52. PALEOMAGNETISM OF DEEP SEA DRILLING PROJECT LEG 60 SEDIMENTS ANDIGNEOUS ROCKS FROM THE MARIANA REGION1

Ulrich Bleil,2 Institut für Geophysik, Ruhr-Universitàt, Bochum, Federal Republic of Germany

INTRODUCTION

As a typical example of an active back-arc basin, theMariana Trough has attracted increasing attention in re-cent years. Intensive surface-ship geophysical observa-tions, including the IPOD site survey along the southPhilippine Sea transect, have resulted in a complex pic-ture of a marginal basin with current crustal extensionabove a Benioff zone (Karig et al., 1978). Several linesof evidence indicate that the major zone of crustal ac-cretion is confined to a central topographic high abouthalfway between the scarp systems bounding the trough,and that spreading was roughly symmetric (Karig, 1971).Estimates of spreading rate on the basis of geologic datafor the widest area of the Mariana Trough, at 18°N,range from more than 6 cm/y. (Karig and Glassley, 1970)to 8-10 cm/y. (Karig, 1975).

It is difficult to specify the mode of opening in manymarginal basins because of rather poorly defined pat-terns of lineated magnetic anomalies, which may resultfrom a diffuse magma injection (Lawver and Hawkins,1978). In general, the magnetic anomalies in the Mari-ana Trough cannot be correlated even over short dis-tances, and a well-defined central anomaly is not ob-served in most available profiles. However, in the partof the basin that has been surveyed in greatest detail,near 18°N, Bibee and Shor (1978) identified a pattern ofanomalies symmetric about the axial graben that aretentatively modeled by spreading at a (full) rate of 3cm/y, for the past 3 m.y.

In this study, paleomagnetic results are reported forboth sedimentary and igneous rock drill cores recoveredat four sites in the Mariana Trough and another twosites in the Mariana fore-arc region during DSDP Leg60 (Table 1). The objectives of this paper are: (1) to dis-cuss the paleomagnetic pattern obtained for the in-dividual drill holes in terms of a recording of the earth'smagnetic field history, paleolatitudes, local and regionaltectonics; and (2) to document a paleomagnetic frame-work for the Mariana transect to further the geologicalunderstanding of the area, and in particular to establishconstraints for the opening of the Mariana Trough.

Rock magnetic data and oxide mineralogy of igneoussamples are reported elsewhere.

Table 1. Site information.

Drilling (m)b

Hole Location Basement Agea Cored Recovered Samples

17°54'N143°41'E

18°01'NI44°32'E

17°55'N145°11'E

17°55'N145°11'E

17°52'N

453 ' ' •r," Oldest Pliocene 455.5/149.5 203.5/33.3 34/22143 41 E

454A , i ‰ " i " Early Pleistocene 38.0/104.5 21.0/20.0 - /24 C

144 32 E

456 ^ < O J ^ E Early Pleistocene 134.5/34.5 28.6/3.7 8/4

456A J45OJ1-E Early Pleistocene 95.0/45.0 35.4/2.3 1 2 / -

Initial Reports of the Deep Sea Drilling Project, Volume 60.Present address: Department of Geological Sciences, University of California, Santa

Barbara, California.

458 146O5^E Early Oligocene 256.5/209.0 133.6/35.8 6/39

459B .l^Ifo-c Middle Eocene 558.9/132.6 158.6/23.5 110/26

a Based on age of overlying sediments, using an adapted Bukry (1975) nan-nofossil time scale (see paleontologic chapters, this volume).

" The first number refers to sediments, the second to basement rocks.c Including interbedded sediments.

EXPERIMENTAL PROCEDURES

Approximately 10-cm3 minicore samples were takenfrom the working half of the split main core. Samplingand orientation techniques used are those described byAde-Hall and Johnson (1976). In the sediments, areasindicating features of a horizontal bedding were prefer-entially sampled. Directions and intensities of naturalremanent magnetization (NRM) were measured with aDigico spinner magnetometer both aboard ship and atthe University of Münster, Germany. Initial susceptibil-ity measurements were made using a calibrated commer-cial suspectibility bridge (Bison Model 3101 A). Ship-board and shorebased alternating field (AF) demagnetization was performed on a Schonstedt GSD-1 single-axis demagnetizer. Systematic stepwise AF demagne-tization was carried out to about 10 percent of the initialmagnetization intensity and at least up to 500 Oe. Thestable remanent magnetization direction for each sam-ple has been defined by means of vector diagram plots(Zijderveld, 1968). The values given in the tables (seeAppendix) are means over several consecutive demagne-tization steps yielding a minimum of directional varia-tion. Whenever the standard deviation (s.d.) exceeded5°, the stable inclination and/or declination valueslisted are bracketed; no results are indicated for s.d.>10°.

Owing to the limited sensitivity of the equipmentavailable, most attempts to isolate satisfactory paleo-magnetic information failed for samples with an initial

855

U. BLEIL

magnetization intensity of less than I0"6 emu/cm3.Several igneous samples recovered at Site 453 were ther-mally demagnetized up to 600°C, using a SchonstedtTSD-1 device.

A mean ambient Earth's magnetic field intensity of F= 0.37 Oe as measured during Leg 60 at the variousdrill sites was used to calculate the Königsberger ratios.

RESULTSThe paleomagnetic data are given on a sample-by-

sample basis in the Appendix. The tables include inten-sity (JNRM), inclination (INRM), and declination (DNRM)of natural remanent magnetization and stable remanentdirections (Istabie> ^stable) a t a n optimum demagnetiza-tion stage. Although the actual declination is arbitraryas only the sense of the vertical axis is known for thedrill cores, the relative change for each sample illus-trates its magnetization stability. For the igneous sam-ples analyzed, additionally the median destructive field(MDF), the alternating field necessary to reduce JNRMto50 per cent, the initial susceptibility (k), and the Königs-berger ratio (Q = JNRM/kF) are listed.

Site 453

This site is situated near the scarp of the West Mari-ana Ridge (remnant arc) marking the western boundaryof the Mariana Trough (Figure 1). Paleomagnetic meas-urements were made on 34 samples from the Pliocenesedimentary sequence overlying the uppermost igneousrock layer recovered at this site. Widely divergent natu-ral remanent intensities were encountered, spanning therange from less than I0"8 emu/cm3 to about 7 × I0"4

emu/cm3 (arithmetic mean: 73.5 × I0"6 emu/cm3).The highest intensities were measured in various vol-canic ash layers. The lowest intensities were found inclaystones and mudstones between about 270 and 295meters below the sea floor, and in sandstone samplesnear the base of the sedimentary section at about 440meters sub-bottom. Most samples with JNRM >10"6

emu/cm3 revealed considerable magnetic stability uponpartial AF demagnetization, median destructive fieldstypically ranging from 200 to 400 Oe. Generally, onlyminor directional changes were observed between theinitial NRM stage and the final stable direction, in-dicating the presence of mostly rather small spuriousmagnetization components.

The variation of magnetic inclination with depth inthe lower sediment layer is shown in Figure 2A. A pat-tern of consecutive sections of positive and negativemagnetization directions is evident. In order to link thissequence of magnetic polarities to a geomagnetic polar-ity time scale, the following paleontological information(see paleontologic chapters, this volume) was used: (1)the 3-m.y. boundary (Discoaster tamalis/Discoasterasymmetricus) occurs at a sub-bottom depth of about170 meters and (2) there is no indication of Miocene(Messinian) species in the lowest sediments. On the basisof these calibrations, the present data closely fit estab-lished Earth's magnetic field chronologies for parts ofthe Gauss normal and Gilbert reversed epoch. The lat-ter comprises four distinct normal polarity intervals

20° N -

15 ;N -

\ \

rU53 45JI 'LL

LEGEND VΔ Recent volcano r

- :;:: Zone of youngest crust /-

\. Trough boundary fault j- .*•;'

• Leg 60 drill sites f ;•S

A \

••-••- A

#456 i l 458 459•••S 4 "

ΔJ% Δ]

M j

// j

U

-

X0HLUcc1-

<2

<C

er<c

140'E 145= E

Figure 1. Sketch map of the Mariana Trough and fore-arc region,showing DSDP Leg 60 drilling site locations and outlines of thegeneral geological setting (after Karig et al., 1978). Arrows atislands indicate amount of rotation since Miocene (see text fordetails).

(events), which all were tentatively identified here (seeFig. 2A for details). Using the geomagnetic polaritytime scales of LaBreque et al. (1977) and McDougall(1977), respectively, the accumulation rate derived fromthe magnetic polarity log of the lower sedimentary se-quence in this hole is in fair agreement with the paleon-tological data. For most of the interval, however, themagnetic ages are consistently younger than those in-dicated by calcareous nannofossils. Provided this is notbecause of a systematic discrepancy between the dif-ferent time scales used, a possible explanation would bethat the ridges surrounding the basin drilled acted as animmediate source for the sediment which accumulatedon these ridges and was later mobilized into the basin. Asomewhat puzzling feature is the anomalously shallowinclinations observed in the sediments as compared withthe actual latitude of the site (see Table 2). This will bediscussed more fully in a later section.

Extrapolation of the sediment data yields an age ofabout 5.0 ± 0.2 m.y. for the sediment/basement con-tact. This agrees fairly well with a recent revised esti-mate of 5.2 ± 0.1 m.y. for the age of the Miocene/Pliocene boundary by McDougall et al. (1977). Whetherthis date is directly related to the basement age at the sitewas not revealed by drilling, because we encountered abreccia zone beneath the sediments which extended

856

100-40

Inclination-20 0 +20

200

300

400

+40Geomagnetic Polarity Time Scale (m.y.)

4

\\Basement

450- 6 0 - 4 0

Stable Inclination (°)-20 +0 +20 +40 +60

£ 500

550

-9.4

-48.4

+23.6•

+56.8

Figure 2. A. Downhole variation of magnetic inclination in Hole 453 sediments (full circles = Istabie> °pen circles = I^RM) and accumulation rate as derived from nannofossil zonation(broken line, see paleontological chapters, this volume) and magnetic polarity log of the sequence, using the paleomagnetic time scales of LaBreque et al. (1977) and McDougall (1977), bothshown on top (shaded: normal polarity, white: reversed polarity). Nomenclature of polarity events according to McDougall et al. (1977). B. Downhole variation of stable magnetic inclina-tion for Hole 453 igneous rocks. Vertical lines indicate mean values of the different inclination ranges. Shaded: " n o magnetization zone" (intensity of natural remanent magnetization lessthan about I0""7 emu/cm3). The bar on the left denotes the "hydrothermal alteration zone" as defined by petrologic criteria.

3>

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CΛ

2O

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tnD

2aZin>Z

o

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U. BLEIL

Table 2. Magnetic inclinations and paleolatitudes.a

Hole

453

454A

456456A

Combined

458

MagneticUnits 1-5MagneticUnits 6-8

459B

MioceneSedimentsOligoceneSediments

CenteredAxial Dipole

±32.9

±33.0

±32.9±32.9

±32.9

±32.8

±32.8

Inclinations

Sediments

±21.8

-23.1

+ 33.0±33.4

±33.2

±17.1

±21.4

±23.3

± 18.0

±

±

±±

±

±

±

±

±

2

2,

4.3,

2.

2.

1.

1.

1,

.5

.8

.69

,8

1

0

2

5

IgneousRocks

±36.4

-25.8

-36.0

-36.0

±20.7

+ 27.9

-8.6

±12.4

± 5.1

± 5.2

± 12.3

± 12.3

± 4.8

± 5.0

± 3.2

± 2.2b

All Data

±23.2

-24.8

+ 33.6±33.4

±33.5

±19.1

±20.6

±

±

±±

±

±

±

2

3

33

2

2

0

.8

.1

.6

.9

.6

.8

,9

Present

+ 17.9

+ 18.0

+ 17.9

+ 17.9

+ 17.9

Latitudes

Sediments

±11.3(±9.9-±12.7)

±12.0(±10.5-±13.6)

——

±18.1(±16.3-±20.0)

±8.7(±7.6-±9.9)

—

±11.1(± 10.5-± 11.6)

±12.2(±11.5-±12.8)

±9.2(±8.4-±10.0)

IgneousRocks

±20.2(±16.9-±23.9)

±13.6(±10.6-±16.7)

——

±20.0(±12.4-±29.3)

±10.7(±8.1-±13.4)

±14.8(±11.9-±17.9)

±4.3(±2.7-±6.0)

±6.3(±5.1-±7.4)

—

All Data

±12.1(±10.5-±13.7)

±13.0(±11.3-±14.8)

——

±18.3(±16.7-±20.0)

±9.8(±8.3-±11.4)

±10.6( ± 10.2- ± 1 1 . 1 )

—

j* All errors listed are standard deviations of the mean.b Inclination variation as representing a true reversal pattern,

latitude becomes -0.2 ± 5.9 (see text for details).the respective value for a recording of paleosecular variation at an equatorial

down to the base of the hole at 605 meters sub-bottom.The close chemical affinity of these rocks to the adja-cent West Mariana Ridge may indicate that they havebeen emplaced by a tectonic event related to an earlystage of opening of the Mariana Trough. The clasticcomponents consist predominantly of more or less ex-tensively altered metabasalt, diabase, gabbro and, to-ward the base of the hole, serpentinized gabbro. Thesize of these clasts varies widely, gabbro pieces ofseveral tens of centimeters long being common. Withbut a few exceptions, the samples for paleomagneticmeasurements were taken from such megaclasts.

The remanent magnetization intensities vary fromless than I0"7 to 7.5 × 10 ~3 emu/cm3. The arithmeticmean value of 1.3 × I0"3 emu/cm3 (s.d. 1.6 × 10~3

emu/cm3) is only about 50 percent lower than averagespreviously obtained in basaltic oceanic crust (Lowrie,1974; Ryall et al., 1977). There is no continuous down-hole trend in NRM intensity, but especially low mag-netization values occur in the uppermost samples andthere is a zone at about 520 meters sub-bottom in whichthe intensities abruptly drop below the sensitivity of themagnetometer system (5 × I0"8 emu/cm3). As withother magnetic properties (see tables in the Appendix),the most conspicuous feature of the magnetic stability isits high variability, resulting principally from the greatvariety of rock types and degrees of alteration repre-sented in the breccia. Most of the samples revealed con-siderable amounts of spurious magnetization compo-nents; these may possibly be attributed to a secondaryremanence introduced during either drilling of the maincore or the minicore sampling. Their instability upondemagnetization did not substantially hinder the deter-mination of a stable remanence direction; however, the

effect largely obscures any reasonable estimate of the insitu magnetization state.

The general aim of the paleomagnetic study onoriented specimens from the breccia zone is to deter-mine the emplacement temperature or to provide infor-mation about possible subsequent magnetization proc-esses. Completely scattered magnetic directions shouldresult from cold emplacement, without, or with, onlyvery limited secondary magnetization components ac-quired after emplacement. On the other hand, hot em-placement of the breccia producing a (partial) thermo-remanence (TRM), would yield coherent magnetic di-rections. However, some directional coherence couldalso be imposed on initially random magnetization di-rections by various secondary effects: (1) hydrothermalreheating resulting in a (partial) TRM, (2) acquisition ofa viscous remanent magnetization (VRM) either at am-bient or elevated temperature, and (3) alteration at am-bient or elevated temperature affecting the magnetic ox-ide mineral assemblage and imprinting a chemical rema-nent magnetization (CRM). Results of stable inclina-tions obtained for the breccia (Fig. 2B) do not reflect aclear and simple pattern of directions but rather indicatethe presence of two coherent inclination ranges. The ob-servation is complicated by a frequent oscillation be-tween positive and negative directions, and the fact thatthe negative inclinations are consistently shallower bysome 10 to 15 degrees. The upper section, from the topof the breccia to about 506 meters sub-bottom, yields anaverage stable inclination of ±16.5° ± 7.7Q. From thelower part, a mean stable inclination of ±54.0° ± 5.7°was obtained. There is a spatial overlap of both theseranges at about 500 meters sub-bottom. The overallmean stable inclination of ±36.4° ± 20.3° is in com-

858

PALEOMAGNETISM OF SEDIMENTS AND IGNEOUS ROCKS

plete agreement with a centered axial dipole inclinationfor the present latitude of Site 453 (see Table 2). If this isa reliable result, the origin of the remanent magnetiza-tion remains obscure. Judging from the geological evi-dence, hot emplacement of the breccia seems highly im-probable. However, some remagnetization may haveoccurred after emplacement. Nevertheless, any processacting at ambient temperature over a long period, wouldnot give the observed distinct inclination ranges andmixed polarities. The close grouping around differentmean directions (see Fig. 2B) may represent a spot re-cording of the Earth's magnetic field. Such an interpre-tation is consistent with the observation that none ofthem directly coincides with the average dipole inclina-tion for the latitude of the site. Accepting this interpre-tation, a possible mechanism could have been an intensehydrothermal activity gradually declining over a longperiod, the different magnetic directions representingroughly two discrete blocking temperature spectra. Infact, samples carrying a shallow inclination generallydisplay much higher magnetic stabilities compared withthose of steep inclinations (see tables in the Appendix).An alternative explanation would be at least two hydro-thermal episodes affecting (mainly) either the upper orthe lower portion of the breccia zone. Areas of specifi-cally low magnetic intensity where the magnetic mineralassemblage has obviously been destroyed to a great ex-tent are likely to have acted as major pathways for theflux of hydrothermal fluids indicative of a continuousor repeated volcanic activity. The most confusing obser-vation, which remains unexplained, is the pattern ofmultiple changes in magnetization polarity within agiven inclination range. As matters stand, any inter-pretation of this most complex situation (e.g., in termsof self-reversal) would be speculative.

Site 454

This site is located in a small sediment pond, theclosest suitable site (about 26 km) to the west of centralgraben of the Mariana Trough, the apparent axis ofspreading in this back-arc basin. According to the sitesurvey data and magnetic profiles obtained under wayby the Glomar Challenger (see site report and underwaygeophysics chapter, this volume), the site is situatedon a negative magnetic anomaly adjacent to the westernflank of the possible central anomaly. A magnetic base-ment age of about 0.7 to 1.6 m.y. in the first half of theMatuyama reversed epoch, is tentatively inferred. Pale-ontological analysis (see Ellis, paleontological chapters,this volume) gave a sediment age of 0.9 m.y. at a sub-bottom depth of about 60 meters, about 8 meters abovethe sediment/basalt contact.

The basaltic basement includes several interlayeredsediment beds. All these sediments belong to the Ge-phyrocapsa caribbeanica nannofossil Subzone (0.9-1.6m.y.). This is in agreement with the interpretation of themagnetic anomaly pattern and would place Site 454 be-tween the Jaramillo and Olduvai events, yielding anaverage basement age of 1.3 ±0.3 m.y.

Due to a badly disturbed recovery, no paleomagneticmeasurements were made on the sediments at this site,

except on a few samples from mudstone beds interlay-ered in the basaltic basement. The upper approximately100-meter basement section drilled consists of basalticflows (Cores 5 and 6, and 11 and 12) interbedded withsediment layers (Cores 7 through 13), and rather poorlyrecovered pillowed basalts (Cores 8 through 10, and 14through 16). Most pillow fragments recovered were toosmall to be positively oriented and, except for twospherulitic pillow samples from Core 16, paleomagneticresults could only be obtained from sediments andbasaltic flow units.

Almost all samples studied, including the sediments,revealed magnetic stability upon AF demagnetization(MDF > 100 Oe), and only minor directional changeswere observed between NRM and stable directions. In-clinations consistently became more steeply negative byseveral degrees upon demagnetization (see tables in theAppendix). This is thought to result from the removalof a downward-directed magnetization component thatwas either acquired as a VRM during the Brunhes epochor as a spurious magnetic overprinting during drilling.On the average, almost identical remanent intensitieswere found for the pillows (JNRM = 5.69 ± 2.47 ×I0"3 emu/cm3) and basaltic flows (JNRM = 6.47 ± 4.61× I0"3 emu/cm3). Similar values have generally beenobtained in young oceanic crust in the Atlantic (Ryall etal., 1977; Day et al., 1979), but such values are distinctlylower than those from the East Pacific Rise (Petersen,1979). Other magnetic properties like MDF, Konigsber-ger ratio, and especially the initial susceptibility of thepillows and basaltic flows display systematic differenceprobably reflecting their different cooling history. Com-pared with previous DSDP results on massive oceanicflows (Bleil and Smith, 1979), the present magnetic dataindicate rather thin (< 3-m) cooling units for the ba-saltic flows.

All samples, including the sediments, consistentlyshow a negative stable inclination (Fig. 3), thus match-ing the sign of the magnetic anomaly at the site. Thissingle paleomagnetic polarity of Hole 454A did not pro-vide direct, independent age-diagnostic information.However, it is in complete agreement with the paleon-tologic results and the interpretation of the regionalmagnetic anomaly pattern as has been described in thispaper.

On the basis of petrologic and geologic criteria, thebasement column encountered in Hole 454A has beendivided into five lithologic units. This scheme was usedas a general framework to define a series of magneticunits. A magnetic unit comprises all consecutive igneoussamples within a basement interval yielding a commonstable inclination range. At most levels lithologic andmagnetic units were found to be in excellent agreement,although lithologic Units 2 and 3 could not be verified,as no oriented samples were available from this part ofthe hole. On the evidence of different magnetic direc-tions, lithologic Unit 4 was subdivided into two mag-netic units (see Fig. 3 and table in the Appendix). Thelogging results obtained in the hole indicate, in fact, thatthese two magnetic units are separated by a thin sedi-mentary layer. Each magnetic unit is thought to repre-

859

U. BLEIL

3 U

100

150

iLith

olog

icJ

Uni

ts

~l M

agne

ticJ

Uni

ts

i

-

1

DB

3

S

3

4

=S=

4

S

5

1

7

2

3

4

5

7//

f

\- 4 0 -30 -20 -10

Stable Inclination (°)±0

DB = drilling brecciaS = sediments

Figure 3. Downhole variation of stable magnetic inclination in thebasement rocks recovered from Hole 454A (squares = basalts,circles = sediments). Columns give lithologic and magnetic units(see text for details).

sent an individual recording of the Earth's magneticfield during a separate short pulse of volcanic activity.

Estimating the thickness of the interlayered sedimentbeds from drilling and logging gave about 20 meters inCores 7 through 10 and 10 meters in Cores 12 and 13,and an average sedimentation rate of 10-25 cm/103 y.(see paleontologic chapters, this volume). The time in-terval represented by the basement section drilled inHole 454A thus amounts to about 1.2-3 × I05 years.Intervals between eruptions, or short eruptive se-quences, at the Mid-Atlantic Ridge crest were evaluatedby Ryall et al. (1977) to be of the order of 10,000 years.As at least five different magnetic/lithologic units wereidentified in the present hole, and there may indeed betwice this number, the present data suggest a somewhathigher average value of 1.2-6 × I04 years between in-dividual pulses of volcanic activity. Together with theuniform magnetic polarity encountered, this result mayput an interesting constraint on the emplacement modeof at least the extrusive portion of new crust in theMariana Trough, as it implies a formation within a nar-row zone rather than a spatially diffuse volcanism assuggested by Lawver and Hawkins (1978).

Including three sediment samples, a total of eightseparate independent recordings of the Earth's paleo-magnetic field directions, distributed over a span ofsome 100,000 years, were obtained in Hole 454A. Forthis reason it seems unlikely that the mean stable incli-nation is still biased by a large amount of secular fieldvariation components. The average stable inclination of-24.8° (s.d. of the mean 3.1°) is significantly differentfrom the centered axial dipole field inclination for thepresent latitude of the site (±33.0°), probably in-dicating that it has been originally situated in a moresouthern position. The significance of this result will bediscussed in a later section.

Site 456Two holes were drilled at Site 456 within a small sedi-

ment pond on a basement ridge about 37 km to the eastof the apparent axis of spreading in the Mariana Trough(Figure 1). Hole 456A is situated some 200 meters eastof the primary Hole 456. The drilling area is located ona negative magnetic anomaly, presumably between mag-netic anomalies 2 (Olduvai event) and 2A (Gauss nor-mal epoch). A basement age of 1.8 to 2.4 m.y. or 1.9 to2.5 m.y. is therefore anticipated, using either thegeomagnetic polarity time scales of LaBreque et al.(1977) or McDougall (1977), respectively. Biostrati-graphic analysis in both holes gave an age of 1.6 m.y. ata sub-bottom depth of about 60 meters (see paleon-tologic chapters, this volume). Down to the basementcontact, the entire lower sedimentary sections belong tothe basement contact, the entire lower sedimentary sec-tions belong to the nannofossil Emiliania annula Sub-zone. According to the zonation scheme of Bukry(1975), it should cover the time interval between 1.6 and1.8 m.y. The latter date thus gives a maximum paleon-tological age for the sediments of Site 456.

The upper 60 meters of sediments showed significantdrilling disturbance and was unsuitable for paleomag-netic study. Below this depth and including the basalticbasement, there was low recovery and most of the coreswere too fragmented or disturbed by drilling to be posi-tively oriented in the vertical sense, making them poorchoices for paleomagnetic work. Conditions were some-what better in Hole 456A sediments and Hole 456 ba-saltic rocks, respectively. Paleomagnetic data for thetwo holes are summarized in the tables in the Appendix;the downhole variation of the stable inclination direc-tion is shown in Figure 4. The sediments in both holeswere found to have a positive magnetization direction.Reversed inclinations were found in only two samplesfrom the upper part of Hole 456A. Almost all the ana-lyzed sediment specimens exhibited magnetic stabilityupon AF demagnetization, indicating that the resultsobtained, though scarce, are reliable.

There is no detailed correlation between the direc-tions of magnetization in the two sedimentary sections,which is probably not surprising in view of the dif-ficulties encountered in obtaining a representative sam-ple collection. With exception of the uppermost twospecimens in Hole 456A, all sedimentary samples ana-

860

PALEOMAGNETISM OF SEDIMENTS AND IGNEOUS ROCKS

?£100Q.

Q

150

Hole 456

_ ^ —

~~~~~~* Basement

i

Hole 456A

\

Basement

-

- 4 0 ±0 +20

Stable Inclination (°)+40 +20 +40

Stable Inclination (°)+60

Figure 4. Downhole variation of stable magnetic inclination in Hole 456 and 456A sediments (circles) andHole 456 basalts (squares).

lyzed have been assigned a biostratigraphic age of 1.6 to1.8 m.y. The positive inclinations found, therefore,most likely refer to the normal Olduvai event within thereversed Matuyama epoch. The two negative inclina-tions encountered in Hole 456A are relatively shallow(about -10°) and may not necessarily represent a re-versed Earth's magnetic field configuration. They couldbe explained equally well by a slightly abnormal paleo-secular field behavior at this low latitude. An alternativeinterpretation relates Sample 5-1, 116-118 cm whichbelongs to the Gephyrocapsa caribbeanica nannofossilSubzone (0.9 to 1.6 m.y.), to the mostly reversed field(Matuyama epoch) during this period. The lower, nega-tive Sample 7-1, 106-108 cm could be an indicationthat magnetic anomaly 2 consists effectively of two nor-mal events, the Gilsa and Olduvai (see e.g., McDougall,1977). Controversy still exists about this in the litera-ture, and the evidence given here—based on one datapoint—is of course tenuous. The mean inclination ofthe sediments in both holes falls well within the antici-pated inclination range for the present latitude of thesite (see Table 2 for details).

Paleomagnetic results from the basaltic basement areavailable only for Hole 456. The four samples measuredgive a consistent reversed inclination, matching the signof the negative anomaly at the site. In the upper fewmeters, the pillow basalts show features of a rather spec-tacular hydrothermal alteration which also affected theoxide mineralogy. There is no indication, however, thatthe paleomagnetic directions determined are unreliableon account of this phenomenon. A single basaltic sam-ple from Hole 456A, tentatively oriented by verticalvesicle traces, revealed an inclination value in reason-able agreement with the other data. On the evidence oftwo different inclination ranges, lithologic Unit 1 wassubdivided into two magnetic units, each representing aseparate episode of volcanic activity. The boundary be-tween lithologic Units 1 and 2 seems not to coincide with

a magnetic boundary (see table in the Appendix), butthe very limited recovery does not permit any detailedinterpretation of this observation. The average stable in-clination of the basalts is in good agreement with thecentered axial dipole inclination for the present latitudeof the site (see Table 2).

Site 458

This site is located between the presently active islandarc and the mid-slope basement high in the Marianafore-arc region (Fig. 1). The drilling area is on a slighttopographic high probably related to pronounced anom-alies in both gravity and magnetics (see Site Report andunderway geophysics chapter, this volume). Site 458 issituated on a negative magnetic anomaly surrounded bya series of positive anomalies all reaching amplitudes ofabout 100 gammas, indicating a kind of dipole magneticsource layer. Similar configurations in most areas be-tween the trench axis and the Mariana island arc, as wellas the presence of seamounts, suggest that the regionwas built up predominantly in an arc-type setting, al-though no existing island arc model addresses the prob-lem of fore-arc volcanism. Biostratigraphic analysisgave early Oligocene ages for both calcareous nannofos-sils and radiolarians in the lowest sediments near thebasement contact in Hole 458. Accordingly, the vol-canic activity here should be younger than the oldest ig-neous formation found on several islands of the Mari-ana arc. Preliminary radiometric dating of two samples(Ozima and Takigami, personal communication, 1979)gave total fusion ages of 31.8 and 40.5 m.y., respec-tively, but showed rather disturbed age patterns.

Similar to the situation in some of the former holesdrilled during Leg 60, most of the sedimentary coresrecovered at Site 458 show an extreme degree of drillingdisturbance. Paleomagnetic results are thus availableonly from the lowest sediment section (Cores 25 through

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U. BLEIL

27). Further limited by a poor magnetic stability en-countered in samples near the basement contact, thepaleomagnetic data obtained in the sediment column areinsufficient for an interpretation in terms of magneticdating. The average inclination is again too shallow forthe present latitude of the site (see Table 2).

About 209 meters of igneous basement was suc-cessfully penetrated in Hole 458; recovery, however,was only low to moderate. The section consists ofseveral pillow sequences and interbedded more massiveflows, occasionally including fragments of limestone inthe uppermost cores. Petrographically the rocks in theupper part are basaltic andesites interlayered withbronzite-andesites (Meijer et al., 1978), showing geo-chemical characteristics of typical island-arc volcanics(Saunders et al., 1978). Material more similar to normaloceanic tholeiites was encountered only near the base ofthe hole. Magnetic oxide mineralogy and several rockmagnetic properties of these andesitic rocks are clearlydifferent compared with typical ocean-floor basalts andalso the basalts recovered from the Mariana Trough.

All basement samples at this site were found to bemagnetically rather stable. Only minor directional vari-ations were observed upon AF demagnetization; the me-dian destructive field averaged 227 Oe (s.d. 77 Oe).Natural remanent magnetization intensities (JNRM =2.23 × 10~3 emu/cm3; s.d. 4.26 × I0"3 emu/cm3) andKönigsberger ratios (Q = 4.2; s.d. 5.2) are at the lowerend of ranges commonly found in oceanic rocks. Theinitial susceptibilities remain remarkably constantthroughout (k = 1.28 emu/cm3 Oe; s.d. 0.41 emu/cm3 Oe) covering an about average distribution withinthe general variability in ocean-floor igneous materials.None of these magnetic properties reveals a consistentdownhole trend. The samples from the last two cores(Cores 46 and 47) show a drastic increase in NRM inten-sity, and it is tempting to identify this interval as the topof the source layer of the magnetic anomaly. Due tovarious factors like low NRM intensity and Königsber-ger ratio, and most important, mixed polarities, onlyminor contributions to the observed magnetic anomalyresult from the upper basement section.

Five major lithologic units were recognized within thebasement section penetrated at Site 458. In general, thisdivision is fully confirmed by magnetic measurements(Fig. 5, table in the Appendix). As before the stablemagnetic inclination was used to define the magneticunits, because it is the only magnetic property likely toremain constant over each volcanic layer erupted andcooled instantaneously on the time scale of geomagneticvariations. At two levels, a magnetic unit was found tocontinue about 50 cm into the lithologic unit beneath.Provided this is not due to an observational error, themost likely explanation would be a reheating of thelower unit during emplacement of the upper (younger)lava. In dividing a magmatic suite accumulated over along period, it is crucial to know what scatter can beassigned to experimental error in the inclination data.Ryall et al. (1977) from their results on Leg 37 materialsgive an error of ±5° in inclination for one sample. Thistotal error includes effects from orientation, meas-

200

250h

300 h

350 h

400 h

-40 ±0 +20Stable Inclination (c

+60

Figure 5. Downhole variation of the stable magnetic inclination forHole 458 sediments (circles) and igneous rocks (squares); trianglesdenote breccia. Columns give lithologic and magnetic units.Dashed lines indicate minor discrepancies between boundariesdefined by the different methods (see text for details).

urement, and determination of stable inclination. Thelatter procedure may effectively introduce large errorsin magnetically unstable materials. For the presentrocks, however, all reflecting a reasonable magneticstability, the experimental error for a single sampleprobably does not exceed about 2 to 3 degrees. Thisestimate seems justified by the small standard deviationsobtained for the inclination variation within the variousmagnetic units (see table in the Appendix).

On the evidence of clearly different inclinationranges, showing no overlap of the respective standarddeviations, three lithologic units required subdivisioninto two magnetic units. On the other hand, no mag-netic unit comprises more than one lithologic unit, in-dicating that only one magma source has been activeduring the same short period. This suggests a somewhatdifferent magmatic and tectonic regime than proposedfor the North Atlantic ridge. Assuming a similar magni-tude of secular geomagnetic variation as observed in re-cent times, the present results indicate a very episodic

862

PALEOMAGNETISM OF SEDIMENTS AND IGNEOUS ROCKS

volcanic activity as seen in most other parts of the worldoceans. Individual magnetic units, some 10-50 metersthick, may represent a single eruption or an eruptionalsequence over a short time span of the order of 100years. As secular variation of the Earth's dipole andnon-dipole field includes a wide range of periods, some1000 to 100,000 years (see results of Hole 454), the inter-vals between different volcanic pulses should be of thesame order of magnitude. This implies that where alithologic unit required subdivision into two magneticunits, the same type of magma source prevailed over along period without showing major evolutionary trends.On the other hand, only a small fraction of the totalperiod in which the basement section was accumulatedis represented by rock, which consequently limits theamount of magnetic information.

For the upper part, magnetic Units 1 through 5, theaverage stable inclination is not significantly differentfrom a centered axial dipole inclination for the presentlatitude of the site. This contrasts with the shallow in-clination found in the sediments, and in particular, thelower basement rocks, magnetic Units 6 through 8 (seeTable 2). Due to the limited number of data, any or allthe mean stable inclinations could be biased by in-completely averaged secular geomagnetic field com-ponents. Another critical factor in deciding whether arepresentative paleomagnetic field direction has beensampled, allowing for a reliable estimate of absoluteplate motions, is to account for possible post-emplace-ment tectonism. The entire basement section recoveredat Site 458, and especially the upper lithologic units,show extensive fracturing and numerous sheared zones;also a somewhat inclined bedding (up to 10°) was ob-served in the lower part of the sedimentary column.

At a sub-bottom depth of about 380 meters (betweenCores 40 and 41), the stable inclinations abruptlychange from positive to shallow negative without dis-playing intermediate, transitional directions (Fig. 5). Atthe same depth a lithologic break is encountered, whichmay mark a substantial time gap during which theearth's magnetic field reversed. Unfortunately, the con-tact itself was not recovered; thus, its nature remainsuncertain. The abundant slickensides and highly frac-tured and sheared zones are indicative of a tectonicevent which may account for the persistent scatter instable inclination seen on a small scale within the mag-netic units, or alternatively be evidence that a largerfault-block rotation has affected the basement section.

Site 459

This site is located just above the slope break of theMariana Trench (Fig. 1). Magnetic site survey datarevealed an extended positive magnetic anomaly in thedrilling area whose nature and source are possiblyrelated to the trench structure itself. According to thepaleontological results, the oldest sediments identifiedwere late to middle Eocene in age indicating that thebasement formed at about 38-40 m.y. ago, beingroughly contemporaneous with the oldest volcanicsfound on the Mariana island arc. One preliminaryradiometric age of 36.1 ± 2.0 m.y. obtained from the

igneous section at this site (Ozima and Takigami, per-sonal communication, 1979) is based only on a ratherdisturbed age pattern.

With the aim of establishing a magnetic basement ageand to elaborate on a possible magnetic recording of anorthward movement of the region as suggested bypaleomagnetics of the adjacent Site 458, at least onesample was taken from every sedimentary core sectionrecovered in Hole 459B for paleomagnetic study. Atotal of 109 specimens were sampled over the 559 metersof drilled and continuously cored sediment sequence.Although a relatively high degree of drilling disturbanceimpaired an adequate sampling at various levels, gen-erally the recovery allowed for a fairly well-defined ver-tical orientation of the specimen. Whenever possible,preference was given to areas where bedding could beused as a horizontal reference for sampling.

Among other factors, an average recovery of lessthan 40 per cent in the sedimentary column considerablyreduced the chance of obtaining a reasonably completepolarity-age pattern in this hole. Biostratigraphic data,therefore, had to be used as calibration marks. Theearly Oligocene through middle Eocene is only repre-sented by the three deepest sediment cores (Cores 58through 60), from which almost no paleomagnetic in-formation is available. The approximately 540 meters ofsediments of Cores 1 through 57 represent the past 30m.y. Referring to the recent polarity-time scale ofLaBreque et al. (1977), the Earth's magnetic field hasreversed at least 120 times during this period. Hence, onthe average each core should include at least a recordingof one positive and one negative geomagnetic field con-figuration. However, the polarity intervals are not ofequal length (see Fig. 6A) and the possibility of identify-ing a short polarity event of the order of 0.1 m.y. isalmost negligible. Extended periods (>0.5 m.y.) ofuniform polarity or a series of closely spaced positive ornegative polarities, each of some intermediate length ingeneral, should be represented by several data points.To some extent, therefore, the present situation is com-parable to the problem of recognizing marine magneticanomalies.

The average sedimentation rate amounts to 18 me-ters/m.y., but the paleontological results obtained re-vealed complexity in detail. From early Pliocene throughmiddle Miocene there may have been a hiatus of 6 to 7m.y. Another shorter hiatus of about 1 m.y. in middle/early Miocene times is probably less well-documentedand not of specific relevance for the discussion of thepaleomagnetic data. For most of the sediments the natu-ral remanent magnetization intensity is of the order of10~4 emu/cm3. Upon AF demagnetization, with but afew exceptions, these samples displayed a reasonablemagnetic stability, and stable magnetic directions couldpositively be defined using the procedures describedabove. Much lower NRM intensities (about 10~6 emu/cm3 or less) were encountered in parts of the upper(Cores 2 through 6) and lower (Core 54 to the basementcontact) sedimentary column, probably indicating a re-duced volcanic ash content at these levels. Those sam-ples frequently were found magnetically unstable, show-

863

DO

Paleomagnetic Polarity Time Scale (m.y.) (LeBreque et al., 1977)

10 20

i•π mimπ xi1 nπnπo E•IΠ5 5A 5B 5C 5D 5E 6 6A" 6B 6C 7 7A

Stable Inclination (°)20 ±0 +20 +40

Figure 6. A. Downhole variation of the magnetic inclination in Hole 459B sediments (full circles = Istabk> °P e n circles = I^RM)- The column on the left gives the biostratigraphic results; thebroken lines indicate the respective accumulation rate of the sediments. Straight lines show sedimentation rates as derived from the magnetic polarity log of the sequence (shaded = normalpolarity, white = reversed polarity), using the paleomagnetic time scale of LaBreque et al. (1977). Only the normal polarity zones identified are indicated as columnar sections referring tothe respective time and depth interval. B. Downhole variation of stable magnetic inclination in Hole 459B basement rocks. Columns give lithologic and magnetic units. Dashed lines indicatepoorly confined boundaries or minor discrepancies between boundaries defined by the different methods (see text for details).

PALEOMAGNETISM OF SEDIMENTS AND IGNEOUS ROCKS

ing larger directional variations upon demagnetization,limiting a reliable definition of stable magnetic direc-tions (see table in the Appendix).

The polarity pattern obtained (Fig. 6A) seems to be infair agreement with established paleomagnetic timescales for most of the Cenozoic time. A magnetic down-hole age scheme was based mainly on the identificationof several longer periods of uniform magnetic polarity,like positive anomalies 5 and 6 and the negative intervalbetween positive anomalies 6C and 7. Compared withthe biostratigraphy, the magnetic data show almostidentical trends in Quaternary, early Miocene, and lateOligocene. Differences of some 1 to 2 m.y. at somestages are probably not significant, as the presentlyavailable time scales are not yet precise enough to drawany conclusions from this evidence. A major discrep-ancy, however, is observed in the middle Miocene. Thepaleomagnetic results here give consistently youngerages and suggest that the hiatus encountered at about 65meters sub-bottom represents only a time span of 2 to 3m.y.

Averaging the stable inclinations of the sediments inHole 459B again yields a mean which is too shallow forthe present latitude of the site. Moreover, calculating in-dividual averages for the Miocene and Oligocenesediments, respectively, reveals a trend toward shal-lower inclinations deeper in the hole—that is, with in-creasing age.

The igneous rocks recovered from Hole 459B areessentially clinopyroxene-plagioclase basalts. The litho-logic sequence comprises pillows and interlayered, moremassive flow units. The natural remanent intensitiesvary over almost four orders of magnitude, from 10~5

to I0"1 emu/cm3. Thus, within only an approximately125-meter igneous section, almost the same total rangeof variability is encountered as has been found in all theoceanic basement so far. The mean of JNRM = 1.4 ×I0"2 emu/cm3 (s.d. 2 × 10~2 emu/cm3) obtained hereis the highest measured on any DSDP material. Otherrock magnetic properties measured cover a more aver-age range of values. This will be discussed in detailelsewhere.

The directional stability, though the median destruc-tive fields are not particularly high (see table in the Ap-pendix), was found adequate in almost all of the rocksto determine a reliable stable inclination direction.

The downhole variation of the stable inclination (Fig.6B) in part reveals similar characteristics as seen inprevious holes of this Leg. In general, there is a closecorrelation between lithologic and magnetic units. Withone exception, all lithologic units comprise two mag-netic units, suggesting that the magmatic source re-mained unchanged over long periods. Although themagnetic units were defined on the same basis as usedbefore—that is, a narrow range of inclination variationover a given basement section—the downhole patternobtained here differs from the others in that betweenevery two magnetic units the inclination seems to changesign. This could of course be due to a somewhat peculiarmode of basement emplacement, but in view of all theother results it appears highly unlikely that the Earth's

magnetic field always reversed between lithologic unitsand additionally several times within the unit. Thedownhole variation in stable inclination, therefore, maybest be interpreted as reflecting in most parts a normalrange of paleosecular variation of the Earth's magneticfield at an equatorial latitude. However, as indicated bythe highly fractured rocks and numerous sheared zonesrecovered, some post-emplacement tectonism may haveaffected this basement section.

A mean stable inclination of -0.2° (s.d. 14.4°) is ob-tained, assuming the observed inclination variation isdue to a normal paleosecular field variation. If thepolarity pattern would entirely result from true rever-sals, the respective mean becomes ±12.4° (s.d. 5.4°).Both values clearly indicate that the site must have beenat low latitude in Eocene times.

DISCUSSION AND CONCLUSIONSThe principal objectives of Leg 60 were (1) to study

the mode of spreading and the character of the crust inthe currently active inter-arc basin of the MarianaTrough, and (2) to trace the structural evolution of thefore-arc region of the Mariana island arc. Paleomag-netic results bearing on these different aspects were ob-tained from the sediment and basement rocks at threedrilling sites within the Mariana Trough (Sites 453, 454,and 456) and from two holes drilled in the Mariana fore-arc area (Holes 458 and 459).

The paleomagnetic study of deep-sea sediments hasled in the past to significant contributions to the mag-netic reversal stratigraphy and the knowledge of paleo-secular variation in the Earth's magnetic field, althoughonly limited information is available concerning thefundamental processes by which the magnetic pattern isrecorded. Various types of remanent magnetization ac-quisition, taking place either during deposition, postde-positional bioturbation, or chemical alteration of thesediments, may be the dominant mechanism in any par-ticular record. Erratic remanent magnetization direc-tions—rather frequently observed in deep-sea sedimentcores—are generally believed to result from physicaldisturbances introduced during the coring operation.Highly unstable magnetic remanences have been foundin "red clay" abyssal sediments by several workers. Ac-cording to Kent and Lowrie (1974), this instability is ofviscous origin and due to the presence of maghemite.No general mechanism, however, has yet been identifiedcausing systematic deviations in magnetic directions(e.g., an inclination error). By detailed susceptibilityanisotropy measurements, Kent and Lowrie (1975)showed that magnetic fabrics acquired as a result of thedepositional process are almost negligible in deep-seasediments.

Special care was taken in sampling the sediments dur-ing this Leg to avoid all disturbed zones and to refer tofeatures of a horizontal bedding whenever possible.This safeguard, together with the overall high magneticstability encountered, suggests that the results mayrepresent a reliable record of the paleomagnetic field in-clination. Because of the lack in azimuthal orientationof DSDP cores, reversal chronologies can only be traced

865

U. BLEIL

by changes in sign of the inclination which may be mis-leading in low latitudes like the present. For the youngMariana Trough sites this probably does not introducetoo much of a problem, because results of a suite ofBrunhes lavas from Pagan Island (Levi et al., 1975) sug-gest that the Mariana region belongs to the Pacificdipole window of low angular dispersion (<10°) inmagnetic field directions. Most of the present data ap-parently indicate a somewhat higher degree of varia-bility, which in part may be due to experimental errors.Aziz-ur-Rahman and McDougall (1973) have presentedevidence that the secular variation in the southwestPacific in early Pleistocene and Pliocene times may havebeen higher. During this period, therefore, there mayhave been significant contributions from non-dipolefield variations in the area. In discussing the relativemagnitude in secular variation, it should be pointed outthat within a sedimentary sequence covering a longertime span, possible plate motions causing a shift inmagnetic mean direction must also be taken into ac-count.

The details of the magnetic record depend upon thesedimentation rate. Low rates of about 0.1 cm/1000 y.will average out most of the secular variation in a stand-ard sample 2.5 cm thick, whereas samples with a highaccumulation rate (> 10 cm/1000 y.) will record detailsof the paleofield variation. All sediment samples takenduring this Leg apparently belong to the latter category;thus, much of the scatter seen in the data should be in-terpreted as the result of discontinuous spot readings offield variations. In this respect, the rather commonplaceintermediate inclination values and several continuoustrends in magnetic directions observed in Hole 453 and459B sediments are difficult to understand, as they sug-gest much longer periods than are generally anticipatedin the Earth's magnetic field secular variations.

At Site 453, adjacent to the West Mariana Ridge,which bounds the Mariana Trough to the west, the mag-netic pattern in the sediments was found to match estab-lished polarity time scales in almost every detail (Fig.2A) between approximately 3 and 5 m.y. B.P. The mag-netic age of the oldest sediments (5.0 ± 0.2 m.y.) is inexcellent agreement with the biostratigraphic results.Beneath the sediments, a polymict breccia layer was en-countered at this site which obviously does not provideuseful paleomagnetic information. The various igneousand metamorphic fragments contained in the breccia allshow compositional affinity to the West Mariana Ridgeigneous rocks drilled during Leg 59 (Saunders et al.,1978). This rock assemblage could have been depositedas landslide debris, or may represent the remnants of adeep-reaching fault, uplifted and exposed during theearly rifting of the Mariana Trough. The drilling did notclarify whether the deeper basement at this site consistsof inter-arc basin crust. Assuming it is situated on theoldest western part of the "true" Mariana Trough, thebasement here should be somewhat older than the sedi-ment age obtained. Extrapolation of spreading ratesdetermined from the two younger Mariana Trough sites

suggests an apparent basement age of about 6.5 m.y.(Fig. 7). This date places a limit on the onset of openingin this back-arc basin which, therefore, appears to bemuch older than previously anticipated (Karig, 1975;Karigetal., 1978).

At the two drilling sites near the center of theMariana Trough (Sites 454 and 456), the paleomagneticanalysis did not provide independent age information.However, the magnetic pattern in the sediments and ig-neous rocks are again in complete agreement with pale-ontologic data and so it was possible to assign a mag-netic basement age at both sites. The magnetic polarityin each igneous section cored was found to match thesign of the magnetic anomaly in the respective drillingarea. This result, together with fairly high remanentmagnetic intensities encountered, suggests that the ex-trusive portion of the igneous basement in the MarianaTrough effectively is a potential source layer con-tributing to magnetic anomaly lineations similar tothose observed at mid-ocean ridges. The recent attemptsby Bibee and Shor (1978) to model these anomalies may,therefore, be valid for at least the densely surveyed areanear 18°N. The lack of published closely spacedgeophysical profiles impairs further constraints on thedetailed character of the magnetic anomaly pattern inthis back-arc basin. The available data indicate a rathercomplex style of crustal dilation along a system ofmultiple short ridges and transform faults similar to thesituation in the Gulf of California or the Gulf of Aden.

Figure 7 summarizes the drilling results obtained inthe Mariana Trough along about 18 °N. The distancesof each site from the axial graben structure, the sus-pected locus of zero age, are plotted versus basementages as inferred from paleomagnetic and biostrati-graphic analyses. On the basis of this limited informa-tion the following tentative conclusions are drawn: (1)The apparent increase of basement ages toward theflanks of the axial zone suggests a uniaxial spreadingsimilar to that occurring at mid-ocean ridges. (2) On theaverage, the spreading was symmetrical and there is noevidence for a spatially diffuse off-axis volcanism, butthe accretion of new crust seems to be confined within anarrow zone at least to the central graben which has ahalf-width of less than 10 km at 18°N. (3) The mode ofspreading was continuous with a constant (half-) rate ofabout 2 ± 0.5 cm/y, in Pliocene and Pleistocene times.This low spreading rate confirms the interpretation ofthe magnetic anomaly pattern given by Bibee and Shor(1978) and also correlates with the rough topographicrelief observed. (4) The present data suggest an overalleast-west spreading direction, but no further con-straints of any specific strike direction on the flow linesare indicated that might decide between the differentopening models proposed for the trough (Karig et al.,1978). (5) Opening of the currently active back-arc basinwas initiated between about 5 and 6.5 m.y. ago. It is in-teresting that this date coincides approximately with amajor change in direction and rate of plate motions inthe Pacific region (Klitgord et al., 1975). The arcuate

866

PALEOMAGNETISM OF SEDIMENTS AND IGNEOUS ROCKS

Hole 453

W150 100

Me 456

50

Figure 7. Distance of drilling sites (km) from the axial graben structure, the apparent spreading center inthe Mariana Trough near 18°N, versus basement ages (m.y.) as inferred from paleomagnetic andbiostratigraphic analyses. Error bars at each site refer to the possible age intervals and distancesassuming an east-west or 30° striking flow line, respectively. Straight lines give visual best fits for con-stant spreading rates (see text for details). Site 456 is also shown in a hypothetical western positionrelative to the suggested axial zero age in the back-arc basin (broken column).

form and rotations of islands in the southern Marianasalso developed since Miocene time, according to Larsonet al. (1975).

As indicated in various chapters discussing results ofthe individual sites, with the exception of Holes 456 and456A, all other stable remanent magnetization inclina-tion measured for sediments and/or igneous rocks gavetoo shallow a mean for the present latitude of the drill-ing area. The simplest explanation is that their forma-tion was at more equatorial positions (see Table 2).However, a critical assumption in the determination ofpaleolatitudes from the direction of remanent magneti-zation is that secular variation has been eliminated.Despite the high accumulation rates, only for the Hole454A and 458 sediments, the number of samples is pre-sumably not large enough to account for this effect. Inorder to avoid a bias of the averages toward shallowdirections, apparent intermediate inclinations < 10° forthe young Mariana Trough sites and <5° for the olderfore-arc sites are not included in the mean values listedin Table 2.

The situation is obviously more complicated for thedifferent basement sections analyzed. The number ofmagnetic units, each representing only one independentspot recording of the Earth's paleomagnetic field, variesbetween two (Hole 456) and eight (Hole 458). Even inthe latter case, it remains doubtful whether the mean in-clination will provide a reliable reading of the axialgeocentric dipole field. A further problem, in particularfor the igenous rock suites, is a possible post-emplace-ment block rotation. A tilting resulting only in an effec-

tive 10° deflection of the median magnetic inclinationwould produce an error in latitude of almost the sameorder of magnitude for the present latitudinal positionof the drilling area.

As previously described, direct evidence for somekind of tectonic event, not necessarily a tilting, was seenin the basement drill cores in Holes 458 and 459B in theMariana fore-arc region. The Mariana Trough base-ment was formed at a spreading center. The results fromthe igneous sections here must therefore also be suspect,as a (slight) tilting is commonly associated with tectonicactivity at ocean ridges. The paleolatitude obtained forHole 454A (Table 2, Fig. 8), which is mainly based onbasement rock paleomagnetics and includes only a fewsedimentary data, is thus not well documented. In con-trast, for Sites 453 and 456 the paleolatitudes are almostexclusively derived from sediment paleomagnetic resultsand should be considered reliable. According to Figure8, for the about 2-m.y.-old Site 456, situated on theeastern spreading flank, no significant deviation fromthe actual latitude is indicated. Site 453 is located in thewesternmost part of the Mariana Trough. Drilling couldnot resolve whether the basement here still belongs tothe "true" Mariana Trough crust or whether it formspart of the West Mariana Ridge structure. This ques-tion, however, is crucial for any interpretation of theapparent large latitudinal offset derived from the sedi-ment paleomagnetic results which suggest a northwardmotion of the drilling area by about 20 cm/y, sinceMiocene. Although high, this velocity is probably nottotally unrealistic; its northward direction would be

867

U. BLEIL

20°N

0°N

Guam (Larson, et al., 1975)

.Hole 459Oligocene Sediments

Hole 458-

Hole 459Basement

i i ^

Hole 458Lower Basement \ I

I "N

I JHole 459Basement

10 20Age (m.y.)

30 40

Figure 8. Age versus latitudes for Leg 60 drilling sites in the Mariana Trough (Holes 453 through 456) andMariana fore-arc region (Holes 458 and 459). Error bars refer to age intervals as indicated bypaleomagnetic and biostratigraphic results and ranges of paleolatitudes as inferred from mean inclina-tion standard deviations (see Table 2). All present latitudes are projected to 18°N, and, unless other-wise indicated, paleolatitudes given were deduced from combined paleomagnetic data of sedimentsand igneous rocks. In addition, results from Guam Island (Larson et al., 1975) are plotted. Straightlines indicate different visual best fit estimates for a continuous northward movement of the drillingarea (see text for details).

compatible with a large post-Miocene clockwise tectonicrotation (about 50°) of the entire southern Mariana arc-trench system as indicated by paleomagnetic data fromGuam (Larson et al., 1975) and possibly Saipan (Dunnet al., 1979). Results of the West Philippine Basin(Louden, 1977) from both a paleomagnetic study ofDSDP sediment cores and phase shifting of marinemagnetic anomalies gave strong evidence for a consider-able northward movement of the Philippine plate dur-ing the past 50 m.y., including a large rotational com-ponent (about 60°). If the same general plate motion iseffectively documented in the Mariana area as proposedby Louden (1977), this would imply that it has not beencompletely disrupted by inter-arc spreading episodesbetween the Mariana Arc and the West PhilippineBasin. On the other hand this idea, like the presentresults from Hole 453, seems to be in conflict with anyopening model proposed so far for the Mariana Trough(Karig et al., 1978).

Paleomagnetic results from both drilling sites in theMariana fore-arc also clearly indicate some northwardmovement of this area. The data are summarized inTable 2 and Figure 8. In particular, the magnetic

records obtained from the Hole 459B sediments providedetailed information about the latitudinal variation inOligocene and Miocene times. In the sedimentary col-umn of this hole the paleomagnetic and biostratigraphicage pattern identified shows generally a nearly perfectcorrelation (Fig. 6A). Because of the close interrelation-ship of the respective time scales, the apparent largediscrepancy around magnetic anomaly 5—where thepaleontologic results indicate considerable older ages—is rather surprising. However, similar observations havealso been reported from other DSDP sites (LaBreque etal., 1977). As the time of magnetic anomaly 5 is con-trolled by radiometric dating, it appears that the bio-stratigraphic zonation scheme needs revision for thisperiod.

The mean inclination of the sediments shows a grad-ual shallowing with increasing age in Hole 459B. Thistrend is confirmed by the basement paleomagnetics atboth fore-arc drilling sites. The respective variation inlatitudes is illustrated in Figure 8. To allow for a directcomparison of the different results, all present latitudeshave been projected to 18 °N. As the various data pointsall cover a more or less extended time interval, and some

868

PALEOMAGNETISM OF SEDIMENTS AND IGNEOUS ROCKS

error has to be assigned to the latitudes according to thestandard deviations in mean inclinations, the northwardmovement—though well documented as a general fea-ture—cannot be traced exactly in space and time. Dif-ferent best visual fits, assuming a constant continuousmotion from Eocene to present, are shown in Figure 8.The resulting velocities are in fair agreement with thoseobtained from the West Philippine Basin (Louden,1977), but as already indicated by his data and thepaleomagnetic results from the Mariana island arc (Lar-son et al., 1975; Levi et al., 1975; Dunn et al., 1979),most of the northward movement could have been com-pleted before Miocene times.

In conclusion, the different results may also placesome interesting constraints on the opening mode of theMariana Trough. The eastern portion, including theisland arc and fore-arc region, has apparently under-gone only a limited northward motion at least in thepast 5 m.y. Results from the western half of the trough(Sites 453 and 454), on the other hand, strongly suggesta marked latitudinal motion to the north. A possible in-terpretation, therefore, would be that spreading even inthis central portion of the Mariana Trough at 18°N wasapproximately in a northwest-southeast direction, arelative northward motion being superimposed, prob-ably associated with a clockwise rotational component.

ACKNOWLEDGMENTS

I would like to thank the many people who were responsible forthe success of DSDP Leg 60. Prof. J. Untiedt kindly made availablethe paleomagnetic laboratory facilities at the University of Munster,Germany. I am also grateful to N. Weinreich and H. U. Worm forassistance with the measurements and to R. Day and M. Fuller forreview of the manuscript.

This study was made possible by funds provided by the DeutscheForschungsgemeinschaft.

REFERENCES

Ade-Hall, J. M., and Johnson, H. P., 1976. Paleomagnetism of ba-salts, Leg 34. In Yeats, R. S., Hart, S. R., et al., Init. Repts.DSDP, 34: Washington (U.S. Govt. Printing Office), 513-532.

Aziz-Ur-Rahman, and McDougall, I., 1973. Paleomagnetism andpaleosecular variation on lavas from Norfolk and Philip Islands,southwest Pacific Ocean. Geophys. J., 33:141-156.

Bibee, L. D., and Shor, G. G., 1978. IPOD Site Survey Along theSouth Philippine Sea Transect: unpublished report.

Bleil, U., and Smith, B. M., 1979. Paleomagnetism of basalts Leg 51.In Donnelly, T. W., Francheteau, J., Bryan, W., Robinson, P.,Flower, M., Salisbury, M., et al., Init. Repts. DSDP, 51, 52, 53,Pt. 2: Washington (U.S. Govt. Printing Office), 1351-1361.

Bukry, D., 1975. Coccolith and silicoflagellate stratigraphy, north-western Pacific Ocean, Deep Sea Drilling Project, Leg 32. In Lar-son, R. L., Moberly, R., et al., Init. Repts. DSDP, 32: Washing-ton (U.S. Govt. Printing Office), 677-701.

Day, R., Halgedahl, S., Steiner, M., Kobayashi, K., Furuta, T.,Ischii, T., and Faller, A., 1979. Magnetic properties of basaltsfrom DSDP Leg 49. In Luyendyk, B. P., Cann, J. R., et al., Init.Repts. DSDP, 49: Washington (U.S. Govt. Printing Office),781-791.

Dunn, J. R., Fuller, M., Green, G., McCabe, R., and Williams, I.,1979. Preliminary paleomagnetic results from the Philippines,Marianas, Carolines and Solomons. Am. Geophys. Union Trans.,60:239. (Abstract)

Karig, D. E., 1971. Structural history of the Mariana Island arcsystem. Geol. Soc. Am. Bull., 82:323-344.

, 1975. Basin genesis in the Philippine Sea. In Karig, D. E.,Ingle, J. C , Jr., et al., Init. Repts. DSDP, 31: Washington (U.S.Govt. Printing Office), 857-879.

Karig, D. E., Anderson, R. N., and Bibee, L. D., 1978. Characteris-tics of back arc spreading in the Mariana Trough. J. Geophys.Res., 83:1213-1226.

Karig, D. E., and Glassley, W. E., 1970. Dacite and related sedimentfrom the West Mariana Ridge, Philippine Sea. Geol. Soc. Am.Bull., 81:2143-2146.

Kent, D. V., and Lowrie, W., 1974. Origin of magnetic instability insediment cores from the Central North Pacific. J. Geophy. Res.,79:2987-3000.

, 1975. On the magnetic susceptibility anisotropy of deep-seasediment. Earth Planet. Sci. Lett., 28:1-12.

Klitgord, K. D., Huestis, S. P., Mudie, J. D., and Parker, R. L.,1975. An analysis of near-bottom magnetic anomalies: Sea floorspreading and the magnetized layer. Geophys. J., 43:387-424.

LaBreque, J. L., Kent, D. V., and Cande, S. C , 1977. Revisedmagnetic polarity time scale for Late Cretaceous and Cenozoictime. Geology, 5:330-335.

Larson, E. E., Reynolds, R. L., Ozima, M., et al., 1975. Japan-U.S.Paleomagnetism project in Micronesia. Paleomagnetism of Mio-cene volcanic rocks of Guam and the curvature of the SouthernMariana Island arc. Geol. Soc. Am. Bull., 86:346-350.

Lawver, L. A., and Hawkins, J. W, 1978. Diffuse magnetic anom-alies in marginal basins: Their possible tectonic and petrologic sig-nificance. Tectonophysics, 45:323-339.

Levi, S., Merrill, R., Aoki, Y., et al., 1975. U.S.-Japan cooperationprogram in Micronesia. Paleosecular variation of lavas from theMarianas in the western Pacific Ocean. J. Geomag. Geoeletr.,27:57-66.

Louden, K. E., 1977. Paleomagnetism of DSDP sediments, phaseshifting of magnetic anomalies, and rotations of the West Philip-pine Basin. J. Geophys. Res., 82:2989-3002.

Lowrie, W., 1974. Oceanic basalt magnetic properties and the Vineand Matthews hypothesis. J. Geophys., 40:513-536.

McDougall, I., 1977. The Present Status of the Geomagnetic PolarityTime Scale: Canberra (Austr. Nat. Univ., Res. School Earth Sci.),Publ. No. 1288.

McDougall, I., Saemundson, K., Johannesson, H., Watkins, N. D.,and Kristjansson, L., 1977. Extension of the geomagnetic polaritytime scale to 6.5 m.y.: K-Ar dating, geological and paleomagneticstudy of a 3,500 m lava succession in western Iceland. Geol. Soc.Am. Bull., 88:1-15.

Meijer, A., and Shipboard Scientific Party IPOD, Leg 60, 1978.Petrology of volcanic rocks from the Mariana arc-trench gap andgabbros from the Mariana inter-arc basin, IPOD, Leg 60. Am.Geophys. Union Trans., 59:1182. (Abstract)

Petersen, N., in press. Paleomagnetism and rock magnetism ofbasalts, Leg 54. In Hekinian, R., Rosendahl, B. R., et al., Init.Repts. DSDP, 54: Washington (U.S. Govt. Printing Office).

Ryall, P. J. C , Hall, J. M., Clark, J., and Milligan, T., 1977. Mag-netization of oceanic crustal layer 2—results and thoughts afterDSDP Leg 37. Can. J. Earth Sci., 14:684-706.

Saunders, A. D., Marsh, N. G., and Tarney, J., 1978. Geochemistryof arc and inter-arc basalts from the east Mariana transect: IPODLeg 60. Am. Geophys. Union Trans., 59:1182. (Abstract)

Zijderveld, J. D. A., 1968. A. C. demagnetization of rocks: Analysisof results. In Collison, D. W., Creer, K. M., and Runcorn, S. K.,(Eds.), Methods in Paleomagnetism: Amsterdam (Elsevier Scien-tific Publ. Co.), pp. 254-286.

APPENDIXThis section lists paleomagnetic parameters on a sample-by-sample

basis for the sediments (Table 1) and basement rocks (Table 2). Forthe igneous samples also basic rock magnetic properties are listed.

Table 3 summarizes the lithologic and magnetic stratigraphy forthe drill holes. Upper and lower limits of magnetic units refer to firstand last sample measured; errors listed for stable inclinations arestandard deviations.

869

U. BLEIL

Table 1. Results of paleomagnetic measurements of Leg 60 sediments. Table 1. (Continued).

Sample(interval in cm)

Depth belowSea Floor (m)

JNRM .(10~6 emu/cm3) 'NRM ^Stable &NRM Datable

Sample(interval in cm)

Depth belowSea Floor (m)

JNRM .(10 ~ 6 emu/cmJ 'Stable &NRM Stable

Hole 453

15-2, 16-1819.CC, 5-820-2, 58-6021-1, 100-10322-2, 17-2023-1, 89-9124-1, 137-14025-1, 118-12026-2, 7-927-1, 23-2528-1, 29-3129-1, 24-2629A 69-7130-1, 92-9431-2, 81-8332-1, 74-7633-1, 59-6133-3, 81-8334-2, 21-2335-1, 126-128354, 101-10336-1, 82-8436-3, 131-13337-1, 18-2038-1, 46-4839-3, 59-6141-5, 120-12242-1, 54-5643-3, 19-2144-1, 65-6745-1, 7-946-1, 19-2147-3, 8-1048-1, 12-14

Hole 456

9-1, 59-6110-1, 117-11910-2, 64-6611-1, 54-5612.CC, 3-413-1, 25-2714-1, 7-914-1, 73-75

Hole 456A

5-1, 116-1185-2, 108-1106-1, 64-666-2, 106-1087-1, 104-1067-2, 58-608-1, 39-419-1, 139-1419-2, 35-3710-1, 41-4310-3, 59-6111-1, 57-59

Hole 458

25-1, 46-4825-2, 48-5026-1, 28-3027-1, 35-3727-1, 106-10827-2, 29-31

Hole 459B

1-1, 130-1321-2, 99-1011-3, 78-801-4, 34-351-5, 8-102-1, 43-452-2, 30-322-3, 104-1062-4, 34-362-5, 10-124,CC, 11-135-1, 9-106-1, 128-1306-2, 84-866-3, 67-696-4, 11-136,CC, 12-148-1, 140-1429-1, 58-6011-1, 47-4911-2, 53-5512.CC, 19-2113-1, 77-7914-1, 55-57

134.17179.87182.09190.52200.69209.40219.39228.69238.58246.74256.30265.75270.70275.93286.92294.75304.10307.32314.72323.77328.02332.83336.32341.69351.47364.10386.71389.55401.70408.66417.58427.20439.59446.13

67.6077.6878.6586.55

104.84105.26114.58115.24

58.1759.5967.1569.0777.0578.0985.9096.4096.86

104.92108.10114.58

228.47229.99237.79247.36248.07248.80

1.312.503.79

4.856.09

7.949.31

11.5512.3513.6126.6236.0946.7947.8549.1850.1251.2865.9174.5993.4895.04

102.70112.78122.06

-0.1136-0.161.1

1956.33.3

49.240.485.87.0

244<O.l<O.l<O.l<O.l57.1

-0.12.5

<O.l695

26.80.4

17978.14.0

216196

0.31.2

29.5186<O.l<O.l

12210989.181.4

1112.52 × 10~ 3

121

14517411811711113269513211115915664.6

57.586.211016.31.48.2

91.574.071.583.368.00.3

-0.10.3

-0.10.2

1681.55.8

-0.11170.2

<O.l367778347443162171378

-41.1-40.7-14.9+ 0.3+ 35.8+ 6.0+ 1.8-0.9-12.1-7 .6-21.1-13.2+ 8.7-17.8-12.5+ 28.8+ 14.7+ 79.6+ 3.4+ 42.0+ 32.1+ 17.0+ 11.7+ 1.2+ 28.9+ 10.6+ 22.2+ 18.8-1.7-16.1-27.5-10.2

no-47.1

(-42.7)-0.7+ 38.9+ 5.3-11.6-5.1-14.8-12.2-18.6-18.2

_

_+ 22.5

(-10.5)(-9.9)

—+ 35.2+ 21.6+ 12.8+ 0.6+ 31.0+ 13.3+ 11.4+ 9.2-14.5-24.7-30.9-14.5

14.3223.8259.9

84.2148.6178.2142.0252.8342.8125.9180.123.386.0

184.682.190.07.3

231.7257.4333.664.4

106.4345.4262.6269.6101.9293.0174.9112.214.7

187.595.1

no228.7

(239.7)84.0

145.9175.7142.1260.6340.0119.8180.027.4

_7.0

(101.4)(262.9)

64.096.1

351.0261.7269.8101.3283.3176.7100.810.6

189.392.8

+ 36.4+ 14.9+ 65.2+ 28.6+ 37.9-34.2+ 39.7+ 36.8

+ 9.1+ 35.5+ 38.3+ 40.7-2.7+ 34.8+ 33.5+ 42.3+ 23.6+ 34.4+ 36.6+ 27.9

-15.8-21.0+ 28.2+ 19.8+ 51.6-55.8

+ 14.4+ 31.2+ 38.5+ 38.1+ 29.2+ 20.8+ 1.6-13.5-22.0-19.5-36.5-19.0-46.1-46.4+ 11.2-19.0+ 39.6-21.0-20.7+ 18.6+ 18.8+ 19.8+ 30.1+ 25.4

+ 31.4+ 14.7+ 55.1+ 23.4+ 32.4+ 43.5+ 31.5+ 31.9

-7.8+ 38.2+ 44.6+ 49.3-10.5+ 33.0+ 35.7+ 45.9+ 35.3+ 40.4+ 34.2+ 25.3

-21.8-21.7+ 15.8

( + 9.2)(-18.3)(-15.6)

+ 22.9+ 19.2

122.9161.5341.8299.5157.2131.1148.7296.0

289.150.5

256.256.0

151.4144.189.356.2

338.2229.7140.552.3

72.27.8

128.540.1

326.5162.7

120.021.313.581.4

101.7237.3268.4180.1220.7354.5352.8209.5352.3355.130.00.1

198.1358.2291.4226.197.1

305.23.6

350.1

108.5168.8299.4299.9153.0131.6150.2296.4

285.755.8

250.456.2

152.0145.989.863.2

353.9232.8142.449.7

67.7

3.6133.0(15.6)

(185.7)(59.5)

Hole 459B

15-1, 22-2415-2, 136-13817-1, 13-1517.CC, 7-919-1, 12-1420-1, 40-4221-1, 80-8224-1, 9-1124-2, 14-1625-1, 49-5125-2, 40-4225-3, 92-9426-1, 33-3527-1, 8-1027-2, 33-3527-3, 99-10127-4, 143-14528-1, 13-1528-2, 62-6428-3, 102-10429-1, 22-2429-2, 86-8830-1, 64-6730-2, 46-4930-3, 100-10330-4, 91-9431-1, 114-11531-2, 78-8033-1, 12-1534-1, 10-1234-2, 28-3035-1, 23-2535-2, 139-14136-1, 18-2036-2, 70-7237-1, 19-2038-1, 34-3639-1, 12-1439-2, 61-6340-1, 4-640-2, 128-13040-3, 15-1740-4, 106-10841,CC, 18-2042-1,40-4242-2, 73-7542-3, 23-2544-1, 69-7144-2, 52-5445-1, 6-846-1, 20-2246-2, 102-10447-1, 59-6148-1, 85-8749-1, 27-2950-1, 25-2750-2, 104-10651-1, 15-1752-1, 6-852.CC, 25-5753-1, 79-8153-2, 25-2753-3, 62-6453^», 46-4854-1, 58-6054-2, 45-4754-3, 42-44544, 65-6754-5, 52-5455-1, 78-8055-2, 17-1955-3, 28-3055-4, 64-6656-1, 55-5756-2, 80-8256-3, 68-70564, 69-7157-1, 34-3657-2, 68-7057-3, 64-6657.CC, 15-1758-1, 68-7058-2, 58-6058-3, 75-7759-2, 33-35

131.23133.87150.14151.33169.13178.91188.81216.60218.15226.50227.91229.93235.84245.09246.84249.00250.94254.64256.63258.53264.23266.37274.16275.48HIM278.93284.15285.29302.14311.61313.29321.24323.90330.69332.71340.20349.85359.13361.12368.55371.29371.66374.07378.19387.91389.74390.74407.20408.53416.07425.71428.03435.60445.36454.28463.76466.05473.16482.57486.50492.80493.76495.63496.97502.09503.46504.93506.66508.03511.79512.68514.29516.15521.06522.81524.19525.70530.36532.19533.65533.76540.19541.59543.26550.84

83.311713822412599.0

25511512019111793.8

11012939.774.4

10710928.9

15712372.1

11721924910610813427262726611711311280.384.5

13611011985.2

10332.196.358.439.2

16310941.4

10317712216510480682.992.7

22227.641.951.722.586.8

11234.26.57.74.4

20.692.4

<O.l<O.l~O.l<O.l<O.l<O.l

3.427.00.72.3

<O.l0.3

46.872.80.4

86.5

+ 47.3-4.4-15.9+ 2.8+ 22.2-17.3-15.4+ 29.6+ 42.1-6 .0-41.6-40.7+ 15.6-24.2-10.6-21.8-15.9-24.8+ 11.5+ 21.7-21.0-41.9+ 48.3+ 19.7+ 21.7+ 27.9-31.5+ 34.7-1.3-50.9+ 1.8+ 17.5+ 30.9+ 25.7+ 9.1+ 14.8+ 43.7+ 26.2+ 42.6+ 26.3+ 31.2+ 18.1-0.6-6.7-7.1+ 6.4+ 26.0+ 22.8+ 32.7-25.1+ 15.6+ 13.3+ 7.3-14.1+ 42.1-27.2-16.2-6.7+ 22.0-10.1-34.2-33.9-36.9-17.1-10.4-49.3-3.5-24.0-31.4+ 25.1+ 25.6+ 30.0+ 36.8+ 14.1+ 42.4+ 5.7+ 23.1-8.5-36.9-27.1+ 2.8-15.6-2 .9-54.9-6.1

( + 27.0)-4 .6

——

+ 18.2-23.6-12.8+ 28.2+ 42.0-10.1-27.5-22.6+ 18.3-17.0

(-27.4)-19.6-19.8-28.2

+ 21.9-22.1

( + 4.6)( + 34.4)

+ 20.8+ 24.8

-18.7-35.7+ 28.1-1.6-39.9-10.2+ 20.6+ 26.3+ 21.0-4.4+ 16.9+ 35.6+ 29.3+ 29.0+ 23.5+ 35.8

-5.0-10.8

(-18.4)+ 18.9+ 18.6+ 22.4+ 19.3-24.2+ 16.3+ 15.4+ 6.1-16.1+ 19.0-19.1-9.9

(+11.0)+ 11.5-8 .0-31.4-28.8-26.5-11.0

(-16.5)-27.1

(-26.2)-25.0-26.8

(-0.2)+ 17.2+ 12.8+ 18.3

(-19.1)-25.8

-21.7-7.4

(-13.5)-11.9

228.835.520.716.0

287.8129.9215.2134.1306.1270.0113.7119.5151.1149.8247.421.5

244.329.2

266.8247.2224.6

94.626.8

5.1125.433.8

348.7292.3265.5

16.070.655.469.6

265.677.0

192.6338.7250.2

72.4154.9220.7230.4330073.04.9

234.474.5

208.2248.9213.7326.3288.9137.9209.955.8

340.594.519.1

314.524.5

234.5243.387.7

266.816.8

330.492.5

171.8156.3237.0

7.6311.0341.9246.5136.7200.3282.8101.919.6

298.4103.7182.6182.0163.893.6

(181.5)13.3—

277.5101.6216.3152.7310.089.2

281.990.2

179.8161.1

(193.2)348.5254.4

6.6

254.4208.2(92.8)(0.4)4.3

126.225.3

3.9293.2268.920.480.072.383.3

268.379.5

182.5353.4254.5

88.9175.2226.8

349.086.7(6.9)

239.085.0

197.0269.3208.3344.1279.5141.5209.4

16.2350.190.8

(18.9)319.2

9.990.0

255.388.5

268.2(180.0)327.6

(191.2)172.2166.9

———

(90.1)174.8219.2273.1

(143.6)1.3

180.7180.6(96-3)92.6

226.096.3

Notes:JNRM &NRM = directions of natural remanent magnetization; / = inclination in degrees with

respect to horizontal, negative above horizon; D = declination in degreeswith reference to an arbitrary zero azimuth.

^Stable ^Stable = s t a ^ ' e magnetization directions as resulting from an AF or thermal demag-netization analysis, brackets indicate a limited degree of reliability.

870

PALEOMAGNETISM OF SEDIMENTS AND IGNEOUS ROCKS

Table 2. Results of paleomagnetic measurements of Leg 60 basement rocks.

Sample(interval in cm)

Hole 453

49-150-150-251-151-351-452-253-353-453-554-254-354^55-155-355-456-156-257-157-357-463-1

, 110-113, 39^2, 40-43, 8-11, 50-52, 7-10, 13-16, 28-31, 5-8, 121-123, 67-69, 5-8, 16-19, 6-8, 28-31, 141-143, 42-44, 36-39, 88-91, 111-113, 59-62, 11-13

Hole 454A

5-1,5-2,5-3,5-4,6-2,8-1,8-1,10-1.11-1,11-2,11-3.11-4,11-4,12-1,12-2,13-1.13-1,14-1,15-1,16-1,16-2,

133-136102-10474-7662-64110-11529-33122-125a

, 51-53, 101-103, 111-113, 91-9324-26

, 95-97112-114

, 39-412-4a

, 80-82a

20-234-882-8414-16

Hole 456

16-1,16-1,16-2,18-1,

20-2225-2799-10153-55

Hole 45828-1,29-1,29-2,29-3,30-1,30-2,31-1,32-1,32-2,32-3,33-1,33-2,34-1,

90-92121-12492-9464-66105-10756-58121-12372-74118-120119-121115-11742-44123-125

Depth belowSea Floor (m)

456.62465.41466.92474.60478.01479.09485.65496.80498.07500.72505.18506.07507.68512.57515.80518.42522.43523.88532.40535.62536.61588.62

68.3569.5370.7572.1379.1395.3196.24

113.52123.02124.62125.92126.75127.47132.13132.90140.03140.81149.22158.06162.83163.65

133.71133.76136.00153.04

257.41267.23268.43269.65276.56277.57286.22295.23297.19298.70305.16305.93314.74

JNRM r.(emu/cm3)

7 ×23 ×

2.18 ×1.85 ×2.41 ×7.50 ×

0.442 ×1.44 ×

0.442 ×34 ×

1.28 ×0.765 ×

1.28 ×2.85 ×2.35 ×

101010101010101010101010101010

<IO<IO<IO

1.03 ×1.29 ×

0.919 ×0.878 ×

1.99 ×2.75 ×2.79 ×3.22 ×2.02 ×2.74 ×

0.222 ×3.85 ×14.1 ×11.3 ×7.30 ×9.86 ×5.49 ×3.18 ×13.6 ×

14 x54 ×

7.22 ×7.33 ×4.02 ×8.95 X

0.386 ×2.99 ×5.92 ×1.94 ×

0.957 ×2.21 ×

0.579 ×1.18 ×1.51 ×1.25 ×

0.656 ×0.630 ×0.465 ×3.31 ×

0.708 ×0.988 ×

1.83 ×

10101010

101010101010101010101010101010101010101010

10101010

10101010101010101010101010

- 6- 6- 3- 3- 3- 3- 3- 3

"3

- 6- 3- 3

3

- 3- 3- 7- 7- 7- 3- 3- 3- 3

- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3

- 3- 6- 6- 3- 3- 3- 3

- 3- 3- 3- 3

- 3— 1

- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3

JNRM

+ 58.7-7.2+ 17.1+ 2.2+ 10.7-2 .2+ 22.4+ 56.1+ 55.1+ 4.3+ 47.0+ 65.8+ 64.3-46.1+ 59.7

+ 43.9+ 57.3+ 7.1+ 39.5

-11.4-2.1+ 2.0-1.9

-17.2-30.4-22.4-38.3-31.5-36.1-31.5-25.0-27.7+ 21.6-5 .9

-15.5-10.4

-45.9-36.1-26.9-23.4

+ 25.7+ 42.0+ 30.3+ 30.0+ 12.1+ 16.6+ 26.0+ 35.0+ 27.3+ 19.9+ 20.1+ 28.0+ 28.3

Stable

no-8.4+ 24.4-11.3+ 23.9-8.2+ 23.9

( + 62.6)-49.6

(-9.8)+ 55.0

( + 22.2)( + 57.3)(-50.1)+ 59.7

———

+ 49.0+ 57.2-45.6

—

-16.2-10.3-11.6-10.7

——

-27.1-31.4-35.4-41.9-36.4-38.5-34.7-30.4-29.5

(-19.2)-23.0

—

-16.6-19.6

-45.9-44.3-27.9-26.6

+ 34.4+ 37.8+ 35.3+ 33.1+ 13.6+ 14.9+ 34.5+ 34.5+ 31.8+ 31.3+ 35.9+ 34.0+ 29.9

&NRM

167.6225.2309.0240.2243.980.7

110.8183.5150.3110.7167.8155.4166.2176.3175.7

——

156.9235.2193.9208.8

135.2300.5325.5156.1

217.1252.6280.0152.013.7

332.949.5

293.627.0

323.0106.8

257.2245.6

333.7328.1180.3236.5

355.3118.5345.9241.937.8

145.8195.466.8

107.3212.5221.895.583.8

^Stable

no236.8315.9241.7262.278.4

108.3(197.3)

87.5(102.8)155.8(14.0)

(161.7)(161.0)190.8

———

113.0262.1207.1

135.4309.1326.3156.6

—

221.9255.7281.3152.8

12.5333.948.9

296.327.7

(352.0)119.4—

256.9245.6

334.7334.0180.4240.8

354.7118.0339.8238.5

39.0147.0192.167.3

107.7212.9226.396.982.1

MDF(Oe)

48125u361b

538281548270

586455

27861

25656

339———77

50713349

288b

1391301511 5 3u566b

28023015713811711813015514188

191b

546b

376227435b

15495

1 4 7 K208b

185211221225261257178197138151228b

252168

5969

0.4491.951.851.68

0.2683.681.50

0.5446.273.462.514.815.18

3.351.552.014.99

0.3801.211.331.231.00

0.1940.2930.348

1.210.987

1.221.321.15

0.9421.24

0.2410.3410.2280.2990.3760.260

0.2052.942.37

0.806

;;]

]

]

]

i]

]

ii

1.311.071.131.191.05[.161.551.44.49.42.70

1.451.32

×××××××××××××××

××××

×××××××××××××××××××××

××××

×××××××××××××

101010101010101010101010101010———10101010

101010101010101010101010101010101010101010

10101010

10101010101010101010101010

-6- 6- 3- 3- 3- 3~~ ^

"1

- 9- 3~ ^- 3- 3- 3- 3

- 3- 3- 3- 3

- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3

- 3- 3- 3- 3

_ ̂- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3

e

0.30.9

13.12.63.5

12.14.51.10.80.20.60.61.41.61.2———

0.82.31.20.5

14.26.15.77.15.5

38.72.0

29.931.530.916.120.212.99.1

29.60.20.4

85.666.328.993.1

5.03.17.17.2

2.05.61.42.73.92.91.11.20.86.31.11.83.8

871

U. BLEIL

Table 2. (Continued).

Sample(interval in cm)

Hole 458

34-2, 30-3235-1, 97-9935-1, 103-10535-1, 133-13535-1, 137-13935-2, 46-4836-1, 55-5736-2, 22-2437-1, 142-14437-2, 77-7938-1, 55-5739-1, 106-10839-2, 51-5339-3, 15-1740-1, 134-13740-2, 72-7540-2, 115-11741-1, 3-543-1, 146-14843-2, 19-2144-1, 6-844-1, 37-3945-1, 117-11945-2, 29-3146-1, 88-9047-1, 36-38

Hole 459B

60-1, 121-12460-2, 61-6361-1, 85-8761-1, 140-14261-2, 81-8262-1, 47-4963-1, 49-5164-1, 23-2565-1, 83-8566-1, 39-4166-2, 137-13967-1, 134-13667-2, 71-7368-1, 33-3569-1, 44-4669-2, 125-12769-3, 79-81'70-1, 46-4871-1, 39-4171-1, 130-13271-2, 30-3272-1, 54-5672-1, 134-13673-1, 25-2773-2, 139-14173-3, 28-30

Depth belowSea Floor (m)

315.31323.98324.04324.34324.38324.97333.06334.23343.43344.28352.06362.07363.02364.16371.86372.74373.16380.04400.47400.70408.57408.88419.18419.80428.39437.37

560.13561.02568.86569.41570.32577.98587.50596.74606.84615.90618.38626.35627.22634.84644.45646.76647.80653.97663.40664.31664.81673.05673.85682.26684.90685.29

JNRM(emu/cm^)

0.479 × 100.183 × 100.468 × 100.313 × 100.892 × 102.12 × 101.59 × 10

0.799 × 103.36 × 105.64 × 10

0.894 × 101.17 × 10

0.776 × 100.529 × 10

1.34 × 101.64 × 101.07 × 102.68 × 101.14 × 101.36 × 102.00 × 101.06 × 101.84 × 10

0.918 × 1025.7 × 1010.7 × 10

4.11 × 1032.0 × 10

0.535 × 109.60 × 10

0.721 × 10'1.24 × 1020.1 × 1035.6 × 1042.0 × 10

6 × 10"0.860 × 10"0.645 × 10"0.983 × 10"0.733 × 10"

2.05 × 10"1.20 × 10"13.2 × 10"3.81 × 10"3.31 × 10"6.92 × 10"5.82 × 10"11.1 × 10"66.3 × 10"11.2 × 10"22.0 × 10"7.33 × 10"

- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3

- 3- 3- 3- 3- 3- 3- 3- 3-3-6-3-3-3-3-3-3-3-3-3-3-3-3-3-3-3-3

JNRM

+ 29.4+ 56.2+ 30.1+ 13.8+ 35.8+ 25.7+ 37.2+ 18.2+ 21.0+ 29.1+ 18.6+ 37.3+ 33.8+ 35.6+ 33.5+ 45.7+ 42.1-16.1- 6 . 6-10.5-11.4-12.6- 3 . 8- 2 . 7- 3 . 6-10.4

+ 27.9+ 10.0- 2 . 9- 7 . 2-15.8-25.6+ 10.4-25.4+ 44.1+ 15.3+ 0.2+ 23.8+ 10.8+ 12.1+ 14.3-19.0- 4 . 3- 2 . 5+ 13.4+ 3.6+ 4.4+ 3.3+ 13.8+ 1.7+ 2.8+ 12.0

Stable

+ 34.7+ 58.2+ 28.4+ 7.5+ 34.5+ 28.2+ 32.5+ 18.4+ 23.3+ 21.0+ 19.3+ 37.5+ 35.1+ 35.7+ 34.7+ 38.0+ 40.2-15 .6- 9 . 3- 9 . 7-12.3-13.2- 4 . 0- 5 . 6- 4 . 2-10.5

+ 13.4+ 11.4- 8 . 7-10.3-15.4-14.5

+ 11.7-23.6+ 45.0

( + 11.1)(+11.1)+ 11.0+ 9.8+ 12.9+ 12.4-12.2- 8 . 9- 3 . 4+ 6.1+ 5.3+ 4.6+ 4.9+ 12.6+ 4.8+ 3.8+ 13.5

DNRM

107.564.042.0

329.08.0

88.2104.352.0

239.890.9

244.8227.8337.4193.0181.5132.989.811.2

344.897.2

105.1345.3352.7181.4346.544.8

8.6122.0113.8337.6343.035.0

158.068.416.5

346.1199.1355.8237.9148.0283.1318.6147.5

4.1187.0189.9235.2232.191.957.0

282.8322.3

DStable

109.249.736.8

318.77.0

87.3104.953.5

238.290.3

245.9228.4335.0193.8180.9132.391.313.0

355.788.396.4

346.4352.5179.1346.842.4

2.8122.489.3

338.4345.0

18.3158.068.6

16.4(113.1)(84.3)

1.5251.1154.4286.9330.0141.7

0.0184.3185.7236.9233.490.650.6

284.4323.7

MDF(Oe)

204152b

189b

179b

183b

24228031018327916930626825326436436121318017518655Ob

230151165151

109b

908790

123b

139157116147—

188b

128162218307132196182182161135144158143180

1.221.151.241.191.231.27

0.8920.682

1.281.331.51

0.9070.877

1.071.02

0.7230.676

1.771.451.601.64

0.2671.471.402.272.53

1.521.701.361.681.281.461.521.762.292.873.792.182.811.901.571.662.281.873.152.322.482.502.402.462.301.90

k

××××××××××××××××X

×××××××××

××××X

×××××X

×××××X

×××××××××

1010101010101010101010101010101010101010101010101010

101010101010101010101010101010"10"10"10"10"10"10"10"10"10"10"10"

- 3- 3- 3-3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3

- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3- 3-3-3-3-3-3-3-3-3-3-3-3-3-3

ö

1.10.41.00.72.04.54.83.27.1

11.51.63.52.41.33.66.14.34.12.12.33.3

10.73.41.8

30.611.4

7.350.8

1.115.41.52.3

35.754.749.6

I0"" 2

0.60.81.01.03.52.0

15.75.52.88.16.3

12.074.412.325.910.4

Notes:DNRM - directions of natural remanent magnetization; / = inclination in degrees with respect to horizontal, negative

above horizon; D = declination in degrees with reference to an arbitrary zero azimuth.^Stable = stable magnetization direction as resulting from an AF or thermal demagnetization analysis, brackets in-

dicate a limited degree of reliability;MDF = median destructive field in Oe; for Hole 453 underlined values give MDT (median destructive temperature) in °C as

resulting from a thermal demagnetization treatment.k = initial susceptibility in emu/cm^ Oe.Q = Königsberger ratio, present ambient field F = 0.37 Oe.

Sediments.Denotes the presence of spurious magnetization components.

872

PALEOMAGNETISM OF SEDIMENTS AND IGNEOUS ROCKS

Table 3. Lithologic and paleomagnetic units, Leg 60.

Lithologic Units Paleomagnetic Units(intervals in m) (intervals in m) ^Stable

Hole 453

Table 3. (Continued).

I455.50-541.00

465.41-500.72II

466.92-506.07III

496.80-535.62IV

498.07-536.61

-9.4

+ 23.6

+ 56.8

-48.4

± 1.4

± 1.0

± 4.6

± 2.5

II541.00-569.50

III569.50-605.00

(no oriented samples)

(no oriented samples)

"no-magnetization" zone: 518.42-523.1

Hole 4S4A

Lithologic Units(intervals in m)

Hole 459B

II587.00-615.50

III615.50-644.00

IV644.00-691.50

Paleomagnetic Units(intervals in m)

III(one sample)

IV(one sample)

V615.90-644.45

VI646.76-653.97

VII663.40-685.29

Stable

+ 11.7

-23.6

+ 11.4 ± 1.1

-8.2 ± 4.5

+ 7.0 ± 3.8

Note: Dashed lines indicate that boundary is not in completeagreement.

67.00-76.50II

78.90-86.00III

86.00-96.00and

104.00-119.00

68.35-72.13

(no oriented samples)

(no oriented samples)II

(one sample)

-12.2 ± 2.7

-31.4

IV119.00-129.00

and131.00-140.00

III123.02-127.47

IV132.13-132.90

-37.4 ± 2.9

30.0 ± 0.6

149.00-171.50 162.83-163.65 -18.1 ± 2.1

Hole 456

I133.50-152.50

I133.71-133.76

II136.00-153.04

-44.7 ± 0.5

-27.3 ± 0.9

III152.50-169.00

(no oriented samples)

Hole 458

I256.50-286.10

I257.41-269.65

II276.56-277.57

+ 35.2 ± 2.0

+ 14.3 ± 0.9

286

351

II.10-351.50

III.50-380.00

III286.22-333

IV334.23-352

V362.07-373

.06

.06

.16

+ 32

+ 20

+ 36

.5

.5

.9

±

±

±

2

2

2

.6

.2

.1

IV380.00-428.10

VI380.04-408.88

VII419.18-428.39

-12.0 ±

-3.4 ±

2.6

0.6

428.10-465.50VIII

437.37-?(one sample)

-10.4

Hole 459B

I558.90-587.00

I560.13-561.02

II568.86-577.98

+ 12.4 ± 1.4

-12.2 ± 3.2

873