50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
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sideritic concretions that occur in Eocene Waikato<br />
Coal Measures in New Zealand.<br />
ORAL<br />
ANTISCARP INITIATION AND EVOLUTION<br />
Verne H. Pere, Tim Davies & Jarg R. Pettinga<br />
Dept. Geological Sciences, University of<br />
Canterbury, Private Bag 4800, Christchurch.<br />
(v.pere*geol.canterbury.ac.nz)<br />
Antiscarps are defined here as any uphill facing<br />
scarp that may be observed on slopes, regardless of<br />
size or scale, and are variously referred to in<br />
literature as sackungen, ridge rents, obsequent<br />
scarps and reverse scarps. Antiscarps are not a<br />
feature of all slopes, but where they are observed in<br />
New Zealand, they typically occur sub-parallel to<br />
the valley axes near the ridge crests in mountainous<br />
regions. In Scotland they have been observed over<br />
the entire slope that is from the ridge crest to the<br />
valley floor.<br />
This research project aims to identify how seismic<br />
activity, slope rock mass properties, and mountain<br />
stress field changes by slope loading and unloading<br />
from glacial and rock avalanche events, contribute<br />
to the initiation and evolution of antiscarps.<br />
Research for this project has so far identified case<br />
study areas in New Zealand and in Scotland<br />
suitable for analysis of antiscarp formation. In all of<br />
the field areas investigated the antiscarps are<br />
evident as linear features across the slope with<br />
observed lengths ranging from 200–1000+m.<br />
Vertical separation from the crest of the antiscarp to<br />
the scarp intersection with the slope ranges<br />
between 1-5m on the antiscarps observed in New<br />
Zealand, and 1-10m for those in Scotland.<br />
None of the chosen case study areas show any<br />
evidence to suggest that antiscarp formation is<br />
driven by translational movement. The case study<br />
antiscarps are typically aligned with defects<br />
observed within the respective slopes suggesting<br />
that these antiscarps are in situ deformations of the<br />
slope rather than secondary features within a larger<br />
slope failure. Primary mechanisms considered to<br />
drive in situ slope deformation are post-glacial<br />
rebound, seismic activity, gravity faulting, and<br />
unloading of the slope for mountain by large<br />
rockfalls.<br />
POSTER<br />
HIGH-TEMPERATURE DUCTILE<br />
EXTENSION PRESERVED IN THE<br />
FOOTWALL <strong>OF</strong> A LOW-ANGLE NORMAL<br />
FAULT, MISIMA ISLAND, PAPUA <strong>NEW</strong><br />
GUINEA<br />
K. J. Peters 1 ,T.A.Little 1 ,S.L.Baldwin 2<br />
&P.G.Fitzgerald 2<br />
1 School of Earth Sciences, Victoria University of<br />
Wellington, PO Box 600, Wellington.<br />
2 Dept. of Earth Sciences, Syracuse University,<br />
Syracuse, N.Y., U.S.A<br />
(peterskati*student.vuw.ac.nz)<br />
The oblique collision of the Australian and Pacific<br />
plates has resulted in a complicated zone of microplates<br />
in SE Papua New Guinea. The actively<br />
spreading Woodlark rift forms the boundary<br />
between the Australian and Woodlark plates to the<br />
southeast of the Papuan Peninsula. Seafloor<br />
spreading rates vary between 30-40 mm/yr at<br />
152°E and ~70 mm/yr at 156°E (Abers et al, 1997)<br />
and are among the fastest in the world. Continental<br />
rifting gives way to sea-floor spreading to the east<br />
of 151.4° E providing a unique opportunity to study<br />
this transition. The rift is a known area of active<br />
low-angle normal faulting and metamorphic core<br />
complex (MCC) formation including Misima<br />
Island, a MCC situated on the Pocklington Rise,<br />
~75 km to the south of the Woodlark spreading<br />
centre and 100 km east of its western tip.<br />
A normal fault, which dips ~25° to the NE, is the<br />
dominant structural feature on Misima Island,<br />
splitting it into two distinct geological domains.<br />
Amphibolite-facies felsic and mafic gneisses occur<br />
in the lower plate and are placed against<br />
greenschist-facies schists and overlying<br />
unmetamorphosed conglomerates, volcanics, and<br />
other sedimentary rocks in the upper plate.<br />
Separating these two domains is a fault zone with a<br />
layer of gouge >3m thick. The measured NNE slip<br />
direction on the fault (slickenlines) is parallel to the<br />
Australia-Woodlark plate vector between 500 ka<br />
and 3.5 Ma, calculated from seafloor spreading data<br />
(Taylor et al., 1999), showing that the development<br />
of this structure was directly related to the<br />
extensional tectonics of the Woodlark rift. Beneath<br />
the fault there is a 2-3 km thick zone of gneisses<br />
that were ductilely deformed in the lower crust<br />
during the extension. These gneisses have<br />
stretching lineations, commonly defined by<br />
elongate hornblende crystals, that trend towards<br />
NNE, in the plate motion direction and a shallow<br />
dipping foliation that is sup-parallel to the<br />
overlying fault plane. Structurally deeper rocks<br />
further to the west retain E-W trending lineations<br />
on a sub-horizontal foliation, which we interpret as<br />
an older, collisional fabric. Directly beneath the<br />
fault the basement gneisses are strongly lineated<br />
and flaggy, appearing mylonitic at outcrop scale,<br />
but lack typical microstructural evidence for noncoaxial<br />
deformation as is commonly seen in the<br />
lower plate of other MCCs, including those in the<br />
nearby D’Entrecasteux Islands. Quartz CPO<br />
analyses by U-stage and electron backscattered<br />
diffraction techniques show that deformation<br />
50 th <strong>Kaikoura</strong>05 -67- <strong>Kaikoura</strong> <strong>2005</strong>