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

Berryman” model was used as the basis for
Loadings Codes from the mid-80s almost to the
present day. The growth and acceptance of
paleoseismology as a discipline eventually resulted
in PSH models that included active faults explicitly
as earthquake sources in the 1990s, initially in
regional studies, and eventually at a national scale.
The most recent national PSH model includes over
300 fault sources, and incorporates the latest
ground motion attenuation model for New Zealand.
Multidisciplinary and multi-institutional
collaborations are the basis for the new breed of
models. Present efforts are focused on improving
the fault and seismicity source input to the national
model. The latest iteration of the model now
includes an additional 100 fault sources largely
from offshore, courtesy of the National Institute of
Water and Atmospheric Research. Equally
important are efforts focused on validating the
national model against independent data, and
relevant research into “time-varying” phenomena
such as earthquake interactions in space and time,
and elapsed time since the last earthquake.
ORAL
CRUSTAL EXTENSION AND THE MOHO IN
THE CENTRAL VOLCANIC REGION
W.R. Stratford 1 &T.A.Stern 1
1. Victoria University of Wellington, PO Box 600,
Wellington.
(Wanda.Stratford*vuw.ac.nz)
Crustal thickness measurements in the Central
Volcanic Region (CVR), the back-arc basin in
North Island, show that here the crust has been
thinned by at least 50 %. This wedge shaped
segment of extended crust is the onshore
continuation of the oceanic Lau-Havre back-arc
spreading centre. A region of negative residual
gravity anomalies clearly outlines the western and
eastern margins of the CVR. Andesite arcs also
define the boundaries of the CVR. An east moving
and rotating subduction zone through time is
reflected in an apparent fan like opening and
migration of the volcanic arc from the ~ 4 Ma
remnant western arc to the present day eastern arc.
Despite the complex opening structure of the backarc
basin, evidence from offshore seismic data
indicates the basement across the whole area has
subsided by similar amounts and hence the region
has undergone similar amounts of extension.
Apparent extension strain rates in the back-arc are
~ twice that of most continental rifts, and the modes
of extension are less clear. The accepted end
member extension models of pure and simple shear
are less applicable in a region of such high
extension rates, where a significant amount of the
extension is expressed by the intrusion of new
material into the crust. What constitutes a Moho in
this region of high extension rates and elevated heat
flow is also less clear. Beneath the back-arc the
most prominent boundary, from long-range
refraction and reflection data, is between the
extended and intruded upper crust, and a layer of
new crust formed by underplating. From the top of
the underplating, seismic velocities increase with
depth, imply an increasingly mafic composition
with depth. Mantle velocities and hence the base of
the crust are reached at ~ 20 km depth. Thus the
transition from lower crust to upper mantle is
broad, and a conventional Moho does not exist
beneath the CVR.
ORAL
THEN AND NOW - AND QUITE A BIT THEN
AND NOW - AND QUITE A BIT BEFORE
AND IN BETWEEN
R. P. Suggate
Institute of Geological & Nuclear Sciences,
P.O.Box 30368, Lower Hutt
(p.suggate*gns.cri.nz)
I present a personal view, starting by setting the
scene over the 25 years before 1955, during which
significant advances held out the promise of far
greater understanding to be obtained by well
focussed studies.
The next 50 years saw the New Zealand Geological
Survey present a series of 1:250 000 maps for the
whole of New Zealand, and become involved with
both important industrial developments and with
“Think Big” projects. It developed specialist
sections in basin studies, engineering geology, earth
deformation and computers, while retaining
previous paleontology and petrology sections. It
balanced application with research. When later the
Survey was joined with DSIR’s Geophysics
Division and then subsumed into the Institute of
Geological & Nuclear Sciences, much of the
Survey’s expertise remained relevant and the
Survey’s records have stood the new institute in
good stead.
University geology staff numbers increased rapidly
when the colleges of the University of New
Zealand gained university status and then Massey
and Waikato universities established departments in
which geology was taught. The staff and the
increasing numbers of research students began to
put out many new data and interpretations. New
ideas, for example plate tectonics in the late 1960s,
were adopted and adapted to New Zealand by the
universities, ahead of the Geological Survey. New
techniques were commonly used as early as
possible.
50 th Kaikoura05 -84- Kaikoura 2005