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 top of the group, which was probably thin and<br />
eroded during the inversion phase prior to<br />
accumulation of younger units. The Taumatamaire<br />
Formation comprises mainly massive mudstone.<br />
Along the western margin (Herangi High) the<br />
mudstone is interspersed with several limestone<br />
members (Awakino Limestone; Black Creek<br />
Limestone); the carbonate sediments were sourced<br />
along the contemporary rocky shoreline of the<br />
Herangi High underlain by Murihiku basement, and<br />
dispersed across a very narrow east-facing shelf and<br />
down the slope into the basin.<br />
Paleocurrent measurements derived from flute casts<br />
and tool marks indicate that the flysch facies were<br />
sourced from the south, and the depositional system<br />
probably prograded to the north. Muddy sediment<br />
and carbonate were locally derived from basement<br />
exposed along the Herangi High, but this was<br />
probably a minor source. The sandstone beds are<br />
cemented with calcite, and glauconite comprises a<br />
surprisingly high proportion of their content (10%).<br />
Mica is an accessory mineral, suggesting that<br />
schistose or gneissic terrane (in South Island)<br />
contributed sediments to the basin.<br />
POSTER<br />
MANTLE OR CRUST? FIRST RESULTS<br />
FROM A DETAILED ACTIVE-SOURCE<br />
SEISMIC EXPERIMENT ACROSS THE<br />
TAUPO VOLCANIC ZONE<br />
Adrian Benson 1 , Tim Stern 1 &Stephen<br />
Bannister 2<br />
1 School of Earth Sciences, Victoria University of<br />
Wellington, PO Box 600, Wellington.<br />
2 Institute of Geological and Nuclear Sciences, PO<br />
Box 30-368, Lower Hutt.<br />
(adrian.benson*vuw.ac.nz)<br />
Seismic studies of the central North Island have<br />
consistently shown the Taupo Volcanic Zone to be<br />
a region of attenuated continental crust underlain by<br />
mantle of anomalously low seismic velocity (~7.4<br />
km.s -1 ). However, the nature of the crust-mantle<br />
boundary beneath the rift and, in particular, the<br />
position of the Moho remain contentious. Active<br />
source data acquired during the 2001 NIGhT<br />
experiment detected two prominent reflectors at<br />
depth: a ~ 15 km deep horizon (PmP1) and an<br />
unusually strong set of reflections from an interface<br />
at ~30 km (PmP2). These NIGhT data have given<br />
rise to divergent models of the lower-crust and<br />
upper-mantle. Harrison & White (2004) place the<br />
base of the crust at 30 km (= PmP2) and infer that<br />
the 15 km interval between PmP1 and PmP2 is<br />
under-plated and/or intruded lower crust. Stratford<br />
& Stern (2004) suggest that the Moho lies between<br />
15 and 20 km depth and interpret the PmP2<br />
reflections as being from the top of an intra-mantle<br />
fluid/melt body of unknown thickness but limited<br />
lateral extent. We present preliminary results from<br />
a recently completed active source seismic<br />
experiment (MORC) that seeks to address the issue<br />
of what constitutes Mantle OR Crust beneath the<br />
TVZ.<br />
Building upon the results of NIGhT, the MORC<br />
experiment was designed to provide a enhanced<br />
data set to examine the character of the 30 km deep<br />
PmP2 reflector. Nine dynamite shots of between<br />
500 and 1300 kg were detonated into a 120 km long<br />
array across the northern end of Lake Taupo. The<br />
shot geometry and the deployment of 714 receivers<br />
(600 IRIS Texans, 96 geophones on multi-channel<br />
systems, and 18 3-component seismometers) with<br />
an average spacing of ~200 m provides a ten-fold<br />
increase in spatial resolution relative to the NIGhT<br />
data. Reflections from the 15 km deep PmP1<br />
horizon are present in the MORC data, however the<br />
dominant features are arrivals from PmP2 at a<br />
depth of ~ 30 km. The occurrence and amplitude of<br />
the PmP2 reflections are strongly dependent on<br />
both the shot position and receiver offset. Initial<br />
ray-tracing of the PmP2 reflections suggests the<br />
interface is of limited lateral extent and located<br />
directly below the active (eastern) part of the main<br />
TVZ rift. For stations with both PmP1 and PmP2<br />
arrivals it is possible to directly compare the<br />
relative amplitude of these two phases. At large<br />
offsets the amplitude of PmP2 reflections are at<br />
least 5 times those of PmP1 phases, which are<br />
modelled as being from a < 6.0 km.s -1 to 6.8 km.s -1<br />
velocity boundary (Stratford & Stern, 2004). These<br />
results indicate the PmP2 reflections must be due to<br />
a large change in the velocity and/or physical<br />
properties of rocks at 30 km depth.<br />
Further work to characterise the crust-mantle<br />
structure beneath the TVZ will include ray-trace<br />
and tomographic modelling of the combined<br />
MORC and NIGhT datasets, the production of a<br />
stacked low-fold seismic section, and detailed<br />
analysis of the AVO properties of the PmP2 and<br />
other reflectors.<br />
ORAL<br />
SEAFLOOR STRUCTURAL GEOMORPHIC<br />
EVOLUTION IN RESPONSE TO<br />
SUBDUCTION PROCESSES, POVERTY BAY<br />
INDENTATION, <strong>NEW</strong> ZEALAND<br />
K. L. Bodger 1 ,J.R.Pettinga 1 andP.M.Barnes 2<br />
1 Dept. of Geological Sciences, University of<br />
Canterbury, Christchurch<br />
2 National Institute of Water and Atmospheric<br />
Research (NIWA), Greta Point, Wellington<br />
(k.bodger*geol.canterbury.ac.nz)<br />
The >4000 km 2 Poverty Bay Indentation, in the<br />
northern sector of the Hikurangi subduction margin,<br />
50 th <strong>Kaikoura</strong>05 -8- <strong>Kaikoura</strong> <strong>2005</strong>