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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3D IMAGING <strong>OF</strong> THE FORE- AND<br />
BACKTHRUSTS <strong>OF</strong> THE GLEN LYON<br />
SEGMENT <strong>OF</strong> THE OSTLER FAULT<br />
David C. Nobes 1 , Douglas W. Burbank 2 ,<br />
Colin Amos 2 & Kenneth Davis 2<br />
1 Department of Geological Sciences, University of<br />
Canterbury, P.B. 4800, Christchurch<br />
2 Department of Geological Sciences, University of<br />
California-Santa Barbara, USA<br />
(david.nobes*canterbury.ac.nz)<br />
A set of closely spaced ground penetrating radar<br />
(GPR) and GPS survey lines were acquired across<br />
the fore- and backthrusts of the Glen Lyon segment<br />
of the Ostler Fault. The project is part of a larger<br />
ongoing study of thrust fault processes.<br />
We collected 21 survey lines, 50-m long and 1 m<br />
apart, and approximately centred on the fore- and<br />
backthrust scarps. In addition, common midpoint<br />
(CMP) velocity surveys were completed at each site<br />
so that the profiles could be migrated, and the travel<br />
time converted to depth. The velocity structure at<br />
the two sites was similar, yet subtly different, and<br />
consistent with mostly unsaturated sandy and silty<br />
gravels. At the forethrust site, the velocity was 110<br />
m/µs (± 10 – 15 m/µs) down to about 4 m depth,<br />
rising to 120 ± 10 m/µs before returning to 110 ±<br />
10 m/µs from 5 to 8 m depth. In contrast, the<br />
backthrust velocities below the scarp were 110 ± 15<br />
m/µs at shallow depths (< 2m), and 120 – 130 ± 15<br />
m/µs at depths down to 6 m. Above the backthrust<br />
scarp, the velocities were more consistent with dry<br />
sands and gravels, of the order of 150 m/µs, but<br />
were not well constrained. Deeper velocities were<br />
not resolved at either site.<br />
The 3D GPR profiles reveal a backthrust with a<br />
relatively simple geometry, and dipping 65° ± 8° to<br />
the east. For the forethrust, on the other hand, there<br />
are from 2 to 4 individual fault displacements<br />
visible in the 21 GPR lines, with dips of the order<br />
of 56° ± 9° to the west-northwest. These results are<br />
consistent with measurements on outcrop (Davis et<br />
al., <strong>2005</strong>) and GPR profiles from the Benmore<br />
segment of the Ostler Fault (Wallace et al., in<br />
submission), which yielded dips of 50° and 51°,<br />
respectively, ± 9°.<br />
The most recent main fault displacement almost<br />
reaches the surface near the top of the current<br />
forethrust scarp, with a thin wedge of sediment in<br />
front of the fault. The other fault displacements,<br />
each with its own sediment wedge, are located at<br />
successively greater depths and in front (to the east)<br />
of the current scarp. The forethrust position appears<br />
to be migrating westward toward the hinterland<br />
with each new rupture sequence.<br />
Davis, Kenneth, Burbank, Douglas W., Fisher, Donald,<br />
Wallace, Shamus & Nobes, David, <strong>2005</strong>. Thrust fault<br />
growth and segment linkage in the active Ostler fault<br />
zone, New Zealand. Journal of Structural Geology,<br />
27: 1528-1546.<br />
Wallace, Shamus C., Nobes, David C., Davis, Kenneth J.,<br />
Burbank, Douglas W. & White, Antony. Threedimensional<br />
imaging of the Benmore segment of the<br />
Ostler Fault, South Island, New Zealand. Geophysical<br />
Journal International: in submission.<br />
ORAL<br />
ACTIVE FAULTING AND<br />
PALEOSEISMICITY <strong>OF</strong> THE <strong>OF</strong>FSHORE<br />
KAPITI-MANAWATU FAULT SYSTEM,<br />
SOUTHERN NORTH ISLAND, <strong>NEW</strong><br />
ZEALAND<br />
S.D. Nodder 1 ,G.Lamarche 1 &J.-N.Proust 2<br />
1 National Institute of Water and Atmospheric<br />
Research, Private Bag 14-901, Wellington.<br />
2 Géosciences Rennes, UMR CNRS, Univ. de<br />
Rennes 1, 35042 Rennes cedex, France.<br />
(s.nodder*niwa.co.nz)<br />
Seismic hazards associated with active faulting in<br />
coastal regions may be under-estimated if recent<br />
offshore deformation is not accounted for in hazard<br />
analyses. At subduction margins, deformation<br />
occurs predominantly in the high-strain forearc<br />
basin, but can also extend considerable distance<br />
away into the back-arc environment. An example is<br />
described here in the offshore, low contractional<br />
strain Kapiti-Manawatu Fault System (KFMS),<br />
which is located along the eastern margin of the<br />
proto back-arc Wanganui Basin, 200 km behind the<br />
Hikurangi margin subduction front. High-resolution<br />
seismic reflection data show the presence of<br />
numerous sea-floor fault scarps and near-surface<br />
deformation of late Quaternary and possibly<br />
Holocene reflectors on high-angle (>60°),<br />
reactivated reverse and normal faults within the<br />
KMFS. Continuous sea-floor scarps, which are<br />
inferred to correspond to earthquake rupture<br />
segments, range from