34. paleomagnetic investigations of volcanic rocks ... · 34. paleomagnetic investigations of...

Haggerty, J.A., Premoli Silva, I., Rack, F., and McNutt, M.K. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 144

34. PALEOMAGNETIC INVESTIGATIONS OF VOLCANIC ROCKS:PALEOLATITUDES OF THE NORTHWESTERN PACIFIC GUYOTS1

Masao Nakanishi2 and Jeffrey S. Gee3

ABSTRACT

Paleomagnetic properties of 340 minicore samples from 5 guyots in the northwestern Pacific Ocean were measured on boardthe JOIDES Resolution and in the paleomagnetic laboratories of the Ocean Research Institute, University of Tokyo, and ofLamont-Doherty Earth Observatory. Stepwise thermal and alternating-field (AF) demagnetizations typically isolate the samecharacteristic magnetization direction. The shipboard and shore-based paleomagnetic measurements suggest a paleolatitude of~10°S for Limalok, Wodejebato, MIT, and Takuyo-Daisan guyots. Lo-En Guyot apparently was constructed at a latitude of 31°S.According to the radiometric age of volcanic rocks from MIT Guyot, the reversed polarity of the volcanic basement of MIT Guyotcorresponds to Chron M1 or older. A reversed polarity remanence of Wodejebato Guyot was acquired during Chron C33R becauseradiometric age determinations for Wodejebato Guyot range from 79 to 85 Ma. Our paleolatitude estimates for Lo-En, MIT, andTakuyo-Daisan guyots differ from those derived from previous seamount magnetic anomaly modeling. The sense and magnitudeof this paleolatitude discrepancy are consistent with a significant contribution from viscous and induced magnetizations. Weestimated the paleolatitudes of the guyots from the age data and from a previously published absolute-motion model for the PacificPlate. Differences between these paleolatitude estimates and those derived from paleomagnetic measurements except for Wode-jebato Guyot are smaller than 6°.

INTRODUCTION

There are many seamounts and guyots in the western PacificOcean. Flat-topped seamounts in the western Pacific Ocean were firstdescribed in the 1940s by H.H. Hess (1946) and were named "guyots"after the 19th-century geographer, Arnold Guyot. Most of the sea-mounts and guyots in the western Pacific Ocean lie on crust ofMesozoic (Late Jurassic to mid-Cretaceous) age and formed duringthe Cretaceous. Previous studies (e.g., Duncan and Clague, 1985)suggest that most of these seamounts were constructed in the South-ern Hemisphere, in a region presently characterized by abundanthotspot volcanism. This region, known as the South Pacific Super-swell (McNutt and Fischer, 1987), may have been active since theCretaceous (e.g., Larson, 1991; Fukao, 1992). An understanding ofpast superswell activity, as manifest by seamounts and guyots in thewestern Pacific Ocean, is necessary for unravelling the evolution andpast motions of the Pacific Plate.

Ocean Drilling Program (ODP) Leg 144 drilled five guyots in thewestern Pacific Ocean (Table 1; see site map preceding title page).With one exception (Site 880), volcanic material was recovered ateach site, providing information on the paleolatitudes of these guyots.The results of shipboard measurements are described in detail in theLeg 144 Initial Reports volume (Premoli Silva, Haggerty, Rack, et al.,1993). Shipboard paleomagnetic studies suggest that these guyotswere all formed in the Southern Hemisphere. Shore-based paleomag-netic studies of discrete volcanic samples have focused on refiningthe paleolatitude estimates and on evaluating the nature of the rema-nence. This article describes the results of paleomagnetic studies thatcombined both on-board and shore-based measurements. The Fe-Tioxide mineralogy and rock magnetic properties of volcanic rocksused in this study are described in Gee and Nakanishi (this volume).

Haggerty, J.A., Premoli Silva, I., Rack, E, and McNutt, M.K., Proc. ODP, Sci.Results, 144: College Station, TX (Ocean Drilling Program).

2 Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai Nakano, Tokyo164, Japan (present address: Scripps Institution of Oceanography, University of California,San Diego, La Jolla, CA 92093-0212, U.S.A.).

3 Lamont-Doherty Earth Observatory, Palisades, NY 10964, U.S.A. (present address:Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA92093-0215, U.S.A.).

METHOD OF PALEOMAGNETIC MEASUREMENTS

Paleomagnetic measurements were conducted on 340 minicoresamples, both on board the JOIDES Resolution and in the paleomag-netic laboratories of the Ocean Research Institute (ORI), University ofTokyo, and of Lamont-Doherty Earth Observatory (LDEO). Samplinglocations and other data are summarized in Table 2. To obtain the mostreliable paleomagnetic directions, all samples were subjected to eitherstepwise alternating-field (AF) or thermal demagnetization. Whole-core remanence measurements on the volcanic rocks of Sites 878 and879 were also made on board with a 2-G Enterprises (Model 760R)pass-through cryogenic magnetometer. The sampling interval of thewhole-core measurements was 5 cm. Shipboard remanence measure-ments of discrete samples were also made with the cryogenic magne-tometer following AF demagnetization with a Schonstedt geophysicalspecimen demagnetizer (Model GSD-1; see "Paleomagnetism" sec-tion, "Explanatory Notes" chapter, in Premoli Silva, Haggerty, Rack,et al., 1993).

Shore-based paleomagnetic measurements for discrete samples atthe ORI were made with a Schonstedt digital spinner magnetometer(Model DSM-2). The AF and thermal demagnetizations in the labo-ratory were conducted with a Schonstedt geophysical tumbling-specimen demagnetizer (Model GSD-5) and a Schonstedt thermalspecimen demagnetizer (Model TSD-1), respectively. Initial suscep-tibilities of samples were measured by a BISON magnetic suscepti-bility meter (Model 3101 A).

Samples were also measured at Lamont-Doherty Earth Observa-tory using a 2-G Enterprises cryogenic magnetometer housed in ashielded room. The AF demagnetizations were conducted with aSchonstedt GSD-1, and thermal demagnetizations were done in alarge-capacity oven with three separate temperature controllers and anambient field of smaller than 5 through 8 nT. Susceptibilities of sam-ples at LDEO were determined using a Bartington susceptibility meter.

DATA ANALYSES

Remanence directions after progressive demagnetization wereplotted on a Zijderveld diagram (Zijderveld, 1967) to determine theoriginal polarity and to select vector endpoints displaying univecto-rial decay, preferably, to the origin, by visual inspection. The charac-

585

M. NAKANISHI, J.S. GEE

Table 1. Site summary, Leg 144. Table 2. Numbers of samples.

Hole Latitude

Limalok Guyot:871A87 IB871C

5°33.43'N5°33.44'N5°33.44'N

Lo-En Guyot:872A872B872C

1O°O5.85'NIO°O5.81'N10°05.86'N

Wodejebato Guyot:873A873B874A874B875A875B875C876A877A

ll°53.80'N11°53.84'N12°00.22'N12°00.23'N12°00.76'N12°06.16'N12°00.76'N12°14.80'N12°01.15'N

MIT Guyot:878A 27°19.14'N878B878C

27°19.14'N27°19.14'N

Takuyo-Daisan Guyot:879A88OA

34°10.46'N34°12.53'N

Longitude

172°20.66'E172°20.66'E172°20.66'E

162°51.96'E162°52.00'EI62°52.OO'E

164°55.19'E164°55.23'E164°56.39'E164°56.39'E164°56.47'E164°56.47'E164°56.47'E164°55.91'E164°55.33'E

150°53.03'E150°53.03'E15O°53.O3'E

144°18.56'E144°18.74'E

Waterdepth(m)

1254.61254.01224.6

1083.61083.61083.6

1334.01334.01374.91374.91410.81409.81408.81398.81354.8

1326.21325.81325.8

1501.01525.0

Holedepth(mbsf)

151.9152.4133.7

144.077.3148.0

54.369.07.0

193.511.240.0133.0154.0190.5

910.05.86.0

226.518.4

Thicknessof volcanic

rocks(m)

48.5

0.357.1

2.0

57.2

15.8

7.08.54.5

204.6 (v)186.2 (b)

35.5

Notes: (v) = volcanic basement from Hole 878A, and (b) = polymicticbreccia from Hole 878A.

teristic magnetization direction for each sample was calculated byapplying the three-dimensional principal component analysis to theset of vectors thus selected (Kirschvink, 1980).

Mean inclinations (and error estimates) were calculated using theMcFadden and Reid (1982) method for azimuthally unoriented databased on flow units identified on board. Although the precise numberof flow units may be slightly different from this shipboard estimate,this method provides a reasonable method for identifying cooling unitsthat may plausibly represent independent samplings of the geomag-netic field. Within a flow unit, all sample inclinations were given equalweight. Pairs of samples were often taken in close (5-10 cm) proximityto facilitate comparison of AF and thermal demagnetization behavior.Consequently, the within-flow scatter may be somewhat underesti-mated. Samples (indicated by asterisks in Tables 3 through 8) were notincluded in the calculation of flow-unit means if they exhibited particu-larly noisy demagnetization behavior or were obviously different fromthe remaining samples within a flow unit. For example, samples frommore altered flow-top breccia often had inclinations distinct from theremainder of the flow unit. Mean inclinations for each guyot weredetermined by simply averaging all flowunit mean inclinations underthe assumption that each represents a distinct sampling of the geomag-netic field. Finally, estimates of directional dispersion for each guyotwere used to evaluate whether paleosecular variation (PSV) may havebeen adequately averaged. Comparison of expected and observed di-rectional dispersion (e.g., McFadden, 1980) provides a necessary, butnot sufficient, criterion for evaluating the validity of the paleolatitudeestimates for the guyots.

RESULTS OF PROGRESSIVEAF AND THERMAL DEMAGNETIZATIONS

AF and Thermal Demagnetizations

Each sample was subjected to progressive AF or thermal demag-netization to isolate the direction of a characteristic magnetization.The maximum angular deviation (MAD) provides a quantitative mea-surement of the precision with which the best-fit line is determinedby principal component analysis (Kirschvink, 1980). Butler (1992)indicated that line fits from principal component analyses that yield a

Site

871872873874875876877878 (b)878 (v)879

Total

Total

444316144

6

10614

340

On boardAF

191354133

56263

132

Onshore

AF

81045111

13326

81

Thermal

17

75242

17

5

127

Notes: (b) = polymictic breccia from Hole 878A, and (v) = volcanic basement from Hole878A. AF = alternating-field demagnetization. Thermal = thermal demagnetization.

MAD greater or equal to 15° are often considered ill-defined and ofquestionable significance. The MAD values for most samples fromLeg 144 are smaller than 10° (Fig. 1), indicating that the characteristicmagnetization directions are generally well defined.

The median destructive field (MDF; the alternating field neces-sary to reduce the magnetization to 50% of its initial value) providesa first-order measure of the stability of the remanence. The MDFs ofmost samples are generally between 10 and 40 mT (occasionally > 50mT; Fig. 2). Figure 3 shows the relation between the MDF and theangular displacement of stable remanence from the NRM direction inindividual samples. Samples from Leg 144 typically show little direc-tional change during demagnetization, with a slight negative correla-tion between these two parameters as has been previously noted(Kono, 1980). For example, the MDFs of the samples that have twocomponents of remanent magnetization (Samples 144-878A-89R-4,4 5 ^ 7 cm; -91A-4, 138-140 cm; and -98R-3, 4 6 ^ 8 cm, etc.) aresmall and their angular displacements are large.

The Königsberger ratio (Q; the ratio of remanent to induced mag-netization) of igneous rocks also provides some indication of thestability of remanence (Stacey and Banerjee, 1974). With the excep-tion of the polymictic breccia unit of Hole 878A, Q ratios of oursamples generally are between 1 and 60 (Fig. 4A). Together with themoderate MDF values and paucity of low-stability secondary mag-netization components, the Q ratios suggest that the remanent mag-netizations of most of our samples are stable enough to determine thepaleolatitudes of the guyots. The Q ratios of polymictic breccia ofHole 878A are smaller than 4, with many samples having values lessthan unity (Fig. 4B). Thus, remanent magnetizations of polymicticbreccia of Hole 878A are not stable enough to estimate a paleolatitudeof MIT Guyot.

Pairs of samples from the same core piece were demagnetized, oneby AF and the other by thermal treatment. In most cases, AF andthermal demagnetization isolate the same components of magnetiza-tion (Fig. 5). Maximum unblocking temperatures in most samples arehigh (500°-650°C), consistent with petrographic evidence for high-temperature oxyexsolution and the presence of secondary hematitein many samples (Gee and Nakanishi, this volume). However, AFdemagnetization sometimes resulted in linear demagnetization trendsin samples that exhibited erratic behavior during thermal demagne-tization (Fig. 6). The MDFs of these samples are smaller than 3 mT.

The results of our shipboard and shore-based paleomagnetic mea-surements for each guyot are summarized in Tables 3 through 8.Average inclinations for each flow unit are displayed in Table 9. Theaverage inclinations of guyots drilled during Leg 144 are shown inTable 10.

Limalok Guyot

Volcanic basement was drilled to a depth of 48.5 m below thelowest sediment at Hole 871C. We obtained 44 discrete samples forshipboard and shore-based measurements from the volcanic base-

586

PALEOMAGNETIC INVESTIGATIONS OF VOLCANIC ROCKS

80 -

60 -

40

20

!

j

!

ikJ

i i i

sa. m i

1 1 1

BUI 1

-

•

0 2 4 6 8 10 12 14

Maximum angular deviation (degree)

Figure 1. Histogram of maximum angular deviation (MAD) for discrete

samples from the volcanic basement of Leg 144 guyots.

35

30 -

25 -

15 -

5 -

II11

Jm

11 i

1

1i

M M

I

π•TTη M M 1 1 II

km

M M II II

20 30 40 50 60

Median destructive field (mT)

70 80

Figure 2. Histogram of median destructive field (MDF) of volcanic rocks from

Leg 144.

ment (Fig. 7). Samples shallower than 465 mbsf (Section 144-871C-35R-1) have NRM intensities smaller than 3 Am"1. The intensities ofmost samples from 470 to 485 mbsf are 1 to 7 Am"1. Below 485 mbsf,NRM intensities of samples from Hole 871C (Sample 144-871C-39R-5, 113-115 cm) gradually increase until they reach 6 Am"1 inSample 144-871C-41R-1,6-8 cm. The MAD angles of samples fromthe volcanic basement of this guyot are generally smaller than 6°,whereas the MDFs of the majority of samples are greater than 10 mT.Susceptibilities range from 800 to 5300 × 10~5 SI. Most Q ratios aresmaller than 6.

Inclinations of samples from 451.58 to 452.02 mbsf (Section144-871C-35R-1) are shallower than those in the lower part of thecore. Those from 461.46 to 463.08 mbsf (from Samples 144-871C-36R-1, 36-38 cm, to -36R-2, 57-59 cm) are about-30°. Inclinationsof the discrete samples below 463.5 mbsf (Sample 144-871C-36R-2,99-101 cm) range from-10° to-25°. We deleted Samples 144-871C-38R-3, 79-81 cm, and -39R-1, 75-77 cm. Sample 144-871C-38R-3,79-81 cm, has noisy data from demagnetization and is distinct fromthe remaining samples in this flow unit. Sample 144-871C-39R-1,75-77 cm, did not yield a characteristic direction. The mean inclina-

100 -

CO

o10

zc -4

σ>

0.1

I I I I I I I I i i i i i i i

"144-878A-91R-4, 138-140 cm

.^144-878A-89R-4, 45-47 cm

144-878A-98R-3, 46-48 cm

144-878A-90R-1,15-17cm

144-871C-39R-1,81-83 cm

•*.

• %•

••••• ••3 4 5 67

10

i—r-f-3 4 5 67

100Median destructive field (mT)

Figure 3. Relation between the MDF and the angular displacement of stable

remanence from NRM in individual discrete samples. Angular displacements

smaller than 0.1 ° are plotted at 0.1 °.

tion calculated from the 12 flow units recovered on Limalok Guyot is-19.1° (cc = 7.1°, k = 37.5, where α is the half angle of the asymmetric95% confidence interval and k is the best estimate of K; McFaddenand Reid, 1982).

Lo-En Guyot

We obtained 43 discrete samples from all three holes of Site 872.The majority of samples are from Hole 872B, which penetrated to adepth of 57.1 m (Fig. 8). Samples shallower than 148 mbsf in Hole872B are weakly magnetized (<3 Am"1). The NRM intensities ofsamples from 149.79 to 164.12 mbsf (from Samples 144-872B-5R-4,67-69 cm, to -7R-1,12-14 cm) are high (>6 Am"1). The intensities ofsamples from 164.29 to 185.82 (from Samples 144-872B-7R-1, 29-31 cm, to -9R-3, 34-36 cm) range from 2 to 9 Am"1. Below 186.58mbsf (Section 144-872B-9R-4), NRM intensities are much lower (<lAm"1) than the other portions. MADs shallower than 147.63 mbsf

587


A 35

20

i B 15 I I I .

I

T

1 '

y

i i | i i i i

l !I [

Lib

'–

0 M ^ M ^ M I M H ' " • " 1 m maw i i i π i a m i a i i a a H I Q

0 10 20 30 40 50 60 0

Q ratio

Figure 4. Histograms of Königsberger ratio (Q) of discrete samples from Hole 878A excluding polymictic breccia (A) and of polymictic breccia only (B).

2 3

Q ratio

Table 3. AF and thermal demagnetizations from Limalok Guyot.

Core, section,interval (cm)

144-871C-35R-35R-35R-35R-35R-35R-'36R-36R-

. 8,29,32. 4 4. 5 2

1,4. 3 6. 5 4

36R-1.5836R-2, 336R-2, 5736R-2, 9936R-2, 10336R-3, 1736R-3, 2536R-3. 3837R-1.5237R-L6537R-1.8537R-1, 12937R-2, 1137R-2, 1637R-2, 8738R-1.7338R-L8538R-2, 10138R-2, 10438R-3, 79*38R-3, 8438R-5, 5638R-6, 1338R-6, 1738R-6, 9638R-6, 9739R-1,75*39R-I.8139R-2, 4639R-2, 5039R-4, 1139R-5. 11339R-6, 11839R-6, 12240R-1, 14140R-4, 5240R-4, 6241B-1.6

Depth(mbsf)

451.58451.79451.82451.94452.02455.91461.46461.64461.68462.54463.08463.50463.54464.11464.19464.32470.32470.45470.65471.09471.38471.43472.14472.53472.65474.17474.20475.45475.50478.01478.52478.56479.35479.36481.15481.21482.17482.21484.59487.11488.66488.70491.11494.72494.82496.00

Flowunit

7 A9999

10131313131315151515151516171717171717191919191921212121212!

2222

23

Volume(cm3)

10.89.78.29.3

11.17.59.39.99.77.89.09.2

10.29.89.3

1 1.010.210.710.28.6

10.610.210.210.69.8

10.29.59.69.19.39.58.79.3

10.810.110.66.88.4

10.49.40.0

10.210.910.010.210.2

Declination(degree)

42.3132.1126.0128.0126.092.5

304.6128.3121.0260.9235.2348.4351.2

80.2260.2

81.073.474.5

279.6263.8130.2132.6176.5132.0160.943.040.2

126.0119.3195.1280.8283.2144.7146.4

227.5TSS 2251.02M.λ291.6153.4156.085.1

257.9243.7218.5

Inclination(degree)

-11.5-11.9

-1.12.2

-4.1-32.6-28.3-31.5-29.7-35.1-33.3-13.0-14.7-22.2-18.3-13.8-14.5-11.4

-6.2-8.9-7.0-6.0-8.0

-12.7-15.7-24.4-20.5-42.5-19.4-15.8-16.3-20.1-21.9-23.7

-22.3-18.2-14.8-19.5-20.4-17.7-17.8-18.3-11.6-13.2-17.9

Intensity(A/m)

0.372.212.711.371.102.652.561.591.792.971.001.261.202.122.811.683.246.465.973.322.013.22

16.801.671.631.652.342.864.282.482.212.583.173.021.43

17.762.892.71

10.112.733.453.534.504.175.725.63

MAD(degree)

6.01.81.64.0

12.21.52.81.20.61.74.50.71.6

12.62.8

12.21.20.61.41.8

10.72.40.71.14.90.81.86.11 31.30.91.41.58.4

2.72.91.5

0.71.82.55.80.91.31.36.8

MDF(mT)

—9.5

9.8

36.017.323.0

22.941.5

19.1

15.7

27.3

24.4

17.419.2

11.910.4

9.015.3

12.612.3

1.510.5

19.437.137.8

32.1

24.63.3

Susceptibility(10"5SI)

5234

276433673369

281630013447

10771078835941

197441484271

42254098

1812

2416261315721577

18441929

220023042654298830394216

4791404249372832

436

Q ratio

0.153.163.62

0.71

2.08: :<>1.88

4.304.125.546.511.862.8«S5.59

1.711.07

2.01

2.533.313.975.92

4.424.94

2.991.35

14.593.573.295.23

1.571.901.993.21

28.18

Demagnetization

ALALTATAA

ALT

ALAT

ALTAT

ALTATTAA

ALA

ALTTAAT

ALATTA

ALTAAATATAA

Notes: MAD = maximum angular deviation, and MDF = median destructive field. A = shipboard AF demagnetization, AL = shore4oased AF demagnetization, and T = shore 4jasedthermal demagnetization. An asterisk (*) = sample excluded from calculation of an average inclination, blanks = not applicable, and dashes = no data.

(Section 144-872B-5R-3) are scattered. Those below 149.12 mbsf(Section 144-872B-5R-4) are smaller than 2°. Susceptibilities aregenerally less than 4000 × 10~5 SI. Most Q ratios for samples fromLo-En Guyot are larger than those of samples from Limalok Guyot.

Most samples from Lo-En Guyot have inclinations from -60° to-40°. A small number of samples (Table 4) have positive inclinations,which we interpret as resulting from accidental inversion of core

pieces during sampling or curation. With the exception of Sample144-871C-7R-2, 57-59 cm (a flow-top breccia with a significantlydifferent inclination than the remainder of the flow), all samples wereincluded in calculating flow mean inclinations. The resulting meaninclinations for 11 flow units yield an average inclination for Lo-EnGuyot of-49.8° (α = 5.4°, k = 72.6), which we interpret as a normalpolarity magnetization acquired in the Southern Hemisphere.

588


Table 4. AF and thermal demagnetizations from Lo-En Guyot.


144-872A-18X1, 1918X-1,22

144-872B-5R-1.265R-1,295R-1.475R-I.545R-3, 115R-3, 145R-3, 985R-3, 1025R-4, 675R-4, 706R-1.406R-1.437R-1.97R-1, 127R-1.297R-1,317R-2, 57*7R-3, 1137R-4. 677R-4. 877R-4, 1257R-7, 507R-7. 547R-7, 687R-7, 748R-l,708R-2, 878R-2, 898R-2, 1398R-4, 68R-4, 439R-1.249R-2, 899R-2, 929R-3, 299R-3, 349R-4, 569R-5, 1319R-5, 1369R-6, 49R-6, 8

144-872C-18X-1.9518X-1, 9818X-2, 2618X-2. 30

Depth(mbsf)

143.89143.92

145.06145.09145.27145.34147.73147.76148.60148.64149.79149.82154.90154.93164.09164.12164.29164.31165.22167.05168.09168.29168.67172.13172.17172.31172.37173.80175.33175.35175.85177.48177.85182.84184.87184.90185.77185.82187.14189.29189.34189.46189.50

146.95146.98147.62147.66

Flowunit

11

66669A9 A9B9B

10B10B10B10B1313131314A14A14BI4B14B15BI5B15BI5B16A16B16B16B16B16B17A17B17B17B17B18B181!1 SB18B18B

1111

Volume(cm3)

10.711.5

11.110.90.09.4

10.410.49.50.0<S.28.20.09.47.39.79.4

11.710.89.7

11.79.29.7

10.710.99.5

11.79.39.5

10.69.79.7

10.77.39.79.79.7

10.29.79.79.5

11.39.7

9.910.20.0

10.3

Declination(degree)

332.2204.6

152.072.176,4

142.523.519.197.599.6

340.5336.9180.9174.4310.0307.0313.2313.9142.0336.694.4

261.18.5

167.7177.5289.9289.0311.2

89.490.8

270.7235.9142.8157.798.599:457.971.5

274.487.082.379.7

34.631.5

144.0137.3

Inclination(degree)

-40.6-24.1

-50.2-46.2^ 7 . 3-45.8-50.5-51.9-51.0-423^9.2^19.7-48.0-50.6^15.7-45.2-45.3-43.3-33.5^ 8 . 4-56.5-56.3-56.0-54.7-54.6-50.9^ 8 . 7-51.7^ 0 . 7-43.4^ 4 . 7-54.7-51.8-60.7-59.0-61.0-58.8-60.5

-53.6-51.2-32.3-57.2

-39.7^ 0 . 4-49.4-52.1

Intensity(A/m)

2.782.33

2.311.871.612.712.212.451.221.53

10.448.497.716.204.284.152.823.52

12.425.064.643.642.684.985.084.864.42

3.042.792.504.815.913.284.536.233.802.830.130.230.310.270.61

1.301.177.112.29

MAD(degree)

3.14.6

1.6

1.66.91.40.63.72.20.60.81.20.70.61.11.10.91.80.51.11.80.8

0.91.31.0037.81.2

0.40.91.91.52.3II 11.1

2.01.10.71.0

1.10.50.40.8

MDF(mT)

31.8

13.2

18.3

24.0

16.1

21.829.0

19.6

19.918.732.731.3

33.3

21.9

19.821.6

10.412.046.530.8

8.537.8

19.4

Susceptibility(10-5 SI)

21492124

1437124817291845566548314442

39182403278418461346131010741442393833662705189815531369I48616871904

4330

6018

1338708

1871

10431682

44

1107390

481428

389

Q ratio

2.822.39

5.895 Ml

3.2014.3216.388.487.559.77

12.966.047.32

11.6511.615.735.32

3.283.744.186.34

13.3512.536.285.06

1.53

1.52

9.6316.975.28

13.386.16

10.74

6.068.08

24.66

9.9010.00

12.85

Demagnetization

TAL

ALTAT

ALTTAT

ALATT

ALTA

ALALAT

ALT

ALTAATA

ALAATTA

ALTAAT

ALT

TALAT

Notes: Abbreviations and symbols as in Table 3.

Wodejebato Guyot

Although the deepest basement penetration on Wodejebato Guyotwas -50 m (Table 1), some volcanic material was recovered at eachof five sites (Sites 873-877). Figures 9 and 10 illustrate the depthvariation of paleomagnetic parameters for samples from the twodeepest holes (Holes 873A and 874B). The NRM intensities of sam-ples shallower than 203.74 mbsf (Sample 144-873A-18R-1, 74-76cm) are from 2.78 to 8.05 Am"1. Samples 144-873A-18R-1, 91-93cm, 97-99 cm, and 124-126 cm, are greater than 12 Am"1. Samplesbelow 216.61 mbsf (Core 144-873 A-19R), which are from a volcanicbreccia, have very weak magnetizations (<l Am"1). The NRM inten-sities of samples of the volcanic basement from Hole 874B rangefrom 1 to 9 Am"1. At Hole 875C, the upper portion of Section 144-875C-15M-1 has NRM intensities smaller than 1 Am"1, but the lowerportion has intensities of 4 to 6 Am"1. Intensities of Samples 144-877A-20R-4, 111-113 cm and 115-117 cm, and -20R-5, 19-21 cm,are greater than 10 Am"1, and those of three other samples from Hole877A are 1.09 to 2.25 Am"1. Most of the samples from WodejebatoGuyot have MAD angles smaller than 3°. The susceptibilities ofsamples from Hole 873A are generally less than 1500 × 10~5 SI. TheQ ratios of samples from the lower part of Hole 874B are larger thanthose from the upper part.

Inclinations of the volcanic basement from Wodejebato Guyotrange from -7.1 ° to 53.2° (Table 5). The top three samples taken fromthe flow-top breccia of the ankaramitic flow(s) sampled at Site 874(Samples 144-874B-22R-4, 17-19 cm; -22R-4, 32-34 cm; and -23R-1, 28-30 cm) were excluded because they are extensively altered

(Gee and Nakanishi, this volume). We also excluded the volcanicbreccia samples from the top of Hole 877A (Samples 144-877A-20R-1,23-25 cm; -20R-2,72-74 cm; and -20R-3,25-27 cm) because theirinclinations are internally inconsistent. Although Unit 9 of Hole 873 Ais a pyroclastic bomb, samples from this flow unit have the sameinclination as the adjacent units. This implies that the unit has cooledand acquired a remanence during or shortly after deposition. Samplesfrom Sites 875 to 876 on the outer perimeter ridge of WodejebatoGuyot have somewhat shallower inclinations than elsewhere on theguyot and include two samples with negative inclinations. Althoughthese data might be taken as evidence for some normal polaritymaterial on this predominantly reversed polarity guyot, the substan-tial number of samples that have been demonstrably inverted duringsampling precludes resolution of this question. We interpret the posi-tive inclination of this guyot as representing a reversed polarity mag-netization acquired in the Southern Hemisphere. Radiometric age de-terminations for Wodejebato Guyot range from 79 to 85 Ma (Pringleet al., this volume), compatible with a reversed polarity remanenceacquired during Chron C33R. The average inclination of WodejebatoGuyot is 16.5° (α = 8.3°, k = 30.3).

MIT Guyot

Hole 878A penetrated 185.26 m into the volcanic basement ofMIT Guyot (722.54-907.80 mbsf). In addition, a polymictic breccia(399.74-604.3 mbsf) rich in volcanic material was also was sampledat this hole. These two units are separated by a limestone with an earlyAptian paleontologic age. We obtained 86 and 106 samples from the

589


o = vertical component• = horizontal component

EDn

144-871C-40R-4

WUp

Thermal52-54 cm

50 mT

EDn

500 °C

B AF67-69 cm

w

3 •

•n

2-

1 •

i\—

Up

" ;

• / 'O70

1—ė

2p

'40

K>

2i-—fc—H

€°0

3 •

-+-H NEDn

144-872B-7R-4 Thermal87-89 cm

50 mT 500 °C

WUp

AF18-20 cm

50 mT

144-874B-24R-1 Thermal22-24 cm

500 °C

Figure 5. Vector projection diagram and stereographic (equal-area) projection of successive demagnetization steps on samples from (A) Limalok Guyot (Site 871),(B) Lo-En Guyot (Site 872), (C) Wodejebato Guyot (Site 874), (D) MIT Guyot (Site 878), and (E) Takuyo-Daisan Guyot (Site 879).

590


AF49-51 cm 144-878 A-89R-1 Thermal

52-54 cm

50 mT 500 °C

AF37-39 cm 144-879 A-23R-5 Thermal

34-36 cm

50 mT 500

Figure 5 (continued).

volcanic basement and the polymictic breccia, respectively. More-over, we conducted shipboard whole-core remanence measurementson the archive halves of cores from the volcanic basement and poly-mictic breccia.

Polymictic Breccia

Figure 11 displays paleomagnetic parameters of the polymicticbreccia from Hole 878A as a function of depth. Most of the samplesof the polymictic breccia are weakly magnetized, with NRM smallerthan 0.1 Am"1. The MDFs of the samples from the polymictic brecciaare smaller (<20 mT) than those of the volcanic basement. Suscepti-bilities are not greater than 100 × 10~5 SI. Most of the Q ratios aresmaller than 2. Maximum unblocking temperatures in samples of thepolymictic breccia are low. The average inclination of discrete sam-ples is -39.2° (α = 1.2°, k = 21.2). The magnetic record obtained fromthe archive halves of cores of the polymictic breccia from Hole 878Aafter AF demagnetization at 15 mT as well as those of discrete sam-ples is shown in Figure 12. The peak of inclination by whole-coremeasurements is -38° to -46° (Fig. 13) and is consistent with theaverage inclination determined from discrete sample demagnetiza-

tion data. Although the normal polarity of this unit is well established,the erratic behavior of some samples during demagnetization and thediscrepancies between immediately adjacent samples suggest that thegeologic significance of the inclination data from the polymicticbreccia unit is uncertain.

Volcanic Basement

The NRM intensities of the volcanic basement of Hole 87 8A arescattered (Fig. 14). The maximum NRM intensity is 12.7 Am"1. Sam-ples from 820.71 to 857.21 mbsf (from Sections 144-878A-88R-3 to-92R-1) are weakly magnetized, with NRM intensities smaller than1 Am"1. The MAD angles of the volcanic basement are generallysmaller than 4°. Some samples have large MDFs, greater than 40 mT.Most susceptibilities are smaller than 6000 × 10"5 SI. The Q ratios ofmost of them are smaller than 10.

Both the magnetic record obtained from the archive halves of corefrom the volcanic basement in Hole 878A after AF demagnetizationat 15 mT as well as data obtained from discrete samples reveal thepresence of normal and reversed polarity material (Fig. 15). Thepolarity of samples shallower than 800 mbsf (Core 144-878A-86R)

591


o = vertical component• = horizontal component

144-871C-39R-1

AF81-83 cm

Thermal75-77 cm

WUp0.6 +

EDn

1.0 1.0

Jo=18A/m

Jo= 1.4 A/m

500 °C

B 144-878 A-98R-3AF

46-48 cmThermal

58-60 cm

400

450

1.0

Jo= 2.1 A/m

50 mT 500

Figure 6. Vector projection diagram and stereographic (equal-area) projection of successive demagnetization steps on samples from (A) Limalok Guyot and (B)MIT Guyot, in which thermal demagnetization produced erratic behavior whereas AF demagnetization resulted in linear demagnetization trends. Indicators sameas Figure 5.

is normal and that below 838.46 mbsf (Section 144-878A-90R-4) isreversed. The intervening zone (801.6 through 838.9 mbsf) containssamples with shallow inclinations (Unit 21) as well as two samplesfrom Unit 18 that have positive inclinations. The latter two samples(Samples 144-878A-86R-3, 39-41 cm and 43-45 cm) were takenfrom a single core piece and so accidental inversion of this piececannot be excluded. This zone of indeterminate polarity, which mightplausibly represent transitional field behavior, was excluded fromcalculation of the mean inclination for the guyot.

Inclinations of discrete samples with a normal polarity range from-11° to -42°, whereas those of discrete samples with a reversed

polarity range from -1 ° to 40°. Most of samples with an intermediatepolarity have shallower inclinations (<10°). The average inclinationof the samples with a normal polarity is -22.2° (α = 7.3°, k = 44.4),whereas that of reversed polarity samples is 22.0° (α = 5.7°, k = 50.8).The average inclination of the volcanic basement, combining bothpolarities from MIT Guyot, is -22.1° (α = 4.1°, k = 50.1). Figure 16shows a histogram of inclinations obtained from the whole-core mea-surement. The peak inclinations of the histogram are -6° to -14° and18° to 26°, inconsistent with the average inclinations determinedfrom the discrete sample data. This probably reflects the inadequacyof the 15-mT AF demagnetization to isolate the characteristic rema-


450

Intensity(A/m)

10 15 20 0 10

MDF(mT)

20 30Q ratio

40 10 20 30

460

470 -

11480

490

500

Run

"l • %

' α: π- DC

- j

- D

_

-

-

Jαn

i

p

D

D

• 1

•

DD

1 i

1 '

i

•

•

•

•

•

, i

ß

•

3 • =a••

•

D

(

• -

-

i i i

i 1

i i i

i

ft .

3 -

_

= Inclination= Intensity

-

-

i i

1 i

i i

i _

- c

> « :

- t. G

-- D

i i i i i

1

•

D3

Dπ

i i

1 '

D

D

C

π

D

D

I

»

d

L

D

•

• =MAD

^=MDF

D

C

D

....

....

C

-

DJ CD

LJ LJ

I ü- CD

α PI

1 D

"on

- α

1 αI

~i i i i i i i

>

f

•l

• • -

*• = S L

D = Q

i

1

MM

I i

1 I 1

I I

I I

sceptibility

ratio

_

-

f

i\t

L 1 1 1 1 1 1 1

-

-30 -20 -10Inclination(degree)

4 8MAD

(degree)

12 2000 4000Susceptibility

(10"5SI)

6000

Figure 7. Paleomagnetic parameters, inclinations, NRM intensities, MADs, MDFs, susceptibilities, and Q ratios of Hole 871C vs. depth.

nence in the archive half data. Most of their MDFs for the discretesamples are greater than 20 mT (Fig. 14), providing further supportfor this interpretation.

The radiometric age of volcanic rocks from MIT Guyot (122.9 ±0.9 Ma; Pringle et al., this volume) implies that the reversed polaritycorresponds to Chron Ml or older. Shore-based measurement of dis-crete samples of the limestone between the polymictic breccia and thevolcanic basement of Hole 87 8 A could not confirm whether a polarityof the limestone is normal or reversed (Nakanishi et al., 1993). Thus,we cannot definitively say which chron corresponds to the reversedpolarity of the volcanic basement of Hole 878A.

Takuyo-Daisan Guyot

Drilling at Takuyo-Daisan Guyot penetrated -35 m into the vol-canic basement, yielding 14 discrete samples for both shipboard andshore-based measurements (Fig. 17). The NRM intensities rangefrom 2 to 5 Am"1 except for samples from Section 144-879A-23R-1,which have intensities smaller than 1 Am"1. The MADs of mostsamples are smaller than 2°. Most MDFs are greater than 20 mT.Susceptibilities range from 100 to 600 × 10~5 SI. Maximum unblock-ing temperatures for most samples are smaller than 540°C (Fig. 5E)and are somewhat lower than those of samples from other guyots. TheQ ratios are less than 50.

Stepwise AF and thermal demagnetizations of all the samplesyield an average inclination of the volcanic basement of Takuyo-Daisan Guyot of-19.2° (α = 6.0°, k = 42.6), which is nearly the sameas that obtained from samples taken at Limalok Guyot. Results fromarchive half-core measurements and discrete samples from the vol-

canic basement are shown in Figure 18. Figure 19 illustrates thehistogram of inclinations obtained from the whole-core measure-ment. The peak inclinations of the histogram are -8° to -12°. Thepeak is different from the average inclination of this guyot, which issimilar to conditions found at MIT Guyot. The MDFs of samples fromTakuyo-Daisan Guyot are mostly greater than 20 mT (Fig. 17).

PALEOLATITUDES OF GUYOTS

Paleolatitudes from the combined shipboard and shore-based dataare generally compatible with the preliminary shipboard paleolatitudeestimates. The paleolatitude Φ was calculated by the dipole formula:

tan Φ = (l/2)tan /, (1)

where Φ is a paleolatitude and / is the average inclination for a guyot.Despite the range of ages and present-day latitudes, the paleolatitudesof four of the five guyots drilled during Leg 144 are all ~10°S (Table10). The average inclination of Lo-En Guyot indicates formation atapproximately 31°S. Although the average inclination values for allfive guyots provide valuable constraints on the latitude of seamountformation and past motions of the Pacific Plate, the shallow depths ofbasement penetration (Table 1) may not have sampled a sufficientnumber of flow units to provide a good estimate of the time-averagedgeomagnetic inclination.

Models of paleosecular variation based on the characteristics ofthe recent geomagnetic field provide an estimate of the expecteddirectional scatter at the latitude of seamount formation. We haveused the paleosecular variation model of McFadden et al. (1988),based on data from lavas over the past 5 m.y., to calculate the expected

593


140

150

Intensity MDF(A/m) (mT)

4 8 12 0 10 20 30 40 50 0 5 10 15 20 25

Q ratio

160

II170

180

190

1 I

αα i

%'

Φ α

ODÇJ

• ia?

α=

4°

Inclination

Intensity

α

M

p αi

-+O-

0°D

• =MAD

ü=MDF

1 1 1 1 1

- CD

Λ L

:B

G O

o ^

GtUd

ü d

• = Susceptibilityα = Q ratio

αα

-60 -50 -40 -30 -20 0Inclination(degree)

2 4 6MAD

(degree)

8 0 2000 4000 6000Susceptibility

(10-5SI)

Figure 8. Paleomagnetic parameters, inclinations, NRM intensities, MADs, MDFs, susceptibilities, and Q ratios of Site 872 vs. depth.

directional scatter (and equivalent k) for each guyot from Leg 144(Table 10). The values of k for all guyots sampled during Leg 144 arehigher than those predicted from the paleosecular variation model.The one-sided F test of McFadden (1980), with N - 1 degrees offreedom appropriate for inclination-only data (McFadden and Reid,1982), indicates that Limalok and Wodejebato guyots have k valuesthat are statistically indistinguishable from the expected k at the 95%confidence level. Therefore, the paleolatitude values from these twoguyots may have adequately averaged paleosecular variation. In ad-dition, the recovery of volcanic material from five sites on the summitof Wodejebato Guyot approaches the number of sites in some land-based paleomagnetic studies, providing additional support for thevalidity of this paleolatitude estimate.

Inclination data from Lo-En and MIT guyots have k values that arestatistically distinct from the expected values. Although the relativelysmall number of flow units sampled on Lo-En Guyot may not haveadequately averaged paleosecular variation, the apparent lack of timeaveraging at MIT Guyot is surprising, given the presence of bothnormal and reversed polarity material. The mean inclinations and kvalues for the normal and reversed polarity lavas at Site 878 arestatistically indistinguishable at the 95% confidence level, providingstrong support for the validity of the paleolatitude estimate for theguyot. The higher than expected k values for lavas from MIT Guyotmay reflect the time averaging inherent in later alteration and acqui-sition of a chemical remanence (e.g., Van Fossen and Kent, 1993) ormay be indicative of lower paleosecular variation in the Early Creta-ceous. Paleosecular variation during the Mesozoic, and particularlywithin the Cretaceous long normal period, may have been somewhat

lower than values determined for the recent geomagnetic field (e.g.,Irving and Pullaiah, 1976).

The paleolatitude of the polymictic breccia in Hole 878 A from theaverage inclination of discrete samples is 22.2°S. The age of thepolymictic breccia is early to late Aptian (Premoli Silva, Haggerty,Rack, et al., 1993). The difference between the paleolatitude deter-mined by the volcanic basement (11.4°S) and that inferred for thepolymictic breccia is large (11°) and is difficult to reconcile withprevious studies of Pacific Plate motion (e.g., Duncan and Clague,1985). Although average inclinations from discrete samples and ar-chive-half remanence measurements are similar, the geologic signifi-cance of the inclination data from this unit is uncertain. Logging dataindicate that Hole 878A deviated approximately 4° to 5° from thevertical (toward 170°). This southerly deviation would result in aslightly higher than average inclination for MIT Guyot than thatreported in Table 10.

The single flow unit recovered at Takuyo-Daisan Guyot showssignificant scatter in inclination values (-2° to -38°); however, theamount of time represented by this unit (amoeboidal pillows thatapparently erupted into soft sediment) is unknown. Furthermore, log-ging results indicate a substantial deviation of the hole from vertical(-10°, azimuth 300°; Premoli Silva, Haggerty, Rack, et al., 1993). Theeffect of the deviation from vertical on the paleolatitude of Takuyo-Daisan Guyot depends strongly on the declination of NRM. With noindependent control on declination, we have not attempted to correctthe paleolatitude data. This 10° deviation from vertical and the pres-ence of only a single flow unit at Site 879 suggest that the paleolatitu-dinal estimate for Takuyo-Daisan Guyot may be poorly constrained.

594


Table 5. AF and thermal demagnetizations from Wodejebato Guyot.


144-873 A-15R-17R-18R-18R-18R-18R-18R-18R-18R-18R-19R-̂21R-21R-21R-21R-

. 2 3

. 5 0

. 10, 19. 6 2, 6 6. 7 4,91,97, 124I, 104,61,65, 121, 126

21R-2, 2621R-2, 2921R-2, 109

144-874B-22R-4, 17*22R-4. 32*23R- .28*23R-3, 223R-3. 4024R-24R-

. 18

. 2 224R-1, 4724R- ,5124R-3, 9124R-4. 324R-4. 1724R-4. 4224R-4, 45

144-875C-15M-15M-15M-15M-15M-

1. 111, 131,241,45*1,72*

144-876 A-15R-16R-16R-16R-16R-16R-17R-17R-17R-17R-

. 2 8

. 6

. 17

.21

. 6 4

.78,7. 10.26,91

144-877 A-20R- ,23*20R-2, 72*20R-3, 25*20R-4, 11120R-4, 11520R-4, 1382OR-5. 620R-5, 1420R-5. 19

Depth(mbsf)

177.63194.30203.10203.19203.62203.66203.74203.91203.97204.24217.65228.81228.85229.41229.46229.89229.92230.72

176.84176.99177.98180.73181.11184.18184.22184.47184.51187.85188.41188.55188.80188.83

126.71126.73126.84127.05127.32

139.68147.88147.97148.03148.44148.60150.07150.10150.26150.91

182.93184.47185.52183.92187.92188.15188.24188.32188.37

Flowunit

478

8

88

99999999

::

:

IAIBIBIB222223

BRBRBR111111

Volume(cm3)

11.87.98.79.19.27.3

10.310.69.58.7

11.38.78.5

10.610.99.77.3

10.3

10.78.88.7

11.810.89.99.99.79.2

10.710.89.89.2

10.4

10.18.7

10.010.87.3

9.79.79.88.2

10.88.28.2

10.28.1

10.2

10.419.09.4

10.08.97.8

Declination(degree)

348.124.3

297.8305.4221.1219.3357.6301.5297.6

2.7287.326.834.627.919.7

217.8220.920.6

334.6323.7

30.017.735.3

186.1183.9229.2228.9191.3271.3275.2248.6251.7

260.7251.8257.4134.6

260.4334.8313.2325.0292.793.6

124.1130.4134.948.6

176.9174.0192.0

180.4144.083.979.775.0

Inclination(degree)

23.425.525.326.914.316.620.816.720.215.224.221.420.819.918.718.019.020.6

53.23(U36.520.919.316.821.023.321.922.9

4.49.6

11.214.7

38.8v < d

-30.5-21.3

12.815.511.313.3-7.1-6.0

2.54.14.93.0

-34.116.163.1

8.210.34.7

12.413.2

Intensity(A/m)

4.134.332.784.45

8.0412.8814.8015.830.130.791.010.870.950.970.770.36

1.363.602.652.113.833.703.273.533.278.492.541.962.222.33

0.700.380.315.384.13

4.612.312.002.902.611.694.292.611.145.65

0.010.000.00

10.4510.901.091.272.25

15.72

MAD(degree)

1.21.02.31.70.50.81.11.31.20.62.51.40.96.00.60.61.42.8

0.90.7

o.s1.30.81.41.20.91.61.91.60.40.90.8

1.54.50.33.2

1.01.1

13.70.60.40.50.9

.02.91.5

3.62.64.6

.40.90.62.20.8

MDF(mT)

—

4.8

27.119.4

19.019.118.8

19.4

17.212.9

23.5

26.633.424.7

25.434.718.527.1

26.9

24.126.879.8

34.3

34.717.4

23.1

19.152.9

3.6

13.313.711.3

27.220.8

Susceptibility(10"5 SI)

56016985

538310826

8611001

721675801

668765639

953998

160225582837967998

107510241962229164183166

15971349881762

1272

3273153011511315965954

13911040560

2009

25467290235395751

2301

Q ratio

2.861.47

55.2335.7521.23

37.4858.47

4.045.532.38

5.383.741.24

5.297.876.111.802.948.357.15

12.1211.819.44

24.2226.1145.0051.88

0.961.050.75

15.3911.99

5.203.293.798 . 1 4

5.896.56

11.395.474.456.13

15.176.02

10.127.016.52

25.26

Demagnetization

TTTATALALATALAALTTATALAL

TALTALAATTALAALATAL

TALAALT

TATALALTALTAA

AAAALATTAAL

Notes: Abbreviations and symbols as in Table 3. BR = breccia.

COMPARISON TO SEAMOUNTMAGNETIC ANOMALY MODELING

Paleopoles derived from seamount magnetic anomalies provide avaluable source of information on the past motions of the Pacific Plate(e.g., Francheteau et al., 1970; Hildebrand and Parker, 1987). Sea-mount paleopoles are available for three of the guyots drilled duringLeg 144. Bryan et al. (1993) reported a paleolatitude of 14°S for Lo-EnGuyot based on least-squares modeling of their preferred subset ofbathymetry and magnetic data. Sager et al. (1993) determined paleo-latitudes for MIT and Takuyo-Daisan guyots of 32.8°S and 2.5°N,respectively. Our paleolatitude estimates for Lo-En, MIT, and Takuyo-Daisan guyots differ from those derived from seamount magneticanomaly modeling. The normal-polarity, Southern Hemisphere mag-netizations of Lo-En and Takuyo-Daisan guyots yield paleolatitudesfarther south than those derived from the sea-surface anomaly data,consistent with the presence of significant viscous or induced magneti-zation (e.g., Gee et al., 1989). Similarly, the paleolatitude of MITGuyot (11°S) is farther north than the paleolatitude derived fromanomaly modeling (32.8°S), as expected if the magnetic anomaly of

this predominantly reversed polarity seamount includes a significantcontribution from viscous/induced magnetizations. The discrepancybetween the paleolatitude estimates for Takuyo-Daisan Guyot may, inpart, be the result of the deviation of Hole 879A from vertical.

COMPARISON TO PREVIOUS ABSOLUTE-MOTIONMODELS OF THE PACIFIC PLATE

We estimated the paleolatitudes of guyots from the age data andtwo previously published absolute-motion models of the Pacific Plate(Duncan and Clague, 1985; Fleitout and Moriceau, 1992). Our resultsare most consistent with the Pacific absolute-motion model ofDuncan and Clague (1985). These authors determined the absolutemotion of the Pacific Plate in the hotspot reference from using agedata of seamounts and their geometry. The differences between ourpaleolatitudes and paleolatitude estimates of Limalok (15°S), Lo-En(25°S), MIT (14°S), and Takuyo-Daisan (7°S) guyots from the modelof Duncan and Clague (1985) are smaller than 6° and are within the95% confidence limits for our average inclinations. The paleolatitudeestimate for Wodejebato Guyot (21°S) from the plate motion model

595


170

180 -

190

200

210

220 -

230

2 4 0 >-1

Intensity(A/m)

8 12 16 5 10

MDF(mT)

15 20 25

Q ratio

20 40 60

(

-

-

-

i i i i

D

•

-

• = Inclination

π = Intensity

•

•

--

•

•b D :

-

-

-

-

-

-

-

<

•

>

>%

\

;

• i

i

1 '

n - M

1

D

π ^

1 1

A

nD

F

1

-

| -

-

i i

i i

-

-α

• = Susceptibility

D = Q ratio

b π r-

10 15 20 25 30 0 1 2 3 4 5 6 7Inclination MAD(degree) (degree)

2000 4000 6000Susceptibility

(10"5SI)

Figure 9. Paleomagnetic parameters, inclinations, NRM intensities, MADs, MDFs, susceptibilities, and Q ratios of Hole 873A vs. depth.

175

180

If185

190

Intensity

(A/m)

2 4 6 8 20

MDF(mT)

24 28 32

Q ratio

10 20 30 40 50

• = Inclination

G = Intensity

1 t•

π_

• =MAD

ü = MDF

i , , , , I •

• = Susceptibilityα = Q ratio

i i i 11 I i i

20 40 60 0.4 0.8 1.2 1.6 2.0Inclination MAD(degree) (degree)

1000 2000Susceptibility

(10"5SI)

Figure 10. Paleomagnetic parameters, inclinations, NRM intensities, MADs, MDFs, susceptibilities, and Q ratios of Hole 874B vs. depth.

596


Table 6. AF and thermal demagnetizations of polymictic breccia from MIT Guyot.

Core, section.interval (cm)

144-878A-44M-1, 12644M-2, 3445M-1.2145M-1.5245 M-1.5445M-2. 6745M-3, 9746M-1.4446M-1.5246.M-1, 11246M-1, 12647R-1, 10447R-I, 10647R-2, 2748R-1. 148R-1.5249R-1, 11749R-2, 1850R-2, 6650R-2, 7051R-1, 2951R-2, 3551R-2, 3851R-3, 2851R-4, 11151R-4, 11451R-5, 951R-6, 7452R-1. 14152R-2. 9253R-1.9653R-2. 7053R-2. 7953R-3, 7053R-4, 4954R-1, 6154R-2, 7554R-3, 10454R-6, 2454R-6, 2855R-2. 11855R-4. 11055R-6. 10055R-6, 10456R-2, 5656R-3. 1656R-3. 2056R-3, 7856R-6, 2857R-1.3757R-3, 6057R-3. 6357R-5, 10258R-2, 10558R-2, 10758R-3, 9658R-5, 5358R-7, 8859R-1.2359R-3. 4059R-4, 7659R-5, 3759R-6, 2159R-6. 2459R-7. 3860R-2. 6060R-3, 10160R-3, 10660R-5. 2260R-6. 2761 R-2, 1361R-3, 8561R-3, 8961 R-4, 9661R-5, 7862R-2, 7862R-3, 12462R-4, 4662R-6, 10762R-6, 1106 3 R - 1 , 7 7

63R-3, 4863R-3, 5263R-5. 9163R-6, 3764R-1, 1364R-3, 8464R-5, 7764R-7, 564R-7. 1465R-3, 7065R-3. 8065R-5, 2365R-5. 10465R-5, 110

Depth(mbsf)

407.36407.94409.11409.42409.44410.95412.67418.94419.02419.62419.76429.24429.26429.97435.51436.02446.27446.80456.94456.98464.69466.22466.25467.54469.71469.74470.06472.11475.51476.50484.66485.71485.80487.16488.40493.81495.32496.88499.95499.99504.95507.74510.37510.41513.90514.96518.96515.58519.20521.57524.39524.42527.71532.95532.97534.30536.48539.35540.33543.04544.86545.67546.55546.58548.13551.77553.62553.67555.26556.8356(184563.06563.10564.63565.91570.59572.56573.26576.66576.69578.87581.57581.61584.82585.36587.73591.44594.24596.29596.38600.78600.88602.81603.62603.68

Flowunit

PBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPB

PBPBPBPB

PBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPBPB

PBPBPBPBPBPBPBPBPBPBPBPBPBPB

PBPBPBPBPBPBPB

PBPB

PBPBPBPBPBPBPBPBPBPBPBPBPB

Volume(cm3)

9.17.2

9.19.37.27.8

10.19.88>)

10.49.59.79.7

9.410.09.2• I S

8.98.88.0

9..811.110.210.012.1

9.89.99.7

9.69.4

8.59.9

9.610.510.8

7.68.58.88.09.29.29.99.68.58.9

11.18.2

10.29.6

9.59.79.8

10.012.112.69.39.89.70.08.98.293

9.7

10.79.69.69.5

9.110.48.49.8

11.611.110.010.39.39.6

10.19.7

10.29.69.7

9.710.39.68.2

10.89.5

10.710.7

9.39.69.6

11.111.6

Declination(degree)

260.9144.7182.2184.4148.2127.3196.968.2

299.854.9

135.0

215.318.9

295.4275.6208.4158.0357.9157.4155.727.316.6

174.5225.1125.3298.2114.444.8

135.5

137.7139.9224.5262.0

280.073.5

208.3140.4

0.5

32.6124.435.2

306.5214.5211.5123.4130.124.4

166.2254.5

152.668.6

271.098.891.611.7

335.9

234.3227.9

32.839.540.639.1

183.7156.0204.6180.5251.5224.7186.1292.9258.0

184.6253 9

19.145.8

182.451.749.7

147.8

222 388.5

—

Inclination(degree)

-33.7-31.9-33.3-23.9-29.2-38.3-63.7^ 3 . 2-60.0-63.7-33.4

-35.4^ 0 . 2-28.6-27.7-15.0-53.0-32.1-43.9-36.7-41.8-52.8

-26.2-32.6-39.8-54.1-50.1-37.3-30.3

-65.9-20.5-40.0-43.0

-41.0-18.9

-15.7-41.0-51.7

-36.0-41.4^ 6 . 5-44.9^ 0 . 8-36.4-38.1-36.7-36.8-47.0^ 4 . 9

-45.2-43.4-44.4-52.2-57.1-39.7-59.9

-41.3-35.2^ 2 . 2-28.7-66.3-36.4-36.3-43.0-62.8-48.6-45.8-35 3-32.4-27.1-45.6

-41.9-24.5-24.3^M).5-55.6-27.6-30.1-27.5

-16.3-0.4—

Intensity(mA/m)

30.6035.3040.0040.0060.8041.5023.7017.40

2212.001290.00

3.002.00

10.103.106.607.40

32.302.004.30

39.7030.0030.0050.20

2.004ΛHI

8.6012.4029.0010.200.801.300.730.051.00

40.0019.5032.0061.7031.2941.8043.50

1.0343.50lU/vO

10.0020.0023.0046.1058.5033.8040.0074.4080.0070.0060.7080.80

107.10

71.9067.9031.2050.0050.0098.4043.9017.8026.8061.9046.40

100.2030.0040.0018.9016.40

1.3046.0048.4030.0030.0023.1010.0010.0011.3020.5025.3017.8023.8010.0020.0020.2014.804.201.000.70

MAD

(degree)

0.92.02.64.9

4.72.67.33.7

3.03.82.0

9.12.64.02.87.1

10.63.11.2

1.61.45.0

6.02.93.93.2

5.74.44.0

0.96.72.71.6

0.83.5

1.26.32.3

2.4

1.42.81.62.51.30.72.40.82.42.3

3.62.44.31.4

4.31.12.8

1.81.61.28.21.2

1.73.7

1.56.85.93.74.21.03.1

5.5

2.54.3u s

2.54.21.02.62.9

5.121.0

—

MDF(mT)

39.339.445.7

31.623.215.311.33.3

15.7

1.6

18.813.3

8.7

15.5

14.113.1

21.2

13.813.712.475.547.231.3

21.616.715.117.6

15.014.8

53.813.5

11.412.414.2

14.414.715.1

15.315.913.8

14.314.115.916.8

15.216.1

22.315.119.425.5

13.613.112.236.114.217.9

15.612.5

14.99.6

12.914.19.5

14.411.2

12.6

37.5

—

Susceptibility(10-5 SI)

1829

34

40

505445

23931163564

1921326027

3515

4u25

4020

14251819171754

3

324287

302718

44

79515573

64102

93

90113104

93105

6488

105122

60130

81106

62

230657322

5315

1148573592921

505329\'i

1272625302469

12ID

9

Q ratio

0.102.30

2.402.651.681.150.161.550.790.510.310.690.110.53

,01

0.290.192.163.512.942.740.270.531.341.083.511.170.100.170.030.040.223.961.932.183.200J93.0-13.510.121.101.040.710.830.981.831.750.741.361.592.902.611.471.562.25

1.691.411.061.901.661.761.600.300.721.271.630.951.721.881.870.670.19

1.241.451.661.742.030.800.461.541.410.3 12.00

1.701.840.470.760 . 1 6

0.26

Demagnetization

AAA

ALT

AAAA

ALT

ALT

AA

ALALA

ATA

ALTA

TAAAAAAAT

AA

AAAA

TAATAA

AL

TAAAT

AA

ALTAAATAAA

AL

TAATA

AA

AT

ALA

AL

AALAT

AL

AT

ALALAA

ALA

AL

TATA

TA

Notes: Abbreviations and symbols as in Table 3. PB = polymictic breccia.

597


Table 7. AF and thermal demagnetizations of volcanic basement from MIT Guyot.

Core, section, Depth Flow Volume Declination Inclination Intensity MAD MDF Susceptibilityinterval (cm) (mbsf) unit (cm3) (degree) (degree) (A/m) (degree) (mT) (10"5SI) Q ratio Demagnetization

144-878A-78R-2,51 723.99 1 9.7 116.4 -18.1 5.20 1.5 3207 5.27 T78R-2,57 724.05 1 8.5 106.5 -16.7 4.98 1.4 10.0 3229 5.01 AL78R-2,71 724.19 1 8.3 100.2 -18.5 4.07 1.2 18.3 3415 2.60 A78R-2,74 724.22 1 9.0 102.2 -17.8 3.56 2.0 4212 1.84 T79R-1, 107 732.77 2 8.0 230.8 -23.8 2.67 2.3 22.9 4253 1.37 AL79R-2.4 733.17 2 7.3 45.7 -21.6 2.40 1.2 4857 1.61 T79R-2.8 733.21 9.2 42.5 -21.8 2.32 1.5 23.7 4015 1.88 AL79R-3,55 735.05 2 10.0 315.1 -19.5 2.18 2.4 5.1 4527 1.05 AL8OR-3,28 744.68 8 8.5 105.1 -13.2 3.41 1.2 25.2 3232 2.30 A80R-3,40 744.80 8 9.2 98.2 -18.5 2.55 1.9 4283 1.94 T80R-5,21 747.17 8 7.3 67.1 -20.7 6.29 1.2 1553 13.16 T80R-5,26 747.22 8 7.4 56.1 -12.6 6.52 1.7 6.9 1538 9.24 A80R-6,67 748.93 8 9.5 — — 5.58 — 1407 8.65 T80R-6,72 748.98 8 9.5 340.2 -23.1 11.08 0.9 5.6 1184 20.40 AL81R-1, 15 751.15 10 9.5 256.6 -14.8 5.80 2.8 6.6 3232 3.91 A81R-1, 19 751.19 10 9.6 248.7 -25.3 5.19 3.1 1538 7.36 T81R-2,78 753.23 10 9.3 331.4 -12.7 4.81 0.8 8.1 3157 3.32 AL81R-3,30 753.95 10 8.7 76.2 -11.5 5.35 0.7 2466 7.05 T81R-3,35 754.00 10 10.5 89.9 -14.0 6.31 2.6 7.4 3243 4.24 A81R-4,53 755.68 11 6.8 88.6 -12.4 12.48 0.5 3345 12.13 T81R-4.58 755.73 11 9.8 94.0 -11.9 4.52 1.4 52.8 1019 9.67 A81R-5, II 756.63 11 9.4 5.5 -15.7 5.07 1.4 18.7 2627 4.21 AL81R-5,79 757.31 11 8.2 330.1 -15.3 4.97 3.2 3102 3.49 T81R-5,84 757.36 11 9.5 341.1 -11.5 5.15 0.6 35.7 3522 3.19 A82R-2,81 762.85 12 10.6 173.9 -14.4 7.10 0.3 43.2 4068 3.81 A82R-2,84 762.88 12 9.3 173.9 -13.8 6.69 1.4 41.3 2891 5.04 AL83R-3,67 773.83 13 8.5 347.9 -37.7 1.86 1.4 14.2 1159 3.50 A83R-4,7 774.59 13 10.2 222.3 -24.1 3.53 0.9 6511 7.64 T83R-4, 11 774.63 13 8.7 215.8 -27.9 3.22 3.1 15.2 7091 4.75 AL84R-1,8 779.98 14 8.2 137.2 -41.7 8.07 0.9 3520 5.00 T84R-1, 13 780.03 14 11.0 136.9 -41.5 10.47 1.2 25.1 3289 6.94 A84R-3, 106 783.92 15 8.6 283.5 -34.8 3.66 0.7 25.2 1372 5.82 AL84R-4,63 784.94 15 9.5 288.1 -23.5 3.44 0.6 1822 6.13 T84R-4,67 784.98 15 8.5 288.5 -27.9 3.82 0.9 27.8 1739 7.13 AL84R-5,87 786.68 15 10.4 217.1 -21.6 3.03 2.0 12.2 1793 3.68 A84R-5, 109 786.90 15 10.3 347.3 -19.8 4.06 1.8 27.1 1918 4.61 AL84R-6,70 787.93 15 9.6 170.6 -22.4 6.82 1.5 39.1 2575 5.77 A84R-6,76 787.99 15 8.7 169.5 -20.2 6.42 0.3 2702 7.73 T85R-1.31 789.61 16 9.8 156.7 -15.1 12.70 1.2 26.0 3640 7.61 AL85R-1, 142 790.72 16 8.2 293.0 -28.5 2.81 2.1 3719 2.46 T85R-1, 146 790.76 16 8.7 294.4 -29.9 2.74 1.1 6.9 3856 2.31 AL85R-2. 82 791.62 16 7.9 263.7 -19.0 5.91 1.7 2429 5.30 T85R-2.90 791.70 16 9.2 272.9 -23.1 6.96 3.8 19.9 2202 6.89 A86R-3,39 801.65 18 9.7 92.2 24.7 1.40 0.8 947 4.80 T86R-3.43 801.69 18 9.6 96.8 27.0 1.60 1.5 55.0 1027 3.40 A88R-3,4 820.75 20 9.9 344.7 -16.2 1.77 2.5 28.4 2899 1.33 A88R-3,64 821.35 20 8.7 284.4 -14.5 1.67 0.8 3392 1.60 T88R-3,66 821.37 20 7.8 284.1 -7.8 2.30 2.0 34.8 3147 2.37 AL89R-L49 828.09 21 78.4 299.1 -10.4 1.91 1.6 35.3 282 14.72 AL89R-1.52 828.12 21 8.1 302.6 -5.2 2.60 1.6 2946 1.92 T89R-3,75 830.96 21 8.7 216.3 -6.8 0.84 1.5 4927 0.55 T89R-4,25 831.77 21 9.0 318.3 -4.7 1.81 0.4 2399 2.45 T89R-4,45* 831.97 21 10.0 134.3 -20.0 1.36 9.1 4.8 2202 1.34 A90R-1, 15 837.35 21 7.7 50.3 -4.7 0.64 4.0 4.6 6615 0.21 AL90R-L99 838.19 21 7.3 191.4 0.6 0.71 0.6 2433 0.95 T90R-1, 105 838.25 21 10.5 203.4 1.3 0.69 1.6 50.2 2899 0.52 A90R-2,36 838.82 21 10.2 286.0 -2.8 0.37 3.3 57.0 669 1.81 AL90R-2,40 838.86 21 7.3 282.0 -0.7 0.49 1.0 1500 1.06 T90R-4,46 838.92 22 9.5 170.9 35.7 1.56 1.0 738 4.61 T90R-4,70 842.16 22 9.6 212.6 35.9 0.09 0.3 — 886 0.21 AL90R-4, 107 842.53 22 10.2 114.1 35.0 0.83 1.1 — 439 6.12 AL90R-4. 115 842.61 22 7.3 110.0 36.2 1.19 0.7 574 6.75 T90R-5,94 843.90 23 7.3 111.0 28.4 1.95 0.5 619 10.21 T90R-5,98 843.94 23 11.1 110.4 31.3 1.55 1.8 25.3 754 6.68 AL90R-6,3 844.39 23 10.7 265.3 33.2 0.33 2.9 15.5 5739 0.12 AL90R-6.49* 844.85 23 9.1 — — 0.63 — 5064 0.27 T91R-1.97 847.47 23 9.2 229.1 30.8 0.30 1.9 2929 0.33 T91R-I, 103 847.53 23 10.4 234.6 39.5 0.41 2.3 — 1837 0.49 A91R-3,83 850.27 23 10.2 92.6 31.1 0.38 2.7 10.1 3019 0.41 AL91R-3,86* 850.30 23 9.2 97.3 -0.8 0.34 9.2 2690 0.41 T91R-4, 138 852.32 23 9.8 9.0 30.1 0.68 3.9 2.9 2604 0.57 AL92R-L83 857.03 24 10.4 210.5 12.5 0.62 12.1 1583 0.85 T92R-L86 857.06 24 10.8 216.4 15.7 0.66 1.4 18.1 1550 0.93 AL92R-I, 101 857.21 24 10.3 216.4 16.7 1.07 0.6 54.0 1777 1.31 A92R-2,4 857.61 25 10.4 240.6 -1.1 4.66 1.5 24.8 1602 6.35 A92R-2,73 858.30 25 7.3 93.7 16.5 6.39 0.8 38.8 1242 16.73 AL92R-2,94 858.51 25 8.7 258.3 15.8 7.68 0.5 1678 14.87 T92R-3. 113 860.03 26 8.2 195.1 10.6 0.74 1.2 2403 1.00 T92R-3, 116 860.06 26 10.4 203.9 10.8 0.43 2.2 29.0 2181 0.65 AL92R-4.4 860.16 26 10.3 40.3 13.1 0.25 2.3 32.3 1598 0.34 AL92R-4, 83 860.95 26 11.6 190.0 14.1 1.09 0.9 25.1 3253 0.73 A92R-4,91 861.03 26 5.1 188.1 11.7 1.78 3.4 3138 1.24 T92R-5, 12 861.73 27 8.0 — — 0.25 — 2084 0.27 T92R-5,21 861.82 27 10.3 54.3 14.8 0.33 1.5 33.2 986 0.73 AL92R-5, 36 861.97 27 9.2 59.4 10.6 0.54 0.7 1061 1.66 T93R-I, 15 865.85 28 8.2 229.3 21.3 5.54 0.5 29.7 1700 7.11 A93R-L21 865.91 28 8.7 224.2 22.5 1.65 1.3 2967 1.81 T93R-L55 866.25 28 8.0 227.2 21.4 4.24 1.0 1441 6.42 T93R-L59 866.29 28 8.6 225.3 22.1 2.80 0.6 49.1 1085 5.63 AL93R-2,22 867.41 29 10.6 176.3 20.5 0.57 1.7 28.2 1683 0.74 AL93R-2, 123 868.42 30 7.4 17.9 20.9 5.99 0.4 34.9 848 15.41 A93R-2, 129 868.48 30 9.5 8.9 15.4 4.82 1.0 1418 7.41 T93R-3,86 869.49 30 11.0 177.2 18.7 2.80 1.7 41.7 574 10.64 AL94R-1.9 875.49 30 8.2 157.5 7.2 1.42 1.4 5081 0.91 T95R-2, 18 886.29 31B 9.7 353.8 22.7 4.21 0.7 47.9 508 26.96 AL95R-2,20 886.31 31B 10.2 0.3 26.3 3.54 0.8 413 27.80 T95R-5, 114 891.36 32 9.5 54.6 21.5 10.93 1.0 37.2 3090 7.71 AL95R-6,2 891.68 32 10.8 96.9 20.3 0.64 3.2 67.7 848 1.64 A95R-6, 10 891.76 32 9.5 92.0 14.7 2.45 2.4 3131 1.71 T

598


Table 7 (continued).

Core, section.interval (cm)

97R-1. 11397R-2. 12497R-2, 12898R-2, 9898R-3, 4698R-3, 5898R-3, 8898R-3, 96

Depth(mbsf)

895.53897.14897.18906.33907.25907.37907.67915.67

Flowunit

3434343535353535

Volume(cm3)

7.39.79.9

10.612.47.88.7

11.1

Declination(degree)

156.5

86.446.1

160.0

255.4—

Inclination(degree)

28.8

19.435.835.6

32.2—

Intensity(A/m)

0.360.38

: o s0.911.361.12

MAD(degree)

2.5

1.2

4.0

0.7—

MDF(mT)

15.444.3

2.3

19.2

Susceptibility(10 SSI)

226019082607142418391309725

1526

Q ratio

0.510.440.570.392.431.516.082.38

Demagnetization

ALTA

ALAT

ALT


ntensity(A/m)

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•P••••

-->tibility -

D

D

C

...J—L

α-

_

-

8 12 0 50 100 150 200

MAD

(degree)

Susceptibility

(10"5SI)

Figure 11. Paleomagnetic parameters, inclinations, NRM intensities, MADs, MDFs, susceptibilities, and Q ratios of polymictic breccia in Hole 878A vs. depth.

599


Table 8. AF and thermal demagnetizations from Takuyo-Daisan Guyot.


Depth(mbsf)

Flow Volume Declination Inclination Intensity MAD MDF Susceptibilityunit (cm1) (degree) (degree) (A/m) (degree) (mT) (10"5)SI) Q ratio Demagnetization

144-879A-22R-1.7322R-2, 9122R-2, 11622R-2, 12622R-3, 12922R-3, 13622R-5, 4422R-5. 6722R-5, 7422R-7, 3823R-I. 10523R-1. 10823R-5, 3423R-5, 37

198.83200.38200.63200.73202.26209.26204.25204.48204.55206.93208.65208.68213.87213.90

8.910.89.7

10.49.2

10.210.59.7

10.210.78.28.79.5

10.9

148.962.9

256.5242.1340.9338.4185.8181.8179.083.0

332.9326.7117.5118.5

-11.4-17.3-26.9-28.1-10.8

-2.2• - u

-25.3-23.4-18.1-37.8-15.3-15.8-15.4

1.3210.284.284.372.072.354.734.014.332.930.150.203.493.70

7.11.91.01.60.90.70.91.01.40.54.36.52.21.1

21.142.5

24.222.1

58.7

29.150.620.4

15.5

437567434396354367244251442154462202562579

6.6039.5321.4924.0516.7018.3142.2345.6328.0241.500.932.79

13.5513.93

AAT

ALALTAT

ALALALTT

AL


4 5 0 -

Q.

5 0 0 ^

400

5 5 0 -

6 0 0 -

-30 0 30Inclination (degree)

Figure 12. Results of the shipboard whole-core measurements of the polymicticbreccia from MIT Guyot (Site 878) after AF demagnetization at 15 mT as wellas those of discrete samples.

of Duncan and Clague (1985), however, is considerably different thanthat derived from samples recovered during Leg 144. Winterer et al.(1993) have suggested that the French Polynesian hotspots havedrifted ~10°-12° south with respect to the paleomagnetic referenceframe, similar to the southward drift suggested in previous studies(e.g., Gordon and Cape, 1981; Sager and Bleil, 1987). The difference

- • •

i

i i i i i i

knil

iI1

1 1 1

tna mi i

-40 0 40

Inclination (degree)

80

Figure 13. Histogram of archive half-core inclinations from polymictic brecciaof MIT Guyot (Site 878).

between our paleolatitudes and those from Duncan and Clague (1985)except for Wodejebato Guyot is smaller (<6°).

COMPARISON TO THE PACIFICAPPARENT POLAR WANDER PATH

Larson and Sager (1992) constructed an apparent polar wanderpath (APWP) for the Pacific Plate based on seamount paleopolesfrom 39 to 88 Ma and on crustal magnetic lineations from 122 to 155Ma. Two separate paths were calculated for the older portion of theAPWP, with anomalous skewness and without anomalous skewness.Figure 20 illustrates the paleocolatitudes of guyots from Leg 144 aswell as the APWP without anomalous skewness (Larson and Sager,1992). The paleocolatitudes of Takuyo-Daisan and Lo-En guyots donot intersect with the APWP. The intersections of the small circles forLimalok, Wodejebato, and MIT guyots with the APWP suggest agesof about 50, 70, and 123 Ma (or 134 Ma), respectively. The ages ofthe Limalok and Wodejebato guyots, as suggested by their intersec-tions with the APWP, are younger than those proposed by the resultsof Leg 144. The age of MIT Guyot determined from the APWPwithout anomalous skewness is similar to that of the radiometric agedetermined by Pringle et al. (this volume), although the age from theAPWP without anomalous skewness is different.

SUMMARY

We conducted paleomagnetic studies of 340 discrete samples ofvolcanic rocks collected during Leg 144. Stepwise thermal and AF

600


Intensity(A/m)

750

800

ifsi

850

20

MDF(mT)

40 60 10

Q ratio

20 30

900

[D

DDPrjD

ED

q D g I

q D

•D D• D D

•• #

ö

•c

-α

«4

• • •

α α

D

MAD

MDFen α

O

DD

D D

=6

D>Ü

CD

ftp

K βfti

α p


α = Q ratio

•P

D DΦ

πn

-40 -20 0 . . . 20 40 0 4 8 12Inclination MAD(degree) (degree)

Figure 14. Paleomagnetic parameters, inclinations, NRM intensities, MADs, MDFs, susceptibilities, and Q ratios of samples from the volcanic basement in Hole878A vs. depth.

2000 400Q 6000Susceptibility

(10"5SI)

demagnetizations typically isolate the same characteristic magnetiza-tion direction. The moderate MDFs and Q ratios and the lack ofsignificant low-stability magnetizations in most samples suggest thatthe magnetization of these samples is relatively stable. In most cases,the observed scatter in inclination values compares favorably withthat predicted from models of paleosecular variation. The presence ofboth normal and reversed polarity lavas from MIT Guyot and theconsistency of results from five sites on the summit of WodejebatoGuyot suggest that the paleolatitudinal estimates for these guyots areparticularly robust. The radiometric age of volcanic rocks from MITGuyot (122.9 ± 0.9 Ma; Pringle et al., this volume) implies that thereversed polarity corresponds to Chron Ml or older. Radiometricage determinations for Wodejebato Guyot range from 79 to 85 Ma

(Pringle et al., this volume), compatible with a reversed polarityremanence acquired during Chron C33R.

Paleolatitudes from the combined shipboard and shore-based pa-leomagnetic data are about 10°S, except for Lo-En Guyot, which hasa paleolatitude of about 31°S. We also estimated paleolatitudes ofguyots from the age data and from a previously published absoluteplate-motion model for the Pacific Plate. Differences between thesepaleolatitude estimates and those derived from remanence measure-ments are smaller than 6°. Our paleolatitude estimates for Lo-En,MIT, and Takuyo-Daisan guyots differ from those derived from pre-vious seamount magnetic-anomaly modeling. The sense and magni-tude of this discrepancy are compatible with a significant contributionfrom viscous and induced magnetizations.

601


7 5 0 -

800-

8 5 0 -

N

R

Table 9. Average inclinations for each flow unit.

Flowunit

Hole871C:17 A9

101315161719212223

Hole 872A:1

Hole 872B:69

10B1314I5B161718B

1

Hole 873A:47

9

Hole 874B:1AIB

Hole 875C:1

Hole 876A:1

3

Hole 877A:1

Hole 878A:PB

1• >

810] ]

121314151618202122232425

2728293031323435

Hole 879A:1

N

5141561555s1

2

4444446444

11

8

74

3

4

1

6

6444555i

Λ

27523

473i

52414232

• ;

14

Average

inclination

-5.3-32.6-31.2-33.3-16.4

-9.0-8.0

-18.6-19.6-19.1-15.7-17.9

-32.7

^17.4-49.1^ 9 . 4-44.9-54.4-52.3—48.1-60.0^ 9 . 6^ 5 . 7

23.425.519.620.3

20.910.0

34.8

13.2-0.3

3.0

8.4

-38.20-17.8-21.7-17.7-15.7-13.4-14.1-30.1

-41.6-24.4-23.2

25.9-12.9

-2.735.732.115.016.212.112.721.820.515.624.518.924.234.5

-19.2

(X

9.8

6.1

6.04.4

7.05.44.44.8

4.29.32 32.28.26.17.31.6

23.613.2

4.72.0

2.59.0

14.9

3.69.0

5.9

32.431.63.77.28.73.1

25.4

6.09.8

16.04.31.14.07.9

2.4

1.2

12.6

13.0

7.3

6.0

k

93.6

380.0

225.0289.0

162.0276.0419.0344.0

931.0165.0

2792.02857.0

211.0379.0104.0

3142.023.493.1

151.0869.0

654.0179.0

190.0

1086.098.8

160.0

0.75509.01063.0

155.0107.0834.0

66.7

115.084.1

167.0153.0

12633.0256.0682.0

1444.0

9971.0

91.1

249.0

802.0

42.0

900-

-90 -60 -30 0 30 60 90


Figure 15. Discrete sample and shipboard archive half-core remanence data

(after 15 mT) from the volcanic basement at MIT Guyot (Site 878).

Notes: PB = polymictic breccia of Hole 878 A. N = number of flow units.

Table 10. Average inclinations of guyots.

Guyot

LimalokLo-EnWodejebatoMITTakuyo-Daisan

N

12111123

1

Averageinclination

-19.0^ 9 . 8

16.5-22.1-19.2

a

7.15.48.34.16.0

k

37.572.630.350.142.6

Paleo-latitude

-9.8-30.6-8.4

-11.4-9.9

Predicted k

26.939.426.427.526.9

Presentlatitude

5.6°N10.1°N11.9°N27.3°N34.2°N

Note: N = number of flow units

602


•

•

1 I • - T • 1 ' ú1111J1 1

I ill

i i i

tf-•

•

•

-.:.:.:..L.:..:I:.:.:.J

•••J.•':J

120

in£100

o 80

| 60

40

20

0

-80 -40 0 40 80


Figure 16. Histogram of inclinations of the volcanic basement from MIT Guyot(Site 878) by whole-core measurements.

ACKNOWLEDGMENTS

We are grateful to ODP for enabling us to participate during Leg144. This study was supported by the Japan Ocean Drilling Program(M.N.). M. Nakanishi thanks Professor A. Taira and the Japan ODPOffice for efforts regarding his participation in Leg 144. This researchwas supported in part by USSAC Grant No. J44-20747 to J. Gee. Wethank Dr. Kazuo Kobayashi for his critical comments. We appreciatethe helpful reviews by Drs. M. McNutt, M. Torii, and H. Inokuchi.

REFERENCES*

Bryan, P.C., Shoberg, T., Gordon, R.G., Petronotis, K.E., and Bergersen, D.D.,1993. A paleomagnetic pole and estimated age for Lo-En Guyot, Republicof the Marshall Islands. In Pringle, M.S., Sager, W.W., Sliter, W.V., andStein,S. (Eds.), TheMesozoicPacific: Geology, Tectonics, andVolcanism.Geophys. Monogr., Am. Geophys. Union, 77:387-400.

Butler, R.F., 1992. Paleomagnetism: Magnetic Domains to Geologic Ter-ranes: Boston (Blackwell).

Duncan, R.A., and Clague, D.A., 1985. Pacific plate motion recorded by linearvolcanic chains. In Nairn, A.E.M., Stehli, F.G., and Uyeda, S. (Eds.), TheOcean Basins and Margins (Vol. 7A): The Pacific Ocean: New York(Plenum), 89-121.

Fleitout, L., and Moriceau, C , 1992. Short-wavelength geoid, bathymetryand the convective pattern beneath the Pacific Ocean. Geophys. J. Int.,110:6-28.

Francheteau, J., Harrison, C.G.A., Sclater, J.G., and Richards, M.L., 1970.Magnetization of Pacific seamounts: a preliminary polar curve for thenortheastern Pacific. J. Geophys. Res., 75:2035-2061.

Fukao, Y, 1992. Seismic tomogram of the Earth's mantle: geodynamicsimplications. Science, 258:625-630.

Gee, J., Staudigel, H., and Tauxe, L., 1989. Contribution of induced magneti-zation to magnetization of seamounts. Nature, 342:170-173.

Hess, H.H., 1946. Drowned ancient islands of the Pacific basin. Am. J. Set,244:772-791.

Hildebrand, J.A., and Parker, R.L., 1987. Paleomagnetism of CretaceousPacific seamounts revisited. /. Geophys. Res., 92:12695-12712.

Irving, E., and Pullaiah, G., 1976. Reversal of the geomagnetic field, magne-tostratigraphy and relative magnitude of palaeosecular variation in thePhanerozoic. Earth-Sci. Rev., 12:35-64.

Kirschvink, J.L., 1980. The least-squares line and plane and analysis ofpalaeomagnetic data. Geophys. J. R. Astron. Soc, 62:699-718.

Kono, M., 1980. Paleomagnetism of DSDP Leg 55 basalts and implicationsfor the tectonics of the Pacific plate. In Jackson, E.D., Koizumi, I., et al.,Init. Repts. DSDP, 55: Washington (U.S. Govt. Printing Office), 737-752.

Larson, R.L., 1991. The latest pulse of the Earth: evidence for a mid-Creta-ceous super plume. Geology, 19:547-550.

Larson, R.L., and Sager, W.W., 1992. Skewness of magnetic anomalies M0 toM29 in the northwestern Pacific. In Larson, R.L., Lancelot, Y, et al., Proc.ODP, Sci. Results, 129: College Station, TX (Ocean Drilling Program),471^181.

McFadden, PL., 1980. Testing a paleomagnetic study for the averaging ofsecular variation. Geophys. J. R. Astron. Soc, 61:183-192.

McFadden, PL., Merrill, R.T., and McElhinny, M.W., 1988. Dipole/quadru-pole family modeling of paleosecular variation. /. Geophys. Res.,93:11583-11588.

McFadden, PL., and Reid, A.B., 1982. Analysis of paleomagnetic inclinationdata. Geophys. J. R. Astron. Soc, 69:307-319.

McNutt, M.K., and Fischer, K.M., 1987. The South Pacific superswell. InKeating, B.H., Fryer, P., Batiza, R., and Boehlert, G.W. (Eds.), Seamounts,Islands, and Atolls. Geophys. Monogr., Am. Geophys. Union, 43:25-34.

Merrill, R.T., and McElhinny, M.W., 1983. The Earths Magnetic Field: ItsHistory, Origin, and Planetary Perspective: London (Academic).

Nakanishi, M., Gee, J.S., and Scientific Party ODP Leg 144, 1993. Paleomag-netic studies of the northwestern Pacific guyots (Part 2) [paper presentedat 94th SGEPSS Fall Meeting, Kobe, October 1993].

Premoli Silva, I., Haggerty, J., Rack, F, et al., 1993. Proc. ODP, Init. Repts.,144: College Station, TX (Ocean Drilling Program).

Sager, W.W., Duncan, R.A., and Handschumacher, D.W., 1993. Paleomag-netism of the Japanese and Marcus-Wake seamounts, Western PacificOcean. In Pringle, M.S., Sager, W.W., Sliter, W.V., and Stein, S. (Eds.),The Mesozoic Pacific: Geology, Tectonics, and Volcanism. Geophys.Monogr., Am. Geophys. Union, 77:401-435.

Stacey, F.D., and Banerjee, S.K., 1974. The Physical Principles of RockMagnetism: Amsterdam (Elsevier).

Van Fossen, M.C., and Kent, D.V., 1993. A palaeomagnetic study of 143 Makimberlite dikes in central New York State. Geophys. J. Int., 113:175-185.

Winterer, E.L., Natland, J.H., van Waasbergen, R.J., Duncan, R.A., McNutt,M.K., Wolfe, C.J., Premoli Silva, I., Sager, W.W., and Sliter, W.V., 1993.Cretaceous guyots in the Northwest Pacific: an overview of their geologyand geophysics. In Pringle, M.S., Sager, W.W., Sliter, W.V., and Stein, S.(Eds.), The Mesozoic Pacific: Geology, Tectonics, and Volcanism. Geo-phys. Monogr., Am. Geophys. Union, 77:307-334.

Zijderveld, J.D.A., 1967. AC demagnetization of rocks: analysis of results. InCollinson, D.W., Creer, K.M., and Runcorn, S.K. (Eds.), Methods inPalaeomagnetism: New York (Elsevier), 254-286.

Abbreviations for names of organizations and publications in ODP reference lists followthe style given in Chemical Abstracts Service Source Index (published by AmericanChemical Society).

Date of initial receipt: 31 January 1994Date of acceptance: 7 July 1994Ms 144SR-022

603


195

200

205

II

210

215

Intensity(A/m)

2 4 6 8 10

MDF(mT)

20 30 40 50

Q ratio

60 0 10 20 30 40 501 1 1

D

1

i i i

1

π

i

1 '

(

D

, ,

0

11 1 I I ii i i

• = Inclination

D = Intensity

f

•

<

i

»

•

m |

I

i

I11

|

-

i i

i i

i i

i i

i i

i i

<

-I

•

, i ,

D

>D

fc

• •

i i i i i i i

• =MAD

3 = MDF

•

D

• -

D

3

•

-

D

g o *

D


α = Q ratio

DO j

G | A

;π

-40 -30 -20 -10Inclination(degree)

0 0 4 6 8 200 300 400 500 600MAD Susceptibility

(degree) (10"5SI)

Figure 17. Paleomagnetic parameters, inclinations, NRM intensities, MADs, MDFs, susceptibilities, and Q ratios of the volcanic basement in Hole 878A vs. depth.

195

1 5 0

ε

o 40

IZ 30

1 1 1 1 1 1 1 1

r••

i•-

: :

>

•••

; ;

1

i i i

• L" • i

i i i

m i i i 1

;

••-.

-80 -40 0 40


-60 -40 -20 0 20Inclination (degree)

40 60

Figure 19. Histogram of inclinations of the volcanic basement from Takuyo-Daisan Guyot (Site 879) by whole-core measurements.

60°N

Figure 18. Result of the shipboard whole-core measurements of the volcanicbasement from Takuyo-Daisan Guyot (Site 879) after AF demagnetization at15 mT as well as those of discrete samples.

30°W

Figure 20. APWP of the Pacific Plate (Larson and Sager, 1992) with thelocations of paleocolatitudes determined in this study. Dashed line = APWP,numbers near the dashed line = ages (Ma), and solid lines = paleocolatitudes.

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