50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
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the bulk strength of the Pacific Plate crust above the<br />
Alpine Fault ramp. The shears also provided a<br />
means for suprahydrostatic metamorphic fluids to<br />
escape upwards, acting as a closely spaced array of<br />
planar, vertical conduits cutting across the<br />
otherwise low permeability (1 x 10 -18 m 2 ) schist.<br />
We calculate that an aqueous fluid volume of 5.6 x<br />
10 10 m 3 was required to deposit the 1-3 mm thick<br />
veins that currently seal the backshears throughout<br />
the 30 km-long by ~2 km-wide array, equivalent to<br />
an integrated fluid flux of 486000 m 3 /m 2 . Upward<br />
draining of this metamorphic fluid into the hangingwall<br />
would have left the underlying source region<br />
near its base residually drier. Fluid expulsion may<br />
thus have accomplished a net devolatilisation and<br />
rheological strengthening along the Alpine<br />
mylonite zone at depth at the same time that<br />
hydrolytic weakening softened structurally higher<br />
rocks in the hanging-wall. These fluid-related<br />
strength changes may have limited the degree to<br />
which the Alpine Fault could slip in the plate<br />
motion direction without some partitioning of<br />
deformation into its hanging-wall.<br />
POSTER<br />
COLLAPSE AND RE-GROWTH <strong>OF</strong><br />
MONOWAI SUBMARINE VOLCANO,<br />
KERMADEC ARC, 1998-2004<br />
Ian C. Wright 1 , William W. Chadwick 2 ,<br />
Cornel E. J. de Ronde 3 , Dominique Reymond 4 ,<br />
Olivier Hyvernaud 4 , Stephen Bannister 3 ,<br />
Peter Stoffers 5 & Kevin Mackay 1<br />
1<br />
National Institute of Water and Atmospheric<br />
Research, 301 Evans Bay Parade,<br />
Greta Point, Private Bag 14-901, Kilbirnie,<br />
Wellington, 6003, New Zealand<br />
2<br />
CIMRS, Oregon State University, 2115 SE OSU<br />
Drive, Newport, OR, 97365, USA<br />
3 Institute of Geological & Nuclear Sciences, 30<br />
Gracefield Road, PO Box 31-312, Lower Hutt,<br />
New Zealand<br />
4 Laboratoire de Geophysique, CEA/DASE/LDG,<br />
Tahiti, P.O. Box 640, Papeete, French Polynesia<br />
5 Institute of Geosciences, University of Kiel, Kiel,<br />
Germany<br />
(k.mackay*niwa.co.nz)<br />
Monowai submarine volcano is located at<br />
25°53.5’S/177°11.1’W, about 1400 km NNE of<br />
New Zealand along the Kermadec arc, and consists<br />
of a shallow symmetrical cone with a summit depth<br />
of ~100 m. Monowai is one of the most active<br />
submarine volcanoes in the Kermadec arc, based on<br />
visual reports from overflights, oceanographic<br />
surveys of hydrothermal plumes, and<br />
seismoacoustic monitoring from French Polynesia<br />
and elsewhere. Since the late 1970’s, Monowai has<br />
been the source of frequent swarms of acoustic Twave<br />
events every few years. On 24 May 2002<br />
there was a particularly large seismoacoustic event<br />
with a duration of 6-8 minutes and an exceptional<br />
amplitude that was 4-5 times larger than any other<br />
T-wave signal recorded from Monowai.<br />
Bathymetric surveys of Monowai that bracket this<br />
event were collected with multibeam sonars in 1998<br />
and 2004 by R/V Sonne (Hydrosweep) and R/V<br />
Tangaroa (EM300), respectively. A new collapse<br />
feature is apparent on the SE side of the volcano in<br />
the 2004 bathymetry. The two surveys were<br />
compared using a quantitative technique that has<br />
been used for documenting depth changes due to<br />
volcanic eruptions on mid-ocean ridges. The<br />
results of this comparison show that the summit<br />
depth of Monowai changed from 69 m below<br />
sealevel in 1998 to 135 m in 2004, a difference of -<br />
66 m, and the location of the shallowest point<br />
moved ~200 m to the NNW. However, the<br />
maximum depth change between the surveys is -<br />
105 m and is located near the 1998-summit, which<br />
in 2004 is south of the new headwall scarp on the<br />
SE flank of the volcano. The total area of<br />
significant depth change is 1.26 x 10 6 m 2 ,andthe<br />
decrease in volume is 6.12 x 10 7 m 3 (or 0.06 km 3 ).<br />
From the distribution of the depth changes it is also<br />
clear that four things occurred between the surveys:<br />
removal of volume from slope failure, the<br />
subsequent addition of volume from an eruptive<br />
vent within the new slide scar, the removal of part<br />
of that additional volume by a further slope failure,<br />
and the addition of more volume within the<br />
eruptive vent. Therefore, the volume removed by<br />
slope failure was probably closer to 0.1 km 3<br />
whereas the volume added back by eruption is<br />
approximately 0.03 km 3 . It is not known whether<br />
the slide triggered the eruption or visa versa, but the<br />
recent history of frequent volcanic activity at<br />
Monowai suggests that an eruption-triggered slide<br />
maybemorelikely.<br />
POSTER<br />
FINGERPRINTING SLAB–DERIVED<br />
AQUEOUS FLUIDS IN THE MANTLE<br />
SOURCES <strong>OF</strong> ARC LAVAS: AN EXAMPLE<br />
FROM THE SOUTHERN KERMADEC ARC<br />
R.J. Wysoczanski 1 ,I.C.Wright 2 ,J.A.Gamble 3 ,<br />
E.H. Hauri 4 , J.F. Luhr 5 ,S.M.Eggins 6 ,<br />
&M.R.Handler 1<br />
1 IFREE, Japan Agency for Marine Earth Science<br />
and Technology, Yokosuka, Japan.<br />
2 National Institute of Water and Atmospheric<br />
Research, Wellington, New Zealand.<br />
3 Dept. of Geology, National University of Ireland,<br />
University College Cork, Ireland.<br />
4 Dept. of Terrestrial Magnetism, Carnegie<br />
Institution of Washington, USA.<br />
50 th <strong>Kaikoura</strong>05 -98- <strong>Kaikoura</strong> <strong>2005</strong>