Geology and geochronology of the line islands

12 Pages·2007·1.53 MB·English

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B13, PAGES 11,261-11,272,D ECEMBER 10, 1984
Geology and Geochronologyo f the Line Islands
S. O. SCHLANGE1R M, . O. GARCIAB, . H. KEATINGJ, . J. NAUGHTON,
W. W. SAGER2 ,J . g. HAGGERT3Y A, NDJ . g. PHILPOTT4S
Hawaii Institute of GeophysicsU, niversityo f Hawaii, Honolulu
R. A. DUNCAN
Schoolo f Oceano(cid:127)Iraphy,O re,ionS tate University,C orvallis
Geologicala nd geophysicasl tudiesa long the entire length of the Line Islands were undertakeni n
order to test the hot spot model for the origin of this major linear island chain. Volcanic rocks were
recoveredi n 21 dredge hauls and fossiliferouss edimentaryr ocks were recoveredi n 19 dredge hauls.
Volcanic rocks from the Line Islands are alkalic basalts and hawaiites. In addition, a tholeiitic basalt and
a phonolite have been recoveredf rom the central part of the Line chain. Microprobe analyseso f
groundmassa ugite in the alkalic basaltsi ndicatet hat they contain high TiO2 (1.0-4.0 wt %) and A1203
(3.4-9.1 wt %) and are of alkaline to peralkaline affinities.M ajor element compositionso f the Line
Islandsv olcanicr ocks are very comparablet o Hawaiian volcanicr ocks. Trace elementa nd rare earth
elementa nalysesa lso indicatet hat the rocksa re typical of oceanici sland alkalic lavas' the Line Islands
lavas are very much unlike typical mid-oceanr idge or fracturez one basalts.D ating of theser ocks by
'(cid:127)øAr-39ArK, -Ar, and paleontologicmale thodsc, ombinedw ith DeepS eaD rilling Projectd ata from sites
165, 315, and 316 and previouslyd ated dredgedr ocks, provide ages of volcanic eventsa t 20 localities
along the chain from 18øN to 9øS,a distanceo f almost4 000 km. All of thesed atesd efinem id-Cretaceous
to late Eocenee dificeo r ridge-buildingv olcanice vents.E ocenev olcanice ventst ook place from 15øN to
9øS, and Late Cretaceouse vents took place from 18øN to 9øS. In the southern Line Islands both
Cretaceousa nd Eocenee ventst ook place on the samee dificeo r ridge, indicatingr ecurrentv olcanisma t
a singlel ocality. The irregular distribution of atolls in the chain, the fact that Late Cretaceousr eefs
flourisheda long a distanceo f approximately2 500 km in the central and southernL ine Islands,a nd the
observationt hat spatiallyc loselyr elated seamountse xhibit different subsidenceh istoriesa re interpreted
as indicatingth at larges egmentosf the chainh aven ot followeda t (cid:127)/2 relateds ubsidencpea th.M agnetic
surveyso f 11 seamountss how that four of the seamounts,f rom the central Line chain, give virtual
geomagneticp oles which fall well to the north of virtual geomagneticp oles of Cretaceouss eamounts.
Thesef our polesa greew ith other paleomagneticd ata of middle-lateE ocene-earlyO ligocenea ge from
the PacificO. ne of thesef our seamountysi eldeda '(cid:127)øAr-39Art otal fusiona geo f 39 Ma. Becausteh e poles
of all four seamountsfa ll into a tight group we infer that they are probably of middle-lateE ocenea ge to
early Oligocenea ge. The other three seamountsa s well as one seamontf rom the Line Islandsa nalyzed
previouslya ll give virtual geomagneticp olesw hich agreew ith Late Cretaceousp aleomagneticd ata from
the PacificO. f thesef our seamountsth, reeg iveL ate Cretaceou's(cid:127)ø Ar-39Ara gesr angingf rom 71 to 85
Ma; another, in the southern Line Islands, is interpreted,o n paleontologicale vidence,t o be of Late
Cretaceousa ge. The origin of the Line Islandsh as beena scribedb y previousw orkerst o the effectso f a
singleh ot spot and to the action of four hot spots.T he singleh ot spot model cannot account for all of
the volcanice dificesi n the Line Islands, although it does explain a general age progressiono f 9.6 _+0 .4
cm/yr from north to south along the chain derived from a number of dated edifices.T he four hot spot
model accountsf or more of the dated volcanice dificesb ut still doesn ot explain all of the availabled ata.
The petrologic data argue against a mid-ocean ridge or transform fault origin proposed by earlier
workers.T he complexv olcanicp rovincer epresentedb y the Line Islandsr emainsa challenget o existing
models for the origin of midplate volcanism in the Pacific. An atoll drilling program which could
determine edifice building histories,p aleolatitudeso f seamount formation, and subsidencer ates and
patternsi n the Line Islandsi s needed.
INTRODUCTION AND BACKGROUND Guyot and JohnstonI sland in the north to the northeaste nd
of the Tuamotu Islands in the south (Figure 1). The Line
The Line Islands geologicalp rovince, capped by the atolls
Islands are a major bathymetric feature of the central Pacific
of Johnston, Palmyra, Washington, Fanning, Christmas, and
comparable in size to the Hawaiian-Emperor chain to the
Caroline islands, is made up of dozens of simple and com-
north and the Marshall-Gilbert-Ellice chain to the west.
posite seamountsa nd linear ridges that extend from Horizon
Morgan [1972], arguing from the fact that the Line-Tuamotu
geometric trends appear to parallel the Emperor-Hawaiian
(cid:127) Now at Departmento f GeologicalS ciencesN, orthwesternU ni-
trends,p ostulatedt hat the Line and Tuamotu chainsw ere the
versity,E vanston,I llinois.
temporal and genetic equivalentso f the Emperor and Ha-
2 Now at Departmento f OceanographyT,e xasA & M University,
College Station. waiian chains, all four of these major featuresh aving devel-
3 Now at Departmento f GeoscienceUs,n iversityo f Tulsa, Tulsa, oped through the actions of two hot spots fixed relative to
Oklahoma. each other for the past 70 Ma. Jarrard and Clague [1977]
'(cid:127) Now at U.S. GeologicalS urvey,N ational Center, Reston,V ir-
pointed out that geometric and paleomagnetic evidence
ginia.
argued against the proposition that the Line and Emperor
Copyright1 984b y the AmericanG eophysicaUl nion. chainsw ere entirely coeval.
At the time the above argumentsw ere put forward, reliable
Paper number4 B0781.
0148-0227/84/004B -0781$ 05.00 dates for volcanic activity over any significantp ortions of the
11,261
11,262 SCHLANGEERT AL.' GEOLOGYA ND GEOCHRONOLOGOYF THEL INE ISLANDS
of midplate volcanisma long such crosst rends were influenced
by older Line Islands structures. He implied that several
chainso f volcanoso n crosst rends were formed by individual
hot spots that crossedt he main Line chain. Both DSDP drill-
/ ing in the central Line Islandsa nd the dating of rocksd redged
from seamounts in the cross trend showed that both Late
Cretaceous and Eocene volcanism had occurred there [Lan-
phere and Dalryrnple, 1976; Saito and Ozirna, 1977]. Further,
paleontological studies of rocks dredged from seamountsi n
the southern Line Islands [Haggetty et al., 1982] showedt hat
both Late Cretaceousa nd Eocenev olcanismh ad taken place
in the vicinity of Caroline Island (Figure 1, RD-44 and RD-
ß 45). The wide geographicd istribution of both Cretaceousa nd
/.O ,(cid:127) ' Eocene volcanism over a distance of several thousands of ki-
lometersa long the Line chain is regardedb y us as establishing
!
Kingman I the fact that the Line Islands were not formed by the action of
Palmyra
/ a single hot spot. That the southern Line Islands as well as the
central Line Islandsw ere formedb y a complexo verprintingo f
(cid:127)0o H(cid:127)a*shingIsto. n (cid:127) ZOo volcanic events was suggestedb y Crough and darrard [1981].
They showed that a depth and geoid anomaly extends from
o o RD-35 IS, the Marquesas Islands into the southern Line Islands and
RD-39 ascribedt his anomaly to the trace of a hot spot which transi-
o PCOD-6
ted the Line Islands in Eocene time. Vivid evidence that the
-- Christmas Is.
JarviIss .e RD_4 Line chain has a complexh istory of volcanisma s comparedt o
o
that of the Emperor chain is containedi n W. Haxby's geotec-
C'/(cid:127)ø(cid:127)e(cid:127)t.o. n (cid:127) o
tonic imagery map of the Pacific Basin [see Francheteau,
e 1983], a sketch map of which is shown here as Figure 2. In
addition to the marked Line cross trend that cuts across the
central Line Islands at 10øN a number of other linear ridges
intersect the northern and central Line Islands; the southern-
most of these extendst oward the Marquesas Islands and lies
along the depth and geoid anomaly of Crough and darrard
[1981]. The en echelon appearance of much of the central
Line Islands could be due to the presence of these short
NW-SE trending features which intersect the main Line Is-
lands trend. Bathymetrically then, the Line chain bears little
resemblanceto the relativelys impleE mperor chain.
Hendersona nd Gordon [1982] and Duncan [1983] have ad-
dressedt he complexity of the Line Islands and argue, based
on plate motion reconstructionso ver the past 150 Ma, that
several hot spots are needed to model the evolution of the
Line Islands. If all linear volcanic features or chains of vol-
Fig. 1. Chart of the Line Islands showingt he track of R/V Kana
Keoki (dashedl ine) through the area. Dredge stations are indicated by
RD numbers; piston core stations by PCOD numbers; seamount
magnetizations urveysb y L numbers.
LineI slands (cid:127)-(cid:127) /(cid:127)'
Cross-Tr.e..n,(cid:127) d G(cid:127)ppetø(cid:127) (cid:127)
Line Islands were not available. Driling along the Line Islands
on Deep Sea Drilling Project (DSDP) legs 17 and 33 (see
Winterer et al. [1973] and Schlangere t al. [1976] for details)
showedt hat volcanic episodesi n the northern Line Islands at ' (cid:127), o(cid:127) IMsailna LnCindeh xa (cid:127) in
site 171 on Horizon Guyot and at sites 165, 315, and 316 in
the central Line Islands were more temporally complex than
could be accountedf or by using the Emperor chain as a
model for the development of the Line Islands. Winterer
[1976] in an extensive review of the data relevant to the for-
Fig. 2. Sketchm ap of major featuresi n the Line Islandsp rovince
mation of the Line Islands emphasizedt he importance of the (unlabeledl inear trends are after the geotectonici magery map of W.
Line crosst rend (Figure 2) and argued that repeatede pisodes Haxby from Francheteau[ 1983]).
SCHLANGERE T AL.' GEOLOGYA ND GEOCHRONOLOGOYF THE LINE ISLANDS 11,263
canic edificeso riginate solely by hot spot activity of the crosst rend is the relic of a central Pacific rift systemt hat
Hawaiian-Emperors tyle, then a number of hot spotsm ust underwent limited crustal extension.
have contributed to the formation of the Line Islands. Other Strontium isotopic ratios have been determinedo n some of
mechanismsfo r the linear localization of volcanic activity these rocks using both untreated and HCl-leached material.
have been put forward.F arrat and Dixon [1981] postulated Unfortunately, most of these samplesa re at least somewhat
the presenceo f a long transformf racturez one that extended altered,a nd the valuesr eportedh ave very large uncertainties
from the Emperor Trough through the Gardner Seamounts (e.g.,0 .7030 _+0 .0009). Thereforei t is difficult to interpret the
and the Line Islands.O rwig and Kroenke[ 1980] also pro- valuesw ith any confidenceN. everthelesst,h e reportedi nitial
posed a fracture zone origin for the Line Islands. Natland 87Sr-SaSrar tiosf or basaltsv aryf rom0 .7028t o 0.7039( all but
[1976] consideredth at the petrologyo f rocksf rom the central one between0 .7035 and 0.7039),v irtually overlappingt he re-
Line Islands indicatedt heir similarity to rift associatedv ol- ported range for Hawaiian volcanic rocks (0.7029-0.7041
canismo f the African type. Winterer [1976] also invokedf is- [Lanpheree t al., 1980]) and generallyh ighert han typical mid-
sures and fractures in the Line Islands to account for some of oceanr idge basalts( MORB) ratios (0.7026-0.7035[ Schillinge t
the volcanica ctivity there. at., 1983]).
It is the purposeo f this paper to presentt he geological, Previousp etrologics tudiesh ave all been on samplesr ecov-
geochemical,a nd palcomagneticd ata that will outline the eredf rom the northernh alf of the Line chain; no samplesh ad
constraintso n any modelsp roposedf or the evolution of the been recovered from the chain south of DSDP site 315 near
Line Islands. Fanning Island at about 4øN. During the R/V Kana Keoki
1979 cruises,s amplesw ere dredged from along the entire
CRUISE OPERATIONS lengtho f the chain (Figure 1).
The rocks recoveredf rom dredge stations and those pre-
The geologica nd geophysicals tudieso f the Line Islands
viouslyr eported from the Line Islands [Basse t al., 1973;Jack-
discussedh ere were undertakend uring three cruises( of R/V
sone t al., 1976; Natland, 1976] are almost entirely moderately
Kana Keoki) to the area in the fall of 1979 (Figure 1). The
to severelya ltered alkalic basalts and hawaiites. In addition,
objectiveso f these cruisesw ere to dredge edifice volcanic
tholeiitic basalts and a phonolite have been recoveredf rom
rocks and cappingl imestone,t ake piston coresi n submarine
the central part of the chain [Natland, 1976]. The alkalic ba-
valleysi ncisedi nto the sedimentarya pron of the central Line
salts contain phenocrystso f plagioclaseo, livine (replacedb y
Islands province, conduct seamount magnetization studies,
iddingsite),a nd, rarely, augitc. The hawaiites are generally
and to collect magnetic,g ravity, and bathymetric data rele-
aphyric, although some contain rare phenocrystso f plagio-
vant to the problem of the evolution of the Line Islands.O ur
clasea nd/or brown hornblendeT. he matrix of both rock types
bathymetricm apping showedt hat some existingb athymetry
consistp redominantlyo f plagioclaset,i taniferousa ugitc,t itan-
was incorrecta nd misleadingA. mong our discoveriesw ere an
iferousm agnetite,a nd, locally, clays replacingg lassa nd oli-
unmappeds eamounta t 1l ø40'N, 160ø40'Wa, ridge whichr ises
vine. Mineralogically,t hese rocks are typical of Hawaiian
to 2700 m in depth connectinga seamounta t 6ø20'N, 158øW
lavas.
with a ridge at 6ø20'N, 159ø30'W;a nd a ridge nearly 300 km
Microprobe analyseso f groundmassa ugitc in the Line Is-
long consistingo f eight peaks ranging from 3000 to 2150 m
lands lavas indicate that they contain high TiO2 (1.0-4.0 wt
depth connectingC aroline Island at 10øS, 150ø20'W with a
%) and A1203( 2.0-9.1 wt %) [Jacksone t al., 1976]. A plot of
seamounta t 7ø35'S,1 51ø35'W.I n addition, Carol Guyot (9øN,
SiO: versusA IeO3 showst he Line Islands lavas to be of alka-
158øW)a nd Vostok Island (10øS,1 52øYW) were found to be
line to peralkaline affinitiesb ased upon the classificationo f
incorrectlyc ontoured.
LeBas [1962] (Figure 3), although two samples,R D-33-4 and
A total of 21 dredgesr ecoveredv olcanicr ocks.S edimentary
RD-43-1 from the central Line Islands are of the nonalkaline
rock samplesw ere recoveredi n 19 dredges.I n addition, sedi-
type.
ments( and in one caseb asaltp ebbles)w ere recoveredf rom six
The freshests amplesf rom eachd redgeh aul were chemically
piston cores.U nderway gravity and magneticsw ere recorded
analyzed( Table 1). All of the samplesa re moderatelya ltered.
over the entire lengtho f the Line chain and over the margin of
Some samplesh ave high Fe:O3/FeO ratios and P:O5 and
the Tuamotu Ridge. Eleven seamountm agnetizations urveys
H20 contents.N everthelesst,h e least altereds amplesa re simi-
were conducted.
lar chemically to Hawaiian alkalic lavas. They plot in the
Hawaiian alkalic lava field and span the range from alkalic
PETROLOGY
olivine basalt to mugearite using the classificationo f Coombs
Previous petrologicw ork on volcanicr ocks from the Line and Wilkinson [1969].
Islands is almost entirely reported on in the DSDP initial Rare earth elements( REE) were determinedb y isotoped ilu-
reports of legs 17 and 33 [Winterer et al., 1973; Schlangere t tion masss pectrometryo n selecteds amples.O ne of the sam-
al., 1976]. At DSDP site 315, six flow units of transitional ples (RD-60-6) has the REE pattern of a typical Hawaiian
basalt (intermediate in composition between Hawaiian tho- tholeiite (Figure 4). The other sampless hown have REE con-
leiitic and alkalic lavas) were recovered[ Jacksone t al., 1976]. centrations similar to those of Hawaiian alkalic lavas.
Hawaiites,s omec ontainingb rown amphibole,w ere recovered Jacksone t al. [1976], in addressingth e suggestionth at the
at DSDP site 165 [Bass et al., 1973]. A wide variety of rocks Line Islands are an abandoned ridge crest, argue that the
dredged from the northern and central portions of the Line chemistryo f the basaltsd rilled at DSDP sites1 65 and 315 are
Islandsi ncludet holeiitesa, lkalic basaltsh, awaiites,p honolites, inconsistenwt ith a ridge cresto rigin. In their review of analy-
and potassicn ephelinitesN. atland [1976] consideredt he po- sesi n hand following leg 33 [Jacksone t al., 1976] they came
tassic nephelinitesd redged at four locations in the Line Is- to the conclusiont hat the Line Islandsa re made up of basaltic
lands area as similar in compositiona nd origin to potassic rockst hat are similar to basaltso f the Hawaiian chain. Analy-
mafic lavas from African rift zones.H e argued that the pres- ses reported here support the argument that the main Line
ence of such rocks in the Line cross trend indicated that the Islandsc hain was built of effusionso f typical alkalic and tho-
11,264 SCHLANGERE T AL.: GEOLOGYA ND GEOCHRONOLOGYO F THE LINE ISLANDS
x
52
51
NONAL
50-
(cid:127) :(cid:127)iii:i!(cid:127)',':,(cid:127)-i';:i:(cid:127)(cid:127)(cid:127)-i'.:(cid:127)i7:-i; ! (cid:127)::!:.(cid:127) i';-:'(cid:127)i'i..(cid:127)'i(cid:127)i::ii iii.!iii:(cid:127)ii (cid:127)i! i:l:(cid:127)!i!i i?ii!i!ii(cid:127):.(cid:127)!(cid:127)i. !i561ii-3i9! ii(cid:127)i-ii1i(cid:127)::.3: : :.
.o ]
(cid:127) (cid:127) .... ,.:.:.::.:....:...: - ?(cid:127)!,;(cid:127),;:.:?-,
ALKALINE
A ,-,,.
.0_ 47
lO
46- E,
PERALKALINE K La Ce Nd Sm Eu Gd Dy Er Yb Lu
45'
Fig. 4. Chondrite-normalized rare earth element patterns for
some representativel avas from the Line Islands. Fields for Hawaiian
44 ' ] [ I ] I ] I ' I (cid:127) I ' I ' I ' I ' tholeiitesa nd alkalics and mid-ocean ridge basalt (MORB) from Ba-
0 I 2 3 4 5 6 7 8 9
saltic VolcanismS tudy Project [1981].
' AI203 Wt.%
Fig. 3. SiO2 and AI203 wt % in groundmassc linopyroxene
[Winterer et al., 1973; Schlangere t al., 1976]. At DSDP site
grains from lavas from the Line chain. Fields for nonalkaline, alka-
165 a complex and long history of Cretaceousv olcanism was
line, and peralkaline rocks are from LeBas [1962]. A, RD-41-1; B,
RD-41-2; C, RD-44-2; D, RD-54-2; E, RD-59-1, H, RD-59-9; I, RD- revealed [Lanphere and Dalrymple, 1976; Winterer et al.,
59-10; J, RD-59-15; K, RD-63-7; X, RD-52-7. 1973]. Three basalt units interbedded with fossiliferouss edi-
ments were cored at the site. Between the two deepestb asalt
flows fossils of the Eiffelithus eximus zone were recovered.
leiitic Hawaiian type lavas and that there is no compelling
Taking the base of this zone to be at the Santonian-
evidencet hat a ridge crest origin is warranted for the Line
Islands. Campanian boundary dated at 83 Ma by Harland et al.
[1982], we can deduce that volcanism was active at least 83
GEOCHRONOLOGY Ma. The age of the deepestb asalt, based on sedimentation
The basic data set constrainingd iscussionso f the origin of rate arguments could be as old as 93-101 Ma [Lanphere and
linear island chains consistso f determinations of the ages of Dalrymple, 1976]. Higher in the section at site 165 are thick
volcanic breccia and sandstone beds which indicate that late-
volcanic edifice formation along the chain. The data used in
stagev olcanismp ersistedi nto the Tetralithus trifidusz one of
this paper consisto f datesd erivedf rom interpretationo f vol-
late Campanian age dated at 74 Ma by Harland et al. [1982].
canica nd sedimentarys equenceds rilled on DSDP legs 17 and
A rock dredgedf rom the large seamountj ust to the east of site
33, edifice ages determined by dating of fossil assemblages
165 was dated at 72 Ma usingt he 4øAr-39Arm ethod[ Saito
dredgedf rom seamountsa, nd radiometrica gesd eterminedb y
4øAr-39Ara nd K-Ar methodso n dredgedr ocks collectedb y and Ozima, 1977]. Thus major volcanism in the immediate
the Hawaii Institute of Geophysics( RD and PC prefix) and vicinity of site 165 occurred over a time span of at least l l
the ScrippsI nstitution of Oceanography( D suffix)( Figures 5 Ma, from 72 to 83 Ma and possibly longer. At site 315
and 6 and Tables 2 and 3). Becausea ge determinationsl ie at [Schlangere t al., 1976] the east flank of the Fanning Island
edifice was drilled. The basalt cored at the bottom of the hole
the heart of any discussiono n the origin of a chain sucha s the
Line Islands, the data used are discussedb elow in the geologi-
180 øW 160 140
cal context of the various locationss hown on Figures 1 and 5.
Three DSDP sitesw ere drilled along the central Line chain
in an attempt to reconstructt he history of edifice building
(cid:127)id_-Paciffoxic 40Ar-3A9Agre s
ß This paper 20 øN
:L/Mr-tl(cid:127) / (cid:127)(cid:127)(cid:127)(cid:127) 8( 143D) 0$ aito&Ozima(1977)
TABLE 1. Major Element Analyseso f Volcanic Rocks From the (cid:127)(cid:127)_(cid:127)4 (cid:127))(cid:127)6(RD63) FosAsgile s
Line Islands Chain 2
(cid:127) (cid:127)81 83(RD61) f(>78)
56 0 ,
Sample
(RDS9) 083 (133D)
(cid:127)(cid:127) x 79(cid:127) 128D)
63-7 61-1 60-6 59-13 41-2 43-1 44-3 45-1 52-2 _& (cid:127) (cid:127)39(cid:127)B(cid:127)33)
SiO2 41.70 45.20 46.45 42.90 45.20 42.75 41.75 38.65 44.55
TiO 2 3.48 3.88 2.02 2.78 2.88 2.60 3.22 2.34 2.93
AI203 13.53 17.02 16.20 17.64 13.74 17.97 15.80 15.72 15.87 (pc(cid:127) 2(cid:127) (RD4')
Fe20 3 9.19 9.48 8.44 5.81 11.12 13.27 13.68 11.36 9.70 o
FeO 3.62 2.60 2.56 5.56 1.96 0.97 0.92 0.48 3.04 DS(cid:127)P31 (cid:127) (cid:127)9(RD43)
MnO 0.14 0.09 0.09 0.18 0.18 0.13 0.12 0.21 0.15 f(>78) .
LINE ISLANDS
MgO 5.48 2.20 4.57 4.57 6.85 0.97 1.92 1.13 4.05
o
CaO 11.85 8.66 10.30 8.11 12.55 8.58 10.19 14.76 11.53
ø(RO44)
Na20 1.97 3.22 2.95 2.86 2.58 3.34 3.02 2.83 2.60 172 Marque
K20 1.21 2.46 1.03 1.94 0.91 1.75 1.29 1.71 0.92 o /(cid:127)._(cid:127)_1 -10
f(44-50) (RD45)
P205 0.93 1.98 0.58 1.88 0.37 2.12 2.81 5.87 1.48
0 1000 km 0 DSDP 318 f(~52)
H 20 + 3.29 1.95 2.43 3.60 0.62 3.39 2.54 2.51 1.50
H20- 2.48 1.26 1.92 1.93 0.74 1.90 2.16 1.37 1.27
CO 2 1.05 0.02 0.08 0.17 0.16 0.29 0.37 0.83 0.24
Total 99.92 100.02 99.62 99.93 99.78 99.85 99.79 99.77 99.83
I I
Analyst' K. Ramlal, Universityo f Manitoba. Fig. 5. Stationl ocationsa nd aged eterminationsin the Line chain.
$CHLANGERE T AL.' GEOLOGYA ND GEOCHRONOLOGOYF THE LINE ISLANDS 11,265
Mid-Pacific
Mtns 40Ar-39Ar Ages
ß This paper
85
0 8aito&Ozima( 1976,1977)
45
motus
al
4500
85 K-Ar Ages
,TH
ß o o H(cid:127)'...
45 o ß o '....
ß
bl ß
ß
4500 a(cid:127)o 27(cid:127)o ,(cid:127)oo 9(cid:127))0 b
Distanc'e from Line-Tuamotu Bend km
Fig. 6. Radiometrica nd paleontologiacg esi n the Line chain.( a) '(cid:127)øAr-39Ara gess howna s solidc irclesw ered eter-
mined in this study; open circlesa re datesf rom Saito and Ozima [1976, 1977]. The solid line showsa rate of migration of
volcanismo f 9.6 + 0.4 cm/yr which would accountf or many of the agesd etermined.( b) K-Ar agesa nd biostratigraphic
age determinations( DSDP sites1 65, 315, and 316 and seamounts( H) from Haggetty et al. [1982]).
yielded a feldspar fraction K-Ar age of 91.2 + 2.7 Ma [Lan- [Schlangere t al., 1976]. In summary the DSDP resultsf rom
phere and Dalrymple, 1976]. Their age convertst o 93.3 Ma the main Line Islandss ystems how the following'
upon usingn ew constants[ Harland et al., 1982], the age used 1. Volcanism at site 165 and the nearby seamount from
in this paper. Nannofossilso f the Marthasteritesfu rcatus zone which dredgeh aul 130-D was taken occurredf rom 83 Ma (or
were recovered 85 m above the basalt. These sediments are of possiblye alier) to 72 Ma.
Coniacian age, the M. furcatus zone being dated at 88 Ma 2. At site 315, major edifice constructionc easedb y ,--93
[Hadand et al., 1982]. The correlationo f the fossila ge and the Ma; initiation of volcanism is undated but could be prior to
K-Ar age at site 315 arguesf or a cessationo f major edifice ,,- 95 Ma.
building activity at ,,-93 Ma. Further, Premoli-Silvaa nd Brusa 3. At DSDP site 316 the cessation of volcanic edifice
[1981] record the presence of a mid-Cretaceous Cuneolina building is estimateda t 81-83 Ma; initiation of volcanismi s
assemblagein turbiditesa t this site,i mplying the existenceo f a undated.
seamount in the photic zone in Cenomanian-Albian time, In our study of rocks dredgedf rom the Line Islands, both
>90 Ma. K-Ar and '(cid:127)øAr-39Ar dating techniquesw ere employedt o
At site 316, basalt basement was not reached. The oldest examinet he distributiono f agesw ithin this volcanicl ineament
sedimentsp enetratedy ielded foraminiferaa nd nannofossilso f (Figures 5 and 6 and Tables 2 and 3). Previous attempts at
early Campanian age (77-80 Ma) associatedw ith basaltic measuring reliable crystallization ages by the conventional
breccias and volcanic sandstones. On the basis of the corre- K-Ar method [Davis et al., 1980] have been frustratedb y the
lation of stratigraphics ectionsb etweens ites3 16, 315, and 165, effects of seawater alteration. Both K addition and Ar loss
basalt basementa t 316 probably lies ,,-75 m below the deep- during low-temperature clay and zeolite formation, in the
est sedimentsr ecovered;t he data of cessationo f major edifice- presenceo f seawater,w ill cause the measureda ge to be sys-
building volcanism at 316 can be estimated at ,,- 81-83 Ma tematicallyl ower than the rock crystallizationa ge.R econnais-
11,266 SCHLANGER ET AL.' GEOLOGY AND GEOCHRONOLOGY OF THE LINE ISLANDS
TABLE 2. The 4øAr-a9ATr otal FusionA ge Data on DredgedV olcanicR ocksF rom the Line Islands
Chain
Percent
Radiogenic Age + 1(cid:127)
Sample Location '(cid:127)øAr-a6Ar '(cid:127)øAr-a9Ar 37Ar-'(cid:127)øAr* ½øAr x 106y ears
143D-102 19ø30'N, 169ø0YW 3011. 32.03 0.0538 90.0 88.1 + 0.4
142D-11 18ø00'N, 169ø05'W 57028. 38.16 0.0092 99.4 93.4 + 1.3
RD63-7 16ø27'N, 168ø1YW 1456. 35.36 0.1525 79.4 86.0 + 0.9
RD59-12 12ø31'N, 167ø0YW 2463. 71.24 0.0106 88.0 85.0 + 1.1
RD61-1 14ø59'N, 166ø27'W 965.9 86.32 0.0149 69.4 81.4 + 1.1
RD61-5 14ø59'N, 166ø27'W 835.3 79.74 0.0221 65.1 82.6 +_0 .7
128D-11 9ø15'N, 160ø45'W 642.1 59.24 0.0089 53.8 78.7 + 1.3
RD33-1 8ø11'N, 161ø55'W 379.8 128.84 0.0371 22.2 39.3 + 1.5
123D-15 5ø50'N, 160ø45'W 2251. 28.74 0.1058 76.4 76.4 + 0.5
PCOD-6-2 2ø35'N, 158ø30'W 2524. 58.05 0.0243 88.2 69.8 + 1.0
RD41-1 2ø06'N, 157ø21'W 705.8 23.57 0.1929 57.7 35.5 + 0.9
RD43-1 0ø42'S, 155ø17'W 702.4 33.15 0.0956 57.6 59.0 + 0.8
RD44-3 7ø35'S, 151ø3YW 461.8 65.24 0.0445 35.9 71.9 + 1.4
RD45-26D 9ø04'S, 150ø42'W 633.7 53.57 0.1238 53.1 70.5 + 1.1
RD52-1 15ø01'S, 149ø02'W 544.1 75.62 0.0869 45.6 47.4 + 0.9
RD52-2 15ø01'S, 149ø02'W 472.7 81.12 0.0914 37.4 41.8 + 0.9
*Ar 37 correctedf or decay sincei rradiation.
sanceK -Ar agesd eterminedin this study( Table 3) were simi- erally fresh,p articularly in the more trachytic-texturedr ocks
larly scattereda nd significantlylo wer than indirect age esti- (143D-102, 142D-11, RD-59-12, and PROD6-2). Olivine
mates basedo n paleontologicd ata [Haggerry et al., 1982] or phenocrystsa re commonly altered to iddingsite.T he matrix
seamountm agnetizations tudies[ Sager, 1983a]. phasess ucha s feldspara nd clinopyroxenea re generallyf resh,
Recentlyt, he applicationo f the '(cid:127)øAr-39Ar total fusion but where devitrified glass occurred, it has been altered to
method to dating moderately altered submarine volcanic clays.Z eolites and calcite are common in vesiclesa nd along
rocks has proven successfuiln determiningc onsistenta ges microfracturesA. further indication of the degreeo f alteration
[Dalrymplea nd Clague,1 976; Duncan,1 978, 1982; Dalrymple of these samplesi s the high-H20 content in most samples
et al., 1981]. According to these studiesi t is probable that (between0 .6 and 3.6 wt %) and in rare cases,h igh-CO2 con-
39Ar, derivedf rom K-bearing alterationm ineralso r from K tent (up to 1 wt %).
residingo n grain boundariese, scapesfr om the sampled uring Argon isotopic compositionsw ere measuredu sing an AEI
neutroni rradiationo r extractionli ne bake-out.R adiogenic MS-10Sm asss pectrometeerq uippedw ith 3BArs pikea nd air
'(cid:127)øAr was lost from preciselyth eses itesd uring alteration,s o calibrationp ipettes ystemsF.o r 4øAr-39Aar gess, plita liquots
the age signalf rom the low-temperaturea lterationp hasesi s of crushed samplesw ere irridiated at the U.S. Geological
essentialleyr asedp rior to samplefu sionT. hust he 4øAr-39Ar SurveyT RIGA Reactor in Denver, Colorado, for 4 hours at 1
age commonlya pproachesth e crystallizationa ge becauseth e MW power. Operating conditions and isotope interference
predominants ourceo f nonatmospherica rgon is the primary correctionsh ave beend escribedb y Dalrymplee t al. [1981].
crystallinep hases. That conventionalK -Ar analysesu nderestimateth e ageso f
Samplesw ere selectedfr om the Line Islandsc ollectionsfo r Line Islands volcanic rocks is particularly apparent in the
K-Ar and '(cid:127)øAr-39Ar geochronologoyn the basiso f thin sec- scattereda geso f samplesf rom the same seamount( RD-45-1,
tion examination.P etrographically,a ll sampless how slight to RD-45-26D) or neighborings eamounts(T able 3 and Figure 6).
moderatea lteration. Augite and feldsparp henocrystsa re gen- Where indirect age estimatesa re available,K -Ar age determi-
TABLE 3. K-Ar Age determinationsF rom Dredged VolcanicR ocksF rom the Line IslandsC hain
Percent
Percent Radiogenic'(cid:127)ø Ar Radiogenic Age* + 1(cid:127)
Sample Location K x 10- s cma/g Ar x 106 years
143D-102 19ø30'N, 169ø0YW 2.097 0.6128 88.8 73.7 + 0.8
RD63-7 16ø27'N, 168ø1YW 1.194 0.2589 78.6 55.0 + 0.6
RD61-1 14ø59'N, 166ø27'W 2.098 0.4840 67.3 59.8 + 0.6
133D-9 12ø04'N, 165ø50'W 1.666 0.4808 65.8 72.8 + 1.3
128D-11 9ø15'N, 160ø45'W 0.725 0.1370 56.6 48.0 + 0.6
RD33-1 8ø11'N, 161ø55'W 0.695 0.0625 44.8 23.5 + 0.3
123D-15 5ø50'N, 160ø45'W 1.996 0.3478 86.6 44.3 + 0.5
PCOD-6-2 2ø35'N, 158ø30'W 1.601 0.3783 90.2 61.2 + 0.6
RD41-1 2ø06'N, 157ø21'W 1.016 0.1005 75.6 25.3 + 0.3
,,
RD43-1 0ø42'S, 155ø17'W 1.411 0.2047 53.8 37.8 + 0.4
RD44-3 7ø35'S, 151ø3YW 1.120 0.1780 32.8 41.4 + 0.5
RD45-1 9ø04'S, 150ø42'W 1.301 0.2259 34.4 45.2 + 0.6
RD45-26D 9ø04'S, 150ø42'W 0.915 0.2133 58.4 59.0 + 0.7
RD52-2 15ø01'S,1 49ø02'W 0.742 0.0712 35.9 25.1 + 0.4
*Agesc alculatefdro mt hef ollowindge caya nda bundancceo nstant2s(cid:127) '= 0.581x 10- (cid:127)ø yr- 2(cid:127) =
4.962 x 10- 'ø yr- '(cid:127)øK/K = 1.167x 10- '(cid:127) mol/mol.
SCHLANGERE T AL.' GEOLOGYA ND GEOCHRONOLOGOYF THE LINE ISLANDS 11,267
nations are always younger,b y 9-30 Ma, and are not reliable studied,b ut a usefulr esultw as obtainedf rom only one, L3 in
indicatorso f samplec rystallizationa ge,d espiteg enerallyh igh Figure 1 [Harrison et al., 1975]. The magnetizationp arame-
proportionso f radiogenica rgon. ters and virtual geomagneticp oles (VGP's) derived from the
The '(cid:127)øAr-39Art otal fusiona gesa re muchm orec onsistent, eight seamountm agneticm odelst hat yieldeda pparentlyr eli-
however( Table 2 and Figure 6). Where multiple samplesf rom abler esultsa re givena longw ith '(cid:127)øAr-S9Ara gesd etermined
the sames eamountw erea nalyzed( RD-61-1, RD-61-5, RD-52- for them in Table 4.
1, and RD-52-2), age agreement was good. Samples from The goodnesso f fit ratio (GFR) [Richards et al., 1967],
neighboring seamounts( 143D-102, 142D-11, and RD-63-7; measuring the match between the observed and calculated
RD-44-3 and RD45-26D) gave similar ages.F urther, the con- anomaliesi,s high for all of the seamountsa nalyzedi n this
cordance of our radiometric dates with age determinations study,r anging from 3.1 to 5.2. The larger the GFR the better
based on paleontologicala nd seamountm agnetizationd ata the agreement between the observed and calculated anoma-
indicatet hat the '(cid:127)øAr-39Ara gesa re reliableH. aggettye t al. lies; usually a GFR of 1.8-2.0 is consideredt o be the mini-
[1982] determineda minimum age of 70-75 Ma, basedo n the mum acceptablev alue for a reliable magnetici nversion[ Har-
occurrence of Late Cretaceous shallow water fossils, for the risone t al., 1975; Sager,1 983a]. Not only do the GFR's of the
seamountf rom which RD-45 was taken; as shown on Table 2, Line seamountsi ndicate excellent results, but the paleo-
RD-45-26D yieldeda '(cid:127)øAr-39Ara ge of 70.5+ 1.1 Ma. Sea- magnetic poles derived from these seamountsa re consistent
mount L8 has a virtual geomagneticp ole which lies closet o a among themselvesa nd with other Pacific seamount VGP's.
Maestrichtian pole of both Gordon [1982a] and Sager Moreover, the VGP's of the reliably dated seamountsfr om the
[1982a];R D-44-3f romt hiss eamounyti eldeda '(cid:127)øAr-S9Aar ge chain that have been studiedp aleomagneticallya re in close
of 71.9 + 1.4 Ma, within the 65-73 Ma span of the Maestrich- agreementw ith other Pacific paleomagneticd ata of the same
tian stagea ccordingt o Harland et al. [1982]. The majority of age,a s is discussedb elow.
samplea gesf alls along a linear array, from older seamountsin Four of the Line seamountsg ave VGP's closert o the geo-
the north (-,(cid:127)93 Ma) to youngers eamountsin the south( -,(cid:127)44 graphicp ole than any VGP derivedf rom a CretaceousP acific
Ma at the Line-Tuamotu bend). The overall consistencyo f seamount( Figure 7) implying that thesef our seamountsa re
this data set encouragesc onfidencein thesee stimateso f vol- younger than Cretaceousa ge. These four VGP's are located
cano ages. It is difficult to imagine, for instance,a process closet o the VGP's of three Pacific seamountsth at probably
which would produce progressivelyg reater alteration to the formed during the late Eocene or early Oligocene.O f the
south to yield the observedp attern. A rate of migration of three,o ne is inferredt o be approximately4 1-42 Ma of age by
volcanismo f 9.6 + 0.4 cm/yr bestf its the linear array of sea- its positionin the Hawaiians eamouncth aina nd by its mag-
mount ages. A number of ages,h owever, do not fall on the netic polarity [Sager, 1984]. The other tWO seamountsa re
main linear trend, these are also considered reliable and are located in the eastern Pacific on seafloor 45 and 37 Ma old as
discussed below. indicated by magnetic lineations [Sager, 1983b]. In addition
Earliers tudiesh aver eported'(cid:127)ø Ar-39Ara ged eterminations to this data, a DSDP sedimentp a!eolatitude( site 166) calcu-
from a number of seamounts in the northern and central Line lated from sampless panningt he late Eocenea nd early Oligo-
Islands[ Saito and Ozirna,1 976,1 977].W hereb oth '(cid:127)øAr-39Ar cene records a paleopole locus in agreementw. ith the four
total fusion and incrementalh eating agesw ere reported,w e Line IslandsV GP's as doesa paleoequatort ransit determined
have plotted the total fusiona gesc onsideredto be reliableb y from sediment facies examined at DSDP Site 163 [Sager,
Saito and Ozima (showna s open circles)f or comparisonw ith 1983b].
the agesd etermineidn the presenst tudy( showna s solidc ir - Of the four seamountst hat give high-latitude VGP's, only
des) in Figure 6. Two seamounts1, 30D and 133D, yield Cre- one has been dredgeda nd dated. SeamountL 4 (RD-33) has
taceousa gesw hich fall along the main linear trend. Another is an 4øAr-39Ator talf usiona geo f 39.3- +_1 .5M a. SeamounLt5
Eocenei n age. Saito and Ozirna [1977] reported an isochron is directly adjacent (and possiblyc onnected(cid:127)t o) to seamount
age of 127.5 + 5.0 Ma and a total fusion age of 114.8 + 1.3 L4 [Sager and Keating, this issue] and is thus likely to be the
Ma for 142D dredged from the northern end of the Line samea ge, particularly consideringt he nearnesso f their VGP's
Islands.W e determineda n age of 93.4 + 1.3 Ma for this site. (Figure 7). Neither L6 nor L7 have been dated, but the prox-
The reasonf or this discrepancyis unknown. Other agesf rom imity of their VGP's to the other Eocene-Oligocenep aleo-
neighboring seamountss upport our younger age determi- magneticp oles coupledw ith the fact that a significantn umber
nations, however. of Eocene ages have been determinedf or seamountsi n the
Line Islands( Table 3) [Saito and Ozima, 1977; Haggerry et al.,
SEAMOUNT PALEOMAGNETISM
1982] suggeststh at these two seamountsa re also likely to be
Measurement and analysiso f the magnetic field of a sea- the samea ge.
mount can yield information on the paleoazimuth and lati- The other four Line Islands seamounts studied paleo-
tudinal transport of a seamounts ubsequentt o its formation magnetically produced VGP's that are located among the
and on the location of its virtual geomagneticp ole (VGP); the VGP's of Pacific seamountso f Cretaceousa ge. Both L1 and
location of its VGP allows inferences to be drawn as to the L8 have VGP's locatedn ear the Maastrichtianp aleomagnetic
age of the seamount. In this paper the implications of the polesc alculatedb y Gordon[ 1982a] and Sager [1983a] (Figure
paleomagneticre sultsf or periodso f volcanisma long the Line 8). L1 is undatedb, ut a rock from L8 yieldeda n '(cid:127)øAr-39Ar
chain are stressed.T he details of the surveys and data re- total fusion age of 71.9 _+1 .4 Ma (RD-44) which agreesw ell
duction methods are describedb y Sager and Keating [this with the age inferredf or the volcanof rom fossils[ Haggerry et
issue]a nd Sager [1983a]. al., 1982] and from its reversedp olarity [Sager and Keating,
To these ends, 11 seamounts in and around the Line chain this issue].
were surveyedf or paleomagnetica nalysis;o f these,s eveng ave Seamounts L2 and L3 are located close to one another in
apparently reliable results.P revious to our work, eight sea- the Line chain( Figure7 ) and have4 øAr-39Atro tal fusiona ges
mounts in and around the chain had been paleomagnetically that are virtually identical, 85.0 _+1 .1 Ma (L2, RD-59) and
11,268 SCHLANGERlE TA L.' GEOLOGYA ND GEOCHRONOLOGOY F THE LINE ISLANDS
TABLE 4. Magnetization Parameters for Line Islands Seamounts
Location VGP
Latitude Longitude Latitude Longitude Inclination Declination Intensity, Age,
Name ID N E N E ( + Down) ( + East) A/m GFR m.y. Reference
Watkins L1 17.5 190.8 68.8 33.5 -4.3 352.0 5.8 5.2 1
Nagata L2 12.5 193.0 61.6 4.0 - 29.0 4.4 3.8 3.7 85.0 + 1.1 2
Kapsitotwa L3 12.0 194.2 47.5 333.5 -36.6 28.0 5.1 3.7 82.7+ 3.6 3
Stanley L4 8.2 198.1 75.6 356.5 - 10.5 5.3 3.2 4.7 39.3 + 1.5 4
Willoughby L5 7.9 198.1 78.2 14.6 - 7.8 0.7 3.5 4.4 4
Chapman L6 3.4 199.9 75.7 37.8 - 19.7 355.6 4.7/8.0 4.3 4
Clarke L7 - 3.3 206.0 80.0 20.4 - 25.3 1.0 3.3 4.0 4
Uyeda L8 - 7.5 208.5 68.5 345.6* 40.1 195.7 6.9 3.1 71.9 + 1.4 4
GFR, goodnesso f fit ratio. References'1 , Keating and Sager [1980]' 2, Sager et al. [1982]' 3, Harrison et al. [1975]' 4, Sager and Keating
[this issue].
*Reverselyp olarized,s outh pole given.
82.7 + 3.6 Ma (L3, 133D) [Saito and Ozirna, 1977], yet their age of two samplesc ould result in a larger separationo f their
VGP's are separatedb y over 22ø . This phenomenonw as puz- paleopoles.C onsequently,w e hypothesizet hat L2 is slightly
zling until it was noted that apparent polar wanderj ust prior youngert han L3, which is allowedw ithin the analyticalu ncer-
to 80 Ma was very rapid (Figure 8) [Gordon, 1983; Sager, taintyo f their 'tøAr-S9Atro tal fusiona ges.
1983a], so that a differenceo f only a few million years in the
VOLCANIC EVENTS
The above describedD SDP results, radiometric ages,b io-
stratigraphic ages, and paleomagneticr esults determined for
+ NP seamounts and linear ridges in the Line Islands reveal the
complexityo f volcanice ventsi n this province.F igures/5a nd 6
+ summarize the relevent radiometric and fossil age determi-
(cid:127)o
nationsf rom the site of Horizon Guyot south to the Tuamotu
Islands. Table 2 lists the data referred to in this section. For
20
the purposeso f this discussionth e well-definedL ine chain is
taken to extend from Horizon Guyot and the northern end of
3
the linear ridge from which dredge 142-D was taken to the
southern end of the line of seamounts from which RD-45 was
+ NP (cid:127)'
L5
L4
Fig. 7. Comparison of high-latitude seamount paleopoles and
Pacific Eocenep aleomagneticd ata. The VGP's of seamountsL 4-L7
are shown by open stars. L4 has a total fusion age of 39 + 1.5 Ma.
SeamountsE l, E2, and HR1 have VGP's shown by the solid stars.
HR1 has been assigneda n age of 41-42 Ma by its position in the
Hawaiian chain [Sager, 1984] and E1 and E2 have maximum ageso f
45 and 37 Ma, respectivelyd, etermined by the age of the seafloor
upon whicht hey rest [Sager,1 984]. The heavyl ine is a segmenot f the
small circle that is the locus of the paleomagneticp oles measured
from sediments amplesf rom DSDP site 166. These samplesr ange in
age from Oligocenet o Eocene [Jarrard, 1973]. The thin line is a Fig. 8. Comparison of Line Islands seamount paleomagnetic
similar segmento f the polar circle estimatedf rom the identificationo f poles with Pacific apparent polar wander paths. The Line Islands
equatoriasl edimentisn the coresf rom DSDP site 163.I t wasd eter- VGP's are shownb y solid stars.S olid circless howt he locationso f the
mined that this site crossed the equator 43 + 6 Ma [Suarez and meanp aleomagnetipco lesc alculatedb y Sager[ 1983a, 1984],w hereas
Molnar, 1980]. The large dot is the mean position of the Eocene the solid squaresr epresentt he mean pole positionso f Gordon[ 1982a,
seamountV GP's and the ellipsei s the 95% confidencer egion of the 1983]. The solid line is the apparent polar wander path of Sager
mean paleomagneticp ole [Sager, 1984]. The small dots connectedb y [1983a]' the dashedl ine, the path of Gordon[ 1983]. The 95% confi-
the dashedl ines are positionso f the paleopolep redictedb y the Paci- dencer egionss urroundingth e mean paleomagneticp olesa re shown
fic plate/hot spot rotation model of Jarrard and Clague [1977] (with as ellipses,a nd the numbersw ithin the ellipsesa re the mean ageso f
rotation rate corrected by Dalryrnplee t al. [1977]) labeled at 10-Ma the poles in millions of years. SeamountsL 2, L3, L4, and L8 have
intervals. The diamonds are the VGP's of Cretaceous seamounts from total fusion ageso f 85.0 + 1.1, 82.7 + 3.6, 39.3 + 1.1, and 71.9 + 1.4
Harrisone t al. [ 1975].M ap projectionis polar equala rea. Ma, respectivelyM. ap projectioni s polar equal area.
SCHLANGERE T AL.' GEOLOGYA ND GEOCHRONOLOGOYF THE LINE ISLANDS 11,269
taken that extendn orth from Caroline Island. The sequenceo f At the southern end of the central Line Islands the cluster of
volcanic events at both the northern and southern boundaries dates represented by PCOD-6 (69.8 _+ 1.0 Ma), RD-41
of the Line chain are also complex. Drilling on Horizon (35.5 _+0 .9 Ma), RD-43 (59.0 _+0 .8 Ma), and DSDP site 316
Guyot [Winterer et al., 1973] showed this ridge to have a (minimum age of cessationo f volcanisma t 81-83 Ma) presents
history of two Cretaceous volcanic events separated by a further complicationsf or a single hot spot linear progression
period of Cenomanian reef growth. Further, dates obtained in model. Sample PCOD-6 consistedo f a relatively fresh cobble
our study from the northeast end of the Tuamotu Islands of of plagioclase-richb asalt in a piston core taken in a turbidite-
41.8 +_0 .9 and 47.4 _+0 .9 Ma (RD-52) complicate the gener- filled basin which lies downslopef rom the site of RD-41. This
ally accepted argument that the Line-Tuamotu elbow is basalt cobble must representd ebris from a seamount.T hus we
coevalw ith the Emperor-Hawaiian elbow dated at 43 Ma. On see in a very close geological conjunction edifice ages of
the basiso f fossile videnced erived from drilling at DSDP site 69.8 + 1.0 and 35.5 + 0.9 Ma. The PCOD-6 and RD-43 dates
318 in the Tuamotu chain [Schlan(cid:127)ter et al., 1976], where late fit on the linear progressionl ine of Figure 6. The cessationo f
early Eocene shallow water fossils in turbidites overlie the volcanism at DSDP site 316 greatly predates both PCOD-6
presumedb asalt basement,i t was argued that the volcanic and RD-43; three distinct periods of volcanisma re seeni n this
edificeso f the Tuamotu ridge could have formed more than 51 region. Seamount L7 of this paper, of Eocenea ge, could also
Ma. Also, Ha(cid:127)l(cid:127)lerty et al. [1982] determined, paleon- be ascribedt o the main age-progressivetr end linear ridge.
tologically,t hat Eocenev olcanism,- ,(cid:127)44-50 Ma, took place in The final set of dates to be discussed are those from the
the southern Line chain at the site of RD-45. Therefore the southernL ine Islands,d redge hauls RD-44 and RD-45. Prior
data available at this time show that volcanism in the area of to the determination of radiometric age dates from these
the Line-Tuamotu elbow took place from more than 51 to 42 dredgeh auls, Ha(cid:127)t(cid:127)terty et al. [1982] had determinedo n fossi.
Ma. evidence that the seamount from which RD-45 was obtained
Within the Line chain the following chronologicd ata needs had a minimum age of 70-75 Ma. This seamounti s capped by
to be taken into account. As shown on Figures 5 and 6, the coarse-grainedb ioclasticl imestoneo f reefal origin which con-
majority of dated volcanic edificesd efine an overall age pro- tains rudists, calcareousa lgae, and volcanic rock fragments.
gression along the Line chain from the location of dredge Further, volcanic peperite was dredged from the RD-45 sea-
142-D (93.4 _+ 1.3 Ma) to the Line-Tuamotu elbow. Watkins mount. Tests of planktonic foraminifera incorporated into this
Seamount( L1 of this paper; Figure 1) is also consideredt o be volcanicr ock have been dated as middle Eocenei n age, 44-50
of Late Cretaceousa ge. A rate of migration of volcanism of Ma [Ha(cid:127)t(cid:127)terty et al., 1982]. The ages of 71.9 q-1.4 and
9.6 +_0 .4 cm/yr best fits this trend. However, deviations from 70.5 q- 1.1 Ma reported here are consistent with the Mae-
the progressivet rend exist. The cluster of agesr epresentedb y strichtian age of shield-buildingv olcanisma t these seamounts
RD-63 (86.0_+ 0.9 Ma), RD-61 (81.4 _+ 1.1 Ma, 82.6 _+0 .7 which must have been above sea level in Late Cretaceous time
Ma), RD-59 (85.0 _+ 1.1 Ma), and 133-D (84.4 _+0 .9 Ma) are as determinedb y the fossile videnceo f Haggerry et al. [1982].
associatedw ith 137D dated at -,(cid:127) 57 Ma by Saito and Ozima These Cretaceous dates fall far above the postulated line of
[1977]. The elongate ridge from which 137-D was dredged progressions hown on Figure 6 and argue persuasivelya gainst
exactly parallels the ridges from which RD-63 and RD-61 the formation of the Line Islands by a single hot spot. In fact,
were taken; RD-59 is from an isolated seamountl ying off the when consideredt ogether with the DSDP results from 165,
southerne nd of the ridge from which 137-D was taken. It is 315, and 316, one could argue that an older progressivet rend
difficult to reconcilet his parallelism of ridgesw ith the path of existedw hich paralleled the trend line defined in Figure 6. The
a single hot spot to account for both the Cretaceous and phase of Eocene volcanic activity at RD-45 can be explained
Paleocene ages in this short sector of the Line Islands. Fur- by hot spot volcanism along the main trend. We wish to
ther, 137-D lies to the north of the well-defined Line cross emphasizet he following points concerningv olcanice vents'
trend (Figure 1). SeamountsL 2 and L3 of this paper (Table 4 1. The majority of dated volcanos can be interpreted as
and Figure 1) yielded Cretaceous ages based on paleo- defining an age-progressivev olcanic trend from 93 (142-D) to
magneticsw hich are in accordw ith the radiometrica ge deter- 44-50 Ma (RD-45 southward along the Line chain, which
minations of RD-59 and 133-D. In the Line cross trend itself supports an origin by hot spot volcanism now active in the
(Figure 2), which has a geometrict rend distinct in the prov- vicinity of Easter Island. During this period the Pacific plate
ince, the only radiometric age obtained at 128D (78.7 +_ 1.3 moved at 9.6 + 0.4 cm/yr acrosst his hot spot.
Ma) plots in the main trend line shown in Figure 6 even 2. Three volcanos exhibit Eocene and Paleocene vol-
though the seamount from which the rock was taken lies far canism (between 38 and 60 Ma) which does not fit this main
to the east of the main Line chain. trend' 137-D at 15øN, RD-33 in the Line Islands cross trend,
Further, the long time span of volcanisma t site 165 and the and RD-41 at 2øN. This small group of agesp ossiblys howsa n
age of 39.3 +_ 1.5 Ma obtained for RD-33 argue against a age progression which could be related to volcanic over-
straightforward age progressiona long the Line cross trend, printing by hot spot volcanism along the Line-Marquesas
although both of these locations,i t could be argued, lie to the Swell [Crou(cid:127)th and Jarrard, 1981]. Alternatively, these vol-
south of the well-definedc rosst rend and may representv ol- canos representa period of rejuvenescencen ot related to hot
canic activity of Cretaceous age along the 9.6 _+0 .4 cm/yr spot volcanism but part of a Pacific-wide Eocene volcanic
progressions hown in Figure 6 complicated by Eocene vol- event [Ha(cid:127)t(cid:127)terty et al., 1982]. In either case, the Eocene and
canism along one of the SE trending ridges which intersects Paleocenee dificesa nd ridges sampled are in close proximity
the Line Islands chain. SeamountsL 4 and L5 of this paper are to older Cretaceous edifices and ridges, as in the case of
of Tertiary age based on paleomagneticd ata. In the central RD-45 and the pair of ages shown by PCOD-6 and RD-41.
Line Islands the date of 76.4 _+0 .5 Ma for dredge 123-D falls The same edificesm ay be compositeso f both Cretaceousa nd
on the line of age progressionss hown on Figure 6. However, Eocene volcanic episodesa nd clearly require a volcanic his-
at site 315 major edifice building ceasedb y 93.3 Ma and a tory requiring more than a singleh ot spot.
nearby seamount,L 6 of this study, is probably of Eocenea ge. 3. The Cretaceous ages determined at DSDP sites 165,
11,270 SCHLANGERE T AL.' GEOLOGYA ND GEOCHRONOLOGOYF THE LINE ISLANDS
315, and 316 and from RD-44 and RD-45 do not fit 9.6 + 0.4 north of Caroline Island (dredge hauls 44 and 45). The eXis-
cm/yr trend and could be interpreted as representinga pro- tenceo f reefso f late Campaniant o early-middleM aestrichtian
gressivea ge trend that is systematically1 0-20 Ma older than age along a 2500-km length of the Line chain from DSDP site
the linear trend shown in Figure 6 which connectst he Tua- 165 to RD-45 shows reef development over this distance be-
motu elbow with the northernmostL ine Islands.I t is particu- tween 70 and 75 Ma.
larly difficult to fit the 93.3 Ma age of volcanisma t DSDP site Evidencef or post-Cretaceousr eef growth is only recorded
315, which is concordantw ith the age of the Cuneolinaa ssem- from DSDP site 165, where late Eocene reef-derived fossil
blaget here,t o the 9.6 + 0.4 cm/yr trend. debris was cored in Oligocene turbidites [Premoli-Silva and
Brusa, 1981] and at Caroline Island (RD-46 and 47) where
REEF GROWTH AND SUBSIDENCE
dredge hauls recovered reef limestones of Eocene through
One of the major implicationso f the hot spot model for the Plio-Pleistocenea ge; Caroline Island is an atoll today [Ha(cid:127)t-
developmento f a linear island chain is that the history of reef (cid:127)terty et al., 1982]. At the site of dredge haul RD-44, late
growth along the chain should reflect the progressivec hrono- Cretaceousr eefs were succeededb y a late Paleocenen eritic-
logy of volcanisma long the chain, provided that the volcanic shelfc arbonateu nit, but by middle Eocenet ime, pelagics edi-
edificesb uilt from the seafloori nto the photic zone in latitudes mentation prevailed at this site. We can assumet hat there are
amenable to vigorous reef growth. The thicknesso f the reef sectionso f post-Cretaceousr eefs below the atolls of the cen-
cap on an edifice would depend on the subsidencep ath of the tral Line chain. It is important to note that dredge hauls
edifice. Along a hot spot path the edifices and the seafloor RD-57 through 64 recovered no reefal limestone debris be-
would be expectedt o sink along an age-depthc urve such as tweenJ ohnstonI sland and Kingman Reef.T he morphologyo f
one of those proposed by Parsons and Sclater [1977], the chain and the pattern of reef growth along it contrasts
Heestanda nd Crou(cid:127)th [1981], and Crou(cid:127)th [1983] where depth with the Emperor-Hawaiian chain. Long sectionso f the Line
is equalt o a functiono f t (cid:127)/2 chain lack atolls, i.e., from Johnston Island to Kingman Reef
The Emperor-Hawaiian chain is the best documented hot and from Christmas Island to Caroline Island. This lack of
spot trace in the Pacific Basin; along this chain the history of atolls, consideringt hat the Line chain was in latitudes amen-
reef growth and development follows a classical Darwinian able to reef growth throughout its history from Late Cre-
pattern [see Jackson et al., 1980]. Around the island of taceoust o the present day, argues against a hot spot model
Hawaii, now over the hot spot, only Recent reefs are known. that would produce a chain capped by a more regularly dis-
At Midway Island, drilling revealedt hat reef growth has been posed chain of atolls. Further, the irregular distribution of
continuous since Miocene time [Ladd et al., 1970]. Drilling atolls arguesf or a subsidenceh istory along the chain at vari-
along the Emperor chain showed that Paleocener eefal car- ance with one predicted by a regular age-depthr elationship
bonates cap seamountsi n that part of the chain. Failure of relatedt o t (cid:127)/2. Where reefingh istoryi s known from closely
atolls to maintain themselvesa t seal evel in the Emperor chain spacede dificess uch as the sites of RD-44, RD-45, and Caro-
can be ascribedt o the transport of the Emperor edificesi nto line Island (RD-46 and 47), the subsidenceh istory appearst o
colder, northern waters as these edificesm oved NW atop the have beenc omplex.A t RD-44 the apparents uccessiono f Late
Pacificp late. Moreover, an almostc ontinuousc hain of closely Cretaceousr eefs followed by late Paleocenen eritic shelf car-
spaceda tolls extendsf rom Midway Island SE along the chain bonatesa nd then by middle Eocenep elagics edimentss uggests
to Gardner Pinnacle where the volcanic edifice breaks sea that the edifice subsidedt oo quickly for the maintenanceo f
level. reef growth; subsidencea t this site amounted to 1500 m since
Therefore,i f the Line chain formed through the action of a Late Cretaceoust ime. At RD-45, reef growth ceasedb y Mae-
single hot spot, we would expect to see a history of reef strichtian time, and the edifice has subsided 1200 m since that
growth and subsidencea long the chain somewhatp arallel but time. At Caroline Island, reef growth has persisteda t least
older than that seen in the Emperor-Hawaiian chain. How- from Eocenet ime to the presentd ay. We note that Epp [this
ever, in the case of the Line chain, two kinds of data needed issue] estimated subsidencer ates at DSDP sites 165 and 316
for a detailed reconstructiono f a subsidenceh istory are large- and at the sites of the seamountsf rom which dredge hauls
ly lacking. First, the age of the seafloor in the vicinity of the RD-44 and RD-45 were taken (see also Haggetty [1982] for
Line chain is poorly known, and second,t he history of reef original data) and came to the conclusiont hat each of these
growth can only be inferred from data from DSDP sites 165, sites underwent different subsidence histories no one of which
315, and 316 in the central part of the chain and dredge haul fit the subsidencep aths predicted by the age-depthc urves of
sitesR D-44, 45, 46, and 47 in the southernp art of the chain. Parsonsa nd Sclater [1977], Heestand and Crough [1981] and
The oldest known reefs in the main Line chain were in the Crough [1983].
vicinity of Fanning Island as shown by data from DSDP site
315. There Prernoli-Silva and Brusa [1981] report a mid- DISCUSSION
CretaceousC uneolinaa ssemblageo f Cenomanian-Albian age The purpose of the researchc arried out by the present au-
in turbidites of both late Campanian-middle Maestrichtian thors was the testing of the various models put forward to
and Oligocene age. The presenceo f this mid-Cretaceousf auna account for the Line chain. Beginningw ith Morgan's [1972]
is in concordancew ith the K-Ar age of 93.3 Ma reported for proposition that the Line chain was generatedb y a single hot
the basal basalt at DSDP site 315. If we consider Horizon spot other models followed. Natland [1976] argued that the
Guyot to be the northern extensiono f the Line chain, then we petrologicc haracter of several basaltic rocks recoveredf rom
must consider the Cuneolina-bearingr eef limestone drilled the Line chain and the crosst rend indicated that at least parts
there at DSDP site 171 [Winterer et al., 1973] to be coeval of the chain were part of a central Pacific rift systemo f limited
with the mid-Cretaceous reef reported at DSDP site 315. crustal extension. Winterer [1976] postulated a ridge crest
During late Campanian through early-middle Maestrichtian origin for the main Line chain at ~ 110 Ma followedb y ridge
time (70-75 Ma) reefse xistedi n the vicinity of Kingman Reef jumps; he recognizedt he importanceo f the NW-SE trending
(DSDP site 165), Fanning Island (DSDP site 315), south of crossr idgesa nd proposedt hat they were youngert han the
ChristmasI sland (DSDP site 316), and on seamountsju st main Line trend and that the complexityo f the chainw as due