50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
active, andesitic White Island volcano has not<br />
widely dispersed tephra, and the oldest primary<br />
deposit found is ~20 ka. Five pre-50 ka rhyolite<br />
eruptions from an unknown Taupo Volcanic Zone<br />
source provide evidence for explosive activity in a<br />
time interval poorly documented on-land. The cores<br />
demonstrate the patchy and uneven preservation of<br />
large magnitude tephra falls caused by local<br />
bioturbation and ponding in bathymetrically<br />
complex regions. Reworked tephra layers are<br />
common and often lack indicative lithological<br />
features. Such units could easily be misinterpreted<br />
as primary events without micro-beam geochemical<br />
analyses of glass shards.<br />
POSTER<br />
3-D STRUCTURAL PERMEABILITY IN<br />
CONTRACTIONAL SETTINGS<br />
R. H. Sibson<br />
Dept. of Geology, University of Otago, P.O. Box<br />
56, Dunedin.<br />
(rick.sibson*stonebow.otago.ac.nz)<br />
Fluid flow in the Earth’s crust is influenced by<br />
anisotropic permeability as well as by hydraulic<br />
gradients in the crust. 2-D modelling of flow<br />
systems based on cross-sections drawn<br />
perpendicular to the strike of orogenic belts can be<br />
misleading when considering fluid redistribution<br />
during contractional orogenesis, because of the<br />
tendency to think of flow as restricted to the plane<br />
of the section. Topographically driven flow and<br />
flow in overpressured systems may both be affected<br />
by directional permeability parallel to strike.<br />
Bedding anisotropy and other forms of primary<br />
rock layering combine with stress/strain-controlled<br />
structural permeability, embracing systems of faults<br />
and fractures as well as fold closures, foliations,<br />
etc. In simple fold-thrust belts associated with<br />
accretionary prisms, collisional, and compressional<br />
inversion orogens, structural permeability is<br />
dominated by fold hinges and duplex structures,<br />
plus fault-fracture intersections aligned parallel to<br />
strike. In both active and ancient fold-belts,<br />
evidence exists for high levels of hydraulic<br />
communication extending along fold hingelines for<br />
kilometres to tens of kilometres along strike. Where<br />
convergence varies along contractional orogenic<br />
belts, fluid may thus be expelled laterally along this<br />
directional permeability from the regions of most<br />
intense shortening. The volume of fluid passing<br />
through a cross-strike section is then likely to be far<br />
greater than that inferred from considering fluid<br />
redistribution restricted to the plane of the section.<br />
Along the New Zealand plate boundary, 3-D<br />
structural permeability affecting fluid redistribution<br />
seems likely to be especially important within the<br />
dewatering accretionary prism of the Hikurangi<br />
Margin, and in areas of active compressional<br />
inversion (e.g. NW Nelson - Taranaki Basin).<br />
ORAL<br />
ACTIVE CRUSTAL FLUID FLOW AROUND<br />
THE <strong>NEW</strong> ZEALAND PLATE BOUNDARY –<br />
A RESEARCH FOCUS FOR THE 21 ST<br />
CENTURY<br />
R. H. Sibson<br />
Dept. of Geology, University of Otago, P.O. Box<br />
56, Dunedin.<br />
(rick.sibson*stonebow.otago.ac.nz)<br />
Crustal-scale fluid flow is a frontier area in Earth<br />
Science, critically relevant to exploration for, and<br />
future exploitation of, energy and mineral resources<br />
(oil, gas, geothermal power, hydrothermal mineral<br />
deposits). The New Zealand plate boundary<br />
(NZPB) is a geochemical factory where the<br />
interplay of tectonic and magmatic processes<br />
promotes fluid redistribution between the<br />
atmosphere, continental and oceanic rock<br />
assemblages, the ocean water mass, and the deep<br />
Earth. The diverse character of the boundary,<br />
comprising opposite-facing subduction zones along<br />
the Hikurangi and Fiordland Margins linked by an<br />
imperfect transform fault system, gives rise to an<br />
array of sites where aqueous and hydrocarbon<br />
fluids are being actively redistributed within the<br />
crust. These include:<br />
i. active hydrothermal circulation coupled to<br />
magmatism in the Taupo Volcanic Zone (TVZ)<br />
(and its northeastward continuation along the<br />
Lau-Havre Trough);<br />
ii. zones of sediment compaction and<br />
compressional ‘squeegee’ deformation with<br />
associated fluid loss along the Hikurangi and<br />
Fiordland subduction interfaces;<br />
iii. areas of ongoing compressional inversion<br />
iv.<br />
associated with hydrocarbon migration in the<br />
Taranaki Basin and in the northwestern and<br />
southern South Island; and<br />
topography-driven flow in the uplifted Southern<br />
Alps and other mountain ranges flanking the<br />
linking transform fault system, and around major<br />
volcanic edifices. Fluid redistribution is<br />
variously driven by topographic relief and<br />
precipitation, upwelling mantle and magmatic<br />
intrusion leading to convective circulation of<br />
hydrothermal fluids, compaction, deformation,<br />
and metamorphic dehydration of thick<br />
sedimentary sequences, and changes in the<br />
regional stress state. While flow in near-surface<br />
systems typically occurs under near-hydrostatic<br />
fluid pressure (the ‘normal state’), fluids at depth<br />
may be structurally compartmentalised and<br />
overpressured well above hydrostatic values.<br />
50 th <strong>Kaikoura</strong>05 -79- <strong>Kaikoura</strong> <strong>2005</strong>