-
Greene, H.G., Collot, J.-Y, Stokking, L.B., et al.,
1994Proceedings of the Ocean Drilling Program, Scientific Results,
Vol. 134
27. ROCK MAGNETIC PROPERTIES, MAGNETIC MINERALOGY, AND MAGNETIC
FABRICOF ROCKS IN THE D'ENTRECASTEAUX COLLISION ZONE1
L.B. Stokking,2,3 D. Merrill,3 X. Zhao,4 and P. Roperch5
ABSTRACT
During Leg 134, the influence of ridge collision and subduction
on the structural evolution of island arcs was investigated
bydrilling at a series of sites in the collision zone between the
d'Entrecasteaux Zone (DEZ) and the central New Hebrides IslandArc.
The DEZ is an arcuate Eocene-Oligocene submarine volcanic chain
that extends from the northern New Caledonia Ridgeto the New
Hebrides Trench. High magnetic susceptibilities and intensities of
magnetic remanence were measured in volcanicsilts, sands,
siltstones, and sandstones from collision zone sites. This chapter
presents the preliminary results of studies of magneticmineralogy,
magnetic properties, and magnetic fabric of sediments and rocks
from Sites 827 through 830 in the collision zone.The dominant
carrier of remanence in the highly magnetic sediments and
sedimentary rocks in the DEZ is low-titaniumtitanomagnetite of
variable particle size. Changes in rock magnetic properties reflect
variations in the abundance and size oftitanomagnetite particles,
which result from differences in volcanogenic contribution and the
presence or absence of graded beds.Although the anisotropy of
magnetic susceptibility results are difficult to interpret in terms
of regional stresses because the coreswere azimuthally unoriented,
the shapes of the susceptibility ellipsoids provide information
about deformation style. The magneticfabric of most samples is
oblate, dominated by foliation, as is the structural fabric. The
variability of degree of anisotropy (P) anda factor that measures
the shape of the ellipsoid (q) reflect the patchy nature of
deformation, at a micrometer scale, that is elucidatedby scanning
electron microscope analysis. The nature of this patchiness implies
that deformation in the shear zones is accomplishedprimarily by
motion along bedding planes, whereas the material within the beds
themselves remains relatively undeformed.
INTRODUCTION
During Leg 134, the influence of ridge collision and
subductionon the structural evolution of island arcs was
investigated by drillingat a series of sites in the collision zone
between the d'EntrecasteauxZone (DEZ) and the central New Hebrides
Island Arc (Figs. 1 and2). The DEZ is an arcuate Eocene-Oligocene
submarine volcanicchain extending from the northern New Caledonia
Ridge to the NewHebrides Trench. Near the New Hebrides Trench, the
DEZ comprisestwo parallel morphologic features that trend
east-west: the fairlycontinuous North d'Entrecasteaux Ridge (NDR)
and the South d'En-trecasteaux Chain (SDC), which are composed of
seamounts andguyots. The impingement of the DEZ upon the central
New HebridesIsland Arc has greatly disrupted and tectonically
modified arc mor-phology and structure. Holes at Sites 827 and 829
were drilled topenetrate the lowermost accretionary wedge and the
interplate thrustfault (décollement), where the NDR collides with
the New HebridesIsland Arc. The primary objective of drilling at
Site 828 was to obtaina reference section of rocks from the north
ridge. Holes at Site 830were drilled to penetrate imbricated arc
rocks in the collision zonebetween the Bougainville Guyot (eastern
member of the SDC) andthe arc.
Preliminary interpretations of the results of Leg 134 suggest
thateach ridge of the twin-ridge DEZ causes different forearc
deformation.The sedimentary and surficial basement rocks of the
NDR, whosebasement rocks (mid-ocean ridge basalts, or MORBs) are
denser thanthose of the Bougainville Guyot, appear to have been
scraped off andaccreted to the forearc during subduction (Collot et
al., this volume;Fisher et al., 1986; Greene et al., this volume).
This accretion has
1 Greene, H.G., Collot, J.-Y., Stokking, L.B., et al., 1994.
Proc. ODP, Sci. Results,134: College Station, TX (Ocean Drilling
Program).
2 ODP, College Station, TX 77845-9547, U.S.A.3 Department of
Geophysics, Texas A&M University, College Station, TX
77843,
U.S.A.4 Earth Sciences Board, University of California, Santa
Cruz, CA 95064, U.S.A.5 Laboratoire de Géodynamique Sous-Marine,
Centre d'Etudes et de Recherches
Océanographiques de Villefranche-sur-Mer, B.P. 48,06230,
Villefranche-sur-Mer, France.
formed the Wousi Bank, which consists of uplifted forearc
rocksand stacked thrust sheets. The SDC impacts the forearc in a
differentmanner: little deformational uplift, but considerable
indenting, hasoccurred compared to the NDR collision zone, although
the SDC isconverging at the same rate and at the same angle as the
NDR (Collot,Greene, Stokking, et al., 1992).
This chapter presents the preliminary results of studies of
mag-netic mineralogy, magnetic properties, and magnetic fabric of
sedi-ments and rocks from Sites 827 through 830 in the collision
zone.Additional work on these units is presented in two chapters
byRoperch et al. (this volume).
LITHOSTRATIGRAPHY
Drilling results are presented in Collot, Greene, Stokking, et
al.(1992), Reid et al. (this volume), and are summarized in Table
1.Typical sediments and rocks are illustrated in Plate 1. Drilling
at Site827 (Figs. 1 and 2) recovered primarily arc-derived
turbiditic volcanicsilts and silty sands. At Site 828 on the NDR
drilling recovered high-susceptibility volcanic silts in the upper
part of Hole 828A and pri-marily low-susceptibility oozes and
chalks in Hole 828B.
At Site 829 collision has formed an accretionary complex that
isat least 590 m thick at a distance of 2 km from the trench (Figs.
1 and2). Site 829 contains 21 lithostratigraphic units, several of
whichrepresent repeated lithologies. Because of these repetitions,
duringLeg 134 the lithostratigraphic units were divided into four
compositeunits (Bigwan Wan, Bigwan Tu, Bigwan Tri, and Bigwan Fo,
namedin Bislama, the official language of the Republic of Vanuatu;
Collot,Greene, Stokking, et al., 1992). These composite units have
beendefined on the basis of age and lithologies. Bigwan Wan
comprisesPleistocene volcanic silt, sandstone, and gray silty
chalk. Bigwan Tucontains upper Oligocene to lower Miocene
foraminiferal and cal-careous chalk. Bigwan Tri is an upper
Pliocene or Pleistocene chalkbreccia with upper Oligocene to lower
Miocene clasts. Bigwan Fo isa breccia of unknown age with clasts of
basaltic rock fragments,microgabbros, and gabbros.
Lithostratigraphic units are numberedconsecutively starting with
Hole 829A and ending with Hole 829C.
475
-
L.B. STOKKING, D. MERRILL, X. ZHAO, P. ROPERCH
Figure 1. The d'Entrecasteaux Zone-central New Hebrides Island
Arc collisionzone. Bathymetry (in kilometers) after Kroenke et al.
(1983). NDR = Northd'Entrecasteaux Ridge, NAB = North Aoba Basin,
SAB = South Aoba Basin.
Initial interpretations of cores suggest that many of the units
iden-tified in Hole 829A are similar to those observed in Hole
828A. Forexample, the calcareous chalk and pale brown chalk of
Bigwan Tuare similar in age and lithology to the nannofossil chalk
of litho-stratigraphic Unit II from Hole 828A. Igneous rocks
collected at bothSites 828 and 829 are similar. Therefore the Leg
134 shipboard party(Collot, Greene, Stokking, et al., 1992) and
post-cruise researchers(Collot et al., this volume; Greene et al.,
this volume; and Reid, et al.,this volume) concluded that an
accretionary prism, composed in partof offscraped rocks and
sediments from the downgoing NDR, waspenetrated at Site 829.
Site 830 was drilled on the forearc slope in a gently
eastward-dipping thrust sheet. Rocks recovered include very coarse,
Cataclas-tic volcaniclastic sandstones that are similar in
composition to rocksexposed on nearby Espiritu Santo and Malakula
islands (Mitchell andWarden, 1971; Mallick and Greenbaum,
1977).
METHODS
Two-hundred-nine samples were studied (Table 2). The set
in-cluded 130 samples of volcanic silts and siltstones, 40 volcanic
sandsand sandstones, 12 breccias and conglomerates, 19 oozes and
chalks,and eight samples of mixed sediment and sedimentary
rocks.
Magnetic hysteresis is measured by applying an increasing
mag-netic field to a sample, then reversing the applied field,
while continu-ously measuring the magnetization of the sample as it
responds to thechanging applied field. Aplotof applied field vs.
sample magnetizationresults in a loop, the shape of which is
determined by the chemicalcomposition, microstructure, and particle
orientation of the magneticmaterial within the sample (Stacey and
Banerjee, 1974; Day et al.,
Site 831 Site 828 SH* 829 Site 827 Site 830
Figure 2. East-west cross sections of the collision zone with
projected simpli-fied lithologic columns. Location of cross
sections is shown in Figure 1.Legend is as follows: 1. Oceanic
crust, 2. Western Belt volcanic rocks, 3.Eastern Belt volcanic
rocks, 4. Central Chain volcanic rocks, 5. Basin fill, 6.Guyot
volcanic rocks, 7. Volcanic sand/sandstone, 8. Volcanic
silt/siltstone, 9.Volcanic sandstone/siltstone/claystone, 10.
Sed-lithic breccia, 11. Volcanicbreccia, 12. Basalt, 13. Multiple
slivers of siltstone and chalk, 14. Foraminiferalooze, 15.
Nannofossil ooze, 16. Foraminiferal chalk, 17. Nannofossil
chalk,18. Calcareous chalk, 19. Pelagic limestone, 20. Lagoonal
limestone, 21.Unconformity, 22. Ash, 23. Thrust fault; 1. Pli./e.
Pie is late Pliocene or earlyPleistocene, 1. O.-e. M. is late
Oligocene to early Miocene, E. is Eocene, m.M.is middle Miocene,
NDR is North d'Entrecasteaux Ridge, BG is BougainvilleGuyot, NAB is
North Aoba Basin, NFB is North Fiji Basin.
1977; Cisowski, 1980; O'Reilly, 1984). Measurement of
magnetichysteresis can provide information about several rock
magnetic prop-erties: coercivity (Hc), saturation remanence (MRS),
and saturationmagnetization (Ms). These parameters, in conjunction
with coercivityof remanence (HCR), then provide constraints on the
domain state ofthe magnetic minerals in the sample (Dunlop, 1969).
Domain state isin turn related to particle size (see "Discussion"
section, this chapter).Hysteresis behavior of nine samples was
measured using a PrincetonApplied Research vibrating sample
magnetometer housed in the Phys-ics Department at the University of
California, Davis.
Magnetic susceptibility provides information about the type
andconcentration of remanence-carrying minerals (such as
titanomag-netite) in a sample. Magnetic susceptibilities of the
archive halves ofcores were measured on board ship using a
Bartington Instrumentsmagnetic susceptibility meter (model MSI)
with an 80-mm sensorloop at a frequency of 0.47 kHz. Magnetic
susceptibilities of 209discrete samples were measured at TAMU using
a Bartington Instru-
476
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ROCK MAGNETIC PROPERTIES, MINERALOGY, AND FABRIC
Table 1. Lithologic summary of Sites 827, 828, 829, and 830.
Lithologicunit
Site 827I
IIIII
IV
Site 828IIIIIIIIIIVIV
Hole 829AIIIIIIIVVVIVIIviπIXX
XIXIIxπiXIVXVXVI
Hole 829BXVII
Hole 829C
xvmXIXXXXXI
Site 830I
If11
Lithologicsubunit
IAIBIC
IIIAI1IBinc
IAIBIC
IC
Core, section,interval (cm)
827A-lH-l,0to-13H-l,60827A-lH-l,0to-3H-6, 150827A-3H-6, 150
to-8H-4, 50827A-8H-4, 50to-13H-6, 60827A-13H-6, 60 to -827B-4R-3,
90827B-4R-3, 90 to -15R-CC, 15
827B-4R-3, 90 to -10R-4, 120827B-10R-4, 120 to -12R-3,
90827B-12R-3, 90 to -15R-CC, 15827B-15R-CC, 15 to -31R-CC, 7
828A-1H-I,0to-8H-1,48828A-8H-l,48to-8H-6,42828A-8H-6,42to-llH-l,92828B-lR-l,0to-2R-l,0828A-llH-l,92to-15N-2,185828B-2R-l,0to-3R-l,
12
829A-lR-l,0to-8R-l,0829A-8R-l,0to-12R-l,40829A-12R-l,40to-19R-4,
88829A-19R-4, 88 to -23R-1, 0829A-23R-l,0to-41R-l,0829A-41R-1, 0 to
-43R-1, 120829A-43R-1, 120 to -43R-CC, 20829A-43R-CC, 20 to -44R-1,
40829A-44R-1, 40 to -45R-CC, 5829A-45R-CC, 5 to -47R-1, 65
829A-47R-1, 65 to -48R-CC, 12829A-48R-CC, 12 to -51R-3,
10829A-51R-3, 10 to -54R-1, 0829A-54R-1, 0 to -55R-CC,
15829A-55R-CC, 15 to -57R-3, 72829A-57R-3, 72 to -64R-CC, 27
829B-lH-l,0to-3H-CC, 50
829C-1H-I,0to-9H-4,0829C-9H-4, 0 to-1
OH-1,0829C-10H-l,0to-llH-l,0829C-llH-l,0to-llH-CC, 15
830A-lH-l,0to-llX-l,34830A-lH-l,0to-3H-3, 150830A-3H-4, 0 to
-6H-5, 110830A-6H-5, 110 to -11X-1, 34
830B-lR-l,0to-14R-l,0
830B-14R-l,0to-24R-CC, 15830C-1R-1, 0 to -12R-CC, 14
Depth(mbsf)
0.0-88.40.0-27.0
27.0-66.066.0-88.488.4-141.0
141.0-252.6
141.0-200.0200.0-218.0218.0-252.6252.6-400.4
0.0-61.961.9-69.369.3-90.890.0-100.090.8-111.4
100.0-119.4
0-60.560.5-99.499.4-171.9
171.9-205.2205.2-378.4378.4-398.9398.9-106.0406.0-413.0413.0-419.6419.6-435.0
435.0-442.0442.0-463.2463.2-484.5484.5-495.6495.6-517.2517.2-590.3
0.0-19.5
0.0-54.64.6-57.3
57.3-58.358.3-58.4
0.0-96.90.0-21.0
21.0-47.047.0-96.9
48.5-174.9
174.9-281.7235.0-350.6
Thickness(m)
88.427.039.022.452.6
111.0
59.018.034.6
147.8
61.97.4
21.510.020.619.4
60.538.972.533.3
173.220.5
7.17.06.6
15.4
7.02.2
21.311.121.673.1
19.5
54.62.71.00.1
96.921.026.049.9
126.4
106.8115.6
Dominantlithology
Volcanic silt, volcanic silty sandFew graded interbedsSeveral
graded interbedsFew graded interbedsVolcanic silt and
siltstoneCalcareous volcanic siltstone,
sed-lithic conglomerateVolcanic siltstoneSed-lithic conglomerate
and brecciaVolcanic siltstoneVolcanic siltstone and sandstone,
breccia
Volcanic silt, volcanic sandy siltForaminiferal oozeNannofossil
chalkNannofossil chalkVolcanic brecciaVolcanic breccia
Clayey volcanic siltForaminiferal chalkClayey volcanic
siltSiltstone-chalk brecciaChalk brecciaCalcareous chalkVolcanic
breccia, clayey chalkForaminiferal chalkSilty chalkCalcareous chalk
and foraminiferal
chalkSilty chalkCalcareous chalkVolcanic sandstoneChalk and
mixed sedimentSandy volcanic siltstoneIgneous sed-lithic
breccia
Clayey sandy volcanic silt
Clayey sandy volcanic siltForaminiferal oozeForaminiferal silty
mixed sedimentForaminiferal chalk
Volcanic silt and volcanic siltstoneVolcanic siltVolcanic sand
interbedsInterbedded sandy silts and silty
sandsInterbedded sandy silts and silty
sandsVolcanic sed-lithic sandstoneVolcanic sed-lithic
sandstone
Age
PleistocenePleistocenePleistocenePleistocenePliocene to
PleistocenePliocene to Pleistocene
PleistocenePliocene or PleistocenePlioceneBarren
PleistocenePleistocene,
PlioceneOligoceneOligocene/EoceneEocene(?)Barren
Pleistocenelate Oligocene to early
MiocenePleistocenePleistocenelate Pliocene or PleistoceneOligocene
and EoceneIndeterminate, Eoceneearly PliocenePleistoceneOligocene
or Miocene
Pliocene (?) and PleistoceneOligocene to MiocenePliocene or
PleistoceneOligocene or MiocenePliocene to PleistoceneEocene
Pleistocene
Pleistoceneearly MiocenePleistocenePleistocene
PleistocenePleistocenePleistocenePleistocene
Pleistocene
BarrenBarren
ments susceptibility meter, model MS2, at a frequency of 0.47
kHz.Data are reported as volume susceptibility (k).
Whole-rock samples from shear zones were observed using a
JEOLJSM 6400 scanning electron microscope (SEM) with Tracor
Northern,Series 2, energy-dispersive X-ray analysis. Magnetic
separates ofvolcanic silt (Samples 134-827A-9H-1,118 cm, and
-829A-6R-4,110cm) prepared by attracting particles in a slurry of
ultrasonically sus-pended sediment and deionized water to a magnet,
were also analyzed.
Alternating field (AF) demagnetization of the natural
remanentmagnetization (NRM) of a sample provides information about
itscoercivity spectrum, a property that, assuming the sample
contains asingle type of magnetic carrier, is itself dependent on
the range of sizesof magnetic particles in the rock. Aboard ship,
both a 2-G Enterprises(Model 760R) pass-through cryogenic rock
magnetometer and a Mol-spin spinner magnetometer were used to
measure NRM. At TAMU,
NRM intensities were measured using a three-axis CTF
cryogenicmagnetometer housed in a shielded room in the
Paleomagnetics Labo-ratory at the Department of Geophysics. An AF
demagnetizer (Model2G600) capable of producing a peak field up to
20 Mt was used on-linewith the pass-through cryogenic magnetometer
on board ship. Ship-board AF demagnetization at higher fields and
AF demagnetization atTAMU were performed using a single-axis
Schonstedt geophysicalspecimen demagnetizer (Model GSD-1) capable
of producing peakfields up to 100 mT. Exposure to strong magnetic
fields in the corebarrel, in the steel drill pipe, and on the rig
floor impart a steep negativeinclination to cores, particularly
those containing low-coercivity mate-rial. The intensity of the
overprint decreases radially from the edge ofthe core to its
center; thus samples taken from the center of the coreare less
affected than those from the edge (Collot, Greene, Stokking,et al.,
1992). This spurious magnetization was removed from most Leg
477
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L.B. STOKKING, D. MERRILL, X. ZHAO, P. ROPERCH
Table 2. List of samples, depths, lithologic information, rock
magnetic data. Acronyms and abbreviations are defined in the
text.
Sample
Hole 827A9H-1, 1189H-2, 8311H-4, 75
Hole 827B5R-1,45R-3, 867R-5, 518R-4, 349R-1,959R-5, 849R-6,
210R-2, 2211R-1,11413R-2, 6213R-3, 1414R-2.11814R-3, 6215R-3,
50
Hole 828A4H-1.835H-4, 578H-1.448H-6, 698H-7, 59H-6, 899H-7,
2910H-l,116HH-1,20HH-3,126
Hole 828B1R-2, 451R-2, 981R-4, 60
Hole 829A5R-1.95R-3, 857R-1.3012R-1, 3517R-1, 13917R-2, 7017R-3,
8817R-4, 2317R-6, 1917R-6, 12317R-7, 2818R-U0718R-3, 7618R-4,
2818R-5.10718R-6, 3019R-1,7319R-4, 3719R-5,
7320R-U1620R-l,13520R-3, 224R-1.4443R-1.7743R-1, 14444R-2,
5751R-1.2351R-1.1O851R-2, 851R-2,
3852R-1,14456R-1.5056R-2.14156R-3, 1157R-2, 6064R-1.102
Hole 829BlH-1,312H-1,702H-2, 512H-3,1002H-4, 273H-1.413H-2,
213H-3, 25
Depth(mbsf)
68.269.382.9
146.5150.4172.3180.2186.1191.9192.6196.4205.5225.8226.8236236.9246.5
24.23861.869.670.579.380.281.690.194.2
9292.595.1
31.134.951.199.4
148.6149.4151.1151.9154.9155.9156.5157.9160.6161.6163.9164.6167.2171.4173.2177.3177.5179.1215.3398.5399.2409.5460.3461.2461.7462466.6504.3506.7506.9515.6582.2
0.31.22.54.55.3
10.411.713.3
Lithology
Vole, siltVole, siltVole, silt
Sltst.Sndst.Sndst.Sltst.Sltst.Mxd. sed.Mxd. sed.Mxd.
sed.Cgt.Mxd. sed.Mxd. sed.Sltst.Sltst.Cgt.
SiltSiltOozeOozeOozeOozeOozeOozeOozeBreccia
ChalkChalkChalk
SiltSiltSiltChalkSiltSiltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.Siltst.BrecciaBrecciaBrecciaSiltst.BrecciaChalkBrecciaChalkChalkChalkChalkChalkSandst.Siltst.Siltst.Siltst.Siltst.Breccia
SiltSiltSiltSiltSiltSiltSiltSilt
NRM(A/m)
0.600.620.19
0.060.120.170.170.070.020.030.040.160.230.130.220.030.09
0.380.330.090.010.030.000.020.010.010.00
0.030.040.04
0.890.680.290.050.370.360.190.230.420.650.400.210.100.350.260.150.481.260.930.020.010.130.050.010.020.160.080.080.070.060.340.290.060.120.480.02
0.550.470.490.450.910.610.59
RM10(A/m)
0.160.120.12
0.040.040.110.080.030.030.010.030.110.060.07
• 0.100.020.04
0.160.200.010.010.010.000.010.010.010.00
0.010.010.02
0.140.180.060.030.120.130.110.040.130.050.260.080.030.240.150.090.140.160.110.010.010.070.030.010.000.080.040.030.030.030.040.190.050.080.170.01
0.130.110.150.150.150.170.140.03
MDF(mT)
53
15
207.5151010557.5
35157.5
12.5101010
7.5203
202.5
405
27.5155
401
15
3052
1055
12555
125
15I0155
104
10101015105
1597873
155558
55555555
IRMIOOO(A/m)
120.66137.43109.81
79.65108.2567.3892.7071.1075.6267.7358.56
104.21138.54125.12129.0799.6795.73
88.10123.74
17.0411.1312.516.867.65
10.1913.650.50
5.686.38
13.85
149.80122.5093.3935.62
128.80144.9986.4182.71
122.95108.2090.64
117.02105.42130.39115.8387.4673.89
117.4981.7012.616.05
97.6124.8728.5540.0070.1949.6947.2540.6635.3752.30
209.93193.90192.19135.60
10.61
120.28125.84135.98120.14138.06131.22131.29105.31
ARM100(A/m)
2.153.682.52
1.842.481.462.461.801.531.531.563.003.153.034.273.902.69
2.323.210.560.560.570.540.470.740.650.03
0.340.340.44
3.994.262.260.923.394.022.822.842.963.323.783.053.593.232.872.313.532.552.210.710.302.300.940.994.021.331.981.911.491.030.953.913.804.325.140.18
2.942.983.102.892.843.043.173.04
K(S1)
0.00830.00410.0051
0.00280.00320.00290.00350.00250.00290.00250.00240.00230.00360.00310.00320.00220.0026
0.00350.00440.00120.00050.00060.00020.00030.00040.00060.0001
0.00030.00030.0006
0.00590.00390.0050.00140.00460.00410.00290.00240.00390.00510.00230.00480.00330.00580.00510.00370.00340.0080.00430.00030.00010.00520.00380.00150.00140.00640.00190.00180.00150.00280.00350.01990.01870.02490.00320.0003
0.00540.00650.0060.0060.00740.00680.00650.0047
Sample
3H-3,493H-4, 583H-5,68
Hnlp S9QP1H-2, 321H-3, 431H-4, 371H-5, 361H-6, 242H-1,792H-2,
722H-3, 792H-4, 792H-5, 792H-6, 793H-2, 803H-2, 1173H-2,1343H-2,
1443H-3,783H-4, 533H-5, 663H-6, 603H-7, 224H-1,354H-1.1194H-2,
294H-2, 1055H-1,295H-1, 1205H-2, 315H-2, 905H-3, 665H-4, 325H-4,
1245H-5, 205H-5, 627H-1,687H-1.1107H-2, 328H-1,289H-1,249H-3,
999H-4, 59
Hole 83OA1H-1, 81H-1,27
IH-l' 1001H-2,' 1031H-3, 821H-4, 711H-5, 402H-l! 862H-2, 632H-2^
1002H-3, 832H-5, 362H-6, 812H-7, 513H-1, 933H-2, 473H-3, 603H-3,
813H-4, 844H-1, 1084H-2, 924H-3, 495H-1, 1235H-2, 385H-2, 1255H-3,
286H-1,746H-2, 147X-1.32
Hole 83OB1R-1,471R-2, 122R-1, 872R-2, 332R-2, 60
Depth(mbsf)
13.515.116.7
1.83.44.96.37.79.1
10.512.113.615.116.620.120.520.620.721.622.824.525.92727.728.529.129.930.631.532.132.73435.13636.536.948.248.649.350.352.25.6
57.1
0.10.30.412.53.85.26.47.99.19.5
10.813.415.316.517.418.520.120.321.827.128.429.531.632.333.233.740.641.548.8
4950.15.959.960.2
Lithology
SiltSiltSilt
SiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSillSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltMxd.
sed.Mxd. sed.Mxd. sed.SiltSiltOoze/siltOoze
SiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSiltSandSandSiltSandSiltSiltSilt
SiltSiltSiltSiltSill
NRM(A/m)
0.290.611.24
0.360.250.260.330.990.630.630.510.450.590.990.520.410.370.420.460.431.110.890.970.990.680.861.300.670.590.610.500.480.520.520.600.430.300.330.501.120.350.240.11
0.340.340.330.380.280.230.330.270.390.600.440.520.490.350.570.590.330.020.140.531.331.071.421.461.791.741.051.961.440.98
1.150.380.310.480.37
RM10(A/m)
0.030.160.17
0.120.110.130.110.110.240.220.200.190.200.200.210.170.130.130.190.200.190.190.160.230.050.100.190.090.110.140.130.100.110.110.130.090.060.070.100.270.090.070.03
0.230.250.220.210.140.090.180.120.110.080.150.230.220.190.240.190.110.010.000.110.160.150.260.200.370.330.280.390.260.28
0.340.180.130.390.29
MDF(mT)
55
30
57
105
1055555
1777555
1055
15255555555555555
55555
20251712107
12733373975
533
101520
33
303020
320
3010102020
IRM1000(A/m)
94.1799.66
106.03
141.21111.41127.87131.38130.76132.21126.45120.02120.18120.54115.72123.84125.38162.29141.78131.51147.53154.41144.96151.98137.48127.44155.05123.08137.47136.70127.51116.95121.78123.48116.55119.4595.5166.1869.7396.4152.1558.2146.6518.13
162.03114.87113.38122.46169.12116.9198.0499.6113.69
111.12154.18115.68115.88124.20115.35136.48121.85
5.626.43
105.74161.73125.31185.31172.81154.83205.46170.35163.20154.66148.92
213.15160.75194.54205.52
98.57
ARM100(A/m)
3.142.793.57
2.852.983.252.732.763.082.902.902.692.652.853.303.343.253.203.073.463.963.894.093.303.153.943.173.073.053.212.923.073.233.163.293.431.581.682.211.281.471.470.53
3.132.712.873.032.372.743.652.222.511.762.672.902.653.052.543.042.770.160.172.222.943.173.472.532.833.653.083.393.043.18
3.513.423.233.853.03
K(S1)
0.0040.00460.0039
0.00710.00380.00510.00650.0060.00620.00610.00570.006
0.00550.00520.0050.00520.00550.00630.00560.00610.00640.00580.00620.00580.00560.00680.00510.00580.00560.00530.00480.00450.00470.00440.00460.00280.00290.00260.00310.00250.00220.00170.0007
0.00330.00320.00330.00380.00380.00410.00220.00380.00420.0060.00470.00490.0050.00360.00430.00510.0050.00030.00020.00530.0090.00580.010.01730.01050.01230.01090.00730.00760.0069
0.01210.00770.01050.00930.0032
478
-
ROCK MAGNETIC PROPERTIES, MINERALOGY, AND FABRIC
Table 2 (continued).
Sample
3R-1, 183R-1,693R-1.983R-1.1314R-1, 154R-1, 834R-2,
315R-2.515R-2, 935R-2, 1016R-l,206R-1,817R-1.878R-1, 1198R-2,
369R-2, 4410R-2,
812R-2.4913R-1,13915R-1.4115R-1,6315R-1.10717R-1,2718R-1.3918R-1.11219R-1.2019R-1.10019R-1,13119R-2,
1520R-1.4320R-l,9322R-1, 8323R-2, 4423R-2, 7724R-1.6024R-1, 69
Hole
830C1R-1,712R-1.712R-1.1162R-U454R-1.1234R-2.1116R-1.1137R-1.207R-1,
668R-1, 1258R-2, 59R-1, 139R-1.269R-1.8310R-1, 1510R-2,
2712R-2,6412R-2, 7412R-2, 111
Depth(mbsf)
67.968.468.76977.778.378.989.489.889.997.397.9
107.7117.7118.4128.1137.4157.3166.6185185.2185.7204.1213.8214.5223.3224.1224.4224.8233.3238.3253.2264.1264.5272.6272.7
235.7245.1245.6245.9264.9266.3284292.8293.3303.6303.9312312.2312.7321.7323.3343343.2343.5
Lithology
SiltSillSillSiltSiltSiltSiltSiltSiltSiltSiltst.Siltst.Siltst.Siltst.Siltst.Sandst.Sandst.Siltst.Siltst.Sandst.Sandst.Sandst.SiltSandst.BrecciaSandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.
Sandst.Sandst.Sandst.Sandst.Sandst.BrecciaBrecciaSandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.Sandst.
NRM(A/m)
0.570.040.011.030.490.440.590.520.750.360.430.380.870.520.400.780.340.150.120.180.170.630.110.120.040.150.210.010.010.000.000.050.010.070.050.07
0.110.080.890.020.080.150.500.040.060.210.070.040.040.101.160.220.490.210.19
RM,0(A/m)
0.260.040.010.470.350.340.390.330.310.260.320.220.250.340.280.210.260.050.110.050.050.040.010.020.030.070.020.010.000.000.000.030.010.030.030.06
0.030.030.090.010.040.020.140.010.010.010.020.010.020.070.080.020.130.020.11
MDF(mT)
7352030202015155
2020125
15175
207
3055555
20105
1057
108
40103020
5453
1155355655
20155
57
IRM1000(A/m)
146.7160.8626.41
169.09143.22126.54166.96147.22157.26138.63170.32150.56153.50186.87243.43144.60128.77152.98127.5186.93
100.8065.9434.54
225.84136.57165.4393.2719.756.212.663.86
197.4168.3851.59
207.38220.21
154.95149.12452.45
39.3384.0339.00
267.4392.38
118.7387.5971.4787.6674.14
205.22212.68
93.72160.3357.86
209.43
ARM100(A/m)
3.712.831.223.233.382.993.393.192.882.594.333.102.896.374.252.682.962.912.972.152.172.682.268.014.087.135.301.380.330.330.467.791.652.415.685.09
3.604.989.031.454.051.837.184.446.71
10.323.776.104.474.785.134.832.242.119.15
K(S1)
0.00560.00140.00120.00790.005
0.00470.00770.00640.0080.00630.0070.00710.00870.00960.01480.00660.00470.00640.00440.00490.00540.00190.00090.00540.00470.00480.00230.00070.00030.00010.00020.00530.00210.00210.00490.0102
0.01090.00380.01440.00170.00360.00190.00510.00310.00570.00230.00280.00260.00220.00920.01180.00340.01030.00190.0065
134 samples by AF demagnetization at a peak field of 10 mT.
Hence,both NRM intensity and intensity remaining after 10-mT (RM10)
de-magnetization are presented.
Ten well lithified mini-core samples of siltstone and sandstone
fromSites 829 and 830 were thermally demagnetized using a
Schonstedtthermal demagnetizer (Model TSD-1) aboard ship. Samples
wereheated at temperature intervals between 20°C and 50°C up to
500°C,and the remanence was measured between steps.
The manner in which isothermal remanent magnetization (IRM)and
anhysteretic remanent magnetization (ARM) are acquired
anddemagnetized also provides information on the type of magnetic
car-riers and the coercivity spectrum of the sample. After AF
demagneti-zation, the acquisition behavior of ARM was studied for a
suite of 10pilot samples. The Schonstedt AF demagnetizer and a
DTECH dou-ble-coil anhysteretic magnetizer were used to produce the
ARM. Thealternating field was progressively increased from 0 to 100
mT in aDC-bias field of 0.1 mT. The ARM was then AF demagnetized so
thedemagnetization behavior could be compared to that of NRM
andIRM. Subsequently, an ARM (ARM100) was imparted to the
209samples in the study using an alternating field of 100 mT and a
DC-
bias field of 0.1 mT. The samples were then AF demagnetized at
peakfields of 95 or 100 mT prior to giving the samples an IRM.
The IRM acquisition behavior of the pilot samples was studied
atTAMU. Impulse fields increasing from 0 to approximately LOT
wereapplied along the -Z-axis of the sample using an ASC impulse
mag-netizer (Model IM-10). The remanence was measured between
stepsusing the spinner magnetometer. The coercivity of saturation
rema-nence (HCR) of the samples was determined by applying an
increasingreverse-field IRM (along the +Z-axis) to the samples. A
second IRMwas then applied (at a saturating field of about 1.0 T)
and demagnet-ized using the Schonstedt AF demagnetizer so that the
demagnetiza-tion behavior of IRM could be compared to that of NRM
and ARM.An IRM was then applied to all samples in the study at a
field of about
LOT(IRMloOo).Anisotropy of magnetic susceptibility of several
samples from
Hole 829A was measured by B. Ellwood at the University of
Texas,Arlington, using a low-field torque magnetometer. All samples
weretaken near or within shear zones. Magnetic fabric is the
three-dimen-sional variation of magnetic properties within a
sample. This variationis determined by measuring the magnetic
property in several differentsample orientations, a procedure that
reveals the anisotropy of mag-netic properties in the sample. The
magnitudes and directions of theproperty being measured define an
ellipse, the shape of which pro-vides information about the
orientation of the magnetic minerals inthe rock (Hrouda, 1982).
This type of analysis is useful for determin-ing orientation of
foliation planes and degree of deformation (Hrouda,1982). The
samples were not corrected for orientation: most samplescame from
cores for which recovery was poor, so that structural
andpaleomagnetic corrections could not be made. Nevertheless,
fabricinformation can be obtained from the shape of the
ellipsoid.
Either ratios or differences among the principal axes
(maximum,intermediate, and minimum) of the susceptibility ellipsoid
may beused as a measure of ellipsoid shape. In the following
equations, KMAXis the maximum axis, KINT is the intermediate axis,
KMIN is the min-imum axis of the anisotropy ellipsoid, and KAVG is
{KMAX + KINT +KM1N)ß. The total anisotropy is defined by (KMAX -
KMIN)/KAVG. Thedegree of anisotropy, P, is defined as the ratio
KMAX/KMIN and de-scribes the amount of preferred orientation of
magnetic minerals inthe sample. The q factor, defined as (KMAX -
K[NT)/[(KMAX + KINT)/2 -(KMIN)], is used to measure the shape of
the ellipsoid in sediments.For most sedimentary rocks, q ranges
from about 0.06 to 0.67 (Tar-ling, 1983, and references therein).
Increases in deformation of sedi-ment in rocks whose degree of
anisotropy (P) is less than about 1.05have been correlated with
increases in q (Hrouda, 1982). Magneticlineation (L) is (KMAX -
KINT)/KAVG × 100%, and measures the degreeto which magnetic
minerals parallel each other in a linear fashion,defining a prolate
anisotropy ellipsoid. Magnetic foliation (F) is (KINT- KMIN)/KAVG ×
100%, and determines the degree of planar parallelismof magnetic
minerals, which describe an oblate anisotropy ellipsoid.
RESULTS
Hysteresis
Hysteresis data from samples from Sites 827 through 829
arepresented in Table 3 and Figure 3. The MRS/MS values all fall
near theboundary between pseudo-single-domain and multidomain
behavior.The shapes of Figures 3A, 3C, and 3D are typical of
samples domi-nated by magnetite, whereas the shape of Figure 3B may
result froma mixture of magnetite and hematite.
Susceptibility
The variation of susceptibility determined using the
shipboardmultisensor track with depth and lithologic type in Holes
827B, 828 A,829A, 830A, and 830B is illustrated and described in
the appropriatesite chapters of the Initial Reports volume (Collot,
Greene, Stokking,et al, 1992) and in two chapters by Roperch et al.
(this volume).
479
-
L.B. STOKKING, D. MERRILL, X. ZHAO, P. ROPERCH
I 4Φ
Sample 828A-1H-1, 58-βOcm B field vs. moment
-2
C
i 1 • 1 r
Moment (emu)
15
-J 0
£ 5
Sample 829A-57R-1, 76-78cm B field vs moment
-5-2000
• Moment (emu)
2000 4000 6000B field (Oβ)
8000
B
10
8
6
4
2
0
-2
D
5
4
3
2
1
0
-1
Sample 829A-18R-2, 71 73cm B field vs moment
Moment (emu)
i
Sample 829A-43R-3, 78-80cm B field vs moment
• Moment (emu)
J i I u10000 -2000 2000 4000 6000 8000 10000
B field (Oe)
Figure 3. Hysteresis loops of representative samples. A. Sample
134-828A-1H-1,58-60 cm. B. Sample 134-829A-18R-2, 71-73 cm. C.
Sample 134-829A-57R-1,76-78 cm. D. Sample 134-829A-43R-3, 78-80
cm.
Table 3. Hysteresis parameters.
Sample
134-827A-9H-1, 24-27 cm827A-12H-3, 73-75 cm828A-1H-1, 58-60
cm828A-3H-1, 62-64 cm829A-4R-3, 98-101 cm829A-18R-2, 71-73
cm829A-43R-3, 78-80 cm829A-51R-3, 24-26 cm829A-57R-1, 76-86 cm1.
Volcanic silt2. Volcanic siltstone3. Ig-lithic breccia4. Volcanic
sandstone
Lith.
111122344
MRSIMS
0.180.140.180.220.160.120.130.190.10
BCR
(mT)
372727273027152121
IRM I O O O(A/m)
79.8102.6112.7110.0103.2111.891.283.083.3
Note: Lith. = lithology, MRS/MS = saturation remanent
magnetiza-tion/saturation magnetization, BCR = coercivity of
remanence,I R M 1 0 0 0 = isothermal remanent magnetization at 1.0
T appliedfield.
Volume magnetic susceptibility of discrete samples of silt and
silt-stone ranges from 0.0002 to 0.0249 S1, with an average of
0.0056S1. The susceptibility of sand and sandstone averages 0.0052
S1 andvaries from 0.0001 to 0.0173 S1, whereas that of breccias and
con-glomerates averaging 0.0022 S1 ranges from 0.0001 to 0.0051
S1.Susceptibility of discrete samples of calcareous oozes and
chalksaverages 0.0013 S1 and ranges from 0.0007 to 0.0064 S1, and
that ofmixed sediment samples varies from 0.0024 to 0.0036 S1 with
an
average of 0.0029 S1 (Table 2). The downhole variation of
magneticsusceptibility in Holes 827B, 829A, 829C, 830A, and 830B is
illus-trated in Figure 4.
NRM
Intensity of NRM ranges from 0.010 to 1.964 A/m (average
0.52A/m) in the silts and siltstones, from 0.12 to 1.794 A/m
(average 0.16A/m) in the sands and sandstones, from 0.0005 to 0.93
A/m in thebreccias and conglomerates, from 0.003 to 0.24 A/m
(average 0.06A/m) in the calcareous oozes and chalks, and from 0.02
to 0.50 A/m(0.20 A/m) in mixed sediment (Table 2). The NRM of all
samples inthis study most probably includes a depositional
remanence (DRM),possibly a post-depositional remanence (pDRM), and
a drilling-induced remanence.
Demagnetization Behavior
Remanent intensity after AF demagnetization at 10 mT (RM
!0)varies from 0.003 to 0.471 A/m (average 0.16 A/m) in the silts
andsiltstones, from 0.001 to 0.366 A/m (average 0.04 A/m) in the
sandsand sandstones, from 0.0005 to 0.14 A/m (average 0.06 A/m) in
thebreccias and conglomerates, from 0.005 to 0.083 A/m in the
calcare-ous oozes and chalks, and from 0.014 to 0.102 A/m (average
0.06A/m) in mixed sediment (Table 2). Vector endpoint diagrams
illus-trating the AF demagnetization behavior of samples from the
pilotstudy are presented in Figure 5. The steep, negative,
drilling-inducedcomponent is evident in all samples and is
significantly reduced by
480
-
ROCK MAGNETIC PROPERTIES, MINERALOGY, AND FABRIC
demagnetization at 10 mT. The downhole variation of RM j 0 in
Holes827B, 829A, 829C, 830A, and 83OB is illustrated in Figure
6.
The median destructive field (MDF) is the peak
demagnetizingfield at which half the NRM has been removed. MDF
values forsamples in this study are provided in Table 2. Although
these dataprovide information about the relative coercivities of
the samples, thevalues should be interpreted with caution because
of the drilling-induced component of the NRM. Nevertheless, the
remanence ofmost samples is demagnetized at relatively low peak
fields, consistentwith the dominant carrier of remanence being
titanomagnetite, witha relatively large multidomain component.
The results of thermal demagnetization of samples of well
lithi-fied siltstone from Core 134-829A-56R are illustrated in
Figure 7.The remanence of all samples measured was unblocked by
heatingat temperatures between 450°C and 500°C, consistent with
eithertitanomagnetite, magnetite, or hematite as the carrier of
remanence.
Laboratory-induced Remanences
Figures 8 and 9 illustrate IRM and ARM acquisition behavior
andremanence decay after AF demagnetization of typical samples
fromthe pilot study. The shapes of the acquisition curves are
typical oftitanomagnetite. The IRM acquisition curves rise sharply,
then flatten,indicating that little, if any, hematite is present.
In most samples,saturation is achieved at an impulse field of
approximately 0.1 T.
IRM1 0 0 0 ranges from 5.62 to 243.4 A/m (average 125. 58 A/m)
insilts and siltstones, from 2.66 to 452.4 A/m (average 121.65 A/m)
inthe sands and sandstones, from 0.50 to 267.4 A/m (average
68.27A/m) in the breccias and conglomerates, from 5.68 to 70.19
A/m(average 25.11 A/m) in the calcareous oozes and chalks, and
from58.56 to 138.5 A/m (average 87.24 A/m) in mixed sediment
(Table2). Especially in the silts, siltstones, sands, and
sandstones, IRM1 0 0 0intensities are quite high, reflecting the
abundance of titanomagnetitein the samples. ARM100 ranges from 0.16
to 6.37 A/m (average 3.05A/m) in silts and siltstones, from 0.33
tolθ.32 A/m (average 3.97 A/m)in the sands and sandstones, from
0.03 to 7.18 A/m (average 2.26A/m) in the breccias and
conglomerates, from 0.34 to 1.98 A/m(average 0.89 A/m) in the
calcareous oozes and chalks, and from 1.53to 3.15 A/m (average 2.03
A/m) in mixed sediment (Table 2). TheARM100 and IRM1 0 0 0 of the
pilot samples were demagnetized so thatthe stability of ARM and IRM
could be compared to that of NRM.
Magnetic Fabric
Results of measurements of anisotropy of magnetic
susceptibilityfrom deformed intervals in Hole 829A are listed in
Table 4, and acomparison of lineation and foliation is presented in
Figure 10. Plate2 contains photomicrographs of samples of breccia
(Samplel34-829A-43R-1, 144-146 cm) and brown clay (Sample
134-829A-47R-1, 57-59 cm) from shear zones.
DISCUSSION
Magnetic Mineralogy
Preliminary scanning electron and energy-dispersive x-ray
micro-analyses of magnetic mineral separates from typical samples
of vol-canic silt from Holes 827A and 829A are illustrated in Plate
3. SEMand qualitative X-ray analysis indicate that titanomagnetite,
relativelylow in titanium, is the dominant carrier of remanence
(Pis. 2 and 3).The shapes of the IRM and ARM acquisition curves and
the behaviorduring AF and thermal demagnetization are consistent
with titano-magnetite as the predominant magnetic mineral.
Particle Size
Rock magnetic data can be used to constrain the size of
magneticparticles in the samples. For equidimensional grains, pure
magnetite
Hole 827B
K(SI)0.002 0.003 0.004 0.005
140
160
180
£ 200
Q.
Q 220
240
260
MIA
1MB
•MIC
Hole 829C
K(SI)
0 0.005 0.01 0.015
0
100
200
300
400
500
600
Hole 829A
K(SI)
0.01 0.02 0.031 1 '
-
- #
• . i
1
l l _
III -
I V -
v -VVI
VIII-XI
XIII
xvr
Holes 830A and 830B
K(SI)) 0.01 0.02 0.03
300
Figure 4. Downhole variation of magnetic susceptibility (K) in
Holes 827B,
829A, 829C, 830A, and 830B.
is superparamagnetic when less than about 0.03 µm in
diameter,single domain from 0.03 to 0.08 µm in diameter, and
typically exhibitsmultidomain at diameters greater than 0.2 µm
(Butler and Banerjee,1975). Particles larger than typical
single-domain grains that showsingle-domain behavior are considered
pseudo-single domain. Thediameters at which transitions between
stable domain states occurdepend upon the composition of the
particle, however, and vary withthe concentration of cations in
solid solution with Fe, particularlyTi, but also Al, Mg, and Mn.
Considering only the influence of Ti,titanomagnetite containing 60%
ulvospinel (Fe2Ti04) does not showsingle domain behavior until its
diameter has increased to 0.08 µm(Dunlop, 1981); the transition
from single domain to multidomainbehavior theoretically should
occur at 0.2 µm (Butler and Banerjee,1975) and has been observed to
occur at about 0.6 µm (Soffel, 1971).In addition, domain behavior
is affected by the elongation of theparticle (Butler and Banerjee,
1975), the structure of the grain,oxidation, or microcracks
(Haggerty, 1970; Johnson and Hall, 1978;Henshaw and Merrill, 1980)
that may produce regions within a largegrain that act as single
domains.
The ratio of hysteresis parameters MRSIMS has been used to
dis-tinguish between single-domain and multidomain behaviors:
valuesgreater than 0.5 indicate single-domain particles, ratios
between about0.1 and 0.5 represent pseudo-single domain particles,
and values lessthan about 0.1 indicate multidomain or
superparamagnetic particles(StonerandWohlfarth, 1948; Day et al.,
1977; Thompson and Oldfield,
481
-
L.B. STOKKING, D. MERRILL, X. ZHAO, P. ROPERCH
NRM
40
Scale: i.β IA/m
40
9 N R M NRM
Scale: 1.β-1 A/m
E40
Scale: 1e-1 A/m Scale: 1.e-1 A/m
oNRM
10
V 20
Scale: 1.e-1 A/m
Figure 5. Vector endpoint diagrams illustrating the results of
AF demagnetization of NRM of representative samples. Solid circles
represent the horizontal
component of remanence and open circles represent the vertical
component. NRM, RMi0, final, and some additional peak demagnetizing
fields (mT) are indicated.
A. Volcanic silt (Sample 134-829C-5H-2, 31 cm). B. Volcanic
siltstone (Sample 134-829A-17R-2, 70 cm). C. Breccia (Sample
134-830C-6R-1, 113 cm). D.
Chalk (Sample 134-829A-44R-2, 57 cm). E. Mixed sediment (Sample
134-827B-9R-6, 2 cm).
Hole 827B
RM 10 (A/m)0 0.05 0.1 0.15
140
Hole 829A
RM 10
0.1 0.2 0.3
Table 4. Anisotropy data. Acronyms and abbreviationsare defined
in the text.
160
c 180< Λ
E200
220
240
260
MIA
1MB'
inc.1
•IV-
VI
Vlll-X
• ,xv
0
100
200
300
400
500
600
o (
50
100
150
200
250
300
Figure 6. Downhole variation of RM 1 0 in Holes 827B, 829A,
829C, and 830A
and 830B.
Hole 829C
RM 100.1 0.2 0.3
Hole 830A and 830B
RM10 (A/m)0.2 0.4 0.6
0
10
ST 20
17
•1
i • •
• * ••t•
i
IAIB
-
IC
, l l ~
Sample
134-829A-12R-1.2412R-1.5543R-1.2043R-1, 13943R-2, 2743R-2,
6947R-1, 5147R-1.5751R-1,851R-1.5951R-2, 13058R-1, 132
Depth(mbsf)
99.2499.55
397.27399.09399.47399.89427.11427.17460.18460.69462.90524.52
P
1.011.061.111.041.061.051.041.081.061.061.051.22
0.760.610.850.610.871.260.590.630.440.190.570.38
F(%)
0.553.324.362.072.241.042.194.123.344.612.7313.2
L(%)
0.682.936.431.793.423.521.823.751.910.982.26.11
Aniso.(%)
1.236.24
10.793.865.664.564.017.875.35.594.92
19.27
Note: Aniso. = total anisotropy.
1986). Although domain state is not a direct measure of particle
size,the ratios may provide some indication of relative changes in
domainbehavior that reflect changes in particle size. Thus, on the
basis ofhysteresis parameters, the samples analyzed in this study
fall near theboundary between pseudo-single domain and multidomain
behavior.
Differences in the stabilities to AF demagnetization of natural
andartificial remanences carried by magnetite are governed by
particlesize, the relative proportions of low- and high-coercivity
particles, thedegree of particle alignment, and the interplay
between internal stresseswithin a particle and magnetostatic
interactions between pinned andunpinned domain walls (Heider et
al., 1992; Moon and Merrill, 1986;Stacey and Banerjee, 1974; Xu and
Merrill, 1990; Xu and Dunlop,1993). Lowrie and Fuller (1971)
demonstrated that the relative stabili-ties to AF demagnetization
of IRM and thermoremanent magnetization(TRM) may be used to
evaluate the domain state that predominatesin the primary carrier
of remanence in a sample, assuming that theprimary magnetization is
carried by magnetite. For single-domain par-ticles, TRM is more
stable than IRM, whereas for multidomain parti-cles, IRM is more
stable than TRM. The stability of ARM to AFdemagnetization is
similar to that of TRM (Johnson et al., 1975; Leviand Merrill,
1976), which enabled the Lowrie-Fuller test to be modifiedto
compare stabilities of ARM and IRM, thereby avoiding
problemsassociated with mineralogical changes caused by heating the
samples.
4S2
-
ROCK MAGNETIC PROPERTIES, MINERALOGY, AND FABRIC
f - Y
Sample 829C-5H-2, 31 cm
50°C
2 - 100
Sample 134-829A-56R-5, 10-12 cm
- 0.1 A/m
Sample 134-829A-56R-4, 122-124 cm
Figure 7. Thermal demagnetization behavior of volcanic siltstone
samples
from Hole 829A. Symbols as in Figure 5, temperature steps in
°C.
The validity of ARM as an analog to TRM may, however, depend
onthe concentration of the magnetic material (Sugiura, 1979).
Recent studies question interpretations of the Lowrie-Fuller
test:the stability of multidomain particles depends on an interplay
betweenthe internal stresses that act to pin domain walls and a
screening effectin which magnetostatic interactions between
unpinned and pinneddomain walls increases the particle's resistance
to AF demagnetiza-tion (Moon and Merrill, 1986; Xu and Merrill,
1990; Heider et al.,1992; Xu and Dunlop, 1993). In grains with high
internal stress, suchas the crushed magnetite particles used by
Lowrie and Fuller (1971),the effect of screening is small, and
stability to AF demagnetizationdepends mostly on microcoercivity,
and hence, particle size. Stabilityto AF demagnetization of
particles whose internal stress is low,however, is controlled by
screening, and thus shows less size depend-ence (Xu and Dunlop,
1993).
Experiments in which the Lowrie-Fuller test was modified by
sub-stituting ARM for TRM were not designed to simulate DRM, and
didnot take into account the effects of concentration or particle
align-ment on stability against AF demagnetization. Nonetheless,
the modi-fied Lowrie-Fuller test has been applied to marine
sediments. Lowrie-Fuller test results were consistent with particle
sizes determinedusing transmission electron microscopy when the
test was applied todeep-sea sediments that contained single-domain
biogenic and multi-
-0.2-200
B
-20
1.2
1.0 "
1 0.6
£ 0.4
200 400 600 800 1000 1200Impulse field (mT)
1.2
inte
nsity
OB
O
5 0.6
11 0.4o
2 0.2
0.0
1 1 '
-
-
1 ' 1
•
t i l l
1 ' 1 '
-
1 1 1 1
20 40 60ARM field (mT)
80 100
1 1 •
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1
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IRMARMNRM
-
-
•
i I i
o.o -
-0.2-20 0 20 40 60 80 100 120
AF demagnetizing field (mT)Figure 8. Rock magnetic behavior of
volcanic silt (Sample 134-829C-5H-2,31 cm). A. IRM acquisition. B.
ARM acquisition. C. Intensity decay, AFdemagnetization of NRM, IRM
1 0 0 0, and ARM1 0 0.
domain lithogenic magnetite (Petersen et al., 1986) and to
carbonatesthat contained single-domain biogenic magnetite (McNeill,
1990).Apparent particle sizes indicated by the hysteresis
parameters differedfrom those implied by the Lowrie-Fuller test,
however, for suboxicsediments that probably contained magnetite
from a variety of sources(Karlin, 1990). The test is probably most
useful in sediments that
483
-
L.B. STOKKING, D. MERRILL, X. ZHAO, P. ROPERCH
.ε
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0 •>
Sample 829A-44R-2, 57 cm• i > i - i i
•---
i i i i i i i i
i i i i i
• ----i i i i i
B
-200 0 200 400 600 800 1000 1200Impulse field (mT)
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•
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i 1 i 1 i 1
i I i | i
Φ
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-
-
-
-
-
-
i 1 i 1 i
-20
-20
20 40 60ARM field (mT)
80 100
1.2
1.0
£0.8
§.1 0.6"SΦ
iθ.2
0.0
-0.2
1 l
•-
-
-
-i i
1 l
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i t i
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OV
1
1 1
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i i
1
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D0
1 •
IRM -ARM -NRM -
-
-
-
U D-
i
1000 20 40 60 80AF demagnetizing field (mT)
Figure 9. Rock magnetic behavior of chalk (Sample
134-829A-44R-2,57 cm).
A. IRM acquisition. B. ARM acquisition. C. Intensity decay, AF
demagneti-
zation of NRM, IRM 1 0 0 0, and ARM1 0 0.
contain uniformly stressed magnetic particles whose stability is
gov-erned by microcoercivity rather than screening, and in which
magne-tostatic interactions between aligned particles are not
significant.
In most Leg 134 samples, the stabilities of IRM and ARM to
AFdemagnetization are similar: ARM is slightly more stable to AF
demag-netization than IRM, and both are much more stable than the
NRM(Figs. 8C and 9C). A simple application of the Lowrie-Fuller
test to
43 R#
-
#43R /
// 47R^3R // 2 R
/ 51R 1R/ 43R
051R
#58R
Figure 10. Hole 829A. Lineation vs. foliation.
these results would suggest that the samples contain
single-domainmaterial. Alternatively, the samples may contain
multidomain particleswhose stability against AF demagnetization is
increased by magneticscreening or particles with a wide range of
internal stresses. Consideringthat hysteresis data implied the
presence of multidomain material, thesamples may well contain
mixtures of particle sizes and domain states.
Ratios of IRMi000 (or ARM100) to susceptibility reflect changes
ingrain size of magnetic particles (Banerjee et al., 1981; King et
al., 1982;Bloemendal et al, 1989; Sager and Hall, 1990). Figure 11
shows thedownhole variation of IRM1000/ÅT and Figure 12 is a plot
of log Kvs. log IRM1000. Susceptibility reflects the concentration
of magneticminerals, whereas IRM1000 and ARM100 are controlled both
by theconcentration and particle size of magnetic minerals. If
grain size isthe same, susceptibility can be used to normalize out
concentrationvariations. In Figures 11 and 12, a large proportion
of fine-grainedparticles results in high ratios, whereas a
preponderance of coarse-grained particles produces low ratios. The
presence of superparamag-netic material will increase
susceptibility, resulting in low ratios thatdo not reflect the
presence of coarse-grained material (Thompson andOldfield, 1986).
This tests elucidates relative changes in particle sizethat
correspond to sedimentological differences discussed below.
Correlation between Magnetic Propertiesand Lithostratigraphy
Figure 4 shows the downhole variation of magnetic
susceptibilitymeasured in discrete samples from Holes 827B, 829A,
829C, and830A and 830B. Susceptibility peaks in all plots
correspond to inter-vals of volcanic silt, siltstone, sand, and
sandstone, and thus indicateincreases in the contribution of
volcanogenic material, either derivedby erosion of arc rocks or
from a volcanic eruption. Oozes, chalks,and calcareous breccias are
marked by low susceptibilities. Intensitiesof NRM and RM10 (Collot,
Greene, Stokking, et al., 1992; and Figure6) also reflect the
abundance of titanomagnetite. Both display trendssimilar to the
susceptibility.
In Hole 827B, Unit III contains highly bioturbated, partially
lith-ified calcareous volcanic siltstone with intervals of
conglomerate.Increases in susceptibility and intensity of remanence
(Figs. 4 and 6)correspond to decreases in carbonate percentage
(Collot, Greene,Stokking, et al., 1992). The IRM1000/£ratio
increases in Subunits IIIAand UIC, suggesting a fining upward of
the relatively dense titano-magnetite grains which was not observed
in sedimentological de-scriptions of the cores.
Units in Hole 829A that contain abundant volcanogenic
mineralsare characterized by high magnetic susceptibilities and
intensities, as
484
-
ROCK MAGNETIC PROPERTIES, MINERALOGY, AND FABRIC
|
20140
160
180
200
220
240
260
10
Hole 827B
SIRM/k30 40 50
Hole 829A
S1 RM/k20 40 60
•
-•
, 1
•
••, 1
-
-
1IIA
IIIB"
-
MIC-
Hole 829C
S1 RM/k20 30
0
100
200
300
400
500
600Hole 830A and Hole 830B
SIRM/k0 10 20 30 40 50
V-VL
VIN—XL
XV
ß xvi
60
Figure 11. Downhole variation of IRM1000/tf in Holes 827B, 829A,
829C, and830Aand830B.
expected. In Unit III, interbeds of volcanic sand are common,
and anincrease in grain size in the lower 15 m of the unit is
reflected by adecrease in IRM1000/ÅT. Differences in magnetic
susceptibility andintensity in Unit XIII, a volcanic sandstone,
reflect differences in theamount of carbonate: intervals low in
carbonate correspond to highmagnetic susceptibility and intensity.
Unit XV, a partially lithifiedsandy volcanic siltstone with chalk
clasts, displays a wide range ofsusceptibility and intensity values
as well as varying IRM QOQ/Kratios.
Cores from Hole 829C are described as structureless clayey
sandyvolcanic silt with some poorly defined ash layers (Collot,
Greene,Stokking, et al., 1992). Trends in magnetic susceptibility
and intensityare similar: both suggest a gradational variation in
the contribution ofvolcanogenic material. Peaks correspond to
intervals described ascontaining ash layers. IRM1000/X" ratios tend
to increase downhole,which corresponds to the observed decrease in
grain size from clayeysandy volcanic silt to clayey volcanic silt
(Collot, Greene, Stokking,et al., 1992). Lower IRMIOOO/K ratios
near the top of Unit XVIII inHole 829C correspond to clay-rich
intervals; higher ratios near thebase of the unit reflect increases
in the amount of sand. Fluctuationsin the IRM1000/K ratio reflect
the variable amounts of clay and sandreported by shipboard
sedimentologists.
Two major lithostratigraphic units were defined and describedat
Site 830 (Collot, Greene, Stokking, et al, 1992; Table 1).
Litho-stratigraphic Unit I is a dark-gray volcanic silt and
siltstone that issubdivided into three subunits based primarily on
the presence and
-2.40
-2.45 -
-1.5
-3..2 1.4 1.6 1.8 2.0 2.2 2.4
log SIRM0.0 0.5 1.0 1.5 2.0 2.5
log SIRMFigure 12. Log K vs. log IRM1Ooo
character of sandy interbeds. Subunit IA consists of nearly
structure-less unlithified silt and contains ash layers 1 to 5 cm
thick. Carbonateis more abundant in the upper part of the subunit.
The decrease incarbonate downhole is reflected by an increase in
magnetic suscepti-bility and intensity. A decrease in IRM1000/^
near the base of Sub-unit IA corresponds to an increase in the
frequency of sandy layers.Subunit IB is clayey sandy silt with
several normally graded interbedsof black sand and an increased
proportion of volcanogenic minerals,notably Plagioclase,
clinopyroxene and opaque minerals. Subunit ICis a sequence of
interbedded sandy silt and sand that contains lessclinopyroxene and
opaque minerals than Subunit IB. Variations inIRM1000//C also
reflect grading observed in Subunits IB and IC. Anincrease in the
contribution of volcanogenic sediment is reflected byan increase in
magnetic susceptibility and intensity in Subunit IB. UnitII
comprises partially lithified, very poorly sorted, very coarse,
vol-caniclastic, silty sandstones. The sand grains in the sandstone
areparticles of well-lithified volcanic siltstone. The substantial
scatter inall rock magnetic properties from Unit II samples results
from thewide range of particle sizes observed in the unit.
Samples from Hole 829A define a fairly linear trend in a plot
oflog K vs. log IRM1000, although some scatter is apparent.
Samplesfrom Holes 829C and 830A and B lie along linear trends in
log K vs.log IRM1000 plots.
Fabric: AMS and SEM Observations
Anisotropy parameters listed in Table 4 vary widely, even
betweenvery closely spaced samples. Figure 10 is a plot of
lineation (L) vs.foliation (F). The straight line drawn on the plot
indicates a slope of
485
-
L.B. STOKKING, D. MERRILL, X. ZHAO, P. ROPERCH
unity and forms the boundary between ellipsoids that are
predomi-nantly prolate and those that are oblate (Hrouda, 1982).
The magneticfabric of Samples 134-829A-12R-1, 24 cm, -43R-2, 27 cm,
and-43R-2, 69 cm, is thus prolate and dominated by lineation,
whereasthat of the remaining samples is oblate, and dominated by
foliation,as is the structural fabric (Meschede and Pelletier, this
volume).Scatter of anisotropy parameters was also observed in
sediments fromabove the décollement in the Nankai Trough, and was
attributed tostrain inhomogeneity; below the décollement at Nankai,
magneticfabric was predominantly oblate (Owens, 1993).
Even in intensely deformed sediments, SEM observations
revealregions within individual beds in which deformation and
parallelorientation of clay particles is not as apparent at high
magnifications(e.g., 2000X) as it is at low magnifications (
-
ROCK MAGNETIC PROPERTIES, MINERALOGY, AND FABRIC
Levi, S., and Merrill, R.T., 1976. Acomparison of ARM and TRM in
magnetite.Earth Planet. Sci. Lett., 32:171-184.
Lowrie, W., and Fuller, M., 1971. On the alternating field
demagnetizationcharacteristics of multidomain thermoremanent
magnetization in magnet-ite. J. Geophys. Res., 76:6339-6349.
Mallick, D.I.J., and Greenbaum, D., 1977. Geology of Southern
Santo. Reg.Rep.—New Hebrides Condominium Geol. Surv.
McNeill, D.F., 1990. Biogenic magnetite from surface Holocene
carbonatesediments, Great Bahama Bank. /. Geophys. Res.,
95:4363^371.
Mitchell, A.H.G., and Warden, A J., 1971. Geological evolution
of the NewHebrides island arc. J. Geol. Soc. London,
127:501-529.
Moon, T, and Merrill, R.T., 1986. Magnetic screening in
multidomain mate-rial. J. Geomagn. Geoelectr., 38:883-894.
O'Reilly, W., 1984. Rock and Mineral Magnetism: New York
(Chapman andHall).
Owens, W.H., 1993. Magnetic fabric studies of samples from Hole
808C,Nankai Trough. In Hill, LA., Taira, A., Firth, J.V., et al.,
Proc. ODP, Sci.Results, 131: College Station, TX (Ocean Drilling
Program), 301-310.
Petersen, N., von Dobeneck, T, and Vali, H., 1986. Fossil
bacterial magnetitein deep-sea sediments from the South Atlantic
Ocean. Nature, 320:611-615.
Sager, W.W., and Hall, S.A., 1990. Magnetic properties of black
mud turbiditesfrom ODP Leg 116, Distal Bengal Fan, Indian Ocean. In
Cochran, J.R.,Stow, D.A.V., et al, Proc. ODP, Sci. Results, 116:
College Station, TX(Ocean Drilling Program), 317-336.
Soffel, H.C., 1971. The single-domain/multidomain transition in
intermediatetitanomagnetites. J. Geophys., 37:451^-70.
Stacey, F.D., and Banerjee, S.K., 1974. Developments in Solid
Earth Geophys-ics (Vol. 5): The Physical Principles of Rock
Magnetism: Elsevier (NewYork).
Stoner, E.C., and Wohlfarth, E.P., 1948. A mechanism of magnetic
hysteresisin heterogeneous alloys. Philos. Trans. R. Soc. London A,
240:599.
Sugiura, N., 1979. ARM, TRM and magnetic interactions:
concentrationdependence. Earth Planet. Sci. Lett., 42:451^455.
Tarling, D.H., 1983. Paleomagnetism, Principles and Applications
in Geol-ogy, Geophysics and Archaeology: London (Chapman and
Hall).
Thompson, R., and Oldfield, F, 1986. Environmental Magnetism:
London(Allen and Unwin).
Xu, S., and Dunlop, D.J., 1993. Theory of alternating field
demagnetization ofmultidomain grains implications for the origin of
pseudo-single domainremanence. J. Geophys. Res., 98:4183-4190.
Xu, S., and Merrill, R.T., 1990. Toward a better understanding
of magneticscreening in multidomain grains. J. Geomagn. Geoelectr.,
42:637-652.
Date of initial receipt: 14 May 1992Date of acceptance: 2
September 1993Ms 134SR-026
487
-
L.B. STOKKING, D. MERRILL, X. ZHAO, P. ROPERCH
Acm
7 0 -
7 5 -
8 0 -
Ccm
65 -
Bcm
25 -
3 0 -
35 -
70 -
7 5 -
80 -J
Dcm
2 0 -
2 5 -
3 0 -
Plate 1. Core photographs illustrating typical sediments and
rocks from Leg 134. A. Volcanicsiltstone with ash layer (Interval
134-830A-3H-3, 70-81 cm). B. Deformed breccia
(Interval134-829A-43R-1,22-39 cm). C. Volcanic sandstone (Interval
134-830C-7R-1,63-80 cm). D. Chalk(Interval 134-829A-51R-1, 18-33
cm).
-
ROCK MAGNETIC PROPERTIES, MINERALOGY, AND FABRIC
Plate 2. SEM photomicrographs illustrating the fabric of
shear-zone sam-ples. A. Sample 134-829A-43R-1,144-146cm,
describedinCollot, Greene,Stokking, et al. (1992) as a highly
deformed breccia. The scale bar is 1 mm.B. Region in center of
right flank of fold in upper-right corner of A. Scale baris 10 µm.
C. Sample 134-829A-47R-1, 55-57 cm, a brown volcanic clay inwhich
wavy lamina were observed (Collot, Greene, Stokking, et al.,
1992).The scale bar is 100 µm.
-
L.B. STOKKING, D. MERRILL, X. ZHAO, P. ROPERCH
Sample 829A-6R-4,110 cm Sample 829A-6R-4,110 cm
Plate 3. SEM photomicrographs illustrating titanomagnetites
separated from volcanic silts. A, B. Sample 134-827A-9H-1, 118 cm.
C, D. Sample 134-829 A-6R-4, 110 cm.
490