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ArticleVolume 5, Number 1230 December 2004
Q12012, doi:10.1029/2004GC000793

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
Published by AGU and the Geochemical Society

ISSN: 1525-2027

Development of oceanic detachment and asymmetricspreading at the Australian-Antarctic Discordance
Kyoko Okino
Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan(okino@ori.u-tokyo.ac.jp)

Kohei Matsuda
Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan
Now at Japan Meteorological Agency, 1-3-4 Otemachi, Chiyoda, Tokyo 100-8122, Japan(kohei.matsuda@met.kishou.go.jp)

David M. Christie
College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Administration Building,Corvallis, Oregon 97331-5503, USA (dchristie@coas.oregonstate.edu)

Yoshifumi Nogi
National Institute of Polar Research, 1-9-10 Kaga, Itabashi, Tokyo 173-8515, Japan (nogi@nipr.ac.jp)

Kin-ichiro Koizumi
Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan(koizumi@ori.u-tokyo.ac.jp)[1] The largest known oceanic detachment terrains occur in Segment B3 of the Australian-Antarctic
Discordance (AAD). Using newly collected bathymetry, magnetic, and gravity data, we show that Segment
B3 is divided into two contrasting second-order segments. The western subsegment, B3W, is characterized
by well-ordered, ridge parallel abyssal hills and low mantle Bouguer gravity anomalies. The eastern
subsegment, B3E, displays rough, chaotic morphology and includes several megamullions characterizedby high mantle Bouguer gravity anomaly values. The crust is estimated to be thinner by a maximum of
3 km in southern B3E. The combination of chaotic morphology with thinner crust supports the idea that themegamullions are exposed footwalls of oceanic detachments. Megamullion terrains are characterized byhigher magnetization than adjacent terrains, most likely as a result of serpentinization of peridotite exposedat the detachment surfaces. Detachment surfaces constitute up to 70% of the total area of both ridge flanks
younger than 2 Ma in B3E, indicating that oceanic detachments have played a major role in its
development. Spreading in B3E has been extremely asymmetric, with higher apparent rates associated withthe large detachment surfaces, where up to 75% of the total extension occurred. Similar asymmetricspreading on oceanic detachments is also recognized in Segment B4, suggesting that this is the dominantmode of extension associated with cold mantle and low magma supply in this deepest part of the AAD,
where it is confined to a mere 100-km-long section of the AAD spreading axis.
Components: 11,394 words, 11 figures, 1 table.
Keywords: detachment; gravity anomaly; magnetic anomaly model; megamullion; mid-ocean ridge; morphology.
Index Terms: 3035 Marine Geology and Geophysics: Midocean ridge processes; 3045 Marine Geology and Geophysics:
Seafloor morphology and bottom photography; 3005 Marine Geology and Geophysics: Geomagnetism (1550).
Received 5 July 2004; Revised 17 October 2004; Accepted 11 November 2004; Published 30 December 2004.

Copyright 2004 by the American Geophysical Union

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Okino, K., K. Matsuda, D. M. Christie, Y. Nogi, and K. Koizumi (2004), Development of oceanic detachment andasymmetric spreading at the Australian-Antarctic Discordance, Geochem. Geophys. Geosyst., 5, Q12012, doi:10.1029/
2004GC000793.

1. Introduction[2] The Australian-Antarctic Discordance (AAD)
is a unique portion of the world mid-ocean ridge
system. The AAD is characterized by its anomalously deep seafloor compared with the standard
age-depth curve [Hayes, 1988; Marks et al., 1990]
and by its rugged, chaotic seafloor morphology[Christie et al., 1998]. Within the AAD, a major
isotope boundary within the upper mantle [Klein etal., 1988; Lanyon et al., 1995; Pyle et al., 1992;
Christie et al., 2004] divides the Indian MORB
province to the west, from the Pacific MORB
province to the east. Within the AAD, the ridge
axis resembles a typical slow-spreading feature
with an axial rift valley [Palmer et al., 1993],
though the full spreading rate is high intermediate(72mm/yr, [Demets et al., 1994]). Elevated abundances of highly incompatible elements in basalt
lavas indicate that low solidus pressures and low
extents of melting characterize the mantle beneath
the AAD [Klein et al., 1991], resulting in a thinnercrust, as interpreted from with observed positive
residual mantle Bouguer anomalies [Cochran etal., 1997; West et al., 1994] and seismic observations of anomalously thin crust [Kojima et al.,
2003; Tolstoy et al., 1995]. Off-axis, the depth
anomaly forms a large, west-pointing, arcuate
shape, indicating that the anomalous lithosphere
formation has existed for at least 25 m.y. and has
migrated westward at a rate of 15 mm/yr relative
to the spreading center [Cochran and Talwani,
1977; Marks et al., 1991]. All these observations
support the presence of a cooler, more viscous
mantle beneath the AAD [Kuo et al., 1995; Markset al., 1990, 1991; West et al., 1997]; however,
the origin of this thermal anomaly is still controversial. Hypotheses that incorporate direct effects
from the mantle flow pattern include: convective
downwelling [Hayes, 1988; Klein et al., 1988;
Kuo et al., 1995; Veevers, 1982; Weissel andHayes, 1974]; reduced upwelling [Kuo, 1993;
Kuo et al., 1996]; cold spot [Hayes, 1976];
and passive along-axis flow [West et al., 1997].
Gurnis et al. [1998] attributed the origin of thecold mantle to a stagnant slab that subductedbeneath the Gondwanaland-Pacific margin during
the Cretaceous era.

[3] The anomalous features observed in the AAD
are closely related to the underlying mantle structureand dynamics. The thermal anomaly may restrain
the melt production beneath the AAD and cause thethin oceanic crust. Spreading rate is in general animportant parameter controlling ridge characteristics, but, despite its relatively high spreading rate,
the AAD shows magma-starved features that are
more usually observed in areas of slow or ultraslow
spreading. Therefore another key parameter, thebalance between the melt supply affected by thermal
anomalies and the plate motion, may play an important role in this anomalous region. The rugged
morphology in the area may be due to magmastarved extension [Christie et al., 1998]; however,the detailed tectonics of the axial zone has not beenclarified. Recent studies show that an importantmode of seafloor spreading at slow and ultraslow
spreading ridges is asymmetric extension along
long-lived, possibly low-angle, normal detachment
faults [Lagabrielle et al., 1998]. These oceanic
detachments generally appear as domed highs, withflow line parallel corrugations (megamullions), on
which lower crust and/or upper mantle materials are
exposed. Oceanic detachment faults are analogous
to those that expose metamorphic core complexes
on land [Davis and Lister, 1988]. This series offault-bounded tectonic blocks that evolve duringlong-lived detachment faulting at mid-ocean ridges
have been called oceanic core complexes. In thispaper, we use the descriptive term ‘‘megamullion
terrain’’ to indicate a domed high with a corrugatedsurface. More than 20 such features have beenreported along the Mid-Atlantic Ridge [Cann etal., 1997; Fujiwara et al., 2003; MacLeod et al.,
2002; Tucholke et al., 1998] and the Southwest
Indian Ridge [Searle et al., 2003]. Proposed regionsof origin for sole-out or rooting of oceanic detachment faults include the brittle-plastic transition[Tucholke et al., 1998], melt-rich zones within thelithosphere [Dick et al., 2000], and shallow serpentinization fronts [Escartı́n et al., 2003]. Exposureson detachment fault surfaces may provide important
information on the composition and structure of thelithosphere, and more certainly on tectonic andmagmatic processes in magma-starved environments. The AAD is unique in the areal extent andlongevity of megamullion terrains, so it is one of the
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best places to study oceanic detachments. Moreover,
the relatively high spreading rate in the AAD allows
us to study the spreading rate dependence of thephenomenon and provides higher spatial/temporal
resolution, potentially allowing for more detailed
knowledge of the formation and evolution of theoceanic detachments.
[4] In this paper, we present new morphology,
magnetic and gravity data, mainly from Segment
B3 of the AAD. Using magnetic and gravity data,
we examine the variability of spreading rate andcrustal thickness, respectively, during the last 2 m.y.
We show that the magma-starved spreading is
restricted to the relatively short (100-km) sectionof the AAD that includes Subsegment B3E andSegment B4. Within this section, spreading is
extremely asymmetric and oceanic detachments
play a key role in the formation of the anomalous
lithosphere.

2. Regional Background[5] The AAD is a part of the Southeast Indian
Ridge (SEIR), which has separated Antarctica from
Australia since the Late Cretaceous (Figure 1a).
The oldest magnetic lineation between Antarctica
and Australia is identified as chron 34 [Mutter etal., 1985], and the breakup between two continents
began between 110 and 90 Ma [Cande and Mutter,1982]. The anomalous, rough seafloor of the AAD
has been recognizable since chron 21/24 period(50 Ma). The current spreading rate and direction
at 125°E are estimated to be 71.6 mm/yr and 11.1°,
respectively, on the basis of the global plate motion
model NUVEL-1A [Demets et al., 1994]. The
current absolute motion of the Australian Plate in
the hot spot reference frame is about 83 mm/yr indirection 11°, in contrast to the almost stationary
Antarctic Plate, so the Southeast Indian Ridge ismigrating northward from Antarctica at about
35 mm/yr [Gripp and Gordon, 1990].
[6] Weissel and Hayes [1971] first divided theSEIR south of Australia into three zones, Zone
A, B, and C from east to west (Figure 1a). Zone A
and Zone C west of 116°W show ‘‘fast spreading’’ features, with no axial rift valley and relatively smooth off-axis morphology. Zone B, from
120°E to 128°E, is characterized by deep seafloor,high off-axis bathymetric relief and prominent
axial valleys, and has therefore been called theAustralian-Antarctic Discordance (AAD) [Hayes
and Conolly, 1972]. Five segments are recognizedwithin Zone B, from the westernmost B1 to the10.1029/2004GC000793

easternmost B5 [Vogt et al., 1983]. The axial zone
between B3 and B5 was mapped by Palmer et al.
[1993]. Segments B3 and B4 are offset by the
Lutana nontransform discontinuity, while the othersegment boundaries are sharp transform faults. Prior
to this study, off-axis areas have been mapped onlyin Segments B4 and B5 [Christie et al., 1998]. The
B4 off-axis area is extremely anomalous, showing
chaotic bathymetry including two megamullion
terrains. In Segment B5 a west pointing V-shaped
boundary marks the transition between the chaotic
AAD and the well-ordered abyssal hill terrain ofZone A. There have been no off-axis surveys in thewestern AAD, so the western limits of the chaotic
terrain and of anomalous crustal accretion were notknown prior to this study.
[7] A number of previous studies contribute to theconclusion that anomalously cool upper mantle andthin crust exist beneath the AAD. Geodynamic
studies by [Kuo et al., 1995; Marks et al., 1991;
Marks and Stock, 1994; West et al., 1994] haveindicated that the mantle beneath the AAD isabout 150°C colder than average using gravity/
topography ratios. Numerical modeling [West etal., 1994] shows that a small decrease in upper
mantle temperature will produce a significantly (atleast 3 km) thinner crust. Refraction seismic studies
in the AAD have also indicated an anomalous
crustal structure. Tolstoy et al. [1995] showed that,within Segment B5, the crust is quite thin (4.2 km),with a relatively thick layer 2 (3 km) and an
unusually thin layer 3 (1.2 km). A more recent
refraction experiment [Kojima et al., 2003] showed
an anomalous velocity model in Segment B4, where
the P wave velocity gradually increases with depth,
reaching 7.8 km/s at only 3.6 km.

3. Data Acquisition and Processing[8] Swath bathymetry, gravity and magnetic data
for Segment B3 of the AAD were acquired in 2002
on board the R/V Hakuho-Maru (KH01-3, Leg. 3).
The track lines were laid out NNE-SSW along flowlines, extending to 75 km off-axis on both ridge
flanks (Figure 1b). Track spacing was 5 nautical
miles, yielding almost complete bathymetric coverage. The survey area covers the whole of B3, fromthe Warringa transform fault at 124°E to the Lutana
nontransform discontinuity (NTD) at 125°050E.

3.1. Bathymetry[9] Swath bathymetry data were collected by the
SeaBeam 2120 system, which comprises 110 beams
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Figure 1. (a) (inset) The plate tectonic setting and the location of the AAD. Yellow and green lines indicate
divergent and transform-fault boundaries, respectively. CIR, Central Indian Ridge; SEIR, Southeast Indian Ridge;
PAR, Pacific-Antarctic Ridge. (b) Bathymetry around the AAD. Our newly obtained data (B3) are combined withprevious multibeam survey data [Christie et al., 1998; L. Géli, personal communication, 2000] and ETOPO2(National Geophysical Data Center, 2001). The AAD (Zone B) is divided into five segments, B1 to B5. Thin blacklines with line numbers indicate the KH01-3 track lines used in this study. Solid circles show ODP Leg 187 drill sites.
Thin black dashed lines with numbers are isochrons by Müller et al. [1998]. Thick gray dotted line is the 400-m depth
anomaly contour by Marks and Stock [1994]. Pink box marks the area shown in Figures 2, 3, 6a – 6f, and 9.

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okino et al.: australian-antarctic discordanceat the surveyed depths operating at 20 kHz. The
horizontal resolution ranges from 70 to 120 m, andthe average swath width is 11 km. The soundvelocity correction used real-time data from thesurface water velocity meter and the results of CTD
and XBT observations in the area. We removed
extreme depth variations from the outer edge of theswath using MB system [Caress and Chayes,
1996]. The chosen grid size is 100 m and the grid
data set is masked to approximately the original
swath coverage allowing interpolation of small
gaps within 1 km. A complete bathymetry gridwithout gaps was also created to do the Fouriertransform analysis described below, by merging thecompiled bathymetry of previous surveys [Christie
et al., 1998; L. Géli, personal communication,
2000] and ETOPO2 (National Geophysical Data
Center, 2001). No adjustment among three data
sets was done during compilation, for the average
difference between newly collected data and datafrom previous surveys is less than 10 m. Depths
from ETOPO2 shows larger disagreement withship-derived data (>60 m), then the areas without
ship bathymetries were masked in results.

3.2. Gravity[10] Gravity data were acquired using the NIPRORI type shipboard gravity meter. Drift and Eötvöscorrections were made and the free-air gravity
anomaly was calculated subtracting the standard
gravity using Gravity Formula 1980. The maximum and root-mean square crossover errors were
8.1 mGal and 3.5 mGal, respectively. Satellite
gravity data [Sandwell and Smith, 1997] weremerged to fill the data gap, and a 1 km spacing
grid was created.
[11] In order to estimate the crustal thickness and/or subsurface density variations we calculated
mantle Bouguer anomalies (MBA) by subtractingfrom free-air gravity anomaly the predicted gravity
effects of seafloor relief and a 4 km thick model
crust [Kuo and Forsyth, 1988; Lin and Phipps
Morgan, 1992; Parker, 1972; Prince and Forsyth,1988]. Assumed water/crust and crust/mantle
density contrasts are 1770 kg/m3 and 600 kg/m3,
respectively. The effect of sediment was neglected,
for the sediment thickness is negligible at the active
spreading axis. We further calculated residual mantle Bouguer anomalies (RMBA) to remove theeffects of lithospheric cooling. The thermal model
was calculated using the three-dimensional passive
upwelling model of [Phipps Morgan and Forsyth,1988], where the transform effect was incorporated.

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The resulting temperature field was converted into
density variation [Prince and Forsyth, 1988], andthen the gravity anomaly caused by density variation on each model subsurface layer was calculated
by upward continuation. We also estimated amodel of crustal thickness variation by downward
continuation of RMBA to an assumed mean depthof 8 km (4 km of average water depth plus 4 km
crust) [Kuo and Forsyth, 1988]. A cosine tapered
low-pass filter between 25 km and 35 km wasapplied to reduce the noise. The MBA, RMBA andcrust thickness variation assuming a 6 km thickcrust were also calculated for discussion.

3.3. Magnetism[ 12 ] The magnetic data were acquired every
30 seconds using a surface-towed proton precession magnetometer. The data were corrected forthe International Geomagnetic Reference Field[Mandea et al., 2000] to obtain magnetic anomalies. On three of the five survey days, the magnetic
data suffered from geomagnetic disturbances whenKp (mean value of the geomagnetic field disturbance levels in the two horizontal field components
observed at selected stations) sum index values
were 28, 28+, 23+, respectively. Crossover errors(total 17 points) were rather large, presumably due
to the disturbance; the maximum crossover error
was 125 nT and the root-mean square error was
52 nT. The closest available magnetic observatory
is at Macquarie Island (54.5°S, 158.95°E) 2500 km
east of the study area, so an appropriate diurnal
correction could not be obtained by directly using
the data from the station. We shifted the Macquarie
station magnetic data, taking the time lag at the
AAD into account, and then calculated our geomagnetic variation model using a stochastic inversion to minimize crossover errors. This correction
reduced the maximum and root-mean square crossover errors to 25 nT and 10 nT, respectively. No
previous off-axis ship-derived magnetic data were
known in segment B3, and the aeromagnetic datain this area [Vogt et al., 1983] show good correlation with the corrected magnetic anomaly pattern.
[13] We compared the corrected magnetic anomalies with synthetic anomaly profiles and identified
a sequence of magnetic anomalies on the basis of
the Cande and Kent [1995] timescale. The identified anomaly chrons and their ages are listed inTable 1. We linked the anomaly picks on eachprofile to draw the isochrons, and interpolated
along flow lines to create a seafloor age grid fromwhich spreading rates were calculated. The degree
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Table 1. Identified Geomagnetic Anomalies
Polarity Chron

Age, Ma

c1o (Brunhes/Matuyama boundary)c1a (Jaramillo subchron)c2 (Olduvai subchron)c2ay (Matuyama/Gauss boundary)c2a-1 (Kaena subchron)c2a-2 (Mammoth subchron)c2ao (Gauss/Gilbert boundary)c3-1 (Cochiti subchron)

0.780
1.030
1.860
2.581
3.075
3.275
3.580
4.235

of spreading asymmetry is calculated for each flankas the ratio (expressed as percentage) of the area
accreted on that flank to the total accreted area for agiven age interval. Thus conjugate values from
opposite flanks should sum to 100%. An asymmetry ratio of 50% equates to symmetrical spreading,while conjugate values of 0 and 100% equate tocomplete asymmetry.
[14] We calculated crustal magnetization to remove
skewness and to correct for effects of bathymetry
using the three-dimensional inversion method of[Parker and Huestis, 1974] and [Macdonald et al.,1980]. The direction of seafloor magnetization
was taken to be the same as the geocentric axial
dipole field, and a 500 m magnetized layer thickness was assumed. The upper surface of themagnetized layer was assumed to be the same as
the seafloor, for the sediment thickness of the area
is very thin. A Taylor expansion up to the fifth
order was carried out, and ten iterations brought
sufficient convergence of the solution. A bandpass filter, cosine tapered at short wavelength
between 8 to 10 km and at long wavelength
between 260 and 300 km, was applied to each
iteration to stabilize the solution. Sufficient annihilator was added so as to give about equalamounts of positive and negative magnetization
amplitude and to give appropriate positions ofgeomagnetic reversals. The aeromagnetic data
[Vogt et al., 1983] were downward continued
and merged with our onboard data set in themarginal areas to create a grid data without gaps
for Fourier transformation.

4. Results
4.1. Morphology[15] The shaded relief map is illuminated from thenorth with a 100 m contour interval (Figure 2)
and the tectonic fabric extracted from the swath

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bathymetry is shown in Figure 3. The ridge axis
is characterized by a 5- to 15-km-wide, 1500- to
2000-m-deep axial rift valley, which is anomalous
for the high intermediate spreading rate. A rightstepping, 8-km nontransform offset at 124°350E
separates second order segment B3W from B3E.
The B3W and B3E axes overlap by 15 km andthe eastern end of the B3W axis is accompanied
by eastward-pointing, V-shaped pseudofaults indicating rift propagation. Minor ridge structures
100 m high within the rift valley, that areinterpreted as neovolcanic zones, are developedin B3W and in the western half of B3E (Figure 4).
The off-axis trace of the nontransform discontinuity(NTD) between B3W and B3E extends across theentire survey area as a highly sinuous, deep region.
The sinuous nature of this trace indicates repeated,‘‘dueling’’ decline and growth of the two subsegment axes. The maximum variation in subsegment
length is about 40 km, and the average periodicityof the fluctuation is about 0.4 m.y.
[16] The morphology of the off-axis area of B3W
contrasts in a striking way with that of B3E. The
B3W off-axis area consists of well-ordered, riftparallel abyssal hills. The average water depth of
the southern flank is shallower (3400 m) thanthat of the northern flank (3700 m). The wavelength and relief of the abyssal hills increase as
they step away from the current rift axis, whichmost likely reflects the continued growth of normal
faults [Macdonald et al., 1996]. In contrast to thenormal abyssal hills of B3W, the off-axis morphology of B3E is highly anomalous. Seafloor relief ishigh and variable and there is no clear correlation
between depth and distance from the axis within
the survey area. The seafloor fabric is a chaotic
mixture of domes, with rift-parallel and rift-obliqueabyssal hills and several megamullions (Figure 5).
Most previously reported megamullions are locatedat inside corners of slow-spreading axial-valleytransform intersections. In contrast, megamullions
within the AAD, including those of B3E shown
here, and those of Segment B4 [Christie et al.,1998] are distributed across the entire (sub)segment. Although the B3E megamullions are developed both on southern and northern flanks, thesouthern megamullions are larger and the corrugations are more continuous.
[17] The largest B3E megamullion, at the easterninside corner, has continuous corrugations 50 
55 km long. It consists of two flow line parallel
bathymetric highs with prominent, smaller-scalecorrugations on their surfaces (Figure 5a). This is
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Figure 2. Shaded relief with bathymetry contours in segment B3. Contour lines are drawn only for the area of the
newly obtained data set. Contour interval is 100 m. Subsegments B3W and B3E show strong contrast in morphology.
Gray lines indicate the locations of across- and along-axis profiles shown in Figure 7.
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Figure 3. Geological interpretation and identification of magnetic anomalies superimposed on shaded bathymetry.
Bathymetry is illuminated from directions 015° and 285°. Thick red lines are rift axes with neovolcanic zones; redwhite lines are rift axes without neovolcanic zones. Blue lines indicate transforms/fracture zones or nontransform
discontinuities (NTDs). Areas with well-developed, flow line parallel corrugations are surrounded by white lines.
Green lines with labels show the identified magnetic anomalies (see Figure 9).

the largest known megamullion in the AAD, and islarger than any known Mid-Atlantic Ridge megamullion [Tucholke et al., 1998]. The termination ofthis megamullion (for terminology, see Tucholke
et al. [1998]) at its northern end coincides withthe axial rift valley (Figure 4), suggesting that the
detachment fault is still active. Farther from theinside corner, an adjacent group of smaller mullion
structures is located next to the largest megamullion terrain. This group consists of several small
corrugated domes, each about 10 km wide (E-W)
and 5–10 km long (N-S), that are cut by three orfour rift parallel escarpments (Figure 5b). To thesouth of this area, a 1000 m high, rift-parallel ridge
marks the breakaway for this terrain. To the north,the corrugations terminate at a distinct E-W lineament marking the termination of the detachment
fault. Three or four normal abyssal hills separate
the termination from the neovolcanic zone in thiswestern part of B3E. The boundary between these
two megamullion terrains on the southern flankappears to coincide with a minor, third ordersegment boundary. On the north flank, this boundary can also be traced as a sinuous deep. South of
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Figure 4. Three-dimensional view of the southern flank of B3E from 085° direction. Area surrounded by graydotted line is the megamullion terrain in southern B3E. Minor ridge structure within the rift valley is interpreted as aneovolcanic zone, which is indicated as NVZ in the figure. Vertical exaggeration is 2.0.

Figure 5. Close-up view of megamullions (a and b) on southern flank of B3E and (c) on northern flank of B3E.
Color scale is as same as that used in Figure 2. Bathymetry is illuminated from direction 285°.
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49°400S, a megamullion terrain consisting of four
10 km wide, 500 m high domes extends E-W
across B3E and south beyond the survey area.
[18] The megamullion terrains on the southernflank of B3E are all characterized by closely spaced
corrugations of 100- to 200-m wavelength, 50-m
relief, and 5- to 10-km minimum length. Megamullion terrains also occur on the northern flank of
B3E, but they are relatively small and widely
separated. Each consists of a single dome, 10–
20 km across and 1000–1500 m high (Figure 5c).
They are sparsely corrugated, randomly distributedand separated by rift-parallel abyssal hills. Breakaway and termination traces are ambiguous. In allthese characteristics, they more closely resemble
megamullion terrains along the slow-spreading
Mid-Atlantic Ridge.

4.2. Gravity Anomalies[19] The free-air, mantle Bouguer and residual
mantle Bouguer gravity anomalies are shownwith estimated variations of crustal thickness
in Figures 6a–6f. The free air anomaly pattern(Figure 6a) correlates well with bathymetry. The
mantle Bouguer anomalies (MBA) are about
10 mGal higher in B3E than in B3W (Figure 6b).
In both subsegments the MBA decreases from thesegment ends to the center. In B3E a weak negative
trough is confined to the western (magmatic) partof the rift valley. The along-axis variations in MBA
values within subsegments B3W and B3E are
17 mGal and 13 mGal, respectively (Figure 7a).
MBA values gradually increase away from theaxis, mainly due to the effect of plate cooling.
[20] The residual mantle Bouguer anomaly (RMBA)
is calculated to remove the effect of plate cooling(Figure 6c). Our calculated RMBA values decreaseaway from the axis; this tendency is likely due toan overestimate of plate cooling rates and/or ourdisregard of buoyancy effects. The RMBA ismarkedly higher in B3E, where the high RMBA
(30 mGal) area is located south of the axis, near
the termination of the western B3E megamullion
terrain. The MBA values in the megamullion on thesouthern flank of B3E are approximately same as
those at same distance from the axis in the northernflank, therefore the high RMBA is consistent withasymmetry of plate cooling, which may indicate
asymmetry of spreading rate described in next
section. However, the megamullion terrains havegenerally higher Bouguer anomalies, with RMBA
values 510 mGal higher than in surrounding
areas. The high gravity anomalies associated with

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megamullions are also clearly shown in across-axis
profiles (Figures 7b and 7c).
[21] Assuming that variations in RMBA are caused
only by differences in crustal thickness, we estimated variations of crustal thickness (Figure 6d).
The crust of B3W is, on average, thicker than that of
B3E. In northern B3W, the crust is estimated to be
thicker by a maximum of 3 km. The megamullion
terrains of southern B3E and those of northern B3E
near the NTD have thinner crust. Thinner crust isalso observed along the Lutana NTD and close to
the axis at the 124°400 (B3W/B3E) NTD. The
Warringa Transform Fault does appear to havethinner crust, but this may be due to the poor data
coverage. Estimates of crustal thickness involve
significant uncertainty, because gravity anomalies
can also be caused by upper mantle temperature anddensity variations, density decreases associatedwith serpentinization or other alteration and otherfactors. The 3 km of crustal variation was obtained
as a result of downward continuation of RMBA
using 600 kg/m3 density contrast across the Moho,so the gravity anomaly can also be explainedby different layer thickness and density contrast
models, for example assuming serpentinized mantle
material for subsurface layer.
[22] The MBA, RMBA and crustal thickness variation assuming a 6 km thick crust were also
calculated (Figure 6e and 6f). The difference of areference depth does not change any of the basic
results of the analysis. The amplitude of the crustal
thickness variation is larger in a 6km thick crustmodel, though the general pattern is as same as in a
4 km thick crust model. We do not have any seismic
profiles in segment B3 to examine the Moho
depth derived from gravity analysis; however, a
4-km-thick crust model is preferable because theprevious refraction results in B4 [Kojima et al.,2003] and B5 [Tolstoy et al., 1995] shows 3.6 
4.2 km crust thickness.

4.3. Magnetic Anomalies[23] We used magnetic isochrons identified fromthe magnetic anomaly map to calculate the detailed
history of plate motion and variations in spreadingrate. Model profiles allowing asymmetric spreading are shown for comparison with observed profiles (Figure 8). Good fits were achieved with little
asymmetry for profiles in B3W (lines 07-10), withpoorer fits for the southern parts of lines 01 and 02
in B3E. Anomalies are more strongly positive
values and less variable over megamullions. The
better fits along lines 01-02 were achieved by
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Figure 6. (a) Free-air gravity anomaly map. Contour interval is 5 mGal. Gray lines show rift axes and transform
fault/NTDs. White dotted circles outline megamullion terrains. (b) Mantle Bouguer gravity anomaly (MBA)contoured at 5 mGal. (c) Residual mantle Bouguer gravity anomaly (RMBA) calculated from MBA and theoreticallithosphere cooling effects. Contour interval is 5 mGal. (d) Relative crustal thickness variations calculated by
downward continuation of RMBA. Contour interval is 500 m. A 4-km-thick crust is assumed for MBA calculation.
For Figures 6e and 6f, a 6-km-thick crust is assumed for MBA calculation: (e) Mantle Bouguer gravity anomaly(MBA) contoured at 5 mGal. (f ) Relative crustal thickness variations calculated by downward continuation of
RMBA.

introducing induced magnetization; however, thiswill be discussed in a later paper. Because the line
06 repeatedly crosses the highly sinuous NTD trace,
there are large discrepancies between model andobserved profiles. Anomalies up to c2 (Olduvai

subchron) were identified on both northern andsouthern flanks of B3W and B3E. Older anomalies
from c2ao to c2 were also identified on the northernflank of B3E using a transit line. Anomalies c1o(B/M boundary) and c1a (Jaramillo subchron) were
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Figure 6. (continued)

identified on most of track lines (Figures 8 and 9a).
The average half spreading rate in segment B3 from
1.8 Ma to present is about 38 mm/yr, consistent
with the global plate motion model [Demets et al.,1994]. During this period, however, marked variations in spreading rate can be clearly recognized.
Figures 9b and 9c show variations in half-spreading
rate and spreading asymmetry, respectively. For
southern B3W, the spreading rate is almost constantat 40 mm/yr from c2 to present, while for northern
B3W, half-spreading rates increased from 30 mm/yr
between c2 and c1o to 38 mm/yr since c1o. Overall,
the southern flank has spread faster, but the degreeof asymmetry is less than 5% (Figure 9c).
[24] In contrast to the relatively small asymmetryin B3W, the spreading rate in B3E has been highlyvariable, both through time and from west to east
across the subsegment. For southern B3E, the halfspreading rate has varied consistently from west toeast as well as with age. Half-spreading rates haveincreased from 32/35 mm/yr (west/east) between
c2 and c1o, to 50/55 mm/yr since c1o. Acrossnorthern B3E, there was no west-east variationfrom c2ao to c2, but the half-spreading rate
decreased from 50 to 30 mm/yr. Between c2 andc1a, the half-spreading rate increased from west
(27 mm/yr) to east (42 mm/yr), but since that time
the half-rate has decreased from west (30 mm/yr)
to east (15 mm/yr). Prior to c1o, the degree of
spreading asymmetry was small (less than 5%),but it has increased dramatically, exceeding 20%
from c1o to present (Figure 9c). During the past one
million years, the spreading in B3E has been highly
asymmetric, with rates exceeding 50 mm/yr acrossthe megamullion terrains to the south. Throughout
this time, however, the full-spreading rate hasremained at 79–80 mm/yr, almost the same as in
the other parts of the Southeast Indian Ridge. The
intense, localized spreading asymmetry over the last
0.5 m.y. is a remarkable contrast to the generally
small asymmetry that characterizes Segment B3
through the time encompassed by the survey area.
The loci of high asymmetry generally correspond tothe major oceanic detachment surfaces, with thefaster spreading flank corresponding to the megamullion terrains. More detailed discussion and theasymmetry in the neighboring segments are written
in the next section.
[25] The magnetization intensity distribution map(Figure 9d) shows high magnetization areas along
the NTDs and in the megamullion terrains ofsouthern B3E. The magnetization at the segment
ends is strongly positive relative to the surrounding
seafloor regardless of whether polarity is normal orreversed. The high positive magnetization, both atsegment ends and across the southern B3E megamullion terrains is most likely caused by induced
magnetization associated with peridotite serpentinization [Pariso et al., 1996; Pockalny et al., 1995;
Tivey and Tucholke, 1998] and the detachment
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Figure 7. Magnetization, bathymetry, mantle Bouguer anomaly (MBA), and residual mantle Bouguer anomaly(RMBA) profiles from Segment B3. The locations of profiles are shown in Figure 2. (a) Along-axis profile from B3W
to B3E. (b and c) Across-axis profiles in B3E, crossing large mullion structures with corrugated surfaces. Horizontal
scale is different between (a) along- and (b and c) across-axis profiles.

surface. Our analysis of the magnetization structureof the area using detailed forward modeling will be
discussed in a later paper.

5. Discussion
5.1. Detachment Hypothesis[26] In recent years, asymmetric extension along
low-angle normal detachment faults has been

increasingly recognized as an important mid-ocean
ridge process, especially in magma-poor and slow
spreading systems. More than twenty seafloor
terrains interpreted as detachment surfaces havebeen reported from on- and off-axis areas along
the Mid-Atlantic Ridge (MAR) [Blackman et al.,
1998; Cann et al., 1997; Fujiwara et al., 2003;
Ranero and Reston, 1999; Tucholke and Lin, 1994;
Tucholke et al., 1998]. These features are usuallycharacterized by dome-like bathymetric highs, with
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Figure 8. Model profiles (blue) using the geomagnetic reversal timescale of Cande and Kent [1995] with observed
magnetic anomaly profiles (red) and bathymetry. We assume a constant 500-m-thick magnetic layer draped on thebathymetry with a uniform ±5 A/m magnetization. Locations of lines are shown in Figure 1.

flow line parallel corrugations and striations, anddelineated from normal abyssal hill terrains by
narrow sublinear valleys that define the ‘‘breakaway’’ and the ‘‘termination’’, the lines at which
the exposure of the detachment surface at theseafloor began and ended, respectively. Most ofthese morphological features, named megamullions,are associated with gravity highs indicating denser
subsurface structure. Dredge and drill sampling
typically returns deep-seated, gabbro and peridotite
samples, in addition to cataclastic (fault-generated)rock types and highly deformed greenschist facies
assemblages [MacLeod et al., 2002]. These obser-

vations indicate that the corrugated surface is the
exposed footwall of an oceanic detachment fault,
analogous to core complexes on land [Davis andLister, 1988]. The oceanic detachments along the
MAR are mostly located at present or remnant inside
corners of ridge-transform intersections, where themelt supply is presumed to be smaller than that at
segment centers. Very similar features have been
also reported at ultraslow spreading systems [Searle
et al., 2003], at some intermediate spreading-rate
systems [Martinez et al., 1998; Mitchell et al., 1998;
Ohara et al., 2001] and from Segment B4 of the
AAD [Christie et al., 1998].
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Figure 9. (a) Magnetic anomaly map. Contour interval is 20 nT. The anomaly profiles along track lines are shownin wiggles (positive anomalies are shaded). Red symbols mark anomalies (see Table 1) picked along ship track. Graylines show rift axes and transform fault/NTDs. (b) Half spreading rates calculated for intervals between anomaly
picks. White dotted lines outline megamullion terrains. Large variations in spreading rate are observed in B3E.
(c) Crustal accretion rates of conjugate plates, expressed in terms of the ‘‘degree of asymmetry’’ (see text). A ‘‘50%’’
asymmetry equates to symmetrical spreading. (d) Crustal magnetization calculated from magnetic anomaly
amplitudes. Contours are at 2 A/m intervals.

[27] The B3E megamullion terrains described here
are extremely anomalous in their broad areal extent
and in their duration. In B3E, domed features withprominent or subtle corrugations occupy about
70% of the seafloor younger than 2 Ma, indicatingthat ‘‘magma-starved’’ extension dominates over‘‘magmatic’’ accretion, and has been the dominantmode of spreading for at least two million years.
The largest mullion structure, at the inside cornerof the eastern end of Segment B3, is about 50 km
long, about twice the average length of megamullions in the MAR. Megamullion structures occur
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Figure 10. One hypothesis for the evolution of B3E and the origin of small detached terrains in the northern B3E(‘‘reversal of detachment polarity’’; see text). IC indicates inside corner. Green lines are isochrons. Red and blue lines
are spreading centers and nontransform/transform offsets, respectively. Megamullions are surrounded by brown lines.
The offset between B3E and B3W is shown by brown arrows.

across the width of southern B3E, in clear contrast
to the well-ordered normal abyssal hills in B3W. A
shortage of melt supply extends along the entire
B3E axis and is not limited to the segment ends.
Small, chaotic domes are also widely distributed on
the northern flank of B3E. We propose two alternative hypotheses for the origin of these small
detached terrains.

5.1.1. Origin of Detached Terrains by
Reversal of Detachment Polarity[28] The small northern megamullions may haveformed when the polarity of detachment was reversed for short periods. Like most megamullion
terrains elsewhere, the major B3E megamullion
adjoins the Lutana Discontinuity, the easternboundary of Segment B3, on the southern flank,at the inside corner of a right-stepping offset. A
small northern terrain also adjoins the Lutana
Discontinuity just after c2 chron. Along the western boundary of B3E, on the other hand, megamullion terrains are distributed on both flanks. The
southernmost megamullion terrain began forming
prior to the survey coverage and terminated justafter c2 chron. A younger southern terrain inwestern B3E began forming close to c1o andterminated relatively recently. The three small

northern megamullions in western B3E formed:
before c2, just before c1a, and between c1a andc1o. The offset between B3W and B3E is small andits sinuous off-axis trace indicates that it is unstable, migrating over 40 km from east to west as aresult of small ridge jumps or propagation events.
Each of the northern megamullions coincides witha westward excursion of the B3W/B3E boundary
trace (Figure 9), suggesting that it occurred at an
inside corner during a period when B3E was active
and growing. As the migration of the offset
reversed, the transformation from inside to outside
corner position may have terminated the active
detachment and initiated a new fault with reversed
polarity. Similar, long-lived magma-starved spreading segments with changing detachment polarity
have been reported from off-axis areas of theeasternmost Southwest Indian Ridge at 64–65° E[Cannat et al., 2003]. A possible model for segment
evolution involving such transient detachmentpolarity reversals is illustrated in Figure 10.

5.1.2. Origin of Detached Terrains by
Lithosphere Transfer During Detachment
Relocation[29] The small northern B3E megamullions may
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okino et al.: australian-antarctic discordanceferred from the southern to the northern plate as thelocus of displacement shifted southward to a newfault plane. As described in the preceding section,
the northern B3E megamullion terrains moreclosely resemble their counterparts along theslow-spreading Mid-Atlantic Ridge than those insouthern B3E. Further, they coincide in apparent
age with the southern flank megamullion terrains,with no complementary gaps on either flank. Their
origin does not seem to be attributable to a simple
detachment fault model. One possible explanationis that the northern terrains were formed duringa transient periods when extension rates wereslowing, immediately prior to deactivation of thedetachment fault plane. Initiation of a new detachment farther to the south would transfer these
localized, ‘‘slow-spread’’ terrains from the southernto the northern flank and reinstate the formation of afaster-spreading terrain on the southern flank. The
transfer of a megamullion terrain from one flank to
the other has also been proposed for the FifteenTwenty Fracture Zone area of the MAR [Fujiwara
et al., 2003].
[30] Although no direct observation was reported
on the crustal structure beneath the B3 segment, theprevious refraction studies in B4 and B5 segments
may give us some indications of the lithospherestructure and composition of the area. Tolstoy et al.
[1995] showed that the crust within Segment B5 of
the AAD is quite thin at 4.2 km. Their model
indicates that an unusually thin (1.2 km) layer 3
underlies a layer 2 of normal thickness. A more
recent refraction experiment in Segment B4[Kojima et al., 2003] yielded an anomalous velocity model, where P wave velocity gradually
increases with depth to 7.8 km/s at a depth of only
3.6 km. Their study indicated that there is nolayering structure beneath B4, and may support
the idea that a compositionally heterogeneous
lithosphere is formed when extension is dominatedby detachment faulting [Cannat, 1993]. The
amplitude of magnetic anomalies in the AAD issignificantly weaker than in the surrounding areas[Vogt et al., 1983; Weissel and Hayes, 1971]. Thisdifference was attributed to the differences in Fe
and Ti content between Zone A lavas and those ofand Zones B and C [Anderson et al., 1980], but
the discovery of megamullion terrains suggests thatthe difference is more likely attributable to theanomalous lithospheric structure.
[31] The geometry and mechanism of the detachment fault plane is an important factor in understanding how oceanic lithosphere is developed

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under magma-starved conditions. Competingmodels with implications for the geometry andthe characteristics on the fault plane at depth
include: rooting of the detachment in the brittleductile transition at the base of the lithosphere[Lavier et al., 1999; Tucholke et al., 1998]; androoting of the detachment in the shallower lithosphere where the serpentinization front may correspond to a rheological boundary and play a keyrole for fault initiation [Escartı́n et al., 2003;
MacLeod et al., 2002]. The former hypothesis
requires a steeply dipping fault plane with large
rotations during fault movement, while the latter
requires a shallow dipping fault plane with potentially much less exposure of deep sections of theoceanic lithosphere. The difference between thesemodels can potentially be resolved by gravity andmagnetic data. Although our data set is not adequate to resolve these models, a cross-axis profile
across the largest B3E megamullion (Figure 7b) is
consistent with a low-angle, shallow rooted detachment model. This profile shows low magnetizationand high MBA on the north, hanging wall side of
the axis (48.85°S  49°S), consistent with thepresence of unaltered peridotite (high density andvery low magnetization) at shallow depth.

5.2. Asymmetric Spreading[32] The recent spreading history of subsegment
B3E is dominated by extension on a detachment
fault, leading to a remarkable degree of spreading
asymmetry and the formation of a 50–55 km long
megamullion terrain in less than 1 million years. In
the same 1 million year period, only 10 km of new
ocean crust has been added to the northern flank,
and the full spreading rate has remained close to
the 70 mm/yr rate predicted by the global platemodel. Extreme asymmetric spreading is not confined to the inside corner of B3E, however. In
Figures 9b and 9c, we show that unusually highhalf-spreading rates, with asymmetry up to 75%,
coincide with some of the northern B3E megamullion terrains in addition to those of southern
B3E. Asymmetric spreading also led to a change inthe sense of the offset (from left to right/right toleft) between B3E and B3W, as described in
section 5.1 and Figure 10.
[33] Intervals during which detachment tectonics
appear to have dominated the spreading process
can also be inferred from existing data for other
AAD segments (Figure 11). For Segments B4 andB5, we have calculated spreading rates and asymmetry factors using magnetic anomaly identifica17 of 22

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a case of extremely asymmetric crustal accretionaccompanying a single, large fault near 29°N on
the MAR. They concluded that magmatic accretionin this area has been effectively one sided, withnew crust being added only to the outside corner,
causing asymmetric spreading with polarity opposite to that of the AAD. This observation, combined with the common occurrence of gabbroic
rocks on MAR detachment surfaces, suggests thatmelt production rates may remain high (relatively
to spreading rate) on the MAR, even during
episodes of detachment faulting.

Figure 11. Crustal accretion rates for Segments B3,B4 and B5. Magnetic anomaly picks are shown as black
symbols. The anomaly picks in B4 and B5 were
provided by D. Wilson (personal communication, 2001)
and were based on data collected during previous
cruises. Double lines mark the ridge axes.

tions by D. Wilson (personal communication,
2001, based on magnetic profiles collected during
previous cruises). Highly asymmetric spreading
occurs in B3E, as reported here, across B4, andfar off-axis (prior to c2) in B5 west. In B4 and B5(as in B3) high spreading asymmetry corresponds
to areas of chaotic seafloor morphology [Christie etal., 1998].
[34] In an idealized model, asymmetry should benearly 100% during intervals when extension isdominated by detachment faulting. Existing studies
outside the AAD show, however, that this is rarely
the case. For oceanic detachments along the MidAtlantic Ridge, spreading asymmetry is generally
small or absent. This appears to be partly due topoor horizontal resolution associated with slow
spreading rates and partly due to magmatic crustal
accretion on the hanging wall. Fujiwara et al.
[2003] identified the magnetic anomalies andshowed that there are significant spreading asymmetries in two segments near the Fifteen-Twenty
fracture zone. The asymmetry is, however, not solarge and the horizontal resolution is not enough tocorrelate the area of apparent fast spreading withcorrugated surfaces. Allerton et al. [2000] reported[35] In the AAD, on the other hand, melt supply
rates are uniformly low, reflecting the anomalously
cold upper mantle beneath the region. Seismic
layer 3 is very thin beneath the chaotic terrain ofsegment B5 [Tolstoy et al., 1995] and seems not toexist at all beneath B4 [Kojima et al., 2003].
Together with magnetic studies, these seismic
observations confirm that melt production duringextensional intervals in the AAD is anomalously
low, especially relative to the faster spreading rate.
This low melt supply, in turn, ensures the dominantrole of detachment faulting in the spreading process. Although our off-axis data are limited to
40 miles off-axis, corresponding to approximately
2 Ma crust, asymmetric spreading has beenrecognized throughout the history of the AAD[Cochran, 1986; Vogt et al., 1983; Weissel andHayes, 1974]. A recent global compilation ofasymmetric seafloor spreading [Müller et al.,1998] shows that the southern flank of the AAD
has spread faster, on average, at least since 10 Ma.
This long-term asymmetry must be attributed primarily to ridge migration required by the absolute
plate motions. It may, however, be accomplishedby the accumulation of episodic, short-term asymmetries that are accommodated by oceanic detachment faults.
[36] Well developed oceanic detachments are now
known to have formed at intermediate spreading
rates in at least three places in addition to the AAD:
the southernmost Central Indian Ridge (full spreading rate 50 mm/yr) [Mitchell et al., 1998]; theChile Ridge (full spreading rate 70 mm/yr)[Martinez et al., 1998]; and at an extinct back arc
spreading center in the Philippine Sea (estimatedfull spreading rate 70 mm/yr) [Ohara et al.,2001]. The large size of the Philippine Sea and
AAD megamullions suggests a tendency toward
larger megamullions at higher spreading rates,
although no detailed analysis has yet been done.
We speculate that the formation period of major
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megamullion terrains may be relatively constant(1–2 million years) at all intermediate to slow
spreading rates, implying that the mode of spreading is not simply a function of spreading rate, butof the balance between melt supply and plate
separation rate.

5.3. Extent of Chaotic Terrain[37] Christie et al. [1998] pointed out that chaotic
seafloor terrain within the AAD is a strong indicator of the dominance of tectonic extension overmagmatic accretion at the spreading axis. A keyobservation of this study is that the chaotic terrain,
and therefore the locus of asymmetric spreading
and thin crust production is presently limited to
Segments B3E and B4. Only 100 km of theaxis appears to have produced anomalous oceanic
lithosphere at least since 2 Ma. There is someuncertainty to this observation as there has been
no off-axis mapping to the west of our survey area.
However, a single multibeam swath along an E-W
track near 48°S appears to record normal abyssal
hill terrain with no indication of chaotic seafloor
topography.
[38] The eastern boundary of the chaotic terrain
was shown by Christie et al. [1998] to havemigrated westward across Segment B5 between
4 Ma and present, although this migration appears
to reflect a transient phenomenon [Christie et al.,1998, 2004]. The terrain boundary in B5 corresponds to a geochemical boundary between Indianand Pacific MORB mantle isotopic domains that iscurrently located at or near the B4/B5 segment
boundary. Leg 187 of the Ocean Drilling Program[Christie et al., 2004] traced this isotopic boundaryfrom 14–28 Ma and showed that, since 28 Ma, it
has remained within the eastern half of the depth
anomaly. In contrast, the western boundary of thechaotic terrain has no known geochemical orisotopic expression, and it appears to have maintained a stable position relative to the axis for atleast 2 Ma.
[39] The chaotic terrain does appear to correspondto the region of lowest melt production. Klein et al.
[1991] showed from axial basalt geochemistry thatthe melt production rate is lowest in Segments B3
and B4. Crustal thickness estimates from geochemical data are also very low beneath this region[Lecroart et al., 1997]. The anomalous shortage ofmelt supply may also be related to an anomalous
thermal structure. Recent surface wave tomographynear the AAD [Ritzwoller et al., 2003] delineated anorthwest-southeast trending low velocity anomaly

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that extends across the ocean basin, through the
AAD. This anomaly is confined to the upper
120 km of the mantle and dips shallowly to thewest. They estimated the crustal thinning averaged
over the AAD to be about 4.8 km on the basis ofan average temperature depression of 80°C in
the upper 175 km. Because of the westward dip,
the temperature depression should be larger and themelt deficit more prominent beneath B3E and B4
but definitive resolution is lacking at this fine scale.
On a broader scale, Gurnis et al. [1998] proposedthat the anomalously cool mantle temperatures
beneath the AAD are derived from a stagnatedsubducted slab in the mantle transition zone. Thisslab originated at the long-lived GondwanalandPacific convergent margin, and has been overridden by the rapidly migrating Australian plate
since the Cretaceous. In their models, a fraction of
the stagnant slab is entrained by upwelling mantle,
creating a narrow, cold band beneath the eastern
AAD. This simulation is, however, based on an
unrealistic, uniform viscosity structure and its significance for realistic mantle conditions has notbeen demonstrated.
[40] Regardless of the validity or resolution of any
given model, the abrupt change of morphologicalcharacter at the chaotic terrain boundaries cannot
be explained solely by regional mantle temperature
anomalies. The large horizontal thermal gradientsrequired are unlikely to be sustainable against thenegative buoyancy of cold mantle [Lin et al., 2002;
West et al., 1997]. The additional, and so far
unknown, dynamic and/or petrological factors thatmust be involved require more detailed data andmodels for their solution.

6. Conclusion[41] New bathymetry, magnetic and gravity data
mainly from Segment B3 of the AustralianAntarctic Discordance (AAD) provide a wealth of
new information on the distribution, mechanisms
and origin of detachment faulting and its role asa dominant mechanism of seafloor spreading.
Key observations and conclusions include thefollowing:[42] 1. Segment B3 is divided into two long-lived
second-order segments of markedly contrasting
character. The flanks of the western subsegment
B3W are characterized by normal well-orderedabyssal hills. The flanks of B3E, on the other hand,are characterized by rough, chaotic terrain including several megamullions. The largest megamul19 of 22

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okino et al.: australian-antarctic discordancelion terrain, at the inside corner of B3E, has 50–
55 km long, continuous corrugations and is morethan twice as large as typical megamullions of theMAR. Similar, but smaller domed structures withflow line parallel striations are distributed on both
northern and southern flanks.
[43] 2. The megamullions in B3E are generallyaccompanied by 510 mGal residual mantle
Bouguer anomaly highs, indicating thinner crust
and/or denser lithosphere. In contrast, a lower
MBA anomaly values along B3W suggest that thecrust is more normal along this subsegment.
[44] 3. High magnetization areas occur along theoff-axis traces of nontransform discontinuities(NTD) and over the megamullion terrains of thesouthern flank of B3E most likely reflecting induced
magnetization caused by peridotite serpentinization.
[45] 4. The association of chaotic seafloor morphology, thinner crust, megamullion structuresand positive magnetization and high gravity
indicate that extension along oceanic detachment
faults has dominated the extension process in
B3E and B4 for at least 2 million years. The
presence of small megamullion terrains on the
nondominant (north) flank indicates either thatthe polarity of detachment has reversed for shortperiods, or that periodic southward relocation of
the detachment plane has isolated small sectionsof footwall and transferred them to the opposite(hanging wall) plate.
[46] 5. Since 2 Ma, the full-spreading rate hasremained close to the 70 mm/yr rate predicted by
the global plate motion model. Despite the rapid
northward motion of the plate boundary, spreadingin B3W is almost symmetrical. In contrast, B3E(and B4) is dominated by highly (and unusually)
asymmetrical spreading, with asymmetries up to
75% over the largest megamullion terrains. These
extreme asymmetries reflect an unusual imbalance
between low magma supply and (relatively) rapid
spreading rate.
[47] 6. The formation of chaotic seafloor terrain,
reflecting a dominance of detachment faulting
at the spreading axis, is presently restricted to a
100-km section of the axis in B3E and B4, apparently coinciding with the most magma-starved
portion of the generally magma deficient AAD.

Acknowledgments[48] We are grateful to the officers and crew of R/V HakuhoMaru for their experienced support in data acquisition in

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very bad weather conditions. We thank the KH01-3 Leg 3
shipboard scientific party, M. Terazaki, M. Shinohara,
K. Mochizuki, T. Yamada, K. Nakahigashi, and C. Igarashi
for collaboration and discussion on board. We are also indebted
to K. Tamaki for his support in planning and post-cruisestudies. We deeply thank L. Géli and D. Wilson for allowing
us to use their unpublished bathymetry and magnetic data in thesurrounding areas, D. Blackman and an anonymous referee for
their helpful reviews, and M. Coffin for his comments on an
early draft of the paper. T. Fujiwara kindly helped us inanalyzing the gravity data. The GMT software [Wessel andSmith, 1995] and MB system [Caress and Chayes, 1996] wereextensively used in this study. Basic data processing andanalysis of this work were mainly done by K. Matsuda as hismaster course study in the University of Tokyo. The study was
supported by Japan Society for the Promotion of Science,
Grant-in Aid for Scientific Researches (A)(2)11304024.
Christie’s participation was supported in part by the U.S.
National Science Foundation.

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asymmetric magmatic accretion of oceanic crust at the ends
of slow-spreading ridge segments, Geology, 28(2), 179 – 182.
Anderson, R. N., D. J. Spariosu, J. K. Weissel, and D. E.
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