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International Research
The slow ship speed and closely spaced tracks resulted in a large
degree of over-sampling with the multibeam, and enabled us
to produce a gridded Digital Terrain Model (DTM) with a
horizontal resolution of ~50 m (Fig. 1).
TOBI position was initially estimated from a combination of
vehicle depth and tow-cable length using a dynamic tow-cable
simulation, and sidescan mosaics were prepared using PRISM
software (Le Bas, 2005). The sidescan and bathymetry were
then compared, and positions of salient features were found to
agree to within 500 m or less. Control points were then manually
selected by visually matching common features in the two
datasets, and used to produce a corrected TOBI track. Subsequently,
GPS/USBL-navigated Isis bathymetry was compared,
and positions of the re-navigated sidescan were found to agree
with the Isis bathymetry to within a few tens of metres.
TOBI total magnetic field data were inverted with bathymetry
to produce crustal magnetisation maps (Parker and Huestis,
1974; Guspi, 1987). Since magnetic palaeointensity increased
approximately monotonically from ~45 ka to ~3 ka (McElhinny
and McFadden, 2000), we took crustal magnetisation as an
approximate proxy for age and used the magnetisation map to
help target Isis dives. The map showed high magnetisation in
the southern half of the AVR (which is also slightly shallower
overall), particularly near its southern tip; there was also a high
magnetisation area on the median valley (MV) floor to the NE
of the AVR.
We used a combination of the EM120 bathymetry, TOBI
magnetisation and especially TOBI sidescan to plan Isis dive
tracks (Fig. 1). The aim was to provide representative imaging
and sampling of different parts of the AVR and its surroundings,
to investigate the relations among and within different
volcanic units, and to sample as broad an age range as possible.
Isis is instrumented with a range of video cameras and CTD;
we added a three-component magnetometer (from WHOI)
and, for some dives, MS2000 high-resolution multibeam bathymetry
and/or MAPRs. Isis also may sample up to ~65 kg:
as well as rocks, we also collected samples of deep-sea corals.
On most dives, we attempted to sample each significantly different
volcanic unit (e.g., individual small cones or haystacks,
major lava flow fronts), either side of significant lithological or
structural boundaries, and other outcrops provided by faults,
etc. The northernmost dive targeted an area of extensive flat
MV floor associated with high magnetisation that might have
represented the beginning of a new cycle of AVR building.
Two dives (near 45°35’N and 45°29’N, Fig. 1) were dedicated
to MS2000 surveys, which were flown some 100 m above the
seafloor, out of visual range of the bottom. With tracks spaced
~180 m apart, we obtained essentially complete coverage with
2.8-m grid spacing.
Finally, during instrument down-time or when the weather
was too poor for Isis operations, we conducted a number of
“tow-yo” MAPR wire-line surveys to supplement the measurements
made during the TOBI survey.
Preliminary results
Thirteen Isis dives were completed. Three included alongaxis
coverage of the AVR, seven investigated the flanks, three
touched on older rocks exposed by the MV wall faults or outcropping
above them, and six included investigation of the MV
floor around the AVR. We recovered 270 rock samples (almost
all basalt) and 56 biological samples. Rock samples were described
on board and any external glass removed and separately
curated.
Typical volcanic “hummocky” terrain extends the whole length
of the AVR from ~45°24’N to ~45°40’N. However, north of
45°33’N, the AVR has lower topographic relief, lower magnetisation,
and increased levels of tectonism, suggesting this is
older than the southern half. The AVR contains only one or
two large (~2 km diameter) flat-topped seamounts, though several
more occur on the surrounding MV floor. Volcanism on
the AVR is largely confined to a very large number (estimated
at ~3000) of small volcanic centres. These tend to be aligned
along a number of N-S ridges, especially in the central part of
the AVR. A surprising observation was the high angle of repose
of many of the seamount flanks. In places, near the summit
of these seamounts, haystacks of basaltic pillow lavas forming
slopes in excess of 60° were encountered. We ascribe these steep
slopes to the high viscosity of erupted magmas due to their
extensive cooling during ascent through the highly permeable
volcanic carapace.
Small spurs aligned at right angles to the main AVR trend
are common on its flanks, and usually comprise single lines
of small volcanoes, often overlapping. The general sense appears
to be of magmatism initially focused near the AVR axis
and subsequently building edifices outwards along the lines of
Figure 2: Hollow lava tube in the median valley (MV) floor flanking
the AVR.
InterRidge News 13 Vol. 17, 2008