50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

the bulk strength of the Pacific Plate crust above the
Alpine Fault ramp. The shears also provided a
means for suprahydrostatic metamorphic fluids to
escape upwards, acting as a closely spaced array of
planar, vertical conduits cutting across the
otherwise low permeability (1 x 10 -18 m 2 ) schist.
We calculate that an aqueous fluid volume of 5.6 x
10 10 m 3 was required to deposit the 1-3 mm thick
veins that currently seal the backshears throughout
the 30 km-long by ~2 km-wide array, equivalent to
an integrated fluid flux of 486000 m 3 /m 2 . Upward
draining of this metamorphic fluid into the hangingwall
would have left the underlying source region
near its base residually drier. Fluid expulsion may
thus have accomplished a net devolatilisation and
rheological strengthening along the Alpine
mylonite zone at depth at the same time that
hydrolytic weakening softened structurally higher
rocks in the hanging-wall. These fluid-related
strength changes may have limited the degree to
which the Alpine Fault could slip in the plate
motion direction without some partitioning of
deformation into its hanging-wall.
POSTER
COLLAPSE AND RE-GROWTH OF
MONOWAI SUBMARINE VOLCANO,
KERMADEC ARC, 1998-2004
Ian C. Wright 1 , William W. Chadwick 2 ,
Cornel E. J. de Ronde 3 , Dominique Reymond 4 ,
Olivier Hyvernaud 4 , Stephen Bannister 3 ,
Peter Stoffers 5 & Kevin Mackay 1
1
National Institute of Water and Atmospheric
Research, 301 Evans Bay Parade,
Greta Point, Private Bag 14-901, Kilbirnie,
Wellington, 6003, New Zealand
2
CIMRS, Oregon State University, 2115 SE OSU
Drive, Newport, OR, 97365, USA
3 Institute of Geological & Nuclear Sciences, 30
Gracefield Road, PO Box 31-312, Lower Hutt,
New Zealand
4 Laboratoire de Geophysique, CEA/DASE/LDG,
Tahiti, P.O. Box 640, Papeete, French Polynesia
5 Institute of Geosciences, University of Kiel, Kiel,
Germany
(k.mackay*niwa.co.nz)
Monowai submarine volcano is located at
25°53.5’S/177°11.1’W, about 1400 km NNE of
New Zealand along the Kermadec arc, and consists
of a shallow symmetrical cone with a summit depth
of ~100 m. Monowai is one of the most active
submarine volcanoes in the Kermadec arc, based on
visual reports from overflights, oceanographic
surveys of hydrothermal plumes, and
seismoacoustic monitoring from French Polynesia
and elsewhere. Since the late 1970’s, Monowai has
been the source of frequent swarms of acoustic Twave
events every few years. On 24 May 2002
there was a particularly large seismoacoustic event
with a duration of 6-8 minutes and an exceptional
amplitude that was 4-5 times larger than any other
T-wave signal recorded from Monowai.
Bathymetric surveys of Monowai that bracket this
event were collected with multibeam sonars in 1998
and 2004 by R/V Sonne (Hydrosweep) and R/V
Tangaroa (EM300), respectively. A new collapse
feature is apparent on the SE side of the volcano in
the 2004 bathymetry. The two surveys were
compared using a quantitative technique that has
been used for documenting depth changes due to
volcanic eruptions on mid-ocean ridges. The
results of this comparison show that the summit
depth of Monowai changed from 69 m below
sealevel in 1998 to 135 m in 2004, a difference of -
66 m, and the location of the shallowest point
moved ~200 m to the NNW. However, the
maximum depth change between the surveys is -
105 m and is located near the 1998-summit, which
in 2004 is south of the new headwall scarp on the
SE flank of the volcano. The total area of
significant depth change is 1.26 x 10 6 m 2 ,andthe
decrease in volume is 6.12 x 10 7 m 3 (or 0.06 km 3 ).
From the distribution of the depth changes it is also
clear that four things occurred between the surveys:
removal of volume from slope failure, the
subsequent addition of volume from an eruptive
vent within the new slide scar, the removal of part
of that additional volume by a further slope failure,
and the addition of more volume within the
eruptive vent. Therefore, the volume removed by
slope failure was probably closer to 0.1 km 3
whereas the volume added back by eruption is
approximately 0.03 km 3 . It is not known whether
the slide triggered the eruption or visa versa, but the
recent history of frequent volcanic activity at
Monowai suggests that an eruption-triggered slide
maybemorelikely.
POSTER
FINGERPRINTING SLAB–DERIVED
AQUEOUS FLUIDS IN THE MANTLE
SOURCES OF ARC LAVAS: AN EXAMPLE
FROM THE SOUTHERN KERMADEC ARC
R.J. Wysoczanski 1 ,I.C.Wright 2 ,J.A.Gamble 3 ,
E.H. Hauri 4 , J.F. Luhr 5 ,S.M.Eggins 6 ,
&M.R.Handler 1
1 IFREE, Japan Agency for Marine Earth Science
and Technology, Yokosuka, Japan.
2 National Institute of Water and Atmospheric
Research, Wellington, New Zealand.
3 Dept. of Geology, National University of Ireland,
University College Cork, Ireland.
4 Dept. of Terrestrial Magnetism, Carnegie
Institution of Washington, USA.
50 th Kaikoura05 -98- Kaikoura 2005