50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
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These areas are also described in “kastom” stories<br />
of vast swamp lands covering the shoreline after the<br />
Kuwae event. From our evidence it is clear that<br />
multiple explosive events took place in this region<br />
from at least two submarine caldera systems.<br />
However, our evidence does not support that these<br />
events were as extremely explosive and as<br />
regionally hazardous as described. Instead these<br />
explosive events produced small- to medium<br />
volume pyroclastic deposits. It is very unlikely that<br />
they could have caused significant climatic effect,<br />
or a significant extra-regional hazard. We suggest<br />
that the mid-Fifteenth Century volcanic signatures<br />
described from ice cores, may fit better to other<br />
volcanic events in the SW-Pacific such as Kaharoa<br />
eruption that clearly was much larger and more<br />
explosive.<br />
ORAL<br />
STACKING THE SCHISTS: PROBLEMS IN<br />
METAMORPHIC STRATIGRAPHY 1865-1955<br />
Heather Nicholson<br />
15A Colonial Rd, Birkenhead, Auckland, New<br />
Zealand<br />
(hnich*xtra.co.nz)<br />
The theoretical problems facing nineteenth and<br />
twentieth century geologists in mapping the schists<br />
of New Zealand and placing them in stratigraphic<br />
order are examined. The settler geologists<br />
considered that the schists and greywackes of<br />
north-west Nelson and Westland were continuous<br />
with those of Otago and Fiordland. The gerologists<br />
were always puzzled by the way in which unaltered<br />
rocks (usually greywackes) passed ‘insensibly’ into<br />
metamorphic rocks, and their inability to find an<br />
unconformity between schists and unaltered rocks.<br />
Nevertheless, they drew stratigraphic boundaries<br />
between the schists and unaltered rocks that were<br />
all arbitrary and all different. The beliefs that the<br />
greater the metamorphism the older the formation,<br />
and that metamorphic rocks were always older than<br />
neighbouring unaltered rocks were used to place the<br />
rocks in a single vertical stratigraphic order and led<br />
to assumptions of ages ranging from Carboniferous<br />
to pre-Cambrian.<br />
Controversy began early in the twentieth century<br />
when a few geologists took up the idea that the<br />
‘insensible’ gradation from unaltered greywackes to<br />
schists was real. Patrick Marshall believed there<br />
were no unconformities between the rocks of<br />
Southland, the greywackes, and the Otago schist,<br />
that all were of the same age and all should be<br />
included in his grand Trias-Jura Maitai System -<br />
except for the metamorphic rocks of northwest<br />
Nelson and Fiordland. Marshall’s prepared<br />
microscope slides showing increasing grades of<br />
metamorphism from Nugget Point northwards<br />
towards Waipori Gorge earned hesitant support<br />
from Noel Benson of Otago. However, Marshall’s<br />
views were regarded with scepticism by Survey<br />
geologists. When Permian fossils were found<br />
between Nugget Point and the schists, Marshall’s<br />
scheme was angrily denounced and the<br />
conventional view that a stratigraphic unconformity<br />
existed (somewhere) between the schists and the<br />
greywackes was reinforced.<br />
During the 1930s, intensive petrological<br />
investigations by F.J.Turner of progressive regional<br />
metamorphism in Otago and Westland led to his<br />
identification of metamorphic zones based on index<br />
minerals. Turner then abandoned any stratigraphic<br />
subdivision of metamorphic rocks in favour of<br />
metamorphic zones and focussed on metamorphic<br />
processes rather than geological history. He<br />
subdivided Otago’s very large area of Chlorite zone<br />
greywackes and schists into three sub-zones and<br />
with his colleague C.O. Hutton identified a fourth<br />
sub-zone. Nevertheless, the New Zealand<br />
Geological Survey map of 1947 maintained that the<br />
Otago schists were older than the greywackes.<br />
Later, the discovery of Triassic fossils in<br />
greywackes that clearly graded into schists<br />
(Wellman, Grindley and Munden 1955) showed<br />
that indeed, the greywackes and schists were both<br />
of the same age as the fossiliferous Hokonui rocks<br />
of Southland. Soon, Wellman’s invention of the<br />
New Zealand Geosyncline (1956) and petrological<br />
studies by J.J.Reed (1958, 1959) and D.S.Coombs<br />
(1960) began answering many questions about our<br />
metamorphic rocks and, happily, also uncovered<br />
lots of new, enticing geological puzzles.<br />
ORAL<br />
TEMPORAL STABILITY <strong>OF</strong> REGIONAL<br />
DEFORMATION RATES WITHIN THE<br />
HIKURANGI SUBDUCTION MARGIN<br />
A. Nicol & L.M.Wallace<br />
Institute of Geological and Nuclear Sciences, PO<br />
Box 30 368, Lower Hutt.<br />
(a.nicol*gns.cri.nz)<br />
Global Positioning System (GPS) geodetic<br />
measurements have provided a wealth of new<br />
information on the horizontal velocity fields of<br />
plates and plate boundary zones over the last 10-15<br />
years. GPS present a snapshot of the finite<br />
deformation and, at different locations, is likely to<br />
include varying components of elastic, aseismic and<br />
seismic strains. Given the short duration of GPS<br />
measurements, which often sample part of the<br />
interseismic periods of the largest active faults<br />
(e.g.,