New Zealand Journal of Geology and Geophysics abstracts
Collapse in a Quaternary shelf basin off East Cape, New Zealand: evidence
for passage of a subducted seamount inboard of the Ruatoria giant avalanche
Keith B. Lewis1*
Serge E. Lallemand2
Lionel Carter1
1National Institute of Water and Atmospheric Research Ltd
(NIWA)
P.O. Box 14 901
Wellington, New Zealand
*email: k.lewis@niwa.co.nz
2Laboratoire de Dynamique de la Lithosphere
Institut des Sciences de la Terre, de l’Environnement et de l’Espace
Université Montpellier 2
CC 60, place E. Bataillon
34095 Montpellier, France
Abstract The Ruatoria margin indentation and its associated
giant avalanche off East Cape, New Zealand, have been inferred to result
from margin instability following oblique subduction of a large seamount.
The earlier studies hypothesise that a diachronous seamount-wake trough formed
the northern part of the indentation, and collapse between the oblique trough
and an oversteepened margin front formed the southern indentation and giant
avalanche. If correct, then the impacting seamount must now be landward of
the indentation. New seismic profiles, supported by multibeam bathymetry
and core samples, from landward of the Ruatoria Indentation, provide support
for the passage of a large seamount deep beneath the continental shelf.The
continental margin around the head of the indentation is underlain by Quaternary
basins that are inferred to result from transpression associated with oblique
plate convergence. One basin underlies the shelf landward of the indentation,
its seaward edge having collapsed into the indentation. There, faults and
a graben with gravitational collapse structures that are transverse to the
northeast-southwest regional trends may be evidence of passage of a seamount.
A tentative sequence stratigraphy, based on Quaternary unconformities and
shelf-edge prograding units, suggests that the main phase of collapse occurred
on the outer shelf before the penultimate, major, glacial sea-level lowering
c. 155-135 000 yr ago. Depths to the post-last-glacial erosion surface indicate
that the basin is subsiding rapidly at >4 m/ka. If a significant part
of the subsidence relates to wake collapse behind a subducting seamount,
then the seamount must now underlie the adjacent land. Onshore, doming hills,
with coastal terraces indicating uplift of 2.6 m/ka, suggest that the seamount
is now c. 10 km west of East Cape.
Keywords Hikurangi margin; New Zealand; Quaternary basin;
subducted seamount; debris avalanche; submarine landslide; Raukumara Peninsula
G03046; Received 12 May 2003; accepted 28 January 2004; Online publication
date 7 September 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 415-429
0028-8306/04/4703-0415 © The Royal Society of New Zealand 2004
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