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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core complex mode, temperature driven viscosity<br />
variations can determine whether the lower crust<br />
flows in a distributed manner or localisation occurs.<br />
This may allow a distinction between ‘cold’ or<br />
‘strong’ core complexes and ‘hot’ or ‘weak’ core<br />
complexes. The latter develop when the lower crust<br />
has a markedly low viscosity. Hot core complexes<br />
in numerical simulations are characterized by largescale<br />
normal faults with great slip and cooling rates<br />
and the de-velopment of an overall symmetric<br />
geometry expressed by a relatively late, secondary<br />
ductile shear zone with an antithetic movement<br />
sense caused by symmetric, plume-like extrusion of<br />
the hot and weak lower crust. In nature, hot core<br />
complexes may be ex-pressed by extension-related<br />
migmatite domes in their footwalls. Most core<br />
complexes worldwide appear to belong to the<br />
monovergent cold case. We report on a hot core<br />
complex exposed on the islands of Naxos and Ios in<br />
the Aegean Sea.<br />
The Aegean Sea is an example of ongoing<br />
continental extension and core complex formation<br />
caused by the retreating Hellenic subduction zone.<br />
The extension-related migmatite dome on Naxos<br />
Island is characterized by cooling rates<br />
>100°C/Myr. Low-temperature thermochronology<br />
reveals great slip rates of ~10 km/Myr associated<br />
with the top-N Naxos detachment, which is the<br />
major detachment of the hot core complex. The<br />
secondary, antihtetic top-S normal fault is exposed<br />
on Ios Island some 40 km south of Naxos. Age data<br />
suggest that the top-S shear zone on Ios formed ~2-<br />
5 Myr after the Naxos detachment. These data are<br />
consistent with the numerical simulations of hot<br />
core complexes. Exhumation of hot footwall<br />
material on Ios did not expose magmatites but<br />
triggered anatexis in the lower crust producing synextensional<br />
S-type granites in the footwall. Naxos<br />
and Ios are presently situated in a central position<br />
above the retreating Hellenic subduction zone. We<br />
argue that asthenospheric flow associated with<br />
subduc-tion-zone retreat caused increased heat<br />
input in the center of the region affected by subduction-zone<br />
retreat causing the anomalous hot<br />
conditions on Naxos and Ios.<br />
The Paparoa core complex in Buller, South Island,<br />
is a symmetric core complex (Tulloch &<br />
Kimbrough 1989) suggesting it fits the hot core<br />
complex case. High cooling rates of the Buckland<br />
granite in the footwall of the Paparoa core complex<br />
(Spell et al. 2000) are in line with this<br />
interpretation. In addition, the Paparoa core<br />
complex should have detachments with slightly<br />
different ages, with large displacements and high<br />
slip rates. Furthermore, high-grade rocks should<br />
occur in the footwall (White et al. 1994) and their<br />
peak metamorphism should be of Late Cretaceous<br />
age.<br />
ORAL<br />
PALAEOMAGNETISM <strong>OF</strong> VOLCANIC<br />
ROCKS IN WAITAKERE AND<br />
COROMANDEL, AND MIOCENE<br />
RECONSTRUCTION <strong>OF</strong> <strong>NEW</strong> ZEALAND<br />
D.J. Robertson<br />
Physics Department, University of Namibia, P/Bag<br />
13301 Windhoek, NAMIBIA.<br />
(DJR*unam.na)<br />
Samples from 8 sites in the Waitakere Group,<br />
located to the west of Auckland, and 8 sites in the<br />
Coromandel Group in the Coromandel Peninsula<br />
have been studied palaeomagnetically using<br />
complete progressive af demagnetisation. The two<br />
Groups lie on opposite limbs of a gentle anticline,<br />
and the respective formation mean palaeofield<br />
directions become more consistent after unfolding<br />
is carried out on a regional basis. After unfolding,<br />
the palaeofield directions are 346.9°, -62.1°, 26.7°<br />
(as declination, inclination and �95) for the<br />
Waitakere Group and 347.5°, -57.9°, 14.1° for the<br />
Coromandel Group. Comparison of these results<br />
with palaeomagnetic data for other parts of New<br />
Zealand indicates that, relative to the southeast<br />
South Island and Chatham Rise, the Northern North<br />
Island has migrated northwards by an amount in the<br />
order of 1000 km and rotated by some 12°<br />
anticlockwise and since Miocene time. This is<br />
consistent with large scale dextral displacement of<br />
the Alpine Fault, compression in the Southern Alps<br />
and dilatation in the North Island. Anomalous<br />
clockwise rotation in the order of 15° to 45° which<br />
has been reported for the eastern North Island and<br />
Marlborough is attributable to dextral shear<br />
deformation in a zone incorporating the Hikurangi<br />
Trench system and likely extensions of the Alpine<br />
Fault.<br />
ORAL<br />
GEOCHEMICAL ASPECTS <strong>OF</strong> THE<br />
COROMANDEL VOLCANIC ZONE: TRACE<br />
ELEMENT AND RARE EARTH ELEMENT<br />
DATA<br />
K.A. Robertson 1 , R.M. Briggs 1 ,<br />
& M.I. Leybourne 2<br />
1 Department of Earth Sciences, University of<br />
Waikato, P B 3105, Hamilton.<br />
2 Dept. of Geosciences, University of Texas at<br />
Dallas, Richardson, TX, USA<br />
(kr48*waikato.ac.nz)<br />
Late Cenozoic volcanic activity in the North Island<br />
of New Zealand has been attributed to a series of<br />
volcanic arcs following the movement of the<br />
subduction zone boundary between the Pacific and<br />
Indo-Australian plates. The Coromandel Volcanic<br />
Zone (CVZ) is one such volcanic arc active from 18<br />
50 th <strong>Kaikoura</strong>05 -75- <strong>Kaikoura</strong> <strong>2005</strong>