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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Sibson, R.H., 1983. Continental fault structure and the<br />
shallow earthquake source. Journal of the Geological<br />
Society of London, 5, 741–767.<br />
Zoback, M.D., and Townend, J., 2001. Implications of<br />
hydrostatic pore pressures and high crustal strength<br />
for the deformation of intraplate lithosphere.<br />
Tectonophysics, 336, 19–30.<br />
POSTER<br />
THE EARLY MIOCENE PLATE BOUNDARY<br />
IN MARLBOROUGH, <strong>NEW</strong> ZEALAND:<br />
THRUST FAULTING AND REGIONAL<br />
CLOCKWISE VERTICAL-AXIS ROTATION<br />
D. B. Townsend 1 &T.A.Little 2<br />
1 GNS, PO Box 30368 Lower Hutt<br />
2 School of Earth Sciences, Victoria University, PO<br />
Box 600 Wellington<br />
(d.townsend*gns.cri.nz)<br />
Plate reconstructions for the Early Miocene suggest<br />
that the Pacific-Australia plate boundary trended<br />
NW-SE and consisted of an array of thrust faults<br />
striking parallel to the paleo-Hikurangi margin.<br />
Parts of that subduction complex are preserved<br />
onshore in the Raukumara Peninsula, Wairarapa<br />
and Marlborough, where it was pinned at its eastern<br />
end by the Chatham Rise. The plate boundary zone<br />
in Marlborough has experienced a complex<br />
structural history, culminating in the late Neogene<br />
development of the NE-striking, dextral strike-slip<br />
Marlborough Fault System (MFS).<br />
Inception of the Early Miocene plate boundary<br />
through New Zealand is recorded in Marlborough<br />
by development of the imbricate Flags Creek Fault<br />
System (FCFS) and coeval deposition of the<br />
(Otaian-Altonian) Great Marlborough<br />
Conglomerate. New structural mapping and<br />
analysis of folded thrust sheets in the FCFS has<br />
yielded a predominant in-situ SE vergence, which<br />
is strongly discordant to the inferred NW-SE strike<br />
and NE (seaward) vergence of the Early Miocene<br />
subduction complex.<br />
Five new paleomagnetic samples from around the<br />
arcuate FCFS coupled with existing paleomagnetic<br />
data from coastal Marlborough suggest that the<br />
entire area has undergone clockwise vertical-axis<br />
rotation of at least 100º during the Middle and early<br />
Late Miocene [Vickery and Lamb 1995; Little &<br />
Roberts 1997]. Late Miocene to Pliocene clockwise<br />
vertical-axis rotation of up to 44º has affected<br />
coastal Marlborough to the north of the FCFS<br />
[Roberts 1995; Little & Roberts 1997]. Late<br />
Pliocene to Quaternary dextral shear strain within<br />
~5 km of the active Kekerengu Fault has locally<br />
produced additional clockwise vertical-axis rotation<br />
(up to 45º) near Woodside Creek.<br />
Restoration of strike-slip on the MFS and of the<br />
above-described rotations realigns the Early<br />
Miocene FCFS back to its original NW-SE strike<br />
and NE- (seaward-) vergent configuration. The<br />
proto-Clarence Fault (Flags Creek Fault) initiated<br />
as a NW-SE striking thrust within the Early<br />
Miocene subduction complex, and accrued >10 km<br />
of NE-vergent slip (with an additional 16 km of<br />
shortening within the FCFS) before being folded,<br />
strongly clockwise-rotated, and locally reactivated.<br />
Subsequently, oblique plate motion has allowed this<br />
and other faults in the MFS to remain active while<br />
transitioning from dip-slip to oblique- or strike-slip<br />
styles of deformation.<br />
Roberts, A. 1995: Tectonic rotation about the termination<br />
of a major strike-slip fault, Marlborough fault system,<br />
New Zealand. Geophysical Research Letters 22 (3),<br />
187-190<br />
Vickery, S., Lamb, S. 1995: Large tectonic rotations<br />
since the Early Miocene in a convergent plate<br />
boundary zone, South Island, New Zealand. Earth and<br />
Planetary Science Letters 136, 44-59.<br />
Little, T., Roberts, A. 1997: Distribution of Neogene to<br />
present-day vertical-axis rotations, Pacific-Australia<br />
plate boundary zone, South Island, New Zealand.<br />
Journal of Geophysical Research 102 (B9), 20447-<br />
20468<br />
POSTER<br />
FABRIC TRANSITIONS IN THE ALPINE<br />
FAULT MYLONITES: VARIATION IN THE<br />
TEMPERATURE <strong>OF</strong> THE BRITTLE-<br />
PLASTIC TRANSITION DUE TO CHANGES<br />
IN STRAIN RATE<br />
V.G. Toy 1 ,R.J.Norris 1 &D.J.Prior 2<br />
1 Geology Department, University of Otago, PO<br />
Box 56, Dunedin.<br />
2 Department of Earth and Ocean Sciences,<br />
University of Liverpool, L69 3GP, U.K.<br />
(virginia*geology.co.nz)<br />
The Alpine Fault is the major structure within the<br />
Australian–Pacific plate boundary and<br />
accommodates c. 25 mm/yr displacement. Due to<br />
the oblique convergence across it, mylonites<br />
formed by ductile shear at depth are exhumed in the<br />
hanging wall immediately east of the present fault<br />
trace. They grade from ultramylonite close to the<br />
fault to protomylonite adjacent to the Alpine Schist<br />
protolith c. 1 km to the east.<br />
In order to gain further information about the<br />
distribution of strain and deformation conditions<br />
within the hanging-wall, Alpine Schist-derived<br />
Alpine fault mylonites, we have collected quartz<br />
crystallographic preferred orientation (CPO) data<br />
from mylonite samples along two transects<br />
perpendicular to the Alpine fault. Analyses were<br />
50 th <strong>Kaikoura</strong>05 -88- <strong>Kaikoura</strong> <strong>2005</strong>