introduction basalts sampling and measurementsabrahamsen, 1992). arason and levi (1990) found a...

Hawkins, J., Parson, L., Allan, J., et al., 1994Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 135

45. MAGNETIC PROPERTIES OF BASALTS AND SEDIMENTS FROM THE LAU BASIN1

Niels Abrahamsen2 and William W. Sager3

ABSTRACT

Paleomagnetic and rock-magnetic investigations of basalts from Hole 834B in the Lau backarc basin and of sediments fromHoles 841A and 841B at the Tonga Ridge are reported. Three groups of blocking temperatures in the basalts suggest the presenceof at least three magnetic phases: pure magnetite, a Ti-poor titanomagnetite, and a Ti-rich phase. The drill-string-inducedremanence in the basalts is typically between three and six times the original normal remanent magnetization intensity, but it ismostly removed by alternating-field (AF) cleaning in 5 mT. Volume susceptibility values range from 0.04 × I0"3 to 4 × I0"3 cgs.The modified g-ratio 75/sus ranges from 0.5 to 10.

The drill-string-induced remanence behaves different in the two sediment cores from Holes 841A and 841B, which may bethe result of differences in the sediment or caused by the different drilling equipment used.

The AF-cleaned inclinations of the sediment in Holes 841A and 84IB suggest a slight flattening with increasing depth (up to6° under a load of 400 m of sediment) to be present. This flattening is likely to be caused by the differential rotation of detritalparticles under compaction during diagenesis.

INTRODUCTION

In this paper we present detailed data from basaltic cores drilledin the Lau Basin (Holes 834B and 835B) and from sediments obtainedfrom the Tonga Ridge (Holes 841A and 841B). The area is a young(< ca. 5 Ma) and quite intensely studied backarc basin (Parson,Hawkins, Allan, et al., 1992) situated in the western south centralPacific, between the remnant arc of the Lau Ridge and the Tonga Arcbetween 18°-22°S and 176°-178°W (Fig. 1).

SAMPLING AND MEASUREMENTS

Shipboard pass-through cryogenic magnetic measurements weremade on archive halves of the sediment cores, as well as on basalt coresand cubes. After having measured the natural remanent magnetization(NRM), they were stepwise partially demagnetized in alternatingmagnetic fields of up to 15 mT and remeasured, as explained in Parson,Hawkins, Allan, et al. (1992, "Explanatory Notes" chapter).

A number of sediment cubes were also collected and measured inthe shipboard cryogenic magnetometer (Holes 841A and 841B),whereas basalt minicores from Holes 834B and 835B were measuredon land in a Molspin spinner magnetometer, undergoing partialstepwise AF demagnetization in small steps of 2.5 mT. In about halfof the cases (see Table 1), the basalt cubes were thermally cleanedafterward in a magnetically shielded Schonstedt furnace. The volumesusceptibility was also measured by means of a Geofyzika Microkap-pameter KT-5.

RESULTS

The rock-magnetic and paleomagnetic properties of the Lau Basinand Tonga Ridge sediments and volcanics, as measured onboard theJOIDES Resolution, are described in detail elsewhere (Parson,Hawkins, Allan, et al., 1992; Abrahamsen and Sager, this volume;Sager et al., this volume).

Hawkins, J., Parson, L., Allan, J., et al., 1994. Proc. ODP, Sci. Results, 135: CollegeStation, TX (Ocean Drilling Program).

2 Department of Earth Sciences, Aarhus University, Finlandsgade 8, DK-8200 AarhusN, Denmark.

3 Departments of Oceanography and Geophysics, Texas A&M University, CollegeStation, TX 77843, U.S.A.

Basalts

The inferred blocking temperatures that resulted from the ther-momagnetic cleaning of the basalt cubes are shown in Figure 2, basedon values from Table 1. The blocking temperatures were estimatedfrom the thermal intensity-decay curves, which are shown in Figure 3.

The blocking temperatures in the basalts from Hole 834B fall intothree rather distinct groups: around 580°C, around 500°C, and around250°C ± 100°C. This indicates the presence of at least three magneticphases: pure magnetite, Ti-poor titanomagnetite, and a phase withrather low blocking temperatures, probably a more Ti-rich titanomag-netite (Creer et al, 1977).

The drill-string-induced remanence may be very strong and domi-nant, typically between three and six times the original NRM intensityof the basalts. As may be seen in the AF intensity-decay curvesillustrated in Figure 4, however, a major part of the drill-string-induced remanence appears to be removed by AF cleaning in 5 mT.This level of cleaning, therefore, seems to restore satisfactorily theoriginal predrilling NRM intensity of most of the basalts.

Hence, to obtain a more realistic estimate of rock-magnetic char-acteristics, such as the median destructive field (mdf) and the modifiedß-ratio (Q = remanence/susceptibility), the remanence intensity afterpartial AF cleaning in a field of 5 mT, J5, was applied in the calculationof the mdf and the modified Q-ratio. From Table 1, we can see that themodified ß-ratio = 75/sus ranges from 0.46 to 10.6 in the basalts.

Volume susceptibilities of the measured basalt cubes range from0.04 to 4 × I0"3 cgs ( 4π S1). More extensive susceptibility data fromwhole-core measurements are given in the Initial Reports volume(Parson, Hawkins, Allan, et al., 1992).

Sediments

The remanent intensity of sediment cubes combined from Holes841A and 84IB vs. depth are shown in Figure 5. Open symbolsindicate the NRM inclination, whereas solid symbols indicate theinclination after 20-mT AF demagnetization.

Above a level of ca. 50 mbsf, the NRM intensity is low (10 to 100mA/ra); between 50 and 230 mbsf the NRM intensity is an order ofmagnitude higher (typically between 200 and 800 mA/m); and belowthis level lower values typically between 50 and 200 mA/m are seen.

The behavior against AF demagnetization in 20 mT is notablydifferent above and below 170 mbsf (Fig. 5). Above this level thedecrease in intensity is close to a factor of 10, indicating very low

717

N. ABRAHAMSEN, W.W. SAGER

Figure 1. Index map of the Lau Ridge, Lau Basin, Tonga Ridge, Tofua Arc,and Tonga Trench, indicating the drill sites (834 through 841) of Leg 135.Central Lau and Eastern Lau spreading centers (CLSC and ELSC), Valu FaRidge (VF), and Mangatolu Triple Junction (MTJ) are also shown. Waterdepths in kilometers (from Leg 135 Scientific Party, 1991).

coercivity, whereas below 170 mbsf the decrease in intensity is onlysome 20% to 50%.

The low intensity values above 50 mbsf compare with lithologicUnit I (Parson, Hawkins, Allan, et al., 1992), which is described asmiddle Pleistocene to ?Pliocene, structureless, grayish brown togreenish gray (between 0 and 17 mbsf) and light yellowish brown andgreenish gray to dark greenish gray (between 17 and 55 mbsf) clayswith very thin- to medium-bedded vitric sand, vitric silt, and coarseand fine ash interbeds. Late Miocene lithologic Unit II (56-333 mbsf)is described as interbedded black, dark greenish gray, and dark grayvitric siltstone, clay, clayey siltstone, vitric siltstone, and vitric sand-stone (between 81 and 187 mbsf), and as vitric sandstones and vitricsiltstones (between 170 and 333 mbsf); this part generally coarsensdownward. The strong contrasts in NRM values and coercivity areprobably caused by variations in the ash content as well as in the grainsize of the sediment.

In Figure 6 the inclination of sediment cubes from Holes 841Aand 84IB are shown before (open symbols) and after (solid symbols)20-mT AF demagnetization, respectively. Above 170 mbsf the NRMinclination is steeply negative because of a dominating drill-string-induced magnetization, which forced most inclinations into steepnegative (updip) values. Below 170 mbsf the drill-string-inducedmagnetization is less dominant; especially below ca. 300 mbsf, scat-tered values of original positive inclinations have survived in theNRM. Thus, the drill-string-induced remanence behaves differentlyin the sediment cores from Hole 841A and 841B. The reason for thismay be the result of differences in the sediment, or it could be causedby differences in the magnetic properties of the drilling equipment

°c700

600

500

400

300

200

100

0

I • • • • • • B ••

•

• ••

••

• • • •

• •

Hole 834B

• •

835B

Sample no. (arbitrary)Figure 2. Distribution of blocking temperatures in basalts from Holes 834Band 835B.

used, the cores of Hole 841A being of the H- or X-type, whereas coresfrom Hole 84IB are of the R-type.

Several sources of uncontrolled inclination errors arising fromincorrect measurements of grain size, compaction, bioturbation, andremagnetization are possible in the sediments (e.g., Gordon, 1990b;Abrahamsen, 1992). Arason and Levi (1990) found a progressivedowncore shallowing of the remanent inclination (from 6° to 8°) in a120-m deep section (DSDP Site 578 in the northwest Pacific), ofwhich only one-fourth could be explained by the northward transla-tion of the Pacific Plate. The inclination shallowing corresponds to a3%-4% decrease in the porosity. Sediment paleolatitudes in a studyby Gordon (1990a) were also found to be systematically more north-erly than those predicted by the reference apparent polar wander path,corresponding to inclinations shallower than expected.

To investigate the possible influence of sediment compaction uponthe inclination, numerical values Norm(/20) of the inclination /20 (cf.Table 2) are shown in Figure 7. Numerical values are used because ashallowing effect by compaction will act in the same sense on bothpositive and negative inclinations (Abrahamsen, 1992). The inclina-tions are shown after AF cleaning in 20 mT to avoid influence fromthe drill-string-induced magnetization. In the graph a seven-pointmoving average is also plotted (thin dotted line), as well as theregression line, Norm‰) = -0.0145 Depth + 41.5 (TV = 134). Thecentral axial dipole inclination at the site is Io = 40.8°, which is closeto the mean of the upper part of the section. A slight but systematicshallowing of the inclination with a depth of ca. 6° of the top 400 msediment thus appears to be present also in these cores, although theindividual data points are very scattered. The inclination shallowingis probably caused by differential rotation of the detrital particlesduring compression and diagenesis, which takes place after the lock-ing in of the depositional remanence.

To get an overall impression of the strong variations in the mag-netic properties of the sediments at Site 841, the whole-core volume

718

MAGNETIC PROPERTIES OF BASALTS AND SEDIMENTS

Table 1. Rock-magnetic properties of basalts from Holes 834B and 835B.

Core, section,interval (cm)

135-834B-8R-1.258R-2, 3410R-1,1310R-2, 1310R-3, 3111R-2, 13712R-1.3312R-4, 2213R-1.4013R-2, 1315R-1.3016R-1.820R-1.9224R-1.5128R-1.14132R-1.5033R-2, 5835R-2, 9041R-1,7643R-1,2148R-1, 3751R-1, 13654R-1, 13856R-2, 7559R-1,114

135-835B-7R-2, 237R-2, 1227R-3, 130

Depth(mbsf)

126.30127.60145.40147.45148.40152.22156.00159.20161.14162.40175.66185.02214.50233.37253.43271.95283.25293.15334.73344.18368.52384.20398.53409.10432.08

180.22181.27182.95

Tmax(°C)

650

650

650

650650

650650

650

650550550

650

650650

Blockingtemperature

(°C)

150250, 500, 580

250, 500, 580

200, 350, 500

300, 500, 580350, 500, 580

250, 500, 580325,580

275, 500, 580

150, 500, 570440, 520440, 550

250, 500, 580

400, 520, 650400,510,650

AF maximum(mT)

0-800-220-800-220-850-220-80

0-800-220-22

0-80

0-70

Jo (mA/m)

23871672370620234173

1792432

111531625823

2698

1321

J5 (mA/m)

985968567

11971458279

1959

100132434125

2343

1377

mdf (mT)(J5 )

9.09.09.09.07.5

10.06.0

8.0>22.0

10.0

8.0

20

Susceptibility(× I0"6 cgs)

594532

124336052250051242855448832438836

308296260Ml76

2121HS

1592170612803800378

144470536

Q (J5/Sus)

1.661.820.463.332.790.563.83

2.0510.0110.63

7.92

9.56

Notes: Jo = natural remanent magnetization intensity; J5 = remanent intensity after alternating-field (AF) demagnetization at 5 mT; and mdf = median destructivfield, supposing J5 to be the original remanent intensity. Q = modified Q-ratio based upon the value of J5.

susceptibilities for Holes 841A and 841B are also shown (Figs. 8-9).Susceptibility ranges over 2 orders of magnitude. The sediment prop-erties therefore are highly variable, as may be expected in this envi-ronment, close to intensive volcanic activity with discrete eruptionevents. The strong variations in susceptibility are most likely causedby the highly fluctuating content of volcanic ash in the sediments.

CONCLUSIONS

We conclude that the blocking temperatures in the basalts inves-tigated fall into three distinct groups: around 580°C, around 500°C,and about 250°C ± 100°C. This may suggest the presence of at leastthree magnetic phases: pure magnetite, Ti-poor titanomagnetite, anda phase with low blocking temperatures, probably Ti-rich titanomag-netite. Volume susceptibility values range from 0.04 × I0"3 to 4 × I0"3

cgs in the basalts. The modified ß-ratio 75/sus ranges from 0.5 to 10.0.Drill-string-induced remanence of the basalts may be as high as

three to six times the original NRM intensity, but it was mostlyremoved by AF cleaning at 5 mT.

Drill-string-induced remanence behaves differently in the sedimentcores from Hole 841A and 84 IB. The reason for this may be the resultof differences in the sediment or of differences in the drilling equipmentused (H- or X-type in Hole 841 A, in contrast to R-type in Hole 841B).

The inclination of the sediments in Holes 841A and 84IB displaya slight flattening with increasing depth (ca. 6° under a load of 400 mof sediment). This flattening is likely to be a result of the differentialrotations of the detrital particles under moderate compaction, whichoccurred during the diagenetic processes in the sediments that tookplace after the locking of the depositional remanence.

An inclination flattening in deep-sea sediments, amounting toseveral degrees of arc, as in the present case, may add up to significantand systematic errors in the paleolatitude and hence cause bias, if usedin plate tectonic reconstructions.

ACKNOWLEDGMENTS

Part of the work was supported by a grant to N. Abrahamsen fromthe Danish Natural Science Foundation and to W. W. Sager by JOI/US-SAC. We thank Anna Norberciak for assisting in the laboratory, andMarek Lewandowski for lending us his program for paleomagneticdata processing.

REFERENCES*

Abrahamsen, N., 1992. On farsidedness of palaeomagnetic poles: magneticrefraction, sediment compaction and dipole off-set. Stud. Geophys. Geod.,36:26-41.

Arason, P., and Levi, S., 1990. Compaction and inclination shallowing indeep-sea sediments from the Pacific Ocean. Eos, 71:381.

Creer, K.M., Hedley, I.G., and O'Reilly, W., 1977. Magnetic oxides in geo-magnetism. In Craik, D.J. (Ed.), Magnetic Oxides: London (Wiley),649-688.

Gordon, R.G., 1990a. Paleomagnetically determined paleolatitudes from Pa-cific Plate Deep Sea Drilling Project sediments. Eos, 71:381.

, 1990b. Test for bias in paleomagnetically determined paleolatitudesfrom Pacific Plate Deep Sea Drilling Project sediments. J. Geophys. Res.,95:8397-8408.

Leg 135 Shipboard Science Party, 1991. A new view of ARC/BACKARCsystems. Geotimes, 36:19-20.

Parson, L., Hawkins, J., Allan, J., et al., 1992. Proc. ODP, Init. Repts., 135:College Station, TX (Ocean Drilling Program).

Date of initial receipt: 6 July 1992Date of acceptance: 4 January 1993Ms 135SR-121

Abbreviations for names of organizations and publication titles in ODP reference listsfollow the style given in Chemical Abstracts Service Source Index (published byAmerican Chemical Society).

719


135-834B-8R-2, 34 cm (TH) 135-834B-10R-2, 13 cm (TH)

Down

W

Down

Irm/Inrm Irm/Inrm

200 400 600 200 400 600

Figure 3. Thermal demagnetization (TH) of 12 basalt samples from Hole 834B and 2 basalt samples from Hole 835B. Stereograms(top), orthogonal plots (middle part), and intensity-decay curves (base) are illustrated. Four of the samples were partly AFdemagnetized in 22 mT, and one in 80 mT, before thermal demagnetization was undertaken (see Table 1).

720


135-834B-16R-1, 8 cm (TH) 135-834B-24R-1, 51 cm (TH)

N

yz

Irm/Inrml -

Irm/Inrm

200 400 6000 I i i I I I i I I i I I INfc ç

Figure 3 (continued).

721


135-834B-28R-1, 141 cm (TH) 135-834B-33R-2, 58 cm (TH)

wN

N

Irm/Inrml i -

Irm/Inrm

0 200 400 600 200 400 600



135-834B-48R-1, 37 cm (TH) 135-834B-51R-1, 136 cm (TH)

Down

Irm/Inrml. r- '

Irm/Inrm

200 400 600 200 400


723


135-834B-54R-1, 138 cm (TH) 135-834B-59R-1, 114 cm (TH)

Irm/Inrm1 r -

Irm/Inrm

200 400 0 200 400



135-835B-7R-2, 122 cm (TH) 135-835B-7R-3, 130 cm (TH)

Down

Irm/Inrm

200 400 600Q I I I I I I I

200 400 600


725


135-834B-8R-1, 25 cm (AF) 135-834B-8R-2, 34 cm (AF)

Up Down

1000 N

Irm/Inrm Irm/Inrm

20

Figure 4. Alternating-field (AF) demagnetization of 11 basalt samples from Hole 834B. Stereograms (top), orthogonal plots(middle part), and intensity-decay curves (base) are illustrated.

726


135-834B-10R-1, 13 cm (AF) 135-834B-10R-2, 13 cm (AF)

3000 -

2000 -

1000 -

Down

Irm/Inrm Irm/Inrm


727


135-834B-10R-3, 31 cm (AF) 135-834B-11R-2, 137 cm (AF)

Down

N

Down

Irm/Inrm Irm/Inrm

o 20 40 •à é ė àmé



135-834B-12R-1, 33 cm (AF) 135-834B-13R-2, 13 cm (AF)

Down Down

N

Irm/Inrm Irm/Inrm

0 20 40 0 20 40 60 80



135-834B-15R-1, 30 cm (AF) 135-834B-16R-1, 8 cm (AF)

xy

1000.

1000.

2000.

yzUp

Irm/Inrm Irm/Inrm

20


730


135-834B-28R-1, 141 cm (AF)

Irm/Inrm


731


1000=1

100 200

Depth (mbsf)300 400

Figure 5. Remanent intensity of sediment cubes from Holes 841A and 841B before (open symbols) and after (solid symbols) 20-mT AF demagnetization. Above170 mbsf the decrease in intensity is as high as a factor of 10, whereas below 170 mbsf the decrease in intensity is only 20% to 50%.

100 200

Depth (mbsf)300 400

Figure 6. Inclination of sediment cubes from Holes 841A and 84IB before (open symbols) and after (solid symbols) 20-mT AF demagnetization. Above 170 mbsfthe NRM inclination is steeply negative because of the dominant drill-string-induced magnetization. Below 170 mbsf the drill-string-induced magnetization ismuch less dominant.

732


Table 2. Inclination and intensity of sediments from Holes 841A and 841B, before ([INRM] and [JNRM]) and after ([I20] and [J20]) 20-mTalternating-field cleaning.


135-841A-

1H-1, 141H-1,2O1H-1,3O1H-1,4O2H-1, 282H-1, 702H4, 703H-1, 503H-2, 503H-3, 503H-6, 604H-2, 304H-2, 505H-1.405H-2, 405H-3, 405H4, 405H-6, 806H-2, 796H-3,1106H-5, 1108H-1.228H-1, 1428H-2, 808H-3, 709X-1, 169X-1, 599X-1, 62HX-1,24HX-1,3611X-1,4812X-?, 912X-?, 1514X-1, 3314X-1, 4915X-1.3815X-1.7616X-1, 816X-1, 1316X-1, 2017X-1.617X-1.3018X-1.618X-1.919X-1.219X-1, 1219X-1.4221X-1, 3321X-1.47

135-841B-

2R-1.212R-1, 662R-1, 1102R-1, 1162R-2, 152R-2, 632R-2, 933R-1, 163R-1.243R-1, 1003R-2, 63R-2, 423R-2, 544R-1, 674R-1, 724R-2, 134R-2, 555R-1.9

Depth I N R M

(mbsf) (o)

0.14 -32.90.20 -59.70.30 -68.20.40 -72.08.80 -84.19.20 -53.014.20 -69.918.50 -73.220.00 -79.821.50 -86.726.10 -49.229.30 -83.829.50 -84.837.40 35.038.90 3.640.40 39.741.90 -67.345.30 -82.248.79 -82.050.60 -86.153.60 -80.365.72 -81.266.92 -79.768.30 -78.569.20 -77.071.86 -66.872.29 -81.272.32 -79.791.24 -87.291.36 -72.791.48 -71.9100.79 -76.1100.85 -71.1120.33 -77.0120.49 -76.6129.98 -81.2130.36 -62.4139.38 -84.2139.43 -75.6139.50 -82.6148.96 -65.6149.20 -80.7153.96 -85.4153.99 67.2159.12 -76.4159.22 -83.7159.52 -74.4177.23 -66.8177.37 -73.2

170.01 -46.0170.46 -43.2170.9 -47.9170.96 -47.2171.45 -3.0171.93 -48.7172.2 -37.9179.56 -33.5179.64 -36.5180.4 -52.6180.96 -47.8181.32 -31.9181.44 -37.5189.77 -43.6189.82 -41.2190.73 -48.7191.05 21.8198.79 -31.7

JNRM(mA/m) I20 (o)

13.348.770.154.313.763.6121.6110.625.291.510.8110.698.848.826.438.441.8704.3228.0303.1335.6432.9433.4303.0405.5134.650.5690.9419.2645.1506.1679.2678.2373.0214.8259.6297.6237.4244.8912.4467.3365.6548.3496.3333.9370.3596.2926.9889.0

504.7290.2387.7223.6203.6222.8213.9569.8464.5428.7647.3655.0797.8292.7273.1108.7123.0214.0

32.4^3.7-55.9-50.3-57.135.829.541.648.646.631.5

-33.6-4.836.729.952.636.1

-75.8-85.8-64.4-37.9-68.236.227.913.337.2

-18.9-73.0-49.1-41.3^4.221.041.222.137.0

-29.3-38.8

15.4

-21.2-53.6-39.815.0

-48.9-56.7^2.9^14.2-29.5

(Im =

-45.9-43.5^2.5-42.25.8

-47.2-34.0-30.3-36.6^9.8^7.2-31.1-36.1-42.1-4-1.2^ 2 . 026.7

-29.6

J20 (mA/m)

9.722.722.513.81.5

32.135.828.02.45.08.41.94.6

40.823.630.415.729.732.736.227.225.342.330.959.240.58.0

82.952.8118.4123.243.5178.041.960.939.992.57.9

32.30.8

111.833.149.1108.0123.2101.4185.3337.7238.3

38.9 + 2.5)

367.4220.4265.8137.1148.7168.5119.6399.8333.6278.0467.1461.8598.9196.4169.869.188.5139.2


5R-1, 155R-1, 375R-2,155R-2, 865R-3, 66R-1, 176R-1,716R-1,1096R-2, 86R-2, 637R-1, 237R-1,1038R-1.118R-2, 219R-1,659R-2, 3610R-1, 310R-1, 62llR-1,1011R-2, 10HR-2,4211R-2, 10212R-1,4612R-2, 8413R-1, 13513R-2, 613R-2, 12413R4, 415R-1, 10415R-3, 115R-3, 3815R-3, 7316R-1, 7816R-1, 9816R-1, 11016R-2, 12416R-2, 13816R-3, 216R-3, 2916R-3, 5316R-5, 3716R-5, 6116R-5, 8417R-1, 10217R-2, 6817R-2, 8317R-3, 3218R-1, 3118R-1, 10518R-1, 10918R-2, 12619R-3, 819R-3, 3419R-3, 5520R-l,7820R-1, 11020R-2, 1221R-1.7621R-2, 222R-3, 2622R-3, 3222R-3, 4722R-3, 6823R4, 3624R-1,9624R-1, 11525R-2, 112

Depth lmM(mbsf) (0)

198.85 -28.6199.07 -40.8199.35 -46.8200.06 -50.9201.76 -33.0208.57 -40.5209.11 -39.8209.49 -42.5209.98 -35.6210.53 -32.6218.33 -22.1219.13 -32.9227.81 -41.8228.51 -47.3238.05 -73.8239.36 -65.4247.03 75.3247.62 -80.2256.8 -40.0257.3 -33.3257.62 -47.1258.22 -37.7266.96 -34.1268.64 -44.5276.95 -6.1277.16 -45.3278.34 -57.5280.14 -65.9295.94 -58.7297.91 -74.4298.28 -64.3298.63 -47.4305.38 -56.0305.58 -37.4305.7 -58.3307.34 -19.6307.48 -54.4307.62 -25.2307.89 20.1308.13 -2.3310.97 -73.8310.21 -50.7310.44 -14.1314.82 -33.3315.98 -41.0316.13 -32.7317.12 -7.7323.81 5.7324.55 12.3324.59 -28.3326.26 -16.4336.28 31.3336.54 -70.8336.75 -62.5343.28 -45.5343.6 -19.9344.12 -8.0252.86 -75.1353.62 21.1364.66 6.4364.72 -15.1364.87 -16.5365.08 13.3375.96 -25.7381.66 19.9381.85 28.9393.02 -3.1

^NRM(mA/m) I20 (0)

311.6526.9277.3408.7733.41223.31205.8323.11074.6946.7720.4416.6134.4289.0117.8126.088.9198.3142.7113.274.0134.6957.4165.129.771.647.2178.388.380.5

224.6147.962.122.830.659.663.941.248.367.484.659.071.6146.224.337.0125.9191.6145.554.7

473.9291.084.5166.1211.064.468.481.7

204.874.991.3137.3134.0683.5589.6346.7986.7

-27.3^0.9^5.9-52.7-34.2-39.9-39.8-43.0-35.1-34.0-22.1-35.8-40.9-44.3-61.6-68.465.3

-86.6-35.6-33.2-43.7-34.3-34.2^6.2-24.526.7-6.8

-55.953.059.9

-53.9-53.7-50.338.435.929.5

-22.332.343.432.6

^0.5-16.627.734.334.639.455.540.044.545.739.142.4

-18.5

35.738.039.8

-20.737.941.517.321.742.519.346.448.534.3

(Im =

J20 (mA/m)

202.9310.3187.2286.8550.51024.0976.9241.3839.1758.9573.6311.390.2

223.163.587.529.0132.9111.575.451.381.1

770.2117.47.715.212.6

147.228.323.3123.296.915.629.228.734.920.318.645.638.930.327.028.484.970.229.019.1

207.0125.947.9

480.7235.130.05.6

126.160.376.625.7154.544.925.275.493.3

379.5544.2336.6729.9

38.4+1.4)

Note: Im is the mean of the numerical values of l20

733


E 50-*

100 200

Depth (mbsf)300 400

Figure 7. Stable inclinations /20 (numerical values only; cf. Table 2) after partial AF cleaning in a 20-mT field of sediment cubes from Holes 841A and 841B. Thindotted line is a seven-point moving average. The thick solid line is the regression line: Norm(/20) = -0.0145 Depth + 41.5 (N - 134). A systematic shallowing ofthe inclination depth of 1.57100 m is seen, although the scatter is high.

1200-

1000-

ü

CO

ó

lity

<ib

i

Q.CUoen

800

600

400

200-

VI '

Mr10 20 30 40

Depth (mbsf)50 60 70 80

Figure 8. Whole-core magnetic volume susceptibility in sediments of Hole 841 A.

734


CO

8>CD

(10-

lity

.Q

ep

ti

uu

2500-

2000-

1500-

1000-

500-

o-

| i

|l|l ll| |

II-|

Iii

1

i

bf•' *ij:(

i1 Λ

f i

j,

ITI >

i

i i

i

ini i

i

i

i

r

1ii i1 i

i

1 1i

• j

i

, j

HU

1

I

11 1. u

u l

1

1

i•!1 i

> jii

I

i I

1

i

i

i ' ' i

150 200 250 300 350 400 450 500 550Depth (mbsf)

Figure 9. Whole-core magnetic volume susceptibility in sediments of Hole 84IB.

600 650 700 750 800 850

735

introduction basalts sampling and measurementsabrahamsen, 1992). arason and levi (1990) found a...

Documents