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

These areas are also described in “kastom” stories
of vast swamp lands covering the shoreline after the
Kuwae event. From our evidence it is clear that
multiple explosive events took place in this region
from at least two submarine caldera systems.
However, our evidence does not support that these
events were as extremely explosive and as
regionally hazardous as described. Instead these
explosive events produced small- to medium
volume pyroclastic deposits. It is very unlikely that
they could have caused significant climatic effect,
or a significant extra-regional hazard. We suggest
that the mid-Fifteenth Century volcanic signatures
described from ice cores, may fit better to other
volcanic events in the SW-Pacific such as Kaharoa
eruption that clearly was much larger and more
explosive.
ORAL
STACKING THE SCHISTS: PROBLEMS IN
METAMORPHIC STRATIGRAPHY 1865-1955
Heather Nicholson
15A Colonial Rd, Birkenhead, Auckland, New
Zealand
(hnich*xtra.co.nz)
The theoretical problems facing nineteenth and
twentieth century geologists in mapping the schists
of New Zealand and placing them in stratigraphic
order are examined. The settler geologists
considered that the schists and greywackes of
north-west Nelson and Westland were continuous
with those of Otago and Fiordland. The gerologists
were always puzzled by the way in which unaltered
rocks (usually greywackes) passed ‘insensibly’ into
metamorphic rocks, and their inability to find an
unconformity between schists and unaltered rocks.
Nevertheless, they drew stratigraphic boundaries
between the schists and unaltered rocks that were
all arbitrary and all different. The beliefs that the
greater the metamorphism the older the formation,
and that metamorphic rocks were always older than
neighbouring unaltered rocks were used to place the
rocks in a single vertical stratigraphic order and led
to assumptions of ages ranging from Carboniferous
to pre-Cambrian.
Controversy began early in the twentieth century
when a few geologists took up the idea that the
‘insensible’ gradation from unaltered greywackes to
schists was real. Patrick Marshall believed there
were no unconformities between the rocks of
Southland, the greywackes, and the Otago schist,
that all were of the same age and all should be
included in his grand Trias-Jura Maitai System -
except for the metamorphic rocks of northwest
Nelson and Fiordland. Marshall’s prepared
microscope slides showing increasing grades of
metamorphism from Nugget Point northwards
towards Waipori Gorge earned hesitant support
from Noel Benson of Otago. However, Marshall’s
views were regarded with scepticism by Survey
geologists. When Permian fossils were found
between Nugget Point and the schists, Marshall’s
scheme was angrily denounced and the
conventional view that a stratigraphic unconformity
existed (somewhere) between the schists and the
greywackes was reinforced.
During the 1930s, intensive petrological
investigations by F.J.Turner of progressive regional
metamorphism in Otago and Westland led to his
identification of metamorphic zones based on index
minerals. Turner then abandoned any stratigraphic
subdivision of metamorphic rocks in favour of
metamorphic zones and focussed on metamorphic
processes rather than geological history. He
subdivided Otago’s very large area of Chlorite zone
greywackes and schists into three sub-zones and
with his colleague C.O. Hutton identified a fourth
sub-zone. Nevertheless, the New Zealand
Geological Survey map of 1947 maintained that the
Otago schists were older than the greywackes.
Later, the discovery of Triassic fossils in
greywackes that clearly graded into schists
(Wellman, Grindley and Munden 1955) showed
that indeed, the greywackes and schists were both
of the same age as the fossiliferous Hokonui rocks
of Southland. Soon, Wellman’s invention of the
New Zealand Geosyncline (1956) and petrological
studies by J.J.Reed (1958, 1959) and D.S.Coombs
(1960) began answering many questions about our
metamorphic rocks and, happily, also uncovered
lots of new, enticing geological puzzles.
ORAL
TEMPORAL STABILITY OF REGIONAL
DEFORMATION RATES WITHIN THE
HIKURANGI SUBDUCTION MARGIN
A. Nicol & L.M.Wallace
Institute of Geological and Nuclear Sciences, PO
Box 30 368, Lower Hutt.
(a.nicol*gns.cri.nz)
Global Positioning System (GPS) geodetic
measurements have provided a wealth of new
information on the horizontal velocity fields of
plates and plate boundary zones over the last 10-15
years. GPS present a snapshot of the finite
deformation and, at different locations, is likely to
include varying components of elastic, aseismic and
seismic strains. Given the short duration of GPS
measurements, which often sample part of the
interseismic periods of the largest active faults
(e.g.,