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

sideritic concretions that occur in Eocene Waikato
Coal Measures in New Zealand.
ORAL
ANTISCARP INITIATION AND EVOLUTION
Verne H. Pere, Tim Davies & Jarg R. Pettinga
Dept. Geological Sciences, University of
Canterbury, Private Bag 4800, Christchurch.
(v.pere*geol.canterbury.ac.nz)
Antiscarps are defined here as any uphill facing
scarp that may be observed on slopes, regardless of
size or scale, and are variously referred to in
literature as sackungen, ridge rents, obsequent
scarps and reverse scarps. Antiscarps are not a
feature of all slopes, but where they are observed in
New Zealand, they typically occur sub-parallel to
the valley axes near the ridge crests in mountainous
regions. In Scotland they have been observed over
the entire slope that is from the ridge crest to the
valley floor.
This research project aims to identify how seismic
activity, slope rock mass properties, and mountain
stress field changes by slope loading and unloading
from glacial and rock avalanche events, contribute
to the initiation and evolution of antiscarps.
Research for this project has so far identified case
study areas in New Zealand and in Scotland
suitable for analysis of antiscarp formation. In all of
the field areas investigated the antiscarps are
evident as linear features across the slope with
observed lengths ranging from 200–1000+m.
Vertical separation from the crest of the antiscarp to
the scarp intersection with the slope ranges
between 1-5m on the antiscarps observed in New
Zealand, and 1-10m for those in Scotland.
None of the chosen case study areas show any
evidence to suggest that antiscarp formation is
driven by translational movement. The case study
antiscarps are typically aligned with defects
observed within the respective slopes suggesting
that these antiscarps are in situ deformations of the
slope rather than secondary features within a larger
slope failure. Primary mechanisms considered to
drive in situ slope deformation are post-glacial
rebound, seismic activity, gravity faulting, and
unloading of the slope for mountain by large
rockfalls.
POSTER
HIGH-TEMPERATURE DUCTILE
EXTENSION PRESERVED IN THE
FOOTWALL OF A LOW-ANGLE NORMAL
FAULT, MISIMA ISLAND, PAPUA NEW
GUINEA
K. J. Peters 1 ,T.A.Little 1 ,S.L.Baldwin 2
&P.G.Fitzgerald 2
1 School of Earth Sciences, Victoria University of
Wellington, PO Box 600, Wellington.
2 Dept. of Earth Sciences, Syracuse University,
Syracuse, N.Y., U.S.A
(peterskati*student.vuw.ac.nz)
The oblique collision of the Australian and Pacific
plates has resulted in a complicated zone of microplates
in SE Papua New Guinea. The actively
spreading Woodlark rift forms the boundary
between the Australian and Woodlark plates to the
southeast of the Papuan Peninsula. Seafloor
spreading rates vary between 30-40 mm/yr at
152°E and ~70 mm/yr at 156°E (Abers et al, 1997)
and are among the fastest in the world. Continental
rifting gives way to sea-floor spreading to the east
of 151.4° E providing a unique opportunity to study
this transition. The rift is a known area of active
low-angle normal faulting and metamorphic core
complex (MCC) formation including Misima
Island, a MCC situated on the Pocklington Rise,
~75 km to the south of the Woodlark spreading
centre and 100 km east of its western tip.
A normal fault, which dips ~25° to the NE, is the
dominant structural feature on Misima Island,
splitting it into two distinct geological domains.
Amphibolite-facies felsic and mafic gneisses occur
in the lower plate and are placed against
greenschist-facies schists and overlying
unmetamorphosed conglomerates, volcanics, and
other sedimentary rocks in the upper plate.
Separating these two domains is a fault zone with a
layer of gouge >3m thick. The measured NNE slip
direction on the fault (slickenlines) is parallel to the
Australia-Woodlark plate vector between 500 ka
and 3.5 Ma, calculated from seafloor spreading data
(Taylor et al., 1999), showing that the development
of this structure was directly related to the
extensional tectonics of the Woodlark rift. Beneath
the fault there is a 2-3 km thick zone of gneisses
that were ductilely deformed in the lower crust
during the extension. These gneisses have
stretching lineations, commonly defined by
elongate hornblende crystals, that trend towards
NNE, in the plate motion direction and a shallow
dipping foliation that is sup-parallel to the
overlying fault plane. Structurally deeper rocks
further to the west retain E-W trending lineations
on a sub-horizontal foliation, which we interpret as
an older, collisional fabric. Directly beneath the
fault the basement gneisses are strongly lineated
and flaggy, appearing mylonitic at outcrop scale,
but lack typical microstructural evidence for noncoaxial
deformation as is commonly seen in the
lower plate of other MCCs, including those in the
nearby D’Entrecasteux Islands. Quartz CPO
analyses by U-stage and electron backscattered
diffraction techniques show that deformation
50 th Kaikoura05 -67- Kaikoura 2005