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geological rates (10 000
years suggesting that previous assertions regarding
the low seismic hazard of the Kapiti-Manawatu
region may not be substantiated, and needs to be reappraised.
The method used here for determining
the paleoseismicity of offshore faults has
potentially wider applications elsewhere, and in
New Zealand has already been applied to normal
(e.g., Cape Egmont Fault, Taranaki; offshore Taupo
Volcanic Zone, Bay of Plenty), reverse (e.g., North
Canterbury shelf; Lachlan Ridge, Hawkes Bay) and
strike-slip faults (e.g., Alpine Fault, Fiordland) in
the offshore environment.
ORAL
THE GROWTH OF ANTICLINAL RANGES
IN AN ACTIVE FOLD-THRUST BELT,
CENTRAL OTAGO, NEW ZEALAND
R. Norris 1 ,E.Bennett 2 , J. Youngson 1 ,
J. Jackson 2 ,G.Raisbeck 3 ,&F.Yiou 3
1 Dept. of Geology, University of Otago, PO Box
56, Dunedin
2 Bullard Laboratories, Dept. of Earth Sciences,
Madingley Road, Cambridge, U. K.
3 Centre de Spectrométrie Nucléaire et
Spectrométrie de Masse, CNRS, Orsay, France
(richard.norris*stonebow.otago.ac.nz)
The Otago fold-thrust belt is situated east of the
Alpine Fault within the zone of distributed
deformation along the Australian–Pacific plate
boundary in South Island, New Zealand. Average
shortening rates across the belt, based on both longterm
geological estimates and on preliminary GPS
data, are only 2-3 mm/yr. We have employed a
combination of geomorphology, structural geology
and cosmogenic isotope dating to determine the
style and rate of fault-fold propagation, and the
evolution of the topography in the central part of
the belt. Cosmogenic 10 Be measurements were
made on “sarsen stones”, remnants of silcrete
deposits in the Tertiary sediments overlying the
basement schist erosion surface. During uplift of
the ranges, the surrounding sediments were rapidly
stripped, leaving the silcretes behind. Previous
work (E. Bennett et al., J. Geophys. Research, v.
110, B02404, doi: 10.1029/2004JB003184, 2005)
has shown that 10 Be ages up to 1.3 Ma are not
saturated and provide valuable data on the timing of
uplift of the underlying ranges.
The data strongly support a model of rapid
establishment of fault length followed by increase
in amplitude and slow tip propagation. The two
ridge systems investigated (Raggedy Range–
Blackstone Hill and Rough Ridge–North Rough
Ridge) have grown since 1.5 Ma, by either rapid
lateral propagation and/or early coalescence of
shorter segments, before continuing to increase in
height with slower tip extension. Two ridges have
developed frontal imbricates within 1 Ma of
initiation. Initial growth of all the central/east
Otago ranges occurred over a short time period
with no systematic progression towards the
foreland. Continuing activity across the region
indicates the whole thrust belt is ‘out-of-sequence’
in terms of classical thrust kinematics.
Erosion of the ridges is very slow and they closely
reflect tectonic deformation above the underlying
fault. Elastic dislocation modelling of the
topography of North Rough Ridge indicates an
underlying fault dipping north-westwards at 45° or
less. The slipping fault modeled extends from a
depth of 1-2 km down to at least 10 km. The
average spacing of the ranges of 18 km, together
with their average length of 45 km, suggests that
they most likely extend to the base of the
seismogenic zone.
The Rough Ridge system of ridges currently forms
the drainage divide between streams draining into
the Clutha system via the Manuherikia River and
streams draining eastwards into the Taieri system.
Prior to the growth of these ridges over the last 1.5
Ma, it is likely that the drainage systems were quite
different. The resulting major drainage changes
may have important ecological effects, for example
on the evolution of indigenous fauna.
ORAL
TUBULAR CARBONATE CONCRETIONS AS
CONDUITS FOR METHANE ESCAPE
FEEDING SEAFLOOR COLD SEEPS: SOME
NORTH ISLAND EXAMPLES
S. Nyman 1 ,C.Nelson 1 ,K.Campbell 2 ,
F. Schellenberg 1,3 ,P.Kamp 1 ,G.Browne 4 ,
P. King 4
1 Dept. of Earth Sciences, University of Waikato,
Private Bag 3105, Hamilton
2 Dept. of Geology, University of Auckland, Private
Bag 92019, Auckland
3 Institut für Geologie und Paläontologie,
Universität Tübingen, Tübingen, Germany
4 Institute of Geological and Nuclear Sciences, PO
Box 30368, Lower Hutt
(s.nyman*waikato.ac.nz)
Several Cenozoic sedimentary formations in the
North Island of New Zealand include locally
prominent development of carbonate concretions
which exhibit a variety of broadly tubular shapes,
and commonly a central conduit that may be empty
or filled with sediment or multiple generations of
later carbonate cements. Stable oxygen and carbon
isotope data suggest that many of the concreting
and later cements were sourced primarily from
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