marquesas Islands MARQ SMT Fischer et al. [1987] Marshall Gilbert seamounts MARS SMT Schlanger et al. [1981] Mascarene Plateau MASC OP Duncan [1990] Mathematicians seamounts MATH SMT Mammerickx [1989] Maud Rise MAUD OP Barker et al. [1988] Meteor Rise METE SR LaBrecque and Hayes [1979] Midcontinent rift volcanics (Keweenawan) MIDC CFB Van Schmus [1992] Mid-Pacific Mountains MIDP SMT Winterer and Metzler [1984] Musicians seamounts MUSI SMT Batiza [1989] Naturaliste Plateau NATU OP Coleman et al. [1982] Natkusiak flood basalt NATK CFB Rainbird [1993] Nauru Basin NAUR OBFB Shipley et al [1993] 4 ß Coffin and Eldholm: LARGE IGNEOUS PROVINCES 32, 1 / REVIEWS OF GEOPHYSICS

TABLE 1. (continued)

Abbreviations LIP in Figure 1 Type Reference

Nazca Ridge NAZC SR Pilger and Handschumacher [1981] New England seamounts NEWE SMT Duncan [1984] Ninetyeast Ridge NINE SR Peirce et al. [1989] North Atlantic volcanic province NAVP CFB Upton [1988]; Dickin [1988] Northeast Georgia Rise NEGE OP LaBrecque and Hayes [1979] Northwest Georgia Rise NWGE OP Brenner and LaBrecque [1988] Northwest Hawaiian Ridge NOHA SR/SMT Mammerickx [1989] Northwind Ridge NOWI SR Hall [ 1990] Ontong Java Plateau ONTO OP Hussong et al. [1979] Osborn Knoll OSBO OP Sclater and Fisher [1974] Paranti flood volcanism PARA CFB Piccirillo et al. [1988] Phoenix seamounts PHOE SMT IHO/IOC/CHS [ 1984] Pigafetta Basin PIGA OBFB Abrams et al. [1993] Pratt-Welker seamounts PRWE SMT Batiza [1989] Rajmahal traps RAJM CFB Mahoney et al. [1983] Rio Grande Rise RIOG OP Gamboa and Rabinowitz [1984] Roo Rise ROOR OP IHO/IOC/CHS [ 1984] Sala y Gomez Ridge SALA SR Pilger and Handschumacher [1981] Shatsky Rise SHAT OP Den et al. [1969] Shona Ridge SHON SR IHO/IOC/CHS [ 1984] Siberian traps SIBE CFB Zolotukhin and Al'Mukhamedov [1988] Sierra Leone Rise SIER OP Kumar [ 1979] Sohm Abyssal Plain SOHM ASSC Hinz and Popovici [1989] Tahiti TAHI SMT Duncan and McDougall [1976] Tasmantid seamounts TASM SMT McDougall and Duncan [1988] Tokelau seamounts TOKE SMT IHO/IOC/CHS [ 1984] Tuamotu Archipelago TUAM SMT Duncan and Clague [1985] Tuvalu seamounts TUVA SMT IHO/IOC/CHS [ 1984] Vit6ria-Trindade Ridge VITR SR/SMT Francisconi and Kowsmann [1976] Wallaby Plateau WALL OP Symonds and Cameron [1977] Walvis Ridge WALV SR Rabinowitz and LaBrecque [1979] Wrangellia? OP Richards et al. [ 1991] Yemen Plateau basalts YEME CFB Mohr and Zanettin [1988]

ASSC, anomalousseafloor spreading crust; CFB, continentalflood basalt; OBFB, ocean basin flood basalt; OP, oceanicplateau; SMT, seamount; SR submarine ridge. *Referred to in the literature as both a submarine ridge and an oceanic plateau. ?Not mapped in Figure 1.

deeper, dynamic upwelling beneath hotspots [Zhang sible environmental consequences.To do so, we use and Tanimoto, 1992]. Furthermore, thermal structure primarily geophysical and geological data and pub- of the upper mantle is heterogeneous,suggesting that lished results, focusing on five major mafic emplace- extensive magmatism requires a combination of hot ments: the Ontong Java and Kerguelen/Broken Ridge upper mantle and suitable lithospheric conditions oceanic plateaus, the North Atlantic volcanic passive [Anderson et al., 1992a, b]. Despite a wealth of data margins, and the Deccan and Columbia River conti- and results on hotspots and mantle plumes (for re- nental flood basalts, which represent the three main views and syntheses, see, for example, White and categoriesof transient LIPs. Of these, the two oceanic McKenzie [1989], Duncan and Richards [1991], Hill et plateaus are the largest igneous provinces and may al. [1992], and Sleep [1992]), not one of the available represent the highestrates of planetary crustal produc- models satisfactorily explains the genesisof all LIPs or tion on timescalesof 106years. even individual categories of LIPs. In this study we will describe and characterize transient LIPs, that is, those LIPs which form by volumi- 2. COMPOSITION, CATEGORIES, AND nous, but probably relatively short-lived, magmatic TECTONIC SETTING episodes and review their crustal structure. Because dimensions and emplacement rates for most LIPs are LIPs are dominated by mafic rocks. Because con- poorly known but are of fundamental importance both tinental flood basalts are far more accessible to sam- for internal processes and external implications, we pling than submarinevolcanic passive margin and oce- calculate LIP dimensions and rates in order to evalu- anic plateau igneousrocks, we know much more about ate and summarize their genesis, evolution, and pos- the petrology and geochemistry of the former. The 32, I / REVIEWS OF GEOPHYSICS Coffin and Eldholm: LARGE IGNEOUS PROVINCES ß 5

TABLE 2. Volcanic PassiveMargins, Including Marginal Plateaus

Continent Abbreviations ( Sub c o ntine nt/Mic roco ntine nO Location in Figure 1 Reference

Africa Abutment Plateau ABUT Hinz [1981] Cape Basin CABA Hinz [ 1981] Gulf of Guinea GULF Rosendahl et al. [1991] Mozambique Basin MOZA DeBuyl and Flores [1986] Antarctica Astrid Ridge ASTR Roeser et al. [1990] Eastern Explora wedge EEXP Hinz and Krause [1982] Western Explora wedge WEXP Kristoffersen and Hinz [1991] Gunnerus Ridge GUNN Roeser et al. [1990] Wilkes Land WILK Eittrem et al. [1985] Australia Cuvier Plateau CUVI Hopper et al. [1992] Scott Plateau SCOT Hinz [ 1981] Greenland Jan Mayen Ridge JANM $kogseid and Eldholm [1987] Morris Jesup Rise MORR Feden et al. [1979] Northeast Greenland NEGR Hinz et al. [1987] Southeast Greenland SEGR Larsen and Jakobsdottir [1988] Southwest Greenland SWGR Chalmers [1991] India Kerala Basin KERA Hinz [ 1981] Kutch Basin KUTC Hinz [ 1981] North America Baltimore Canyon trough BALT Sheridan et al. [1993] Carolina trough CATR Austin et al. [1990] Newfoundland Ridge NEWF Grant [1977] Northwest Europe Bear Island margin BEAR Faleide et al. [1988] Hatton Bank HATT Roberts et al. [1984] Lofoten margin LOFO Eldholm et al. [1979] MOre margin MORE Smythe [1983] V0ring margin VORI Hinz and Weber [1976] Yermak Plateau YERM Feden et al. [1979] Seychelles Seychelles Bank SEYC Devey and Stephens [1992] South America Argentine margin ARGE Hinz [ 1990] Brazilian margin BRAZ Chang et al. [1992] Falkland Plateau FALK Lorenzo and Mutter [1988]

most abundant rock types are typically subhorizontal, ble MORB lavas derived from depleted mantle. Many subaerial basalt (mostly tholeiitic) flows, some of continental flood basalts have chemistries characteris- which extend for many hundred kilometers and have tic of lithosphericfractionation and isotopic signatures volumes as great as several thousand cubic kilometers more closely resembling those of continental crust [e.g., Tolan et al., 1989]. Felsic and intermediate rocks than of convecting mantle. Differences among LIP also occur as lavas and intrusives and are mostly chemistry and isotopes may be explained by varying associated with initial and late emplacement stages, degrees of crustal or lithospheric-mantle contamina- while flood basalts largely uncontaminated by crustal tion and different subcontinentallithospheric and as- components represent the most intense, transient thenosphericsource compositions. eruption period. Relative to mid-ocean ridge basalt Intermediate and mafic sills are well developed in (MORB), major element compositions of continental continental flood basalt provinces that overlie sedi- flood basalts are much more diverse [Macdougall, mentary sequences and along the offshore Norwegian 1988a]. Fractionated componentsare more common, and British volcanic margins and may contribute a and both alkalic and tholeiitic differentiates occur. significant fraction of the total igneous volume. Off Petrologic and geochemical studies of rocks from Norway, flood basalts of a dipping reflector wedge are LIP environments suggest lower to upper mantle locally underlainby dacitic-andesiticlavas interpreted sourcesfor the magma. A consensushas developed on as having been emplaced during the late rift stage the basis of isotopic taxonomy that at least four dif- [Eldholm et al., 1989] by partial melting of a compo- ferent sourcereservoirs persist in the mantle: depleted nent of continental material in a shallow crustal MORB mantle (DMM), high U/Pb ratio (HIMU) and magma chamber [Parson et al., 1989]. Dacite also enriched mantle 1 and 2 (EM1 and EM2) [Zindler and underlies basalt in the northern Rockall Trough off Hart, 1986;Hart et al., 1992]. In chemical and isotopic Great Britain [Morton et al., 1988]. Some provinces character, some continental flood basalts mimic in- also contain significantamounts of rhyolite, probably traoceanic hotspot basalts along the same plume trace erupted as high-temperature (---1100øC) lavas. [Carlson, 1991]. Other continental flood basalts resem- Whether dacite and/or rhyolite is present in purely 6 ß Coffin and Eldholm' LARGEIGNEOUS PROVINCES 32, I / REVIEWSOF GEOPHYSICS

120 LIP/Mid-Ocean Ridge EmplacementRate 11o ONTO LIP

lOO

Early-Middle Eocene- Cenomanian- Aptian- Berriasian- Turonian Albian Miocene Oli8øcenePaleocene- Maastrichtian- Valanginian • • Eocene ß / Danian

DECC

KERG LIP 10

0 ]COLRD I o lO 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Figure 2. LIPs as a percentageof mid-oceanridge crustalproduction versus time. Thin line indicatescurve derivedfrom the work by Larson [1991b]. Thick line depictsresults of Coffin and Eldholm [1993]. The solid oceanicplateau (Kerguelen large igneousprovince (KERG LIP) and OntongJava (ONTO) LIP) columnsindicate off-ridge emplacement; solid and open parts depict on-ridge emplacement.Solid part of continentalflood basalt DECC and COLR) columnsindicates minimum rate; solidand openpart indicatesmaximum rates. Maximum and minimum rates were calculatedusing 1.5- and 11.5-m.y. emplacement periodsfor ColumbiaRiver basalts(COLR) and 1.0 and4.0 m.y. for Deccantraps (DECC). Compare with Figure 6. Solid horizontal bars indicate Cenozoic and Cretaceous mass extinctions [after Rampino and Stothers, 1988; Thomas, 1992]; note the correlation of COLR, DECC, and Kerguelen Plateau (KERG) LIP with extinctions. ONTO indicates OntongJava Plateauonly; ONTO LIP includesOntong Java and Manihiki plateaus,and Mariana, Nauru, and Pigafettabasin flood basalts.KERG indicatesKerguelen Plateau only; KERG LIP includesKerguelen Plateau, Elan Bank, and Broken Ridge.

mafic oceanic plateausis uncertain. No intrusive rocks consistprimarily of horizontal and subhorizontalflows. have been recovered from LIPs; these deeper rocks, Volcanicpassive margins, situated on the trailing, rifted presumably gabbros and ultramafics, are probably at edgesof continents,are characterizedby excessivevol- least as voluminousas the extrusive components. canismand by uplift and/or lack of rapid initial subsid- LIPs are distributed worldwide, occurring on both ence during continental breakup [e.g., Roberts et al., continentaland oceaniccrust in purely intraplate set- 1984; Eldholm, 1991]. Margin formation is associated tings, on present and former plate boundaries, and with both intrusive and extrusive activity [Larsen and along the edges of continents [Co•n and Eldholm, Marcussen, 1992; Skogseid et al., 1992a]. The oldest 1992] (Figure 1 and Tables 1 and 2). Continental flood oceaniccrust is thicker than adjacent "normal" oceanic basalts, the most intensively studiedLIPs, are pre- crust, and the lower crust beneath the extrusives is dominantly tholeiitic magmas erupted on continental commonly characterizedby bodies with high seismic crustover timescalesof 105-106years. Generally be- velocities [e.g., Mutter and Zehnder, 1988; White and lieved to be the product of fissure eruptions, they McKenzie, 1989; Eldholm and Grue, 1993]. 32, 1 / REVIEWS OF GEOPHYSICS Coffin and Eldholm- LARGE IGNEOUS PROVINCES ß 7

In deep ocean basins, the largest LIPs are oceanic ing, persisted thereafter. Such transient events, which plateaus: broad, more or less flat-topped features also include oceanic plateaus, have been commonly which generally lie 2000 rn or more above the sur- attributed to plume "heads" manifesting themselves rounding seafloor. They are formed by mafic volca- in the crust after arriving from the lower mantle [e.g., nism and associated intrusive activity; their extrusive Richards et al., 1989]. cover may be emplaced either in subaerial (e.g., Ker- Persistent magmatism resulting in LIP formation is guelen Plateau) [Coffin, 1992] or submarine (e.g., On- in most cases related to lithosphere moving over more tong Java Plateau) [Tarduno, 1992] environments. or less steady state mantle plumes or plume "tails" in Oceanic plateaus are commonly isolated from major the terminology of Richards et al. [1989]. Through continents. Their crustal thickness is anomalously Early Cretaceous and younger plate reconstructions greater than that of adjacent oceanic crust, and their [e.g., Duncan and Richards, 1991], each of several age may or may not be similar to that of adjacent crust. mantle plume sources, or hotspots, can be tied to a Another category of oceanic LIP is submarine ridges, group of LIPs, including (Figure 1) (1) the Iceland which are elongated, steep-sidedelevations of the sea- source (North Atlantic volcanic province including floor. They may be of continental or oceanic origin, North Atlantic volcanic margins and the Greenland- and we only consider the latter herein. Submarine Scotland ridge), (2) the Kerguelen source (Bunbury ridges are commonly characterized by varying topog- basalt (Australia), Naturaliste Plateau, Rajmahal raphy, and those of basaltic nature may be created traps, Kerguelen Plateau/Broken Ridge, Ninetyeast either on or off the axes of spreading centers. As with Ridge, and northernmost Kerguelen Plateau), (3) the oceanic plateaus, their volcanic upper crust may be R•union source (Deccan traps, western Indian volca- emplaced either subaerially or under water [Coffin, nic margins, Chagos-Laccadive Ridge, Mascarene Pla- 1992]. Closely related to submarine ridges are sea- teau, and Mauritius), and (4) the Tristan da Cunha mounts, which are local elevations of the seafloor. source (Paranti and Etendeka basalts, South Atlantic Seamounts are flat-topped (guyot) or peaked mafic volcanic margins, Rio Grande Rise, Walvis Ridge). volcanoes whose last stages of construction were in a The Iceland source has been active for at least 60 m.y. subaerial or a submarine environment, respectively. [e.g., Upton, 1988], the Kerguelen source for 135 m.y. They may be discrete, arranged in a linear or random [e.g., H. L. Davies et al., 1989], the R6union source grouping, or connected at their basesand aligned along for 65 m.y. [e.g., Courtillot et al., 1988], and the a ridge or rise. The least studied LIPs are ocean basin Tristan da Cunha source for 120 m.y. [e.g., O'Connor flood basalts, which are thick, extensive submarine and le Roex, 1992]. Each of these hotspots was at flows and sills lying above and postdating oceanic some point during its evolution temporally and spa- igneous basement. Recently, K. Hinz (personal com- tially associatedwith continental breakup. munication, 1992) has discovered thickened oceanic Not all LIPs, however, have obvious connections crust and dipping intrabasement reflectors in deep to hotspots. In particular, some volcanic margins, for ocean basins with linear seafloor spreading-typemag- example, the U.S. East Coast [Sheridan et al., 1993] netic anomalies. We term this crustal type anomalous and northwest Australian Cuvier [Hopper et al., 1992] seafloor spreading crust. margins, appear to have formed away from known Surface manifestations of the LIP types above hotspots. These observations, together with well-doc- record important spatial and temporal characteristics umented transient and persistent LIP-forming magma related to magma production. Many LIPs result from sources, suggestthat more than one source model may long-lived magmatic sources in the mantle, sources be required to explain LIPs in general and volcanic which initially transfer huge volumes of mafic rock margins in particular. into the crust over short intervals (• 1 m.y.) but which Mesozoic/Cenozoic LIP emplacement peaked near later transfer material at a far lesser rate, albeit over the middle of Cretaceous time (Figure 2) [e.g., Larson, long intervals (10-100 m.y.). 1991b]. To date, there has been no detailed explanation Transient magmatism during LIP formation is most as to why this peak occurred, although recent specula- clearly documented for continental flood basalts and tion has centered on heightened activity at the core- volcanic passive margins (Figure 1 and Tables 1 and mantle boundary (D" layer) preceding crustal emplace- 2). Numerous4øAr-39Ar dates, for example,show that ment of numerous LIPs in the Pacific Ocean [Larson, most basalts of the Deccan traps [e.g., Duncan and 1991a, b], includingthe largestknown igneousprovince, Richards, 1991] and Siberian traps [e.g., Campbell et the Ontong Java Plateau. Crustal production at mid- al., 1992] were erupted during -1-m.y. periods indis- ocean ridges has varied far less than LIP crustal pro- tinguishable from the Cretaceous-Tertiary and Per- duction over the past 150 m.y. [Larson, 1991b], sug- mian-Triassic boundaries, respectively. Similarly, the gesting that LIP production and seafloor spreading North Atlantic volcanic province and other volcanic reflect fundamentally different mantle processes. margins were emplaced during and immediately sub- LIPs are found in a variety of tectonic settings, sequent to continental breakup, but little or no volca- including the axes of seafloor spreading centers (e.g., nism, aside from that associatedwith seafloor spread- Iceland), triple junctions (e.g., Shatsky Rise) [Nakan- 8 ß Coffin and Eldholm: LARGE IGNEOUS PROVINCES 32, 1 / REVIEWS OF GEOPHYSICS

ishi et al., 1989], old oceanic lithosphere (e.g., Ha- is much more common than marine erosion. Thus waii), passive margins (e.g., North and South Atlantic uncertainty in the areas originally covered by conti- volcanic margins), and cratons (e.g., Siberian traps). nental flood basalts increases with increasing age of Those situated at spreading centers, such as Iceland, the province. The other two main categories of LIPs, Azores, and Galapagos, are readily distinguishable volcanicpassive margins and oceanicplateaus, suffer from "normal" oceanic crust by their anomalous vol- from severe undersampling of igneous basement and umes and geochemical signatures [e.g., Zindler and overlying sediment. For example, volcanic margins of Hart, 1986; Hart et al., 1992]. Recent discussion has the North Atlantic, the best studied of any worldwide, focused on whether mantle plumes and plate tectonics have been penetrated by fewer than ten drill holes of operate largely independently [e.g., Hill et al., 1992] or which only two have sampled more than 100 m of are intimately linked [e.g., Anderson et al., 1992a, b]; basement. Igneous basement of the two largest oce- no consensus has been reached. anic plateaus, Ontong Java and Kerguelen, which to- LIPs that are indisputably emplaced at plate bound- gether cover an area nearly half the size of the conter- aries include those now observed to lie on active minous United States, has been sampled at only seven spreadingcenters, those which are reliably dated to be sites, of which only one was drilled more than 100 m. the same age as adjacent "normal" oceanic crust, and Yet seismic refraction, reflection, and gravity data, volcanic passive margins. As higher-quality seismic necessaryfor investigatingcrustal structure and hence reflection data become available on passive margins, volumes of LIPs, are easier to obtain and thus more an increasing number appear to be "volcanic" [Coffin prevalent from oceanic than from continental LIPs. and Eldholm, 1992]; thus massive volcanism may be Furthermore, the simpler lithospheric setting of oce- quite common when continents separate. anic plateaus permits less ambiguous evaluation of Many LIPs, however, are difficult to associate with crustal structure than of continental flood basalts or plate separation. Part of the reason is that most oce- volcanic passive margins. To study temporal and spa- anic crust is Cretaceous or younger; nearly all older tial aspects of LIP development, we examine charac- oceanic crust with accompanying LIPs has been sub- teristic examples which are among the best known ducted. This makes tying older LIPs to younger LIPs LIPs (Figures 1 and 3): Ontong Java and Kerguelen- or to presently active hotspots difficult. The paucity of Broken Ridge oceanic plateaus, North Atlantic rifted hotspot tracks on continents, however, could suggest volcanic margins, and Deccan and Columbia River that plate separationsproduce favorable conditions for continental flood basalts. thermally anomalous mantle to upwell. Alternatively, continental lithosphere might be robust enough to de- LIP Parameters:Methodology flect all but the most vigorous mantle upwellings to On the basis of existing data and new calculations oceanic areas [Thompson and Gibson, 1991]. [Coffin and Eldholm, 1993] we determine the magni- Given the potential characteristics required for a tude and range of key LIP parameters. Seismic (e.g., mantle upwelling to reach the surface and form a LIP Figure 4), outcrop, and borehole data from LIPs show (the size of the mantle upwelling, its temperature, and that they are major constructionsof extrusive igneous its temporal variability, as well as the heterogeneous rock underlain by intrusive rock with crustal thickness vulnerability of the lithosphere), it is remarkable that ranging from 20 to 40 km. Despite scarce crustal data, hotspot trails are in some cases traceable for more an expanded crustal thickness and a lower crustal than 100 m.y. It further implies that the long-lived bodycharacterized by highvelocities (7.0-7.6 km s-1) tracks, including those emanating from Hawaii, Ker- appear typical for many LIPs (Figure 5). These fea- guelen, R6union, Iceland, and Tristan da Cunha, are tures exist on Hawaii, the best known mid-oceanic among the most stable and vigorous. LIP. Lower crustal bodies are believed to be added to the crust as part of the magmatic event, which also results in massive extrusive activity [Watts and ten 3. DIMENSIONS, CRUSTAL STRUCTURE, Brink, 1989; White and McKenzie, 1989]. AGES, AND EMPLACEMENT: A STUDY OF FIVE LIPS Surface areas of the five LIPs examined are either calculated or taken from published work (Figures 3 Evaluation of how LIP emplacements affect Earth and 6 and Table 3). For the giant oceanic plateaus we history depends on key parameters, especially size, determine LIP boundaries from bathymetry. For on- composition, emplacement rate, and emplacement en- shore and offshore portions of the North Atlantic vol- vironment. Information on these parameters in differ- canic province, we employ published values based on ent provinces is highly variable, and each LIP cate- drilling results and seismic data. Areas of continental gory offers particular advantages in understanding flood basalts are also taken from the literature. These LIPs as a whole. Continental flood basalts provide by estimates do not take into account any rock that may far the most data on composition and age. The areal have been eroded. We compile both present and extent of continental LIPs is easier to measure than preerosional estimates for the Deccan traps and that of submarine LIPs, but subaerial erosion of basalt present estimatesfor the Columbia River flood basalts. 32, 1 / REVIEWS OF GEOPHYSICS Coffin and Eldholm' LARGE IGNEOUS PROVINCES ß 9

River basalts (COLR), Dec- -. 'x,,x cantraps (DECC), East Mar- ; J ,•-'•••.; "•. •0o iana(EMAR),BasinElan Bankflood (ELAN), basalts '; •' //', HawaiianKerguelen IslandsPlateau (HAWA),(KERG),

Nauru Basin flood basalts (NAUR), North Atlantic vol- _ 0canicManihiki provincePlateau (NAVP),(MANI), On- •"•. • '- _KE • tongJava Plateau (ONTO), • •__-PI.AT•S/LrrIG '0•.•----•.ø ELAN,,• , • •- -600 andsaltsPigafetta (PIGA).Basin Digital flood mapba- courtesy of the Plates project (Institute for Geophysics, University of Texas, Austin).

LIP volumes are calculated from areal and vertical on seismic data and interpretations for quite some dimensions,the latter obtained primarily through seis- time. It is important to bear in mind that only the mic reflection and refraction investigations of LIP uppermost extrusive components of all of these LIPs crustal structure. For the upper crust these geophysi- have been sampled and dated; intrusive material forms cal data are commonly correlated with outcrop and/or a large proportion of volcanic passive margins and drilling results to arrive at structural and stratigraphic oceanic plateaus, if not continental flood basalts, and interpretations. Absence of such data from the deeper remains unsampled and undated. crust forces geological interpretations to rely more on Another approach for determining oceanic LIP di- ideas and forward models. Given that drilling has not mensionshas been to use global topographic data and yet penetrated an entire crustal section [e.g., Dick et assumptionsof Airy isostasy [Schubert and Sandwell, al., 1992], even where it is relatively thin, for example, 1989]. The latter method, however, is unable to distin- along the flank of the East Pacific Rise, our knowledge guish different age provinces within an individual LIP, of the crust of much thicker LIPs is likely to rely solely to uniquely sensethe presence of lower crustal bodies,

Figure 4. Dipping reflector sequences (below thick black lines) consisting primarily of basalt flows on (top) the Kerguelen oceanic plateau and (bat- tom)the Norwegian volcanic passive margin.The dippingwedge aft Nor- way was sampled at Ocean Drilling Project site 642; na dippingreflectors were drilled an Kerguelen.

lO km 10 ß Coffin and Eldholm- LARGE IGNEOUS PROVINCES 32, I / REVIEWS OF GEOPHYSICS

7.1 km thick [White et al., 1992] and no preexisting oceanic crust, respectively-. Furthermore, transient 1 '..-.:..::::.::•::.....:....::...... ,,..,•..•ß Moho events may have generated excess crust outside the 2 •HAWAII S• NE 0-'ONTONG JAVA plateausproper. Where adjacent coeval features exist, '[SW--' ..... ---" NE we include them in our calculations (Figures 3 and 6). Seismic refraction data show a lower crustal body beneath the Columbia River basalts [Catchings and Mooney, 1988]. To calculate crustal volume beneath o KERGUELEN E S..-•.•_• BROKEN RIDGE the extrusive cover, we assume an identical configuration beneath the entire province (Figure 5). A similar procedure is applied to the Deccan traps. Crisp [1984] examined ratios of intrusive to extrusive mafic rock volumes worldwide and documented a loosely con- NORTH ATLANTIC strained range of 3:1 to 16:1. We note that our calcu- 1 NWCO• SE lated volumes for the intrusive components of the two • NEGreenland •..• • V•ring continental flood basalt provinces fit within this range ,-• o, '•::• •vioho (Table 3). Finally, we use the volumes of Eldholm and Grue [1993] for the conjugate North Atlantic volcanic o- DECCAN margins. lO- We employ published data and results in assessing the ages of the five LIPs examined. Radiometric, biostratigraphic, magnetostratigraphic, and marine mag- 30- netic anomaly methods have all been employed; the 40. adventof the4øAr/39Ar mineral separate technique has begun to have a particularly large impact on the dating o- COLUMBIA RIVER of LIPs. More precise and reliable dates [e.g., Renne SW NE 10. et al., 1992] will allow more accurate estimates of emplacement durations of LIPs and the correlation of 2o- ;':-• LCB their emplacement with other geological, biological, 3o- oceanographic,and environmental events. At present, 4O- '•Moho• Sediment however, dating of submarine LIPs is limited by a VE: 10:1 dearth of samples rather than a lack of appropriate 5O i i i I i i 6•0 400* •o o •o •o •o ' techniques. Emplacement rates are calculated using our volume determinations and published age information. Given Figure 5. Simplified crustal structuresof five LIPs examined existing data, we believe our estimated dimensionsof plus Hawaii. LCB indicates the lower crustal body charac- the five LIPs are the best now available, despite sig- terized by high (7.0-7.6 km s-•) seismicvelocities. Open nificant uncertainties. Nonetheless, absolute values areas above lower crustal bodies indicate other igneous crust will undoubtedly change with new data. and possible sedimentary and metamorphic rocks beneath Below we describe and discuss parameters of rela- Deccan traps and Columbia River basalts. For Ontong Java tively well-sampled LIPs representing each of the Plateau, crustal thickness is maximum Airy compensation depth [Sandwell and Renkin, 1988]. For North Atlantic vol- three major categories: oceanic plateaus, volcanic canic province, dipping reflector wedges are outlined, and margins, and continental flood basalts. continent-ocean boundaries (COB) are indicated. Giant Oceanic Plateaus The Ontong Java Plateau (Figure 3) is bounded to or to calculate volumes of ocean basin flood basalts. It the southwest by the north Solomon trough and else- also relies on more assumptionsthan the method com- where is defined by the 4000-m contour [International bining seismic, topographic, and samplingdata, which HydrographicOrganization/Intergovernmental Oceano- we prefer to use on the five representative LIPs (Fig- graphic Commission/Canadian Hydrographic Service ure 6 and Table 3). (IHO/IOC/CHS), 1984]. It includes only that part of For oceanic plateaus we calculate crustal volumes the Caroline seamount chain which impinges on the from available refraction and geoid studies (Figures 5 northwestern plateau. The Ontong Java Plateau sensu and 6 and Table 3), employing Hawaii as a template in strictoencompasses 1.86 x 106 km2, roughlyone-third determining geometries of the lower crustal bodies and of the conterminous United States (Figure 3 and Table Moho. Volume calculations for oceanic plateaus de- 3). Datable igneousrock has only been recovered from pend strongly on whether the features formed off or on three scientific drill sites on the Ontong Java Plateau ridge; therefore we calculate for both cases, assuming (289, 803, and 807), one in the Nauru Basin (462), one 32, 1 / REVIEWSOF GEOPHYSICS Coffinand Eldholm' LARGEIGNEOUS PROVINCES ß 11

Volume Area

before erosion

Figure 6. LIP areas, volumes, crustal emplacement rates, and emplacement rates relative to mid-ocean ridge system (last values from Larson [1991a]). Minimum values are indicated by EmplacementRate LIP/Mid-Ocean Ridge Emplacement Rate solid columns; solid and open parts depict maximum values. Key to shadingand abbreviations as in Figures 2 and 3.

.• 8

in the East Mariana Basin (802), two in the Pigafetta tains (Figure 1), which suggeststhat the Ontong Java Basin (800 and 801), and one on the Manihiki Plateau LIP (sensu lato) may be larger still. (317). Mahoneyet al. [1993] reportthat 4øAr/39Ar Furumoto et al. [1976] determined crust as thick as dates of igneousbasement at Deep Sea Drilling Project 42.7 km on the central Ontong Java Plateau from (DSDP) site 289, Ocean Drilling Program (ODP) site refraction data. Maximum average thickness is 39 km 807, and on Malaita Island are all •122 Ma. Igneous includinga •16-km-thick lower crustal body (7.4-7.7 basement at DSDP site 803, however, yielded a date of km s-•). They noted,however, that the free-airgravity •90 Ma, suggestingthat somevolcanism continued for value of + 15 mGal differs by •200 mGal from that 30 m.y. or more. Tarduno et al. [1991] have argued expected from the crustal structure. Sandwell and that the plateau is the same age (early Aptian) as Renkin [1988] examined satellite geoid data and topog- Nauru Basin flood basalts and the Manihiki Plateau raphy and determineda maximumAiry compensation based on radiometric, magnetostratigraphic,and bios- (Moho) depth of 25 km. Basalt flows and sills have tratigraphic dating of sediment and basement. Ac- been identified in the adjacent Nauru [Shipley et al., knowledging uncertainty in absolute ages, they pre- 1993], East Mariana, and Pigafetta [Abrams et al., ferred emplacement between 121 and 124 Ma 1993] basins, but no deep crustal data are available (timescale of Harland et al. [1990]), although they from them. Refraction Moho beneath the Manihiki raised the possibility of emplacementbetween 117.7 Plateau has not been determined [Sutton et al., 1971]; and 118.2 Ma (timescale of Harland et al. [1982]) studies of global topography assumingAiry isostasy (Table3). Although4øAr/39Ar dates of ODP samplesin have estimated an average crustal thickness of only the Pigafetta and East Mariana basins [Pringle, 1992] 10.8 km [Schubert and Sandwell, 1989]. differ from those of Tarduno et al. [1991] by <7 m.y., Crustal thickness is thus uncertain because of dif- their proximity and similarity to Nauru basaltssuggest fering results from seismic refraction experiments that they may belong to the same LIP, which would (<42.7 km) and gravity studies (•25 km). Because of affect 4.9 x 106 km2, or closeto 1% of the Earth's this, we have calculated volumes for both thicknesses surface (Figures 1, 3, and 6 and Table 3). Recently, (Table 3). Neither of these estimatesinclude crust that Winterer [1992] has reported a date of •121 Ma for may have been subducted.For comparisonwith other Resolution guyot in the western Mid-Pacific Moun- LIPs, however, we employ the conservative value 12 ß Coffinand Eldholm-LARGE IGNEOUS PROVINCES 32, 1 / REVIEWSOF GEOPHYSICS 32, I / REVIEWS OF GEOPHYSICS Coffin and Eldholm: LARGE IGNEOUS PROVINCES ß 13

(Figure 6). For the Manihiki Plateau, the same age as Ridge and Kerguelen Plateau. Turonian (88.5-91 Ma) Ontong Java [Tarduno et al., 1991], we assumed a sediment recovered from Broken Ridge [Peirce et al., maximum value of 25 km (Table 3), identical to the 1989; Resiwati, 1991], however, implies older base- lower value for Ontong Java. Volume estimates of ment. Furthermore, Royer and Sandwell's [1989] plate basalt flows and sills are available for the adjacent model suggeststhat rifting between Broken Ridge and Naum [Shipley et al., 1993], Pigafetta, and East Mar- Kerguelen Plateau continued from Early Cretaceous iana basins [Abrams et al., 1993], but it is not possible to Eocene time; we conclude that both age intervals to estimate volumes of any deeper crustal compo- [Duncan, 1991] represent rifting events and that the nents. For the entire Ontong Java LIP, calculated basement age of Broken Ridge is similar to that of crustalaccretion rates are 12.1and 18.0km 3 yr-• for Kerguelen Plateau. off- and on-ridge emplacement, respectively (Figure 6 Seismic refraction experiments on the Cretaceous and Table 3). Using refraction, Moho for the Ontong Kerguelen Plateau [Charyis and Operto, 1992] indicate Java Plateau would increase these rates by --•60 and 23- to 25-km-thick crust (Figure 5), including a variable •40%, respectively. thicknesslower crustalbody (7.3 km s-•). Refraction Three drill sites on the Ontong Java Plateau recov- crustal thickness concurs with results of gravity mod- ered tholeiitic basalt presumed to represent basement. eling, which indicate 20- to 23-km-thick crust [Houtz et The basalt appears to record high degrees of partial al., 1977]. We do not consider refraction results from melting (--•30%), and isotopic signatures indicate a the Kerguelen Islands, because Quaternary volcanism hotspot-like mantle source [Mahoney et al., 1993]. No suggestsa different thermal and, probably, a different shallow water sediment overlies basement at these seismic crustal structure. Refraction data from Broken drill sites [Tarduno, 1992], and at each site, basalt Ridge [Francis and Raitt, 1967] reveal a 20.5-km-thick apparently erupted in a submarine environment crust, including a 5.9-km-thick lower crustal body (7.3 [Berger et al., 1992]. Seismic data do not indicate km s-•). The differencein our on-ridgevolume (Figure subaerial exposure and erosion of basement [Kroenke 6) and the ---57 x 106 km3 value of Schubert and et al., 1991], but subsidenceanalysis indicates that at Sandwell [1989] occurs mainly because we considered least part of the Ontong Java Plateau was emplaced only the Cretaceous portions of the features. A 4.5- near sea level [Tarduno, 1992]. m.y. igneous event results in emplacement rates for Kerguelen Plateau and Broken Ridge are conjugate, the entireLIP of 3.4 and 5.4 km3 yr-•, dependingon Early Cretaceous LIPs in the Indian Ocean (Figures 1 whether it was constructed off- or on-ridge (Figure 6). and 3) which separated in Eocene time [Houtz et al., Four drill sites on the southern Kerguelen Plateau 1977; Mutter and Cande, 1983]. Cretaceous parts of (738, 747, 749, and 750) recovered basalts, mostly the features are difficult to distinguish on the basis of tholeiites, interpreted as basement. A lava flow drilled bathymetry (Schlich et al. [1987] for Kerguelen and at ODP site 748 is an alkali basalt. The rocks show IHO/IOC/CHS [1984] for Broken Ridge) alone, be- large geochemical and isotopic variations; however, cause they are continuous with younger parts. How- with the exception of site 738 basalts they appear ever, using dates from rock samples (Table 3), plate hotspot related [Storey et al., 1992]. Extreme isotopic reconstructions [e.g., Royer and Sandwell, 1989; compositions of a site 738 tholeiite may indicate sub- Royer and Coffin, 1992], and changes in bathymetry, continental lithospheric contamination [Alibert, 1991]. we interpreted the break between Mesozoic and Ce- Terrestrial, terrigenous, and/or shallow-water sedi- nozoic parts to be just south of the Kerguelen Islands ment overlies basement at two of the drill sites (738 and between Broken and Ninetyeast ridges. Elan and 750), and at the other two sites (747 and 749) a Bank, a distinct structural entity of unknown age ad- significant hiatus separates basement and sediment jacent to the southern Kerguelen Plateau [Coffin et al., [Barron et al., 1989; Schlich et al., 1989]. Weathering 1986], was treated independently but assumed to be characteristics of the basalt at all four sites hnd sub- part of the Early Cretaceous Kerguelen Plateau. To- sidence analysis suggest subaerial or shallow water gether, the two conjugate Cretaceous features cover emplacement [Coffin, 1992] in a near-tropical to tem- 2.3 x 106 km2 (Figures3 and6 andTable 3). perate climate with high orographic rainfall [Holmes, Whitechurchet al. [1992]employed 4øAr?Ar tech- 1992]. Angular unconformities observed on seismic niques on whole rock ODP basement samples and reflection data (Figure 4) support subaerial exposure obtained a wide variety of dates but concluded that the and erosion of basement [Coffin et al., 1990], which bulk of the plateau formed at --•110Ma (Table 3). Their may have lasted as long as 50 m.y. following emplace- most reliable date, 109.5 Ma, and the 114-Ma K-Ar ment [Coffin, 1992]. basement date of Leclaire et al. [1987] have been used to bracket the igneous event. Duncan [1991] deter- North Atlantic Volcanic Margins mined two 4øAr-39Arage rangesfor Broken Ridge Widespread occurrence of early Tertiary continen- dredge samples, 83-89 and 62-63 Ma, and proposed tal flood basalts comprising the North Atlantic volca- that the ridge formed during the older interval, while nic province (Figures 1 and 3) ranks it as a major the younger dates mark early rifting between Broken continental flood basalt [Dickin, 1988]. Nonetheless, 14 ß Coffin and Eldholm' LARGE IGNEOUS PROVINCES 32, 1 / REVIEWS OF GEOPHYSICS

Figure 7. Extent of early Tertiary basalts plotted on a reconstructionat ---3 m.y. after breakup [after Eldholm and Grue, 1993]. Double circle indicates initial Iceland hotspot location interpreted by White and McK- enzie [1989]. BB indicates Baffin Bay basalts.

-lO o

its area is smaller than that of the Columbia River not be characterized by dipping and/or layered reflec- basalts,and its volumeis only---2.3 x 105km 3. Drilling tions; these wedges are only the most prominent and and seismic data show that subsequent to Late Creta- recognizable component of the magmatic events. ceous-Paleocene uplift coeval with rifting (synrift), Temporally, the igneous activity has two compo- continental breakup was accompaniedby massivevol- nents. Along an almost 3000-km-long rifted plate canic activity which formed oceanic crust above or boundary, lavas were extruded largely subaerially be- just below sealevel alongthe almost3000-km-long rifted tween 57.5 and 54.5 Ma (timescale of Berggren et al. plate boundary (Figure 7) [e.g., Eldholm et al., 1989]. [1985]), with most lavas having erupted during chron Calculations of North Atlantic volcanic province 24r. This transient component contrasts with the cen- dimensions, including the coeval volcanic margins, tral region, where conjugate continental flood basalts show its main part lies offshore [Eldholm and Grue, are connected by the broad submarine ridge on which 1993]. By including continental crust covered by flood Iceland lies (Figures 1 and 3). The ridge reflects the basalt and oceanic crust emplaced during the transient persistentsubaerial volcanism over ---60 m.y. that is event, the area presently covered by extrusives is commonly attributed to the Iceland plume. > 1.3 x 106 km2 (Figures6 and 7 and Table 3). The On the Norwegian margin, site 642 on the inner total crustal volume of---6.6 x 106 km 3 is based on dippingwedge (e.g., Figure 4) and 643 on crust formed velocity profiles along margin transects (e.g., Figure 5) ---3 m.y. after breakup have provided key information and includes expanded oceanic crust, parts of the about early Tertiary environments [Eldholm et al., extrusive cover and the lower crustal body extending 1989]by penetratingan entire (800 m) seaward-dipping landward of the defined continent-ocean transition, wedge below early Eocene basal sediment. This sedi- and onshore flood basalts. Our values are minima, be- ment includes continental soils formed under a hot, cause the amount of melt intruded into continental crust seasonablyhumid climate overlain by restricted ma- after breakup cannot yet be calculated. Nevertheless, rine facies [Thiede et al., 1989]. Volcanism abated 2-3 the North Atlantic volcanic province ranks among the m.y. after breakup, and the injection center sub- world's most prominent volcanic margin LIPs. merged with sporadic but waning volcanism. Subse- Sediment-covered lavas on the margin form a quently, a deepwater injection center developed, leav- smooth acoustic basement locally underlain by intra- ing an isostatically compensated igneous complex basement reflectors, including the series of seaward trailing behind new oceanic crust accreted at a normal dipping reflectors typical of volcanic margins (Figure spreadingaxis. As the ocean basin matured, the high 4) [Hinz, 1981; Coffin and Eldholm, 1992]. The igneous subsided at rates similar to normal oceanic crust, event also included sills and dikes in the continental maintaining the difference in basement relief. crust, as well as subextrusive thickened crust which in The wedge is composed of transitional mid-ocean most places contains a lower crustal body (Figure 5). ridge basalts and altered, interbedded, basaltic vitric We note that igneous crust associated with volcanic tuffs. Many flows have reddened tops, while some passivemargins in particular and LIPs in general, need flows and sediments indicate shallow marine deposi- 32, 1 / REVIEWSOF GEOPHYSICS Coffinand Eldholm'LARGE IGNEOUS PROVINCES ß 15

tional environments. Subaerial and neritic environ- bia River basalts(Figures 1 and 3) once covered 1.64 x ments are inferred during and subsequentto breakup 105km 2 (Figure 6 and Table 3) With an extrusive when the wedge was constructedby an intensephase volumeof 1.74 x 105km 3. A seismicrefraction profile of explosive, subaerialvolcanism. The basalt rests on across the Columbia Plateau shows a lower crustal daciticlavas, emplacedby infrequenteruptions during body(7.5 km s-1) (Figure5) interpretedas a "rift pil- the late rift stage, and interbeddedsediments indicat- low" [Catchings and Mooney, 1988]. To calculate ing fluvial or shallow-water deposition. crustal volume beneath the extrusive cover, we assumed an identicalconfiguration beneath the entire province. Continental Flood Basalts The total LIP volume amountsto 1.3 x 106km 3. TheDeccan traps, covering ---5 x 105km 2 in west- The great majority of Columbia River basalts are ern India (Figure3), may havecovered >1.5 x 106 tholeiites, although some flows are characterized by km2 priorto erosion(Figure 6 andTable 3) [Mahoney, transitional to alkali basalt mineralogy [Hooper, 1988]. 1988]. These estimates do not include any offshore Variable isotopic values and trace element ratios re- componentof Deccanmagmatism, which is suggested quire a variable source,either in the form of a heter- by seaward-dippingreflectors on the Indian margin ogeneousmantle or different degreesof crustal con-

[Hinz, 1981]. At the time of eruption, the Seychelles taminationß or both. Basalts range in age from 6 to 17.5 were attached to India; the coeval basalt there is esti- Ma, but most erupted between 15.7 and 17.2 Ma matedto cover2.5 x 105km 2 [Deveyand Stephens, [Baksi, 1989]. Corresponding emplacement rates are 1992].Apparently, the Deccan volcanicevent covered 0.1-0.9 km3 yr-• (Figure6). a much larger area than reported in the literature, althougha paucity of data offshoreIndia and on the Discussion MascarenePlateau preclude estimatingboth the area Areas and volumes of transient LIPs (Figure 6), as and volume of the total event. well as previousestimates for other continentalflood The Deccantraps are predominantlytholeiitic basalts, basalts[e.g., White and McKenzie, 1989], reveal vari- althoughalkalic, felsic, and ultramaficintrusive suites ations over ! to 2 orders of magnitude. Crustal struc- form lithologicallydistinct but volumetricallynegligible tures of the three main LIP types are similar, compris- components[Mahoney, 1988].Trace elementsand iso- ing an extrusive upper crest (Figure 4) and a lower topic ratiosvary considerablywithin the province,and crust(Figure 5) characterizedby highseismic veloci- bulk compositionschange as well, but most evidence ties (7.0-7.6 km s-•), and differfrom "normal" oce- suggeststhat the depletedsource of most tholelitesis anic or continental crust. Thus these LIPs may share a homogeneous,arguing that continentallithosphere is not common genesis.Construction of the largest igneous the major basaltsource. According to Biswasand Tho- provincesduring transient magmatic episodes signifi- mas [1992], most basalt eruptedbetween 65 and 69 Ma, cantly increasesglobal crustal productionrates (Fig- while Baksi [1990] and Vandamme et al. [1991] suggesta ure 2). The North Atlantic volcanic province and re- periodof < 1m.y. (Table 3). An 4øAr-39Ardate of 64Ma cent studies of the U.S. East Coast continental margin hasbeen reported from MascarenePlateau basalts •500 [Holbrook and Kelemen, 1993] show that volcanic km southwestof the Seychelles(Figure 1 and Table 1) marginsmust be included in the LIP inventory. [Duncanand Hargraves, 1990],implying that a signifi- Compressionalwave velocities of 7.0-7.6km s-• in cantportion of thatfeature may have been created by the lower crustal bodies are not those of normal oceanic or transient Deccan event. continental crust, although they are found in some Verma and Banerjee [1992] have reviewed deep continentalsettings, mainly beneathrifts [e.g., Catch- seismic reflection data and modeled gravity data ingsand Mooney, 1988].Composition of lower crustal across the Deccan traps. Interval velocities show bodies on volcanic margins is unknown; gabbroic, lowercrustal bodies (7.0-7.3 km s-I), but thicknesses stronglymafic, and hot ultramaficrocks are possibili- are not well defined. Gravity modeling suggests an ties [Meissner and K6pnick, 1988]. Some lower crustal anomalouslyhigh density crustal body beneath the bodieshave been explainedas magmaticunderplating traps, but geometry is poorly constrained.We esti- by accumulatingmantle-derived material below the mated a lower crustal body volume by applying the originalcrust [Large ApertureSeismic Experiments ratio of surface area to deep crustal volume for the (LASE) Study Group, 1986; White et al., 1987]. Un- Columbia River basalts to the original surface area of derplatingsuggests emplacement of mafic magma at the Deccan flood basalts, excluding the Seychellesand the base of continental crust [Furlong and Fountain, the Mascarene Plateau (Figure 6). The total Deccan 1986].Density contrastsbetween the old crust and the trapsvolume of 8.2 x 106 km3 conservativelyincludes melt determine whether melts are ponded below the only extrusive rocks and lower crustal bodies. If the crust or intruded into the crust [Herzberg et al., 1983]. Deccan traps formed during 1- and 4-m.y. intervals, For example, White and McKenzie [1989] suggested emplacementrates were 8.2 and2.1 km3 yr-• respec- that decompressionalmelting of hot asthenospheric tively (Figure 6 and Table 3). mantlewould produce 7.1- to 7.2-kms -• velocitiesin Tolan et al. [1989] have calculated that the Colum- pondedbasaltic melt becauseof increasedMgO con- 16 ß Coffin and Eldholm: LARGE IGNEOUS PROVINCES 32, 1 / REVIEWS OF GEOPHYSICS

tent. Thus underplating should only occur beneath 4. MANTLE DYNAMICS continental crust, contradicting observed lower crustal body continuity into oceanic crest in the North Atlan- LIPs are crustal manifestations of dynamic pro- tic [Eldholm and Grue, 1993] and off the U.S. East cesses in the Earth's mantle; hence LIP parameters Coast [LASE Study Group, 1986; Tr•hu et al., 1989]. may be applied as boundary conditions to invert such The underplating hypothesis suggeststhat melts are processes. Presently, no consensus has developed trapped by a crustal density filter during rifting. Fol- concerning the nature and dimensions of mantle circu- lowing breakup, no such filter exists in oceanic crust, lation required to emplace LIPs. In general, however, although similar anomalous velocity-density struc- more effort has been devoted to development of man- tures are found beneath oceanic crust adjacent to vol- tle models that may produce transient and/or persis- canic passive margins. Clearly, the underplating hy- tent magmatism than to verification of such models pothesisdoes not adequately explain the occurrenceof through tests employing geological data. Despite the lower crustal bodies beneath oceanic crest. It is also fragmentednature of the LIPs global database,data exist arguable whether magmatic underplating takes place that directly or indirectly relate LIPs to mantle pro- when the crust is strongly attenuated, for example, cesses.Particularly relevant are (1) dimensionsand em- during the late rift stage, because crustal strength placement rates (see section 3), (2) composition of the rapidly becomes negligible when massive mafic up- igneous complexes (see sections 2 and 3), (3) spatial welling reaches the brittle crust, causingthe assump- location relative to known hotspots(see section2), and tion of conductive heat transfer to break down [Dixon (4) geologicalsetting prior to and duringemplacement (in et al., 1989]. Although the lower crustal body is most particular, the relationshipof LIPs to rifting and other types of lithosphericdeformation) (see section2). likely emplaced during breakup, we consider the term Transient LIPs exist on many scales, reflecting a "underplating" a misnomer. wide size range of mantle melt anomalies. Recognition Uncertainty in absolute dimensions, however, is of this dimensional variability is fundamental in eval- dramatically illustrated by the biggest LIP, Ontong Java. We estimate a conservative volume based on uating mantle processesleading to LIP emplacement. Volumetric variationsamong LIPs (Figure 6 and Table 3) Airy isostasy, off-ridge emplacement, and a 3-m.y. are influenced by at least three factors: mantle source magmatic event leading to an emplacementrate of 12.1 intensity, vulnerability of the lithosphere to asthenos- km3 yr-• (Figure6). On-ridgeemplacement and refrac- pheric penetration, and lithospheric plate speed over tion Moho might increase the rate by a factor of 3 to the source. Seismic tomography provides a broad, --•36km 3 yr-•. In addition,emplacement rate is par- three-dimensional measure of source intensity, that is, ticularly sensitive to construction duration. For exam- volumesand relative magnitudesof asthenosphericther- ple, the alternate 0.5-m.y. duration considered by Tar- mal anomalies. The method outlines such features as duno et al. [1991] results in an off-ridge emplacement mid-oceanridges, dead slabs, and cratons. Global stud- rate of 72.8 km3 yr-•. We stress,however, that well- ies indicate that the upper mantle is dominatedby sub- dated basement at four sites over an area greater than duction, whereas upwelling appears to dominate circu- half the size of the conterminous United States and lation in the lower mantle. It appearsthat the main mode penetrating, at most, the upper --•0.5% of the total of mantle convection is flow controlled and scaled by igneous section does not rule out a somewhat longer plates [e.g., Dziewonski and Woodhouse, 1987;Ander- emplacement period, which would result in values son et al., 1992a; Davies and Richards, 1992]. resembling those of other LIPs. Lithospheric vulnerability is somewhat difficult to Comparison of LIP crustal production with that of assess;it was once thought that younger lithosphere the global mid-ocean ridge system over the past 150 should be more vulnerable than older lithosphere to m.y. [Larson, 1991 a] (Figures 3 and 6) shows that the asthenosphere-derivedvolcanism [Gass et al., 1978; largest LIPs had the potential to significantlyalter the Pollack et al., 1981; Vogt, 1981], but recent analyses hydrosphereand atmospherethrough their mass, heat, of present-day heat flux from oceanic lithosphere sug- chemical, fluid, and particulate fluxes during isolated gest rather a correlation between plate speed and vul- time periods [e.g., Larson, 1991 a, b; Eldholm, 1990]. nerability [Sleep, 1990; McNutt, 1990]. Absolute In this context, the extmsive crustal component is a global plate motions in the hotspot reference frame are key factor [e.g., Officer et al., 1987], and extrusive well constrainedfor the past --•90 m.y. [e.g., Miiller et eruption rates for Deccan traps, North Atlantic volca- al., 1993]; plate speed over the source does not appear nic margins, and the two giant plateaus show less to influence the volumes of transient features. How- variability(0.3-2.6 km3 yr-•) than do total crustal ever, lithospheric structures, for example, fracture production rates [Eldholm and Grue, 1994]. This ob- zones and rifts, may act as preferential conduits for servation, and the temporal correspondenceof Deccan asthenosphericmelts [Sykes, 1978; Okal and Batiza, and North Atlantic eruptions with major environmen- 1987;McNutt et al., 1989]. In most intraplate settings, tal change, may imply that LIPs smaller than Ontong melting of the plume head is inhibited by the > 125- Java have also contributed to these changes. km-thick mechanical boundary layer. Therefore Kent 32, 1 / REVIEWS OF GEOPHYSICS Coffin and Eldholm: LARGE IGNEOUS PROVINCES ß 17

et al. [1992] proposed a plume incubation period to to lithospheric extension; maximum melting occurs dur- thin and partly remove the boundary layer prior to ing crustal breakup. This model has both "active" and massive continental flood basalt production. "passive" elements and has been used to explain continental flood basalts and volcanic margins. A similar Mantle Convection and Plumes effect may be achieved if hot asthenosphereis trapped The original hotspot/mantleplume concept [Wilson, by a relict lithospheric relief, or "thin spots," from 1963; Morgan, 1971] has recently been developed fur- previous tectonic episodes [Thompson and Gibson, ther. Although models for structure and temporal ev- 1991; Skogseidet al., 1992a]. olution of mantle plumes vary considerably, a com- Another concept, rooted in seismic tomography, monly observed feature is the capability of a plume to relies on a thermally, chemically, and isotopically het- generate large quantities of melt by decompressionof erogeneous asthenosphere [e.g., Anderson et al., upwelling, thermally anomalous mantle. Where the 1992b]. Initially weak cratonic lithosphere or litho- thermal anomaly is associatedwith continentalbreakup, sphere weakened by plate reorganizations may be in- it may induce the formation of a volcanic margin and vaded by material from hot regions of upper mantle, adjacent continental flood basalts distinguishedby tran- allowing melts to surface and produce LIPs. sient magmatism over a wide region. If the plume ini- These models reflect fundamentally different pri- tially surfacesthrough oceanic lithosphere,an oceanic mary convection systems in the mantle (Figure 9). plateau may form, and as the plate migrates over the Davies and Richards [1992], for example, visualize focus of upwelling, a submarineridge and/or seamounts "whole mantle" circulation and interpret the mantle may be constructed. Finally, interaction of plumes and transition zone at the base of the asthenosphere (670 thick continental lithospheremay, under certain condi- km depth) as an isochemical phase change. On the tions, allow emplacementof a continentalflood basalt. other hand, the "layered mantle" circulation model Several plume models which induce a broad region assumes the mantle transition zone to be a thermal of hot mantle beneath the lithosphere have been ad- boundary layer. Anderson et al. [ 1992b] state that the vanced to explain emplacement of LIPs (Figure 8). major predictions of the deep plume model are untest- One concept [Richards et al., 1989; Griffiths and able and that geophysical techniques cannot resolve Campbell, 1990], primarily based on results of labora- the narrow, 10- to 200-km-diameter plume tails. Fur- tory experiments, invokes a secondarymode of mantle thermore, they claim that the effects of the large flat- convectionin addition to the dominantplate scalemode. tened plume heads, which seismic tomography only The former initiates LIP-forming, upwelling, buoyant images to 200- to 300-km depths, may be produced by plumes which detach from the weak, heterogeneous a number of phenomena consistent with upper mantle thermal boundary layer D" at the base of the mantle. convection. Courthey and White's [1986] plume model Such plumes have a large, hot head over a narrow stem is also restricted to the upper mantle. (Figure 8a). When the deep plume impingeson the me- Mantle plumes in various forms represent the most chanical boundary layer at the base of the lithosphere, plausible mechanism for explaining the large amounts conductive heating and thinning of the lithosphere in- of thermal energy required by massive melting anom- duce large-scalemelting. The model suggests"active" alies. Although mantle plumes provide a convenient rifting, that is, stressand strain are transferredfrom the source for the excess melting, observational data are plume to lithosphericplates. Uplift precedesand accom- required in order to examine whether focused mantle panies volcanism, but rifting may postdate the main plumes are necessary to generate LIPs. In particular, volcanic event [Griffiths and Campbell, 1991a; Hill, the concept that all LIPs are connected to hotspots 1991; Hill et al., 1992]. The "active" plume model also [White and McKenzie, 1989] must be tested. Mutter et suggestsoccasional coupling between core, mantle, and al. [1988] claim that some volcanic margins cannot crustal processes.Although not covered in this review, convincingly be associated with known mantle magnetic field reversals, presumably originatingin the plumes. For example, the U.S. East Coast margin core, have been related to major plate tectonic changes [Holbrook and Kelemen, 1993] and the Cuvier margin [e.g., Vogt, 1975] as well as LIP emplacements [e.g., off northwest Australia [Hopper et al., 1992] are can- Larson, 1991a, b; Larson and Olson, 1991], reviving and didates for nonhotspot LIPs. Furthermore, the North refining the ideas of Vogt [1972]. Atlantic volcanic margins extend beyond the maxi- Another plume model [Courthey and White, 1986; mum diameter of the thermal anomaly predicted from White and McKenzie, 1989], developed from volcanic common plume models [Eldholm and Grue, 1993], an margin crustal structure and petrologic modeling, pro- argument that may also apply to the greater Ontong poses adiabatic upwelling and decompressional melt- Java LIP [Coffin and Eldholm, 1993]. Vogt [1991] has ing of hot asthenosphereas a result of extending litho- pointed out that most of the midplate igneous activity sphere. The plume-derived thermal mushroom (Figure and all topographic swells in the western North Atlan- 8b) causesdynamic uplift which accelerates the rate of tic and eastern North American region are difficult to extension and thereby the amount of melting. Thus reconcile with simple hotspots or plumes. He suggests magmatismis not driven by the plume but is a response they are caused either by episodic midplate stress 18 ß Coffinand Eldholm'LARGE IGNEOUS PROVINCES 32, 1 / REVIEWSOF GEOPHYSICS

After plume head dispersion Beforeplume head dispersion

lithosphere

asthenosphere

ascent o! hot source material up axial conduit

Figure 8. Three different models for LIP generation and emplacement: (a) Ther- mally induced "active" mantle plume entraining cooler material within the "head" mantle material during its as- of the starting cent to the base of the litho- plume sphere where a large tran- hot source material in conduit or plume sient plume head develops "tall" (left), rifts the lithosphere (right), and causes excessive magmatism [Campbell and Grijfiths, 1990], (b) steady state "passive" thermal plume over which rifting eventually causes excessive melting and volcanism [White and McKenzie, 1989], and (c) convective overturn caused by steep, cold lithospheric edges asthenosphere 500 km [Mutter et al., 1988].

500 km

Prebreakup Tectonism Breakup with Postbreakup Vigorous Convection Abating and Melting Convection

•• crustlithosphere asthenosphere

intensificationor by shallowasthenospheric convec- A further feature of recent mantle plume models,as tion travelingwith the plate and controlledby vertical opposedto Morgan's [1981] ideas, is that plumesand thermal boundaries.Finally, observationsof anoma- plate kinematicsare unrelatedphenomena. Some ex- lous seafloor spreadingcrust [Hinz et al., 1993; K. amples, such as the Hawaiian-Emperor seamount Hinz, personalcommunication, 1992] (Figure 1 and chain, stronglysupport this view, but someLIPs, such Table 2) may suggestthat some smaller oceanic LIPs as the OntongJava Plateau specificallyand the Early simply reflect periods of increasedglobal mid-ocean CretaceousPacific volcanic events in general, are so ridge crustal productionassociated with divergent large that they probably reflect primary modifications plate boundarieslocated over mantle regionscharac- of mantle dynamics. Emplacement of the latter LIPs terized by high melt potentials. may be connectedto changesin spreadingrates in the 32, I / REVIEWS OF GEOPHYSICS Coffin and Eldholm: LARGE IGNEOUS PROVINCES ß 19 I mid-ocean• Litho•sphere Layered-mantle riag••__--• ups, Whole-mantle circulation

Figure 9. Cartoons of (left) layered [e.g., Anderson et al., 1992a, b] and (fight) plume whole [e.g., Davies and Ri- chards, 1992] mantle circulation. descending plate

Early Cretaceous Pacific (W. J. Morgan, personal orites, comets, and asteroids, have been proposed communication, 1990); one suggestionis that spread- [e.g., Altet al., 1988; Rampino and Stothers, 1988; ing rates increased during the Cretaceous magnetic Rampino and Caldeira, 1993]. A major impact would quiet zone, because excessive plume melt lowered create a crater large e.noughto cause pressure relief asthenosphericviscosity and hence resistanceto plate melting in the asthenosphere,which would create the motion (R. L. Larson, personalcommunication, 1993). terrestrial equivalent of a lunar mare in the crust and Although some workers have proposed that plate re- persistent low-pressure cells in the mantle, that is, adjustmentsmay in places tap anomalouslyhot mantle hotspots. Although some workers accept impacts as and thus initiate a LIP [e.g., Anderson et al., t992a, causesof lunar maria and flood basalts on Venus [e.g., b], such a model has yet to be rigorously tested. Other Solomon, 1993], a strong case for impacts causing work has indicated that mantle plumes such as Hawaii terrestrial LIPs has yet to be made, and devising ex- must deflect in response to changes in direction of periments and models to test the hypothesis has lagged plate motion and that the shape of the bend in the behind investigation of internal origins and sources. surface track reflects this readjustment [Griffiths and Richards, 1989]. Lithosphere Upper Mantle To compare dimensionsof LIP sourcesin the mantle, we show minimum and maximum spherical thermal anomalies (Figure t0) based on volumes estimated by assuming 5-30% partial melting for basalts [e.g.,

McKenzie and Bickle, 1988; Liu and Chase, 1991; ONTO COLR McKenzie and O'Nions, 1991; Watson and McKenzie, 670 km Discontinuity 1991; S. Eggins, personal communication, 1992]. The observed scale range and magnitude of these estimates imply that some LIPs can be explained by temperature Lower Mantle and/or fluid anomaliesrestricted to a convectingupper mantle. Ontong Java and probably Kerguelen, however, appear to have been generated, at least in part, from the lower mantle (>670 km deep) (Figure t0). Furthermore, we note that degrees of partial melting Outer Core are likely to be higher beneath thinner oceanic lithosphere than beneath thicker continental lithosphere, which suggeststhat thermal anomalies responsiblefor Figure 10. Minimum and maximum spherical diameters of the North Atlantic volcanic province, Deccan traps, five LIPs calculated assuming 5% and 30% partial melting and Columbia River basalts may tend toward our max- (Table 1) plotted on a cross section of the Earth. For sim- imum dimensions (Figure t0) and thus involve the plicity, spheresare placed below the base of the lithosphere, lower mantle as well. as experiments suggest[Griffiths and Campbell, 1991a, b]. External causes for LIP magmatism, namely mete- Abbreviations as in Table 1. 20 ß Coffin and Eldholm: LARGE IGNEOUS PROVINCES 32, 1 / REVIEWS OF GEOPHYSICS

LithosphericDeformation: Uplift, Extension, atures, although they would facilitate large melt vol- Rifting, and Subsidence umes. Implicitly, it requires little lithospheric exten- Whether rifting precedes or accompaniesemplace- sion prior to a relatively sudden breakup. However, ment of oceanic plateaus in intraplate settings is not recent work by Skogseid et al. [ 1992a] on the V0ring possible to evaluate from available data. Where the margin and its conjugate records a 16- to 18-m.y. rift mantle thermal anomaly is associated with continental phase prior to excess magmatism during and subse- breakup or located below thick continental crust, how- quent to early Tertiary continental breakup. Further- ever, the relationship between thermal plumes and more, they suggest that Late Cretaceous-Paleocene rifting has been much debated [e.g., Duncan and Ri- North Atlantic rifting affected a 300- to 350-km-wide chards, 1991; Hill, 1991; Hill et al., 1992]. We have crustal region. The rift zone is thus wider than the previously shown that for LIPs erupted both in intra- thickness of the lithosphere, and a wide region of plate and plate boundary settings, the tectonomag- asthenospheric upwelling has been predicted by El- matic history of LIP formation depends on interaction dholm and Grue [1994], applying the concept of Keen of the mantle asthenosphere anomaly, configuration [1987]. As the region of lithospheric extension nar- and composition of mantle lithosphere and crust, and rowed and the deformation rate increased with time, the amount of stress in the lithosphere. Nonetheless, melt production also increased, climaxing during one would expect volcanism and faulting to precede breakup. Alkali basalts and gabbros, emplaced in con- active rifting, while they would follow passive rifting tinental crust, were produced during early rifting and [Dixon et al., 1989]. for low stretching factors; melts became tholeiitic The main tholeiitic phases of many continental when the lithosphere was extended further [McKenzie flood basalts, such as Columbia River [Hooper, 1990], and Bickle, 1988]. Final crustal extension, or the late Deccan [Hooper, 1990], and Siberian [Campbell et al., rift stageleading to breakup, occurred rapidly, and the 1992], were erupted in what have been interpreted as main excess melts may have surfaced within ---1 m.y. largely nonextensional settings [Hill, 1991;Hill et al., [McKenzie, 1984]. 1992]. Other investigators connect continental flood Each mantle plume model should predict character- basalts to rifting: examples are Paranti [Peate et al., istic lithospheric uplift and subsidencehistories asso- 1990; Kazmin, 1991; Harry and Sawyer, 1992; Renne ciated with LIP emplacement and evolution. Experi- et al., 1992], Karoo [Kazmin, 1991], and Deccan [Ka- mental results of "active" plume modeling predict zmin, 1991; Biswas and Thomas, 1992]. Because many surface uplift on a wavelength comparable to the di- continental flood basalts show little evidence of litho- ameter of the plume, uplift which commences before spheric extension, Duncan and Richards [1991] con- extension and ---25 m.y. prior to maximum uplift (Fig- clude that they result from melting events at the base ure 11a) [e.g., Griffiths and Campbell, 1991a]. The of the lithosphere, whether or not such thermal events uplift is controlled by the buoyancy of the hot plume precipitate subsequent rifting. On the other hand, a material and lateral variation in lithospheric rheology nonextensionalsetting can probably only be expected and thickness. Maximum uplift can reach 1000 m in if plume impingement at the base of the lithosphere continental, and likely more in oceanic, lithosphere and volcanism are nearly coeval events, whereas a and precedes initiation of major basaltic volcanism by more protracted deformation period, for example, the 3 to 30 m.y. [Hill et al., 1992]. Results of numerical plume incubation model of Kent et al. [1992], would "active" plume modeling demonstrate that a plethora lead to regional uplift, extension, and intrusion of of uplift and subsidencehistories are possible, given alkalic melts prior to volcanism. From field evidence various lithospheric ages, plate speeds, and many in the Siberian, Karoo, Paranti, Rajmahal-Kerguelen, more loosely constrained model parameters [Mon- and Deccan-Seychelles-Mascarene Plateau continen- nereau et al., 1993]. tal flood basalt provinces, Kent et al. [1992] conclude Volcanism is a "passive" response to lithospheric that the latter emplacement history is most appropriate thinning in White and McKenzie' s [1989] plume model; for continental flood basalts. they predict that the bulk of LIP emplacement occurs Various plume and upwelling models may all be contemporaneous with the main rifting. Uplift and applied to asthenospheric behavior during formation subsidenceare affected by lithospheric thinning, addi- of volcanic margins. However, Mutter et al. [1988] tion to the crust of igneous material produced by have proposed an alternative model depending on de- adiabatic decompression,dynamic support of the man- velopment of short-lived, small-scale convection close tle plume, and reduction in density of residual mantle to the trailing edges of cold, thick continental litho- after removal of melt. Interaction of the various fac- sphere (Figure 8c). Convection induces a pulse of tors produces a variety of uplift and subsidencemod- excess magmatism during the onset of seafloor spread- els (Figure 11b). Uplift and subsidencehistories have ing. This entirely "passive" model, derived from data not yet been predicted for the Mutter et al. [1988] and on the North Atlantic V0ring and northwest Australian Anderson et al. [1992a, b] models. margins [Hopper et al., 1992] and driven by plate Data with which to test the various models' uplift separation, does not require increased mantle temper- and subsidence histories are sparse. The geological 32, I / REVIEWSOF GEOPHYSICS Coffin and Eldholm' LARGEIGNEOUS PROVINCES ß 21

A B

======

5 m.y. :.:.:.:.:.:.:.:.: ./.-.'ii ...... :::::::::::::::::::::

ß ...... :::::::::::::::::::::::::::::::::::::::::::::::::::::::::: + thermal

...... _ illII

•1=3x102ø 3 4 0.0 •l=102• STRETCHING FACTOR

? -3 m.y. + residual lOOO . lilttillI . - mantle 0.5 ...... 1360øC

+ thermal anomaly / \ 15oo - i ' I-:25m.y. / / ,.///

2ooo

2.0

residual mantle 1280oC I I 1000 500 0 500 1000 2.5 HORIZONTAL DISTANCE (km)

Figure 11. Vertical tectonic historiespredicted by (a) "active" and (b) "passive" plume models. Figure 1la represents surfaceuplift as a function of plume headlocation in the mantle [after Griffithsand Campbell, 1991a]. Figure 11b represents uplift and subsidenceas a function of lithosphericextension (beta) and residualmantle potential temperature[after White and McKenzie, 1989]. record of uplift is not commonly preserved, and sub- to volcanic margins, the tectonomagmatichistory can sidence studies have commonly focused on decay of a only be appreciated if flood basalts are studied in the "normal" mid-ocean ridge thermal anomaly and on framework of the entire rift system, that is, in a con- mechanical loading by sediment and water. Oceanic jugate margin setting [Eldholm, 1991]. Except for the plateaus, in contrast to thermal/dynamic swells, offer North Atlantic, no such studies have been made. arguably the best opportunity for testing and improv- Nonetheless, many continental flood basalts arriving ing transientplume modelsin the simplestlithospheric from the same mantle anomaly which causescontinen- setting; subsidence studies [Coffin, 1992; Derrick et tal breakup and volcanic margin formation actually al., 1977] using DSDP and ODP material suggestthat rest on crust outside the rift zone or in the part of the oceanic LIPs follow a subsidencecurve dominated by rift zone characterized by low stretchingfactors. This thermal decay (Figure 12) which appears similar to effect is amplified if breakup occurs along a detach- that of "normal" oceanic lithosphere [Parsons and ment plane near the flank of the rift zone where the Sclater, 1977]. Preemplacement and synemplacement crust is weak, for example, along a preexistingfault or uplift of oceanic plateaus has not yet been investi- thrust plane, possibly induced by small-scaleconvec- gated. tion near the edges of the wide upwelling zone [Keen, Although the question of how lithospheric uplift, 1987]. Many rift models predict extension over a zone extension, rifting, and subsidence relate to LIP em- of upwelling.Other modelsshow the locusof crustal placementand evolution is not fully resolved, a review failure displaced laterally because of a preexisting of these LIP categories reveals a preponderance of weak zone or a detachment [Bassi and Bonnin, 1988; extensional settings [Coffin and Eldholm, 1992]. The Braun and Beattrnont, 1989; Dunbar and Sawyer, interpretation of purely nonextensional continental 1989; Lister et al., 1991; Sawyer and Harry, 1991; flood basaltsmay result from low stretchingfactors for Harry and Sawyer, 1992]. This breakup mode does not provinces unrelated to plate boundary events, such as depend on a low-angle fault through the entire litho- Siberian traps. For continental flood basalts adjacent spherebut rather on a fault passingthrough the crust 22 ß Coffin and Eldholm' LARGE IGNEOUS PROVINCES 32, I / REVIEWS OF GEOPHYSICS

Figure 12. Age-depth curves for LIP volcanic basement at Indian Ocean drill sites [after Coffin, 1992]. Num- bers identify Deep Sea Drilling Project and Ocean Drilling Program sites; letters and numbers indicate industry wells. Calculated basement -1 © depths for all sites are given for zero age (boxes) and the present (cir- 749 •© (•) SM-1 cles); where possible, calculated 756 basement depths at the end of shelf •> ©7o7 © 747 deposition are indicated (triangles). 214 Theoretical subsidence curves ac- (•> 750 (•) 706 715 (•) 216 cording to the equation of Hayes [1988] for crust emplaced 2000 m 713 above sea level and at a depth of (•> 758 1000 m (C = 3000) enclose at all points.

-40! 0 20 40 60 80 lOO 12o Age (m.y.)

[Dixon et al., 1989]. However, since simple shear Also problematic in a convecting upper mantle is the extension is always less effective than pure shear in apparent fixity of hotspots within two distinct groups, inducing partial melting [Buck et al., 1988], a relatively an Atlantic/Indian and a Pacific group, over the past 90 steep detachment is required. A similar asymmetry m.y. [Mt;iller et al., 1993]. may originate if the pre-LIP lithosphere contains a thin One might envision a convecting upper mantle oc- spot which laterally offsets the maximum melt zone casionally being penetrated by a plume originating and surface location of the LIP [Thompson and Gib- deep in the mantle (Figure 13). This would be consis- son, 1991; Skogseid et al., 1992a]. tent with a heterogeneous upper asthenosphere, as documented by the various mantle end-members [e.g., Discussion Hart et al., 1992], in which temperature, composition, Various LIPs are associated with continental and and fluid content vary regionally. These factors, to- oceanic breakup, but causal mechanisms have yet to gether with the history and rate of lithospheric exten- be well documented. The active plume head, steady sion prior to continental separation, would then deter- state plume, or hot mantle models could account for all mine the amount, timing, and position of igneousrocks LIPs; the secondary convection model could apply emplaced during breakup. A heterogeneous astheno- exclusively to some volcanic passive margins. Mantle tomography suggests, however, that subduction, not sphere would produce the observed excess magma at plume upwelling, is the dominant upper mantle pro- and in the vicinity of the crustal focus of deep mantle cess today [e.g., Dziewonski and Woodhouse, 1987; plumes. On the other hand, it would also allow forma- Anderson et al., 1992a], which suggeststhat at least tion of volcanic passive margins by increased melt some plumes originate in the lower mantle. A lower production away from a plume if rifting occurs within mantle origin is also supported by the huge calculated a region of " abnormal" asthenosphere.On the basis of melt volumesfor the largestigneous provinces; how- numericalmodels, Tackley et al• [1993]have suggested ever, most LIPs might originate equally well in the heterogeneousmantle circulation in which cold down- upper mantle (Figure 10). Furthermore, longevity of welling plumes and hot upwelling plumes, but not hotspot sources is hard to reconcile with an exclusive linear forms (sheets), penetrate the 670-km disconti- plume model in a convecting mantle: Why, for exam- nuity, accounting for long-wavelength lateral hetero- ple, should the thin plume stem be maintained over 100 geneity in the mantle. Nevertheless, additional obser- m.y. or more [e.g., Richards and Griffiths, 1988]? And vational data and modeling are clearly required to how do the different scales of convection interact? adequately address the origin and evolution of LIPs 32, 1 / REVIEWS OF GEOPHYSICS Coffin and Eldholm- LARGE IGNEOUS PROVINCES ß 23

Heterogeneous hotspot, mantle transient LIP circulation mid-ocean ridge

descending Dlume mid-ocean ridge plate plate Lithosphere Figure 13. Cartoon of heterogeneous mantle circulation proposed in this paper.

km hotspot, discontinuity LIP

Mantle D' UpperLower Mantle Outer Core lnn• and their relationship to continental and oceanic [Lockley, 1990]. Moreover, the fact that many oceanic breakup. plateaus are isolated from continental detritus and Although available data may be interpreted both in usually lie above the surrounding seafloor and well terms of "active" and "passive" asthenospheric up- above the calcium carbonate compensation depth welling, melt volumes and geological settings of many makes them repositories of sediment deposited at LIPs suggest a combination of the two geodynamic varying paleodepths, commonly without major hia- models by an active thermal source impinging on litho- tuses. Thus LIPs which are "active" in forming or sphere under extension. Therefore different models for changingthe environment also provide an opportunity generating LIPs are not necessary mutually exclusive, to study environmental change by serving as "pas- and components of them may contribute to the ob- sive" sediment repositories [Coffin and Eldholm, served tectonomagmatic emplacement history. 1991]. The intense debate on extraterrestrial impact versus endogenous mechanisms for changes at the Creta- 5. ENVIRONMENTAL IMPLICATIONS ceous-Tertiary (K-T) boundary [e.g., Lockley and Rice, 1990; Sharpton and Ward, 1990] has to some Construction of transient LIPs has the potential to extent overshadowed the important message: the tem- induce environmental change and affect biotic evolu- poral proximity of LIP emplacement and environmen- tion and extinction primarily because of (1) geometri- tal change (e.g., Figures 2 and 15). Durations of these cal changes of the Earth's surface, in particular, mechanisms are distinctly different, and also, it is changing of the ocean basin geometry, (2) chemical difficult to clearly determine the cause and effect rela- and physical changes of the hydrosphere caused by tionships of tectonomagmatic events and global envi- interaction of lava and seawater, and (3) increased ronmental events. LIP formation, in particular, is as- transfer of gases and particulate matter to the atmo- sociated with many individual factors of different sphere during eruptions (Figure 14). Consequencesof duration and polarity that alone or combined, impact LIP emplacement on basin geometry and on the bio- on the environment; the geologic record reflects the sphere may occur on local, regional, and global scales sum of these factors and provides few clues for differ-

LIP Environmental Parameters Scales VE: -40:1 Local Regional Figure 14. Cartoon of environ- particulates Global chemicals mental parameters illustrated by emplacement of transient LIP heat •yroclasticsanddebrisflows sea level end-members continental flood land bridges • basalts and oceanic plateaus. All sea surface continental parameters are relevant for vol- flood basalt canic margins; absolute and rela- upwelling• hydrothermal dynamic uplift activity tive impact depends on tectono- gateways erosion/redeposition magmatic setting and magmatic LIP construction intensity. isostaticcompensation geometry thermal subsidence seafloor mechanical subsidence oceanicplateau 24 ß Coffin and Eldholm: LARGE IGNEOUS PROVINCES 32, I / REVIEWS OF GEOPHYSICS Sea Level Large Atlantic5]s0 (metersabove present-day) Igneous 4 3 2 1 0 -1 0 50 100 150 200 250 Extinctions Provinces o I 'i '-•'• '1'''1 10 • Ead•;MiddleMiocene! 1 20 ColumbiaRiver

3O • OiigoceneEocene- I 4O

• 60 •% Paleocene-Eocene--tlant•c, 70• aastrichtian-Danian1 / 80"'1 BlackShales • • • Conlaoan;oniacian •. Cenomanian- 901 -'• • Cenomanian-'• Turonian• 1 / Bae.,an-/ /Ontong Java 301 I I /I I '1 • ValanginianBerriasian- Paleoternperature(øC,ice free) '•

150.-• ,... Figure 15. Cenozoic and Cretaceous climate, black shales, sea level, mass extinctions, and five LIP emplacementsplotted on the timescaleof Harland et al. [1990].Atlantic •sO is after Raytooand Ruddiman[1992], paleotemperature is after Arthur et al. [1985], black shalesare after Jenkyns [1980], sea level is after Haq et al. [1987], extinctions are after Rampino and Stothers [1988] and Thomas [1992], LIP emplacementsare from Coffin and Eldholm [1993] and this paper.

entiation. Therefore we focus on temporal and spatial and Stothers, 1988; White, 1989a; Rampino and Cal- similarities without excluding other causeseither sep- deira, 1993] (Figure 2). Although most such correla- arated in time or coeval with LIP formation and draw tions are based on few and relatively uncertain age on examples from the three major LIP categories. determinations, a general relationship is apparent. Three spectacular events are the synchronism of the Temporal Correlations greatest known extinction of biota on Earth at the Environmental impact of LIP emplacement is sup- Permian-Triassic boundary (•248 Ma) and the Sibe- ported by temporal correlations of continental flood rian traps [Campbell et al., 1992] and the biotic basalt volcanism and periods of major biotic changes changes at the K-T [e.g., Officer et al., 1987] and or extinctions of terrestrial and marine organisms Paleocene-Eocene [e.g., Rea et al., 1990; Thomas, [e.g., Rich et al., 1986; Officer et al., 1987; Rampino 1990] transitions during eruptions of the Deccan flood 32, 1 / REVIEWSOF GEOPHYSICS Coffinand Eldholm:LARGE IGNEOUS PROVINCES ß 25 basalts and the North Atlantic volcanic province, re- factors that influence basin geometry and eustatic sea spectively. level [Pitman, 1978]. However, other significant fac- The geologicalrecord showschanges in populations tors or combinations of factors may not be directly of marine and terrestrial biota at or near the K-T related to LIP size [Self and Rampino, 1988]. For transition[e.g., Raup and Sepkoski,1986]. Extinctions example, geological setting governs interaction be- were particularlysevere among calcareous planktonic tween lavas and the biosphere,as well as the impact of organismsand among many species of reptiles. In volcanism on atmospheric and oceanographiccircula- contrast, some biotic groups show no significant tion patterns. For the latter, we considercontinental changes, for example, benthic organisms. However, flood basalts and intraplate oceanic plateaus to be •10 m.y. later, within chron 24r, many benthic fora- end-members, while the impact of the intermediate minifera experienced global mass extinction [Kennett volcanic margin members depends on the magnitude and Stott, 1991; Thomas, 1992]. In fact, there is a of the mantle thermal anomaly, mode of rifting, plate compelling temporal correspondencebetween em- tectonic setting, and explosivity of the volcanism. In placementof the North Atlantic volcanicprovince and particular,subaerial flood basalt activity may havehad major paleoceanographicchanges comprising this a larger effect than submarine eruptions because of "Paleocene-Eocene boundary event." Among the direct input of climatically important volatiles into the most significant events are massive extinctions of atmosphereinstead of into the oceans. deep-seaorganisms and land mammals,a large shift in Physical and chemical properties of the lava com- oceaniccarbon isotopes,greatly increasedhydrother- prise another key factor. Although the major propor- mal activity, and reduced atmospheric circulation tion of extmsive lava at LIPs has a basaltic composi- [Rea et al., 1990; Thomas, 1992]. Increased ocean tion, volumetrically smaller acidic and intermediate temperature corresponds with Paleocene-Eocene varieties contribute greater amounts of gasesand par- warming(Figure 15) and with the early Eocene climate ticles that affect biota, atmospheric chemistry, and maximum which marks the warmest period on Earth insolation. On the other hand, lava composition also during the last 70 m.y. [Owen and Rea, 1985]. As governsflow properties, and basalts in particular are another example of temporal correspondence,the able to flow for great distances. For example, flow peak eruptive phase of the Columbia River basalts lengthsof >750 km have been reported from the Co- correlates with early and middle Miocene extinctions lumbia River province [Tolan et al., 1989], whereas [Rampino and Stothers, 1988]. acidic lavas are more viscous and remain closer to No similar temporal correlations have been well feeders. documentedfor emplacementperiods of the giant oce- Volume and rate of emplacement of extrusive cover anic plateaus. Tarduno et al. [1991], in fact, note that the differencesbetween biotic responseto interpreted may be equally as important in affecting the environment as crustal volume and total crustal production submarine Ontong Java Plateau volcanism and sub- rate. Eldholm and Grue [1993] have estimated extru- aerial volcanism at the K-T transition are much greater than the similarities, suggestingthat emplacement of sive parametersfor the North Atlantic volcanic prov- the largest igneous provinces may occur without a ince and noted that volumes and geometries of extru- major negative global impact on the marine ecosys- sive cover on the two giant oceanic plateaus, Ontong tem. Diversification of nannoplankton after emplace- Java and Kerguelen, remain virtually unknown. None- ment of the Ontong Java Plateau may represent, in theless, by recognizing similarities in crustal velocity fact, a positive impact on the marine ecosystem[Erba structure among volcanic margins and oceanic plateaus, they inferred that the upper crustal layer on and Larson, 1991]. The igneous event would cause -1 atmosphericCO2 levels to rise, increasingthe global theseplateaus, with an averagevelocity of 5.5 km s mean temperature. Sea level would also rise, mobiliz- [Hussong et al., 1979; Recq et al., 1990], roughly ing nutrients from the shelf. Both of these factors correspondsto flood basalts.Assuming average thick- would promote biologic evolution. In contrast, Raup nesses of 4.5 and 5.0 km for Ontong Java and Ker- and Sepkoski [1986] report an extinction event in the guelen, respectively, they estimated extrusive vol- Aptian which correlateswith emplacementof the Ker- umes and eruption rates (Table 3). Although these guelen Plateau and Broken Ridge. values are still considereduncertain, eruption rates are of the same order of magnitudefor the three types of tip Properties LIPs studied and the giant oceanic plateaus are char- Great variability in LIP magnitude and emplace- acterized by maximum average rates. We emphasize ment rates (Figures 2, 6, and 10 and Table 3) does not that the rates in Table 3 are averaged over large areas have to translate directly to a corresponding scale of and time periods, whereas the actual eruption history environmentalimpact; such a direct correlation with probablyconsisted of many intense,voluminous, dis- size may, in fact, be misleading. Admittedly, the larg- crete events separatedby quiet intervals [Eldholm et est igneous province represents greatly increased al., 1989; White, 1989b]. Regardless, these calcula- crustal production (Figure 2) and associateduplift, tions and the variable biotic response to LIP emplace- 26 ß Coffin and Eldholm: LARGEIGNEOUS PROVINCES 32, 1 / REVIEWSOF GEOPHYSICS

ment show that LIPs smaller than the giant oceanic way development,have beenrelated to plate tectonic plateaus must be considered. events and the subsidencehistory of persistent LIPs, Although possibleeffects on global environment or hotspottrails (e.g., the Greenland-Scotlandridge, may directlyresult from LIP "forcing," it is likely that Walvis Ridge-Rio Grande Rise). Formation of the the impactof individualLIP eventsis a functionof the North Atlantic volcanic province (Figure 7), however, state of the global environmentduring emplacement. introduced an additional component of basin fragmen- In periodsof environmentalstress close to ill-defined tation associatedwith synrift uplift and buildup of thresholdlevels, LIP genesismight trigger nonlinear extrusivecomplexes which significantlyinfluenced re- responsesand rapid global change. gional circulationand sedimentationand may have delayedopening of gatewaysas well as contributedto Basin Geometry biotic endemism [Eldholm, 1990]. Rapid emplacementof offshoreLIPs changesocean Comparedwith a "standard" plate tectonicmodel basingeometry by the morphologyand locationof the of ocean basin development characterizedby initial new constructional features, which in turn displaces subsidenceand basin widening and deepening with seawater and thus modifies sea level. These changes time, oceanic LIP formation, either at plate bound- can directlyinfluence water massformation and circu- aries or intraplate, representsan additionalevent of lation, and erosion and sedimentation,influences that basin deformation which might change water mass might reach far beyond the LIP proper. circulationpatterns both on regionaland globalscales. Initial thermal and dynamic uplift and subsequent AlthoughLIP emplacementnormally will affectboth rapidbuild-up of the LIP abovenormal oceanic crustal surfaceand deep waters, the impact on surfacewater depthsresult in a eustaticsea level rise. Such rises is most significantduring the constructionalphase, might be detectedon continentalmargins. Rate and while the impact on bottom-watercirculation might magnitudeof the rise dependson the volumeand rate last for longer periods. In addition, the surfacewaters of LIP emplacementand on isostaticand flexural re- above some LIPs may be affected by topographic sponsesof the lithosphere.The volumeof water-dis- upwellingwhich may enhanceproductivity. placingigneous rock on the OntongJava Plateau,for example,is •4 x 106km 3 [Schubertand Sandwell, Impacton the Biosphere 1989], which would elevate sea level by •10 m. If the Submarine volcanism might contribute to biotic entire OntongJava LIP is included,this rise is larger. change,including mass extinction events, by addition The magnitudes,however, are much smaller than of trace metals such as As, Sb, and Se, which are those calculatedby Haq et al. [1987] (lower Aptian poisonousto marinelife. Throughheat output,under- black shaledeposition, for example, correspondsto a sea volcanism also destabilizes the water column, in- short-term eustatic rise of--•75 m), but they are com- ducinglarge-scale overturn affecting surface-dwelling parableto those of Pitman and Golovchenko[1991]. organisms.Furthermore, release of volatilessuch as Because fluctuationsin global crustal accretion (sea- CO2and their redistributionthrough the oceanmight floor spreading)rates are more gradual(Figure 2) than lead to changesin the alkalinity of the oceanaffecting LIP emplacement(Table 3), we considerthe latter climate and marine life. Increased CO2, in particular, more likely to produce"events" in the stratigraphical may causedissolution of carbonate.This would aug- record on continental margins. When the transient ment high-productivityepisodes recorded as black igneousepisode ceases, LIP subsidencecontributes to shalesduring oceanic anoxic events [Schlangerand eustatic sea level fall. Thus the net effect of a LIP Jenkyns,1976]. Pacific Ocean sediment records inter- emplacementwould be a gentleeustatic sea level rise vals of major carbonburial at 91, 112, and 117.5 Ma, (lithosphericuplift) culminatingwith a rapid flooding which correspond to patterns of biotic evolution event (LIP emplacement)and a subsequentgentle [Sliter, 1992].The two oldestevents are approximately eustatic sea level fall (LIP subsidence). contemporaneouswith formation of the OntongJava Oceanicplateaus and volcanicmargins affect water and KerguelenLIPs, respectively(Figures 2 and 15). mass circulation by creating physical barriers. The CO2 content in the oceansand atmospheremay barriers sometimes function as gatekeepers between changein responseto seafloorhydrothermal activity major ocean basinswhich over short time intervals [Berner et al., 1983] and volcanism [Arthur et al., might inhibit existing or create new water massroutes. 1985]. Noting that hydrothermalactivity is most in- The significanceof such gatekeepers,which signifi- tense during continental rifting and oceanic plate cantly affect the global transportsystem of heat and boundaryrearrangement, Owen and Rea [1985] and moisture,depends on their location, areal extent, and Rea et al. [1990] postulatedthat the geochemicalim- depth profile. The marine environmentresponds rap- pact of these tectonic events could affect the bio- idly to physicaloceanographic events of this kind, and sphere.Hydrothermal activity alongthe presentmid- establishmentof gatewaysis well documentedin the ocean ridge system accounts for -20% of the sedimentary record [Hay, 1988]. Previously, ocean preindustrialatmospheric CO2 level, and enhanced basin evolution and fragmentation,and thereby gate- hydrothermalactivity might significantlyraise global 32, 1 / REVIEWS OF GEOPHYSICS Coffin and Eldholm: LARGE IGNEOUS PROVINCES ß 27

CO2 levels, potentially inducingglobal warming [Owen silicic magma [Devine et al., 1984]. Thus the gasesand and Rea, 1985]. On the other hand, Varekamp et al. particles from both central vent and fissure eruptions [1992] infer that the combined effects of mid-ocean might, if carried to the stratosphere, cause dramatic ridge volcanism, subductionmagmatism, and slab al- short-term effects such as acid rain, darkening, and teration have little impact on atmosphericCO2 content climatic cooling spells [Officer et al., 1987]. Stothers et and climate, whereas intense flood basalt volcanism al. [1986] proposed that convective plumes over vents may influenceclimate on timescalesin excessof 104 with very high eruption rates and associatedlarge fire years. Thus transient volcanism may force rapid envi- fountains provide the injection mechanism and calcu- ronmental change. lated that the larger Columbia River flow units were Continental flood basalts and the subaerial part of able to trigger environmental effects of this kind. oceanic LIPs transfer CO2 directly to the atmosphere Synrift uplift at some volcanic margins and conti- [McLean, 1985; Rampino, 1991]. For periods > 1 m.y., nental flood basalts is another tectonic mechanism that atmosphericCO2 levels are governed mainly by mag- might influence environmental parameters by modify- matic sources and chemical weathering [Raymo and ing atmospheric circulation [Ruddiman and Kutzbach, Ruddiman, 1992]. The Deccan traps, for example, 1989] and by increasingchemical weathering, resulting couldhave transferred as much as 2 x 1017mol of CO2 in withdrawal of CO2 and global cooling [Raymo, over a period of several hundred thousand years, a 1991]. volume which has been estimated to create a global greenhouse effect of---2øC [CaMeira and Rampino, Resources 1990]. Coeval volcanism along the conjugate passive Relations between LIP emplacement and regional margins of western India and the Seychelles-Mas- and global environmental events may have implica- carene Plateau might have greatly enhanced this ef- tions for resource generation and accumulation [Keith, fect. Furthermore, Eldholm and Thomas [1993] have 1982]. Larson [1991a] pointed out the coincidence of suggestedthat CO2 output of North Atlantic volcanic Early Cretaceous superplume formation, global tem- province basalts could have triggered rapid environ- perature increase, black shale deposition, eustatically mental change through increased high-latitude surface transgressive sea level, and oil generation. The result- temperatures, in turn changing deep-sea circulation ing flooded continental platforms would promote ma- and productivity. The Early Cretaceous superplume rine deposition of organic carbon (phytoplankton) that [Larson, 1991 b] provides another source of potential would later mature and form oil. He also explained the large-scale atmospheric impact. It may have led to large Pennsylvanian-Permian coal deposits by a simi- CO2 levels from 3.7 to 14.7 times the modern prein- lar superplume coinciding with sea level fall. dustrial value, resulting in a global warming of 2.8ø- On volcanic margins, several factors may relate to 7.7øC with respect to the present global mean temper- hydrocarbon potential [Eldholm, 1991; Skogseid et al., ature. Including changesin paleogeographyand higher 1992b]. First, LIP emplacement affects prerift sedi- sea level, the temperature range increases to 7.6 ø- mentary basins on the incipient margin by regional 12.5øC [Caldeira and Rampino, 1991]. It is probably uplift, erosion, tectonism, and emplacement of intru- not coincidental that the Cretaceous greenhouse cul- sive and extrusive rock complexes. Second, the ther- minated from late Aptian through late Albian time mal anomaly responsible for LIP formation might af- [Arthur et al., 1991] following Ontong Java LIP for- fect maturation. Third, regional synrift uplift provides mation and that the early Eocene greenhousefollowed an additional source area for synrift and postrift sedi- the North Atlantic volcanic margin LIP emplacement. ment. If basin fragmentation during rifting and initial Volcanic eruptions also transfer other volcanic continental margin development forms restricted and gases, aerosols, ash, and heat to the atmosphere [Sig- poorly ventilated basins during hot and humid climates urdsson, 1990]. Aerosols are commonly generated thereby enhancingbiological productivity, local poten- from violent, explosive silicic eruptions which trans- tial lacustrine or marine source rocks may develop. port sulfuric volatiles to the stratosphere where they This setting existed during breakup of the North At- are converted to sulfuric acid aerosols [Rampino and lantic, and a similar setting should be considered for Self, 1984; Self and Rampino, 1988]. Furthermore, anoxic sequencesalong other passive continental mar- widely dispersedearly Tertiary tholeiitic tephras in the gins, such as the Early Cretaceous South Atlantic. North Atlantic realm [Knox and Morton, 1988] imply that the North Atlantic volcanic province had at least a regionally significantenvironmental impact [Eldholm 6. CONCLUSIONS and Thomas, 1993]. In addition, basalts drilled at ODP site 642 reflect phreatomagmatic eruptions [Viereck et Evidence that LIPs both manifest a fundamental al., 1988] where local magma-water interaction ampli- mode of mantle circulation commonly distinct from fied the explosive nature. Sulfur contribution per unit that which characterizes plate tectonics and contribute volume from basaltic eruptions like those at LIPs is episodically, sometimes catastrophically, to global en- about 1 order of magnitude greater than that from vironmental change is accumulatingrapidly. Dating of 28 ß Coffin and Eldholm: LARGE IGNEOUS PROVINCES 32, I / REVIEWS OF GEOPHYSICS

many continental flood basalts, from the Deccan traps system as more a "regulator" and LIPs as more "in- in particular, have convincingly shown the geological stigators" of global environment change. suddenness of LIP emplacement. Geophysical and drilling data have demonstrated in one case, the North ACKNOWLEDGMENTS. The senior author gratefully Atlantic volcanic province, that a large volumetric acknowledgesthe support of the Norwegian Research Coun- percentage of those continental flood basalts associ- cil for Science and the Humanities and of the sponsorsof the ated with continental breakup lie offshore, that the Plates global reconstruction project. We thank Roger Lar- uppermost extrusives were erupted subaerially, and son, Cliff Frohlich, John Ewing, Nathan Bangs, and an that the intrusive component of LIPs is at least as anonym for reviews. Comments from Bob White and John voluminous as the extmsive component. Geophysical Tarduno were helpful, as were discussions with Shen-su and drilling data from two oceanic plateaus, Ontong Sun, Paul Mann, Greg Houseman, Steve Eggins, Bob Dun- can, Sherm Bloomer, John Bender, and Jamie Austin. Lisa Java and Kerguelen-Broken Ridge, have illustrated Gahagan and Wayne Lloyd ably supplied technical services. that volumetrically these LIPs dwarf all others known, This is the University of Texas at Austin Institute for Geo- that much of Kerguelen's uppermost crest was physics contribution 986. erupted subaerially, and that extrusives of the Ontong Alan Chave was the editor responsible for this manu- Java LIP affected -1% of the Earth's surface. How- script. 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