Full version, lower resolution, 3.25MB - InterRidge

International Research
greatest topographic gradient. A few much larger spurs occur
(e.g., near 45°31’N, 27°51’W), and usually trend NNE. They
appear to be controlled by underlying oblique tectonic trends
associated with the dextral non-transform offset to the next
segment north.
In some places the hummocky volcanoes of the AVR abut directly
against the major normal faults of the MV walls. Elsewhere,
extensive flat-lying lavas were found and, where possible,
we investigated their contacts with the AVR. In several
places minor faulting revealed the internal structure of these
lavas, including open lava tubes and feeder dykes (Fig. 2).
The TOBI magnetisation map revealed a central anomaly magnetic
high with a width (trough-to-trough) of 7 km, much less
than the 18 km predicted for the Brunhes chron at this spreading
rate. This narrow magnetisation high coincides closely with
the AVR, and suggests there may have been a hiatus between
previous magmatism and construction of the current AVR.
MS2000 bathymetry from the two Isis survey dives has revealed
a wealth of new information, including relationships
between volcanic and tectonic features. It also shows that
TOBI sidescan reliably images all volcanic cones down to a
diameter of ~20 m. One surprising feature was vertical failure
scars, evidence for extensive volcanic flank collapse in young
cones on the AVR. This indicates that tectonic erosion of the
crust begins almost as soon as it is formed, and long before it is
affected by major faulting at the edges of the MV floor (Yeo et
al., 2008). Magnetisation maps from the Isis surveys show more
detail; in particular, that magnetisation highs are not confined
to the AVR crest but also occur on its flanks and lateral spurs,
in various relationships to the volcanic architecture.
Visual estimates of sample ages were made in two ways: 1) by
using the degree of sediment cover around the outcrop, and 2)
estimated from the appearance (degree of freshness, etc.) of the
hand specimen. Both estimates show a considerable amount
of scatter. Although the youngest estimates fall on the axis of
the central and southern AVR crest, young ages also occur in
places on its flanks and south of its southern tip, while some of
the oldest estimates also occur on the AVR crest.
MAPRs on the TOBI survey showed hydrothermal plume
signals in the NE of the survey and on the eastern flank of
the AVR near 45°29’N, 27°51’W. However, despite a detailed
MAPR tow-yo grid and Isis dives in the plume vicinity (one of
which encountered a buoyant plume), we were unable to precisely
locate the plume source on the seafloor.
Future work
Post-cruise analysis at Durham (RS, KA, IY) will focus on analysing
the tectonic and volcanic structure of the AVR, building
a volcano-stratigraphic model of it, and incorporating the
results of the other studies into the model. U-series radiometric
dating will be carried out on selected samples by PVC, LT and
KS. Analysis of magnetic survey data will be led by MT with
additional magnetic palaeointensity measurements conducted
by MC, to provide detailed magnetic age proxies. Geochemical
analyses will be carried out at Southampton (BM, NS), Durham
(JD, YN) and Lisbon (PF) to investigate melt sources,
magma storage and fractionation processes, to fingerprint individual
volcanic units, and to provide the analyses needed to
support the U-series dating. KM will analyse the coral samples,
and BM will analyse the MAPR data with Ed Baker at
NOAA.
Acknowledgements
We are greatly indebted to the Captain, officers and crew of
RRS James Cook cruise JC24 and to the shipboard technicians
for their excellent support. The cruise was funded by the Natural
Environment Research Council.
References
Baker, E.T., and H.B. Milburn (1997), MAPR: A new instrument
for hydrothermal plume mapping, Ridge Events, 8,
23-25.
Ballard, R.D., and T.H. Van Andel (1977), Morphology and
tectonics of the inner rift valley at lat. 36°50'N on the Mid-
Atlantic Ridge, Geological Society of America Bulletin, 88,
507-530.
Barclay, A.H., et al. (1998), Seismic structure and crustal magmatism
at the Mid-Atlantic Ridge, 35°N, J. Geophys. Res.,
103(B8), 17827-17844.
Briais, A., et al. (2000), Accretionary processes in the axial valley
of the Mid-Atlantic Ridge 27°-30°N from TOBI side-scan
sonar images, Marine Geophysical Researches, 21, 87-119.
Flewellen, C., et al. (1993), TOBI, a vehicle for deep ocean
survey, Electronics and Communication Engineering Journal
(April 1993), 85-93.
Guspi, F. (1987), Frequency-domain reduction of potential
field measurements to a horizontal plane, Geoexploration,
24, 87-98.
Head, J.W., et al. (1996), Mid-ocean ridge eruptive vent morphology
and structure: Evidence for dike widths, eruption
rates, and evolution of eruptions and axial volcanic ridges, J.
Geophys. Res., 101(B12), 28,265-228,280.
Hussenoeder, S.A., et al. (1996), Near-bottom magnetic survey
of the Mid-Atlantic Ridge axis, 24°-24°40'N: Implications
for crustal accretion at slow spreading ridges, J. Geophys.
Res., 101(B10), 22,051-22,069.
Keeton, J.A., and R.C. Searle (1996), Analysis of Simrad
EM12 multibeam bathymetry and acoustic backscatter data
for seafloor mapping, exemplified at the Mid-Atlantic Ridge
at 45°N, Marine Geophysical Researches, 18(6), 663-688.
Lawson, K., et al. (1996), Detailed volcanic geology of the
MARNOK area, Mid-Atlantic Ridge north of Kane transform,
in Spec. Publ. 118, edited by C.J. MacLeod, et al.,
InterRidge News 14 Vol. 17, 2008