New Zealand Journal of Geology and Geophysics abstracts
Crustal structure in the central South Island, New Zealand, from the
Lake Pukaki seismic experiment
STEFAN KLEFFMANN1
FRED DAVEY2
ANNE MELHUISH2
DAVID OKAYA3
TIM STERN1
and the SIGHT Team
4
1Institute of Geophysics
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
2Institute of Geological & Nuclear Sciences
P.O. Box 1320
Wellington, New Zealand
3University of Southern California
University Park
Los Angeles, CA 90089-0740
U.S.A.
4South Island GeopHysical Transect working group
Abstract The crustal structure of the central South Island,
New Zealand, has been investigated using explosion seismology measurements. A
series of 114 shots were fired in Lake Pukaki and recorded in three modes: on a
120 channel, 6 km long, seismic reflection array rolled along a 27 km
profile on the eastern margin of Lake Pukaki; on a Reftek seismograph array
consisting of 40 units spread over a 52 km long line, partially coincident
with the reflection profile; and wide angle reflections from the lower crust
recorded on permanent stations of the New Zealand Seismograph Network that were
located between 80 and 120 km from the shots. The data show that the
greywacke-schist forming the mid-upper crust of the Pacific plate is
c. 25 km thick near the east coast and thickens as a crustal root to
>35 km beneath the Southern Alps. A strong reflection was recorded from
a depth of 25 km and is interpreted to be from the base of the
greywacke-schist crust overlying an old oceanic crust.
Below 6 km depth the compressional seismic velocity is relatively uniform
and increases only slightly from 6.2 km/s to 6.25 km/s at the lower
crustal interface. A relatively large velocity gradient of 0.22 (km/s)/km
is derived for the upper 6 km of the greywacke basement. Whether this
velocity gradient, derived from a relatively short profile, is regional in
extent needs to be verified. A Poisson's ratio of 0.21 +/- 0.03 was determined
from apparent velocities of Pg and Sg for the greywacke/schist and is
interpreted to indicate fracturing in this layer.
Reflections, inferred to be from segments of the downdip extension of a broad
Alpine Fault Zone, occur at depths of c. 22 and 28 km and indicate a
dip of 33 +/- 5deg.. A width of 7.5 km is estimated for the fault zone at
this depth. The dipping reflector segments would intercept the surface west of
the Alpine Fault, and suggest that the fault zone is steeper at shallower
depths.
The total amount of material, which has been subjected to uplift, has been
estimated from the amount of crustal shortening and the shape of the crustal
root. These estimates indicate that 50% of this material has not been accounted
for in previous studies, and suggests either a non-exponential distribution, or
larger rates of uplift southeast of the Alpine Fault than thought previously.
Keywords crustal structure; active source seismology;
tectonics; reflection; refraction; central South Island; Lake Pukaki
New Zealand Journal of Geology and Geophysics, 1998, Vol. 41: 39-49
0028-8306/98/4101-0039 $7.00/0 (c) The Royal Society of New Zealand
1998
PDF file of entire paper: medium quality (3507K); (scanned from paper original: notes about this process)
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